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Slumping Ice Cliffs May Trigger Rapid Sea Level Rise

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Ice cliffs melt and flow into the ocean in a process called calving, but scientists in Greenland believe another process could trigger rapid sea level rise.

A team of researchers that flew over the Helheim and Jakobshavn glaciers on Greenland's eastern coast determined in a new study that a process called slumping could break off much larger chunks of ice at a faster pace. The implications of this process can be huge.

Richard Alley, one of the co-authors of the new study published in the journal Geology, said that if what happened in Helheim will also occur in Antarctica over the next hundred years, models suggest that rapid sea level rise could surpass those that were already predicted.

What Is Slumping?

Glaciers that drain ice sheets often end in near-vertical cliffs. As the glacier moves into the sea, chunks of it break off.

Such calving events often occur when the sea melts the front of the ice and the ice cliff falls down. However, Alley and his teammates determined that large mass does not need to melt to flow into the ocean because it requires something solid. This is what slumping is all about.

The Helheim glaciers end abruptly in the ocean in ice cliffs that reach at least 100 meters high. In a recent flight, scientists saw large cracks called crevasses on top of the Helheim ice which marched toward the end of the glacier.

Alley explained that slumps occur when a mass of sediment or rock loses its strength, breaks away from land, and slides down a slope. Slumps are usually marked by a scarp where the material broke away, which is followed by material that moved downslope.

The features of Helheim glacier are usually what can be found in a slump-prone area. Because of this, Alley and his fellow scientists wonder if the ice will suffer the same fate. He explained that there is a head scarp and then the stresses within the ice that is maximized down at water level.

How Slumping Occurs In The Arctic

Alley said geologists have been worrying over slumps for decades. He and his team decided to test if slumps indeed occurred on ice cliffs by monitoring the Helheim glacier during a calving event. They measured the position, speed, and motion of the ice.

What they saw was that there had been a significant ice flow acceleration before the initial slump happened and then there, the thickness of the calving of the glacier rotates. The remaining ice cliff rotates, reaching both below and above the sea.

Alley and his colleagues realized that slumping occurs among tall cliffs that reach 100 meters of ice above water. Regular calving events usually happen very slowly because the ice needs to melt. On the other hand, slumping happens without even waiting for the ice to melt, researchers have found.

"We'll go slump... basal crevasse... boom," said Alley.

In some places in Antarctica, the glacial bed can be found at least 2,000 meters above sea level, creating a much taller ice cliff. Alley said the worry is that taller cliffs are more susceptible to slumping. If slumping occurs in Antarctica, it could lead to more rapid sea level rise.

In the meantime, Alley and his team are gearing up for more investigations in the future. He said they still want to understand ice breakage and how slumping applies to it. They plan to collect more observational data to refine their models.

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