Formation of volcanoes

This occurs when magma from the earth's interior is able to make its way to the surface, via a vent. This is usually associated with plate boundaries. Volcanoes vary greatly in their shape, as does the type of material emitted during an eruption. Volcanoes range from steep to gently sided.

Types of volcano

Two contrasting types exist; those dominated by lava, usually found at diverging boundaries, where basalt can rise freely to the surface (volcanoes of Iceland) and those dominated by Ash, usually found along subduction zones where large amounts of pyroclastic material is ejected (Japan - Fuji Yama)

The cone shapes of volcanoes are summarised in the table below:

Cone Shape:	Characteristics:
Fissure	Very gentle slope, found at diverging ocean plates, basaltic lava, can flow over large distances.
Basic / Shield	Have gentle slopes, steeper than fissure due to repeated explosions and subsequent build up of basalt based lava (Mauna Loa Hawaii).
Cone	Symmetrical in shape, A) acid where thick viscous lava, rapidly cools, B) Ash / Cinder.
Composite	Very large old volcanoes. Both ash and lava are deposited (Mt. St. Helens).
Crater / Caldera	Form when a very violent eruption occurs after a build up of gas beneath the volcano. Can destroy the magma chamber leaving a large crater.