Late in 1996, the media showed us a special kind of catastrophe: a battle between ice and fire. In Iceland, the volcano Grimsvötn awoke beneath the ice cap Vatnajökull, melting a huge quantity of ice. On 5 November that meltwater burst out from under Vatnajökull's edge and rushed to the sea, carrying away bridges, an optical data cable, and other pieces of infrastructure. The Icelanders calmly rebuilt—it was just another jökulhlaup.
A jökulhlaup is not merely a big flood in one odd country. It is a peculiar type of flood with rich significance for this interglacial age, and that is why geologists the world over know the word, though only Icelanders can pronounce it.
The Jökulhlaup Mechanism
Jökulhlaups (jökull = glacier, hlaup = floodburst) are inevitable when ice becomes a dam. At Vatnajökull, the ice cap dammed meltwater inside itself. In other cases a glacier might surge into a narrow river valley, like a landslide across a highway, and the resulting dam collects itself a lake. But ice has no strength, and the lake quickly destroys the dam when it can overtop it.
Another thing about a dam made of ice is that ice is lighter and colder than water. Water both melts and floats an ice dam, and once the water first trickles underneath it the dam, no matter how large, swiftly fails. Hence the weird word for this special event.
Jökulhlaups in Other Lands
Jökulhlaups happen elsewhere than Iceland. In the Alps, several "cursed glaciers" were named for their history of advancing into the valleys and blocking rivers. The floods that came down when the ice gave way were jökulhlaups.
In southeastern Alaska in May 1986, the Hubbard Glacier had a vigorous surge that took its snout into the shallows of Disenchantment Bay, cutting off Russell Fiord at its entrance. In that instant Russell Fiord became Russell Lake, which began to rise. The rising lake threatened to overflow elsewhere, into an ancient spillway that had been active in similar previous situations. The brackish water would flood the Situk River and ruin its world-class fishing—and the economy of Yakutat, the nearest town.
But on 7 October, a jökulhlaup saved the day. At one point it carried 1 million cubic meters of water per second into the Gulf of Alaska, along with much of Hubbard's snout. The U.S. Geological Survey said "the peak flow rate may have been the greatest water discharge to occur in North America and the largest glacier outburst in the world during the past few centuries" (it happened again in 2002).
In 2007 a small glacial lake in the Chilean Andes suddenly drained, giving rise to a flurry of news stories, some of them quite outlandish. But it was another jökulhlaup.
But we know of some jökulhlaups that make pipsqueaks of these recent ones. Late in the last glacial period, about 13,000 years ago, part of the Cordilleran icesheet stoppered up the Columbia River, near present-day Lake Pend Oreille, Idaho. After some years Glacial Lake Missoula formed behind the dam. When it broke through, a wall of water hundreds of meters high, and of many days' duration, rearranged the landscape of central Washington state. The effects are as if you dumped a wheelbarrow full of water into a sandbox, but the scale is so huge that only a picture from outer space or a regional map makes it clear. (See more from the Ice Age Flood Institute.)
A very dogged geologist named J Harlan Bretz spent several decades documenting the evidence of this event: oversized river channels and colossal riverbed potholes that present-day streams could not have cut; ripple marks big enough to be levees; gigantic strips of land where all soil was stripped to bedrock and bedrock itself plucked out in house-sized chunks. (Some photos of these are here on About.com and elsewhere.) Downstream were widespread layers of sand and clay many meters thick without soil horizons—the flood was so great that the whole Columbia River Gorge became a bottleneck, and the backed-up flow had to drop its sediment load. More enormous deposits of Washington sediment lie in the Pacific off the Columbia's mouth.
It took Bretz his whole long life to win acceptance because his scenario was so big, it sounded too much like Noah's flood. It still sounds that way to creationist cranks. But it was a jökulhlaup. Actually, the whole process repeated dozens of times. What might be called a biblical cataclysm if it happened once was—like asteroid impacts—just another of nature's big power tools.
The largest events that act like jökulhlaups occurred during the Ice Ages, when the North American continental ice cap trapped water beneath it. The resulting floods were so immense that they have their own name, Heinrich events. Flotillas of icebergs dropped stones on the seafloor across the whole North Atlantic Ocean. But worse, the addition of so much freezing freshwater may have knocked the ocean out of whack. Changes in sea surface temperature, salinity, and climate have been documented around all six known Heinrich events.
The relatively rapid loading and unloading of the Earth's crust in the jökulhlaup cycle may trigger earthquakes in sensitive regions, as well as volcanic disturbances in areas like Iceland. Other areas known to have jökulhlaups besides Iceland and the Americas include the Alps, central Asia and the Himalayas.
At smaller scales, the sudden surge of ice and sediment from a jökulhlaup might have more effects than washed-out roads and landscapes. For instance, the new load of mud and gravel on the seafloor might trigger offshore landslides, which are a major cause of tsunamis. If the media report a jökulhlaup tsunami, though, they will call it something more pronounceable.
PS: If jökulhlaups and asteroid impacts can be compared in the same breath, consider the case of Mars, where it seems that impacts on frozen ground have created gargantuan floods in a kind of cosmic double-whammy, not once but many times.