Slides in this set
Background of St Helens
Mount St Helens, Washington State, NW
USA is located in the Cascade mountain
range and prior to its eruption in 1980 it
had been active for over 100 years
The volcano sits on a destructive boundary
where the Juan de Fuca plate meets the
North American plate.
The volcano itself is a stratovolcano…read more
Nature of volcanic hazard
Mount St Helens erupted on May 18th 1980 following a period of
activity which began in March 1980 with an earthquake measuring 4.0
on the Richter scale.
What followed was 3 months of seismic activity as magma rose within
As the magma rose, a large bulge grew on the north flank of the
volcano, this was due to a blockage in the main vent resulting in the
growth of a crypto dome (mound of viscous lava) in the side of the
On May 18th, an earthquake measuring 5.1 on the Richter scale
caused a landslide on the northern flank of the volcano, which in turn
exposed the crypto dome below, resulting in a sudden release of
pressure and a cataclysmic eruption in the form of a lateral (sideways)
The blast zone consisted of 230 square miles with the eruption leaving
a 'lunor' landscape in its wake.
The eruption packed the force of 27,000 Hiroshima bombs.…read more
A large number of wildlife were killed by the blast and the
volcanic ash with nothing surviving in the blast zone
The eruption melted snow, ice, and glaciers on the
mountain resulting in mudslides.
Lava flows and ash filling in Spirit Lake and log jams and
ash blocking the channel of the Toutle River
Flooding resulting from blocked rivers washed away road
and rail bridges
The Toutle River was flooded by melting snow and ice
from the mountain.
The soil in the surrounding area became more fertile
for farming…read more
57 people died in the eruption - most from poisonous
300km of main roads destroyed
200 homes were destroyed or damaged
Crops were ruined and livelihoods of loggers were
devastated with large areas of trees being flattened like
Shipping was stopped on the Columbia River and some
vessels were stranded.
In eastern Washington, falling ash stranded 5,000
motorist. Ash had to be cleared from runways and