A volcanic eruption under Vatnajokul glacier (Europe’s largest), Iceland, in October 1996 topped Lake Grimsvotn with meltwater.  The meltwater flowed from the lake at an estimated 45,000m3/sec (1.6 million ft3/sec), making it the greatest melting of a glacier in recorded history.

Glaciers are large moving masses if ice formed in (usually mountainous) regions where the rate of snowfall is greater than the rate of snow melting. The snow piles up, is compressed into ice by the weight of more snow above it, and begins to slowly flow downhill under the force if its own weight.