Presentation on theme: "Mt. St. Helens. Mt. St. Helen's, May 1980 (The U.S.A.) Erupted on the morning of May 18 th 1980, after being inactive for over 120 years. Caused by the."— Presentation transcript:
Mt. St. Helen's, May 1980 (The U.S.A.) Erupted on the morning of May 18 th 1980, after being inactive for over 120 years. Caused by the subduction of the oceanic Juan de Fuca plate under the continental North American plate, on the West Coast of the United States. The volcano forms part of the Rockies Range of mountains.
How It Happened After minor earthquakes and a small eruption during March, scientists carefully monitored the mountain. By the beginning of May a bulge had developed on the North side of the mountain. This activity forced the Local Authorities to create a "Red Zone" around the volcano, from where any residents were evacuated, and no one was allowed to re-enter. This saved a lot of people from the disaster.
The Effects At approximately 8.30 a.m. on the 18 th May, the volcano erupted a small amount of ash and stem. Then, a couple of minutes later, something occurred that nobody could have predicted. An earthquake caused the bulge to collapse down the side of the mountain. This released the full power of the explosive gases behind as the mountain literally blew its own side off.
The Effects The material from the massive landslide sped down the mountain, filling Spirit Lake and then racing on as a mud flow (it had mixed with the lake water) down the Toutle River and Kalama River. The explosions from the side of the mountain sent a speeding cloud of gas, steam and dust across the ground to the North of the volcano, flattening everything in its path. The trees of the forests around the mountain were toppled as if they were matchsticks, with the majority of them facing in the same direction.
The Effects These explosions also instantly melted all of the snow on the mountain, adding to the mudflows that were racing down the mountain and into the rivers. These flows took all with them, including huge trees and even bridges. During the remainder of the day huge gas and ash eruptions occurred, spreading a massive cloud of ash that took 7 days to entirely circle the world. The town of Yakima, 120 miles away, was so badly affected by the dust and ash that residents could only go outside if they wore facemasks.
The Aftermath The mountain managed to reduce its height by nearly 400 metres and blew an amphitheatre shape hole in its side, 3km long and 500 metres wide. 61 people were killed by the eruption, mainly by the poisonous gases, but also by being caught up in the massively swollen rivers. No wildlife, either plants or trees, survived within the 25km blast zone. However vegetation has already made great strides to re-colonise the area.
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