The debris avalanche deposit is
made of rocks and rock layers that once filled the crater and formed the
top 1,300 feet of volcano.
The debris avalanche deposit is
made of rocks and rock layers that once filled the crater and formed the
top 1,300 feet of volcano.
The most obvious characteristic
of the deposit is the hundreds of mounds and hills that geologists call
hummocks. They range in size from 25 to 600 feet.
The hummocks formed as whole
pieces of the avalanching mountain came to rest, and then slumped. This
left behind an elevated area or hill.
A second, less obvious,
characteristic of this deposit is the volcano's old rock layers. These
old lava flows, pyroclastic flows, debris flows, and ash layers were
preserved in layers, even though they slid several miles from the
volcano.
The layers can be found right side
up.
They can be found upside
down.
And they can even be found tilted
sideways.
Most of the rocks in the layers
are thoroughly shattered and many are offset by thousands of small shear
zones.
A third characteristic of the
debris avalanche is the horse-shoe shaped crater that it left behind.
When such a crater is found above
a landscape filled with hummocks made of layers of volcanic rocks, a
debris avalanche almost certainly occurred in the past.
These distinct characteristics
have helped geologists document more than 200 other volcanic debris
avalanche deposits around the world.
The discovery of so many
deposits clearly shows that large scale debris avalanches occur more
frequently than scientists previously recognized. These discoveries also
show that debris avalanches are a new hazard that must be taken into
account when a volcano becomes active.
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Mount St. Helens & Other
Volcanoes
Debris Avalanche
During the Eruption
Mount St. Helens
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