Ice-covered volcanoes like the one in Iceland that brought European air traffic to a standstill are the center of an emerging branch of volcano science that seeks to answer important questions about climate change.

Scientists believe the rocks created when volcanoes erupt beneath glaciers contain distinct chemical signatures that indicate the thickness of the ice that was above the volcanoes when they blew. By correlating the thickness with the age of the rocks, researchers can estimate the degree to which Earth was covered by glaciers thousands — or even millions — of years ago. That information is crucial to climatologists who want to understand how ice and temperature conspire to make the globe cool down or heat up.

"In the big global climate models that they run on supercomputers, ice cover on the Earth is very important," said Ian Skilling, a volcanologist at the University of Pittsburgh.

Paleoclimatologists already have a good idea about temperatures in the planet's distant past, thanks to sediment samples from the ocean floor. But ocean temperatures, though an important factor in understanding the history of the planet's climate, are only part of the story.

Scientists don't understand very well how changes in ocean temperature are linked with ice conditions on land, said Ben Edwards, a volcanologist at Dickinson College in Carlisle, Pa. Combining the ocean data with information from volcanic rocks "will give us a much better feel for how global climate changes in the course of time," he said.

The rocks formed by volcanoes covered by glaciers have some distinctive features. They have relatively smooth edges, on account of banging into ice on their way out of the crater. Those edges also have a glassy rind that forms because ice causes magma to cool extremely quickly.

The glass contains water, carbon dioxide and sulfur dioxide. The concentration of water is determined in part by the pressure above the volcano before it erupts — an indication of the thickness of the ancient ice layer that covered it.

"Higher water content means higher pressure on that glass when it was forming," Skilling said. "It gives you some idea of just how far beneath the surface it was."

Ice thickness is important because it helps scientists figure out how long a glacier lasted in a particular place. It also gives scientists clues about how warm the air was, how much sunlight was reflected back into space, and how high sea levels were, Skilling said.

Edwards recently collected rocks from about 40 volcanoes in British Columbia that were covered by glaciers when they erupted. "We want to correlate when there were big ice sheets on land with the cold spells recorded in the ocean," he said.

But North American volcanoes erupt somewhat sporadically. To get a more continuous record of glacial conditions, the neighbors of the erupting Eyjafjallajokull volcano could be particularly helpful, he said.

Volcanologists have calculated glacier thickness for several times and places, and so far they match up with ocean temperatures from the same periods. Edwards said he expects to find many more matches.