Above are two false-color Landsat satellite images of Mount St. Helens and vicinity. The first image is from August 29, 1979. Just months later, in March 1980, the ground began to shake. Red indicates vegetation; patches of lighter color are where the region was logged.
The second image is from September 24, 1980, four months after the large eruption on May 18. The relics of the eruption are everywhere. The mountain’s northern flank has collapsed, leaving a horseshoe shaped crater. Rock and ash have blown over 230 square miles. Dead trees are floating in Spirit Lake and volcanic mudflows clog the rivers. A more recent image shows that vegetation has begun to colonize at the farther reaches of the area affected by the eruption.
Volcanoes are a vibrant manifestation of plate tectonics processes. Volcanoes are common along convergent and divergent plate boundaries. Volcanoes are also found within lithospheric plates away from plate boundaries. Wherever mantle is able to melt, volcanoes may be the result.
World map of active volcanoes.
See if you can give a geological explanation for the locations of all the volcanoes in Figure above. What is the Pacific Ring of Fire? Why are the Hawaiian volcanoes located away from any plate boundaries? What is the cause of the volcanoes along the mid-Atlantic ridge?
Volcanoes erupt because mantle rock melts. This is the first stage in creating a volcano. Remember from the chapter “Rocks” that mantle may melt if temperature rises, pressure lowers, or water is added. Be sure to think about how melting occurs in each of the following volcanic settings.
Convergent Plate Boundaries
Why does melting occur at convergent plate boundaries? The subducting plate heats up as it sinks into the mantle. Also, water is mixed in with the sediments lying on top of the subducting plate. This water lowers the melting point of the mantle material, which increases melting. Volcanoes at convergent plate boundaries are found all along the Pacific Ocean basin, primarily at the edges of the Pacific, Cocos, and Nazca plates. Trenches mark subduction zones, although only the Aleutian Trench and the Java Trench appear on the map above (Figure above).
Remember your plate tectonics knowledge. Large earthquakes are extremely common along convergent plate boundaries. Since the Pacific Ocean is rimmed by convergent and transform boundaries, about 80% of all earthquakes strike around the Pacific Ocean basin (Figure below). Why are 75% of the world’s volcanoes found around the Pacific basin? Of course, these volcanoes are caused by the abundance of convergent plate boundaries around the Pacific.
The Pacific Ring of Fire is where the majority of the volcanic activity on the Earth occurs.
A description of the Pacific Ring of Fire along western North America is a description of the plate boundaries.
- Subduction at the Middle American Trench creates volcanoes in Central America.
- The San Andreas Fault is a transform boundary.
- Subduction of the Juan de Fuca plate beneath the North American plate creates the Cascade volcanoes.
- Subduction of the Pacific plate beneath the North American plate in the north creates the Aleutian Islands volcanoes.
The Cascades are shown on this interactive map with photos and descriptions of each of the volcanoes:http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/interactive/6.Volcanoes4Rollover.swf.
This incredible explosive eruption of Mount Vesuvius in Italy in A.D. 79 is an example of a composite volcano that forms as the result of a convergent plate boundary (3f): http://www.youtube.com/watch?v=1u1Ys4m5zY4&feature=related (1:53).
Divergent plate boundaries
Why does melting occur at divergent plate boundaries? Hot mantle rock rises where the plates are moving apart. This releases pressure on the mantle, which lowers its melting temperature. Lava erupts through long cracks in the ground, or fissures.
Footage of Undersea Volcanic Eruptions is seen in National Geographic Videos, Environment Video, Habitat, Ocean section: http://video.nationalgeographic.com/video/player/environment/.
- Fantastic footage of undersea volcanic eruption is in the “Deepest Ocean Eruption Ever Filmed.”
- “Giant Undersea Volcano Revealed” explores a volcano and its life off of Indonesia.
Volcanoes erupt at mid-ocean ridges, such as the Mid-Atlantic ridge, where seafloor spreading creates new seafloor in the rift valleys. Where a hotspot is located along the ridge, such as at Iceland, volcanoes grow high enough to create islands (Figure below).
A volcanic eruption at Surtsey, a small island near Iceland.
Eruptions are found at divergent plate boundaries as continents break apart. The volcanoes in Figure beloware in the East African Rift between the African and Arabian plates.
Mount Gahinga and Mount Muhabura in the East African Rift valley.
Although most volcanoes are found at convergent or divergent plate boundaries, intraplate volcanoes are found in the middle of a tectonic plate. Why is there melting at these locations? The Hawaiian Islands are the exposed peaks of a great chain of volcanoes that lie on the Pacific plate. These islands are in the middle of the Pacific plate. The youngest island sits directly above a column of hot rock called a mantle plume. As the plume rises through the mantle, pressure is released and mantle melts to create a hotspot (Figure below).
(a) The Society Islands formed above a hotspot that is now beneath Mehetia and two submarine volcanoes. (b) The satellite image shows how the islands become smaller and coral reefs became more developed as the volcanoes move off the hotspot and grow older.
Earth is home to about 50 known hot spots. Most of these are in the oceans because they are better able to penetrate oceanic lithosphere to create volcanoes. The hotspots that are known beneath continents are extremely large, such as Yellowstone (Figure below).
Prominent hotspots of the world.
A hot spot beneath Hawaii, the origin of the voluminous lava produced by the shield volcano Kilauea can be viewed here(3f): http://www.youtube.com/watch?v=byJp5o49IF4&feature=related (2:06).
How would you be able to tell hotspot volcanoes from island arc volcanoes? At island arcs, the volcanoes are all about the same age. By contrast, at hotspots the volcanoes are youngest at one end of the chain and oldest at the other.
- Most volcanoes are found along convergent or divergent plate boundaries.
- The Pacific Ring of Fire is the most geologically active region in the world.
- Volcanoes such as those that form the islands of Hawaii form over hotspots, which are melting zones above mantle plumes.
- Why are there volcanoes along the west coast of the United States?
- Why does melting occur at divergent plate boundaries?
- In Figure above, explain the geologic reason for every group of volcanoes in the diagram.
- How did the Pacific Ring of Fire get its name? Does it deserve it?
- What is a mantle plume?
- Suppose a new volcano suddenly formed in the middle of the United States. How might you explain what caused this volcano?
Points to Consider
- Some volcanoes are no longer active. What could cause a volcano to become extinct?
- Hot spots are still poorly understood by Earth scientists. Why do you think it’s hard to understand hotspots? What clues are there regarding these geological phenomena?
- Volcanoes have been found on Venus, Mars, and even Jupiter’s moon Io. What do you think this indicates to planetary geologists?
- active volcano
- dormant volcano
- effusive eruption
- explosive eruption
- extinct volcano
- magma chamber
- pyroclastic flow
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).
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.
Volcanoes do not always erupt in the same way. 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).
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?
A large explosive 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).
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 apyroclastic 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.).
(a) 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. (b) 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 (Figurebelow).
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).
The ash plume from Eyjafjallaj
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).
In effusive eruptions, lava flows readily, producing rivers of molten rock.
- The Kilauea volcanic eruption in 2008 is seen in this short video: http://www.youtube.com/watch?v=BtH79yxBIJI.
- A Quicktime movie with thermal camera of a lava stream within the vent of a Hawaiian volcano is seen here: http://hvo.wr.usgs.gov/kilauea/update/archive/2009/Nov/OverflightFLIR_13Jan2010.mov.
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).
- Undersea eruption videos are seen here http://news.discovery.com/videos/earth-undersea-eruption-now-in-stereo.html and here http://news.discovery.com/videos/earth-underwater-volcano-caught-on-video.html
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 (Figurebelow).
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.
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.
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.
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.
Scientists monitoring gas emissions at Mount St. Helens.
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.
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.
- 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.
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?
Pompeii the last day.
3. What is pyroclastic material?
4. Name three substances that have low viscosity and three that have high viscosity.
5. Why might the addition of water make an eruption more explosive?
6. What are three names for non-explosive lava?
7. What factors are considered in predicting volcanic eruptions?
8. Why is predicting a volcanic eruption so important?
9. 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
- The top five volcano web cams and video: http://news.discovery.com/videos/earth-top-5-volcano-webcams-and-videos.html
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?