Geotimes - October 2003 - New model for glacial erosion

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Glaciology
New model for glacial erosion

Glaciers are the most powerful and efficient erosive agents on the planet — literally moving mountains and reducing them to rubble, as the ice sculpts and carves the land. Historically, glaciologists have lacked knowledge about what governs glacial erosion, hindering their attempts to model the erosive processes. What has been needed, they say, is a set of rules such as those that govern flow rates, erosion and deposition in graded rivers and streams. Understanding what controls glacial erosion may have important implications for understanding glaciated mountain belts and modeling both ancient and current ice sheets.

Glaciologists have studied the Matanuska glacier margin shown here, finding that the slope of a glacier’s bed plays a key role in whether the ice mass erodes or deposits the sediment beneath it. Photo courtesy of Daniel Lawson, Cold Regions Research and Engineering Lab.

Now, glaciologist Richard Alley of Penn State and colleagues have proposed that the relationship between the slope of a glacier’s bed and its surface determines whether a glacier is eroding or depositing sediment beneath it and that, over time, these slopes will tend toward equilibrium angles.

“We find that the long profiles of beds of highly erosive glaciers tend towards steady-state angles opposed to and slightly more than 50 percent steeper than the overlying ice-air surface slopes,” the authors reported in the Aug. 14 Nature.

The study is yet another round in the debate over how glacial erosion occurs, which, in the last twenty years, has seen the emergence of two main positions.
“Some researchers consider glaciers and ice sheets rather firmly coupled to their beds — shear stresses are transferred to the substratum and cause erosion of both hard and soft beds,” says Jan Piotrowski, a glaciologist at the University of Aarhus in Denmark. “Others envisage meltwater flowing in channels or in sheets at the ice-bed interface as the important, if not dominant, agent of glacial erosion.”

The Nature study follows the second camp, and is extremely important, says Öskar Knudsen, a glaciologist with Klettur Consulting Engineers in Reykjavik, Iceland. “It explains sedimentation at Vatnajökull ice cap, which is the largest temperate body of ice on Earth and possibly the best analogue to the Laurentide and Scandinavian ice sheets,” he says. “The hypothesis therefore helps us to understand processes that operated at these ice sheets 10,000 years ago.”

Knudsen adds that the paper has been met with some skepticism because it presents an entirely new paradigm in glaciology and deals with the often-overlooked process of sedimentation beneath glaciers.

The new, so-called “graded glacier hypothesis” argues that the steeper the slope at the glacier’s surface, the more erosion is occurring down below. The slope controls both the pressure and water temperature at the base of the glacier, which in turn control the opening and closing of the meltwater channels that distribute sediment.

Normally, when a glacier is advancing and eroding, “the bed slope is at a relatively low angle and the ice slope is at a relatively high angle,” says co-author Daniel Lawson, a glaciologist with the Cold Regions Research and Engineering Lab in Hanover, N.H.

As meltwater flows down this slope from the glacier’s surface, it erodes and transports sediment along the ice-bed interface. Eventually, the meltwater begins to create a slight depressed area under the toe of the glacier. This gouge, called an overdeepening, can be several square kilometers or larger in size; meltwater along with its load of sediment must pass through it before being discharged from the glacier.

Under a glacier, pressure from the overlying ice depresses the freezing point and allows meltwater to remain liquid even at subfreezing temperatures in a process called glaciohydraulic supercooling. As the pressure decreases along the flow path, the
melting point is raised and the water, now below the new freezing point, must either warm or freeze. If the water is flowing in small channels and the pressure change is not too steep, friction will generate more than enough heat for the water to stay liquid. In fact, it produces excess heat that then further melts and widens the channel.

Over time the channels will grow to carry more and more water with a higher capacity to erode sediment, and the overdeepening will deepen and the down-glacier slope will steepen. Eventually, the slope will reach a critical angle at which, in its new wider channel, the water can no longer be warmed enough by friction to remain liquid. When the water ascends the slope, the pressure drops and frazil ice will begin to form on the walls of the channel. The effect is somewhat similar to the champagne slush that might result from suddenly uncorking an overchilled bottle.

“When you form frazil ice, you have individual crystals, and we think that the void space between the crystals can trap materials,” Lawson says, “which then freezes to the basal layer.” As the sediment-filled frazil ice builds up, the meltwater arteries become further narrowed and clogged, forcing the meltwater into a much more distributed shallow flow system, like the flow of a very thin braided stream. When this occurs, the glacier has entered what scientists call the supercooled phase.

Deposition begins because a distributed system is less capable of carrying sediment than the larger channels are. That deposition, in turn, will eventually shallow the slope of the overdeepening back toward the critical angle, called the supercooling threshold.

Alley’s team says that through this series of feedbacks, the glacier will tend toward an equilibrium state, which they estimate could take more than 50 years to achieve. However, a change to the slope of the glacier’s surface, due to climate change, for example, could disrupt the equilibrium. A cooler climate would cause ice to build and steepen the surface slope, which would increase the rate of erosion. A warmer climate would cause the surface slope to shallow, which would increase the rate of deposition.

Previous studies of glaciers in Iceland and Alaska, including the Matanuska glacier, have shown that glaciohydraulic supercooling is occurring and that slopes and sediment discharge rates are consistent with the new hypothesis. However, researchers say that they need additional data on the processes and the nature of the ice-bed interface.

Sara Pratt
Geotimes contributing writer

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