Why is the movement of plates so important to the location of volcanoes?

Answer 1

Volcanoes do mostly occur very close to plate boundaries. Furthermore, some very large volcanic zones or supervolcanoes (like Yellowstone in the U.S. or Taupo in New Zealand) may have played a large role in breaking up continental crust into smaller plates in Earth's history (e.g. the Central Atlantic Magmatic Province CAMP at the transition from the Triassic to the Jurassic Period).

Volcanoes can be created on places where crust is stretched (divergent plate boundaries) because here material from the Earth's mantle can ascend and magma can rise through fractures and weak zones created by the stretching of the crust. An example is the Afar rift in Ethiopia (Africa).

Volcanoes are also a very common feature close to subduction zones. The subduction process (and angle and speed of the subducting plate) is believed to play a major role in the formation of volcanoes on the overriding plate, because material that is subducted (partly weathered oceanic crust with a sedimentary cover) is not stable under the pressures and temperatures in the mantle and thus starts to melt. The melt that is extracted from the subducting plate ascends and ultimately creates volcanoes on the overriding plate as seen e.g. in the Pacific ring of fire.