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Mt St Helens
Mt St-Helens, a volcano cloaked with ice, snow and forest rested quietly for 123 years until rattled by an earthquake on March 20 th, 1980. The horseshoe-shaped crater formed when the top (1300 ft / 396 m.) of the volcano collapsed in a debris avalanche during the eruption on May 18 th, 1980. During the first nine hours of the eruption, the volcano spewed out 540 million tons of ash which fell over an area of more than 22,000 square miles. In a matter of minutes, the mountain's green forested slopes were transformed into a desolate gray moonscape : the eruption wiped out 234 square miles of forests.
elevation of crater rim = 8,365 ft / 2,550 m. elevation of the dome = 885 ft / 270 m. Lava too sticky to flow : thick, pasty lava forming the crater dome is almost one million times more viscous than the lava erupting from Hawaiian volcanoes. The dome's dacite lava has a much higher silica content than Hawaii's fast flowing basalt lava. Consequently, dacite lava piles up around the vent and moves away very slowly, building a mound shaped dome instead of a thin lava flow. Lava Dome erupted numerous times between October 1980 and October 1986. Steam is still sometimes going out of the dome...
When Mt St-Helens erupted, the volcano did not produce flowing molten lava, it produced super-heated glowing avalanches of pumice rocks, gasses and ash called "pyroclastic flows". During the eruption, pumice flows of 1600�F slid down the volcano's slope at over 150 mph. Two weeks later, the flows were still 780�F. |
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