Download Volcanoes occur where underground molten rock erupts at

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 smallscale 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
1
Less than 104
104-106
Less than 0.1
0.1 –1
Several per year
Several per year
2
106-107
1–5
3
107-108
Nevado del Ruiz,
1985
8
10 -109 (10-1-1)
Galunngung, 1982
109- 1010 (1-10)
Mt St Helens, 1980
1010-1011 (10-102)
Krakatau, 1883
1011-1012 (102-103)
3 – 15
Non-explosive
Small
(Strombolian)
Moderate
(Strombolian)
Moderate-large
(Sub-Plinian)
Large
(Plinian)
Very large
(Plinian)
Giant
(Plinian)
Colossal
(Ultra-Plinian)
Super Eruption
(Ultra-Plinian)
4 per decade
4
5
6
7
8
More than 1012
(more than103)
Toba, 74,000 yr ago
10 – 25
More than 25
More than 25
Climatic Effects
More than 25
Climatic Effects
More than 25
Climatic Effects
10 per year
2 per year
5 per century
2 per 300 years
1 per 1,000 years
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
Basalt
Andesite
Dacite
Rhyolite
Water at Earth's surface
Crude oil at Earth's surface
Eruption
temperature (°C)
Eruption viscosity, including the effect of
crystals (Pa s)
1050-1200
950-1170
900-1100
700-900
102-103
104-107
105-108
109-1013
20
20
10-3
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 500600°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
EUROPE
Vesuvius, Italy
Date
Impact
1794
60% of Torre del Greco buried by lava flows. San Giuseppe, Ottaviano and
Somma also damaged. 400 deaths.
1986
Cloud of carbon dioxide released from lake rolled downslope and killed 1,700.
CAMEROON
Lake Nyos
MELANESIA
Ritter island
Rabaul
INDONESIA
Makian
Papandayan
Gamalama
Awu
Tambora
1888
1937
Volcanic avalanche triggered tsunamis. As many as 3,000 killed.
Pyroclastic flows, ash fall and tsunamis claim nearly 500 lives.
1760
1772
1775
1812
1815
Galunggung
1822
Awu
Ruang
Krakatau
Awu
1856
1871
1883
1892
Kelut
1919
Merapi
Agung
Iliwerung
PHILIPPINES
Mayon
Taal
Pinatubo
1930
1963
1979
Lahars kill about 2,000 people.
Flank collapse destroys 40 villages and kills about 3,000 people.
Pyroclastic flows kill as many as 1,300 people.
Pyroclastic flows and lahars kill more than 900 people.
Pyroclastic flows and tsunamis kill 10,000. Subsequent famine may have taken
another 82,000 lives.
Plinian eruption destroyed 114 villages and killed more than 4,000 people. See
also 1856 and 1892.
Pyroclastic flows kill more than 2,800 people. See also 1812 and 1892.
Tsunamis triggered by collapse of lava dome kill as many as 400 people.
Plinian eruption and tsunamis kill more than 36,400 people.
Pyroclastic flows and lahars destroy 12 villages and claim more than 1,500
victims. See also 1812 and 1856.
Lahars and, possibly, pyrocl;astic flows destroy 104 villages, killing more than
5,000 people.
Pyroclastic flows wipe out 13 villages and damage 29 others. 1,369 victims.
Pyroclastic flows and lahars claim at least 1,100 lives.
Landslides and tsunamis kill at least 500 people.
JAPAN
Bandai San
Asama
Unzen
USA
Mount St Helens
MEXICO
El Chichon
GUATEMALA
Santa Maria
COLOMBIA
Nevado del Ruiz
Nevado del Ruiz
CARIBBEAN
Mt Pelée, Martinique
Soufriere, St Vincent
ICELAND
Lakagigar
1814
1911
1991
Pyroclastic flows and lahars kill 1,200 people.
Pyroclastic flows kill about 1,335 people.
Plinian eruption kills about 350; subsequent disease claims another 450 lives.
65,000 people evacuated.
1888
1783
1792
Pyroclastic flows and lahars kill 461 people.
Pyroclastic flows and floods claim nearly 1,500 lives.
Flank collapse and tsunamis kill more than 14,500 people.
1980
Plinian eruption claimed 57 lives and caused losses of about US$ 1,000 million.
1982
Pyroclastic flows, ash fall and lahars kill at least 1,600 people.
1902
Ash fall and gas kill 1,500 people.
1845
1985
Lahars kill about 1,000 people
Lahars destroy Armero and kill 23,000 people. Total economic loss estimated at
US$7,700 million
1902
1902
Pyroclastic flows destry St Pierre and kill nearly 30,000 people.
Pyroclastic flows kill nearly 1,700 people.
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
Pyroclastic Flows
Ash Fall
Lava flows
Lahars
57
9
1
33
1900-2000
52
5
much less than 1
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
Vesuvius, Italy 1944
Kilauea, Hawaii, 1960
Heimaey, Iceland, 1973
Nyiragongo, Congo, 1977
Etna, Italy, 1983
Etna, Italy, 1991-93
Etna, Italy, 2002-03
Pu'u 'O'o, Hawaii, 1983-Present
Nyiragongo, Congo, 2001
Impact and Cost (at time of eruption)
Massa and parts of San Sebastiano destroyed at northern foot of
volcano. The area had previously been overrun by lavas in 1855
and 1872.
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.
Town partially destroyed. Extended attempts to halt flows by
water spraying. Damage, mitigation and repair costs estimated at
US$ 35 million.
Several villages destroyed and at least 60 killed on flanks of
volcano.
South-flank tourist structures destroyed. Attempts to divert flows
with barriers and explosives. Damage, mitigation and repair costs
estimated at US$ 5 million.
Environmentally sensitive land lost. Attempts to divert flows with
barriers and explosives. Damage, mitigation and repair costs
estimated at US$ 5 million.
NE-flank tourist facilities devastated; southern tourist facilities had
previously also been threatened by lavas in 2001.
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.
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: 12311238.
Simkin T, Siebert L 1996. Volcanoes of the world (2nd Edition). Smithsonian Institution and
Geoscience Press, pp 349.
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