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Mount St. Helens
Washington, USA
18th May 1980…read more

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Facts about the volcano:
· An active stratovolcano.
· Had fairly symmetrical sides, with alternating layers of lava and ash
· Prior to 1980 the 5th highest peak in Washington. (Before - 9,677ft, after
· Had andesitic lava, common in explosive volcanoes where the viscous
magma blocks the main vent of the volcano
· Began growing around 37,000 years ago ­ one of the youngest in the
Cascade Range. The summit cone started growing less than 3,000 years
ago, also making it the most active within the Cascade Range.…read more

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Slide 4

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Facts about the 1980 eruption:
· 20th March 1980 ­ a 4.1 magnitude earthquake signalled that the volcano
was active for the first time in more than 100 years.
· All through April, the north side of the volcano grew by up to 5ft a day.
· 18th May 1980 ­ a magnitude 5.1 earthquake triggered the largest
landslide in recorded history as the volcano erupted.
· 57 people killed.
· The lateral blast flattened 9 million m3 of timber, enough to build 300,000
two bedroom houses.
· The most economically damaging volcanic eruption in US history - $2.74
· The eruption column of ash rose over 80,000ft in less than 15mins.
· Released 24 megatons of thermal energy. This is equivalent to 1,600 times
the size of the atomic bomb dropped on Hiroshima.…read more

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Landslide debris covered 23 square miles and travelled up to 150mph.
· The lateral blast covered 230 square miles and travelled at least 300mph.
· Lahars travelled up to 50mph, destroying27 bridges and 200 homes.
· Pyroclastic flows reached as far as 5 miles from the crater, travelled
between 50-80mph and reached temperatures of 700°C.…read more

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