VHP Photo Glossary: Hummock
Photograph by E. Klimasauskas on 30 April 1998
Hummocks
are rounded or conical mounds 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.
More information about hummocks
- Texture of hummocks at Mount St. Helens
- Examples of hummocks and other volcanic landslide deposits from around the world
Did you know?
- Studies of the hummocky debris avalanche deposit at Mount St. Helens have led to the identification of hundreds of volcanic debris avalanche deposits around the world.
- The 1980 Mount St. Helens debris-avalanche deposit contains as many as 625 individual hummocks.
- The debris-avalanche deposit at Mount St. Helens blocked tributary river valleys, which filled with water to form two new lakes within a few months of the 1980 eruption.
