Lesson Objectives

  • Explain how magma composition affects the type of eruption.
  • Compare the types of volcanic eruptions.
  • Distinguish between different types of lava and the rocks they form.
  • Describe a method for predicting volcanic eruptions.


In 1980, Mount St. Helens blew up in the costliest and deadliest volcanic eruption in United States history. The eruption killed 57 people, destroyed 250 homes and swept away 47 bridges (Figure below).
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Mount St. Helens on May 18, 1980.
Mt. St. Helens still has minor earthquakes and eruptions. The volcano now has a horseshoe-shaped crater with a lava dome inside. The dome is formed of viscous lava that oozes into place.

Magma Composition

Each volcanic eruption is unique, differing in size, style, and composition of erupted material. One key to what makes the eruption unique is the chemical composition of the magma that feeds a volcano, which determines (1) the eruption style, (2) the type of volcanic cone that forms, and (3) the composition of rocks that are found at the volcano.
Remember from the Rocks chapter that different minerals within a rock melt at different temperatures. The amount of partial melting and the composition of the original rock determine the composition of the magma. Magma collects in magma chambers in the crust at 160 kilometers (100 miles) beneath the surface.
The words that describe composition of igneous rocks also describe magma composition.
  • Mafic magmas are low in silica and contain more dark, magnesium and iron rich mafic minerals, such as olivine and pyroxene.
  • Felsic magmas are higher in silica and contain lighter colored minerals such as quartz and orthoclase feldspar. The higher the amount of silica in the magma, the higher is its viscosity. Viscosity is a liquid’s resistance to flow (Figure below).
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Honey flows slowly. It is more viscous than water.
Viscosity determines what the magma will do. Mafic magma is not viscous and will flow easily to the surface. Felsic magma is viscous and does not flow easily. Most felsic magma will stay deeper in the crust and will cool to form igneous intrusive rocks such as granite and granodiorite. If felsic magma rises into a magma chamber, it may be too viscous to move and so it gets stuck. Dissolved gases become trapped by thick magma. The magma churns in the chamber and the pressure builds.


The type of magma in the chamber determines the type of volcanic eruption. Although the two major kinds of eruptions – explosive and effusive - are described in this section, there is an entire continuum of eruption types. Which magma composition do you think leads to each type?

Explosive Eruptions

A large explosive volcanic eruption creates even more devastation than the force of the atom bomb dropped on Nagasaki at the end of World War II in which more than 40,000 people died. A large explosive volcanic eruption is 10,000 times as powerful. Felsic magmas erupt explosively. Hot, gas-rich magma churns within the chamber. The pressure becomes so great that the magma eventually breaks the seal and explodes, just like when a cork is released from a bottle of champagne. Magma, rock, and ash burst upward in an enormous explosion. The erupted material is called tephra (Figure below).
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Ash and gases create a mushroom cloud above Mt. Redoubt in Alaska, 1989. The cloud reached 45,000 feet and caught a Boeing 747 in its plume.
Scorching hot tephra, ash, and gas may speed down the volcano’s slopes at 700 km/h (450 mph) as a pyroclastic flow. Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,000oC (1,800oF) (Figure below.) and (Figure below.).
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An explosive eruption from the Mayon Volcano in the Philippines in 1984. Ash flies upward into the sky and pyroclastic flows pour down the mountainside.
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The end of a pyroclastic flow at Mount St. Helens.
A pyroclastic flow at Montserrat volcano is seen in this video: http://faculty.gg.uwyo.edu/heller/SedMovs/Sed%20Movie%20files/PyroclasticFlow.MOV
Prior to the Mount St. Helens eruption in 1980, the Lassen Peak eruption on May 22, 1915, was the most recent Cascades eruption. A column of ash and gas shot 30,000 feet into the air. This triggered a high-speed pyroclastic flow, which melted snow and created a volcanic mudflow known as a lahar. Lassen Peak currently has geothermal activity and could erupt explosively again. Mt. Shasta, the other active volcano in California, erupts every 600 to 800 years. An eruption would most likely create a large pyroclastic flow, and probably a lahar. Of course, Mt. Shasta could explode and collapse like Mt. Mazama in Oregon (Figure below).
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Crater Lake fills the caldera of the collapsed Mt. Mazama, which erupted with 42 times more power than Mount St. Helens in 1980. The bathymetry of the lake shows volcanic features such as cinder cones.
Volcanic gases can form poisonous and invisible clouds in the atmosphere. These gases may contribute to environmental problems such as acid rain and ozone destruction. Particles of dust and ash may stay in the atmosphere for years, disrupting weather patterns and blocking sunlight (Figure below).
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The ash plume from Eyjafjallaj

Effusive Eruptions

Mafic magma creates gentler effusive eruptions . Although the pressure builds enough for the magma to erupt, it does not erupt with the same explosive force as felsic magma. People can usually be evacuated before an effusive eruption, so they are much less deadly. Magma pushes toward the surface through fissures. Eventually, the magma reaches the surface and erupts through a vent (Figure below).
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In effusive eruptions, lava flows readily, producing rivers of molten rock.
Low-viscosity lava flows down mountainsides. Differences in composition and where the lavas erupt result in three types of lava flow coming from effusive eruptions (Figure below), (Figure below), (Figure below), and (Figure below).
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Mafic lava that erupts underwater creates pillow lava. The lava cools very quickly to roughly spherical rocks. Pillow lava is common at mid-ocean ridges.
Although effusive eruptions rarely kill anyone, they can be destructive. Even when people know that a lava flow is approaching, there is not much anyone can do to stop it from destroying a building or road (Figure below).
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A road is overrun by an eruption at Kilauea volcano in Hawaii.

Predicting Volcanic Eruptions

Volcanologists attempt to forecast volcanic eruptions, but this has proven to be nearly as difficult as predicting an earthquake. Many pieces of evidence can mean that a volcano is about to erupt, but the time and magnitude of the eruption are difficult to pin down. This evidence includes the history of previous volcanic activity, earthquakes, slope deformation, and gas emissions.

History of Volcanic Activity

A volcano’s history -- how long since its last eruption and the time span between its previous eruptions -- is a good first step to predicting eruptions. Which of these categories does the volcano fit into?
  • Active: currently erupting or showing signs of erupting soon.
  • Dormant: no current activity, but has erupted recently (Figure below).
  • Extinct: no activity for some time; will probably not erupt again.
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Mount Vesuvius destroyed Pompeii in 79 AD. Fortunately this volcano is dormant because the region is now much more heavily populated.
Active and dormant volcanoes are heavily monitored, especially in populated areas.


Moving magma shakes the ground, so the number and size of earthquakes increases before an eruption. A volcano that is about to erupt may produce a sequence of earthquakes. Scientists use seismographs that record the length and strength of each earthquake to try to determine if an eruption is imminent.

Slope Deformation

Magma and gas can push the volcano’s slope upward. Most ground deformation is subtle and can only be detected by tiltmeters, which are instruments that measure the angle of the slope of a volcano. But ground swelling may sometimes create huge changes in the shape of a volcano. Mount St. Helens grew a bulge on its north side before its 1980 eruption. Ground swelling may also increase rock falls and landslides.

Gas Emissions

Gases may be able to escape a volcano before magma reaches the surface. Scientists measure gas emissions in vents on or around the volcano. Gases, such as sulfur dioxide (SO2), carbon dioxide (CO2), hydrochloric acid (HCl), and even water vapor can be measured at the site (Figure below) or, in some cases, from a distance using satellites. The amounts of gases and their ratios are calculated to help predict eruptions.
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Scientists monitoring gas emissions at Mount St. Helens.

Remote Monitoring

Some gases can be monitored using satellite technology (Figure below). Satellites also monitor temperature readings and deformation. As technology improves, scientists are better able to detect changes in a volcano accurately and safely.
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An Earth-Observation Satellite before launch.
Since volcanologists are usually uncertain about an eruption, officials may not know whether to require an evacuation. If people are evacuated and the eruption doesn’t happen, the people will be displeased and less likely to evacuate the next time there is a threat of an eruption. The costs of disrupting business are great. However, scientists continue to work to improve the accuracy of their predictions.

Lesson Summary

  • The style of a volcanic eruption depends on magma viscosity.
  • Felsic magmas produce explosive eruptions. Mafic magmas produce effusive eruptions.
  • Explosive eruptions happen along the edges of continents and produce tremendous amounts of material ejected into the air.
  • Non-explosive eruptions produce lavas, such as a'a, pahoehoe, and pillow lavas.
  • Volcanoes are classified as active, dormant, or extinct.
  • Signs that a volcano may soon erupt include earthquakes, surface bulging, and gases emitted, as well as other changes that can be monitored by scientists.

Review Questions

1. What are the two basic types of volcanic eruptions?
2. Several hundred years ago, a volcano erupted near the city of Pompeii, Italy (Figure below). Archaeologists have found the remains of people embracing each other, suffocated by ash and rock that covered everything. What type of eruption must have this been?
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3. What is pyroclastic material?
4. Name three substances that have low viscosity and three that have high viscosity.
5. What is the difference between a magma chamber and a mantle plume?
6. Why might the addition of water make an eruption more explosive?
7. What are three names for non-explosive lava?
8. What factors are considered in predicting volcanic eruptions?
9. Why is predicting a volcanic eruption so important?
10. Given that astronomers are far away from the planets they study, what evidence might they look for to determine the composition of a planet on which a volcano is found?

Further Reading / Supplemental Links


viscosity The thickness of a liquid; its resistance to flow. tephra Fragments of material produced in a volcanic eruption. pyroclastic flow Hot ash, gas, and rock that race down a volcano’s slopes during an eruption. magma chamber A region below a volcano where magma and gases collect. lahar A volcanic mudflow containing ash, rock, and water that races down river valleys. extinct volcano A volcano that has not erupted in recorded history, and is unlikely to erupt again. explosive eruption A potentially devastating eruption of rock, lava, ash, and gas exploding from a volcano. eruption The release of lava, tephra, and gases from a volcano. effusive eruption A relatively gentle, non-explosive volcanic eruption. dormant volcano A volcano that is not currently erupting, but that has erupted in the recent past. active volcano A volcano that is currently erupting or is just about to erupt.

Points to Consider

  • What would you look for to determine if an old eruption was explosive or non-explosive?
  • Given the different styles of eruptions discussed above, what do you think the shapes of volcanoes are?
  • Where do you think the names a’a and pāhoehoe came from?
  • Do earthquakes always indicate an imminent eruption? What factors about an earthquake might indicate a relationship to a volcanic eruption?