View of the hummocks that mark the surface of the 1980 landslide deposit at the base of Mount St. Helens, Washington. The deposit buries the North Fork Toutle River valley (foreground) with rock debris as deep as 200 m. Individual hummocks in the foreground are 20-40 m wide (only a few of the largest hummocks are indentified by a line). Note the deep channels eroded into the landslide deposit by running water in only 5 years.
A hummocky topography with many hills and closed depressions is probably the most distinctive feature of a volcano landslide deposit. The hills vary widely in shape from circular to elliptical or oval, and range in dimension from a few meters to more than 1 km at their bases. In many cases, the long axis of the hills point radially away from the volcano parallel to the flow direction. The hills vary in height from a few meters to more than 200 m tall.
Three mechanisms have been proposed for the formation of the hummocks at Mount St. Helens:
More about the landslide deposit at Mount St. Helens
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