Rare melt key to Ring of Fire?

Science 07 Oct 10 Oxford University scientists have discovered the explanation for why the world's explosive volcanoes are confined to bands only a few tens of kilometres wide, such as those along the Pacific 'Ring of Fire'. Most of the molten rock that comes out of these volcanoes is rich in water, but the Oxford team has shown that the volcanoes are aligned above narrow regions in the mantle where water-free melting can take place. They publish a report of their research in this week's Nature . These volcanic chains have been responsible for the most devastating eruptions in history, such as that of Krakatoa in 1883, and the huge eruption of Toba about 74,000 years ago, whose aftermath may have come close to extinguishing the human race. 'It has been recognised for almost 50 years that the volcanic arcs form where one oceanic plate sinks beneath another,' said Professor Philip England of Oxford University's Department of Earth Sciences, an author of the report, '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.' The eruptions of volcanoes in the Ring of Fire are extremely violent (in contrast with the relatively gentle eruption in Iceland that paralysed European air travel in April) because the molten rock contains a high proportion of water which, as superheated gas, provides the power for the explosive eruptions. This water is liberated from the plates descending beneath the volcanoes and lowers the melting point of rocks in the mantle.