Mud volcano stews in chilly Arctic waters.

A team of U.S., Norwegian, and Russian scientists has found an unusual layer of snowlike natural gas draped across a warm mud volcano--the deep-sea equivalent of apple pie a la mode. Oceanographers have never before witnessed this contrast of an icy coating on top of a seething underwater volcano.

Researchers from the Naval Research Laboratory (NRL) in Washington, D.C., and their Norwegian colleagues discovered the 1,250-meter-deep mud volcano in 1995, while conducting a sonar study between Norway and the island of Spitsbergen. Unlike more familiar volcanoes, which eject rock and ash, mud volcanoes spew out a slurry of seafloor sediments mixed with water. The scientists named the 1-kilometer-wide circular feature the Hakon Mosby mud volcano, after the Norwegian research vessel used in the expedition.

Last year, the oceanographic team returned on a Russian ship with cameras and instruments to probe the seafloor. The scientists presented their findings last week at a meeting of the American Geophysical Union in Baltimore.

Pictures of the mud volcano show a white substance, believed to be methane hydrate, covering the seafloor like freshly fallen snow. Methane hydrate is a solid that forms when high pressures and low temperatures squeeze water molecules into a crystalline cage around a methane molecule. Cores drilled into the sediments beneath the surface of the mud volcano contained radish-size clumps of hydrates, which fizzled and evaporated quickly when brought to the ship.

Methane hydrates are capturing increasing attention, in part because they represent the largest untapped source of fossil fuels left on Earth (SN: 11/9/96, p. 298). Oceanographers suspect that methane hydrates hide in vast subseafloor deposits around the continents, but it is extremely rare to find the icy substance sitting on the ocean floor, says Peter R. Vogt of NRL, one of the scientists leading last year's expedition.

Methane seeping up from the mud volcano supports a rich community of organisms, including a new species of tiny tube worms, report the Russian scientists on the team. Members of the animal phylum Pogonophora, tube worms have neither mouths nor digestive tracts. They get their energy from symbiotic bacteria, which live inside the worms and oxidize methane.

Mud volcanoes exist in many places around the world, but the Hakon Mosby is unusual, says Vogt. Unlike most other such volcanoes, which develop above rising blobs of salt or near ocean trenches, the Hakon Mosby has no clear geologic feature forcing the mud to erupt at the surface. The team of researchers is planning another expedition in 1998, during which they will dive to the volcano inside two Russian submersibles.

As ocean temperatures warm in the next century, shallow deposits of methane hydrates could melt and destabilize sediments on the continental slopes. If the seafloor gives way, massive submarine landslides would trigger giant waves that would inundate coastal communities.

"The consequences of hydrate decomposition in the not-too-distant future are going to be a problem for society," says Peter G. Brewer of the Monterey Bay Aquarium Research Institute in Moss Landing, Calif. Sites like the Hakon Mosby mud volcano allow scientists to Study flow hydrates form and decompose in their natural environment, he says.