Pennsylvania State University glaciologist Sridhar Anandakrishnan was standing atop a shifting mountain of ice in west Antarctica as scientists halfway across the Earth were tracking its seismic activity.

The glacier, called the Whillians Ice Stream, unleashed a seismic wave equal to a magnitude 7 earthquake, enough ruinous force to crack the ground, topple houses and snap pipes.

Anandakrishnan hardly moved.

He never even felt the force as the giant glacier stopped and then slipped forward.

He was safe because the glacier took 10 to 25 minutes to move, while an earthquake, caused by movements in plates of the Earth's crust, causes havoc in seconds.

The glacier moved twice a day on its 60-mile-wide, half-mile-thick ancient ice stream.

The findings show that glaciers move in fits and starts, rather than the steady crawl that scientists once thought.

The geospatial data collected by Anandakrishnan for an experiment led by Douglas A. Wiens of Washington University in St. Louis shows that the giant sheets move about 18 inches within 10 minutes, and then rest for 12 hours.

"It's important to understand and model this movement so we can understand what future effect global warming may have on glaciers," said Anandakrishnan, who led an expedition in 2004 to gather the data.

Each time the icy mass moves, it produces seismic waves recorded up to 4,000 miles away in Australia.

When the waves were first detected, many thought they came from giant chunks of ice falling into the sea, a phenomenon known as calving.

Now it seems that the seismic waves are caused by the sudden slipping of giant ice sheets.

- John Sullivan