| Summary: | Active volcanoes and hydrothermal systems underlie ice caps in Iceland. Glacier–volcano interactions produce meltwater that either drains toward the glacier margin or accumulates in subglacial lakes. Accumulated meltwater drains periodically in jökulhlaups from the subglacial lakes and occasionally during volcanic eruptions. The release of meltwater from glacial lakes can take place in two different mechanisms. Drainage can begin at pressures lower than the ice overburden in conduits that expand slowly due to melting of the ice walls by frictional and sensible heat in the water. Alternatively, the lake level rises until the ice dam is lifted and water pressure in excess of the ice overburden opens the waterways; the glacier is lifted along the flowpath to make space for the water. In this case, discharge rises faster than can be accommodated by melting of the conduits. Normally jökulhlaups do not lead to glacier surges but eruptions in ice-capped stratovolcanoes have caused rapid and extensive glacier sliding. Jökulhlaups from subglacial lakes may transport on the order of 107 tons of sediment per event but during violent volcanic eruptions, the sediment load has been 108 tons.
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