composite volcano

Steep-sided conical volcano formed above a subduction zone at a destructive plate margin. It is made up of alternate layers of ash and lava. The magma (molten rock) associated with composite volcanoes is very thick and often clogs up the vent. This can cause a tremendous build-up of pressure, which, once released, causes a very violent eruption. Examples of composite volcanoes are Mount St Helens in the USA, and Stromboli and Vesuvius in Italy.

Composite volcanoes are usually found in association with island arcs and coastal mountain chains. The magma is mostly derived from plate material, and is rich in silica. This makes a very viscous lava, such as andesite, which solidifies rapidly to form a high, steep-sided volcanic mountain. This magma often clogs the volcanic vent. After the eruption, the crater may collapse to form a caldera.