Volcanoes

Introduction

A volcano is a vent in Earth’s crust through which molten rock, gas, or ash flow onto the surface or are injected into the atmosphere. Volcanoes vary in size and violence, with some flowing steadily for centuries and others, termed supervolcanoes, capable of exploding with a force equal to that of thousands of nuclear weapons.

For billions of years, volcanism has reshaped the surface of Earth and several other bodies in the solar system. Eruptions have destructive short-term environmental and human effects, and can change global climate for years, but their long-term effects are not destructive. The history of life on Earth has been altered by periods of intense volcanism that contributed to mass extinctions.

Historical Background and Scientific Foundations

The molten rock or magma that is expelled by most volcanoes is either forced up through cracks from the mantle (the layer of Earth’s structure just below the crust) or, working its way upward through the denser, solid rocks around it, accumulates in pockets within the crust. An underground pocket of magma is termed a magma chamber. When a magma chamber becomes pressurized enough to force some of its contents to the surface, a volcanic eruption occurs. Often, volcanologists can predict a volcanic eruption by measuring the swelling of Earth’s surface above a growing magma chamber. For example, the center of Yellowstone National Park, whose central region resides inside the caldera (crater) of a gigantic volcano 40 mi (70 km) across, has been rising at 3 in (7.5 cm) per year since 2004 as a large magma chamber beneath it swells.

The molten rock expelled by a volcanic eruption—called lava as soon as it emerges—begins as rock in Earth’s mantle, about 40 to 120 mi (70 to 200 km) underground. For material to travel through the crust to the surface and emerge from a volcano, it must be mobile—namely, gas, liquid, or a mixture of both. The heat that melts the rock comes from the decay of radioactive elements over geological time. There is not enough radioactive material inside Earth to make lava significantly more radioactive than surface rock, but inside the planet, heat from radioactivity has nowhere to go, and so accumulates.

There are several kinds of volcano. The most common type is the subduction-zone volcano. A subduction zone is a linear (long, narrow) region where one large, semi-rigid piece of Earth’s rocky crust—termed a plate—is being pushed against another so that its leading edge is forced down into the mantle. At a typical subduction zone, crust forming an ocean floor is forced down into the mantle at an inch or two per year. Some water is carried down with the subducted rock.

Mixing rock with even a small amount of water lowers its melting point significantly, so some of the descending crust turns into magma at a depth where the surrounding, hot, dry rocks of the upper mantle are still solid. These pockets or droplets of liquid rock also contain gases such as water vapor and carbon dioxide. Being less dense than the surrounding solid rock, they migrate upward through the crust. These ascending masses of magma are termed plutonic diapirs. Most do not reach the surface, but cool and solidify while still deep within the crust, forming large masses of igneous rock. If a diapir gets close enough to the surface, however, it may form a magma chamber and be joined by other diapirs, becoming pressurized. This pressure may build until a narrow channel to the surface is forced open or the overlying rock is blown away in a massive explosion. Volcanic eruptions of magmas containing water tend to be violent: Without steam (hot water vapor) or large amounts of some other gas, magma does not explode, but flows.

Because subduction zones more or less surround the edges of the Pacific Ocean, volcanoes are common along much of the ocean’s rim. This area is often termed the Ring of Fire.

Another kind of volcano is the shield volcano. This type includes the largest volcanoes on Earth and in the solar system: Earth’s largest volcano, Mauna Loa in Hawaii, is a shield volcano, as is Olympus Mons on Mars. A shield volcano is formed by hotspot volcanism, a “hotspot” being a place in the mantle where an upwelling of liquid rock persists for geologic time (millions of years). A runny form of lava is usually produced by these volcanoes, causing the resulting mountains to have a wide, round, spread-out shape reminiscent of an ancient hand-carried shield.

As an oceanic or continental plate moves slowly above a hotspot, a series of volcanoes may be formed above it as the hotspot burns a series of holes in the moving plate. A live volcano will usually be found right above the hotspot and inactive (extinct) volcanoes trailing away from it in the direction of the plate’s motion. The Hawaiian Island chain has been formed by this process. On Mars, which does not have moving plates, the now-extinct Olympus Mons volcano sat motionless upon its hot spot for millions of years and grew to be the largest mountain in the solar system. Shield volcanoes, because their lava is so runny, tend not to become plugged up and build gas pressure, so they erupt steadily and calmly, rather than explosively. However, the Yellowstone super-volcano is a hotspot volcano that erupts explosively due to the presence of steam in its magma. Yellowstone explodes every 20,000 years or so but has undergone three especially large explosions, the most recent of which, about 640,000 years ago, spread ash over most of North America.

Impacts and Issues

Violent venting or outright explosion pulverizes rock around the volcanic vent, turning it to a powder termed ash. This ash, lofted into the atmosphere, can settle around a volcano and be carried far downwind from it, smothering animals and vegetation. Volcanic explosions can cause great damage by direct force: in the vicinity of Mt. St. Helens, a volcano in the state of Washington that erupted in 1980, the initial explosion killed 62 people and destroyed about 150 square mi (390 square km) of surrounding forests, a loss to the timber industry of about $1 billion.

Volcanoes also emit gases, including carbon dioxide, water vapor, sulfur dioxide, and hydrogen sulfide. Over billions of years, these emissions have helped form Earth’s atmosphere. Hot gases mixed with pulverized

WORDS TO KNOW

CALDERA: Volcanic crater that has collapsed to form a circular depression greater than 1 mi (1.6 km) in diameter.

MANTLE: The thick, dense layer of rock that underlies Earth’s crust and overlies the core.

rock can sweep down the slopes of a volcano at many miles an hour, often traveling far from the caldera: This is called a pyroclastic flow, and can be one of the most dangerous volcanic events. On the Caribbean island of Martinique, a pyroclastic flow from the volcano Mt. Pelee completely wiped out the town of Saint-Pierre in 1902, killing about 30,000 people. About 640,000 years ago, a pyroclastic eruption of the Yellowstone volcano blanketed the surrounding landscape with 240 cubic mi (1,000 cubic km) of ash. Another hazard from volcanic eruptions is the sudden melting of snow and ice or the mixing of heavy rain with ash, which can produce floods or lahars (hot mudflows).

Over the middle term—several years, as opposed to the hours or days of the eruption itself—individual volcanoes can affect the climate of the entire world. The eruption of Mt. Pinatubo in the Philippines in 1991 ejected about 20 million tons of sulfur dioxide aerosol particles into the upper atmosphere. These bright particles reflect sunlight into space and infrared light back toward Earth, tending to make summers cooler and winters warmer. In the Northern Hemisphere, summer was about 3.6°F (2°C) cooler the year after the Pinatubo eruption and winter was about 5.4°F (3°C) warmer. The main effect of bright aerosols such as sulfur dioxide, as predicted by theory and observed following the Pinatubo eruption, is to cool climate. After a few years, the climatic effect of such an eruption fades away.

Particularly massive and prolonged volcanic eruptions can alter the history of life, wiping out thousands of species. In the largest mass extinction in Earth’s history, the Permian-Triassic extinction event about 251 million years ago, about 70% of all land-dwelling plants and animals and 96% of all sea-dwelling creatures died. Geologists state that the extinction coincided and may have been caused by the most massive volcanic eruption in Earth’s history, the Siberian Traps, which covered tens of thousands of square miles with lava and darkened the skies of the world with ash. Several other mass extinctions have coincided with large volcanic eruptions, as well as with strikes by asteroids. The extinction of the dinosaurs was probably caused by an asteroid impact, not volcanism.

IN CONTEXT: ACTIVE AFRICA

Given the intense activity and frequent eruptions related to the Pacific rim “Ring of Fire,” most people do not think of Africa as volcanically active. Yet much of the African landscape and rich mineral deposits are related to volcanic and tectonic activity. Both Kilimanjaro and Africa’s second highest peak, Mount Kenya (17,058 ft; 5,117 m) sitting astride the equator, are actually composite volcanos, part of the vast volcanic field associated with the East African rift valley.

The most distinctive and dramatic geological feature in Africa is undoubtedly the East African rift system. The rift opened up in the Tertiary period, approximately 65 million years ago, shortly after the dinosaurs became extinct. Seismically, the rift valley is very much alive. Lava flows and volcanic eruptions occur about once a decade in the Virunga Mountains north of Lake Kivu along the western stretch of the rift valley. One volcano in the Virunga area in eastern Zaire that borders Rwanda and Uganda actually dammed a portion of the valley formerly drained by a tributary of the Nile River, forming Lake Kivu as a result.

Despite volcanoes’ destructive effects, the long-term environmental results of volcanic eruptions are usually beneficial. Forests quickly re-colonize ash-covered ground and lava flows, especially in tropical regions. Lava and ash are rich in minerals, and when weathered produce rich soils that are excellent for farming.

See Also Climate Change; Earthquakes; Extinction and Extirpation; Tsunami Impacts

BIBLIOGRAPHY

Books

de Boer, Jelle Zeilinga Volcanoes in Human History: The Far-Reaching Effects of Major Eruptions. Princeton, NJ: Princeton University Press, 2004.

Schmincke, Hans-Ulrich. Volcanism. New York: Springer, 2004.

Solomon, Susan, et al, eds. Climate Change 2007: Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Kerr, Richard A. “Did Volcanoes Drive Ancient Extinctions?” Science 289 (2000): 1130–1131.

McEwen, Alfred S. “Active Volcanism on Io.” Science 297 (2002): 2220—2221.

Robock, Alan. “Pinatubo Eruption: The Climatic Aftermath” Science 295 (2002): 1242–1244.

Web Sites

U.S. Geological Survey. “Volcano Hazards Program.” http://volcanoes.usgs.gov/ (accessed May 12, 2008).

Larry Gilman