Photo Information

View of a landslide hummmock, Mount St. Helens, Washington
Hummock on 1980 landslide deposit at Mount St. Helens
Photograph by S.R. Brantley on July 21, 1982

Scientists at the base of a single hummock with internal rock layers and contacts exposed. This hummock represents a large block from the cone of Mount St. Helens that was deposited relatively intact despite traveling about 8 km and losing more than 1,000 m in elevation! The dark rocks on the left are lava flows that were erupted within the past 2,500 years and the light rocks on the right are parts of dacite lava domes that erupted 3,000-2,500 years ago. Note the irregular contact between the two units; the original contact was subhorizontal, indicating the block rotated as it slid to this location. These same rock layers are now exposed in the crater of Mount St. Helens.

Typically, two end-member types of deposits are used to describe the internal texture of landslide deposits:

  1. Block facies consists entirely of landslide blocks, coherent but shattered (unconsolidated) pieces of a volcano that were transported relatively intact.
  2. Matrix facies consists of completely mixed parts of the landslide deposit, generally consisting of all rock types from the original sliding debris and fragments of wood and bits of soil and rock from the ground beneath the deposit.
The hummock shown above is part of the block facies of the deposit at Mount St. Helens.

More about the landslide deposit at Mount St. Helens


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U.S. Department of the Interior, U.S. Geological Survey, Menlo Park, California, USA
Contact: VHP WWW Team
Last modification: 13 December 1999 (SRB)