The May 18th eruption began when an
earthquake, magnitude 5.1 on the Richter scale, jolted the volcano.
This caused the north flank of the mountain to break loose. This enormous
block slid off the mountain at
more than 100 miles an hour. A second block collapsed behind the first
one. Then a third block broke free. It was completely obscured by the
lateral blast.
Together, these blocks slid from
the volcano as a massive
avalanche of rock, ice, snow, and soil. Geologists call this a debris
avalanche.
The jumbled rocks and mounds of the
debris avalanche deposit fill
the upper 14 miles of the North Fork of the Toutle River valley.
The average depth of the deposit is
about 150 feet. In places it is as deep as 600 feet.
This remarkable deposit has led to
the discovery of more than 200 similar deposits at other volcanoes around
the world.
And it has had severe consequences
for people living downstream along the Toutle and Cowlitz Rivers.
After the eruption, the potential
for large floods and debris flows
in the Toutle River valley increased dramatically.
There are two reasons for this.
The unstable debris dammed streams, forming new lakes...
And the deposit is made of loose sediment
that is easily eroded and transported downstream.
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Mount St. Helens & Other
Volcanoes
Debris Avalanche
During the Eruption
Mount St. Helens
VolcanoWorld