When lava and ice clashed on Mount Rainier.

During prehistoric eruptions of Mount Rainier, rivers of lava poured out over the glacier-shrouded volcano, leading to epic battles of fire against ice. In most of those contests, the ice won, according to new geologic research on the mountain.

Unable to burn its way through the thickest part of the glaciers, the lava was forced to skirt the edges of the ice fields, leaving behind unusual rock formations that have deceived geologists for decades, report David T. Lescinsky of Arizona State University in Tempe and Thomas W. Sisson of the U.S. Geological Survey in Menlo Park, Calif.

The new understanding "very much changes how you think about the history of the volcano," says Sisson.

Lescinsky and Sisson made their discovery by studying the Washington volcano's large lava flows, many of which sit on top of ridges separating major valleys. These formations have intrigued geologists because lava, like any other fluid, should flow through a valley rather than along the top of a ridge.

When researchers first mapped Mount Rainier in the late 1950s, they tried to explain the unusual deposits by suggesting that the current topography represents the reverse of the former landscape. According to this theory, the molten rock originally flowed through low-lying valleys, shifting the rivers that had occupied them. Over time, the hardened lava resisted erosion, and the displaced rivers ate away at the softer rock on either side of the deposits. Eventually, the ancient lava flows ended up high on the ridge tops.

Lescinsky and Sisson abandoned this explanation when recent dating of the lava flows revealed that they are much younger than previously supposed--in some cases only 40,000 years old. Erosion could not have formed major valleys on either side of the lava deposits in so short a time, say the researchers in the April Geology.

When they examined the deposits in detail, evidence of an icy past emerged. Along the sides of the flows, the lava has a glassy texture and breaks into telltale hexagonal columns--twin indications that the flows cooled rapidly.

Lescinsky and Sisson hypothesize that the lava erupted at a time when glaciers covered more of the mountain than they do now. As the lava crept down the slope, its front hardened where it hit thick, glacier-filled valleys, forcing the lava to flow along the line of least resistance, usually the ridges between the valleys. There, where the ice layer was thinnest, the lava cut steep-sided channels.

As it flowed along the ridge top, the ice-cooled lava hardened in place. When the glaciers retreated, the lava deposits remained perched on top of the ridges, say Lescinsky and Sisson. They think the same process has occurred on many other glaciated volcanoes.

Their explanation has won over Richard S. Fiske, one of the researchers who proposed the original river valley hypothesis almost 40 years ago. Fiske and his coworkers had assumed that the lava flows were extremely old, but modern dating techniques were not widely available then. "With these new ages, I don't see how our earlier interpretation can be sustained," says Fiske, a geologist with the Smithsonian Institution in Washington, D.C.

The new hypothesis forces geologists to revise their ideas about past eruptions. "What it says to me is that a lot has happened on Mount Rainier just in the last 100,000 years rather than being stretched out over a long period of time," says Fiske.

Geologists consider Mount Rainier one of the greatest volcanic hazards in the United States (SN: 5/28/94, p. 341). Water from its glaciers has seeped into the mountain and corroded much of the rock, making it prone to collapse. During large landslides, glacial ice and rock combine to form massive mudflows. Future slides could sweep over nearby towns.