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:

1. Some hummocks are bounded by faults or slumping, which suggests that they formed as the landslide spread laterally--the slope of the hummock represents the slip surface of the fault or slump.
2. Some hummocks represent the surface topography of individual landslide blocks with no faulting between hills; the blocks may be suspended in rock debris that has been thoroughly mixed (i.e. the original layers of the volcano are not preserved).
3. Some hummocks formed as the basal part of the landslide slowed and the faster-moving upper part either sculpted the material into elongate hills parallel to the direction of flow or piled up the material into randomly-oriented elongate hills.

More about the landslide deposit at Mount St. Helens

• Detailed report, available online in PDF