Plate tectonics brought about a revolution in science, perhaps as important as Einstein╒s theory of relativity, because it enabled scientists to understand, amongst other things, the reasons for the location and distribution of volcanoes and earthquakes on the surface of the globe. Previously, weird and wonderful theories had abounded including the belief that, because many volcanoes were close to the sea, it must follow that volcanic activity is linked to the interaction between water and fire!
WHAT IS PLATE TECTONICS?
Plate tectonics, first described only twenty years ago, is the study of the earth╒s crust and the movements of its lithosphere (plates).
The earth╒s internal structure can be subdivided into three clearly defined and separate concentric parts: the lithosphere, the mantle and the core:
The lithosphere is the outermost shell of the earth, consisting of the first 70 to 100 km including the crust.
The mantle, which comprises 83% of the earth╒s volume, is about 2800 km thick. The uppermost part of the mantle is called the asthenosphere, a ╥plastic╙ layer about 600 km thick which maintains a temperature of around 1,300-1,400íC and a pressure of about 40 kbars.
The core, about 3,500 km in radius, is the solid iron ╥heart╙ of the earth surrounded by liquid iron and nickel.
Although the mantle is solid in the short time scale, over a long period (measured in millions of years) it is ╥plastic╙ and undergoes convection. As a result of the mantle╒s movements, the lithosphere is gradually rearranged on its surface. It is broken up into pieces (plates) and moved at speeds varying between 1cm and 20cm per year. (The earth╒s crust is made up of about 12 large plates and a few smaller ones.) There are three types of plate boundaries: constructive, destructive and conservative.
Constructive plate margins are found where these plates move away from each other - at mid-ocean ridges or continental rifts. These areas are sometimes called divergent boundaries where oceanic crust is formed.
Destructive plate margins occur where the plates move towards each other and one plate is forced underneath the other. These areas are sometimes called convergent boundaries.
Conservative plate margins are found where the plates move past each other without construction or destruction of the crust. These areas are sometimes called transform boundaries.
CONSTRUCTIVE PLATE MARGINS
At constructive margins (such as the Atlantic Ocean), where the plates are drifting away from each other, new crust is formed as hot mantle material rises up to fill the rift between the two plates and hardens as it cools. Large ridges, as much as 3000 km above the ocean floor are formed. This process is known as sea floor spreading. Land masses lying on these plates have gradually been pushed apart. As the Atlantic Ocean has become wider, so the continents of South America and Africa, one single land mass 100 million years ago, split away from each other and are being forced apart at the rate of 10 mm to 50 mm a year. Even more rapid rates occur in the East Pacific... up to 90 mm a year. This is known as continental drift.
The ridges that are created at constructive plate margins are up to 4,000 km wide in places and form a chain of more than 60,000 km around the globe. This is the planet╒s most important volcanic system. Practically the whole of the ocean floor (two-thirds of the earth╒s surface) is made up of basaltic volcanic rocks that have been emitted in volcanic eruptions on the ocean ridges in the last 200 million years. Only two of these ridges occur on land: Iceland and the Afar Depression in East Africa.
Example: In 1963, an undersea eruption created the island of Surtsey which lies just off Iceland, a very volcanic area. Pressure from within the mantle pushed to the surface a section of the mid-Atlantic ridge and formed the new island which has now grown to 2.6 sq km and is still dominated by the cone of its volcano.
DESTRUCTIVE PLATE MARGINS
At destructive margins, one plate slides beneath another to form what is known as a subduction zone. Subduction volcanoes are more explosive and dangerous than the gentler chain of mid-ocean ridge volcanoes. This is due to the presence of water and the build-up of gases which are dissolved in the thick, sticky magma associated with subduction. If subduction occurs in an oceanic area, then island arcs and trenches are formed as on the margins of the Pacific Ocean. On a continental plate, a line of mountains will arise at the edge of the overriding plate, such as the Andes where the Nazca Plate is subducted beneath the South American Plate. Frequent volcanic and earthquake activity occurs here. Elsewhere, when two continental plates collide, soft rock sediments fold and push up to form fold mountains such as the Rockies, the Andes and the Alps. The most recent occurrence of this was 65 million years ago, in the Tertiary period, when the Himalayas were formed. The Indian Plate moved northwards to collide with the Eurasian Plate which used to be separated by the Tethys Ocean.
As the descending plate sinks into the mantle, earthquakes are caused along the upper edge of the plate. This is called the Benioff zone. The sinking plate melts as it meets the molten magma and mixes to produce a new type of magma which has a lower density. This new magma may then rise through any cracks in the crust above it and cause volcanic activity. These volcanoes may then form island arcs like those associated with the Tonga Trench in the southern Pacific. Some of the deepest trenches (and deepest earthquakes) of the earth╒s ocean floors are formed at subduction zones.
Example: The islands of Japan are the site of extreme tectonic disturbance and suffer more earthquakes than any other country - more than 7,500 a year. People in Japan feel earth tremors every few weeks and cities like Tokyo have disaster teams at the ready at all times. Most people take part in regular earthquake drills. Four major plates occur in this part of the world resulting in two subduction zones causing oceanic crust to be subducted under Japan itself. Japan also has many volcanoes and hot springs (a major tourist attraction). It is actually part of the Pacific ╥Ring of Fire╙ - a circle of volcanoes which encircle the Pacific Ocean. At this time, there are eighty active volcanoes in Japan, including the most famous, Mount Fuji. The most recent earthquake occurred on Awaji Island, just outside Kobe, southern Japan, at 5.46pm on 17th January 1995. Measuring 7.2 on the Richter Scale, the damage was estimated at around ú64 million (10,000 billion Japanese Yen). 5,500 people were killed and 35,000 injured. 171,000 houses collapsed and 7,500 houses were destroyed by fire. 130 km of bullet train network was closed and the Hanshin Highway completely closed when 1 km of the elevated section crashed to the ground. Japan is also vulnerable to tidal waves (or tsunamis) caused by seismic disturbance on the seabed. A 30 metre high tsunami devastated the west coast of Hokkaido in July 1993 as it followed on the heels of a nearby undersea earthquake. When it hit the island of Okushiri, it was travelling at 500 km per hour and caused 158 deaths.
CONSERVATIVE PLATE MARGINS
In some areas of collision, such as the western coast of North America, no subduction or crustal deposits occur. At conservative (or transform) margins, the plates grind past each other causing web-like cracks in the crust and producing powerful earthquakes.
Example: The San Andreas Fault in California is a typical example of this type of plate movement and has been the site of many earthquakes including the 1906 San Francisco earthquake disaster. The fault line slashes through the state for 1,100 km. Each year, scientists record 20,000 earth tremors although most of these are slight and only detected by sensitive instrumentation. Los Angeles is situated on the Pacific plate whilst San Francisco sits on the North American plate. It is possible that, in millions of years or more, the two cities might meet!
HOTSPOTS
Although hotspot volcanoes occur in random spots in the middle of the plates, they are still affected by plate movement. Hotspots remain stationary and the plates above move over the top of them so that, over millions of years, chains of volcanoes are formed. As one volcano is created, the plate moves away taking the volcano with it. Then, at the next eruption, a new volcano is formed and so on. As each volcano is moved away from the hotspot it becomes extinct.
Example: The Hawaiian Island chain on the Pacific plate was formed in this way. Hawaii╒s Mauna Loa and Kilauea volcanoes, now active, will gradually become extinct as Hawaii moves away from the hotspot. Hotspot volcanoes are very spectacular but less dangerous than those in a subduction zone. The magma source pierces the plate dramatically (rather like a blow-torch) and produces a fountain of lava which cascades down the cone at speeds of 100 km per hour.
Task A
PREDICTION OF EARTHQUAKES AND VOLCANIC ERUPTIONS
The unpredicted eruption of Vesuvius in AD79 claimed about 2,000 victims. Had it occurred today, the death toll might be closer to 200,000 due to the increase in habitation in the areas surrounding volcanoes. This land is extremely fertile and associated with high population density. It is therefore important that we are able to monitor and predict seismic activity accurately in order to save lives. In 1975, in Haicheng in China, a successful prediction saved up to 90,000 lives when the town was evacuated prior to an earthquake which destroyed 90% of the town. (On a lighter note, it appears that animals may be better than humans at predicting seismic activity although there is no concrete evidence to substantiate this. During the winter preceding the February earthquake at Haicheng, chickens, frogs, snakes and dogs seemed to be unusually agitated.) In recent years, prediction has become more and more reliable and scientists are now employing space-age technology in their work. Satellites bounce laser beams off the surface of the earth and NASA radio telescopes used timed radio signals to detect minute changes in rock movement. The tiltmeter is a simple but very effective instrument for measuring changes in the tilt of the ground. It consists of a water pipe with a container of water at each end in which sensitive equipment indicates any change in water level. Seismometers record underground tremors and strainmeters (positioned across a fault line) measure movement both sides of the fault. Volcanic activity can be predicted by measuring many different signs including the emission of gases which are released prior to an eruption, the levels of water in wells, seismic shocks, structural deformation (magma under pressure will cause swelling of the earth╒s crust before its emergence) and changes in the earth╒s magnetic field. Monitoring and observation of these parameters increases our understanding of which particular changes are significant, thus assisting in prediction of future eruptions.
