Main Eruption On May 18, a quiet Sunday morning, a few
volcanologists were at their stations, watching Mount St. Helens. A few tourists and
loggers were also nearby. At 8:32 A.M. a small aircraft with two geologists aboard flew
directly over the central cone. Eleven seconds later, a strong earthquake shook Mount St.
Helens, and the whole north face of the mountain broke free and slid downward as a giant
rock avalanche. In seconds, as the rock slid off the mass of hot lava inside the mountain,
pressure in the lava dropped, and water that had been dissolved in the lava turned
into superheated steam, which formed bubbles that violently expanded and fragmented the
lava into a fine powder ash. This mass of superheated steam and ash blasted upward and
outward over the top of the avalanche, roaring to the north and west at speeds reaching
hundreds of miles an hour. The pilot of the small aircraft narrowly avoided disaster by
putting the "plane into a steep dive to gain speed" and turning sharply south,
away from the expanding ash cloud. Photo: Courtesy of NGDC/NOAA, (D. Wellman).
Every living thing within about 10 miles of the volcano on the north side--tree or bush, human or beast, scientist or layman--was doomed. Some of the people took a few quick pictures. Then, realizing their situation, most ran or tried to drive away from the approaching cloud of dust and steam. The near-supersonic lateral blast of rock, ash, and hot gas engulfed the area with a force sufficient to strip huge trees bare and uproot or break them off at ground level. The temperature within the cloud reached 500ºF, sufficient to kill, cause serious burns, and start fires. The rock avalanche roared over Spirit Lake and the valley of the North Fork of the Toutle River, burying them under layers of rock up to several hundred feet thick. Photo: Dept. of Natural Resources, State of Washington. Courtesy of NGDC/NOAA.
Moments after the outrush of the avalanche and ash cloud, enormous mudflows slid off the mountain. These pasty mudflows formed when the glacial ice and snow that had capped the mountain were melted by the intense heat and mixed with the powdered and fragmented rock created by the eruption. The resulting hot and cold masses of mud poured down adjacent river valleys, sweeping away buildings, vehicles, trees, and bridges. One flow even blocked the shipping channel of the Columbia River, 55 miles downstream. Photo: Dept. of Natural Resources, State of Washington. Courtesy of NGDC/NOAA.
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, (D. Schoolcraft).
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