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Narrative of Mount St. Helens Moments after the outrush of
the avalanche and ash cloud, enormous mudflows slid off the mountain down several of the
adjacent river valleys. These flows were caused by water from blast-melted glaciers and
snow that mixed with the already powdered rock to form pasty, muddy flows. These hot and
cold masses of mud swept down the valleys of several rivers, sweeping away buildings,
vehicles, trees, and even bridges. Trees amounting to more than four billion board feet of
salable lumber were damaged or destroyed by the near-supersonic lateral blast of rock,
ash, and hot gases. One of these flows even reached and blocked the shipping channel of
the Columbia River, 55 miles downstream. Photo: Courtesy of NGDC/NOAA.
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In addition to the ash cloud
that stayed near the ground, millions of tons of fine ash were thrown high into the air,
and carried hundreds and thousands of miles downwind. These clouds, easily seen in
satellite images, dropped several inches of ash over many communities and agricultural
areas, ruining machines and crops. Photo: Courtesy of NGDC/NOAA.
"Surface details of the
debris (mud) flow on the North Fork of the Toutle River near Coldwater Creek (click on
map). There is approximately 50 feet of relief between the pond and the mud flow surface.
Pyroclastic flows together with hot ash from nuČes ardentes(superheated masses of
superheated masses of gas-charged ash that has been explosively expelled) melted the snow
and ice on the flanks of the mountain and created these mud flows. Debris filled the North
Fork valley of the Toutle River to a depth of 200 feet." Photo:
Courtesy of NGDC/NOAA.