Some undersea landslides ride a nearly frictionless slick of water.

Hydroplaning, the traction-sapping phenomenon that makes high-speed driving dangerous on rainy days, may be responsible for the unexpectedly large distances covered by some undersea avalanches, according to new computer simulations.

Sediments carried by rivers often accumulate in thick layers on the sloping seafloors surrounding the continents. When large deposits of that material break loose, the huge flows of silt, clay, and mud that result wreck everything in their paths. Among the victims can be entire communities of seafloor life as well as ocean-floor pipelines and communications cables, says Anders Elverhoi of the University of Oslo.

Sediment can slide hundreds of kilometers, even across nearly level slopes. In the so-called Storegga slide, which occurred in the Norwegian Sea more than 8,000 years ago, about 2,500 cubic kilometers of material---enough to make up several sizable mountains-broke free, some of it sliding nearly 500 km toward Greenland.

Using computer models they developed, Elverhoi and his colleagues have identified what may be a major factor enabling such landslides to travel so far: a thin layer of water that insinuates itself between the floor and the overlying mass of quick-moving sediment. No longer in contact with the floor, the sediment is akin to a speeding, multi-ton vehicle hydroplaning atop a thin slick of rain on pavement. The Norwegian researchers describe their model in an upcoming Journal of Geophysical Research-Oceans.

The scarce, indirect observations of submarine landslides now available suggest the slumping sediments can travel swiftly, says Steven N. Ward, a geophysicist at the University of California, Santa Cruz. In 1929, between 300 and 700 k[m.sup.3] of sediment slid off the continental shelf south of Newfoundland, snapping several transatlantic telegraph cables. The timing of the cable breaks indicated that the sediment traveled across the ocean floor at nearly 80 km per hour, Ward notes.

Scientists have long sought an explanation for the anomalously long travel distances of submarine landslides over gentle

slopes, says David Mohrig, a marine geologist at the Massachusetts Institute of Technology. Small-scale lab experiments that he, Elverhoi, and other researchers have conducted to simulate underwater landslides show that the front edge of slumping sediments can hydroplane. During hydroplaning, frictional forces that might otherwise bring the landslide to a halt would be greatly diminished, he notes.

The hydroplaning phenomenon may also explain how large chunks of sediment apparently can skate far beyond the main flow of material before they drop to the seafloor, says Mohrig. These so-called outrunner blocks, which aren't seen in avalanches on land, often show up on seafloor maps of ancient landslides.