Within a few hours of the
lateral blast, hot mixtures of volcanic gas, pumice, and ash swept down
the north flank of the volcano at speeds up to 100 miles an hour. These
mixtures of gas, pumice and ash are called pyroclastic flows. The
temperature of these pyroclastic flows was extremely hot -- more than
1200 degrees Fahrenheit.
The huge ash plume rising from
the volcano hid the pyroclastic flows that occurred throughout the
day.
North of the volcano, these flows
deposited pumice and ash in layers as thick as 60 feet.
Pyroclastic flow deposits are
easily eroded by wind and water because they are made of loose ash and
pumice.
North of the volcano and directly
in front of the crater is a fan-shaped deposit area known as the Pumice
Plain, because it is covered with pyroclastic flow deposits.
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Mount St. Helens & Other
Volcanoes
During the Eruption
Mount St. Helens
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