‘It has been recognised for almost 50 years that the volcanic arcs form where one oceanic plate sinks beneath another, but while many models of this process have been put forward, none has been able to explain the location, and narrowness, of the volcanic arcs,’ said Professor Philip England, one of the authors of the study, which was published in Nature.

Volcanic eruptions in the Ring of Fire are extremely violent – examples include the 1883 eruption of Krakatoa and that of Toba about 74,000 years ago, which may have almost wiped out the human race. This is caused by the high proportion of water in the molten rock beneath the volcanoes, which turns into explosive superheated steam. The water is liberated when the plates descend into the mantle and lowers the melting point of the surrounding rocks.

Previous theories suggested that this ‘wet melting’ was responsible for the formation of the volcanoes, but this sort of melting takes place over wide areas of the mantle, so it doesn’t explain why the volcanoes tend to be so localised. The Oxford team used mathematical models of heat transport in the regions where plates collide and found that the observed pattern could only be explained if the volcanoes are forming in regions where the mantle is melting in the absence of water. Molten rock rising from these regions blazes a trail that more water-rich magma can follow to the surface, where it erupts to form volcanoes.

December 2010