7 December 2011

Antarctic ice streams start and stop as they slide

Posted by kramsayer

Ice streams like the Whillans Ice Plain in West Antarctica provide insight into how glaciers flow, generating seismic activity that indicates “slip-stick” behavior. (Credit: Jake Walter)

Glacier motion is not always graceful motion. Some glaciers are downright jerky, slipping along in fitful bursts. To better understand the process, scientists are studying ice streams: regions of ice that move faster than their surroundings.

An ice stream is like a duffel bag on a sloping surface: sliding a little, then stopping, then sliding again. Each slip creates waves of seismic energy that ripple outward in all directions as gravity pulls the icy mass downhill.

Jake Walter, a geophysics graduate student at the University of California, Santa Cruz, presented a poster Monday morning about the movement mechanisms of these icy bodies during the American Geophysical Union’s Fall Meeting. Rather than flowing smoothly and continuously like honey, many glaciers may follow the “stick-slip” model of motion.

“We wanted to understand better the physics of how glaciers flow,” Walter says.

Analyzing the Whillans Ice Plain, an ice stream in West Antarctica 600 meters deep and about the width of California, Walter and colleagues determined the slipping motions originate in the southernmost part of the region known as the Ross Ice Shelf.

As the Whillans Ice Plain slipped, it sent out seismic waves at speeds of 100-300 meters per second, according to a March paper by Walter that was published in the Journal of Geophysical Research. The further apart in time the slips occurred, the faster the waves.

The whole ice-slipping phenomenon resembles an earthquake. In fact, scientists use some of the same instruments to measure ice stream flow as they use for earthquakes. Walter’s team monitored slipping energy waves using a network of seismometers, devices designed to measure motion of the ground. The seismometers detected a wave tumbling outward through the ice stream, from the point where the sliding began. Using a GPS much more accurate than the average Garmin, the scientists tracked the movement of the ice stream itself.

The seismic data helped pinpoint the origin of ice slipping, placing it further south on the Ross Ice Shelf than previously estimated. As for why the seismic waves traveled faster after longer pauses between slips, it could come down to freezing cycles. The intense pressure underneath the ice stream actually melts the water at the base, Walter explains. He hypothesizes that the longer intervals between slips might let that water refreeze, allowing for faster waves.

Compared with the glacial melts of global warming, the type of glacier flow studied here can stabilize the glacier as it slows down.

“In a way, you’re listening to the death of an ice stream,” Walter mused.

-Tanya Lewis is a science communication graduate student at UC Santa Cruz.