When a volcano erupts, hot gases and liquefied rock from deep within the Earth find their way up to the surface, where they rapidly cool. The material may flow down from the volcano's vent as lava, or it may spew into the air as ash. Volcanoes may cause total destruction in their path. Gases from the largest eruptions can lower air temperatures worldwide for a time by filtering the rays of the sun. But volcanoes also build up new islands and mountains, and they provide nutrients for the surrounding soil, making it a good place to grow crops.

Where volcanoes are found

Of the nearly 1,900 volcanoes active today or known to have been active in historical times, about 90 percent can be found in the parts of South America, North America, Asia, and Oceania that are close to the Pacific Ocean. When plotted on a map of the world, this line of volcanoes looks a bit like a vast open circle. For this reason it is known as the Ring of Fire.

The Ring of Fire, like many other volcanic regions, marks the boundaries between sections of the Earth's crust. As these sections, called plates, collide in slow motion, volcanoes and earthquakes are the result. However, not all volcanoes occur at plate boundaries. Some, such as those of Iceland, were formed over undersea ridges in the middle of great oceans. Still others, such as those that built up the Hawaiian Islands, sit above an isolated “hot spot” in the Earth's crust.

Volcanic eruptions

The hot liquid rock that lies below the Earth's surface is known as magma. Magma released from a volcano is known as lava. All lavas contain a high percentage of silica, a compound made up of the chemical elements silicon and oxygen. Hawaiian names have been given to the two main kinds of lava flows. Hot, very fluid lava, released in a relatively continuous stream, produces pahoehoe flows, which have smooth to ropy surfaces. Thicker lava breaks into chunks or blocks and forms blocky lava flows called aa.

Fresh lava ranges from 1,300° to 2,200°  F (700° to 1,200°  C) in temperature. It glows red hot to white hot as it flows. When lava comes out of a volcano, it is usually mixed with steam and gas. The gas may boil off very forcefully and expel large amounts of glowing ash and cinders. The particles of hot debris thrown out of a volcano are called pyroclastics. Sometimes the gas is released more slowly or is even trapped as bubbles when the lava hardens. When the bubbles are tiny and closely packed, lightweight stone called pumice is formed.

Some volcanoes are more intense than others. In the least explosive kind of activity, lava flows from cracks in the ground and spreads widely. This is called the Icelandic type because it is seen frequently in Iceland. In the Hawaiian type, lava flows from a gently sloping cone. In the Strombolian type, there are frequent small eruptions of ash and gases. The Pelean type is marked by avalanches of intensely hot rock fragments and gas. Also quite violent are Plinian eruptions, which are marked by flows of lava and skyward explosions of gases. Earthquakes, lightning, and strong whirlwinds may accompany a powerful eruption.

Volcanic landforms

The word volcano is also applied to the land formations that result from volcanic eruptions. Such volcanoes come in many types, but the two most common are stratovolcanoes and shield volcanoes.

The cone-shaped form of a stratovolcano begins gradually and becomes steeper toward the summit, which generally contains a crater. These volcanoes are formed by alternating types of eruptions. When a quiet lava flow ends, it leaves a seal of hardened lava within the volcano. Pressure gradually builds up from beneath such a plug, setting the stage for a violent blast of ash and other debris. The volcano is therefore built up from sections of ash and lava. For this reason, stratovolcanoes are also referred to as composite volcanoes.

Shield volcanoes are large, dome-shaped mountains built of lava flows. They are not as steep as stratovolcanoes, though they can be quite large. Some shield volcanoes that pour forth under the sea may create volcanic islands. Eventually the islands can grow high enough to rise above sea level. The Hawaiian Islands were formed in this way. The islands are the world's most massive volcanoes and the tallest, rising some 32,000 feet (9,750 meters) from the ocean floor.

Studying volcanoes

Volcanology is the branch of geology that is focused on volcanoes. Many volcanologists work in observatories from which they keep track of earth tremors and other signs of volcanic activity. Others venture forth to the slopes and craters for an even closer look. On the basis of what they measure and see, they try to predict when an eruption might take place, how severe it will be, and which places will be in the danger zone. Their job is important because it is difficult or impossible to get out of the path of a big eruption once it begins.

Uses of volcanoes

The effects of volcanoes are not entirely harmful. Volcanic ash soil—called andisol—is good for growing crops. In addition, the volcanic glass called obsidian has been used by many of the world's peoples for weapons, tools, and ornaments. People also use pumice for cleaning wood, metal, and other surfaces and in producing building materials.

The heat within the Earth that is released in volcanoes is an enormous potential source of energy. This energy, called geothermal energy, is difficult for people to control. However, hot water and steam trapped below the surface have been used to heat homes and greenhouses and to produce electric power in several countries, including Italy, New Zealand, Japan, Iceland, and the United States.

History

The word volcano comes from the name of Vulcan, the ancient Roman god of fire and metalworking. The Romans believed that volcanic eruptions resulted when Vulcan made thunderbolts and weapons for the gods. Other cultures explained volcanoes as outbursts of anger from a god or goddess. Pele was the name of the volcano goddess of the native Hawaiians.

Volcanoes have a long history of destruction. In AD 79 the eruption of Mount Vesuvius destroyed the Roman cities of Pompeii and Herculaneum. In 1815 Mount Tambora in what is now Indonesia released more than 12 cubic miles (50 cubic kilometers) of matter, making 1816 a “year without a summer” and a disaster for farmers as far away as New England. In 1883 the volcano Krakatoa, which is also on an Indonesian island, unleashed a colossal sea wave known as a tsunami. The tsunami drowned more than 36,000 people as it swept along the coasts of Java and Sumatra.

On May 15, 1902, Mount Pelée erupted on the Caribbean island of Martinique. Although very little lava flowed, an unstoppable black cloud of hot gases and ash engulfed the city of St. Pierre, killing almost all of its 30,000 people. The birth of a volcano was witnessed between 1943 and 1952, when a smoking hole in a Mexican farmer's cornfield erupted into a new mountain called Parícutin that eventually stood 1,400 feet (425 meters) above the level ground around it. Another notable event took place in 1963, when a new volcanic island called Surtsey rose up from the Atlantic Ocean near Iceland. Within a few years it built up to an area of 1 square mile (2.5 square kilometers), with a peak more than 560 feet (170 meters) above sea level. In 1991 the largest eruption of the 20th century occurred at Mount Pinatubo in the Philippine Islands. The volcano left about 100,000 people homeless.

The best-known American volcano is Mount Saint Helens in the Cascade Range in the state of Washington. This volcano's eruption on May 18, 1980, was the first in the United States, outside Alaska and Hawaii, in almost 60 years. More than 50 people were killed and the mountain's cone was blasted away, replaced by a horseshoe-shaped crater. Related mudflows and flooding caused more than 1 billion dollars in property damage.