A Volcano is an opening in the earth surface through which lava, hot gases, and rock fragments erupt burst forth. The name comes from Vulcan, Roman god of the forge. Such an opening forms when melted rock from deep within the earth blast through the surface. Most volcanoes are mountains, particularly cone shaped, which are built around the opening by lava and other materials thrown out during eruptions.

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.

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 fracture, 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 bowl like crater generally forms at the top of the volcano. The vent lies at the bottom of the crater.

Types of volcanic materials

Three basic kinds of materials may erupt from a volcano: 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 reached 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 (pronounced (pa-hoy-hoy). 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.

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 0.25 millimeter in diameter. Volcanic dust can be carried great distances. In 1883, the eruption of Krakatau in Indonesia shot dust 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. It is likely 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 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 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 1.2 meters across and weigh up to 91 metric tons. Small volcanic bombs are generally called cinders.

Gas discharges out of volcanoes in large quantities during most eruptions. The gas is mainly made up steam. But it also includes carbon dioxide, nitrogen, sulfur dioxide, and other gases. Most of the steam comes from a volcano's magma.

Types of volcanoes
Volcanoes are divided into three main groups based on the shape of the volcano and the type of material they are made of: 1) shield volcanoes, 2) cinder cones, and 3) composite volcanoes.

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 115 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 410 meters above its base.

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  Mount Fuji; Mayon Volcano in the Philippines; and Italy's Vesuvius. Mount St. Helens, which has erupted several times since 1980, is one of the most active composite volcanoes in the United States.

Plate tectonic setting of volcanism: Why volcanoes occur in restricted areas
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.

The theory of plate tectonics explains why most volcanoes as well as most earthquakes and mountains 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 plates is generally about l 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.
Most volcanoes are formed at convergent plate boundaries 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 at divergent plate boundaries 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.  Such volcanoes develop at hot spots, when a huge column of magma rises from inside the earth toward the surface. This column, called a mantle plume, may measure up to 160 kilometers in diameter and rise 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.

Classification of prediction of volcanic activity. This classification is based on the activity of a volcano according to how often it erupts:  (1) active, (2) intermittent, (3) dormant, or (4) extinct

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

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

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 Parcutin 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.

Classification of volcanic eruptions. Volcanic eruptions can be divided into four basic groups based on the violence of the eruption and the type of material that erupts: (1) Hawaiian, (2) Strombolian, (3) Vulcanian/Peleean, and (4) Plinian. Other variations include steam-blast (phreatic euruptions).

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. Vulcanian eruptions are some time referred to as Pelean 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 Peleean eruptions, much of the volcano blows apart. Peleean eruptions are 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 Peleean eruption, much of the volcano blows apart.

Steam-blast (Phreatic) eruptions are violent, explosive, glowing avalanches; include powerful blasts of rock, magma, and steam, rising magma come into contact with seawater or groundwater; usually steep-sided domes or collapse calderas. They often erupt viscous, silica-rich magma. Examples include: Surtsey (Iceland); and Krakatau (Indonesia)

Plinian eruptions  are exceptionally powerful blasts from the eruptive column; includes viscous, gas-rich magma and voluminous pyroclastics. Usually stratovolcanoes. Examples include: Mount Pinatubo (Philippines); Mount St. Helens (Washington State, USA).

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. Then 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
 Several devices are used to predict when such a volcano will erupt. An instrument called a tiltmeter to measure the expansion of a volcano is often used. 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; however, they can 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 electricity 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. Volcanoes serve as 'windows" to the earth's interior. The materials they erupt help scientists learn about conditions within the earth. Also, they help to release excessive pressure from the earth’s interior.