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Volcanoes occur around the Earth. But they are not random located and occur in volcanic zones associated with boundaries of crustal plates. These often form liner volcanic mountain chains thousands of kilometres long on the edges of continents, along the ocean floor or as island chains. The locations of these are closely related to the 12 major plates (a number of smaller ones) that form the Earth’s crust. The relatively movement and relationships of these plates is explained by the theory of Plate Tectonics.

Plate tectonics is the theory which described the motion of the relatively brittle, rigid crustal plates that forms the Earth’s crust, about 80 km thick, rest above the relatively more plastic asthenosphere. Most of the Earth’s active volcanoes are located along these plate boundaries where they spread or collide. Plate boundaries may be continental, oceanic, or both continental and oceanic. Different types of plate boundaries give rise to different types of volcanic eruptions. Oceanic crust is mainly composed of basaltic material whereas continental crust varies from granitic to basaltic in composition.

At destructive plate margins continental plates override oceanic plates. The descent of oceanic crust and associated sediments and sea water into higher temperature zones results in melting and the formation of magma. These magmas vary in composition but tend to have higher silica content. These usually produce andesitic magmas, which are responsible for violent volcanic eruptions. By contrast, at constructive plate margins, where new oceanic crust is formed, and the plates are diverging, the magma is of basaltic composition. This is less viscous than the andesitic magma and there are relatively less violent eruptions. As a result the lavas generated tend to be more mobile and capable of flowing.

Some volcanoes however, such as those of the Hawaiian Islands, do not occur at the margins of plate boundaries, but instead are located within the interior parts of crustal plates. They are generated at hot spots in the Earth’s interior, which have intruded the Earth’s crust as the plates move over them.

Basaltic magmas are generated in the upper mantle, where the temperatures and pressures cause the melting of the parent rock from which they were derived. Magma generation appears to be related to major discontinuities in the Earth’s crust, whereby faults and rifts extend from the crust deep (several tens of kilometres) into the upper mantle. However, the magma chambers that supply individual volcanoes are not located at such great depths and may be located just a few kilometres below the Earth’s surface.

(Image Source: Smithsonian Institute Global Volcanism Programme http://www.volcano.si.edu/gvp/volcano/index.htm)

Volcanoes and thermal fields that have been active during the past 10,000 years. (Image Source: Encyclopædia Britannica, Inc.http://www.britannica.com/EBchecked/topic-art/632078/3256/The-major-types-of-volcanic-eruptions )

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