Introduction to Volcanic Hazard Christopher Kilburn Earth is populated by some 10,000 active volcanoes. Volcanic activity has been described from at least 1,500 of these and about 40-50 are in eruption each year. Volcanoes occur where underground molten rock erupts at the surface. The eruption may be gentle and produce flows of oozing lava or, at the other extreme, it may be explosive and violent enough to send pulverised rock beyond the stratosphere. The hazards from volcanoes have thus a spectacular range, from the destruction of land in the immediate vicinity to the disruption of world climate. How a volcano erupts depends on the history of molten rock (or magma) while still underground and this, in turn, is strongly influenced by plate tectonics. Types of volcanic eruption How a volcano erupts depends on the explosive potential of its magma while approaching the surface. Explosive potential is controlled by the gas pressure in bubbles growing in the magma and this, in turn, depends on the quantity of bubbles present and on the fluidity of the magma. The more fluid a magma, the easier it is for bubbles to migrate upwards under buoyancy and to escape before the magma reaches the surface, especially if the magma is also rising slowly. The result is magma emerging as a lava flow (Fig. 1). As a magma's fluidity decreases and its ascent velocity increases, the bubbles are less able to escape. The bubble pressure increases until it is high enough to drive out the magma in a series of explosions.

Figure 1. Lava flow (moving left to right) near the vent of Mt Etna's 1983 eruption. Part of the tourist ski-lift system damaged by lavas can be seen in the background. (Photo: C.R.J. Kilburn.) The style of explosion depends on how the bubbles are packed together. When very tightly packed, they form a froth (as do bubbles in champagne when the bottle is opened. The magma erupts as a jet of gas and molten fragments. The jet soon loses its larger and heavier fragments, which rain down close to the vent, while the hot gases continue to buoy the lighter fragments aloft. The result is a billowing eruption column that may rise 20 km or more through a thinning atmosphere until it is forced to spread laterally (Fig. 2). Fine particles (volcanic ash) and larger fragments of froth (pumice) fall out from the eruption cloud to form

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a surface veneer that may cover 100s-1000s km2. Together with droplets of condensing gas, the finest ash may be blown by winds around the world if carried into the stratosphere. The West's earliest written account of a billowing eruption column is Pliny's description of the 79 AD outburst of Vesuvius. Such events are thus now known as plinian eruptions. A plinian column cools as it rises and some parts may eventually become heavy enough to collapse and, crashing from heights of several kilometres, send clouds of gas and ash racing over the ground at hurricane speed. Such pyroclastic flows are the greatest killers in volcanic eruptions and were responsible for most of the deaths at Vesuvius in 79 AD. Historical pyroclastic flows have travelled as much as 30 km, although commonly they reach only a few kilometres. However, as they push away the air ahead of them, pyroclastic flows may be preceded by extreme winds that can flatten forests at greater distances from the volcano.

Figure 2. Plinian eruption column above Klyuchevskoi, in Kamchatka, 1994. (Photo: NASA.)

When the blobs of magma between bursting bubbles are too large to be buoyed up as a plinian column, they are instead fired out either as a continuous stream of volcanic bombs and scoria, producing lava fountains, or as a series of discrete explosions. The most celebrated lava fountains have been seen in Hawaii, where they reach heights of several hundred metres (Fig. 3). The discrete explosions reflect the bursting of giant bubbles, the broken skins of which are hurled away as bombs sometimes metres across (Fig. 4). Stromboli island, north of Sicily, has shown this behaviour almost continuously for at least 2,000 years and so the style of eruption is described as strombolian. Although lava fountains and strombolian explosions are both impressive, their bombs normally land less than a kilometre away. Their hazard is thus restricted to the immediate vicinity of the eruptive vent. On impact, however, larger bombs can break roofs and kill.

Intermittent explosions also occur among very resistant magmas in which trapped bubbles gather below a thick magmatic cap, similar to the cork in a bottle. The bubbles develop pressures larger than those for strombolian eruptions and may trigger explosions violent enough to shatter the volcanic edifice. Pulverised magma may also produce a small-scale eruption column and pyroclastic flows. Such activity is described as vulcanian, following descriptions of the 1888-90 eruption of Vulcano, one of Stromboli's island sisters.

When the magma is very viscous, the developing froth can evolve into a gaseous network for feeding gas from the magma into fractures in the surrounding rock. The magma loses its

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gases and erupts as a creeping lava dome (Fig. 3.5). Such domes may grow to a few hundred metres tall and more than a kilometre across. While stable they present a minor hazard, but should they collapse or explode under the pressure of residual gas, they may disintegrate into clouds of ash, which, still at several hundred degrees Celsius, can evolve into pyroclastic flows travelling for several kilometres (Fig. 3.5).

The escaping gases generally leak out through the ground or volcanic vent. They can also collect as large gas pockets underground or at the bottom of craters filled with water. Along the Cameroon chain of volcanoes in West Africa, clouds of carbon dioxide escaped from the crater lakes Monoun (1984) and Nyos (1986). Heavier than air, the suffocating gas swept through villages killing together more than 1,700 people.

Figure 3. Lava fountain from the early vent of the Pu`u `O`o eruption, Hawaii, in 1984. The falling magmatic fragments are piling-up around the vent to form a cone. (Photo: J.D. Griggs, U.S. Geological Survey.)

Figure 4. Strombolian explosion from a summit vent on Stromboli. The arcs trace the trajectories of individual bombs. (Photo: B. Chouet, U.S. Geological Survey.)

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Figure 5. (a) Lava dome growing at the summit of Soufriere Hills volcano, on Montserrat, in 1996. (b) When unstable, the edge of the dome collapse and disintegrates to feed a pyroclastic flow. (Photos: C.R.J. Kilburn.) Sizes of volcanic eruptions Historical eruptions have expelled volumes of magma from a few cubic metres to 50 km3 (the 1815 Tambora eruption in Indonesia). In Geological Time, however, eruptions have expelled thousands of cubic kilometres explosively (e.g., the 1,000 km3 eruption at Yellowstone, in the USA, 640 thousand years ago) and as much as 10,000 km3 for individual lava flows (e.g., the Roza lava flow in the northwestern USA, 7 million years ago). Fortunately, larger-volume eruptions are less frequent than smaller ones. Among historical effusions, lava flows of about 10 million m3 have occurred every few years, whereas those of cubic kilometres have erupted every few centuries. In comparison, explosive eruptions worldwide tend to be more frequent (Table 1), eruptions of 10 million m3 occurring almost each year and those of cubic kilometres about 5 times a century. Eruptions of 10-100 km3 or more (about 1 or 2 per century) may be driven by the collapse of the crust into a giant reservoir of magma only a few kilometres below the surface. When this occurs, the eruption produces a giant depression, or caldera, at least several kilometres across. For thousands of years or more after they have formed, such calderas are often the site of smaller eruptions from the rim of the collapsed region. The peculiarities of these structures makes forecasting eruptions even more uncertain than normal, as seen in three of the most restless calderas since 1970: Campi Flegrei, just west of Naples, Rabaul in Papua New Guinea, and Long Valley in California. Where volcanoes occur: the role of plate tectonics Most volcanoes occur along the boundaries between tectonic plates, where intense deformation favours both rock melting and crustal fracturing (thus helping magma to reach the surface). How quickly magma rises depends in part on how neighbouring plates interact. Where plates are pulling away from each other (as along the mid-Atlantic ridge), or are being torn across their interiors (as above the Hawaiian Islands), they provide easy access to the surface. Where plates are riding over each other in subduction zones, magma has to overcome a greater resistance and so tends to take longer to ascend.

Magmas normally form by melting rock about 60-100 km below the surface. As these magmas rise, they cool, crystallise and become more difficult to flow (Table 2). The longer a magma takes to reach the surface, the greater the chance it has to become more viscous, so that volcanoes on crust tearing itself apart are dominated by eruptions of lava flows,

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strombolian activity and lava fountaining (e.g., Iceland and Hawaii), whereas volcanoes above subduction zones are characterised by plinian eruptions and the extrusion of lava domes (e.g., the rim of the Pacific Ocean). The chief hazards from volcanoes are thus not the same across the world, but depend heavily on tectonic setting. Table 1. The size and frequency of explosive volcanic eruptions. The erupted volumes have been grouped together by volume and energy into to produce the Volcano Explosivity Index, VEI. The style of eruption changes from strombolian to plinian as VEI increases (modified from Newhall and Self (1982) and Simkin and Siebert (1994)).

VEI Volume of Tephra m3 (km3), with

example

Height of Eruption Column km

General description

Expected frequency

0 Less than 104 Less than 0.1 Non-explosive Several per year 1 104-106 0.1 –1 Small

(Strombolian) Several per year

2 106-107 1 – 5 Moderate (Strombolian)

10 per year

3 107-108

Nevado del Ruiz, 1985

3 – 15 Moderate-large (Sub-Plinian)

2 per year

4 108-109 (10-1-1) Galunngung, 1982

10 – 25 Large (Plinian)

4 per decade

5 109- 1010 (1-10) Mt St Helens, 1980

More than 25 Very large (Plinian)

5 per century

6 1010-1011 (10-102) Krakatau, 1883

More than 25 Climatic Effects

Giant (Plinian)

2 per 300 years

7 1011-1012 (102-103) More than 25 Climatic Effects

Colossal (Ultra-Plinian)

1 per 1,000 years

8 More than 1012

(more than103) Toba, 74,000 yr ago

More than 25 Climatic Effects

Super Eruption (Ultra-Plinian)

1 per 100,000 years

Table 2. The composition and viscosity of common magmas. While magmas cool during ascent, their liquid portion changes composition as crystals form and the whole molten rock becomes more viscous and difficult to flow. Changes in composition normally follow one of three or four main trends. The most common trend is from basalt, through andesite and dacite to rhyolite. Basalt is the most fluid type and is associated with lava flows, strombolian activity and lava fountains. Andesites and more evolved rocks are associated with lava domes and plinian eruptions.

Composition Eruption temperature (°C)

Eruption viscosity, including the effect of crystals (Pa s)

Basalt 1050-1200 102-103

Andesite 950-1170 104-107

Dacite 900-1100 105-108

Rhyolite 700-900 109-1013

Water at Earth's surface 20 10-3

Crude oil at Earth's surface 20 10-2

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Water and volcanoes The greatest hazard to human activity occurs from subaerial volcanoes, despite the fact that Earth's most extensive volcanic products are the lava flows that make up the ocean floors. Volcanoes erupting on the ocean floor are effusive because the weight of the overlying sea water stops bubbles forming in the magma and, hence, prevents explosive activity. External water, however, can make subaerial eruptions more powerful as it flashes to steam and adds to the existing pressure from magmatic gas (hydromagmatic eruptions). Melted snow and ice can in addition trigger deadly mudflows or lahars and major floods. Eruptions beneath the Vatnajökull glacier in Iceland frequently trigger giant floods or jökulhlaups. Discharge rates of 300,000-400,000 m3 s-1 have been measured, about 20 times the discharge rate of the Mississippi. Groundwater also acts to destabilise volcanic edifices and may trigger giant landslides, especially if heated by magma close to the surface.. The volcanoes Shasta (California) and Popocatépetl (Mexico) have produced landslides of 25-30 km3, among the largest known collapses on land. Collapses from volcanic islands into the sea, however, can be at least ten times larger. Examples occur across the globe, including Hawaii, the Canaries, the Cape Verdes and Aeolian Islands. As well as decimating an island, such collapses may produce energetic tsunamis with a lethal range that can extend across entire ocean basins. They are a dramatic example of how one natural hazard can trigger another that is even more dangerous.

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Impact from Volcanic Hazards Almost one person in ten lives within range of an active volcano. Most will never suffer the impact of an eruption. When a volcano does strike, however, it may annihilate entire communities, and major disasters have occurred across the globe in the last 250 years (Table 3). The prospect of future such disasters is increasing, especially around cities that continue to grow in the shadow of a volcano (Fig. 6).

Pyroclastic flows, ash fall and lahars are responsible for about 95% of human victims (Table 4). Lava flows tend to move slowly enough for people to escape, but their destruction of property is total; they also incur large economic loss through attempts to divert or halt their advance (Table 5). The greatest impact from volcanoes is thus linked with plinian and effusive eruptions, together with eruptions through water and ice that can trigger major mudflows.

Figure 6. About ten percent of Earth's population lives within range of a volcano. About ten percent of these live in just 15 cities with populations of 1 million or more. The positions of the cities are shown relative to their nearest volcano (placed at the centre). The cities are (with population and closest volcano in brackets): 1. Tokyo, Japan (25m; Hakone volcano). 2. Mexico City, Mexico (15m; Popocatépetl). 3. Manila, Philippines (8m; Taal). 4. Yokohama, Japan (3.3m; Hakone). 5. Seattle, USA (3m; Ranier). 6. Surabaya, Indonesia (2.7m; Arjuno-Welirang). 7. Bandung, Indonesia (2.4m; Tangkuban Prahu). 8. Naples, Italy (2m; Vesuvius). 9. Vancouver, Canada (1.6m; Garibaldi). 10. San Salvador, El Salvador (1.5m; San Salvador). 11. Quito, Ecuador (1.4m; Guagua Pichincha). 12. Semarang, Indonesia (1.3m; Sundoro, Sumbing); 13. Guatemala City, Guatemala (1.2m; Pacaya). 14. Kawasaki, Japan (1.2m; Hakone). 15. Managua, Nicaragua (1m; Masaya). (After Chester et al., 2001). Pyroclastic flows Pyroclastic flows are lethal because they are hot and fast. At temperatures approaching 500-600°C, they can kill almost instantaneously through burning and suffocation (Fig. 7), while at speeds of up to 400 km h-1 they are not only difficult to escape, but can also flatten even masonry buildings (Fig. 7). Between 29 March and 04 April, 1982, El Chichón, in Southern Mexico, claimed about 2,000 lives in pyroclastic flows that swept over 100 km2. Among the deadliest plinian eruptions of the second half of the Twentieth Century, it occurred from a volcano previously believed to have been extinct.

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Table 3. Major volcanic disasters, 1750-2003. The 32 listed eruptions have either claimed 400 or more victims or incurred losses exceeding US$1000 million. During the same period, several hundred eruptions have killed at least one person. See Fig. 13.1 for locations of volcanoes. Data to 1993 from Simkin and Siebert (1996).

Region, Volcano Date Impact EUROPE Vesuvius, Italy 1794 60% of Torre del Greco buried by lava flows. San Giuseppe, Ottaviano and

Somma also damaged. 400 deaths. CAMEROON Lake Nyos 1986 Cloud of carbon dioxide released from lake rolled downslope and killed 1,700. MELANESIA Ritter island 1888 Volcanic avalanche triggered tsunamis. As many as 3,000 killed. Rabaul 1937 Pyroclastic flows, ash fall and tsunamis claim nearly 500 lives. INDONESIA Makian 1760 Lahars kill about 2,000 people. Papandayan 1772 Flank collapse destroys 40 villages and kills about 3,000 people. Gamalama 1775 Pyroclastic flows kill as many as 1,300 people. Awu 1812 Pyroclastic flows and lahars kill more than 900 people. Tambora 1815 Pyroclastic flows and tsunamis kill 10,000. Subsequent famine may have taken

another 82,000 lives. Galunggung 1822 Plinian eruption destroyed 114 villages and killed more than 4,000 people. See

also 1856 and 1892. Awu 1856 Pyroclastic flows kill more than 2,800 people. See also 1812 and 1892. Ruang 1871 Tsunamis triggered by collapse of lava dome kill as many as 400 people. Krakatau 1883 Plinian eruption and tsunamis kill more than 36,400 people. Awu 1892 Pyroclastic flows and lahars destroy 12 villages and claim more than 1,500

victims. See also 1812 and 1856. Kelut 1919 Lahars and, possibly, pyrocl;astic flows destroy 104 villages, killing more than

5,000 people. Merapi 1930 Pyroclastic flows wipe out 13 villages and damage 29 others. 1,369 victims. Agung 1963 Pyroclastic flows and lahars claim at least 1,100 lives. Iliwerung 1979 Landslides and tsunamis kill at least 500 people. PHILIPPINES Mayon 1814 Pyroclastic flows and lahars kill 1,200 people. Taal 1911 Pyroclastic flows kill about 1,335 people. Pinatubo 1991 Plinian eruption kills about 350; subsequent disease claims another 450 lives.

65,000 people evacuated. JAPAN Bandai San 1888 Pyroclastic flows and lahars kill 461 people. Asama 1783 Pyroclastic flows and floods claim nearly 1,500 lives. Unzen 1792 Flank collapse and tsunamis kill more than 14,500 people. USA Mount St Helens 1980 Plinian eruption claimed 57 lives and caused losses of about US$ 1,000 million. MEXICO El Chichon 1982 Pyroclastic flows, ash fall and lahars kill at least 1,600 people. GUATEMALA Santa Maria 1902 Ash fall and gas kill 1,500 people. COLOMBIA Nevado del Ruiz 1845 Lahars kill about 1,000 people Nevado del Ruiz 1985 Lahars destroy Armero and kill 23,000 people. Total economic loss estimated at

US$7,700 million CARIBBEAN Mt Pelée, Martinique 1902 Pyroclastic flows destry St Pierre and kill nearly 30,000 people. Soufriere, St Vincent 1902 Pyroclastic flows kill nearly 1,700 people. ICELAND Lakagigar 1783-84 The largest historical effusion produced 13-14 km3 of lava. Gases destroyed

crops and led to famine, killing some 9,350 people.

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Figure 7. The impact of pyroclastic flows. (a) Skeletons, at Herculaneum, of people engulfed by pyroclastic flows during the AD 79 plinian eruption of Vesuvius in Southern Italy (Photo: C.R.J. Kilburn). (b) St Pierre, flattened by pyroclastic flows released during the 1902 eruption of Mont Pelée on Martinique in the West Indies. Triggered by the explosive failure of a lava dome, the flows killed some 30,000 people. Ash fall Pulverised magma swept up to tens of kilometres settles back through the atmosphere as pumice and ash. Thicknesses of tens of metres can accumulate on the volcano itself, while ash carpets 30 cm thick may extend 100 km or more distant.

Falling ash dramatically reduces visibility (Fig. 8) and, during the climax of an eruption, can prevent people from seeing their hands at arm's length. When wet, ash blankets behave as cement and even 30-cm accumulations on buildings can cause roofs to collapse. During the 1991 eruption of Pinatubo, in the Philippines, that expelled more than 5 km3 of magma, some 65,000 people were successfully evacuated from the volcano. About 300 people were directly killed by the eruption, a few dozen by pyroclastic flows, but most by the collapse of roofs under piles of ash. Dry ash is also extremely hazardous to humans and animals. The fine particles lodge in the lungs and respiratory tract, causing breathing difficulties and laying the foundations for later silicosis; they are also small enough to infiltrate machinery (from engines to computers), often leading to power failures, and to clog water-filtration plants, contaminating water supplies and destroying fisheries. Even thin layers of ash can kill crops and, because the particles are extremely abrasive, they can grind the teeth of grazing livestock until they are no longer able properly to pulp their food for digestion. The infiltration of ash and its abrasive properties are a major hazard to aircraft caught in a spreading eruption cloud. During the 1982 eruption of Indonesia's Galunggung, a British Airways jumbo jet temporarily lost power to all four engines after they had become blocked by volcanic ash. The plane made a successful emergency landing, but with the pilots being unable to see through the windshields, since these had become completely crazed-over by abrasion.

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Figure 8. A curtain of falling ash moving away from Soufriere Hills volcano on Montserrat (left to right in the photo). This is a light ash fall, material falling back from clouds released by a modest pyroclastic flow. Major ash falls during plinian eruptions can induce an effective blackout (Photo: C.R.J. Kilburn). Table 4. Deaths by volcanic hazard. The relative proportions remain similar comparing 1600-2000 and 1900-2000.

Hazard Percentage Loss 1600-2000 1900-2000 Pyroclastic Flows 57 52 Ash Fall 9 5 Lava flows 1 much less than 1 Lahars 33 43 Excludes indirect effects, such as famine and tsunamis. Total deaths, 1600-2000: 566,000.

Lava flows Most lava flows advance at less than a kilometre a day and so are not normally a threat to life. Occasionally, however, a flow can race forward at 10-20 km h-1. Such rapid flows have been seen on Mauna Loa, in Hawaii, and on Congo's Nyiragongo when, in 1977, a lava killed at least 60 villagers in their sleep (Table 5). The slow evolution of lava flows not only reduces the threat to life, but also provides the opportunity to make attempts at changing the natural development of a flow. Concerted efforts have been carried out particularly on Mt Etna, in Sicily, where attempts to halt and divert lava flows using barriers and explosives were conducted in 1983, 1991-3 and 2001 (Table 5). The strategies employed were virtually identical to those used on Etna in 1669, the earliest record of people trying to change the course of a volcanic eruption.

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Table 5. The cost of selected lava flows. For comparison, plinian eruptions may cause damage worth US$ 100 millions or billions.

Erupton Impact and Cost (at time of eruption) Vesuvius, Italy 1944 Massa and parts of San Sebastiano destroyed at northern foot of

volcano. The area had previously been overrun by lavas in 1855 and 1872.

Kilauea, Hawaii, 1960 Eruption destroyed Kapoho village at eastern tip of island. About 5 km of earthen barriers were built in attempt to constrain lava. Damage, mitigation and repair costs estimated at US$ 6 million.

Heimaey, Iceland, 1973 Town partially destroyed. Extended attempts to halt flows by water spraying. Damage, mitigation and repair costs estimated at US$ 35 million.

Nyiragongo, Congo, 1977 Several villages destroyed and at least 60 killed on flanks of volcano.

Etna, Italy, 1983 South-flank tourist structures destroyed. Attempts to divert flows with barriers and explosives. Damage, mitigation and repair costs estimated at US$ 5 million.

Etna, Italy, 1991-93 Environmentally sensitive land lost. Attempts to divert flows with barriers and explosives. Damage, mitigation and repair costs estimated at US$ 5 million.

Etna, Italy, 2002-03 NE-flank tourist facilities devastated; southern tourist facilities had previously also been threatened by lavas in 2001.

Pu'u 'O'o, Hawaii, 1983-Present In its first 20 years of activity, Pu'u 'O'o has expelled 2.1 km3 of magma, making it the largest effusion on Earth for 200 years. More than 100 km2 have been covered, including several residential areas on the island's South coast.

Nyiragongo, Congo, 2001 At least 200 deaths in the city of Goma, many when people crossing stagnant flows were caught in explosion at petrol station surrounded by lava.

Lahars Volcanic mudflows are among the most dangerous types of landslide. They can be triggered by eruptions melting snow and ice, or by rain washing away piles of accumulated ash, even long after an eruption has ceased. Since its 1991 eruption, lahars from Piantubo's ash deposits have each year destroyed 10s-100s km2 of vital farmland around the volcano during the monsoon and typhoon seasons. The deadliest lahar on record occurred on 13 November 1985 at Nevado del Ruiz, in Colombia. Standing 5 km above sea level, the top of the volcano is above the snowline. A minor eruption (VEI 3) dispersed ash and pyroclastic flows over the summit, generating 20 million m3 of meltwater. The flood soon developed as a lahar, by mixing with the ash and mud picked up as it travelled down valleys. Just 2.5 hours later, the lahar sent a series of waves 2-5 m thick crashing through the town of Armero, 74 km away. Within 90 minutes, 23,000 people lay dead and Armero ceased to exist (Fig. 9). Sixty percent of the region's livestock was killed and the total economic loss estimated at US$ 7,700 million.

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Figure 9. Only a handful of buildings remain among the desolation where Armero once stood. The 1985 lahar from Nevado del Ruiz destroyed more than 5,000 homes, rendered 7,700 useless and killed about 23,000 people. (Photo: US Geological Survey). Giant, catastrophic collapses can also trigger other forms of natural hazard, the impact of which is greater than that from the landslide alone. Especially pernicious are flank failures at active volcanoes and giant landslides into the sea. On 18 May 1980, a magnitude 5.1 earthquake triggered the collapse of about 3 km3 from the northern flank of Mount St Helens in the Washington State section of North America's Cascade mountains. The landslide exposed the gas-rich magma that had been accumulating inside the volcano for at least two months. Exploding laterally, the magma disintegrated into a cloud of gas and volcanic debris that, at temperatures of several hundred degrees Celsius, raced downslope at 300-500 km an hour. This cloud, together with the blast from the air pushed ahead of it, laid waste some 600 km2 of land within two minutes (Lipman and Mullineaux, 1981). Super-eruptions Since records began, the largest witnessed eruption occurred in 1815 when Tambora, in Indonesia, expelled 50 km3 of magma. In the Geological Record, however, explosive eruptions 100 times larger have occurred at intervals of about 100,000 years, the most recent being the Toba eruption (2,800 km3), also in Indonesia, about 74,000 years ago. A similar eruption today would be catastrophic. The surrounding region would be devastated, while ash and volcanic gas forced into the upper atmosphere would significantly block incoming sunlight and cause average world temperatures to drop several degrees Celsius for many years. Global crop failure would result and, in the worst case, the polar ice caps may grow sufficiently for cooling to continue after the ash and gas have dissipated, producing conditions that might herald a new Ice Age. Indeed, it is possible that the climatic effects of the Toba eruption almost wiped out the ancestors of Homo Sapiens. Although not expected to occur soon, a super-eruption is virtually inevitable at some time in the future.

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References Chester DK, Degg M, Duncan AM, Guest JE., 2001. The increasing exposure of cities to the effects of volcanic eruptions: a global survey. Environmental Hazards, 2: 89-103. Francis P, Oppenheimer C 2004. Volcanoes. Oxford, pp 521. Lipman P, Mullineaux DR (eds) 1981. The 1980 eruptions of Mount St Helens, Washington. US Geol Survey Prof Paper 1250: 1-844. Newhall CG, Self S 1982. The Volcanic Explosivity Index (VEI): an estimate of the explosive magnitude for historical volcanism. Journal of Geophysical Research, 87: 1231-1238. Simkin T, Siebert L 1996. Volcanoes of the world (2nd Edition). Smithsonian Institution and Geoscience Press, pp 349.

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