Propagation of a subglacial flood wave during the initiation of a jôkulhlaup

Observations from the jôkulhlaup from Grimsvôtn in Vatnajôkull, southeastern Iceland, in 1996 indicate that the jôkulhlaup was initiated by the movement of a localised pressure wave that travelled 50 km in 10 h from Grimsvôtn to the terminus, forming a subglacial pathway along the glacier bed. Shortly after this wave reached the terminus, the jôkulhlaup was flowing at a high discharge through a tunnel that would have needed much longer time to form by ice melting as assumed in existing theories of jôkulhlaups. Frozen sediments formed in crevasses and frazil ice on the surface of the flood waters indicate the flow of supercooled water in the terminus region, demonstrating that the rate of heat transfer from subglacial flood water to the overlying ice is greatly underestimated in current theories.