Picture of the Day: The Dirty Tracks of Alaska's Susitna Glacier

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Glaciers, like rivers, flow downhill, gathering dust and dirt along the way -- and leaving a trail. This image, a composite of several infrared snapshots captured by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite on August 27, 2009, shows the long, slow journey of Alaska's Susitna Glacier.

"Vegetation is red and the glacier's surface is marbled with dirt-free blue ice and dirt-coated brown ice," NASA explained. "Infusions of relatively clean ice push in from tributaries in the north. The glacier surface appears especially complex near the center of the image, where a tributary has pushed the ice in the main glacier slightly southward."

Geologists once believed that some of the slopes and steep cliffs seen on the glacier's surface were caused by a 7.9-magnitude earthquake that shook the region in November 2002. It seems, though, that they were in fact caused by surges, or quick bursts of movement, in tributary glaciers. "Glacier surges -- typically short-lived events where a glacier moves many times its normal rate -- can occur when melt water accumulates at the base and lubricates the flow," according to NASA. "This water may be supplied by meltwater lakes that accumulate on top of the glacier; some are visible in the lower left corner of this image. The underlying bedrock can also contribute to glacier surges, with soft, easily deformed rock leading to more frequent surges."

View more Pictures of the Day.

Image: NASA.

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Nicholas Jackson is a former associate editor at The Atlantic.

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