Pressure and stress can lead to a crack-up, or several, if you are a glacier. Researchers have discovered a system of deep cracks extending from the ground up into the overlying ice of southern Alaska’s Bench Glacier. The crevasses are described in the Sept. 30 Nature.
The scientists aren’t certain how widespread such cracks are, how long the fissures persist or how glacier movement might be influenced. The cracks hold a considerable amount of water and could help scientists better predict how and when glaciers move and melt, which in turn could affect how much sea levels rise in the future.
Water pressure below the glacier and stress from the massive weight above probably produces the cracks, say the scientists. The cracks, some of which are 80 meters deep, could act as a buffer, allowing the glacier to absorb, sponge-style, a sudden inrush of water from melting or rain. This draining may prevent the sliding and lurching that can happen when water rushes into the bed of soil and rock under a glacier. But under certain conditions, the cracks might suddenly drain, and that could spill water into the bed and send a glacier sliding.
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“Water is tricky business,” says Jack Kohler of the Norwegian Polar Institute in Tromsø, who was not involved with the research. “It short-circuits things and allows things to happen very rapidly. Water is at the heart of really all the dynamic behavior of a glacier. But the full physics of the links between the drainage system and the sliding is still an unsolved problem.”
The researchers who discovered the cracks weren’t looking for them. Led by Joel Harper of the University of Montana in Missoula, the team set out to drill holes through Bench Glacier’s roughly 200 meters of solid ice to see how the holes would drain. Because the drill bit was made of hot water — a jet of snow melted to 160 degrees Celsius —the bore hole stayed filled to the brim during the drilling.
“But then all of a sudden, boom, it drains right down,” says Harper. The scientists stuck a video camera down the hole and realized they had hit a huge crevasse, he says. “It sucked all the water out of our hole.”
The researchers had drilled only 120 meters — still 80 meters from the bottom of the bed.
“We thought, ‘Well, that’s one in a million,’” Harper says. “But sure enough, it happened again. And it just kept happening.”
The team drilled nearly 30 holes at various spots on the glacier — the majority of which drained prematurely, suggesting numerous cracks spike up into the ice. To figure out how much water was in the crevasses the team used radar and seismic equipment to probe what was water and what was ice deep in the heart of the glacier.
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The crevasses all appear to be oriented at 45 degree angles from the glacier’s side walls. Within the cracks was enough water to cover the entire glacier to a depth of 10 centimeters, the team reports. Research suggests that flooding the bed of a glacier with just 4 centimeters of water can increase the typically plodding pace of the massive hunk of ice by a factor of five.
But key to that increase in velocity is the water’s sudden arrival. “A little water suddenly has more of an effect than a lot of water slowly,” say Kohler. “Generally, the more water that goes through a system, the more it can handle pressure and perturbations.” Typically after an onslaught from spring rains and melts, a glacier may lurch forward. But eventually a drainage system develops and water flowing through the system can run out. “It’s not static,” says Kohler. “It’s not like a cast iron pipe system.”
This new work goes a long way toward explaining where in a glacier water goes, be it from rain, surface melt or melting at the ice-earth interface, says Andrew Fountain of Portland State University in Oregon.
“All our calculations show a large amount of water at the bed. And we all knew it could not be accommodated by conduits and subglacial cavities, but nobody knew where to put the water,” says Fountain. “We would all just talk about it being stored someplace in the body of the glacier.”
Much more work needs to be done on other glaciers and ice sheets to figure out how widespread such glacial cracks are, how much water hides within them, and what role they play in glacier and sea level dynamics, says Harper. His team spent the summer on the western side of the Greenland ice sheet boring more holes. The preliminary data suggest there are grounded crevasses there as well.
“Now we’re saying to the world, keep an eye out,” he says. “These could be in other places.”