VOLCANOES WHAT ARE VOLCANOES? Deep in the earthÕs mantle, magma under pressure rises upwards, collecting in magma chambers within or below the crust. Cracks in the rocks (either existing cracks or those caused by the force of the magma) of the crust provide an outlet for the intense pressure and an eruption occurs. Steam, smoke, gases, ash, rock and lava are ejected into the atmosphere. TYPES OF VOLCANIC ERUPTION Hawaiian - Large amounts of runny lava creating large but low volcanoes. PelŽean - Blocks of thick and sticky lava, closely followed by a cloud of ash and gases. Strombolian - Small, sticky lava bombs and blocks. Also ash, gases and cinders. Vulcanian - Very thick lava and large lava bombs. Extremely violent. Plinian - Ash, gases and cinders erupt high into the atmosphere. Task B TYPES OF LAVA Lava is the generic name given to magma once it has erupted, but there are many different kinds of lava, each named according to how it looks when it cools and hardens. Three of the most interesting are: Pillow lava is the commonest type of lava on earth. It is found underwater (or on land that was once under the water). Magma oozes through cracks in the ocean floor and hardens into pillow shapes as it cools. Pahoehoe lava is fast flowing and develops a cooling skin on top of the still running lava underneath. It forms characteristic twists and coils like rope as it cools. Aa lava moves slowly and hardens into sharp chunks of rock with a rough surface which can be as thick as 100 metres. TYPES OF VOLCANIC ROCK Rock formed from hardened lava is known as igneous rock (from the Latin... ignis, meaning fire). Dark, runny lava cools and hardens into an igneous rock called basalt. Lava that contains a lot of silica is lighter in colour. Granite is a rough textured rock, formed when magma cools within the crust. If gas is trapped in the lava it hardens as pumice, a rock so light that it can float in water. CRATERS AND CALDERAS Craters are the funnel-shaped hollows that form at the opening (or vent) of a volcano. These will typically have a diameter of about 1 km. Craters can also be formed on the side of a cone. Calderas are very large craters caused by an explosion or massive eruption. When this happens, sometimes the magma chamber empties suddenly, leaving an empty space at its top which cannot now support the weight of the cone, and the crater collapses in on itself. The worldÕs largest caldera which measures 23 km by 16 km is at Aso in Japan. LAVA FLOWS Lava varies in consistency depending on the type of eruption that produces it. In Hawaiian eruptions, for instance, the lava is runny and cools to form basalt rocks. In subduction zones the lava is a thicker mixture of magma and melted lithosphere, high in silicates. This cools and hardens to form rocks called rhyolite, and volcanic glass called obsidian. GASES, ASH AND PYROCLASTIC FLOW Gases are present in lava and sometimes include carbon dioxide and hydrogen sulphide which are harmful to humans and may cause breathing difficulties in the vicinity of an eruption. Sulphur dioxide can cause acid rain when mixed with water vapour. These gases expand in the lava and try to escape. In runny lava, the gases escape easily and cause only mild eruptions but in sticky lava, the gases build up and explode violently. These explosive eruptions throw volcanic debris including rock fragments and hardened lava froth (pumice) high into the air. Ash is formed in the eruption and is also jettisoned into the atmosphere, often causing climatic changes all over the globe as it is carried far and wide by the wind. There is also a real danger to aircraft in the immediate vicinity whose engines can become clogged and may stall. Pyroclastic flow is an ash cloud containing rocks and gases that has dropped to the ground. It plummets down the slope of a volcano at speeds of up to 250 km per hour (far too fast to run or drive away from) and a temperature of 100¡C. More lives are lost due to pyroclastic flow than lava. MUDFLOWS, AVALANCHES AND FLOODS Mudflows (or lahars) are caused by the mixing of volcanic ash with water. They can be every bit as lethal as lava flows but in a different way. During an eruption or following it, ash can mix with water from a melted ice cap or glacier, a thunderstorm or river to create a moving stream of debris - rocks, tree trunks and building rubble are picked up along the way - which eventually hardens to the consistency of concrete. In November 1985, the eruption of Nevado del Ruiz melted ice and snow which caused a mudflow that completely engulfed Armero in Colombia, 50 km away, in a 40 metre deep blanket of mud. Japan is leading the world in mudflow control. Dams made of steel and concrete have been built around the very active Mount Sakurajima. Although mudflows sometimes spill over the dam, the time saved allows for evacuation of nearby residents. Similarly, ash that collects near the crater may easily become an avalanche when disturbed by a shock wave, a further eruption, rainfall or the melting of snow. When sufficient snow and ice are melted, flooding may also compound the damage and loss of life caused by a volcanic eruption. Iceland suffered massive flooding in 1996 due to the melting of an ice-cap. EFFECTS ON THE WEATHER Volcanic eruptions affect more than just the surrounding landscape where static electricity caused by colliding particles of volcanic material can spark off thunder and lightning. Clouds of gases and ash may be ejected high enough into the atmosphere to be blown all around the globe, affecting weather patterns far and wide by reflecting the sunÕs rays back into space. This process can produce astoundingly colourful sunsets but also widespread cooling of the earthÕs average temperature. Following the huge Tambora eruption in 1815, summer frost and snow damaged crops in the northeast of the United States, Scandinavia and parts of Europe. There may also be an effect on the ozone layer. There is a theory that the extinction of the dinosaurs may have been brought about by volcanic activity dramatically altering the worldÕs weather patterns. SO WHY DO WE LIVE NEAR VOLCANOES? In the face of all this mayhem and destruction, we still find communities in close proximity to volcanoes - why ? Many of the reasons are connected with the optimism that is inherent in human nature - to a certain extent, an established community will remain in situ rather than Òup sticksÓ and move away, because that particular place is a home, not just a place to live. Also, seismic activity can be erratic (and, until recently, unpredictable) - a volcano may remain dormant for generations, often leading the inhabitants of nearby settlements to believe that it is truly extinct. The citizens of Pompeii and Herculaneum had been totally unaware of the potential danger when Vesuvius erupted unexpectedly in AD79. Their communities were completely destroyed and resettlement was not possible but, had they been able, they might conceivably have returned to rebuild their formerly prosperous cities. WHAT ARE THE ADVANTAGES? Why were settlements established near volcanoes in the first place? Some communities, as outlined above, may have settled through ignorance of the hazards. However, there are also many advantages of living in an area of volcanic activity, the most important being soil fertility. Soils that are formed from volcanic material are very rich and therefore attractive as farm land, for both grazing and crop production. In Indonesia, more people live on those islands with active volcanoes than on the islands with none because soil fertility is much better. Some villages in Indonesia - Pinggan in Bali, for instance - are built within the cadera of extinct volcanoes. The slopes of Mount Etna in Italy are planted with vines, orange trees and lemon groves. And there are other advantages. Energy from the hot underground rocks is often harnessed by power stations. This is known as geothermal energy and is doubly advantageous due to its being ecologically sounder than other more traditional power generation methods. Iceland, in particular, uses this system extensively. The volcanic rocks themselves have diverse uses - pumice stone is used as an abrasive; basalt and granite are useful as building materials; gemstones, formed under extreme heat and pressure in the mantle (opal, beryl, zircon, tourmaline, aquamarine, topaz, moonstone, sapphires and diamonds) and precious metals (copper, silver and gold) can be found buried in layers of volcanic rock. Some of the largest diamonds in the world have been found in volcanic rock called kimberlite near the town of Kimberley in South Africa. The mineral sulphur is also mined for use in the manufacture of fertiliser and as an additive to rubber. VOLCANIC ACTIVITY AND TOURISM People have always been fascinated by volcanoes and tourism in those areas is not a new phenomenon. Kilauea in Hawaii was a popular attraction last century. Visitors from America and Europe would light their cigars and fry eggs in pans on hot lava, and press lucky coins into soft rock. Relatives would receive postcards that had been singed at the edges on hot lava. At the turn of the 20th century, KilaueaÕs lava lake provided a compelling attraction for tourists seeking adventure and new experiences. Nowadays, Yellowstone National Park in Wyoming, U.S.A. - one of the best known continental hotspots - provides a very popular leisure area. Its hot springs and geysers attract tourists from all over the world. Old Faithful is probably the most famous geyser, shooting steam and hot water up to 46 metres high into the air (faithfully) every hour for the last 80 years. Similarly, JapanÕs hot springs enjoy much popularity (the city of Beppu has over 4,000 hot springs) and Mount Fuji which erupted last in 1707 has half a million visitors every summer. SOME NOTABLE VOLCANOES Mount Mazama (Crater Lake) In 5,000BC, America was plunged into darkness when Mount Mazama, (Oregon, U.S.A.) erupted. Over centuries, its lava plug has hardened and the caldera has filled with melted ice and snow and rain water to form Crater Lake which has a small island in the centre - Wizard Island. Santorini One of the western worldÕs most enduring legends is connected with volcanic activity - that of Atlantis, a mythical Òland of milk and honeyÓ inhabited by a sophisticated and cultured civilisation which was said to have been destroyed suddenly by a volcanic eruption and flooded by a tidal wave. Scientists and archaeologists believe now that, if it ever existed at all, its location might have been the island of Santorini (in those times, known as Thera) in the Aegean Sea off Greece, which suffered a succession of massive eruptions, around 1628 BC, which buried the island 30 metres deep in pumice fragments. Following these eruptions, the coasts of the eastern Mediterranean were struck by a 30 metre tidal wave. Vesuvius The sister cities of Pompeii and Herculaneum in Italy were destroyed on 24th August AD79 when Vesuvius erupted. Pompeii (8 km away), although further than Herculaneum (5 km away) from the volcano itself, suffered first as hot pumice, ash (or tephra) and rock rained down during the first afternoon and evening onto the panic-stricken, fleeing residents. By midnight, a pyroclastic flow of ash and gases followed by a river of hot mud descended on the city. By morning, Herculaneum was buried 20 metres deep under volcanic ash and deposits. A final surge of ash and gases killed the last few inhabitants of Pompeii and it too was buried - three metres deep. Vesuvius claimed at least 2,000 victims. Although the geographer Strabo (58BC - AD51) had recognised Vesuvius as volcanic, at the time of the AD79 eruption it had been presumed, covered in vegetation up to its summit, to have been profoundly dormant. For 1,700 years, the two towns lay buried until, in the eighteenth century, scholars began excavations and found that the streets were perfectly preserved beneath the killer ash and mud. Casts were made of the spaces where bodies of the townspeople and their animals had fallen and many everyday items (e.g. bells, liquid heater, cooking utensils) have been recovered from the site which give us an accurate picture of life in those times. Tambora The eruption of Tambora (1815) on Sumbawa Island in the Sunda arc segment of the Indonesian Archipelago, ranks as the most destructive volcanic eruption to date. Ten thousand died during the eruption, but a further 82,000 died in the aftermath of starvation or disease. Krakatau Although the 1815 eruption of Tambora was much more violent, the Krakatau eruption (1883) was brought more closely to the publicÕs attention by improved methods of communication and still remains more famous today. Krakatau is located in the Sunda Strait, between the islands of Sumatra and Java, in an unstable subduction zone where the Indo-Australian plate subducts under the Eurasian plate. In May 1883, a volcano on the island began erupting, but these early rumblings were the precursors of a much more violent eruption. On 27th August 1883, an enormous blast (10,000 times more powerful than the Hiroshima bomb) blew the island apart and was heard 3,500 km away (as far away as Madagascar), the loudest explosion ever recorded. Clouds of dust and ash were ejected up to 80 km into the air which circled the globe for several years, affecting weather patterns all around the world. After the volcano collapsed in on itself, 40 metre high tsunamis destroyed 163 villages along the coasts of Java and Sumatra, killing 36,000 villagers. Its extraordinary effects around the world included: lowering of the earthÕs average temperature for several years; in Alice Springs, central Australia the explosion sounded like rifle shots; Perth Harbour - destroyed by a tsunami; Indian Ocean - floating pumice blocked some shipping lanes up to a year later; in Trinidad, on the other side of the globe, the sun appeared blue, in Sri Lanka green; England was dazzled by flame red and purple sunsets, and waves raised the tides in the English Channel. When it exploded, Krakatau collapsed in on itself and formed a 6.5 km undersea caldera. Then in 1927, smoke began to rise again from the caldera and, within a year, a new volcanic island began to appear above sea level. The new island is named Anak or Child of Krakatau and continues to grow by depositing volcanic material during its frequent but mild eruptions. Mount PelŽe The eruption of Mount PelŽe in Martinique on 8th May 1902 was the most catastrophic of this century - 28,000 dead. Even today, the devastated town of Saint-Pierre is a mere shadow of its former self - then known as the ÒParis of the AntillesÓ, as it resists development in fear of further volcanic activity. Mount St. Helens Mount St. Helens, in the north west United States, lies in a subduction zone where the Juan de Fuca plate subducts beneath the North American plate. It is one of 15 volcanoes in the Cascade Range. Dormant since 1857, its awakening was signalled on 20th March 1980 by a series of earthquakes to the north west. Then, a week later, a small eruption shot ash and steam into the air. By early May, the cone had started to bulge - a sure sign that magma is under pressure and looking for an outlet. The bulge increased in size each day until a huge explosion (possibly triggered by a small nearby earthquake) on 18th May blew out the northern side of the mountain at a speed of 1,000 km per hour and a temperature of 300¡C creating a landslide which engulfed 600 sq km. Minutes later, a second eruption, which lasted for nine hours, sent a huge Plinian column of ash and gases 25 km into the air. Mudflows and floods, caused by the melting of snow and ice descended on the North Toutle River Valley. More than 6 million trees were flattened or uprooted. Volcanic ash fell like snow as far away as Montana, Idaho, Oregon and Washington. The sound, however, travelled less than 10 km due to the muffling effect of the huge cloud of volcanic dust and ash. El Chich—n The eruption of El Chich—n in Southern Mexico in 1982 spat ash and sulphurous gas up to a height of 35 km and prompted a turning point in the investigation of climatic change in relation to volcanic activity. So much ash shot into the sky that it remained dark for 44 hours. Nevado del Ruiz On 13th November 1985, in Colombia, the summit glacier of Nevado del Ruiz was melted by a volcanic explosion. This led to enormous mudflows that engulfed Armero, killing 22,000. Although the eruption has been predicted in time to evacuate the inhabitants, vulcanologists failed to convince the authorities that the eruption was imminent. Mount Pinatubo A major volcanic explosion took place at Mount Pinatubo (Luzon, Phillipines) on 15th and 16th June 1991. Eruptions sent ash columns more than 30 km into the atmosphere. Locals were found to be sifting through the volcanic ash for diamonds, rumoured to have been spewed out during the eruption. The ÒdiamondsÓ proved to be quartz crystals formed by magma hardening within the volcano. Because it had been predicted beforehand, only 300 people died when Mount Pinatubo erupted. Iceland Iceland is an island which straddles the northern Atlantic section of the mid-ocean ridge - one of the few locations on earth where an ocean ridge appears above water. A large crack is appearing as the island is gradually pulled apart by the North American plate to the west and the Eurasian plate to the east. Ravines and cliffs are appearing at the edges of the two plates and Iceland is becoming more volcanic and unstable. Iceland is the source of one-fifth of the entire worldÕs lava output and it has been suggested that it sits not only on a mid-ocean ridge but possibly also a hotspot! Icelandic scientists have been studying the Vatnajškull glacier, the largest of 4 ice-caps, where a huge block of ice is melting under the volcano. The seismic activity has advantages as well as disadvantages - Iceland harnesses the geothermal energy of its volcanoes for heating and electrical power. A new eruption in 1996 caused the melting of EuropeÕs largest ice-cap and resulted in widespread flooding. Task C Volcanoes in Space Mount Olympus on Mars is the largest volcano in our solar system - nearly three times the height of Mount Everest. Venus has some hotspot volcanoes that erupt continuously. Maxwell is the tallest on Venus at about 2 km higher than Everest. A ÒVoyagerÓ spacecraft observed that Io, one of JupiterÕs moons, has at least six vents which erupt gases up to an amazing 150 km above its surface.