Volcanic Eruptions

In 1980, Mount St. Helens erupted in one of the most deadly and costly volcanic eruptions in the United States ever. The eruption was particularly deadly since Mount St. Helens, one of the Cascade Range, is in a populated area between Portland, Oregon and Seattle, Washington. The eruption killed 57 people, destroyed 250 homes, and swept away 47 bridges. The elevation of the volcano dropped by over 400 meters (1,300 feet) because of the immense explosion created by the eruption. Today Mt. St. Helens is still active (Figure 8.8). The volcano now has a horseshoe-shaped crater that is 1.5 km (nearly one mile) across. Within the crater, a new lava dome has formed. How did this eruption occur? Why aren’t all volcanoes explosive like Mt. St. Helens? Why did so many people perish if we knew that it was going to erupt? The study of volcanoes has many questions still unanswered. However, scientists have studied volcanoes for many years and are piecing together evidence that explains these powerful geologic phenomena.

Figure 8.8: Mount St. Helens, Washington, two years after its eruption.

Lesson Objectives

  • Explain how volcanoes erupt.
  • Describe and compare the types of volcanic eruptions.
  • Distinguish between different types of lava and understand the difference between magma and lava.
  • Describe a method for predicting volcanic eruptions.

How Volcanoes Erupt

All volcanoes share the same basic features. The magma collects in magma chambers that can be 160 kilometers (100 miles) beneath the surface. As the rock heats, it expands, which creates even more pressure. As a result, the magma seeks a way out pushing toward the surface, the magma seeps through cracks in the Earth’s crust called vents. Eventually, the magma reaches the surface; when it comes out, we call it an eruption. The word eruption is used in other contexts, as well. An eruption can be an outburst or explosion, a violent and sudden occurrence, like when a crowd erupts in anger. But an eruption can also be a spreading of something like a rash on your skin, gradual and relatively calm. These two definitions are similar to the two kinds of eruptions that we see in volcanoes.

Types of Eruptions

Every geological formation is unique. Their composition and construction depend on so many factors, that it would be impossible for two formations to be exactly alike. In the same way, each volcano and its eruptions are unique. However, we tend to see two major kinds of eruptions. We talked about eruption to mean both a violent explosion or a sort of silent spreading. These are the two types of volcanic eruptions that we see–explosive and non-explosive eruptions. When we think of volcanic eruptions, we often think of huge clouds of volcanic ash ejected high into the atmosphere and then thick rivers of red lava snaking down the mountainside. In reality, these two phenomena rarely occur in the same volcano. Volcanic eruptions tend to be one or the other.

Explosive Eruptions

Imagine the devastation and force caused by the atom bomb dropped on Nagasaki at the end of World War II in which over 40,000 people died. Now imagine an explosion 10,000 times as powerful. Explosive volcanic eruptions can be that powerful (Figure 8.9). As hot magma beneath the surface interacts with water, gases accumulate and the magma pressure builds up. This pressure grows and grows until these dissolved gases cause it to burst in an enormous explosion.

Figure 8.9: An explosive eruption from the Mayon Volcano in the Philippines in 1984.

This great explosion takes with it the magma and volcanic gases, which can shoot many kilometers into the sky and forms a mushroom cloud, similar to that formed by a nuclear explosion (Figure 8.10). The debris travels up into the air at very high speeds and cools in the atmosphere to form solid particles called pyroclasts. Some of these particles can stay in the atmosphere for years, which can disrupt weather patterns and affect the temperature of the Earth. The rest of the debris comes falling back to Earth where it rains down for kilometers and kilometers around.

Figure 8.10: Explosive eruption of Mt. Redoubt in Alaska, 1989. This huge mushroom cloud reached 45,000 feet and caught a Boeing 747 in its plume.

Sometimes secondary explosions occur that are even greater than the first. Additionally, volcanic gases like water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide, and hydrogen chloride can form poisonous and invisible clouds that roam about the atmosphere. These gases contribute to environmental problems like acid rain and ozone destruction, and can actually cool the Earth’s atmosphere.

In the Cascade Range, the explosive eruption of Mount St. Helens was preceded by the eruption of Lassen Peak, one of the three Cascade Volcanoes in northern California. On May 22, 1915, an explosive eruption sent a column of ash and gas 30,000 feet into the air and triggered a high-speed pyroclastic flow, which melted snow and created a lahar. Lassen continues to have geothermal activity and could erupt explosively again. Mt. Shasta erupts every 600 to 800 years. An eruption would most likely to create a large pyroclastic flow, and perhaps a lahar. However, the volcano could explode like Mt. Mazama, which blew itself in an eruption about 42 times more powerful than Mount St. Helens in 1980, to create Crater Lake.

Non-explosive Eruptions

A second type of volcanic eruption is a non-explosive or effusive eruption (Figure 8.11). Because the composition of magma is different in different volcanoes, the properties of the lava are different. In effusive eruptions, lava flows are relatively calm and do not explode out of the volcano. As a result, people generally have a great deal of warning before lava reaches them, so non-explosive eruptions are much less deadly. That does not keep them from being destructive, however. Even when we know that a lava flow is approaching, there are few ways of stopping it, given the huge quantity and temperature of lava.

Figure 8.11: In effusive eruptions, lava flows more readily, producing rivers of molten rock.

Magma and Lava

Volcanoes wouldn’t be nearly as interesting without the great explosions they create and the glowing red rivers of lava. All igneous rock comes from magma or lava. The next time you go hiking near a volcanic zone, you might try to identify the types of lava that the volcano erupted, based on the types of igneous rocks you find.

Figure 8.12: When lava flows readily, pressure does not build up so great explosions do not occur.


Deep beneath the Earth, magma forms as the first stage in creating a volcano. This occurs because rock below the surface is subjected to great amounts of pressure from gravity. The decay of radioactive materials generates additional heat. The substantial heat and pressure melt the rock below the surface to form a taffy-like substance. You may have seen a candle that has been left out in the hot sun too long. It becomes softer and more like a liquid. As the molecules absorb heat, they begin to slide past one another becoming more fluid. A similar process occurs with magma. However, different substances melt at different temperatures. For that reason, the temperature at which rocks melt depends on the specific types of rocks. The Earth’s crust and mantle are made of many substances so the temperature required to create magma varies. Most magmas are formed between 600°C and 1300°C (Figure 8.13).

Figure 8.13: Cutaway of the Earth. The melting of rock in the crust and upper mantle create magma.

Melted rock or magma can be found in magma chambers beneath the Earth. Since the magma chambers are so far beneath the Earth’s surface, it is difficult for scientists to study them. Scientists know that magma chambers are created where the heat and pressure are greatest. When tectonic plates collide and rub against each other, magma is formed there. That is how the Pacific Ring of Fire was created. We also know there are volcanoes far away from plate boundaries, so we know there are magma chambers in these areas as well. Magma chambers can be found where there are mantle plumes or hot spots.

Just how or why these hot spots are created isn’t exactly known. However, because different substances melt at different temperatures, the creation of magma depends on what substances make it up—its composition. Just like the flavor of a cake depends on the ingredients that you put in it, the behavior of magma and lava depends on its composition. Certain melted rocks act in certain ways. So when the magma becomes lava, not all lava acts the same.


Figure 8.14: Honey flows slowly; it is more viscous than water.

Once magma reaches the surface, it becomes lava. Consider different liquids that you might see in your house—honey and a bottle of cola, for example. You might agree that the two liquids are different in many regards. They taste different, have different colors, have different gases in them, and they flow differently. In fact, honey is a liquid that resists flowing, whereas cola flows easily. Honey has a higher viscosity than the cola; it resists flowing (Figure 8.14). Cola has a low viscosity because it flows easily. One of the major differences in different types of lava is their viscosity.

A highly viscous lava is one that doesn’t tend to flow easily. It tends to stay in place. Lavas with high silica contents tend to be more viscous. Since it is so resistant to moving, it clogs the vents in a volcano. The pressure becomes greater and greater until the volcano finally explodes. This type of lava is found in explosive eruptions. It also tends to trap a lot of gas. When the gas is released, it makes the eruption more explosive. Most of this lava is shot up into the air where it hardens and becomes solid rock. This molten rock that solidifies in the air is known as pyroclastic material. In an igneous rock like pumice, small holes in the solid rock show where gas bubbles were when the rock was still liquid lava.

Low-viscosity lava slides or flows down mountainsides. There is more than one type of low-viscosity lava. The differences between them come from the lavas’ different composition and different spots where they come to the surface. The type of igneous formations formed depends on which type of lava it is. The three major categories are a’a, pahoehoe, and pillow lava.

A’a Lava

A’a lava is the more viscous of the non-explosive lavas (Figure 8.15). This lava forms a thick and brittle crust which is torn into rough and jagged pieces. The solidified surface is jagged and sharp. It can spread over large areas as the lava continues to flow underneath.

Figure 8.15: A’a lava flow.

Pāhoehoe Lava

Pāhoehoe lava is less viscous than a’a lava, and flows more readily. Its surface looks more wrinkly and smooth than the jagged a’a lava. Pāhoehoe lava flows in a series of lobes or rounded areas that form strange twisted shapes and natural rock sculptures (Figure 8.16). Pāhoehoe lava can also form lava tubes beneath the ground (Figure 8.17).

Figure 8.16: Pāhoehoe lava.

Figure 8.17: The Thurston Lava Tube in Hawaii Volcanoes National Park.

Pillow Lava

Pillow lava is lava that comes out from volcanic vents underwater (Figure 8.18). When it comes out underwater, it cools down very quickly and forms roughly spherical rocks that resemble pillows, from which more lava leaks and creates more pillows. Pillow lava is particularly common along underwater spreading centers.

Figure 8.18: Pillow lava.

Predicting Volcanic Eruptions

Volcanic eruptions can be devastating, particularly to the people who are closer to volcanoes. As meteorologists attempt to predict, or forecast, hurricanes and tornadoes, so too do vulcanologist attempt to forecast volcanic eruptions. Although predicting volcanic eruptions is far from perfect, many pieces of evidence can indicate that a volcano is about to erupt. Some of those factors are hard to measure, contributing to the difficulty in predicting eruptions.

History of Volcanic Activities

One important factor in predicting eruptions is a volcano’s history. That is, we consider how long since it has erupted and the time span between its previous eruptions. Volcanoes are categorized into three subdivisions—active, dormant, and extinct. An active volcano is one that is currently erupting or shows signs of erupting in the near future. A dormant volcano no longer shows signs of activity, but has erupted in recent history (Figure 8.19). Finally, an extinct volcano is one that has not erupted in recent history and will probably not erupt again in the future. Both active and dormant volcanoes are heavily monitored because even dormant volcanoes could suddenly show signs of activity.

Figure 8.19: Vesuvius is a dormant volcano near the city of Naples. Although it shows no current signs of eruption, it could one day become active.


As magma beneath a volcano pushes upward, it shakes the ground and causes earthquakes. Although earthquakes probably occur every day near a volcano, the quantity and size of the earthquakes increases before an eruption. In fact, a volcano that is about to erupt may produce a continuous string of earthquakes, as magma moving underground creates stress on the neighboring rocks. In order to measure these earthquakes, scientists use seismographs that record the length and strength of each earthquake.

Slope Deformation

All that magma and gas pushing upwards can make the ground or the volcano’s slope begin to swell. Sometimes, ground swelling reveals huge changes in the shape of a volcano. Most cases of ground deformation are subtle, though, and can only be detected by tiltmeters, which are instruments that measure the angle of the slope of a volcano. Additionally, ground swelling may cause increased rock falls and landslides.

Gas Emissions

Oftentimes, gases are able to escape a volcano before magma reaches the surface in an eruption. So, scientists can measure gas output, or gas emissions, in vents on or around the volcano. Gases, like sulfur dioxide (SO2), carbon dioxide (CO2), hydrochloric acid (HCl) and even water vapor can be measured at the site or, in some cases, at a distance with satellites. The amounts of gases and their ratios are calculated to help predict eruptions.

Remote Monitoring

As mentioned, some gases can be monitored using satellite technology (Figure 8.20). Satellites are able to measure other factors, too, like temperature readings of particularly warm spots at a volcano site or areas where the volcano surface is changing. As our technology continues to improve, scientists are better able to detect changes accurately and safely.

Figure 8.20: An Earth-observation satellite before launch.

Although monitoring methods are getting better and better, it is still difficult to predict a volcanic eruption with certainty. No scientist or government agency wants to be considered alarmist by announcing that an eruption is going to occur and then it really doesn’t. The cost and disruption to society of a large-scale evacuation would leave many people displeased and the scientists embarrassed. However, the possibility of saving lives and property most certainly makes the pursuit of eruption prediction a worthy cause.

Lesson Summary

  • Volcanoes are produced when magma rises towards the Earth’s surface because it is less dense than the surrounding rock.
  • Volcanic eruptions can be non-explosive or explosive depending on the viscosity of the magma.
  • Explosive type eruptions happen along the edges of continents and produce tremendous amounts of material ejected into the air.
  • Non-explosive type eruptions mostly produce various types of lava, such as a’a, pāhoehoe and pillow lavas.
  • Some signs that a volcano may soon erupt include earthquakes, surface bulging, 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. 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?
  3. What is pyroclastic material?
  4. Name three liquids that have low viscosity and three that have high viscosity.
  5. What is the difference between a magma chamber and a mantle plume?
  6. The boiling point of water is 100°C. Why might 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 volcanoes so important?
  10. Given that astronomers are far away from the subjects they study, what evidence might they look for to determine the composition of a planet on which a volcano is found?


active volcano
A volcano that is currently erupting or just about to erupt.
dormant volcano
A volcano that is not currently erupting, but that has erupted in the recorded past.
effusive eruption
A relatively gentle, non-explosive volcanic eruption.
The release of magma onto the Earth’s surface. Usually an eruption is accompanied by the release of gases as well.
explosive eruption
A volcanic eruption that releases large amounts of gas, so that magma is violently thrown up into the air.
extinct volcano
A volcano that has not erupted in recorded history, and is considered unlikely to erupt again.
magma chamber
A region within Earth surrounded by solid rock and containing magma.
A rock made up of fragments of volcanic rock thrown into the air by volcanic eruptions.
The “thickness” or “stickiness” of a liquid. The more viscous a liquid is, the harder it will be for the liquid to flow.

Points to Consider

  • What types of evidence do you think would tell scientists whether an ancient volcanic eruption was explosive or non-explosive?
  • Are all volcanoes shaped like tall mountains with a crater on the peak?
  • What do you think is the origin of the names A’a and Pāhoehoe?
  • Earthquakes do not always indicate that a volcano is going to erupt. What factors about an earthquake might indicate a relationship to a volcanic eruption?