National Geographic : 1997 May
Anatomof a metdown Located between two major volcanoes- Grimsv6tn and Bardarbunga-the fissure that erupted last autumn, following a telltale series of earthquakes (red dots), tore open the rocky rind that separates Vatna j6kull's frozen underbelly from Iceland's fiery underworld. Intense melting fueled by the eruption produced vast vol umes of water, which amassed in Grimsvotn's ice-capped lake. Under normal conditions geothermal heat rising beneath the glacier melts enough ice to fill the lake and trigger a small flood every few years. But an eruption fast-forwards the process. 60 feet a year," says Magnus Tumi Gudmundsson, a geo physicist at the University of Iceland. "But during the first days of the eruption it was ris ing 60 feet a day." The ascending water, in turn, lifted the 800-foot-thick lid of ice covering the caldera. When at last the swelling lake floated the overlying ice off its footing, nearly a cubic mile of water burst from Grimsvotn and surged beneath the ice some 30 miles to the glacier's edge. Such was the power of the j6kulhlaup that blocks of ice 30 feet high and judged to weigh a thousand tons were torn from the glacier and deposited three miles away.