A large hummock on the 1980 landslide deposit from Mount St. Helens stands above a wetland area (foreground) that lies between other hummocks. These hummocks once formed part of the volcano's summit, which was removed by an enormous landslide on 18 May 1980. The landslide scattered the summit rocks widely in the North Fork Toutle River valley. Credit: E. Klimasauskas / US Geological Survey.

A hummock is a rounded or conical mound within a volcanic landslide or debris avalanche deposit. Hummocks contain a wide range of rock debris, reflecting the variation of deposits that previously formed the flanks of the volcano. Some hummocks contain huge intact blocks tens to hundreds of meters in diameter. Some of the original layering of lava flows and other deposits can be seen in these large hummocks, but most of the large rock fragments are thoroughly shattered. In other hummocks the rock debris is thoroughly mixed as if the material had been in a blender and thoroughly mixed together.