Another factor might be contributing to the thinning of some of the Antarctica's glaciers: volcanoes.

In an article published Sunday on the Web site of the journal Nature Geoscience, Hugh Corr and David Vaughan of the British Antarctic Survey report the identification of a layer of volcanic ash and glass shards frozen within an ice sheet in western Antarctica.

"This is the first time we have seen a volcano beneath the ice sheet punch a hole through the ice sheet" in Antarctica, Vaughan said.

Volcanic heat could still be melting ice to water and contributing to thinning and speeding up of the Pine Island glacier, which passes nearby, but Vaughan said he doubted that it could be affecting other glaciers in western Antarctica, which have also thinned in recent years. Most glaciologists, including Vaughan, say that warmer ocean water is the primary cause of thinning.

Volcanically, Antarctica is a fairly quiet place. But sometime around 325 B.C., the researchers said, a hidden and still active volcano erupted, puncturing several hundred yards of ice above it. Ash and shards from the volcano carried through the air and settled onto the surrounding landscape. That layer is now out of sight, hidden beneath the snows that fell during the next 2,300 years.

The thickness of ice above the ash layer provided an estimate of the date of the eruption: 207 B.C., give or take 240 years. "It's probably within Alexander the Great's lifetime, but not more precise than that," Vaughan said.