Mount St. Helens Eruptive Activity, 1980-1984
Explosive Eruptions
Following May 18, Mount St. Helens erupted explosively five times
during 1980. None of these eruptions was as large as the events on May
18, but each eruption produced ash columns 25,000-50,000 feet above sea
level and hot, dry pyroclastic flows of pumice and ash that swept down
the north flank as fast as 60 miles per hour. These pyroclastic flows
deposited ash and pumice fragments in fan-like patterns of sheets,
tongues, and lobes in an area extending up to five miles north of the
vent. Individual pyroclastic-flow units were generally less than 15 feet
thick, and maximum temperatures recorded several hours after their
deposition ranged from about 570 to 1,350 degrees Fahrenheit. The
thickness of air-fall deposits ranged from one-third to one-fortieth
that of the May 18th air-fall deposit at a given distance from the
volcano.
Lava extruded from the vent and formed lava domes within a few days
after the June 12, August 7, and mid-October explosive eruptions. The
June and August domes were blown away by subsequent explosive eruptions,
but the October dome survived to form the core of the present dome.
Domes are formed by thick, pasty masses of lava too sticky to flow
very far from the vent. Lava of the Mount St. Helens dome is dacite. It contains a higher
percentage of silica than the Hawaiian basalts
and is about one million times more viscous.
Dome-building Eruptions
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Eruptive Activity
Living Lab Curriculum
Mount St. Helens