As the ash column rose, the
edges collapsed onto the slopes of the newly formed crater.
As the ash column rose, the
edges collapsed onto the slopes of the newly formed crater.
This superheated ash quickly
melted and mixed with existing snow and ice. Then it began to flow
downslope.
The water quickly picked up
rock debris as it sped from the volcano. This debris included boulders as
large as 20 feet in diameter.
Mudflows can travel as fast as
30 miles an hour. They can easily rip trees, houses, and bridges from
the ground.
Mudflows sped from the
volcano's west, south, and east sides within minutes of the eruption.
The largest mudflow started on the debris-avalanche deposit about 4 hours
later.
Most of the water that poured
across the surface of the debris avalanche deposit came from the deposit
itself. This included water that had been trapped inside the volcano,
and melting blocks of ice that had been glaciers on the volcano.
The mudflow sloshed from side-to-side
as it rushed through forests and clearcuts. It left its
unmistakable mark on the land.
From the toe of the deposit,
millions of tons of debris broke loose and devastated downstream
communities.
Homes were carried away and deposited many miles downstream, or
simply, destroyed by
bridges.
The mighty Columbia River was
closed to freighter traffic for several days as the debris was dredged
out.
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Mount St. Helens & Other
Volcanoes
During the Eruption
Mount St. Helens
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