volcanic eruptions

costs and benefits of predictions

No scientist or government agency wants to announce an eruption and then be wrong. There is a very real cost and disruption to society during a large-scale evacuation. If the scientists are wrong, people would be less likely to evacuate the next time scientists predicted an eruption. But if scientists predict an eruption that does take place it could save many lives.

gases

Scientists measure the gases that escape from a volcano to predict eruptions. Gases like sulfur dioxide (SO2 ), carbon dioxide (CO2 ), hydrochloric acid (HCl), and water vapor can be measured at the site. Gases may also be measured from satellites. The amounts of gases and the ratios of gases are calculated to help predict eruptions.

remote monitoring

Satellites can be used to monitor more than just gases (Figure 8.13). Satellites can look for high temperature spots or areas where the volcano surface is changing. This allows scientists to detect changes accurately and safely.

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predicting volcanic eruptions

Volcanic eruptions can be devastating, particularly to the people who live close to volcanoes. Volcanologists study volcanoes to be able to predict when a volcano will erupt. Many changes happen when a volcano is about to erupt.

earthquakes

Earthquakes may take place every day near a volcano. But before an eruption the number and size of earthquakes increases. This is the result of magma pushing upward into the magma chamber. This motion causes stresses on neighboring rock to build up. Eventually the ground shakes. A continuous string of earthquakes may indicate that a volcano is about to erupt. Scientists use seismographs to record the length and strength of each earthquake.

slope tilt

All that magma and gas pushing upwards can make the volcanos slope begin to swell. Ground swelling may change the shape of a volcano or cause rock falls and landslides. Most of the time, the ground tilting is not visible. Scientists detect it by using tiltmeters, which are instruments that measure the angle of the slope of a volcano.

how volcanoes erupt

All volcanoes share the same basic features. First, mantle rock melts. The molten rock collects in magma chambers that can be 160 kilometers (100 miles) beneath the surface. As the rock heats, it expands. The hot rock is less dense than the surrounding rock. The magma rises toward the surface through cracks in the crust. A volcanic eruption occurs when the magma reaches the surface. Lava can reach the surface gently or explosively.

history of volcanic activities

Scientists study a volcanos history to try to predict when it will next erupt. They want to know how long it has been since it last erupted. They also want to know the time span between its previous eruptions. Volcanoes can be active, dormant, or extinct (Figure 8.12). An active volcano may be currently erupting. Alter- natively, it may be showing signs that it will erupt in the near future. A dormant volcano no longer shows signs of activity. But it has erupted in recent history and will probably erupt again. An extinct volcano is one that has not erupted in recent history. Scientists think that it will probably not erupt again. Scientists watch both active and dormant volcanoes closely for signs that show they might erupt.

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explosive eruptions

An explosive eruption produces huge clouds of volcanic ash. Chunks of the volcano fly high into the atmosphere. Explosive eruptions can be 10,000 times as powerful as an atomic bomb (Figure 8.6). Hot magma beneath the surface mixes with water. This forms gases. The gas pressure grows until it must be released. The volcano erupts in an enormous explosion. Ash and particles shoot many kilometers into the sky. The material may form a mushroom cloud, just like a nuclear explosion. Hot fragments of rock, called pyroclasts, fly up into the air at very high speeds. The pyroclasts cool in the atmosphere. Some ash may stay in the atmosphere for years. The ash may block out sunlight. This changes weather patterns and affects the temperature of the Earth. For a year or two after a large eruption, sunsets may be especially beautiful worldwide. Volcanic gases can form poisonous, invisible clouds. The poisonous gases may be toxic close to the eruption. The gases may cause environmental problems like acid rain and ozone destruction. Mt St. Helens was not a very large eruption for the Cascades. Mt. Mazama blew itself apart in an eruption about 42 times more powerful than Mount St. Helens in 1980. Today all that remains of that huge stratovolcano is Crater Lake (Figure 8.18).

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types of eruptions

Eruptions can be explosive or non-explosive. Only rarely do gentle and explosive eruptions happen in the same volcano.

magma and lava

Great volcanic explosions and glowing red rivers of lava are fascinating. All igneous rock comes from magma or lava. Remember that magma is molten rock that is below Earths surface. Lava is molten rock at Earths surface.

nonexplosive eruptions

Some volcanic eruptions are non-explosive (Figure 8.7). This happens when there is little or no gas. The lava is thin, fluid and runny. It flows over the ground like a river. People generally have a lot of warning before a lava flow like this reaches them, so non-explosive eruptions are much less deadly. They may still be destructive to property, though. Even when we know that a lava flow is approaching, there are few ways of stopping it!

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lava

The way lava flows depends on what it is made of. Thick lava doesnt flow easily. It may block the vent of a volcano. If the lava traps a lot of gas, the pressure builds up. After the pressure becomes greater and greater, the volcano finally explodes. Ash and pyroclasts shoot up into the air. Pumice, with small holes in solid rock, shows where gas bubbles were when the rock was still molten. Fluid lava flows down mountainsides. The rock that the flow becomes depends on which type of lava it is and where it cools. The three types of flows are aa, pahoehoe, and pillow lava. Aa Lava Aa lava is the thickest of the non-explosive lavas. Aa forms a thick and brittle crust, which is torn into rough, rubbly pieces. The solidified surface is angular, jagged and sharp. Aa can spread over large areas as the lava continues to flow underneath. Pahoehoe Lava Pahoehoe lava is thinner than aa, and flows more readily. Its surface looks more wrinkly and smooth. Pahoehoe lava flows in a series of lobes that form strange twisted shapes and natural rock sculptures (Figure 8.9). Pahoehoe lava can form lava tubes. The outer layer of the lava flow cools and solidifies. The inner part of the flow remains fluid. The fluid lava flows through and leaves behind a tube (Figure 8.10). Pillow Lava Pillow lava is created from lava that enters the water. The volcanic vent may be underwater. The lava may flow over land and enter the water (Figure 8.11). Once in the water, the lava cools very quickly. The lava forms round rocks that resemble pillows. Pillow lava is particularly common along mid-ocean ridges.

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magma

Magma forms deep beneath the Earths surface. Rock melts below the surface under tremendous pressure and high temperatures. Molten rock flows like taffy or hot wax. Most magmas are formed at temperatures between 600o C and 1300o C (Figure 8.8). Magma collects in magma chambers beneath Earths surface. Magma chambers are located where the heat and pressure are great enough to melt rock. These locations are at divergent or convergent plate boundaries or at hotpots. The chemistry of a magma determines the type of igneous rock it forms. The chemistry also determines how the magma moves. Thicker magmas tend to stay below the surface or erupt explosively. When magma is fluid and runny, it often reaches the surface by flowing out in rivers of lava.

instructional diagrams

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questions

Which statement about Mount St. Helens is false?

a. It is an active volcano.

b. Its 1980 eruption was explosive.

-->  c. It is unlikely to erupt again.

d. all of the above

Non-explosive eruptions

a. cause little damage

-->  b. rarely kill anyone

c. have lavas that contain a lot of gas

d. all of these

Compared with the solid rock around it, magma is

a. cooler.

b. harder.

-->  c. less dense.

d. two of the above

A volcano that has had no activity for quite a long time is said to be

a. active

b. sleeping

c. dormant

-->  d. extinct

The deepest magma chambers are about

a. 16 km below the surface.

-->  b. 160 km below the surface.

c. 1600 km below the surface.

d. 16,000 km below the surface.

An ash plume from a volcano in Iceland

-->  a. disrupted air travel across Europe for six days in 2010

b. mixed with pollutants in the atmosphere to cause excess acid rain

c. created pillow lavas offshore

d. created Aa and pahoehoe lavas offshore

To see if a volcano will soon erupt, satellites can sense

a. earthquakes

b. rock fall

-->  c. temperature, deformation and gases

d. nothing; satellites are too high up

A non-explosive volcanic eruption may release a large quantity of

a. ash.

b. gases.

-->  c. molten rock.

d. pyroclasts.

The chemistry of magma determines

a. how it erupts.

b. how thick it is.

c. what type of igneous rock it forms.

-->  d. all of the above

The ability of scientists to predict volcanic eruptions is

a. excellent in most locations

b. about 50-50

-->  c. an area of science that needs improvement

d. never going to improve; its just too har

For magma to form, solid rock must reach a temperature of at least

-->  a. 600 C.

b. 1600 C.

c. 6000 C.

d. 16,000 C.

Signs that a volcano may soon erupt include

a. earthquakes.

b. ground tilting

c. release of gases.

-->  d. all of the above

At least one of the Cascades volcanoes has been actively erupting for at least a century.

a. true

-->  b. false

Gases from volcanoes can be poisonous.

-->  a. true

b. false

Volcanic eruptions in Hawaii are usually explosive.

a. true

-->  b. false

Eruptions that are non-explosive have little or no gas.

-->  a. true

b. false

An increase in earthquake activity is a sign that a volcano may be about to erupt.

-->  a. true

b. false

The 1985 Mount St. Helens eruption was the biggest volcanic eruption that ever occurred.

a. true

-->  b. false

All volcanic eruptions involve explosions.

a. true

-->  b. false

A volcanic eruption may be more powerful than a nuclear explosion.

-->  a. true

b. false

Gases form in a volcano when magma boils and evaporates.

a. true

-->  b. false

Ash from a volcanic eruption may stay in the atmosphere for years.

-->  a. true

b. false

Gases from a volcano may cause environmental problems.

-->  a. true

b. false

A volcano is more likely to be explosive when lava is thin and runny.

a. true

-->  b. false

When aa lava cools, it forms lava tubes.

a. true

-->  b. false

Pillow lava is especially common along mid-ocean ridges.

-->  a. true

b. false

A volcanos history can help scientists predict whether the volcano is likely to erupt again.

-->  a. true

b. false

thickest type of non-explosive lava

a. magma chamber

b. volcanic eruption

c. pyroclast

-->  d. aa

e. dormant volcano

f. pillow lava

g. extinct volcano

volcano that has not erupted in recent history and probably will not erupt again

a. magma chamber

b. volcanic eruption

c. pyroclast

d. aa

e. dormant volcano

f. pillow lava

-->  g. extinct volcano

region within Earth that is surrounded by solid rock and contains magma

-->  a. magma chamber

b. volcanic eruption

c. pyroclast

d. aa

e. dormant volcano

f. pillow lava

g. extinct volcano

hot volcanic rock thrown into the air by an explosive eruption

a. magma chamber

b. volcanic eruption

-->  c. pyroclast

d. aa

e. dormant volcano

f. pillow lava

g. extinct volcano

lava that enters water

a. magma chamber

b. volcanic eruption

c. pyroclast

d. aa

e. dormant volcano

-->  f. pillow lava

g. extinct volcano

any release of magma onto Earths surface

a. magma chamber

-->  b. volcanic eruption

c. pyroclast

d. aa

e. dormant volcano

f. pillow lava

g. extinct volcano

volcano that has erupted in recent history and probably will erupt again

a. magma chamber

b. volcanic eruption

c. pyroclast

d. aa

-->  e. dormant volcano

f. pillow lava

g. extinct volcano

diagram questions

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