Mount St. Helens

Mount St. Helens

Mount St. Helens is an active volcano. It is a tall conical volcano built up by many layers of hardened lava, tephra, pumice and volcanic ash (a stratovolcano). This stratovolcano is located in Skamania County, Washington, in the Pacific Northwest region of the United States of America. It is 96 miles south of Seattle, Washington. The volcano is located in the Cascade Range and is part of the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active volcanoes. This volcano was very well known for its ash explosions and pyroclastic flows.

Before the 1980 blast, surrounding the mountain was an area of outstanding beauty filled with a huge forest of trees and wildlife.

But before the huge explosion this is what was happening:

Magma or molten material was moving up from a deep reservoir beneath the mountain up into the volcano itself and it began to grow and form a cryptodome inside the volcano.

As we can see from this diagram the Juan de Fuca plate was pushed under the North American plate which was directly under Mount St Helens which caused the magma to build up inside before it release the colossal explosion.

We can also see that there are also many volcanoes nearby so there could have been many eruptions in the area but Mount St Helens was the closest one to the plate boundary which made it the biggest and most powerful explosion.

For over 120 years the volcano was dormant. Nobody was sure what to expect but on Sunday morning at 8:32am, may 18 1980, a 5.1 magnitude earthquake rocked the mountain and within 10 seconds, the volcano's northern flank collapsed.

This was the largest landslide in recorded history. It released millions of tons of magma causing a colossal explosion as you can see from picture 4. A cloud of searing hot gas and rock shot up into the stratosphere whilst a huge pyroclastic flow going at several hundered milles an hour raced across the countryside. The forest was flattened. Across more than 200 square miles, the surge of ash and rock incinerated trees.

4 miles from the volcano was the enormous spirit lake which was choked with debris and was scarcely recognisable. The avalanche lifted its bed more than 200 feet and now the surface is smothered in dead trees and lots of ash from the pyroclastic flow. All of the ash from the pyroclastic flow mixed with the water and therfore hundreds of species of aquatic life including insects, amphibians and fish were all killed.

'It was black water, if you put your hand in up to your wrist, you wouldn't even be able to see your finger that's how grossly modified it was.' - Charlie Crisafulli, ecologist, USDA forest service.

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