The amount of ash produced by the Bardarbunga volcano would depend on the thickness of the ice, Icelandic Met Office vulcanologist Melissa Pfeffer has said, after Iceland raised its aviation alert to red.
Seismologists say magma is moving under the glacier but so far has travelled horizontally at a depth of three to six miles. The volcano will erupt if the magma rises and melts the ice above.
The thicker the ice, the more water there is, the more explosive it will be and the more ash-rich the eruption will be.
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Planning on going to the London Marathon tomorrow? Read our handy guide first!