25154500 What is a Supervolcano

THE TERM supervolcano was first used in the year

2000 to describe a small number of giant volcanoes that have produced rare but catastrophic eruptions. Somewhere in the world a supervolcano erupts roughly every 50,000 to 100,000 years and blasts over 1,000 cubic km (km3) of material into the air. Supervolcanoes are very large and usually 50 to 100 km in diameter. Figure 1 shows the location of the world’s known supervolcanoes. Some supervolcanoes are located in areas of dense human population, in South-east Asia, western USA, Japan and Italy, and they have attracted considerable interest from geographers and geologists because they are potentially very dangerous and not fully understood. The dates of historic supervolcano eruptions in the last million years are shown in Figure 2. As no supervolcano has produced a full-scale eruption in the last 2,000 years, we can only work out from rocks and fossils how extreme their eruptions can be.

Geologists classify volcanoes into eight types using the Volcano Explosivity Index (VEI), which is based on the types of eruption (Figure 3). Runny lava (Hawaiian-type) volcanoes found along mid-ocean ridges let gases escape easily. Cone-shaped composite volcanoes found along destructive plate boundaries have sticky lavas and produce explosive eruptions graded from 2 to 7. Supervolcanoes are graded VEI 8, the most explosive category. Figure 4 shows the main differences between a composite volcano and a supervolcano.

How do supervolcanoes erupt?Geologists believe that beneath each supervolcano there is a very large bubble of very sticky magma tens of kilometres across. This bubble of magma becomes trapped under overlying rock, with water vapour, carbon dioxide and sulphur dioxide trapped within it.

Supervolcanic eruptions may be started in three ways:

1 An earthquake cracks the rock above it and suddenly releases the magma which explodes out,

rather like air rushing out of a balloon that has been burst.

2 The pressure from the rising bubble of magma underneath literally pushes off the lid of overlying rock, in the same way that floodwater in an underground drain can lift up manhole covers in a road. With the overlying lid gone, the whole magma bubble bursts out.

3 A sudden drop in pressure, produced as the magma nears the surface, may cause dissolved gases in the liquid magma to be released from solution. The whole magma mass then

Series 21 Spring issue Unit 432 What is a Supervolcano? © 2010 Nelson Thornes GeoActive OnlineThis page may be photocopied for use within the purchasing institution only. Page 1 of 4

by John DavidsonWhat is a supervolcano?

GeoActive OnlineGeoActive Online

432

Supervolcano Country Date of eruption (years before present)

Volume of ejected material (km3)

Long Valley caldera, California

USA 760,000 560

Lava Creek, Yellowstone

USA 640,000 1,000

Whakamaru New Zealand 254,000 2,000

Aso Japan 80,000 600

Mount Toba, Sumatra

Indonesia 74,000 2,800

Campi Flegrei Italy 39,000 500

Lake Taupo New Zealand 26,500 1,170

GeoActive Series 21 Issue 2Fig 432_01 Mac/eps/illustrator 11 s/s

NELSON THORNES PUBLISHINGArtist: David Russell Illustration

Toba

Taupo

A

B

Supervolcano eruption in the last 2 million yearsKey

C

DE

F

H

G Long Valley Yellowstone

AsoCampiFlegrei

Figure 1: World distribution of supervolcanoes

Figure 2: Dates of historic supervolcano eruptions

becomes a massive explosive froth which blasts out of the ground – rather as a bottle of fizzy drink can explode if it is shaken very violently.

Scientists believe that a supervolcano produces a massive explosion which literally blasts the crust and molten froth into the air. Ash is blown over 30 km up into the atmosphere. Some remains airborne for many months, drifting around the Earth and gradually falling like grey snow, but large amounts collapse back down to Earth while still very hot and then travel long distances over the ground as pyroclastic flows. When all the magma has been erupted, a deep crater is left behind called a caldera (from the Latin word for cauldron). These depressions are often over 50 km in diameter, and sometimes fill with water. Figure 5 shows an example of such a depression as a cross-section through the Long Valley caldera in the Sierra Nevada mountains east of San Francisco. Figure 6 show how supervolcano eruptions compare with more recent volcanic events.

How and why do supervolcanoes form?Supervolcanoes mainly occur in areas of continental crust in two types of location:

1 Near destructive plate boundaries where a large

amount of material from a descending plate begins to rise back up to the surface. This magma rises up under the continental crust and produces a very sticky mass of molten rock rich in silica.

2 Above continental ‘hotspots’ where a large bubble of rising magma is pushing up through the plate and heating the continental crust.

In both these cases, the crust is being extended locally or ‘stretched’, producing weaknesses that allow the magma bubbles (sometimes called plumes) to rise up towards the surface over a long period of time. This steady flow of hot rock over thousands of years forms the large magma lakes beneath supervolcanoes.

What are their effects?Using evidence such as ash and fossil remains, geologists have worked out that a supervolcano eruption could produce the following effects:

• Between 1,000 and 3,000 km3 of ash are blasted up into the atmosphere.

• Ash fall between 1 and 5 metres deep can occur up to 1,000 km from the supervolcano.

• The cloud of ash can block out solar radiation causing a ‘volcanic winter’, which may last from one to five years. Estimates suggest worldwide falls in temperature of between 5° and 10°C, possibly resulting in an ice age.

• The explosive eruption could destroy everything within a radius of 500 km.

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VEI Classification Description Height of ash plume

Amount of material ejected

Example

1 Hawaiian Non-explosive/gentle

100–1,000 metres Less than 1 million m3 Mauna Loa, Hawaii

2 Strombolian Explosive 1–5 km 1 million to 10 million m3

Stromboli, Italy

3 Vulcanian Severe explosive 3–15 km 10 million m3 to 0.1 km3

Vulcano, Italy

4 Pelean Cataclysmic 10–25 km 0.1 km3 to 1 km3 Mount Pelée, Martinique 1902

5 Plinian Paroxysmal Over 25 km 1 km3 to 10 km3 Mount St Helens, USA 1980

6 Ultra-Plinian Colossal Over 25 km 10 km3 to 100 km3 Mount Pinatubo, Philippines 1991

7 Ultra-Plinian Super-colossal Over 25 km 100 km3 to 1,000 km3 Tambora, Indonesia 1815

8 Supervolcano Mega-colossal Over 25 km Over 1,000 km3 Mount Toba, Sumatra, 74,000 years ago

Figure 3: Volcano Explosivity Index (VEI)



Localrocks

Localrocks

FaultsMagmabubble

Magmachamber Rising magma from below

LakeRising dome

Caldera

Possibleside vents

Main vent

COMPOSITE VOLCANOExample, Mt St Helens

SUPERVOLCANO

Layers of ashand lava Smaller volcano

or steam eruption/geyser

Figure 4: Differences between composite volcanoes and supervolcanoes

Series 21 Spring issue Unit 432 What is a Supervolcano? © 2010 Nelson Thornes GeoActive OnlineThis page may be photocopied for use within the purchasing institution only. Page 3 of 4

• An erupting supervolcano on the coast could trigger massive wave surges (tsunamis) which may travel up to 5,000 km.

• Poisonous gases could contaminate water supplies and rainfall.

• People and animals would be severely affected worldwide. The effects would include burying crops and other plants, damage to health from gases and breathing in ash, polluted water supplies and climate change.

Can we predict supervolcano eruptions?In recent years scientists have been studying volcanoes and attempting to predict their behaviour by looking at how rising magma may be linked to

earthquakes and the changing shape of the volcano, but it is difficult to predict precisely when a volcano will erupt and how massive the eruption will be. Some supervolcanoes like Yellowstone are now intensively monitored, but less is known about those in remote areas where there is a lack of equipment. Scientists know that supervolcanoes will erupt in the future – and the Earth is five times more likely to be affected by a supervolcano eruption than to be hit by an asteroid! Figure 7 on page 4 is a summary of the last major supervolcano eruption on Mount Toba in Sumatra, and the case study of Yellowstone suggests how human populations are at risk.

Case Study

YellowstoneThe Yellowstone caldera is located in the north-west corner of the state of Wyoming in the USA and measures 55 km by 72 km. Yellowstone lies over a hotspot where a large mass of magma is rising up from the mantle. The Yellowstone supervolcano has erupted at least twelve times in the last 15 million years, at regular intervals of about 600,000 years. The last supervolcano eruption was 640,000 years ago. Previous eruptions have produced a blast zone of 1,000 km and ash fall up to 1,600 km from Yellowstone (see Figure 8) and today 20 million people live within 1,000 km of the supervolcano. Earthquakes occur frequently and Yellowstone is swelling by 1–3 cm every year, which may indicate increasing magma pressure below. The Yellowstone Volcano Observatory will issue warnings based on earthquake readings if an eruption is believed to be imminent and the State of Wyoming has evacuation plans in place, but a supervolcano eruption here could destroy much of the farming across the USA, and make large areas of the continent uninhabitable.

ConclusionSupervolcanoes are hard to predict. Geologists have calculated that there could be at least one VEI 7 or VEI 8 super-eruption in the next hundred years and it is very unlikely that humans would be able to prevent or stop a supervolcano eruption. However, research may make the prediction of future eruptions easier.



Caldera

Glass Mountain

Faults

Deposits of ash/lava

Solid magma

Risingmagmabubble

Rising magma

20 km

15 km

10 km

5 km

Local extension (stretching) of crust

Faults

Mammoth MountainDome

Localrocks

Localrocks

FaultsMagmabubble

Magmachamber Rising magma from below

LakeRising dome

Caldera

Possibleside vents

Main vent

COMPOSITE VOLCANOExample, Mt St Helens

SUPERVOLCANO

Layers of ashand lava Smaller volcano

or steam eruption/geyser

Figure 5: Cross-section through a caldera (Long Valley, California)

Volcano Country Date Volume of material ejected (km3)

Mount St Helens, Washington

USA 1980 0.25

Mount Pinatubo Philippines 1991 5

Tambora Indonesia 1815 160

Campi Flegrei Italy 39,000 years ago 500

Long Valley caldera, California

USA 760,000 years ago 560

Lava Creek, Yellowstone

USA 640,000 years ago 1,000

Lake Taupo New Zealand 26,500 years ago 1,170

Mount Toba, Sumatra

Indonesia 74,000 years ago 2,800

Figure 6: Comparison of supervolcano eruptions and some recent eruptions

1 On a copy of Figure 1, name:(a) the oceans A, B and C(b) the continents D, E and F(c) the countries G and H.

2 Using the data in Figure 2, plot a supervolcano time line on graph paper, to show the amount of material ejected by the supervolcano on the vertical axis, and the date of the eruption in years before present on the horizontal axis. You could use a computer spreadsheet to help you with this task.

3 Using Figures 4 and 5, describe three main differences between a composite volcano such as Mount Pinatubo and a supervolcano.

4 Referring to Figure 7, suggest three reasons why scientists think that so many plants and animals, including humans, were killed by the supervolcano eruption of Mount Toba 74,000 years ago.

5 Using Figure 8 and the internet, write a description of the Yellowstone supervolcano using the following headings:

• What is the present volcanic activity in the region?

• What are the present land uses that an eruption might affect?

• How is volcanic activity in the area being monitored?

6 Before answering this question, try to view an extract of the BBC Horizon TV programme on Yellowstone. Extracts are available on the web: www.bbc.co.uk/science/horizon/1999/supervolcanoes.shtmlUsing Figure 8, suggest five different ways in which the present human population of the USA would be at risk if a supervolcano eruption occurred at either Yellowstone or Long Valley in the USA.

7 Write a short article for a magazine suggesting why money should be spent by governments on finding out more about supervolcanoes and their eruptions.

8 Imagine you had the job of explaining to people in an area near a supervolcano who did not want to move, why they might need to be evacuated in the future if the volcano suddenly became dangerous. Design a poster or cartoon strip to show the main hazards that could affect people if they did not leave.

9 Using the internet, find out what is meant by a ‘volcanic winter’. Write a short explanation of how a supervolcano eruption could affect the whole planet. (There are articles on this on the BBC News website: http://news.bbc.co.uk)

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Where? In the northern part of the Indonesian island of Sumatra (an area of continental crust).

When? 74,000 years ago.What happened? A VEI 8 eruption blasted out nearly 3,000 km3 of ash

and pyroclastics which were deposited over a large area of Asia. About two-thirds of this raced across the ground as scorching hot pyroclastic flows destroying everything in their path. The remaining third fell as ash covering South-east Asia and India in layers several metres thick. Large quantities of sulphuric acid were ejected into the atmosphere producing acid rain around the world. The volcano finally collapsed into a caldera 100 km long and 30 km wide, now called Lake Toba.

What were the effects? The ash clouds from Mount Toba cooled the Earth by 3–4°C for several years, causing a ‘volcanic winter’. Fossil evidence suggests that very few plants and animals survived in South-east Asia. Some scientists believe that the eruption may have led to the deaths of up to a staggering 80% of our human ancestors on the planet at this time.

Figure 7: Factfile: Mount Toba, Sumatra



Mt St Helens1980 Yellowstone

supervolcano

Long Valley caldera

0 800 km

U S A

CANADA

MEXICO

Area of ash deposits from YellowstoneLava Creek eruption 640,000 years ago

KeyN

Figure 8: Location of volcanoes and supervolcanoes in the USA

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25154500 What is a Supervolcano