Composite Volcano (Stratovolcano): Key Facts and Formation

There are several different types of volcanoes, including shield volcanoes, composite volcanoes, dome volcanoes, and cinder cones. However, if you ask a child to draw a volcano, you'll almost always get a picture of a composite volcano. The reason? Composite volcanoes form the steep-sided cones most often seen in photographs. They are also associated with the most violent, historically important eruptions.

Composition

Composite volcanoes—also called stratovolcanoes—are named for their composition. These volcanoes are built from layers, or strata, of pyroclastic material, including lava, pumice, volcanic ash, and tephra. The layers stack on each other with each eruption. The volcanoes form steep cones, rather than rounded shapes, because the magma is viscous.

Composite volcano magma is felsic, which means it contains silicate-rich minerals rhyolite, andesite, and dacite. Low-viscosity lava from a shield volcano, such as might be found in Hawaii, flows from fissures and spreads. Lava, rocks, and ash from a stratovolcano either flow a short distance from the cone or explosively eject into the air before falling back down toward the source.

Formation

Stratovolcanoes form at subduction zones, where one plate at a tectonic boundary is pushed below another. This may be where the oceanic crust slips below an oceanic plate (near or underneath Japan and the Aleutian Islands, for example) or where the oceanic crust is drawn below the continental crust (underneath the Andes and Cascades mountain ranges).

Water is trapped in porous basalt and minerals. As the plate sinks to greater depths, temperature and pressure rise until a process called "dewatering" occurs. Release of water from hydrates lowers the melting point of rock in the mantle. Melted rock rises because it is less dense than solid rock, becoming magma. As magma ascends, lessening pressure allows volatile compounds to escape from the solution. Water, carbon dioxide, sulfur dioxide, and chlorine gas exert pressure. Finally, the rocky plug over a vent pops open, producing an explosive eruption.

Location

Composite volcanoes tend to occur in chains, with each volcano several kilometers from the next. The "Ring of Fire" in the Pacific Ocean consists of stratovolcanoes. Famous examples of composite volcanoes include Mount Fuji in Japan, Mount Rainier and Mount St. Helens in Washington State, and Mayon Volcano in the Philippines. Notable eruptions include that of Mount Vesuvius in 79, which destroyed Pompeii and Herculaneum, and that of Pinatubo in 1991, which ranks as one of the biggest eruptions of the 20th century.

To date, composite volcanoes have only been found on one other body in the solar system: Mars. Zephyria Tholus on Mars is believed to be an extinct stratovolcano.

Eruptions and Their Consequences

Composite volcano magma isn't fluid enough to flow around obstacles and exit as a river of lava. Instead, a stratovolcanic eruption is sudden and destructive. Superheated toxic gases, ash, and hot debris are forcefully ejected, often with little warning.

Lava bombs present another hazard. These molten chunks of rock may be the size of small stones up to the size of a bus. Most of these "bombs" don't explode, but their mass and velocity cause destruction comparable to that from an explosion. Composite volcanoes also produce lahars. A lahar is a mix of water with volcanic debris. Lahars are basically volcanic landslides down the steep slope, traveling so quickly that they are difficult to escape. Nearly a third of a million people have been killed by volcanoes since 1600. Most of these deaths are attributed to stratovolcanic eruptions.

Death and property damage aren't the only consequences of composite volcanoes. Because they eject matter and gases into the stratosphere, they affect weather and climate. Particulates released by composite volcanoes yield colorful sunrises and sunsets. Although no vehicle accidents have been attributed to volcanic eruptions, the explosive debris from composite volcanoes poses a risk to air traffic.

Sulfur dioxide released into the atmosphere can form sulfuric acid. Sulfuric acid clouds can produce acid rain, plus they block sunlight and cool temperatures. The eruption of Mount Tambora in 1815 produced a cloud that lowered global temperatures 3.5 C (6.3 F), leading to the 1816 "year without a summer" in North America and Europe.

The world's biggest extinction event may have been due, at least in part, to stratovolcanic eruptions. A group of volcanoes named the Siberian Traps released massive amounts of greenhouse gases and ash, starting 300,000 years before the end-Permian mass extinction and concluding half a million years after the event. Researchers now hold the eruptions as the principal cause for the collapse of 70 percent of terrestrial species and 96 percent of marine life.

Sources

• Brož, P. and Hauber, E. "A unique volcanic field in Tharsis, Mars: Pyroclastic cones as evidence for explosive eruptions." Icarus, Academic Press, 8 Dec. 2011. 
• Decker, Robert Wayne and Decker, Barbara (1991). Mountains of Fire: The Nature of Volcanoes. Cambridge University Press. p. 7.
• Miles, M. G., et al. "The significance of volcanic eruption strength and frequency for climate." Quarterly Journal of the Royal Meteorological Society. John Wiley & Sons, Ltd, 29 Dec. 2006.
• Sigurðsson, Haraldur, ed. (1999). Encyclopedia of Volcanoes. Academic Press.
• Grasby, Stephen E., et al. “Catastrophic Dispersion of Coal Fly Ash into Oceans during the Latest Permian Extinction.” Nature News, Nature Publishing Group, 23 Jan. 2011.