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# Types of Volcanoes

A volcano is a cone-shaped mountain formed by surface eruptions of magma from inside the earth. The magma that reaches the surface in an eruption is called lava and is one of the many different products that can be thrown out, including ash, pumice, cinders, dust, steam, and gasses. The world distribution of active volcanoes shows an almost perfect fit with the locations of the tectonic plate margins. When you picture a volcano, what do you see? If you imagine a fountain of lava squirting out of a peak, you're partly right. You may be picturing a shield volcano. There are three common types of volcanoes: the composite volcano, the cinder cone volcano, and the shield volcano. Geologists classify volcanoes based on their size, shape, and formation.

## Composite Volcano

A composite volcano is a volcanic cone that has a rather complex history. Although the volcano itself always has tapped the same magma chamber, the composition and nature of its products might have changed greatly over time, as has its eruptive style. Composite volcanoes go by many names, including composite cone, stratocone, and stratovolcano. All of these terms describe the same type of volcano. Composite volcanoes are composed of layers of lava flows and pyroclastic materials. They can have cinder cones or lava domes on their flanks. They might or might not have craters or calderas at their summits. They can start as shield volcanoes or cinder cones and develop into a composite volcano after hundreds of eruptions over thousands of years of activity. Because this type of volcano is complex by nature, there is no 'typical' way in which one develops.
Mount St. Helens in Washington is an example of a composite volcano. Early in its history, Mount St. Helens erupted a great deal of basalt, which still can be found around the base of the volcano. Their slopes are made of innumerable layers of rubble derived from the flow and break up of brittle lava and of dome rocks that are interspersed with some explosively produced pyroclastic layers and a few lava flows. Some of these volcanoes grow to greater than 3000 m above their bases. Composite volcanoes are constructed from multiple eruptions, sometimes occurring over hundreds of thousands of years, sometimes over a few hundred. In fact, a set of lava tubes in these basalt flows- called the Ape Caves- is a popular destination within the Mount St. Helens National Volcanic Monument. After the basalt phase, the volcano began erupting andesite and built up an impressive symmetrical cone composed of andesite lava flows and pyroclastic material. Within the last few thousand years, the eruptive material has been dacite. This is the material that makes up the many lava domes found on the flanks and within the current crater of the volcano. The eruptive style has changed from gentle to explosive, the magma type has changed, and the materials erupted from the volcano have varied throughout its long history. If there is a typical history of a composite volcano, then Mount St. Helens has that history; it is one of variety and change. Most volcanoes around the edge of the Pacific Rim are composite volcanoes.

## Cinder Cone Volcano

Cinder cone volcanoes form from ejected lava fragments. They are largely composed of loose pyroclastic material and, in contrast to the gently sloping shield volcanoes, can have slopes of $40^{\circ}C$.
In 1943, residents of Paricutin, Mexico, had the opportunity to observe firsthand the construction of a cinder cone volcano. The volcano began to build in a farmer's confield when tremors were followed by the emission of sulfurous gasses from a small hole in the field. Hot rock fragments began to eject from the hole, and the cone was build. The cone reached a maximum height of 400 meters (1300 ft) over a period of 2 years, but grew as much as 40 meters (130 ft) in 1 day! Large amounts of ash were also reported to cover the entire area.

## Shield Volcano

Shield volcanoes form when fluid, basaltic lava flows through the main vent, exists and produces a broad, slightly domed structure. These volcanoes are so named because they resemble a warrior's shield. Mauna Kea on the Big Island of Hawaii is a classic example of the shield volcano. It is also quite large, as it rises nearly 10000 meters (30000 ft) from the seafloor to a height of over 4100 meters (13000 ft) above sea level.

## Dome (Acid Lava Cones)

Acid lave is highly viscous and can flow over a short distance. It is rich in silica but poor in iron and magnesium. When the volcano erupts with a great explosion, this type of lava forms high, steep- sided cones and solidifies n the vent, forming a plug through which it may erupt again. During and explosion, the lava consisting of molten rocks and minerals is ejected with great force, e.g. Mt. Pelee on the Island of Martinique was the most catastrophic of modern times. St Pierre, the capital, was completed destroyed.

## Different Types of Volcanoes

• Volcanoes can be classified on various bases. On the basis of the frequency of eruption they are grouped into three types- active volcanoes, dormant volcanoes and extinct volcanoes.
Active Volcanoes:
These are the ones that have erupted in the recent past. Some of these volcanoes keep erupting periodically. Mt. Vesuvius, Mt. Stramboli, Mt. Etna in Italy and Mauna Loa or Mauna Kea in Hawaii Islands are examples of active volcanoes. Barren Island in India is also an active volcano. In all there are about 500 active volcanoes in the world.

Dormant Volcanoes:
These are the ones that have erupted in the historical periods, but have remained inactive for a fairly long time. However, they may erupt ant time in future. If a dormant volcano erupts again it becomes an active one. The volcano in the Barren Island in India was considered a dormant volcano befor it become active again recently. Some of the volcanoes in West Indies, Philippines and Japan are considered dormant.

Extinct Volcanoes:
These are those volcanoes which have not erupted since remote geological times when they were active. They have been inactive for so long that they are not likely to erupt in future. Mt. Kilimanjaro in Africa is an example of such volcanoes.
• Another basis of classification of volcanoes is the nature of eruption. On this basis they are classified into two categories- the central type and the fissure type.
Central type of Volcanoes:
In this type of volcanoes the eruption occurs at one point and the magma and other volcanic material rises to the surface through a narrow opening or pipe type of vent. The material thus ejected gets accumulated around the point of eruption in the form of a conical hill. Mt. Vesuvious is this type of a volcano.

Fissure type of Volcanoes:
Here, the magma and other material is ejected through a long narrow opening. A large amount of magma is ejected to the surface and it generally spreads over a wide area forming vast plateaus of shields. It is believed that the lava forming the Deccan Trap of India had emerged through a fissure type volcano.
• On the basis of the nature of eruption itself, the volcanic activity can also be classified into explosive type and placid type.
Explosive Type:
Some of the volcanoes erupt explosively with great force. Loud explosions are caused due to sudden escape of gases and steam. Sometimes such explosive eruptions blow up a part of the volcanic cone. Large amount of rock particles and dust is thrown up to great heights in the atmosphere. Mt. Krakatau in Indonesia erupted in a highly explosive manner in 1883 blowing more than half of the island on which it is located. The fine dust thrown up in the air during this eruption remained suspended in the air for a very long time causing colorful sunsets and sunrises in many parts of the world.

Placid Type:
In other instances the magma rises to the surface quietly without any explosions. This happens generally when the magma does not contain large amount of gases. The volcanoes of the Hawaii Islands are of this type.
 More topics in Types of Volcanoes Shield Volcanoes Composite Volcanoes Cinder Cones Lava Domes
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