Why does volcanic activity vary in nature over the earths surface?

Dan

Good question. The style of volcanism is controlled by the tectonic setting and the composition of the magma. Volcanoes occur where plates are subducted (an ocean plate pushed under a continental or another oceanic plate) or move apart (mid-ocean ridges or rifts in continents). Volcanoes also form above hot spots, stationary upwellings of hot mantle rocks. Effusive, mostly gentle eruptions made of basalt are common at mid-ocean ridges, rifts in continents, and volcanoes above most hot spots (one exception being the extremely violent eruptions of Yellowstone where a hot spot is benrath a continent). Basalt has a relatively low viscosity and most gas escapses before the eruption starts. Explosive, violent eruptions are associated with andesite magma produced at subduction zones and released as tephra at stratovolcanoes. Gas stays traps in the magma until it violently expands, driving the eruption. See kinds of Eruptions (http://volcano.und.edu/vwdocs/vwlessons/kinds/kinds.html).

Sincerely,
Steve Mattox

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