Volcanoes from The New Book of Knowledge®

While millions of years are needed for natural forces to build mountain ranges and erode canyons, an erupting volcano can build or destroy large, sometimes massive, structures in a matter of minutes or days. A volcano is a place where molten (melted) rock, pieces of hot solid rock, and hot gases formed inside the Earth erupt through its surface. Volcanic activity is the most dramatic and rapid of all the geologic processes that shape the surface of the Earth. For example, on May 18, 1980, the eruption of Mount St. Helens, in the northwestern United States, destroyed much of the mountain and killed about sixty people. How Volcanoes are Formed

The Earth's interior is very hot, and the heat left over from its formation continually escapes toward the surface. The escape of this heat combined with the heat released by decaying radioactive elements in the interior provides the energy for volcanic activity. As the heat rises, it warms rock in the Earth's interior, and this heated rock begins rising slowly toward the surface. Much of this molten rock, or magma, gradually cools within the Earth. Sometimes, however, magma continues to push upward.

While heat is what causes magma to rise to the surface, escaping gases provide the driving force for volcanic eruptions. The most common volcanic gas is steam. This is formed when water from within the Earth or rainwater soaking into the ground is absorbed by magma and then heated.

When gas-filled magma rises from the Earth's interior, it collects in large pools called magma chambers that are several miles below the surface. As the amount of gases increases, the magma becomes lighter, causing it to rise through a system of conduits, or channels. When the magma reaches the surface, it spews out of vents, or openings, at the tops of the magma conduits. Magma that erupts from a vent is known as lava. Lava and other material, such as ash and rock fragments, ultimately build up to form a volcanic cone. Types of Volcanoes

Some volcanoes are large mountains with summits, or peaks, reaching 20,000 feet (6,000 meters) or more above sea level. Others are small domes of ash or lava that on average are only about 100 feet (30 meters) high. Scientists recognize four main types of volcanoes: stratovolcanoes, shield volcanoes, cinder cones, and ashflow calderas. Ash Flows and Mudflows

In large volcanic eruptions, huge flows of hot ash, pumice, and gases mixed together can be propelled outward by hurricane-force winds. Known as ash flows, they can move along the ground at hundreds of miles per hour, sweeping down slopes, through valleys, and over hills. Sometimes the flows glow red hot and are known as glowing avalanches. Ash flow eruptions are among the most dangerous types of volcanic activity. The combination of high temperatures, high speeds, and gases makes them very destructive and deadly.

The summits of some volcanoes are covered with snow and ice. Other volcanoes are located in rainy regions. When these erupt, they sometimes produce large mudflows composed of pyroclastic material and water. Mudflows can be either hot or cold. Hot mudflows generally form when heat from an eruption melts snow or ice and the water mixes with volcanic debris. Cold mudflows can occur when heavy rain loosens cold cinder and ash, sending it racing down the mountainside.

Heat and Gas

Heat and gas are two important products of a volcanic eruption. Enormous amounts of heat are released during a volcano's lifetime. Typically, most of the heat escapes into the air. However, in some volcanic regions, much of the heat remains in the ground. The captive heat produces a natural form of energy known as geothermal energy. These geothermal regions often contain hot springs and geysers, natural springs that erupt at intervals, throwing up fountains of hot water and steam. Gas also rushes out of a volcano's vents during an eruption, often carrying ash and fragments of rock. Most of the gas produced during an eruption is steam and carbon dioxide. Much of the water now on the surface of the Earth is the product of steam given off during 4.5 billion years of volcanic activity. Another common gas produced during eruptions is sulfur dioxide, which has a strong, suffocating odor. Some gases produced, such as chlorine and fluorine, can be very deadly.

Where Volcanoes Occur

Volcanoes have always been a part of Earth's long history. Volcanoes or the evidence of volcanoes has been located almost everywhere on the Earth's surface. Ancient lava flows have been found in many places where there are no volcanoes today, and most of the deep ocean floors are made up of thick layers of lava underneath much thinner layers of sediment. Today, however, volcanoes do not occur everywhere. Most are located near the edges of continents.

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