<P> The flow deposits were still at about 300 to 420 ° C (570 to 790 ° F) two weeks after they erupted . Secondary steam - blast eruptions fed by this heat created pits on the northern margin of the pyroclastic - flow deposits, at the south shore of Spirit Lake, and along the upper part of the North Fork Toutle River . These steam - blast explosions continued sporadically for weeks or months after the emplacement of pyroclastic flows, and at least one occurred a year later, on May 16, 1981 . </P> <P> 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 . </P> <P> Strong high - altitude wind carried much of this material east - northeasterly from the volcano at an average speed of about 60 miles per hour (100 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 eastward, 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 . </P> <P> During the nine hours of vigorous eruptive activity, about 540,000,000 tons (540 × 10 ^ short tons or 490 × 10 ^ t) of ash fell over an area of more than 22,000 square miles (57,000 km). The total volume of the ash before its compaction by rainfall was about 0.3 cubic miles (1.3 km). The volume of the uncompacted ash is equivalent to about 0.05 cubic miles (0.21 km) 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 . </P>

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