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.
|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 / U.S. Geological Survey
AND PLANETARY SCIENCE