Seismic wave paths through the Earth's interior. Two of the four kinds of seismic waves, P and S waves, travel through the interior of the Earth. By measuring the time it takes these waves to reach seismograph stations around the world, scientists can trace the paths the waves take, observe how their velocities vary, and hence determine the Earth's structure.
The Earth's Structure and Atmosphere (1 of 5)
Moving outwards from the Earth, man has been to the Moon, landed spacecraft on planets, and sent space probes to the outermost reaches of the Solar System. But in the opposite direction the story is very different. Man's direct access to the Earth's interior is limited to the depth of the deepest mine, which is less than 4 km (2.5 mi). The Russians spent most of the 1980s drilling a hole in the crust to a target depth of 15 km (9.3 mi), but in doing so they have penetrated no more than the upper 0.24% of the Earth, the average radius of which is 6371 km (3956 mi).
Unable to visit the Earth's deep interior or place instruments within it, scientists must explore in more subtle ways. One method is to measure natural phenomena - the magnetic and gravitational fields are the chief examples - at the Earth's surface and interpret the observations in terms of the planet's internal properties.
A second approach is to study the Earth with non-material probes, the most important of which are the seismic waves emitted by earthquakes. As seismic waves pass through the Earth, they undergo sudden changes in direction and velocity at certain depths. These depths mark the major boundaries, or discontinuities, that divide the Earth into crust, mantle and core.
The crust
The outermost layer of the Earth, the crust, accounts for only about 0.6% of the planet's volume. The average thickness of the oceanic crust is 5-9 km (3-5 1/2 mi) and varies comparatively little throughout the world. By contrast, the continental crust has the much higher average thickness of 30-40 km (18 1/2-25 mi) and varies much more. Beneath the central valley of California, for example, the crust is only about 20 km (12 1/2 mi) thick, but beneath parts of major mountain ranges such as the Himalaya it can exceed 80 km (50 mi).
The rocks that form the continental crust are highly varied, including volcanic lava flows, huge blocks of granite, and sediments laid down in shallow water when parts of the continents were inundated by the sea. Despite the diversity of materials, the average composition is roughly that of the rock granite, and the two most common elements (in addition to oxygen) are silicon and aluminum.
The oceanic crust is much more uniform in composition and, apart from a thin covering of sediment, consists largely of the rock basalt, possibly underlain by the rock gabbro (which has the same composition as basalt but is coarser grained). Oxygen apart, the most common elements in the oceanic crust are again silicon and aluminum, but there is markedly more magnesium than in the upper continental crust.
The composition of the lower crust, which cannot be sampled directly, is uncertain, but the predominant rock is probably gabbro. The lower crust is certainly different from the upper crust because seismic waves pass through it at a higher velocity.
* THE SUN AND THE SOLAR SYSTEM
* THE INNER PLANETS
* PLATE TECTONICS
* THE FORMATION OF ROCKS
* THE WEATHER
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p076-2
ftsTitleOverride
The Earth's Structure and Atmosphere (page 2)
ftsTitle
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The Earth's Structure and Atmosphere (2 of 5)
The mantle
The mantle extends from the base of the crust to a depth of about 2900 km (1800 mi) and accounts for about 82% of the Earth's volume. The sharp boundary between the crust and the mantle is called the Mohorovicc discontinuity (or Moho for short) after the Yugoslav seismologist who discovered it in 1909.
The mantle is thought to consist largely of peridotite, a rock that contains high proportions of the elements iron, silicon and magnesium, in addition to oxygen. The mantle is inaccessible, but evidence of its composition comes from surface rocks thought to have originated there. Although mostly solid, the mantle contains a partially molten layer.
* THE SUN AND THE SOLAR SYSTEM
* THE INNER PLANETS
* PLATE TECTONICS
* THE FORMATION OF ROCKS
* THE WEATHER
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p076-3
ftsTitleOverride
The Earth's Structure and Atmosphere (page 3)
ftsTitle
The Earth's Structure and Atmosphere (3 of 5)
The core
The core extends from the base of the mantle to the Earth's center and accounts for about 17% of the Earth's volume. The discontinuity between the mantle and core is called the core-mantle boundary or, sometimes, the Gutenberg discontinuity, after the German-American seismologist Beno Gutenberg. The core actually comprises two distinct parts. The outer core - which extends down to a depth of about 5155 km (3200 mi) - is liquid. The inner core is solid.
The main constituent of the core is iron, although measurements of the Earth's rate of rotation show that the density must be slightly lower than that of pure iron. The core must therefore contain a small proportion (5-20%) of some lighter element - possibly sulfur, silicon, carbon, hydrogen or oxygen.
An alternative view
The division of the Earth into crust, mantle and core is based on the fact that the three zones have different chemical compositions. However, there is another way of looking at the Earth, in terms of its physical state.
In the upper mantle, at depths of 75-250 km (46 1/2-155 mi), the velocity of seismic waves is slightly lower than in the zones just above and below. Scientists believe that this layer of the upper mantle is partially molten, and they have named it the asthenosphere. It is this layer that is the source of volcanic magma. The rigid layer above the asthenosphere, the lithosphere, comprises the crust and uppermost mantle. The solid region of the mantle below the asthenosphere is called the mesosphere.
* THE SUN AND THE SOLAR SYSTEM
* THE INNER PLANETS
* PLATE TECTONICS
* THE FORMATION OF ROCKS
* THE WEATHER
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p076-4
ftsTitleOverride
The Earth's Structure and Atmosphere (page 4)
ftsTitle
The magnetic field of the Earth (shown here in cross-section) is mainly that of a dipole - it has the same shape as that of a giant bar magnet at the center of the Earth (although the field is actually produced by motions of liquid iron within the Earth's core). The lines emerging from the magnet are lines of induction; they show the directions in which magnetic compass needles (red arrows) would point if placed in the field.
Most navigational compasses are mounted on a vertical pin, and so can only move on a horizontal plane. However, if a compass needle is suspended on a thread, it will point downwards towards the Earth (in the northern hemisphere) or upwards (in the southern hemisphere), as well as towards the magnetic north pole.
The Earth's Structure and Atmosphere (4 of 5)
The magnetic field
The Earth has a magnetic field, which is why a compass needle points approximately north at most places on the Earth's surface. But where, and how, is the field generated?
The magnetic field has two parts. Most of it is that of a simple dipole; it is as if a giant bar magnet were placed at the center of the Earth (although the magnet slopes at 11 deg to the Earth's axis of rotation). But a small proportion of it is much more complicated and changes very rapidly. This is why a compass needle points in a slightly different direction each year.
The rapid changing indicates that the magnetic field must be produced in a part of the Earth that is fluid, for no solid region could reorganize itself rapidly enough without shaking the planet to pieces. The only liquid zone inside the Earth is the outer core.
This fits in with something else. The only conceivable way in which a magnetic field could be generated within the Earth is by the flow of very large electric currents, and electric currents need a conductor. The Earth's core is the most conductive zone in the whole Earth, because it consists largely of iron. The silicates of the mantle would simply not conduct well enough.
* THE SUN AND THE SOLAR SYSTEM
* THE INNER PLANETS
* PLATE TECTONICS
* THE FORMATION OF ROCKS
* THE WEATHER
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p076-5
ftsTitleOverride
The Earth's Structure and Atmosphere (page 5)
ftsTitle
The layers of the atmosphere, showing temperature changes from 15 deg C (59 deg F) at sea level to -85 deg C (-12 deg F) in the lower thermosphere.
The Earth's Structure and Atmosphere (5 of 5)
The atmosphere
The atmosphere is easier to investigate than the Earth's interior because it is directly accessible to instruments carried by kites, balloons, airplanes and, latterly, rockets and satellites. What these instruments have shown is that traces of the atmosphere extend for thousands of kilometers above the Earth's surface. There is no sharp boundary between the atmosphere and 'interplanetary space'.
The atmosphere may be divided into four layers on the basis of temperature. In the layer closest to the Earth, the troposphere, temperature decreases with altitude to the top of the layer, which on average is at a height of 10-12 km (6-7 1/2 mi), although the thickness of the layer varies from more than 16 km (10 mi) in the tropics to less than 9 km (5 1/2 mi) in polar regions. Most weather phenomena occur in the troposphere.
Above the troposphere lies the stratosphere. In the stratosphere the temperature remains more or less constant up to 20 km (12 1/2 mi) and thereafter increases to the top of the layer at 45-50 km (28-31 mi). The reason for the temperature rise is that the stratosphere is the home of the atmosphere's ozone (O3), and the ozone absorbs dangerous ultraviolet radiation from the Sun, protecting life on Earth in the process In the next layer, the mesosphere (not to be confused with the mesosphere in the deep Earth), temperature again decreases to the top of the layer at 80-85 km (50-53 mi). Above this lies the thermosphere, throughout most of which the temperature again rises. The thermosphere fades out over thousands of kilometers, gradually merging with 'space', although the region above about 500 km (310 mi) is sometimes called the exosphere. In the altitude range 80-400 km (50-250 mi) - but the upper limit is very poorly defined - the atoms of oxygen and molecules of nitrogen are electrically charged (ionized). This layer, part of the thermosphere, is known as the ionosphere. The ionosphere reflects radio waves and hence makes long-range communications possible.
THE COMPOSITION
OF DRY AIR
Constituent Percentage
by volume
Nitrogen (N2) 78.084
Oxygen (O2) 20.946
Argon (A) 0.934
Carbon dioxide (CO2) 0.034
Neon (Ne) 0.00182
Helium (He) 0.000524
Methane (CH4) 0.00015
Krypton (Kr) 0.000114
Hydrogen (H2) 0.00005
* THE SUN AND THE SOLAR SYSTEM
* THE INNER PLANETS
* PLATE TECTONICS
* THE FORMATION OF ROCKS
* THE WEATHER
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p078-1
ftsTitleOverride
Plate Tectonics (page 1)
ftsTitle
Plate Tectonics (1 of 4)
Throughout almost the whole of human history, most people have imagined the continents to be fixed in their present positions and the ocean floors to be the oldest and most primitive parts of the Earth. In the space of a few years during the early 1960s, however, both of these assumptions were overthrown in an intellectual revolution. It suddenly became possible to prove that the continents are drifting across the Earth's surface, that the ocean floors are spreading, and that none of the oceanic crust is more than about 200 million years old - less than 5% of the age of the Earth (4600 million years). Continental drift was not a new idea in the 1960s - it had been proposed by Antonio Snider of Paris in 1858. But it was not taken seriously until, in 1915, the German meteorologist Alfred Wegener (1890-1930) wrote a book drawing together all the scientific evidence for drift then available.
Many of Wegener's arguments are still valid today. He pointed out, for example, that the rocks along the west coast of Africa are very similar to those along the east coast of South America, suggesting that the two continents were once one. He also noted that certain identical fossils are found on continents now separated by thousands of kilometers of ocean. The animals concerned could never have swum so far; so the continents involved must once have been joined.
Wegener proposed that about 200 million years ago there was just one supercontinent, which he named Pangaea. Subsequently, Pangaea split into smaller landmasses, which then drifted to their present positions (and are still drifting, at rates of a few centimeters a year). But for over 40 years Wegener's arguments were rejected by most geologists, who could not envisage how solid continents could possibly plow their way through equally solid ocean floor.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* EARTHQUAKES
* VOLCANOES
* THE FORMATION OF ROCKS
* MOUNTAINS
* ISLANDS
* THE OCEANS
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p078-2
ftsTitleOverride
Plate Tectonics (page 2)
ftsTitle
The Earth's tectonic plates.
1. Transform fault
2. Constructive (Oceanic ridge)
3. Destructive (Subduction zone)
4. Undifferentiated
5. Uncertain and incipient
6. Direction of plate movement.
Plate Tectonics (2 of 4)
The revolution
In the early 1960s scientists managed to prove continental drift by making use of the weak magnetism that many rocks contain. Once this had been done, it was no longer possible to use the problem of how drift occurs as a reason for rejecting it. A solution must exist and had to be found. It was not long in coming. Scientists soon realized that continents did not have to plow through ocean floors, because the ocean floors are moving too. Indeed, it is the spreading oceanic lithosphere that pushes the continents along.
The secret lay in the huge ocean-floor mountain ranges, known as oceanic ridges, discovered by oceanographers during the 1950s. These are now known to be the sites at which magma rises from the asthenosphere below, cools, and solidifies to form new oceanic lithosphere. Once solid, the lava moves away on each side of the ridge, and more magma rises into the gap to take its place. Oceanic lithosphere is thus being created continuously at oceanic ridges.
But unless the Earth is expanding, lithosphere must also be destroyed at the same rate as it is created. This happens at subduction zones , most - but not all - of which lie around the margins of the Pacific. As the spreading oceanic lithosphere reaches the edges of the Pacific continents, it is forced down into the Earth's interior where it gradually melts and loses its identity. All the ocean floor is recycled in this way in less than about 200 million years.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* EARTHQUAKES
* VOLCANOES
* THE FORMATION OF ROCKS
* MOUNTAINS
* ISLANDS
* THE OCEANS
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p078-3
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Plate Tectonics (page 3)
ftsTitle
Constructive plate boundary: new lithosphere is formed at oceanic ridges by molten rock rising from the asthenosphere.
Destructive plate boundary: at subduction zones oceanic lithosphere is forced beneath continental lithosphere, descending into the asthenosphere at approximately 45
angle.
Conservative plate boundary: at transform faults, plates slide past each other, with lithosphere being neither created nor destroyed.
Plate Tectonics (3 of 4)
Plate tectonics
By the late 1960s, continental drift and ocean-floor spreading had come to be seen as two aspects of a wider phenomenon - plate tectonics.
The Earth's lithosphere (not just the crust) is divided into 15 major plates of various sizes. The plates 'float' on the partially molten asthenosphere below, and it is because they are floating that they have the freedom to move horizontally. A few of the plates (for example, the Pacific) are almost completely oceanic, but most include both oceanic and continental lithosphere. There are no completely continental plates.
The boundaries between the plates are of three types. The oceanic ridges are known as constructive plate boundaries, because they are where new lithosphere is being created. The subduction zones are known as destructive plate boundaries, because they are where lithosphere is being consumed by the Earth's interior. Finally, there are conservative plate boundaries, also known as transform faults, along which lithosphere is neither being created nor destroyed, but where the plate edges are simply sliding past each other. Most transform faults are on the ocean floor, where they offset sections of oceanic ridge, enabling the ridges to adjust to the curvature of the Earth. Occasionally, however, they impinge on land. The notorious San Andreas fault of California is a transform fault.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* EARTHQUAKES
* VOLCANOES
* THE FORMATION OF ROCKS
* MOUNTAINS
* ISLANDS
* THE OCEANS
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Constructive plate boundary
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ftsTitleOverride
Plate Tectonics (page 4)
ftsTitle
200 million years ago: virtually all the Earth's dry land is contained in Pangaea, the original supercontinent.
110 million years ago: Pangaea has split into two smaller continents, Laurasia and Gondwana, which themselves begin to split up.
0 million years ago: the continents have assumed their present positions, but are continuing to move.
Plate Tectonics (4 of 4)
The plate boundaries are the most tectonically active parts of the Earth - they are where most mountain building, earthquakes and volcanoes occur. The plates floating on the asthenosphere may be regarded as jostling against each other, generating tectonic activity at their margins. However, because plates have destructive and constructive plate boundaries, they are also continuously changing their sizes and shapes.
Not even the continents themselves are immune from change. Running for more than 6400 km (3975 mi) up eastern Africa, from the Zambezi to Syria, is a giant rift valley, where a long strip of crust has sunk between more or less parallel faults. Many scientists believe that this, the East African Rift, represents an early stage in the break-up of Africa, leading to the creation of a new spreading ocean.
PJS
HOW CONTINENTAL DRIFT WAS PROVED
Many rocks contain minute magnetic particles, usually oxides of iron and titanium. When a rock forms, these particles become magnetized in the direction of the Earth's magnetic field at the particular site. Using highly sensitive instruments, it is possible to measure this weak magnetism and from it determine the position of the north pole at the time the rock was formed.
Scientists were surprised to discover that for rocks older than a few million years the north poles determined in this way did not lie at the present north pole, and that the older the rocks the greater was the discrepancy. They were even more surprised to find that rocks of the same age from different continents gave ancient north poles in quite different positions. There can only be one north pole at any given time, however, and that must lie close to the north end of the Earth's rotational axis. The only way of explaining the rock magnetic data, therefore, was to assume that the continents have drifted with respect to both the present north pole and each other.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* EARTHQUAKES
* VOLCANOES
* THE FORMATION OF ROCKS
* MOUNTAINS
* ISLANDS
* THE OCEANS
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Earthquakes (page 1)
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An earthquake measuring 7.2 on the Richter scale caused destruction in several cities in western Japan.
Earthquakes (1 of 8)
An earthquake is a sudden release of energy in the Earth's crust or upper mantle. As the planet's tectonic plates jostle against each other and become distorted, tremendous strain builds up - and from time to time the strain energy is discharged in zones where the rocks are weakest. The result is a sudden violent shock that can have highly destructive effects on the Earth's surface nearby.
The damaging effects of an earthquake are due to the vibrations (seismic waves) emitted by the shock. For a brief moment the waves shake the ground close to the earthquake, frequently producing permanent effects. Few people are ever killed or injured directly by an earthquake; death and injury are more likely to result from the collapse of buildings caused by the earthquake.
Whether or not there are people or buildings present, earthquakes may cause fissures to appear in the ground, produce changes in the level and tilt of the ground surface, divert rivers and streams, and trigger landslides and avalanches. Undersea earthquakes may also give rise to tsunami - huge sea waves that can travel across the oceans for thousands of kilo meters, causing devastation when they hit land.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
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Earthquake measuring 7.2Q
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Earthquakes (page 2)
ftsTitle
The massively destructive power of earthquakes can be seen in this photograph of the Californian earthquake in January 1994.
Earthquakes (2 of 8)
Where earthquakes occur
Most earthquakes take place along the boundaries of the tectonic plates - along oceanic ridges, transform faults and subduction zones - because this is where the plates interact most intensely, and hence where distortion and strain build-up are greatest.
However, not all earthquakes occur along plate margins. In North America, for example, the most damaging earthquakes of historic times have taken place not in California, through which runs a transform fault (the San Andreas fault), but in South Carolina and Missouri, both of which are far from plate margins. The reasons for this are unclear, but earthquakes within the interiors of plates may be due to deep, still active faults remaining from a much earlier phase of plate tectonics. California is still America's most notorious seismic area, however, because it is there that earthquakes are most frequent.
The point at which an earthquake occurs is called the focus, or hypocenter. The point on the Earth's surface directly above the focus is called the epicenter. A world map of epicenters is largely a map of the Earth's plate boundaries.
All earthquake foci lie within about the upper 700 km (435 mi) of the Earth. Within this range, earthquakes are classified as shallow (focal depths of 0-70 km / 0-43 mi), intermediate (70-300 km / 43-186 mi), or deep (below 300 km / 186 mi). There are about three times as many intermediate earthquakes as there are deep ones, and about ten times as many shallow ones. It is the shallow shocks that produce most of the damage at the Earth's surface, for the obvious reason that they are closer to it. Collectively, the shallow earthquakes also release the most energy - about 75% of the total, compared to 3% for deep earthquakes.
Earthquake foci at the various depths are not distributed uniformly along the plate boundaries. Almost all the deep earthquakes, about 90% of the intermediate ones and about 75% of the shallow ones occur along the subduction zones around the Pacific Ocean. Oceanic ridges and transform faults, on the other hand, are generally the sites of shallower and smaller events.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
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p080-3
ftsTitleOverride
Earthquakes (page 3)
ftsTitle
The four different types of seismic waves differ in their velocities and in the ways that the Earth's rock particles vibrate as the waves pass through.
P and S waves are known as body waves, because they travel through the interior (body) of the Earth. Indeed, the body waves emitted by very big earthquakes can be detected by sensitive instruments on the opposite side of the globe.
Love and Rayleigh waves are known as surface waves, because they are restricted to the vicinity of the Earth's surface.
Earthquakes (3 of 8)
Measuring earthquakes
The size of an earthquake is specified by its magnitude, sometimes called the Richter magnitude after the American seismologist, Charles Richter, who devised the scale in the 1930s. Magnitude is actually a measure of the size (amplitude) of the waves emitted by the earthquake. However, the magnitude scale is logarithmic. This means that each step up the scale represents a ten-fold increase in the amplitude of the emitted waves. Thus the waves from a magnitude-7 earthquake are 10 times bigger than those from a magnitude-6 shock, 100 times bigger than those from a magnitude-5 event, and so on.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
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ftsTitleOverride
Earthquakes (page 4)
ftsTitle
Earthquakes (4 of 8)
Magnitude can also be regarded as a measure of the energy released by an earthquake, because energy is related to wave size. The relationship is such that each division on the magnitude scale represents an approximately thirty-fold difference in energy. Thus a magnitude-7 earthquake releases about 30 times more energy than a magnitude-6 shock and about 30 x 30 = 900 times more energy than a magnitude-5 event. This explains why most of the energy released by earthquakes comes from the very few big shocks that occur each year rather than from the million or so smaller earthquakes.
In principle, there is no upper limit to the possible magnitude of earthquakes, although in practice there are no shocks with magnitudes greater than about 9. On the basis of magnitude, earthquakes are classified as great (magnitude above 7.5), major (6.5-7.5), large (5.5-6.5), moderate (4.5-5.5) or small (below 4.5). Magnitudes may be determined from the amplitudes of either body waves or surface waves, whichever happen to be the most convenient.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
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ftsTitleOverride
Earthquakes (page 5)
ftsTitle
The four types of Seismic waves.
Earthquakes (5 of 8)
Earthquake damage
Although magnitude is a fairly accurate scientific measure of the strength of an earthquake, it does not necessarily relate directly to the amount of death and damage that the earthquake causes, because the destructive power of a seismic disturbance depends on more than the quantity of energy released. For example, a magnitude-7 earthquake can, and often does, produce more devastation than a magnitude-8 shock, even though the latter releases about 30 times more energy than the former. This is because as important as the energy are the characteristics of the ground in the epicentral region, the population density there, and the nature of the buildings in the area.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
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Seismic waves Love wave
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p080-6
ftsTitleOverride
Earthquakes (page 6)
ftsTitle
The four types of Seismic waves.
Earthquakes (6 of 8)
However big an earthquake is, it will cause no damage or death at all if it takes place in an uninhabited wilderness. By contrast, a much smaller earthquake can produce havoc in a major city. Moreover, two earthquakes of the same magnitude may have quite different effects on two more or less identical cities if one of the cities is built on soft sediment (making it very vulnerable to vibrations) and the other on hard rock (less susceptible). The effects of the earthquakes will also depend on such factors as whether the shocks happen during the day (people at work, possibly in high-rise offices) or at night (people asleep in low-rise houses) and whether or not the cities concerned have made any attempt to construct earthquake-resistant buildings.
To specify the size of an earthquake in terms of its effects, an intensity scale is used. In the West (but not in Japan or the former Soviet republics, which use slightly different systems) this is usually the Modified Mercalli Scale .
After a big earthquake there is frequently a survey to discover how intensity varies with distance from the epicenter. The intensity is determined at many points (by observing ground effects and questioning local inhabitants), and points with the same intensity are joined by lines to make an intensity 'contour' map, known as an isoseismal map. Intensity decreases away from the epicenter. If 'the intensity of the earthquake' is specified, it means the maximum intensity - i.e. that at the epicenter.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
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Seismic waves P wave !
**++*
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Earthquakes (page 7)
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The four types of Seismic waves.
Earthquakes (7 of 8)
CAN EARTHQUAKES BE PREDICTED?
The short answer to this question is 'no'. Although scientists have managed to predict a few specific earthquakes, they have been quite unable to devise a generally applicable method of prediction.
The Americans, Japanese, Chinese and Russians have been researching into prediction for several decades. They have discovered that before some earthquakes ground level and tilt may change, tide levels may fluctuate, the seismic velocities in nearby rocks may vary, and the local magnetic field may waver. But no one of these applies to all, or even the majority of, earthquakes, and some earthquakes appear to have no early warning signs at all.
The most spectacularly successful prediction was that of the magnitude-7.3 earthquake at Haicheng, China, in 1975. By mobilizing scientists and many members of the public to observe a large number of natural phenomena, the Chinese were able to predict the location and timing of the event quite accurately. They could thus evacuate the population, reducing deaths from a likely million or so to just 1328. Unfortunately, they have had little success since. In 1976, for example, they failed to forecast the magnitude-7.8 earthquake in Tangshan, and at least 240 000 people died.
In the early 1960s scientists were optimistic enough to suggest that prediction would be possible within a decade or so. Nowadays, however, they will not even guarantee that it will ever be possible.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
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Seismic waves Rayleigh wave
**++*
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Earthquakes (page 8)
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The four types of Seismic waves.
Earthquakes (8 of 8)
THE MODIFIED MERCALLI SCALE
I Not felt except by a few people
under favorable circumstances
II Felt by a few people at rest. Delicately
suspended objects swing.
III Felt noticeably indoors. Standing cars
may rock.
IV Felt generally indoors, and sleeping people
are woken. Cars rocked, windows rattle.
V Felt generally. Some plaster falls and dishes
and windows are broken. Pendulum clocks
stop.
VI Felt by all - many frightened. Chimneys and
plaster damaged. Furniture moved and
objects upset.
VII Everyone runs outdoors. Felt in moving
cars. Moderate structural damage.
VIII General alarm. Weak structures badly
damaged. Walls and furniture fall over.
Water level changes in wells.
IX Panic. Weak structures totally destroyed,
extensive damage to well-built structures,
foundations and underground pipes. Ground
fissured and cracked.
X Panic. Only strongest buildings survive.
Ground badly cracked. Rails bent, and water
slopped over river banks.
XI Panic. Few buildings survive. Broad fissures
in ground. Fault scarps formed. Under-
ground pipes out of service.
XII Panic. Total destruction. Waves seen in
ground, and lines of sight and level are
distorted. Objects thrown in the air.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
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Seismic waves S wave
**++*
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Volcanoes (page 1)
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A river of volcanic lava from the Kilauea-Iki volcano on Hawaii. Although most lava solidifies close to the volcano, some lava can travel several miles. Here the lava is flowing into the sea.
Volcanoes (1 of 3)
The popular image of a volcano is of a conical structure hurling ash, steam, fire and molten rock from a crater in the top, often with explosive violence. Such volcanoes do indeed exist, but they account for less than 1% of the world's volcanic activity. In fact more than 80% of molten rock, or magma, reaching the Earth's surface does so through long fissures in the Earth's outer shell, the lithosphere; this is called fissure volcanism. As the most important of such fissures lie along the axes of the oceanic ridges, the bulk of the Earth's volcanism occurs, unseen, on the floors of the oceans.
Volcanism occurs where magma from the Earth's interior is able to force its way through a weak zone in the lithosphere. Because such weaknesses are most likely to occur where the Earth's tectonic plates interact and become distorted, most volcanism occurs in the vicinity of plate boundaries. Continental volcanoes are usually associated with subduction zones and regions in which continents are colliding.
The ultimate source of most magma is thought to be the partially molten asthenosphere, the layer immediately below the lithosphere. However, beneath the majority of volcanoes there appears to be a reservoir, or magma chamber, that acts as a staging post for magma between the asthenosphere and the Earth's surface. Between the magma chamber and the surface there is a narrower passage called a vent.
Volcanoes that have long ceased to erupt are said to be extinct. Other volcanoes that have been quiet for a very long time but that may erupt again are described as dormant, while those that have erupted in historic times are said to be active.
Volcanic products
The material in a magma chamber is liquid, but by the time it reaches the Earth's surface it can be liquid, solid or gaseous. Magma contains dissolved volatiles such as water and carbon dioxide. As the magma rises towards the surface it experiences a reduction in pressure, and the volatiles are released, often with explosive force. The explosion then shatters the magma and shoots the pieces into the air. By the time they reach the ground they are often solid, albeit still very hot. Such solid fragments are known as pyroclasts or, as a group, tephra. In order of increasing particle size, tephra may consist of dust (less than 0.35 mm / 1/70 in in diameter), ash (less than 4 mm / 1/6 in), lapilli (less than 32 mm / 1 1/4 in), and bombs or blocks (more than 32 mm / 1 1/4 in). In particularly violent explosions, bombs weighing over 100 tons are known to have been thrown several kilometers. The finer particles can travel much further, however, carried by the wind.
In some explosive eruptions there is no sudden blast but rather a continuous stream of hot gases and rock fragments issuing from the volcanic vent at high velocity for up to several hours. Both types of explosive activity may carry tephra up to a considerable height before they fall back to the surface. In some less violent cases, however, the volcanic fragments may stay close to the ground where, in deadly association with hot gases, they roll along destroying everything in their path. This is known as a nuue ardente (French, 'burning cloud').
A volcanic landscape (not to scale), showing some of the major features commonly associated with volcanic activity.
A lake-filled caldera shown in cross section. Calderas are formed by the collapse of the cone of a volcano after a violent eruption.
Volcanoes (2 of 3)
Not all continental volcanic eruptions are explosive. In many there is simply a quiet extrusion of magma. Magma erupted onto the Earth's surface is usually called lava, both in its molten state and when it has cooled and solidified into a lava flow.
Although flow rates of molten lava are generally quite low, speeds of up to 100 km/h (62 mph) have been observed on occasions; and although most lava solidifies in close proximity to the volcano, some is known to have traveled up to 50 km (31 mi) from the vent.
Lava is usually extruded at temperatures of 800-1200 deg C (1450-2200 deg F), but as it flows it loses heat to the atmosphere and the ground, thus cooling and solidifying from the outside inwards. As it solidifies, its surface takes on a variety of textures, depending largely on the viscosity of the lava in its molten form.
When the molten lava is highly mobile (that is, a relatively thin fluid, with low viscosity) it acquires, on cooling, a thin plastic surface layer that gets dragged into rope-like folds by the continued flow of the still-molten lava beneath. When finally solid, this is known as a pahoehoe lava. With thicker, more viscous and thus slow-moving lava, cooling leads to a thicker and harder skin that the continued flow of still-molten lava beneath breaks into a fragmented surface. If the surface is very jagged, the solid lava is known as aa lava; if the surface is more lumpy, the lava is block lava.
When lava is extruded under water, it often acquires an altogether different form. As a result of very rapid cooling by the water, it splits into sack-shaped segments, and so the solid lava is called pillow lava.
Volcanic forms
There are three main types of continental volcano. The simplest is the steep cone (a cinder-cone volcano) built from layers of tephra ejected from a succession of explosive eruptions. A well-known example is Paricutn, in Mexico, which began erupting in 1943 and in 10 years produced a cone over 460 m (1500 ft) high.
Few volcanoes emit only tephra at every eruption. Many eject tephra on some occasions and extrude lavas on others. The result is a cone with alternating layers of tephra and lava (a composite volcano). Well-known examples are Vesuvius and Stromboli (Italy), Etna (Sicily) and Fujiyama (Japan).
Where lava is plentiful and eruptions frequent - through several vents - the result is likely to be a shield volcano, a large structure up to tens of kilometers across and with gentle slopes constructed from hundreds to thousands of successive lava flows. Shield volcanoes are often found in mountain ranges adjacent to subduction zones (for example, the Andes), although Mauna Loa, Hawaii, is also a shield volcano.
Most volcanoes have a crater at, or near, the top, resulting from the sinking of solid lava back into the volcanic vent. However, if there has been a particularly violent explosion, or if the top of the volcano has collapsed because the lava has retreated a long way down the vent, a very large basin-shaped depression known as a caldera may be formed.
Oceanic volcanoes
About 6% of the Earth's volcanism takes place on the ocean floor away from plate margins. Such volcanoes are called sea-mounts if their tops fail to reach the surface, although some build up above sea level. More than 10 000 seamounts have been mapped on the Pacific Ocean floor alone, although most of them are now extinct. Some of these volcanoes are generated by magma derived, as for continental volcanoes, from the asthenosphere. Seamounts originating this way are distributed randomly. However, some oceanic volcanoes are produced by magma that comes not from the asthenosphere but from far deeper. These are hot-spot volcanoes. In addition to these oceanic volcanoes, there are two other forms of oceanic volcanism: fissure volcanism, and island-arc volcanism at subduction zones.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* PLATE TECTONICS
* EARTHQUAKES
* THE FORMATION OF ROCKS
* MOUNTAINS
* ISLANDS
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Volcanoes (page 2)
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Volcanoes (page 3)
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This volcano on the Oshima Islands of Japan, with its spectacular eruptions, caused all residents to be evacuated.
Volcanoes (3 of 3)
SOME MAJOR VOLCANIC ERUPTIONS
SANTORINI (THERA)
Height: 584 m (1960 ft)
Location: Cyclades, Greece
Date: c. 1550 BC
A massive explosion virtually destroyed the island, and is thought by some to have contributed to the demise of Minoan civilization on nearby Crete. The disaster may also have given rise to the legend of the lost city of Atlantis.
VESUVIUS
Height: 1280 m (4198 ft)
Location: Bay of Naples, Italy
Date: AD 79
The towns of Pompeii, Herculaneum and Stabiae were completely buried, and thousands died. In 1631 3000 people were killed, since when there have been around 20 major eruptions, the last in 1944.
UNNAMED
Height: unknown
Location: North Island, New Zealand
Date: c. AD 130
Around 30 million tons of pumice were ejected, creating the vast caldera now filled by Lake Taupo. An area of c. 16 000 km2 (6180 sq mi) was devastated - the most violent of all documented volcanic events.
ETNA
Height: 3308 m (10 853 ft)
Location: Sicily, Italy
Date: 1669
20 000 people were killed, and lava overran the west part of the city of Catania, 28 km (17 mi) from the summit.
KELUD
Height: 1731 m (5679 ft)
Location: Java, Indonesia
Date: 1586
10 000 people killed. Another eruption in 1919 killed 5000 people.
TAMBORA
Height: 2850 m (9350 ft)
Location: Sumbawa, Indonesia
Date: 1815
An estimated 150-180 km3 (36-43 cu mi) were blasted from the cone, which dropped in height from 4100 m (13 450 ft) to 2850 m (9350 ft) in minutes. About 90 000 people were killed in the explosion and subsequent giant wave, or died later of famine.
KRAKATAU
Height: 813 m (2667 ft)
Location: Krakatau, Indonesia
Date: 1883 1
63 villages were wiped out and 36 380 people killed by the giant wave caused by this, the greatest volcanic explosion recorded - although possibly only one fifth of the explosion that destroyed Santorni. Rocks were thrown 55 km (34 mi) into the air, and dust fell 5330 km (3313 mi) away 10 days later. The explosion was heard over one thirteenth of the Earth's surface.
MONT PELEE Height: 1397 m (4582 ft)
Location: Martinique, West Indies
Date: 1902
Within three minutes a nuue ardente destroyed the town of St Pierre, killing all 26 000 inhabitants - except for one, a prisoner who survived in the thick-walled prison.
MOUNT ST HELENS
Height: 2549 m (8360 ft)
Location: Washington State, USA
Date: 1980
66 people were presumed dead and 260 km2 (100 sq mi) of forest destroyed. Smoke and ash rose to a height of 6000 m (20 000 ft), depositing ash 800 km (440 mi) away.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* PLATE TECTONICS
* EARTHQUAKES
* THE FORMATION OF ROCKS
* MOUNTAINS
* ISLANDS
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Volcano on the Oshima Islands of Japan
**++*
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The Formation of Rocks (page 1)
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1. Heat from the Sun causes evaporation. Water vapor rises and condenses into clouds.
2. Water in cloud precipitates as rain or snow.
3. Water erodes rock, and rivers carry away sediment.
4. Rivers deposit sediment as alluvium on flat ground, or transport it to lakes and seas where it settles on the bottom as clay or sand.
5. As sediment builds up, increasing pressure changes lower layers into sedimentary rock.
6. Deeper sedimentary rock is turned into metamorphic rock by pressure from above and heat from below.
7. Magma - molten rock from deep inside the Earth - rises towards the surface. Some is trapped underground and hardens into intrusive igneous rock.
8. Some magma reaches the Earth's surface via volcanoes and fissures as lava and is classified as extrusive igneous rock.
9. Some intrusive igneous rock is forced deeper by the pressure of sedimentation, and is changed into metamorphic rock. This metamorphosis may be assisted by thermal energy from below.
10. Pressure from colliding continental plates pushes all kinds of rock to the surface, and forces them upwards, where they are eroded. The rock cycle begins again.
The Formation of Rocks (1 of 5)
The Earth is perpetually recycling its rocks. Material brought to the surface is eroded, transported and ultimately returned to the Earth's interior, where it becomes available to begin the cycle all over again.
The series of processes by which rock is created, destroyed and reformed is known as the rock cycle, or geological cycle. The energy to maintain it comes partly from the Sun (to fuel the erosion processes) and partly from the Earth's interior (to generate volcanic activity and uplift).
The rock cycle
The creation and destruction of rocks is a continuous process. Heat from the sun causes evaporation of water, which then rises and condenses into clouds. When this water has precipitated as rain or snow, it erodes the rock it falls on, and runs off into rivers which carry away the rock particles as sediment. The sediment is deposited as alluvium on flat ground, or carried to lakes and seas where it settles on the bottom as clay or sand. As pressure builds from accumulation the lower layers of sediment change into sedimentary rock. Deeper sedimentary rock is turned into metamorphic rock by pressure from above and heat from below.
Sedimentary and metamorphic rock are pushed to the surface by pressure from colliding continental plates. Molten rock also rises to the surface from deep inside the Earth via volcanoes and fissures, and then solidifies. Once on the surface all types of rock are susceptible to erosion and the rock cycle begins again.
* The Earth's Structure
* Plate Tectonics
* Volcanoes
* Mountains
* Caves
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The Formation of Rocks (page 2)
ftsTitle
IGNEOUS ROCK FORMS
1 Plug: vent of extinct volcano blocked with solidified lava
2 Lava flows: sheets of lava from volcanic activity that have spread across the surface.
3 Lopolith: a saucer-shaped body of intrusive igneous rock
4 Layers of stratified sedimentary (or metamorphic) rock.
5 Dykes: vertical or near-vertical bodies of igneous rock that have intruded into cracks under older rocks.
6 Sill: a usually horizontal body of igneous rock that has been forced between two layers of older rock.
7 Laccolith: a dome-shaped mass of igneous rock that has been forced between older layers of rock, pushing the strata above into a dome
8 Batholith: a very large mass of igneous, originating from a an intrusion of magma deep under the Earth's surface.
The Formation of Rocks (2 of 5)
Igneous rock
Igneous rock starts deep in the Earth as molten magma, which then forces its way up through the crust to cool and solidify. Magma - which comes from the Earth's surface via volcanic activity - comprises a mixture of oxides (compounds with oxygen) and silicates (compounds with silicon and oxygen). When it cools and solidifies, the oxides and silicates produce a complex mixture of mineral crystals. The nature and properties of the crystals in any particular igneous rock depend partly on the composition of the original magma and partly upon the physical conditions under which the magma crystallized. As compositions and conditions vary greatly, there are thousands of different igneous rock types.
Igneous rocks that form on the Earth's surface are known as extrusive. Those that form within the crust from magma that never reached the surface are known as intrusive. Intrusive rocks cool more slowly because, being surrounded by other rock rather than being open to the air, the heat cannot escape so readily. As a result, the crystals have longer to grow, and the mineral grains are larger (coarser).
Despite the many varieties of igneous rock, just six account for most of the igneous components of the Earth's crust. These are granite, diorite and gabbro, which are coarse-grained intrusive rocks, and rhyolite, andesite and basalt, which are fine-grained extrusive rocks.
Most of the lava produced at constructive plate boundaries is basalt. Both basalt and andesite are generated at destructive plate boundaries, and rhyolite is sometimes also produced. Granite is common in the upper continental crust, and gabbro probably dominates in the lower continental crust.
* The Earth's Structure
* Plate Tectonics
* Volcanoes
* Mountains
* Caves
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ftsTitleOverride
The Formation of Rocks (page 3)
ftsTitle
The Formation of Rocks (3 of 5)
Sedimentary rock
Sedimentary rock is mostly formed when rock of any type is weathered down into fine particles that are then re-deposited under water and later compacted. At least 75% of all sedimentary rock is known as clastic sedimentary rock, which means that it is derived from the erosion products of other rocks. All rocks, even those in the most massive of mountain ranges, are ultimately broken down into smaller and smaller fragments. When the particles become small enough they are then transported by water, wind or ice, usually ending up in the ocean, where they fall as sediment to the ocean floor. Under the pressure of subsequent deposits, they are compacted there into hard rock. The most common sedimentary rock is sandstone.
The remaining 25% of sediment is either chemical or organic. Rivers dissolve minerals out of the rocks through which they pass, and the mineral solutions end up in the oceans. When the oceans reach their saturation limit for the particular mineral concerned, the excess mineral is precipitated out chemically as solid particles, which fall to the ocean floor. The most common chemical sedimentary rock is limestone (calcium carbonate - CaCO3).
Not all limestone is precipitated chemically, however. Many ocean organisms extract calcium carbonate from the water to build their shells, and when they die the shells sink to the ocean floor to form sediment in their own right. The most common organic sedimentary rock is again limestone, but there are other organisms that in a similar way generate silica (SiO2) sediments.
Most sedimentary rocks are a mixture of clastic, chemical and organic, although one type usually predominates.
Metamorphic rock
Metamorphic rock is igneous or sedimentary rock that has been subjected to high pressure and/or temperature. When igneous or sedimentary rocks are subjected to high temperatures and pressures, especially in the presence of percolating fluids, their internal structures, and sometimes even their mineralogical compositions, may be changed. The processes involved are known collectively as metamorphism. The sort of temperatures and pressures required are, respectively, 300
C (572
F) and 100 megapascals (equivalent to 100 atmospheres).
The most extreme conditions in the Earth's crust occur at plate boundaries where continents collide. Most metamorphic rocks are thus generated in the roots of mountains. Depending upon temperature and pressure, there are various grades of metamorphism; but in the most intense (high-grade) metamorphism, rock structures, holes and even fossils are so completely obliterated that the original rock type can no longer be identified.
As a result of the realignment of minerals under pressure, many metamorphic rocks are layered, or banded. Sometimes the layering is visible; but even when it is not, it can often be detected by the way that the rock breaks. An example is slate, which easily breaks into thin sheets along the layering.
Not all metamorphic rock is layered, however. Common examples of non-layered metamorphics are marble, formed by the metamorphism of limestone, and quartzite, derived from sandstone.
* The Earth's Structure
* Plate Tectonics
* Volcanoes
* Mountains
* Caves
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The Formation of Rocks (page 4)
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The Formation of Rocks (4 of 5)
Faults and folds
As soon as rocks form they not only begin to erode, they are also subject to faulting and folding. The most intense faulting and the most intense folding both occur at plate boundaries, but these pressures are also very common within plates, on scales ranging from centimeters to thousands of kilometers.
Faults are fractures along which opposing blocks of rock are moving or have moved in the past. The surface over which the slippage occurs is called the fault plane, and the line along which the fault plane cuts the Earth's surface (if it does; not all faults reach the surface) is known as the fault trace.
* The Earth's Structure
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ftsTitleOverride
The Formation of Rocks (page 5)
ftsTitle
The Formation of Rocks (5 of 5)
Faults are classified according to the direction in which the blocks of rock slip. If the movement is basically vertical (up or down the fault plane) the result is a dip-slip fault, of which there are two basic types - normal and reverse. Horizontal movement gives rise to a strike-slip fault. Where there are two parallel faults, the result can be either a horst or a graben (rift valley).
Folding is the bending of rock without fracturing. The two sides of a fold are called limbs, and the surface that bisects the angle between the limbs is known as the axial plane. Folds are classified according to the severity of the folding, the shape of the folds, and the angle of the axial plane. There are a few basic types of fold and a great number of variations.
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p086-1
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Mountains (page 1)
ftsTitle
The Dolomites, a mountain group lying in the eastern section of the northern Italian Alps, are formed of light-colored dolomitic limestone. The erosion of the limestone has produced grotesque shapes, jagged landforms, ridges, rocky pinnacles and deep gorges. The Dolomites' highest peak, Marmolada, is 3342 m (10 964 ft) high.
Mountains (1 of 5)
Mountains and mountain ranges are largely formed by the interaction of mountain-building processes (orogeny) and the subsequent erosional processes that tend to destroy them. The distribution of the world's major mountain ranges generally follows those belts of the Earth's landmasses where earthquakes and volcanoes are common. These phenomena are in turn caused by the collision of the moving plates that make up the Earth's lithosphere.
Such collisions often result in the margin of one plate being forced upwards, and this process has resulted in the formation of many mountain ranges - although other processes may also play a part in mountain building.
The Earth's largest mountain ranges today - the Alps, Himalaya, Rockies and Andes - are all relatively young, resulting from plate collisions in the last 25 million years or so. Much older ranges include the Scottish Highlands, the Scandinavian mountains and the Appalachians in the USA, which are all around 300-400 million years old. The deeply eroded remnants of even older ranges - up to 3000 million years old - occur in many parts of Africa and Australia.
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
* ICE
* THE OCEANS
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ftsTitleOverride
Mountains (page 2)
ftsTitle
Mountains (2 of 5)
Fold mountains
The world's largest and most complex continental mountain ranges are the result of the collision of tectonic plates. Mountains formed directly by plate collisions are known as fold mountains, because they are conspicuously folded, faulted and otherwise deformed by the huge collision pressures. In some cases the collision is between landmasses. Thus India is pressing into the rest of Asia to form the Himalaya, and Africa is being forced into Europe, producing the Alps. In other cases the collision is between an oceanic plate and a continent. Thus the Pacific plate is spreading towards South America, forcing up the Andes. The Himalaya, the Alps and the Andes are still being formed, but some mountain ranges - for example, the Urals and the Appalachians - are the products of older, long-ceased plate collisions.
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
* ICE
* THE OCEANS
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Mountains (page 3)
ftsTitle
Mountains (3 of 5)
Fault-block and upwarped mountains
Other types of mountain exist where plate collisions are at best only marginally involved. In fault-block mountains a central block of the Earth's crust has sunk and the adjacent blocks have been forced upwards. Mountains of this type define the Basin and Range Province of the western USA (Nevada and parts of Utah, New Mexico, Arizona and California) and form the Sierra Nevada of California and the Teton Range of Wyoming.
In upwarped mountains, on the other hand, a central block has been forced upwards. Examples are the Black Hills of Dakota and the Adirondacks of New York State.
Volcanic mountains
Spectacular mountains may also be built by volcanic action. For example, Mauna Loa in Hawaii, is, at 10 203 m (33 476 ft), the world's highest mountain if measured from the Pacific Ocean floor, although less than half is above sea level. Much more important than such isolated volcanoes, however, are the oceanic ridges, the undersea mountain ranges along which the bulk of the Earth's volcanism takes place. Intense volcanism also occurs where oceanic and continental plates collide. The Andes, for example, owe not a little of their mass to volcanic activity.
The erosion and destruction of isolated continental volcanoes can be a rapid process. Some volcanoes are partially self-destructive - for example, Mount St Helens in the northwest USA, which blasted part of its side out in 1980, or Vesuvius, whose crater top disintegrated in AD 79. Others are completely self-destructive - for example, Krakatau (sometimes incorrectly called Krakatoa), Indonesia, which in 1883 blasted itself entirely out of existence.
Apart from these spectacular episodes, the erosion of volcanoes can be fast, the loose ash of which they are partly built being easily transported by runoff of rainwater. In the case of some Andean volcanoes, earth tremors set off avalanches, which tear down into nearby valleys. Such an event occurred in 1970, when an estimated 40 million m3 (52 million cu yd) of rock and ice avalanched off Huascaran in Peru, completely obliterating several towns and villages and killing many thousands of their inhabitants up to 20 km(12 mi) away.
Volcanoes built up in the sea are sometimes easily eroded by wave action, as in the case of Surtsey, which grew out of the sea off Iceland in the 1960s and has largely been worn away now.
On the other hand, volcanoes built of harder rock can persist indefinitely. On the Pacific Ocean floor alone there are tens of thousands of extinct volcanic cones (seamounts), and in various parts of the world there are the remains of volcanoes up to hundreds of millions of years old.
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
* ICE
* THE OCEANS
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Mountains (page 4)
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Monument Valley, Arizona. These spectacular sandstone towers have been sculpted by water and trimmed by the wind.
Mountains (4 of 5)
Wind and water erosion
As soon as a mountain range starts to rise, the forces of erosion commence operation. Water, wind, ice and vegetation are all agents of erosion, often acting in unison. Mountains are the intermediate result of erosive processes that will ultimately wear everything away to sea level. Young mountain ranges are those which have only been uplifted to somewhere near their present height in the last 25 million years or so, and they tend to be high and jagged. Old mountain ranges are those that have suffered the processes of erosion for hundreds if not thousands of millions of years, and they tend to be lower and more rounded.
The processes of erosion start with weathering of exposed rocks. Rainfall provides water, which reacts chemically with many rocks and minerals. The loose rock fragments are then transported by water down streams into rivers and eventually to the sea, with the more resistant rock masses left upstanding as individual mountains. Mountains that have been worn away by water action alone are generally rounded, with shallow gullies radiating outwards.
Wind sculpture of whole mountain ranges is unusual on its own, but bare and polished surfaces on jagged rocks in desert regions may result from the wind hurling sand grains at rock faces at high speed as, for example, in Death Valley in California and the Hoggar mountains of the central Sahara. Wind sculpture in combination with storm-water runoff (flash floods) can cut canyons into the mountains. At the point where the canyon opens out, the water deposits its sediments in alluvial fans, whilst the wind-blown material covers adjacent lowlands with sand dunes.
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
* ICE
* THE OCEANS
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Mountains (page 5)
ftsTitle
The spectacular formations in Monument Valley are in a "rain-shadow" desert in the Southwest USA. They were formed when soft rock surrounding was eroded by wind and water.
Mountains (5 of 5)
The action of ice
When water freezes in cracks in rocks, it expands and forces the rocks apart, thus causing erosion of mountain peaks. (In a similar way, vegetation may also contribute to erosion - for example, roots may force open cracks.) As it accumulates, ice, in the form of glaciers, sculpts most high mountain ranges - for example, the Alps, Himalaya, Rockies (particularly in Canada), and the Scandinavian mountains. Many other ranges were sculpted to their present shapes by glaciers during the last Ice Age (some 100 000 to 10 000 years ago), for example, the Scottish Highlands, the US Rockies, much of the Andes, the Caucasus and the Ural Mountains.
Headward erosion by glaciers (i.e. erosion of the slope on which the glacier originates) etches out corries (also known as cirques or cwms) and, when two or three corries impinge, the result may be a pyramidal peak such as the Swiss Matterhorn. The high passes between corries cutting into a range are called cols, and gullies leading from them are called couloirs. Steep slopes on such mountains often yield avalanches, in which rock and soil, as well as snow and ice, may be swept away.
The lower parts of mountains that either have or have had glaciers tend to be characterized by terminal moraines, mounds of ice-borne debris across a valley at the melting limit of the glacier. Moraines may hold back lakes but the meltwater streams eventually breach these and outwash plains spread out from the valley mouths.
Thus, as mountain ranges are gradually worn down, it is the more resistant parts that linger longest, giving their present shapes. A peculiarity is that mountains may continue to grow higher as their sides are worn away. The Alps and Himalaya, for example, are still being raised by plate-tectonic processes - Everest is getting higher in spite of the glaciers carving away its sides.
* PLATE TECTONICS
* VOLCANOES
* THE FORMATION OF ROCKS
* ICE
* THE OCEANS
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Caves (page 1)
ftsTitle
Tham Lot Cave, Thailand, which shows the extensive formation of stalactites and stalagmites.
Caves (1 of 5)
Caves are naturally occurring holes in the ground, generally large enough to be entered by humans. They are often linked into complex systems of chambers and passageways, and these systems can extend many kilometers in length, and penetrate deep into the Earth. The entrances of many caves have provided shelter for both animals and humans in the past, and their accumulated remains can tell us much about extinct animal forms and the life of prehistoric man. Some caves are also noted for their animal life today: bats, birds, snakes and even crocodiles, in addition to numerous invertebrates, may make their homes in caves.
By far the majority of caves occur in limestone areas. This is because of the solubility of limestone in rainwater (H2O) containing carbon dioxide (CO2) in solution. This solution is carbonic acid (H2CO3), a weak acid that can attack limestone on its own, but its effects are much greater if it is augmented by acids from soil and vegetation. Limestone consists almost entirely of calcium carbonate (CaCO3), which undergoes a reversible reaction with carbonic acid to form calcium bicarbonate - Ca(HCO3)2 - which is soluble in water. Because the reaction is reversible, if the calcium bicarbonate dissolved in the water reaches excessive proportions then calcium carbonate can be precipitated elsewhere in cave systems as stalactites (see below) or at springs as tufa (a soft porous rock), or it can be carried away downstream.
Not all limestones have caves as some, such as chalk, are mechanically weak and will not support cave roofs. Others have few caves owing to their high porosity, which allows the acidic water to pass through the whole rock mass without concentrating at any particular points. Massive, low-porosity, well-jointed limestones, such as Carboniferous limestone, are the most favorable rock types for caves. Bedding planes and joints (see diagram), together with faults, are weaknesses through which rainwater can percolate, and where its acid can attack the limestone. Prolonged attack may lead to the formation of a cave.
When a limestone mass is first uplifted above sea level it has no caves, but they begin to form as soon as rainwater percolates down joints and bedding planes. Water movement through a limestone mass is very slow at first, but as joints are widened so the flow rates increase. Once the conduits reach a diameter of a few millimeters, free and relatively fast flow is possible. The increased flow rate leads to enhanced solution and erosion of the walls by rock particles, for as the flow increases, sediment grains from overlying rock formations wash in. Once the conduits are large enough for distinct streams to flow through, they are enlarged relatively rapidly to the size of caves.
Free stream flow through caves leads to gradual drainage of the higher parts of the system, with the streams eventually rising at springs near the base of the limestone. Renewed uplift of the limestone may lead to the underground streams finding still lower routes through the rock, and thus the old routes are abandoned.
abandoned.
* THE FORMATION OF ROCKS
* EARLY MAMMALS
* HUMAN PREHISTORY
* PREHISTORIC ART
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p088-3
Karst formations in Guilin, China. Karst landscapes are limestone areas with cave systems, dry valleys and underground streams. In places such as China and Southeast Asia such landscapes are characterized by sheer towers and cones, formed by intense downward erosion.
Caves (3 of 5)
Cave types
Cave passages evolve from the totally submerged (phreatic) state to the free-flowing stream (vadose) state. Each has its own features, and from these it may be possible to work out the cave's history. Very rapid drainage may leave the cave with phreatic features only, most notably passages that are more or less circularin cross section. Free-flowing vadose streams only cut down the floor, giving a trench or canyon-like cross section to the passage. Undermining of the walls may lead to the collapse of parts of the roof and the gradual enlargement of caverns, possibly resulting in openings to the surface known as potholes.
Cave entrances may be at the water-inflow (input) end, when they are known as swallets or sinks, at the water-out flow (resurgence) end, or at intermediate points through collapses or abandoned sinks. The profile of a cave system may show a steady gradient, as is characteristic of vadose caves, or up-and-down loops, typical of phreatic caves in steeply dip ping limestones.
* THE FORMATION OF ROCKS
* EARLY MAMMALS
* HUMAN PREHISTORY
* PREHISTORIC ART
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Caves (page 3)
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Caves (page 4)
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Karst formations in Guilin, China. Karst landscapes are limestone areas with cave systems, dry valleys and underground streams. In places such as China and Southeast Asia such landscapes are characterized by sheer towers and cones, formed by intense downward erosion.
Caves (4 of 5)
Karst landscapes
Limestone landscapes with cave systems are known as karst landscapes, named after an area of Croatia and Slovenia. Karst landscapes are typified by a lack of surface streams, the presence of swallets and collapse potholes, dry valleys (which once had streams now flowing underground), resurgences, and bare rock pavements. These limestone pavements are dissected into areas known as clints by fissures about 50 cm (20 in) wide known as grikes, this process being caused by theetching out of joints, often with subsequent glacial smoothing.
Karst landscapes may also have numerous dolines (funnel-shaped hollows at joint intersections) and poljes (enclosed valleys with internal drainage through caves). Tropical karst is typified by towers and cones formed by intense downward erosion, with 'cockpits' separating cone-shaped hills.
The Lechugilla cave in the Guadeloupes mountain range of New Mexico has 32 miles of passageways and many spectacular formations.
Caves (5 of 5)
Stalactites and stalagmites
Stalactites and stalagmites (collectively known as speleothems) are caused by the precipitation of calcium carbonate from water rich in calcium bicarbonate percolating through the cave roof. Stalactites on the ceiling start as straw-like tubes with drops running down the inside, but as crystals grow inside the tubes they become blocked, and the stalactites thicken. Stalagmites grow where drops fall to the floor.
If drops of lime-saturated water fall into small pools with particles of grit, the latter may become coated with layers of lime and so form cave pearls. Other forms of speleothem include curtain stalactites along rock edges, columns where stalactites have met stalagmites, flowstone sheets (rippled sheets of precipitated calcium carbonate) and helictites. Helictites consist of clusters of irregular, branching rod-like structures that appear to defy gravity, their growth being fed by capillary flow of water through very narrow tubes.
Many speleothems contain a small proportion of uranium, and measurement of the radioactive decay can give the age of the speleothem. This is one of the few ways of dating cave formation, as stalactite growth rates are much more variable than generally thought.
Non-limestone caves
Caves in rocks other than limestone include a variety of sea caves where erosion has etched out weaknesses in the rocks of sea cliffs. Lava caves occur in many basalt volcanic areas, such as Iceland, Hawaii, Kenya and Australia. They are generally tubes within lava flows where the molten material has flowed out from beneath the solidified crust. Fissure caves occur in a few hard-rock areas where fault zones have been widened by erosion or by mass movement pulling rock masses apart.
Ice caves are of two sorts. First, there are englacial tubes through which streams of meltwater run beneath glaciers. Though entirely in ice, they show many of the features of limestone caves, although rapid changes may take place owing to glacier movement. Second, there are caves in high mountain regions where the air within the cave rarely if ever rises above freezing point, so that water percolating in from the surface during the summer freezes into icicles, often very large, and sometimes joining into ice masses underground.
* THE FORMATION OF ROCKS
* EARLY MAMMALS
* HUMAN PREHISTORY
* PREHISTORIC ART
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Caves: The Lechugilla cave i
**++*
p090-1
ftsTitleOverride
Ice (page 1)
ftsTitle
An iceberg off the coast of Newfoundland. Icebergs are floating masses of ice that have broken off from polar glaciers. About 80% of every iceberg is submerged.
Ice (1 of 6)
It has been estimated that over a tenth of the Earth's land surface - some 15 600 000 km2 (6 021 000 sq mi) - is permanently covered with ice. Ice is in fact the world's biggest reservoir of fresh water, with over three quarters of the global total contained in ice sheets, ice caps and glaciers.
These range in size from the huge Antarctic and Greenland ice sheets to the small glaciers found in high-latitude and high-altitude mountain ranges.
Ice bodies develop where winter snowfall is able to accumulate and persist through the summer. Over time this snow is compressed into an ice body, and such ice bodies may grow to blanket the landscape as an ice sheet or ice cap. Alternatively, the ice body may grow to form a mass that flows down a slope - a glacier - often cutting a valley and eroding rock material that is eventually deposited at a lower altitude as the ice melts.
The formation of ice bodies
Ice bodies develop mainly through the accumulation of snow, or sometimes by the freezing of rain as it hits an ice surface. Obviously, not all the snow that falls is turned into ice - during the northern-hemisphere winter over half the world's land surface and up to one third of the surfaces of the oceans may be blanketed by snow and ice. Most of this snow and ice is only temporary, as the Sun's warmth and energy are able to melt the cover during warm winter days or as winter passes into spring and summer.
In some places, however, the summer warmth is unable to melt all the snowfall of the previous winter. This may be because summer temperatures are rather low, or summer is very short, or because winter snowfall is very high. Where this occurs, snow lies all year round (this snow is sometimes called firn or nuvu) and becomes covered by the snow of the next winter. As this process continues from year to year, the snow that is buried becomes compressed and transformed into glacier ice. Latitude and altitude both affect where permanent snow can accumulate. The level that separates permanent snow cover from places where the snow melts in the summer is called the snowline or firnline. The snowline increases in altitude towards the equator: in polar regions it lies at sea level, in Norway at 1200-1500 m (4000-5000 ft) above sea level, and in the Alps at about 2700 m(9000 ft). Permanent snow and ice can even occur in the tropics close to the equator: in East Africa, for example, the snowline lies at about 4900 m (16 000 ft), so that glaciers are found on Mount Kenya, Kilimanjaro, and the Ruwenzori Mountains.
* MOUNTAINS
* RIVERS AND LAKES
* THE OCEANS
* THE MAKING OF THE WEATHER
* CLIMATIC AND VEGETATION REGIONS
* ECOSYSTEMS: EXTREME
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p090-2
Ice (2 of 6)
Ice sheets and ice caps
Ice sheets and ice caps are ice bodies that have grown into domes that blanket an area of land, submerging valleys, hills and mountains. Occasionally, islands' of land, called nunataks, protrude through the sea' of ice. Ice sheets are defined as having an area over 50 000 km2 (19 000 sq mi); ice caps are smaller.
The continent of Antarctica is covered by an ice sheet, which rises to about 4200 m (13 800 ft) above sea level and spreads over an area of 12.5 million km2 (4.8 million sq mi). Much of Greenland is covered by an ice sheet (1.7 million km2 / 660 000 sq mi in area), while ice caps occur in Norway, Canada and Iceland. Together, the Antarctic and Greenland ice sheets account for 94% of the Earth's land area covered by ice bodies.
Sea ice
There is no ice sheet over the North Pole because there is no land there - however, the Arctic Ocean is always frozen and, during the winter, Arctic sea ice covers about 12 million km2 (4.6 million sq mi).
An area of sea ice that is joined to a coast is called an ice shelf. Ice shelves occur in the Arctic, joined to the coasts of northern Canada and Greenland, and in the Antarctic - notably the Ross Ice Shelf, which has an area greater than France. Ocean currents and seasonal melting can cause ice sheets to break up, creating areas of pack ice or smaller ice floes.
* MOUNTAINS
* RIVERS AND LAKES
* THE OCEANS
* THE MAKING OF THE WEATHER
* CLIMATIC AND VEGETATION REGIONS
* ECOSYSTEMS: EXTREME
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p090-3
Ice shaping a landscape.
Ice (3 of 6)
Types of glacier
In comparison to ice sheets and ice caps, glaciers are small, relatively narrow bodies of ice, which flow down slopes. Some Arctic and Antarctic glaciers reach several hundred kilometers in length, but the longest glacier in the European Alps, for example, is only 35 km (22 mi) long.
There are several types of glacier: outlet glaciers extend from the edges of ice sheets and ice caps; valley or alpine glaciers are confined within valleys for much of their length; and cirque (sometimes called corrie or cwm) glaciers are totally confined to a small rock basin and may cover an area of less than 1 km2 ( 1/3 sq mi).
Most glaciers terminate on land, but some - especially those that are outlets of extensive ice sheets or ice caps - may reach the sea. Where this occurs, large blocks of ice may break off the end of the glacier (the snout) to form icebergs, which are carried away by the tide and ocean currents.
* MOUNTAINS
* RIVERS AND LAKES
* THE OCEANS
* THE MAKING OF THE WEATHER
* CLIMATIC AND VEGETATION REGIONS
* ECOSYSTEMS: EXTREME
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Ice shaping a landscape after the glacier has melted.
Ice (4 of 6)
Ice movements
Ice bodies move and flow under the influence of gravity. The movement of frozen water is obviously much slower than when it is in its liquid form. Most glaciers flow at a velocity between 3 and 300 m (10 and 1000 ft) per year. Glaciers on steep slopes may move much faster, and the Quarayaq Glacier, which is supplied with ice from the Greenland Ice Sheet, averages 20-24 m (65-80 ft) per day. Many glaciers experience surges - which may last a few days or several years - when flow is extremely rapid, often equivalent to rates of up to 10 km (6 mi) a year.
Glaciers move in two ways. The first, called glacial sliding, occurs when the base of a glacier slides over the rock beneath it. The other, called internal deformation, involves movements within the glacier, caused by the stresses resulting from the weight of the ice body itself. Many glaciers flow through the combination of both mechanisms, but in very cold environments, where a glacier may be frozen to its rock bed, internal deformation may account for all the movement that occurs.
At the steeper points in a glacier, deep cracks called crevasses will form, usually at right angles to the direction of flow. Where the glacier tumbles over an underlying cliff, an icefall will form, characterized by many crevasses and unstable towers of ice called seracs. Sometimes a glacier will flow round either side of an area of harder rock known as a rognon.
As a glacier flows downhill it will extend the snowline. The area below the snowline will be subject to greater melting than that above it, so the glacier is kept in equilibrium and prevented from growing indefinitely in size.
* MOUNTAINS
* RIVERS AND LAKES
* THE OCEANS
* THE MAKING OF THE WEATHER
* CLIMATIC AND VEGETATION REGIONS
* ECOSYSTEMS: EXTREME
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Ice (page 4)
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p090-5
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Ice (page 5)
ftsTitle
The Franz Josef Glacier in New Zealand. Movement of the glacier causes large chunks of ice to break off its face.
Ice (5 of 6)
Glaciers and landscape
Glacier ice is a very powerful erosional agent, smoothing rock surfaces and cutting deep valleys. Fjords (for example, along the coasts of Norway and Alaska) are U-shaped glacial valleys that become submerged by the sea after the melting of the ice that produced them. U-shaped valleys are classically regarded as glacial features, but they can be formed by other processes - for example, by rivers in their middle and lower reaches.
A sliding glacier erodes by plucking blocks of rock from its bed and by abrading rock surfaces, i.e. breaking off small particles and rock fragments. The rock that is eroded is transported by the ice and deposited as the glacier travels down slope and melts. Glacial deposits can form distinct landforms such as moraines and drumlins, or they may simply be deposited as glacial till, a blanket of sediment covering the landscape. As glacial ice melts, especially during the summer months, the meltwater that emerges from beneath a glacier can carry large quantities of sediment away from the glacier system.
* MOUNTAINS
* RIVERS AND LAKES
* THE OCEANS
* THE MAKING OF THE WEATHER
* CLIMATIC AND VEGETATION REGIONS
* ECOSYSTEMS: EXTREME
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Glacier breaking off
**++*
p090-6
ftsTitleOverride
Ice (page 6)
ftsTitle
The northeast coast of Greenland, photographed by the Landsat-1 satellite from an altitude of 914 km (568 mi). The ice sheet on the left feeds numerous glaciers that flow down to the sea (right). Here, icebergs break off from the snouts of the glaciers. The sea ice in this photograph includes large floes, together with newer ice in the process of formation.
Ice (6 of 6)
THE ICE AGES During colder periods of the Earth's history, ice sheets, ice caps and glaciers have covered much larger areas of the land surface than they do today. There have been several ice ages or glacial periods in the past: scientists now believe that there have been between 15 and 22 glacial periods during the last 2 million years.
The last Ice Age ended about 10 000 years ago and, at its height, great ice sheets covered most of Canada and Scandinavia, ice caps covered Highland Scotland, Snowdonia and the English Lake District, and large glaciers extended into southern England. Throughout Europe, the ice extended roughly to the latitude of the English Midlands, the Netherlands and northern Germany, although in North America it extended even further south, into the northern USA. There are no ice bodies (apart from high-mountain glaciers) so far south today, but the landscape still records the presence of ice in the past, in the form of erosion valleys and large volumes of transported sediment.
* MOUNTAINS
* RIVERS AND LAKES
* THE OCEANS
* THE MAKING OF THE WEATHER
* CLIMATIC AND VEGETATION REGIONS
* ECOSYSTEMS: EXTREME
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p092-1
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Deserts (page 1)
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A massive sand dune rises above a solitary tree in the Namib Desert of Namibia. Despite the popular image, only a relatively small proportion of the world's deserts are sandy - for example, only 28% of the Sahara is composed of sand dunes and sand plains.
Deserts (1 of 2)
Deserts are areas of the world where there is a considerable deficiency of water. The major cause of this aridity is low precipitation, particularly rainfall, but desert areas also frequently experience a great variation in rainfall amounts from year to year. Although deserts are not necessarily hot, many are found in hot climates, which increases the shortage of water because of high rates of evaporation. The lack of water in deserts makes conditions difficult for human, animal and plant life. As a result, living organisms are less common than in wetter areas, with special adaptations necessary in order to allow survival.
Because water deficiency is the main characteristic of deserts, Arctic and Antarctic areas are sometimes called polar deserts because water is not generally available in its liquid form.
What causes deserts?
Many of the world's deserts coincide with areas characterized by stable atmospheric high pressure, conditions that not do not favor rainfall. These subtropical high-pressure belts are responsible for deserts such as the Sahara and Kalahari in Africa and the deserts of Australia and Arabia.
Other deserts - for example, the Gobi Desert in Asia - exist because of their continentality, that is, their distance from the sea. This prevents them being reached by moisture-bearing winds from the oceans. This effect may be enhanced by the shape of the landscape: for example, moist air coming in from the sea will precipitate on mountains as rain or snow, and by the time the air has reached the far side of the mountains it will be dry, so forming a rain-shadow desert. Such deserts occur, for example, to the north of the Himalaya.
The deserts of the west coasts of southern Africa and South America - the Namib and Atacama Deserts - are affected by the presence of cold ocean currents running along the coast. These cool the air that they come into contact with, so preventing evaporation of moisture from the ocean surface and the formation of rain. At some places in the Atacama Desert, no rain was recorded in the 400 years prior to 1971. The cold ocean water does, however, cause a high frequency of fog, which is the major source of moisture in these extremely dry or hyper-arid deserts.
Human activity may also contribute to the creation of new desert areas - a process known as desertification .
Desert climates
Some deserts are drier than others. Because of this it is usual for a distinction to be made between semi-arid areas, which receive on average 200-500 mm (8-20 in) of rainfall per year; arid areas,with an average annual rainfall of 25- 200 mm (1-8 in); and hyper-arid areas, which are so dry that rainfall may not occur for several years on end. Together, arid and hyper-arid areas form the world's true deserts. Semi-arid areas, which are often on the margins of deserts, cover about 15% of the world's land area, while arid and hyper-arid desert areas respectively cover about 16% and 4%.
Most deserts tend to experience warm or hot summer months, with mean temperatures greater than 20 degC (68 degF), and maximum temperatures sometimes reaching over 50 degC (122 degF) in the hotter deserts. However, temperatures in the winter months can vary widely, because of the range of latitudes in which deserts are found. The deserts formed by the subtropical high-pressure belts generally have the warmest winters; indeed parts of the Arabian Desert do not experience anything that really deserves to be called winter, with the mean temperatures of the coldest month being over 20 degC (68 degF). Some deserts do, however, experience cold winters. Parts of the central Sahara are extremely mountainous, so that high altitudes contribute to low winter temperatures, while the mean temperature of the coldest month in the Gobi Desert falls below -20 degC (-4 degF) owing both to its great distance from the sea and to its high altitude.
Many deserts also experience very high daily temperature ranges, with hot days and cold nights. This is due to clear, cloudless skies allowing heat to escape, combined with the lower ability of ground without vegetation to absorb heat. A daily temperature range of 55 degC (99 degF) has been recorded in the central Sahara: from 52 degC to -3.3 degC (126 degF to 26 degF).
Popular images of deserts paint a picture of vast plains of sand dunes without a plant or rock in sight. Although some deserts are composed of huge dunes shaped by the wind, it is by no means the case that all deserts are, or that this type of feature is typical of deserts as a whole.
Nevertheless, the wind can be an important landscaping agent in deserts because of the limited presence of vegetation to protect the ground surface. The wind can erode by sandblasting bare rock faces, creating smooth rock faces and upstanding features such as yardangs. If a sufficient supply of sediment is available, the wind can also transport and deposit sand to form dunes. The sand making up desert dunes often comes from dry river courses and lake beds, or the coast. Rock weathering - which is encouraged by high daily temperature ranges - can also be effective on the bare rock surfaces, adding to the supply of sand-sized material.
Although rainfall is limited in deserts, the lack of vegetation and the high intensity of desert rainstorms does mean that water plays an important part in shaping desert landscapes. Desert river courses, often called wadis, can carry large volumes of water and sediment during storms. This can lead to the formation of canyons and heavily gullied areas, called badlands, in areas where soft and highly erodable sediments are found. Where mountains are present, highly erosive flash floods may occur. The large loads of sediments carried by these floods may be deposited where a river leaves the mountains and passes onto gentler, flatter ground, forming an alluvial fan. Over time, the work of wind and water can cause a desert landscape to become dissected. In areas where the bedrock consists of horizontally bedded strata - for example in the desert areas of Arizona and New Mexico in the southwestern USA - this can lead to the development of isolated, flat-topped hills called mesas and buttes Where the rocks are not stratified in this way, more rounded 'island hills' (sometimes called inselbergs) may develop. A famous example is Ayers Rock in Australia.
Plants, animals and people
Plants and animals can survive in deserts if they become specially adapted to the harsh conditions that occur. People, too, have adapted to desert conditions by finding ways of using the water that is available - for example, by living at an oasis (a constant spring) or digging wells in the beds of dry rivers to tap water supplies. Today, technology has enabled more people than ever to exist and travel comfortably in deserts, as the cities in the North American deserts testify. However, desert life can remain precarious, and is always prone to the dangers of drought, as the human tragedies in recent years on the southern margins of the Sahara have so clearly demonstrated.
PLATEAU AND PLAINS
Some parts of the world's land surface appear to be very flat and level, or gently rolling without any major hills or depressions. These features may be very extensive, covering tens of thousands of square kilometers - for example, the Great Plains of North America. Generally, areas with a level surface that are relatively higher than neighboring areas are called plateau, and those at a relatively low altitude are called plains. Plateau and plains can have various origins, the most important of which are given below.
PLATEAU
There are two types of tectonic plateau. The first are shield areas, formed of ancient upstanding rock masses, with well-defined edges that drop to lower ground. Examples include the Deccan Plateau in India, the Arabian Plateau, and the Canadian Shield. The second type are intermontane plateau, which, despite having a relatively high altitude, have their margins defined by even higher mountains. An example of this is the Great Basin of the southwestern USA, which, because of the fringing mountains, is a rain-shadow desert.
Volcanic plateau result from an outpouring of lava that blankets the landscape and forms a flat basalt plateau. Examples include the Antrim Plateau of Northern Ireland.
Residual plateau are formed by the erosion and dissection of the landscape, leaving behind residual plateau areas that have the same altitude. In arid areas, the dissection of horizontally bedded rocks can result in the formation of flat-topped residual mesas and buttes.
PLAINS
Structural plains result from areas of horizontal rocks being undisturbed by tectonic activity. Examples include the Great Plains of the American Midwest.
Erosional plains can be formed by river or glacier erosion. In deserts, the removal of fine sediment by the wind can result in flat rock or gravel plains - called hamada or reg in the Sahara.
Depositional plains occur where sediment has been deposited to blanket the ground below, forming a flat surface, for example as a river flood plain.
THE LANDSCAPES OF FOUR DESERT AREAS
(percentages of total areas)
Southwest Sahara Arabia Australia
USA
Mountains 38.1 43 47 16
Gentle rock slopes 0.7 10 1 14
Alluvial fans 31. 4 1 4
Ephemeral rivers
and floodplains 4.8 2 2 13
Dry lakes 1.1 1 1 1
Gullied areas 2.6 2 1 0
Sand dunes
and sand plains 0.6 28 26 38
Other miscellaneous 20.7 13 18 1888
* MOUNTAINS
* ICE
* THE MAKING OF THE WEATHER
* CLIMATIC AND VEGETATION REGIONS
* ECOSYSTEMS: EXTREME
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ftsTitleOverride
Rivers and Lakes (page 1)
ftsTitle
Rivers and Lakes (1 of 4)
Rivers and lakes are the most important bodies of surface water on land masses. A river is a freshwater body confined in a channel that flows down a slope into another river, a lake or the sea, or sometimes into an inland desert. Small, narrow rivers may be called brooks, streams or creeks.
A lake is an inland body of water occupying a depression in the Earth's surface. Usually, lakes receive water from rivers, but sometimes only directly from springs. Lakes normally lose water into an outlet or river, but some, called closed lakes, have no outlet and lose water only by evaporation - for example, Lake Eyre in Australia and Great Salt Lake in Utah, USA.
Where do rivers get their water from?
Rivers may receive their water from several sources, but all of these are indirectly or directly related to precipitation - a collective term for the fall of moisture onto the Earth's surface from the atmosphere. Rain falling on the ground may immediately run down slopes as overland flow, becoming concentrated and eventually forming a stream. This tends to occur where the ground surface is impermeable (i.e. water cannot pass through it, as is the case with some kinds of rock). It may also occur where the ground is already saturated with water, or when rainfall is very heavy.
Often, however, rivers receive their water from springs. This is because rainfall will commonly soak into the ground, to accumulate in the soil or to pass into permeable and porous rocks as groundwater. In permeable rock, water can pass right through the rock itself, whereas in porous rock there are holes and fissures through which water can pass. A deposit of rock containing groundwater is known as an aquifer. Springs occur where the top of the aquifer intersects with the ground surface. Groundwater is important as a source for rivers in that it can supply water even when precipitation is not occurring, thereby constantly maintaining river flow.
A third source of water for rivers is the melting of solid precipitation (snow) or snow that has been turned to ice to form a glacier or ice sheet. This is particularly important in high-latitude and mountainous areas.
* ICE
* COASTS
* THE OCEANS
* ECOSYSTEMS: AQUATIC
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Rivers and Lakes (page 2)
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Niagara Falls. Waterfalls are formed when rivers pass over bands of harder, more resistant rock. The softer rock downstream from the hard rock is more quickly eroded, but even the harder rock is eventually undercut and eroded, and the waterfall gradually moves upstream and decreases in height.
Rivers and Lakes (2 of 4)
Perennial, seasonal and ephemeral rivers
Rivers occur in all the world's major environments, even in polar areas and deserts. In temperate areas, such as Western Europe, northeastern USA and New Zealand, and in the wet tropics, enough precipitation tends to fall fairly evenly throughout the year to replenish ground-water constantly, and therefore to allow rivers to flow all year round. These perennial rivers do, however, experience seasonal and day-to-day variations in the volume of water they carry (the flow regime), due to seasonal fluctuations in precipitation and additional inputs from individual storms.
Some rivers may only flow seasonally, particularly in environments with Mediterranean-type climates, which have a very distinct wet, winter season and a dry summer. Rivers in glaciated areas may also have very seasonal flow regimes. Glacial meltwater streams, which receive their water directly from glaciers, usually only flow during the few months in the summer when the ice melts.
In dry desert climates, rivers may not flow for years on end, because of the infrequency of desert storms, and then only for a few days, or even hours. However, when storms do occur these ephemeral rivers may flow at great rates, because desert rainfall is often very heavy. This gives them considerable power and the ability to erode and transport large quantities of sediment.
Some deserts do possess perennial rivers. The Nile, for example, despite experiencing a distinctly seasonal flow regime, flows all year round through the Egyptian Desert; likewise, the Colorado River passes through desert areas of the southwestern USA. The reason that these and other rivers can successfully exist in deserts is that their catchments (source areas) lie in areas with wetter climates.
* ICE
* COASTS
* THE OCEANS
* ECOSYSTEMS: AQUATIC
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Rivers and Lakes (page 3)
ftsTitle
Tributaries of the Amazon from space, showing the numerous meanders typical of rivers flowing over soft, flat ground. In such terrain, rivers frequently change their courses.
c Rivers and Lakes (3 of 4)
River basins
Only some very short rivers are able to flow from a source to the sea without either being joined by others or becoming a tributary of a larger river. Most rivers therefore form part of a drainage network, occupying a drainage basin. In fact, the whole of the Earth's land surface can be divided up into drainage basins, and these basins are separated by areas of relatively high ground called watersheds. Some drainage basins occupy only a few square kilometers, but others are enormous - the largest, the Amazon basin, covers over 7 million km2 (2.7 million sq mi).
Rivers and landscapes
Rivers are a major force in shaping landscapes. They erode rock and sediment, thereby cutting channels and even valleys and shaping the landscape of upland areas. Such channels may be very shallow, but they may be as deep as the Grand Canyon, which the Colorado River has cut to a depth of up to 1500 m (5000 ft) in places. River valleys are commonly regarded as V-shaped, but in fact their shape can vary according to the position along the river's course, the size of the river, and the rock types in the landscape through which the river passes.
Rivers also transport vast quantities of material that has been eroded by other agencies - for example, rock-weathering processes, glaciers and the wind. This sediment may in turn be deposited by the rivers themselves, within river channels or as flood plains , or carried into lakes or the sea.
The deposition of sediment in a valley to form a flood plain tends to result in a lessening of the gradient of a river channel. A gentler gradient means that a river may cut meanders (wandering channels) in the soft flood-plain deposits. Over time, erosion of the river bank of the outside bend of a meander (where flow is fastest) may cause the channel to straighten and the meander to be 'cut off' to form an oxbow lake.
Mud islands may form within the channel of a river carrying a particularly large load of sediment, and a pattern of braided channels may develop. Where a river with a large load of sediment meets the sea, the loss of river energy can cause sediment to be deposited at the coast, forming a delta, as in the cases of the Mississippi and Nile. Over the centuries, deltas - so named because their shape resembles the Greek letter delta () - can build up large areas of land where once there was sea.
* ICE
* COASTS
* THE OCEANS
* ECOSYSTEMS: AQUATIC
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Rivers and Lakes (page 4)
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Rivers and Lakes (4 of 4)
Lakes
Lakes can occur along the course of a river, where it flows into a depression. In some circumstances, a lake can mark the end point of a river course. Such depressions can be erosional, formed by the action of glaciers or wind. They may also be depositional, formed, for example, by a landslide blocking the course of a river. Finally, they may be structural, formed by Earth movements, for example in rift valleys. Lakes may also be formed behind the terminal moraines of retreating glaciers. A volcanic lake, formed by the accumulation of rainfall in a volcanic crater, may even have no inflowing or outflowing river.
In hot, relatively dry climates, lakes will lose a lot of water through evaporation. This results in the concentration of salts and the lake water becoming saline. The Caspian Sea, Aral Sea and Dead Sea are in fact all saline lakes. These three lakes do have outlets, but lakes that lose as much water through evaporation as they gain from inputs will in fact be closed, that is, without an outflow.
Rivers, lakes and people
Rivers and lakes are used by humans for a variety of purposes. Ever since mankind has existed, they have provided water and food. Over time their importance and the diversity of usage have both increased. They have provided water for irrigation and also natural transport routes. In North America, the Mississippi River and the Great Lakes are important routes for the transport of agricultural and industrial produce; in addition, the construction of canals has improved the 'linkages' in the water transport system.
Rivers have been straightened, widened and deepened, and also dammed to create reservoirs and sources of hydroelectricity. Humans have, however, misused rivers and lakes too, by using them to dispose of sewage and industrial waste so endangering natural plant and animal communities.
THE HYDROLOGICAL CYCLE
Water exists in three states: liquid, gaseous (water vapor or steam) and solid (snow and ice). It can also pass from one state to another by freezing, melting, condensing and evaporating. New water is not created on the Earth's surface or in its atmosphere; nor is 'old' water lost. Rather, there is a finite amount, and this circulates in what is known as the hydrological cycle. Water moves around the cycle both by physically moving and by changing its state, as the diagram shows.
Today, 97% of the water in the hydrological cycle is contained in the world's seas, oceans and saline lakes. The remaining 3% is fresh water. About 75% of all fresh water is contained in glaciers and ice sheets, and just over 24% is groundwater (i.e. underground). The rivers, lakes, soil and atmosphere therefore contain a very small amount (less than 0.5%) of the world's fresh water at any one time.
During glacial periods of the Earth's history, the amount of water contained in ice sheets and glaciers has been greater, and the amount in oceans smaller.
* ICE
* COASTS
* THE OCEANS
* ECOSYSTEMS: AQUATIC
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Hydrological cycle
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p096-1
ftsTitleOverride
Coasts (page 1)
ftsTitle
Coasts (1 of 4)
Coasts are where the land meets the sea. The coastline is a zone of landscape activity where new land can be created by the deposition (laying down) of sediment, but also where existing land can be lost through the processes of marine erosion. But what happens where on the coast depends on many factors such as climate, coastal geology, the orientation of the coast to wind and waves, and the history of human activities.
With so many factors affecting the nature of the processes at work on coastlines, it is not surprising that the world's coasts are extremely diverse. In polar regions, the coastline may be protected from the direct impact of the sea by the presence of ice shelves , while in many tropical areas the arrival of great volumes of muddy sediments, transported to the sea by rivers, has allowed the development of extensive mangrove swamps that both protect the coast and make human access to the sea difficult. But in other locations the coastline is actively changing and evolving through the direct impact of the sea's energy.
The energy of the sea
Tides and currents contribute some of the sea's energy at the coast, but waves are the most important factor in shaping the coastline, contributing most of the energy that erodes, transports and deposits sediment. Also important, in areas where winds and waves from one direction dominate, is the sea's ability to transport material along the coast in a preferred direction, by the process known as longshore drift.
Waves are generated by the wind in the open sea where water is deep, and are driven onshore by it. The height of waves and the distance between them (the wave- length), together with wave energy, are largely determined by wind strength and the distance, called the fetch, over which they have been transported. The fetch can extend for many thousands of kilometers; for example, the predominant westerly winds that affect the coastline of Western Europe can transport waves generated off the east coast of North America. Waves with a long fetch tend to possess considerable energy with which to shape the coast.
As the sea becomes shallower at the coast and waves move onshore, they lose energy and 'break', and water rushes onshore as swash. On a sloping beach, the water returns to the sea under the effect of gravity as backwash; in some circumstances this can become concentrated in rip currents, which are particularly erosive.
An important factor in determining the effect of waves on the coast is whether waves are destructive or constructive. Destructive waves are relatively high compared to their wavelength, and break with a force that generates a strong, erosive backwash that tends to remove material from a beach. Constructive waves break more gently, pushing material up the beach, building it up.
* ICE
* RIVERS AND LAKES
* ISLANDS
* THE OCEANS
* ECOSYSTEMS: AQUATIC
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Coasts (page 2)
ftsTitle
Surfer's Paradise, a magnificent beach in Queensland, Australia, is a classic example of longshore drift and a river estuary combining to build up a spit parallel to the coast.
Coasts (2 of 4)
Beaches
Beaches result from the depositional effects of constructive waves, usually in low-energy coastal environments. They can be made out of fine sediments such as mud and sand, or coarser material such as pebbles, or a combination of all three. The type of material present on any one beach is determined by the type of sediment available to the waves that construct it. This in turn is affected by the sediment source, which can be a river bringing sediment to the sea, the ocean floor, or the coast itself, in the form of eroded material transported to the beach by longshore drift.
The type of sediment, wave energy, the tidal range and the effect of storms will determine beach shape. The upper beach is generally steepest and is affected only by high tides and storm waves. It usually consists of coarse material, pushed up the beach by constructive waves, and possesses several parallel ridges (berms) produced by successive high tides. At the back of the beach, a storm ridge may be present. The lower beach is usually more gentle and consists of finer sediments. On some coasts, the beach is backed by sand dunes, which accumulate from sand that is blown onshore by the wind at low tide.
Longshore drift can result in the growth of a beach along the coast. Where this occurs, and the coastline changes direction, for example at an estuary (river mouth) or bay, the growth of the beach can continue to form a spit, a depositional landform that extends out into the sea. A spit can even grow to rejoin the land again, forming a sand bar or barrier beach. Some bays and estuaries are so sheltered from wave action that beaches do not form. However, if there is a source of fine sediment, such as mud supplied by a river, these sheltered places can see the growth of tidal salt marshes, which support types of plants that can withstand the twice daily incursion of salt water during high tides.
Erosional coasts
Erosion occurs where coasts are exposed to waves that have a long fetch, or where strong onshore winds generate high-energy waves over relatively short distances. Erosional coastlines are commonly dominated by cliffs, at the foot of which a wavecut platform may be exposed at low tide. This feature represents the seaward remnant of the eroded cliffs. In areas where the cliffs are formed of rocks that are well jointed and susceptible to erosion - for example, chalk, limestone and some sandstones - sea stacks (free-standing pinnacles), arches and caves may be formed by the sea's erosive action.
Coastal erosion occurs through hydraulic action (the pressure of waves breaking at the foot of the cliff) and by corrasion (a process in which sediment in the water is thrown against the rock surface). Erosion at the cliff foot may slow down over time as the wavecut platform widens and is able to absorb the energy of the waves being driven onshore. This can even result in the formation of a beach at the foot of the cliff.
* ICE
* RIVERS AND LAKES
* ISLANDS
* THE OCEANS
* ECOSYSTEMS: AQUATIC
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Coasts (page 3)
ftsTitle
Tokyo Bay, with Tokyo city shown as the grey area. The port of Yokohama is located next to the deeper water, which provides better access for ships. Tokyo Bay is a natural harbor, and like many such harbors, its shoreline has been altered by reclamation and port construction.
Coasts (3 of 4)
Coastal landscapes
Coastal landscapes are not only determined by the processes of deposition and erosion but also by the nature and structure of the coastal geology. Where the rock structure generally lies parallel to the coast, the processes affecting it will generally be constant along its length, so the coastline will be concordant, i.e. relatively straight.
On the other hand, a discordant coast develops where the rock structure meets the coastline at an angle. This allows the sea to exploit the relative strengths of different rocks, leading to greater erosion of those that are less resistant. A coast characterized by bays and headlands will result and, as the erosion progresses over time, the bays will become more sheltered. These will then become the sites of sediment deposition and beach development.
Humans also affect coastal landscapes. One important way is in the construction of coastal defenses, either to reduce the risk of storm flooding in low-lying areas, or in an attempt to control coastal erosion. Sediment deposition and land reclamation can also be influenced by human actions, as in the Netherlands where 2227 km2 (860 sq mi) of the Zuider Zee have been reclaimed from the sea.
* ICE
* RIVERS AND LAKES
* ISLANDS
* THE OCEANS
* ECOSYSTEMS: AQUATIC
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Coasts (page 4)
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Coasts (4 of 4)
CHANGES IN SEA LEVEL
Throughout geological history, sea level has changed significantly for a number of different reasons. An obvious one is that, during ice ages, water is withdrawn from the oceans to form ice sheets, ice caps and glaciers, causing sea level to fall. However, recent research has indicated that no less important is the fact that during particularly cold periods the ocean water contracts, again resulting in lower sea level. Conversely, when the Earth warms up again the ice melts and the water expands, leading to a rise in sea level.
But not all sea-level changes are caused by climatic effects. For example, at times when the movement of tectonic plates is particularly vigorous, the increased volume of magma rising at oceanic ridges will displace more ocean water than usual, causing sea level to rise.
Such worldwide changes in sea level are known as eustatic changes. However, there can also be more local changes in sea level, known as isostatic changes. The growth of ice on land results in the depression of the Earth's crust beneath the great weight of ice sheets and ice caps, and when the ice melts the crust beneath rises again. The melting of ice caps in North America and Scandinavia at the end of the last Ice Age 10 000 years ago caused the crust to rebound, resulting in coastal areas rising relative to the sea - by as much as 300 m (1000 ft) in Hudson's Bay and the Baltic Sea. Moreover, crustal rebound in some areas can cause subsidence in others by way of compensation. This has occurred in Britain; uplift in Scotland has contributed to the subsidence of the south coast of England.
Rising sea levels can cause coastal areas to become flooded, drowning valleys to form narrow sea inlets called rias. Conversely, falling sea levels or rising land can sometimes create raised beaches, former beaches stranded high above the present sea level.
* ICE
* RIVERS AND LAKES
* ISLANDS
* THE OCEANS
* ECOSYSTEMS: AQUATIC
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p098-1
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Islands (page 1)
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Volcanic activity on Iceland. The island of Iceland started forming about 20 million years ago, and is still growing today. Iceland was largely built up by the slow extrusion of lava at the boundary of a tectonic plate, rather than by violent volcanic eruptions.
Islands (1 of 3)
An island is a body of land, smaller than a continent, that is completely surrounded by water. Islands occur in rivers, lakes, and the seas and oceans. They range in size from very small mud and sand islands of only a few square meters, to Greenland, which has an area of 2.2 million km2 (840 000 sq mi).
Islands, especially those in seas and oceans, have a range of origins. Islands can develop through constructional processes, involving the deposition (laying down) of sediment or the building up of volcanic or organic material. They may also be formed by erosional processes that cause an area of land to become separated from the mainland. Rising sea levels can also lead to the development of islands, by drowning lowlying areas of land and separating higher areas from the main land mass.
Volcanic islands
When volcanic activity occurs beneath the oceans, it can lead to the growth of islands. This is often closely linked to the movement of the Earth's crustal plates, with island-building (e.g. Iceland) occurring both at constructive plate margins and at destructive margins. Volcanic islands (e.g. Hawaii) can also form far from any plate boundary.
Iceland, situated on the mid-Atlantic ridge, is the largest example of a volcanic island formed at a constructive plate margin. About 100 000 km2 (38 600 sq mi) in area, Iceland started forming about 20 million years ago - the age of the oldest rocks on the island. It is still growing in size today, as new material is periodically added, along a line of volcanic activity running from the southwest to the northeast of the island. Much of the volcanic activity responsible for Iceland's growth has not been in the form of spectacular eruptions, but rather as quiet extrusive fissure eruptions, involving the outpouring of large quantities of lava from cracks in the Earth's surface, giving rise to basaltic rocks.
Spectacular eruptions, have, however, also played their part. For example, in 1963, eruptions occurred off the south coast of Iceland. In the space of a few weeks, ash and lava built up on the sea floor and a new, small island named Surtsey was born.
* PLATE TECTONICS
* VOLCANOES
* ICE
* COASTS
* THE OCEANS
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p098-2
ftsTitleOverride
Islands (page 2)
ftsTitle
The islands of Hawaii (above) photographed from the space shuttle. The Hawaiian-Emperor island chain was formed as the Pacific plate passed over a 'hot spot' in the Earth's mantle. Periods of volcanic activity arising from this hot spot have created a succession of islands.
Islands (2 of 3)
Island archipelagos
The collision of crustal plates at destructive margins can generate significant volcanic activity. If this occurs at the edge of a land mass it can cause mountain building, but when the collision zone lies beneath an ocean, island development can result.
Islands that are born in this way do not occur singly, but in chains or archipelagos ('arcs') that parallel the plate boundary. This is well illustrated on the western side of the Pacific Ocean. Here thousands of islands - most of them volcanic but some formed by the folding up of the ocean floor - mark the western edge of the Pacific plate. These islands start in the south at New Zealand, run north to the Tongan chain before heading west to New Guinea, and north again through the Philippines, Japan, the Kurile island chain and finally the Aleutian Islands, which continue to the mainland of North America. The Indonesian archipelago, which extends westwards into the Indian Ocean from the island chains of the west Pacific, is the world's largest archipelago, its 13 000 islands stretching over a distance of 5600 km (3500 mi).
Hot-spot islands
The Hawaiian-Emperor island chain and some other mid-Pacific islands owe their existence to volcanic hot-spot activity. Volcanic activity has erupted through the Pacific plate as it has passed over areas of the Earth's mantle that are particularly active. As the plate has moved, so has the location of volcanic activity and island construction. The Hawaiian-Emperor chain is more than 6000 km (3700 mi) long and includes more than 100 islands and seamounts (volcanoes that have not reached the ocean surface). The youngest island, which is the furthest east, is Hawaii itself, where volcanic activity is still occurring.
* PLATE TECTONICS
* VOLCANOES
* ICE
* COASTS
* THE OCEANS
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p098-3
ftsTitleOverride
Islands (page 3)
ftsTitle
Coral islands in the Maldives, Indian Ocean. In the distance is a more-or-less totally submerged coral atoll, surrounding a lagoon.
Islands (3 of 3)
Coral islands
Coral islands and reefs are an important component of warm tropical and subtropical oceans and seas. They are formed from the skeletons of the group of primitive marine organisms known as corals. Coral islands develop where coral grows up towards the ocean surface from shallow submarine platforms - often volcanic cones. If the cone is totally submerged, then a coral atoll will develop - a circular or horseshoe-shaped coral ring that encloses a body of sea water called a lagoon. Upward growth of the coral ceases once sea level has been reached. Coral islands are therefore flat and low, unless a change in sea level has caused their elevation to change.
Sea level and islands
Changes in sea level can cause new islands to appear or existing ones to disappear. During the last Ice Age eastern Britain was joined to mainland Europe, because sea levels were lower as much of the world's water was frozen in the ice caps and glaciers. As the ice melted and the sea level rose, the North Sea and the Straits of Dover were re-established. By about 8500 years ago Britain was again an island. DT
CORAL
Coral reefs, coral atolls and coral islands are the largest living structures on Earth, although any one structure will probably contain far more skeletons of dead coral than live individuals. The Great Barrier Reef of northeastern Australia is 2027 km (1260 mi) long. The largest coral atoll is Kwajalein, in the Marshall Islands, of the Pacific Ocean. The 283 km (176 mi) long arc of coral encloses a lagoon that covers 2850 km2 (1100 sq mi). The coral body of Christmas Island, in the central Pacific Ocean, has the greatest coral area, covering 323 km2 (125 sq mi).
As well as being the largest living bodies, coral reefs are amongst the most diverse and spectacular of natural habitats. In amongst the many species of hard coral - each adapted for slightly different conditions on the reef - are to be found soft coral, sea anemonies and a host of other species. Of these, the myriads of fish are perhaps the most spectacular.
Corals will not survive in fresh water nor in water that is cooler than 20 degC (68 degF). They are therefore restricted to warm tropical and subtropical seas, though living coral reefs do occur around the coast of Bermuda in the Atlantic (latitude 32 deg N), because of the warming effect of the Gulf Stream. They also require light, so living corals are not usually found in water deeper than about 55 m (180 ft), or in water that contains a lot of sediment.
Corals feed off plankton and also gain food in a symbiotic relationship with small algal cells. Corals reproduce not only sexually but also by budding and by splitting in half, so creating large colonies of genetically identical individuals. During the day, corals appear inactive, with their tentacles retracted, but at night their tentacles extend to feed on the plankton that emerge from crevices in the reef. Many other invertebrates emerge at this time, to feed on plankton and other materials, the threat of predation by fish being largely removed by darkness. Space is at a premium on coral reefs, so individual corals fight each other for the right to grow and reproduce, by using stinging cells, releasing toxic chemicals and even by trying to digest their neighbors.
* PLATE TECTONICS
* VOLCANOES
* ICE
* COASTS
* THE OCEANS
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p100-1
ftsTitleOverride
The Oceans (page 1)
ftsTitle
Icebergs are a hazard to shipping as most of their bulk is hidden under water. Icebergs form as parts of the floating ice shelves around Antarctica break off, or as terrestrial glaciers in both the Arctic and Antarctic feed into the sea.
The Oceans (1 of 4)
The oceans cover a greater area of the Earth than does the land - 71% or almost three quarters of the Earth's surface.
The three major oceans are the Pacific, Atlantic and Indian Oceans. The Pacific is the largest ocean, and covers more than one third of the surface of the Earth. The Arctic Ocean is smaller than the other three and is covered almost entirely by ice. Seas are smaller than the four oceans.
The depth of the oceans is very small compared with their area. The deepest part, the Mariana Trench in the western Pacific, is only about 11 000 m deep(36 000 ft). However, this is greater than the height of the highest mountain on land, Mount Everest at 8863 m (29 078 ft).
Sea water
Sea water has solid substances dissolved in it. Sodium and chlorine are the most abundant of these (which combine in their solid form to make up sodium chloride - common salt), and together with magnesium, calcium and potassium they make up over 90% of the elements dissolved in sea water. Other elements are present only in very small amounts.
The saltiness, or salinity, of sea water depends on the amount of these substances dissolved in it. An average of about
3.5% of the volume of sea water consists of dissolved substances. High evaporation removes more of the pure water, leaving behind the dissolved substances, so the salinity is higher where evaporation is high - particularly if the sea water is also enclosed or cannot mix easily with the sea water of a larger ocean. This occurs, for example, in the Mediterranean and Red Seas. Low values of salinity occur in polar regions, particularly in the summer months when melting ice dilutes the sea water. Low salinity also occurs in seas such as the Baltic, which is linked to the Atlantic Ocean only by a narrow channel and which is fed by a larger number of freshwater rivers.
Most of the water on the Earth, about 94% of it, is in the oceans. More pure water is evaporated from the oceans than is returned as precipitation (rain, snow, etc.), but the volume of water in the oceans remains the same because water is also returned to the oceans from the land by rivers.
* WAVE THEORY
* PLATE TECTONICS
* RIVERS AND LAKES
* COASTS
* ISLANDS
* PRIMITIVE ANIMALS
* ARTHROPODS 1
* FISHES
* MARINE MAMMALS
* ECOSYSTEMS: AQUATIC
* FISHING
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p100-2
ftsTitleOverride
The Oceans (page 2)
ftsTitle
Waves, generated by the wind far out to sea, are the most important factor in shaping the coasts. Tides and currents also play a part.
The Oceans (2 of 4)
Waves
Sea water is rarely still: it is usually moving in waves, tides or currents. Waves are caused by wind blowing across the surface of the ocean. The height of a wave is determined by the wind speed, the time the wind has been blowing, and the distance the wave has traveled over the ocean. The highest wave ever recorded had a height of 34 m (116 ft), although usually they are much smaller. Waves play a very important role in the shaping of coastlines.
Water does not move along with waves. Instead the water changes shape as a wave passes, moving in a roughly circular motion, rising towards a wave crest as it arrives and falling as it passes. This motion can be seen by watching a boat: the boat bobs up and down as the waves move past it but does not move along with the waves.
There is another type of wave in the ocean, which is not generated by winds. These are tsunami. They are also popularly called tidal waves, but this name is quite wrong because they are not caused by tides. Tsunami are due to earthquakes or the eruption of undersea volcanoes. Such events move a large amount of water rapidly, disturbing the sea surface and creating waves that travel away from the area of the earthquake or volcano. Tsunami travel at very high speeds, around 750 km/h (470 mph). However, in the open ocean they cause little damage because their wave height is very low, usually less than 1 m (3 1/4 ft), but in shallow water they slow down and their height increases to 10 m (33 ft) or more, and they can cause extensive damage when they hit a shore.
* WAVE THEORY
* PLATE TECTONICS
* RIVERS AND LAKES
* COASTS
* ISLANDS
* PRIMITIVE ANIMALS
* ARTHROPODS 1
* FISHES
* MARINE MAMMALS
* ECOSYSTEMS: AQUATIC
* FISHING
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p100-3
ftsTitleOverride
The Oceans (page 3)
ftsTitle
Spring tides occur when the Moon and Sun are in line, and neap tides when the Moon and Sun form a right angle with the Earth.
The Oceans (3 of 4)
Tides
Tides are caused by the gravitational pull of the Moon and the Sun on the Earth, causing the level of the oceans to change. The pull is greatest on the side of the Earth facing the Moon, and this produces a high tide. The pull is weakest on the side away from the Moon, where the sea water rises away from the Moon, and this also gives a high tide.
The Sun is much further away than the Moon, so although it is much larger its effect on tides is less than half that of the Moon. When both the Moon and the Sun are on the same or opposite sides of the Earth, the pull is greatest, producing very high tides called spring tides. Weaker tides, called neap tides, occur when the Moon and the Sun form a right angle with the Earth, because the pulls of the two are in different directions. Spring tides occur every 14 days and neap tides half-way between each spring tide.
There are two high tides and two low tides every day in most parts of the Earth, but a few areas have only one high tide and one low tide, or a mixture, with one high tide being much higher than the other.
The tidal range (the difference between the high and the low water levels) varies from place to place, from less than 1 m(3 1/4 ft) in the Mediterranean Sea and Gulf of Mexico to 14.5 m (47 1/2 ft) in the Bay of Fundy on the east coast of Canada.
Currents
The currents near the surface of the oceans, like waves, are driven by the winds. The wind drags the water along with it. Currents move much more slowly than the wind, with speeds of less than 8 km/h (5 mph). They do not flow exactly in the same direction as the wind, but are deflected to one side by the Earth's spin.
There are two main wind systems in each hemisphere. The trade winds, between latitudes 0 deg and 30 deg, blow from the northeast in the northern hemisphere, and the southeast in the southern. The westerlies, between latitudes 30 deg and 60 deg, blow from the southwest in the northern hemisphere, and the northwest in the southern. These two wind systems produce a circulating system of currents that flow from equatorial regions to latitudes of about 50 deg and return to the equator.
Currents may have a significant effect on climate. For example, the North Atlantic Drift, flowing from the Caribbean, gives northwest Europe much milder winters than other parts of the world at the same latitude. Conversely, cold currents, such as the Humboldt Current on the Pacific coast of South America, have a cooling effect on climate.
* WAVE THEORY
* PLATE TECTONICS
* RIVERS AND LAKES
* COASTS
* ISLANDS
* PRIMITIVE ANIMALS
* ARTHROPODS 1
* FISHES
* MARINE MAMMALS
* ECOSYSTEMS: AQUATIC
* FISHING
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p100-4
ftsTitleOverride
The Oceans (page 4)
ftsTitle
The Oceans (4 of 4)
Features of the sea bed
The region of the sea bed closest to land is the continental margin, which is divided into the continental shelf, the continental slope and (sometimes) the continental rise . The continental shelf is the shallowest - around 130 m (430 ft) deep - and is relatively flat. It is about 100 km(60 mi) wide. The sea water over continental shelves usually has abundant marine life and most fishing is done here. About a quarter of the world's supply of oil and gas comes from the rocks beneath the continental shelves.
Oceanic ridges are vast, rugged, undersea mountain chains often, but not always, at the center of oceans. On average they are some 1000 km (620 mi) wide and stand up to 3000 m (10 000 ft) above the adjacent ocean basins. They form a more-or-less linked system about 80 000 km (50 000 mi) long, and this system enters all the major oceans. Different parts of it have different names: in the center and South Atlantic, for example, it is called the Mid-Atlantic Ridge, in the North Atlantic to the southwest of Iceland it is the Reykjanes Ridge, and in the Pacific it is known as the East Pacific Rise. On average, ridge crests lie some 2500 m (8200 ft) below the ocean surface, but there are a few places, such as Iceland, where the rocks have risen above the water surface, forming an island. Oceanic ridges are sites at which new oceanic lithosphere is continuously being created.
Between the ocean ridges and the continental margins there are abyssal plains. These are very flat and featureless parts of the sea floor, around 4000 m (13 000 ft) deep. Abyssal plains are broken in some places by seamounts, underwater volcanoes that have erupted from the sea floor. Seamounts may rise above the sea surface to form islands, such as Hawaii.
The deepest parts of the oceans are the ocean trenches. These are on average about 100 km (62 mi) wide and 7000-8000 m (23 000-26 000 ft) deep, and may be thousands of kilometers long. Trenches occur in two different kinds of location: parallel to the edge of a continent, at the bottom of the continental slope; or in the open ocean, where they are arc-shaped, and are parallel to an island arc.
AREA, VOLUME AND AVERAGE DEPTH
Pacific Atlantic Ocean Indian
Ocean (including Ocean
Arctic Ocean)
Area millions km2 180 106 75
millions sq mi 70 41 29
Volume millions km3 724 355 292
millions cu mi 174 85 70
Average m 3940 3310 3840
depth ft 12 930 10 860 12 60000000000000000000
* WAVE THEORY
* PLATE TECTONICS
* RIVERS AND LAKES
* COASTS
* ISLANDS
* PRIMITIVE ANIMALS
* ARTHROPODS 1
* FISHES
* MARINE MAMMALS
* ECOSYSTEMS: AQUATIC
* FISHING
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100T10
0080x0x
00TT0
-001T0
p102-1
ftsTitleOverride
Weather (page 1)
ftsTitle
Fronts mark boundaries between air of different temperatures. Where warm air is replacing cold air there is a warm front. It extends into the atmosphere well ahead of the surface warm front. Clouds are usually associated with the front, as shown in the diagram.
A cold front occurs where cold air replaces warm air. The temperature change can be quite sudden - several degrees in a few minutes. Cumulonimbus clouds often mark the line of the cold front and give a short period of heavy rain.
In many depressions the cold front moves faster than the warm front, gradually squeezing out the warm sector. When this has taken place, there is an occluded front. Its precise form depends on whether the air following the original cold front is warmer or cooler than the air ahead of the depression.
Weather (1 of 5)
The weather is the atmospheric conditions we experience at any one time. These can vary rapidly as rain gives way to sunshine or snow starts to melt. Such sudden changes of weather are more common in temperate latitudes than in the tropics. Weather is the result of air movements in the atmosphere. The atmosphere responds to the differences in heat received from the Sun between the warm tropics and the cold poles. The Earth's rotation and the nature of the ground surface - whether it is land or sea, mountain or lowland - will all affect the way the atmosphere moves. Viewed from space, cloud patterns show the way in which this movement takes place to give day-to-day variations called weather.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* THE HYDROLOGICAL CYCLE
* CLIMATIC AND VEGETATION REGIONS
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Weather (page 2)
ftsTitle
Weather (2 of 5)
Atmospheric pressure
In temperate parts of the world, the most important feature of the weather is atmospheric pressure. Atmospheric pressure represents the force exerted by a column of the atmosphere on the Earth's surface. If pressure is measured using a barometer and compared with readings taken elsewhere at the same time, patterns appear showing areas of higher and lower surface pressure.
From a pressure map it is possible to find wind direction. As a rough guide, winds blow parallel to the isobars - the lines that link points experiencing the same pressure. In the northern hemisphere, the low pressure lies to the left of the wind direction, while in the southern hemisphere it is to the right. The strength of the wind depends upon the pressure gradient. If the pressure gradient is steep, that means that the isobars are close together and winds will be strong.
Temperature
A pressure map can also inform us about other aspects of weather. Temperature is affected by the origin of air. Air blowing from polar latitudes will be cold, while air from tropical latitudes will be warm. Winds blowing from the Atlantic Ocean onto northwest Europe are relatively cool in summer but relatively mild in winter, while winds from the east are very cold in winter but warm in summer. Temperatures will also depend upon how much the Sun is obscured by clouds. However, nocturnal cloud cover helps to retain the daytime warmth.
Weather forecasters use pressure charts to help them decide what the weather is going to do. The satellite image and superimposed pressure chart show the way in which cloud systems and pressure systems are related. The main areas of thicker cloud are associated with the rising air near the low-pressure center to the west of Ireland. The center of the low marks the start of the typical spiral cloud pattern, with the cold front being a distinctive feature sweeping in from the west.
Ahead of the low pressure, high-level ice-crystal clouds cover much of Britain. They are often an indication of approaching rain - which duly fell later in the day. Behind the depression (to its west), the cellular cloud pattern produced by air rising from the relatively warm sea is typical. Occasional showers will fall. Further to the west a band of thicker cloud shows a weak cold front sweeping eastwards giving more frequent shower south of Iceland. Over the Mediterranean, high pressure keeps the skies clear.
Weather (3 of 5)
Clouds and precipitation
Clouds form when air cools so much that it can no longer hold all its water as a vapor. Water droplets then appear, which we see as clouds. To produce a cloud, air needs to rise or be forced to rise. On sunny days, the ground will warm. Thermals of warm, moist air may then rise sufficiently to produce clouds. Clouds often develop over hills or mountains where air is forced to rise.
Some clouds, though by no means all, produce precipitation (rain, snow, sleet, etc.). To do this, the right sort of conditions have to exist in the cloud. There appear to be two main processes. One method of precipitation formation is through the collision and coalescence of water droplets of different size within the cloud. The rising air that is producing the cloud may be strong enough to carry some of the smaller droplets upwards whilst larger droplets start falling as they become too heavy to be supported by the rising air. Through collision they grow and may become large enough to fall out of the base of the cloud and not be evaporated before reaching the ground.
The other method of precipitation takes place in clouds that consist of a mixture of water droplets and ice crystals. Not all droplets freeze at 0 degC (32 degF) - some remain as droplets down to -38 degC (-36 degF) because they are so small. Under these conditions, water tends to transfer from the droplets onto the larger, already frozen droplets, and these ice crystals may eventually fall out of the cloud. If they reach the ground we get snow. Much more frequently, however, the ice crystals melt as they fall to produce rain. Occasionally, some raindrops get swept into the upper parts of the cloud, where they freeze. Several phases of rising and sinking can produce a hailstone consisting of layers of ice. Hailstones only occur in certain types of cloud when the air is rising very rapidly. It is in this type of cloud - the cumulonimbus cloud - that thunder and lightning may occur. A separation of electrical charge (positive from negative) develops during hail formation. The flash of lightning is a spark of electricity between cloud and Earth or from cloud to cloud resulting from the build-up of charges and potential differences of up to 1000 million volts. The air is heated by the lightning, and the sudden expansion of the air produces the thunder that we hear.
Cloud types
Clouds are identified on the basis of their appearance and height. The highest clouds are called cirrus clouds. They form at heights between 6 and 10 km (3 1/2 and 6 mi), where the temperatures are well below freezing point. They consist of ice crystals, which often tend to be spread out by the strong winds at those levels. Hooked cirrus ('mare's-tails') are usually found ahead of a depression and so indicate imminent rain. When the sheets of ice crystals thicken and cover more of the sky, they are called cirrostratus clouds. Middle-level clouds of uniform greyness that totally obscure the Sun are known as altostratus. They often follow cirrostratus clouds. As rain approaches, the cloud base lowers to give a sheet of thick cloud called nimbostratus, which can give much rain. Where clouds are not able to develop upwards, as in anticyclones, stratocumulus clouds may form. These clouds indicate that air has only been able to rise to a certain level before sinking in the clear zones between the clouds. This gives a fish-scale effect ('mackerel sky') if the clouds are relatively high.
The most impressive clouds are those associated with rapidly rising air. If the clouds have not developed very far, they are called cumulus. They have sharp outlines, and often resemble cauliflowers. Cumulus clouds can build up to enter the ice-crystal zone or even reach the base of the stratosphere. When this happens they cannot rise further but spread out to produce an anvil-like cloud that is known as cumulonimbus. Most give showers, and hail and thunder are possible.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* THE HYDROLOGICAL CYCLE
* CLIMATIC AND VEGETATION REGIONS
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p102-4
ftsTitleOverride
Weather (page 4)
ftsTitle
Tropical storms can have a number of names, including hurricane, typhoon and cyclone. Tropical storms normally occur in summer or autumn when the tropical seas are the warmest.
Weather (4 of 5)
Temperate weather systems
In temperate latitudes, one of the main areas of uplift and cloud formation - and so of rainfall - is the low-pressure area or depression (also called a cyclone). Seen from space, depressions often possess a distinctive spiral or swirl of clouds showing where the air is rising. With depressions the wind blows anticlockwise in the northern hemisphere and clockwise in the southern. Most cloud occurs near frontal surfaces, where temperatures change rapidly. In a typical low-pressure system, a warm front, a cold front, an occluded front and a warm sector are found. Away from the cloud bands of the depression, showers still occur but become less frequent as pressure rises away from the low-pressure center.
High-pressure areas or anticyclones contain generally sinking and warming air. As a result air does not rise enough for deep cloud to form, so rain is rare. In all temperate land areas, anticyclones can bring hot, sunny weather in summer or cold weather in winter, but they are always dry. Within anticyclones the wind blows clockwise in the northern hemisphere and anticlockwise in the southern.
Tropical weather systems
In the tropics, distinctive pressure patterns are less common. Pressure gradients are usually much weaker than in temperate latitudes, so winds are generally light. One major low-pressure system that is found is the tropical storm. It has a number of names, including hurricane, typhoon and cyclone. Tropical storms normally occur in summer or autumn when tropical seas are warmest. At their center is an area of very low pressure surrounded by strong winds, which can exceed 240 km/h (150 mph) in the worst storms. At the center or eye of the storm, winds are light. Away from the eye sweep vast bands of cloud spiraling like a giant Catherine wheel. Heavy rain falls from these cloud bands and more damage is usually caused by flood water following the heavy rain than from the strong winds.
A less dramatic feature of the tropics is the seasonal change of winds known as the monsoon. Over parts of Africa, India, Southeast Asia and northern Australia, the monsoons bring rain during the summer season. The rest of the year is dry with winds blowing in the opposite direction.
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* THE HYDROLOGICAL CYCLE
* CLIMATIC AND VEGETATION REGIONS
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Weather tropical storms
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p102-5
ftsTitleOverride
Weather (page 5)
ftsTitle
Worldwide cloud patterns are seen in this image from January 1979. The cloud density and movements can be seen on a global scale. These movements and densities have a great effect over the weather that we experience on the ground below.
Weather (5 of 5)
THE BEAUFORT SCALE FOR MEASURING WIND STRENGTH
Beaufort Description Speed
Number (or
wind force)
km/h mph
0 Calm less than 1
1 Light air 1-5 1-3
2 Light breeze 6-11 4-7
3 Gentle breeze 12-19 8-12
4 Moderate 20-29 13-18
breeze
5 Fresh breeze 30-39 19-24
6 Strong breeze 40-50 25-31
7 Near gale 51-61 32-38
8 Gale 62-74 39-46
9 Strong gale 75-87 47-54
10 Storm 88-101 55-63
11 Violent storm 102-117 64-73
12 Hurricane 118+ 74++
* THE EARTH'S STRUCTURE AND ATMOSPHERE
* THE HYDROLOGICAL CYCLE
* CLIMATIC AND VEGETATION REGIONS
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Weather Worldwide cloud patterns
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p104-1
ftsTitleOverride
Climatic and Vegetation Regions (page 1)
ftsTitle
Climatic and Vegetation Regions (1 of 4)
The average weather condition found in a region is known as its climate, and this is based on long-term records, usually 30 years. In contrast, weather is the day-to-day variation in atmospheric conditions. Climate is the weather we might expect in a given area at a particular time, while the weather is the actual condition that prevails.
There are many different climates around the world, from arctic to tropical. Climate has a crucial effect on the kinds of vegetation found in a particular region, although as the maps show, climate and vegetation zones do not always coincide.
Classification of climate
Climate can be classified in many complex ways. The broadest and most general method is to divide each hemisphere into broad belts or climatic zones. The ancient Greeks made the earliest attempts at classifying climate. They recognized a winterless tropical region located in the low latitudes, a summerless polar region where temperatures are usually very low, and an intermediate or middle-latitude region, now called the temperate latitudes, with cool summers and mild winters.
A simple classification can be based on two climatic elements, namely temperature and precipitation (rain, snow, dew, etc.). When both average temperature and precipitation are known it is possible to classify a particular location into a climatic type.
* THE MAKING OF THE WEATHER
* NON-FLOWERING PLANTS
* FLOWERING PLANTS
* THE BIOSPHERE
* ECOSYSTEMS
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ftsTitleOverride
Climatic and Vegetation Regions (page 2)
ftsTitle
Climatic and Vegetation Regions (2 of 4)
The seasons
Apart from those locations at or very near to the equator, all climatic regions show seasonal variation. Generally, the further away from the equator, the greater the seasonal variation becomes.
Seasons are caused by the annual revolution of the Earth in a slightly elliptical orbit around the Sun, and by the daily rotation of the Earth on its axis. The axis of rotation is inclined at 23.5 deg from the vertical. The effect of the Earth's rotation and revolution around the Sun is to produce changing day length and varying angles at which the Sun's rays strike the surface of the Earth. Together these two factors cause a seasonal variation in climate.
Twice during each year, on 21 March and 23 September, the Sun's rays are directly overhead at the equator. These two days are the spring and autumnal equinoxes. On 21 June the Earth is midway between the equinoxes and the North Pole is inclined at 23.5 deg towards the Sun; the Sun's rays are overhead at the Tropic of Cancer (latitude 23.5 deg N) and the summer solstice occurs in the northern hemisphere (and the winter solstice in the southern hemisphere). By 21 December the position is reversed and the Sun is overhead at the Tropic of Capricorn (23.5 deg S) and the winter solstice occurs in the northern hemisphere (summer solstice in the southern hemisphere).
Controls on climate
Climatic controls include the proximity of land to water, and the effects of elevation, mountain barriers and ocean currents. Climatic effects include the seasonal or daily ranges of temperature and precipitation, together with humidity, winds, etc.
Even though two places may have similar average yearly temperature and precipitation values, or share the same latitude, they can experience different climates. If the climatic controls of the two places are not alike, then neither will be the resulting climatic effects.
As water is slower to heat up than land and slower to cool down, places in the mid-latitudes near the sea will have cooler summers and milder winters than those far from the sea. The former are said to have maritime climates while the latter have continental climates. Ocean currents can either give a location a milder climate than would be expected at that latitude (for example, the effect of the warm North Atlantic Drift on northwest Europe), or a cooler climate (for example, the effect of the cold Labrador Current on Newfoundland).
Temperature decreases with altitude. High ground may also be wetter, because warm moist air will condense as it rises over a cool land mass, so producing rain or snow. If the rain-bearing winds mostly come from one direction, the land on that side will be wetter than the land on the opposite side, which will be in a rain shadow. On the South Island of New Zealand, for example, there is heavy precipitation to the west side of the New Zealand Alps, but on the east side precipitation in places is as low as 330 mm (13 in).
* THE MAKING OF THE WEATHER
* NON-FLOWERING PLANTS
* FLOWERING PLANTS
* THE BIOSPHERE
* ECOSYSTEMS
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p104-3
ftsTitleOverride
Climatic and Vegetation Regions (page 3)
ftsTitle
We dive into the cloud layers over the continents and see the terrain below. (The vertical scale of the terrain is greatly exaggerated.) This shows how terrain is one of the key factors which determines our climate.
Climatic and Vegetation Regions (3 of 4)
Climate and vegetation
The map showing the world's vegetation regions is complicated by the fact that it attempts to show the original distribution of natural vegetation, that is, the vegetation as it was before being greatly altered by human interference such as deforestation and agriculture. Elevation, slope, drainage, soil type, soil depth and climate all influence the vegetation distribution.
Climate is a major factor in determining the type and number of plants (and to a lesser extent animals) that can live in an area. Three main terrestrial ecosystems can be recognized: deserts, grasslands and forests. Precipitation is the element that determines which vegetation type will occur in an area. If the annual precipitation is less than 250 mm (10 in) then deserts usually occur. Grasslands can be found when precipitation is between 250 and 750 mm (10 and 30 in) per annum, while areas that receive more than 750 mm (30 in) rainfall a year are usually covered by forests.
The average temperature and the nature of the seasons in a region are important in that they can determine the type of desert, grassland or forest. Wherever the monthly average temperature exceeds 21 degC (70 degF) then hot deserts, savannah grasslands or tropical forests occur.
In the middle latitudes, the winter temperatures are low enough (one month or more below 5 degC / 41 degF) to cause vegetation to become dormant. In autumn, growth stops, leaves are often shed and the plant survives the unfavorable winter months in a resting or dormant phase. In spring, when temperatures rise, new growth begins. In high latitudes, the winter conditions are such that between four and six months are dark and average temperature falls well below 0 degC (32 degF). The evergreen conifers can survive these conditions but growth is very slow and confined to the short, cool summers. In the highest latitudes, trees disappear and only small, low-growing plants can survive. GJ
* THE MAKING OF THE WEATHER
* NON-FLOWERING PLANTS
* FLOWERING PLANTS
* THE BIOSPHERE
* ECOSYSTEMS
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p104-4
ftsTitleOverride
Climatic and Vegetation Regions (page 4)
ftsTitle
The diagram shows the order of the seasons
Climatic and Vegetation Regions (4 of 4)
The Seasons
The diagram shows the order of the seasons in the northern hemisphere; in the southern hemisphere the seasons are reversed. The 23.5
tilt of the Earth's axis means that in summer not only is there more daylight, but the Sun's rays reach the Earth's surface more directly through the atmosphere and so less of their warmth. In winter, not only is there less daylight, but the Sun's rays strike the Earth more obliquely, so having to pass through more of the atmosphere and loosing more of their warmth. Beyond the Arctic and Antarctic Circle (66
32' N and S), the Sun does not set in summer or rise in winter; the periods over which this occurs each season increases as one approaches the Poles. Conversely, the nearer one approaches the Equator, the less the seasonal variation.
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