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
Did you know?
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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.
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The 1980 Mount St. Helens debris-avalanche deposit contains as many as 625
individual hummocks.
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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.
Related photo glossary terms: