English: This false-colour image of Susitna Glacier looks similar to an areal photograph, except that vegetation is red. Susitna Glacier’s surface is marbled, combining dirt-free pale blue and dirt-coated dark brown ice. Infusions of relatively clean ice push into the glacier from tributaries flowing from the north. The glacier surface appears especially complex near the centre of the image, where ice from a tributary has pushed ice in the main glacier slightly southward. Susitna Glacier flows over a seismically active area. In fact, a 7.9-magnitude quake struck the region in November 2002, along the previously unknown Susitna Glacier fault. Although geologists surmised that this and other earthquakes left steep cliffs and slopes on the glacier surface, the complex glacial surface apparent in this image results from surges of 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 of the glacier. The water provides lubrication that quickens flow. This water may be supplied by melt-water lakes that accumulate on top of the glacier, and melt ponds appear on the Susitna Glacier, in the lower left corner of this image. The nature of the underlying bedrock might also contribute to glacier surges, with soft, easily deformed rock leading to more frequent surges.