Case Study: Mount St. Helens

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  • Topic: Volcano, Mount St. Helens, North America
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  • Published : November 28, 2012
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Case Study of Mount St. Helens

Date: 18th May, 1980
Time: 8:30-8:33
8: 30—ash and steam erupted.
8:32—earthquake of magnitude 5.1 on the Richter scale caused the bulge on the north side of the mountain to move forwards and downwards, releasing material that formed a landslide of rock, glacier, ice, and soil that moved downhill to fill Spirit Lake. However, the water only reinforced it, and it moved rapidly down the northern fork of the Toutle Valley. The mudflow reached Baker Camp, but the floodwater continued down the valley and the sediment blocked Portland’s port on the Columbia River. 8:33—The exposed magma exploded sideways, which sent out blast waves of volcanic gas, steam, and dust, which is called a ‘nuée ardente’. This moved northwards for 25 km. Within this range every form of life, like plants and animals, were destroyed.

For the rest of the morning, a series of eruptions took place, which ejected gas, ash, and volcanic ‘bombs’, or simply rocks. The thicker ash rose 20 km into the air and drifted eastwards before settling. The volcanic ‘plume’, or could, of fine ash reached the eastern coast of the USA three days later, and several days later, the ash had completely encircled the world.

Location: Mount Saint Helens (stratovolcano) is in North America, in the Cascada mountain range. Skamania County, Washington State, USA. Plates involved: Juan de Fuca plate and the North American Plate. Types of boundary and crust: Oceanic (Juan de Fuca) and Continental (North American) crusts. The margin is destructive, also known as a convergent boundary. It’s also a Subduction Zone, as the Juan de Fuca plate is subducted under the North American Plate.

Warning Signs
On March 20th there was a minor earthquake, which measured 4.1 on the Richter scale. Tremors happened for the next few days until the 27th of March, when there was actually a small eruption of steam and ash, which left a crater around 250 feet wide. On...
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