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Mount St Helens 18 May 1980 eruption Mount St Helens 18 May 1980 eruption
The beast sleeps...
On 20 March 1980, after a quiet period of 123 yr, earthquake activity resumed. Small phreatic (steam) explosions began 7 days later.
Bulge developed as magma pushed up within the peak. It grew at up to 1.5 m/day. By May 17th it had inflated by 135 m.
at 08:32, 18 May 1980... …B A N G ! (get out now)
Ash plume erupted for > 9 hours Reached 12-15 miles high Rolled east, averaging 60 mph.
Horseshoe crater from NE
Volcanic ash deposited several 100 miles downwind. Noticeable ash fell in 11 states. Total volume of ash ~1 cubic km before compaction by rainfall.
Trees flattened by lateral blast
Debris avalanche travelled ~24 km downstream at >150 mph. It left behind 2.3 cu km of a hummocky deposit with avg. thickness of 45 m and max thickness of 180 m.
Mudline left by lahars (volcanic mudflows)
Lahar from 19 March 1982 eruption
More than 200 homes and over 185 miles of roads were destroyed by the 1980 lahars.
At least 17 separate pyroclastic flows descended the flanks of Mount St Helens during the 18 May eruption. Typical flow speed > 60 mph, Typical temperature > 400°C.
Pumice blocks at toe of pyroclastic flow
The action continued... 5 more explosive eruptions during 1980...
After the May 1980 eruption... Activity continues, erupting DOMES
300 m wide 34 m high (9 storeys) 1980
Dome growth via stubby lava lobe, extruding from a “spreading centre”. Lobe growth rate 1-3 m/hr. 1981
400 m wide 163 m high (44 storeys) 1981
By 1990, the dome had replaced 3% of the volume removed by the 18 May 1980 eruption. At this rate it will take over 200 years to rebuild Mount St Helens to its pre-1980 size. 1983
800 m wide 230 m high (66 storeys) 1985
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