Ash Column Grows
As the avalanche and initial pyroclastic flow were still advancing, a huge ash column grew to a height of 12 miles (19 km) above the expanding crater in less than 10 minutes and spread tephra into the stratosphere for 10 straight hours. Near the volcano, the swirling ash particles in the atmosphere generated lightning, which in turn started many forest fires. During this time, parts of the mushroom-shaped ash-cloud column collapsed, and fell back upon the earth. This fallout, mixed with magma, mud, and steam, sent additional pyroclastic flows speeding down St. Helens' flanks. Later, slower flows came directly from the new north-facing crater and consisted of glowing pumice bombs and very hot pumiceous ash. Some of these hot flows covered ice or water which flashed to steam, creating craters up to 65 feet (20 m) in diameter and sending ash as much as 6,500 feet (2,000 m) into the air.
Strong high-altitude wind carried much of this material east-northeasterly from the volcano at an average speed of about 60 miles per hour (97 km/h). By 9:45 a.m. it had reached Yakima, Washington, 90 miles (140 km) away, and by 11:45 a.m. it was over Spokane, Washington. A total of 4 to 5 inches (100 to 130 mm) of ash fell on Yakima, and areas as far east as Spokane were plunged into darkness by noon where visibility was reduced to 10 feet (3.0 m) and 0.5 inches (13 mm) of ash fell. Continuing east, St. Helens' ash fell in the western part of Yellowstone National Park by 10:15 p.m. and was seen on the ground in Denver, Colorado, the next day. In time ash fall from this eruption was reported as far away as Minnesota and Oklahoma, and some of the ash drifted around the globe within about 2 weeks.
During the nine hours of vigorous eruptive activity, about 540,000,000 short tons (4.9×1011 kg) of ash fell over an area of more than 22,000 square miles (57,000 km2). The total volume of the ash before its compaction by rainfall was about 0.3 cubic miles (1.3 km3). The volume of the uncompacted ash is equivalent to about 0.05 cubic miles (210,000,000 m3) of solid rock, or about 7% of the amount of material that slid off in the debris avalanche. By around 5:30 p.m. on May 18, the vertical ash column declined in stature, but less severe outbursts continued through the next several days.
Read more about this topic: 1980 Eruption Of Mount St. Helens
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