Volcano - Mount St Helens
Mt St Helens like many other volcanoes was formed by the melting of an oceanic plate (Juan de Fuca
Plate) as it subducted beneath a continental plate (North American Plate). The newly melted magma
rose to the surface, burning through the crust and leading to the formation of an explosive volcano.
Leading up to an eruption millions of gallons of liquid magma swells up into the main chamber of the
volcano. The pressure inside the volcano builds up continuously, only calmed temporarily by small
eruptions of ash, but even these eruptions cannot stop the immense pressure building up. In the end
all volcanoes, like a bottle of coke after being shaken, explode in violent eruptions of boiling ash,
rocks and lava.
The first eruption
In 1975, scientists warned that Mt St Helens may well erupt again before the end of the century and
in 1980 after a period of 120 years with not activity at all, Mt St Helens began to give a few warning
signs of the things to come.
On March 20th 1980 there were several small earthquakes. These earth tremors continued until on
the 27th March several small eruptions steam and ash occurred as the bulk of the magma rose up into
the volcano.
Mount St. Helens becomes active
Seven days after the earthquakes another small eruption exploded cold black ash 3 km into the air,
and the whole of the north side of the volcano began to bulge outwards, by 1.5 metres a day!!!
At this moment scientists knew that Mt St Helens was winding up to violently explode at any time.
After seeing these small eruptions, most people, most sensible people evacuated the area. However
some people, like this old man, who lived half way up the volcano, refused to leave.
Monitoring the eruption
The government decided that an area 13 km wide around the volcano should be completely cleared of
all people. As April rapidly approached people were still fleeing from the area. The north side of the
volcano was still bulging at about 5 feet a day and the earthquakes under the volcano continued,
getting stronger and stronger every day.
The only people who stayed behind were the scientists who were determined to carrying on watching
the volcano and a few lumber jacks working in the forests.. Months passed and the volcano continued
to grow more and more uneasy, bulging along the whole of one side and suffering frequent
earthquakes. Then on the 14th May 1980 the earthquakes stopped. To an observer the volcano looked
peaceful, the rapid swelling of the north side of the volcano was the only clue that anything at all was
wrong, but the scientists knew this ‘was the calm before the storm’.
The final moments
The morning of the 18th May 1980 was a beautiful morning, there were no clouds and the sun was
shining, the bulging of the volcano continued but there were still no earthquakes. Everything looked
peaceful.
But at 8.32 am all that changed…. At 8.32 in the morning an earthquake, magnitude 5 on the Richter
scale caused the bulge on the northern slopes of the volcano to move forwards and downwards.
The earthquake caused the entire northern flank of the volcano to collapse. As it did so the pressure
which had been building up inside the volcano was violently released sending billions of tonnes of rock,
ice, and soil racing away from the volcano.
This massive blast sent a wave of material rushing down the volcano. This blast wave known as a
pyroclastic flow travels at over 100 km an hour, and is over 1000 degrees Celsius in temperature, and
is the volcanoes most violent weapon.
The blast from the eruption in 1980 was so powerful it could be clearly seen from space. For the rest
of the morning a series of eruptions sent gas, ash and lumps of red hot volcanic rock known as lava
bombs hurtling 20 km into the air.
Failure to escape
Due to the knowledge, understanding and warnings of the scientists studying the volcano the loss of
life was very low. Still, the blast was more intense than anyone had anticipated and the death toll
reached 60 people
The blast not only killed people but completely flattened the entire area surrounding the volcano. As
you can see from pictures, anything in its way was levelled. Forests over 10km away from the volcano
were completely flattened. All of trees and everything that lived in them was destroyed. But the
volcano had one more surprise.
When the volcano erupted, the sides of the mountain were still covered with ice and snow from the
winter. The intense heat melted the ice and snow to release 200,000 million litres of water. The
water turned the rock, ash and destroyed trees from around the volcano into huge mudflows which
poured down the river valleys raising the water level by up to 200 metres in places.
These raging torrents destroyed roads, bridges, railways and swept people away to their deaths.
After the eruption had died down several days later, the ash that it had thrown into the atmosphere
had completed circled the world. This ash blocked the light from the sun and lowered the
temperature of the earth.
Mt St Helens is a volcano success story. Despite being one of the most explosive and violent
eruptions in the last century, blowing away nearly half a kilometre in height of its summit, it killed
very few people and all because of the successful monitoring and later evacuation.