Mount St Helens New Ppt

Mount St Helens is a volcano which is part of the Cascades Mountain range in Washington State in the USA

There are fourteen main volcanoes found in the Cascades Range. This mountain range runs along the north west coast of the USA.

Mount St Helens is the most famous as it erupted most spectacularly in 1980.

The Cascades Range is formed as part of a subduction zone where two plates meet at a destructive plate boundary. The smaller Juan de Fuca plate is subducted beneath the larger North American Plate. The Juan de Fuca plate is composed of more dense oceanic crust so it is forced below the less dense continental crust of the North American Plate.

As the crust is subducted there is friction between the two plates. This causes earthquakes.

The land on the edge of the continental crust gets squashed up to make fold mountains, like the Cascades Range.

The subducting oceanic crust melts as it goes into the mantle. As the plate is subducted it takes some sea water with it this makes it less dense than the mantle around it. This allows the material from the melting plate to rise. It goes up through the crust and explodes at the surface as a volcano. The trapped sea water turns into steam and this makes the volcanoes very explosive. There isn’t much lava but there is a lot of ash, steam and gas.

* On 20th March 1980 there was an earthquake under the mountain. It was caused by the magma beginning to move.

* On 25th March there were 47 earthquakes!

* So, on the 26th March, scientists issued a warning to the local people that they might need to evacuate the area.

* On 27th March gas and steam belched out of the top of the mountain.

* On 3rd April a bulge started to appear on the side of the mountain. It kept growing and by 12th April it was 100m high.

* On 30th April scientists gave another warning and the local authorities put a 30km danger zone around the volcano that people were not allowed to enter.

* On 10th May there were several earthquakes under the mountain and the bulge kept growing by 1.5m a day.

* On 18th May the volcano erupted…………

Problem 1

The volcano did not just erupt from its top. It erupted from its side too. This is called a lateral blast. It caused the largest landslide ever recorded.

Problem 2

Because the volcano is over a destructive margin there was a lot of gas and steam in the magma chamber. When the volcano erupted, the gas, steam and ash burst out and flowed down the side of the volcano like a rolling cloud of burning gas. This is called a pyroclastic flow.

It moved at 300km/hr. It flattened and burnt trees over 360 square kilometres! 7000 animals were killed in the forests. 12 million salmon in a fish farm were killed. 61 people were killed.

Problem 3

The hot magma came to the surface next, and melted the snow that had been on top of the mountain. This made mudslides called lahars that flowed down river valleys at a speed of 35m per second!

Problem 4

The smoke and dust that was ejected from the volcano made a cloud 24km high in the sky. Planes had to be diverted. The settled ash formed a layer 15cm deep. Roads were unusable and crops and farm machinery was ruined. The cost of the damage caused by the ash was $175 million.

The mountain looked very different, the side had been blown out and it was 365m lower than it used to be.

Before the eruption After the eruption

The US Government gave $951 million in aid to rebuild industry in the area and to compensate people.

The area is now a major tourist attraction. This means the local economy is wealthier than it was before the eruption.

There is now an increased risk of flooding, due to the shape of the new landscape.

An exclusion zone was set up around the volcano.