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VOLCANOES

The development of a volcano begins with cracking (also know as a fissure) of the earths crust as the magma forces it’s way to the earth’s surface. As the magma moves upwards it is usually constrained into a circular pipe which reaches the surface as an opening called a vent. Eruptions through this vent of magma and/or pyroclastic flow over time build the volcanic structure. At the apex of the volcano is the crater. A crater is a shallow circular depression around the vent. The crater may be developed by either the deposition of eruptive material around the vent forming a rim or by explosive forces caused by violent eruptions removing material from the crater walls. A very large crater is known as a caldera, while smaller craters may range from meters in width through to hundreds of meters a caldera may extend for over a kilometre up to over 50 kilometres. Throughout the early years of the volcano’s development most volcanic material comes from the vent as discussed earlier. As the volcano matures smaller cones may develop from secondary vents on the sides of the volcano (flanks). These are known as parasitic cones. Those that only emit gasses are known as fumaroles. There are three main types of volcanoes:

Shield Volcanoes

Generally shield volcanoes have developed from the ocean floor to form seamounts of volcanic islands. There structure is broad and dome shaped with sides that gently slope. Examples of these include the Galapagos Islands and the Hawaiian Islands.

Cinder Cones

Cinder cones are amongst the most common of the volcano types. They are characterised by steeply sloping sides which is determined by the angle of repose of pyroclastic material at rest (in other words pyroclastic material can be stable at a high angle). These types of volcanoes are often constructed in a relatively short space of time and frequently within one or two eruptions. Once the magma in the vent and below solidifies there is rarely a subsequent eruption. Consequently these types of volcanoes are not very high with average heights between 30m – 300m. Cinder cones may also be a parasitic cone on a larger volcano.

Composite Cones/Stratovolanoes

This type of volcano is considered to be the most deadly and is commonly found around the Pacific ‘Ring of Fire’ where the Pacific Plate is met by other tectonic plates. Composite cones are composed of both pyroclastic and volcanic material. These volcanos have a steep summit and spreading flanks. This shape is a combination of the type of magma which silica rich giving rise to thick lava which covers short distances and the large pyroclastic flows typical of a composite cone. Mt Fuji in Japan is a classic example of the shape of a composite cone.