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disaster alert systems This section of our technical library presents articles written about Emergency Alert Systems and Disaster Recovery definitions, terms and related information.

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Volcano is an opening in the earth's surface through which lava, hot gases, and rock fragments erupt (burst forth). Such an opening forms when melted rock from deep within the earth blasts through the surface. Most volcanoes are mountains, particularly cone-shaped ones, which were built up around the opening by lava and other materials thrown out during eruptions.

Eruptions of volcanic mountains are spectacular sights. In some eruptions, huge fiery clouds rise over the mountain, and glowing rivers of lava flow down its sides. In other eruptions, red-hot ash and cinders shoot out the mountaintop, and large chunks of hot rock are blasted high into the air. A few eruptions are so violent they blow the mountain apart.

Some eruptions occur on volcanic islands. Such islands are the tops of volcanic mountains that have been built up from the ocean floor by repeated eruptions. Other eruptions occur along narrow cracks in the ocean floor. In such eruptions, lava flows away from the cracks, building up the sea bottom.

People have always been both fascinated by the spectacle of volcanic eruptions and terrified of their power. Eruptions have caused some of the worst disasters in history, wiping out entire towns and killing thousands of people. In early times, volcanoes played a role in the religious life of some peoples. The word volcano, for example, comes from Vulcan, the name the ancient Romans gave to their god of fire. The Romans believed the god lived beneath a volcanic island off the Italian coast. They called the island Vulcano.

How a volcano is formed

Powerful forces within the earth cause volcanoes. Scientists do not fully understand these forces. But they have developed theories on how the forces create volcanoes. This section describes how most scientists explain the beginning and eruption of a volcano.

The beginning of a volcano. A volcano begins as magma, melted rock inside the earth. Magma results from the extreme heat of the earth's interior. At certain depths, the heat is so great it partly melts the rock inside the earth. When the rock melts, it produces much gas, which becomes mixed with the magma. Most magma forms 37 to 100 miles (60 to 160 kilometers) beneath the surface. Some develops at depths of 15 to 30 miles (24 to 48 kilometers).

The gas-filled magma gradually rises toward the earth's surface because it is lighter than the solid rock around it. As the magma rises, it melts gaps in the surrounding rock and forms a large chamber as close as 2 miles (3 kilometers) to the surface. This magma chamber is the reservoir from which volcanic materials erupt.

How a volcano erupts

The eruption of a volcano. The gas-filled magma in the reservoir is under great pressure from the weight of the solid rock around it. This pressure causes the magma to blast or melt a conduit (channel) in a fractured or weakened part of the rock. The magma moves up through the conduit to the surface. When the magma nears the surface, the gas in the magma is released. The gas and magma blast out an opening called the central vent. Most magma and other volcanic materials then erupt through this vent. The materials gradually pile up around the vent, forming a volcanic mountain, or volcano. After the eruption stops, a bowllike crater generally forms at the top of the volcano. The vent lies at the bottom of the crater.

Once a volcano has formed, not all the magma from later eruptions reaches the surface through the central vent. As the magma rises, some of it may break through the conduit wall and branch out into smaller channels. The magma in these channels may escape through a vent formed in the side of the volcano. Or it may remain below the surface.

Kinds of volcanic materials

Three basic kinds of materials may erupt from a volcano. They are (1) lava, (2) rock fragments, and (3) gas. The material that erupts depends chiefly on how sticky or fluid a volcano's magma is.

Lava is the name for magma that has escaped onto the earth's surface. When lava comes to the surface, it is red hot and may have a temperature of more than 2012 °F. (1100° C). Highly fluid lava flows rapidly down a volcano's slopes. Sticky lava flows more slowly. As the lava cools, it hardens into many different formations. Highly fluid lava hardens into smooth, folded sheets of rock called pahoehoe «pah HOH ee HOH ee». Stickier lava cools into rough, jagged sheets of rock called aa «AH ah». Pahoehoe and aa cover large areas of Hawaii, where the terms originated. The stickiest lava forms flows of boulders and rubble called block flows. It may also form mounds of lava called domes.

Other lava formations include spatter cones and lava tubes. Spatter cones are steep hills up to 100 feet (30 meters) high. They build up from the spatter of fountainlike eruptions of thick lava. Lava tubes are tunnels formed from fluid lava. As the lava flows, its outer surface cools and hardens. But the lava underneath continues to flow. After the flowing lava drains away, it leaves a tunnel.

Rock fragments, generally called tephra «TEHF ruh», are formed from sticky magma. Such magma is so sticky that its gas cannot easily escape when the magma approaches the surface or central vent. Finally, the trapped gas builds up so much pressure that it blasts the magma into fragments. Tephra includes, from smallest to largest, volcanic dust, volcanic ash, and volcanic bombs.

Volcanic dust consists of particles less than 0.0625 millimeter (1/400 inch) in diameter. Volcanic dust can be carried great distances. In 1883, the eruption of Krakatau in Indonesia shot dust 17 miles (27 kilometers) into the air. The dust was carried around the earth several times and produced brilliant red sunsets in many parts of the world. Some scientists believe that large quantities of volcanic dust can affect the climate by reducing the amount of sunlight that reaches the earth.

Volcanic ash is made up of fragments less than 1/5 inch (0.5 centimeter) in diameter. Most volcanic ash falls to the surface and becomes welded together as rock called volcanic tuff. Sometimes, volcanic ash combines with water in a stream and forms a boiling mudflow. Mudflows may reach speeds of 60 miles (97 kilometers) per hour and can be highly destructive.

Volcanic bombs are large fragments. Most of them range from the size of a baseball to that of a basketball. The largest bombs may measure more than 4 feet (1.2 meters) across and weigh up to 100 tons (91 metric tons). Small volcanic bombs are generally called cinders.

Gas pours out of volcanoes in large quantities during most eruptions. The gas is made up chiefly of steam. But it includes carbon dioxide, nitrogen, sulfur dioxide, and other gases. Most of the steam comes from a volcano's magma. But some may also be produced when rising magma heats water in the ground. Volcanic gas carries a large amount of volcanic dust. This combination of gas and dust looks like black smoke.

Kinds of volcanoes

Scientists divide volcanoes into three main groups: (1) shield volcanoes, (2) cinder cones, and (3) composite volcanoes. These groups are based on the shape of the volcanoes and the type of material they are built of.

Shield volcanoes are formed when a large amount of free-flowing lava spills from a vent and spreads widely. The lava gradually builds up a low, broad, dome-shaped mountain. The famous Mauna Loa in Hawaii is a shield volcano. Thousands of separate, overlapping lava flows, each less than 50 feet (15 meters) thick, formed Mauna Loa.

Cinder cones build up when mostly tephra erupts from a vent and falls back to earth around the vent. The accumulated tephra, which is generally cinders, forms a cone-shaped mountain. Paricutin in western Mexico is a well-known cinder cone. It began in 1943, when a crack opened in the ground of a cornfield. When the eruptions ended in 1952, the top of the cone was 1,345 feet (410 meters) above its base.

Mount St. Helens

Composite volcanoes are formed when both lava and tephra erupt from a central vent. The materials pile up in alternate layers around the vent and form a towering, cone-shaped mountain. Composite volcanoes include Japan's beautiful Mount Fuji; Mayon Volcano in the Philippines; and Italy's Vesuvius. In A.D. 79, Vesuvius erupted, burying the nearby towns of Pompeii, Herculaneum, and Stabiae under a mass of ashes, dust, and cinders. Mount St. Helens, which erupted frequently between 1980 and 1986, is one of the most active composite volcanoes in the United States.

Occasionally, the magma chamber of a shield volcano, cinder cone, or composite volcano may become nearly empty. This happens when most of a volcano's magma erupts onto the surface. Because the chamber is empty, it can no longer support the volcano above. As a result, a large part of the volcano collapses, forming a huge crater called a caldera. Scenic Crater Lake in Oregon is a caldera that has filled with water. It is about 6 miles (10 kilometers) across at its widest point and 1,932 feet (589 meters) deep.

Why volcanoes occur in certain places

Most volcanoes are found along a belt, called the Ring of Fire, that encircles the Pacific Ocean. Volcanic activity also occurs in such places as Hawaii, Iceland, and southern Europe and at the bottom of the sea.

Scientists have developed a theory, called plate tectonics, that explains why most volcanoes&emdash;as well as most earthquakes and mountains&emdash;occur only in certain places. According to this theory, the earth's outer shell is divided into a number of rigid sections of rock, called plates. The plates slide or drift about continuously over a layer of partly melted rock. Relative movement at the boundary between two plates is generally about 1/2 to 4 inches (1 to 10 centimeters) a year. As the plates move, their boundaries collide, spread apart, or slide past one another. Most volcanoes occur at the plate boundaries. The map Where volcanoes occur in the print version of World Book shows the plate boundaries and the volcanic activity along them.

Most volcanoes are formed where two plates collide. One of the plates is then forced under the other. As the plate sinks, friction and the earth's heat cause part of it to melt. This melted part then rises as magma. When it reaches the surface, it produces a volcano.

Volcanic activity also occurs when two plates spread apart. Most such movement takes place on the ocean floor. As the plates move apart, magma below the crust moves up between the plates. Large amounts of lava pour onto the surface and build up the ocean floor. Magma sometimes creates an underwater mountain range, such as the huge Mid-Atlantic Ridge that runs down the length of the Atlantic Ocean. Iceland and the volcanic islands nearby are exposed parts of this ridge.

A number of volcanoes - for example, those in Hawaii - lie far from plate boundaries. Some scientists believe such volcanoes develop when a huge column of magma rises from inside the earth toward the surface. This column, called a mantle plume, may measure about 100 miles (160 kilometers) in diameter and rise 5 to 10 inches (13 to 25 centimeters) yearly. In some cases, the plume comes close enough to the surface so that part of the magma breaks through and forms a volcano.

For additional information on the plate tectonics theory, see the article Plate tectonics.

The study of volcanoes

The scientific study of volcanoes is called volcanology. It includes investigating the nature and causes of eruptions and has saved many lives. To aid them in their work, scientists have set up observatories on the slopes or rim of several volcanoes, including Mount Asama in Japan, Kilauea in Hawaii, and Vesuvius in Italy.

Classifying volcanic activity. Scientists classify the activity of a volcano according to how often it erupts. A volcano may thus be classed as (1) active, (2) intermittent, (3) dormant, or (4) extinct.

Active volcanoes erupt constantly. The eruption is generally quiet but occasionally becomes violent. A famous active volcano is Stromboli, which lies on an island off the coast of Italy.

Mount Etna erupts

Intermittent volcanoes erupt at fairly regular periods. Such volcanoes include Mount Asama in Japan, Mount Etna in Sicily, and Hawaii's Hualalai.

Dormant volcanoes have become inactive, but not long enough to know whether they will erupt again. Such "sleeping" volcanoes include Lassen Peak in California and Paricutin in Mexico.

Extinct volcanoes have been inactive since the beginning of recorded history. Aconcagua in Argentina and Mount Kenya in Kenya are extinct volcanoes. They probably will not erupt again.

Classifying volcanic eruptions. Scientists divide volcanic eruptions into four basic groups: (1) Hawaiian, (2) Strombolian, (3) Vulcanian, and (4) Peleean. These groups are based on the violence of the eruption and the type of material that erupts.

Hawaiian eruptions are named after the volcanoes in Hawaii and are the least violent type. In such eruptions, highly fluid lava flows quietly from several vents and gradually builds up a shield volcano.

Strombolian eruptions are named after Stromboli. Such eruptions result from the continuous escape of gas from the magma. As the gas escapes, it produces tephra that piles up into a cinder cone.

Vulcanian eruptions get their name from Vulcano, a volcanic island off the Italian coast. These eruptions occur when sticky magma plugs up the central vent. The magmatic gas gradually builds up pressure until it blasts the magma into volcanic dust and bombs.

Peleean eruptions are the most violent. Their name comes from the eruption in 1902 of Mont Pelee on Martinique, an island in the West Indies. The eruption killed about 38,000 people. A Peleean eruption occurs when the gas in highly sticky magma builds up tremendous pressure. This pressure causes violent explosions that produce glowing clouds of hot ash and dust. In a Peleean eruption, much of the volcano blows apart.

Predicting volcanic eruptions is one of the chief concerns of volcanology. When a volcano erupts, little can be done to prevent property damage in the surrounding area. But many lives can be saved if people in the area are evacuated before the eruption begins.

Most volcanic eruptions cannot be predicted. However, some volcanoes, such as those in Hawaii, have a built-in warning system. Before such a volcano erupts, it expands slightly as magma collects in the magma chamber. As the magma rises, many small earthquakes occur. The temperature in the surrounding area also begins to increase, and clouds of gas start to pour from the vent. Scientists use several devices to predict when such a volcano will erupt. They use an instrument called a tiltmeter to measure the expansion of a volcano. A device called a seismograph helps detect earthquakes. Thermometers check temperature increases in the area, and gas detectors measure the amount of gas.

Benefits of volcanoes

Volcanoes are among the most destructive natural forces on the earth. Since the 1400's, they have killed almost 200,000 people. But volcanoes also produce benefits. For example, many volcanic materials have important industrial and chemical uses. Rock formed from lava is commonly used in building roads. Pumice, a natural glass that comes from lava, is widely used for grinding and polishing stones, metals, and other materials. Sulfur deposits from volcanoes are used in making chemicals. Weathered volcanic ash greatly improves soil fertility.

In many volcanic regions, people use underground steam as a source of energy. This geothermal energy is used to produce electric power in such countries as Italy, Mexico, New Zealand, and the United States. In Reykjavik, Iceland, most people heat their homes with water piped from volcanic hot springs.

Finally, volcanoes serve as "windows" to the earth's interior. The materials they erupt help scientists learn about conditions within the earth.


Katharine V. Cashman, Ph.D., Associate Professor of Geological Sciences, University of Oregon.

How to cite this article:

To cite this article in a footnote, World Book recommends the following format:

Katharine V. Cashman, "Volcano," World Book Online Americas Edition:

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