Mount St. Helens. Copyright 2010 LessonSnips

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Aubrey Francis
6 years ago
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1 Mount St. Helens Washington State is home to the Cascade Mountains, a range of mountains that extends from the Canadian province of British Columbia to northern California. Many of these mountains are actually volcanoes, part of the volcanoes on land that circle the Pacific Ocean and are called the Ring of Fire. Volcanoes in Washington State that have been active in the last 200 years include Mount Rainier and Mount St. Helens as well as Mount Hood in Oregon. A huge eruption of Mount St. Helens began on March 20, 1980 with an earthquake and finally the mountain exploded on May 18 and the destruction covered about 230 square miles. Residents of the area knew that Mount St. Helens was a volcano but no one was particularly concerned. Scientists and historians knew that Mount St. Helens had experienced a few small eruptions between 1832 and 1857, but there had not been any volcanic activity there since then. However as scientists studied Mount St. Helens they realized that the volcano became active in a regular pattern of years, and they predicted it would erupt again sometime before the year These scientists were proven correct when the huge eruption occurred in The Eruption of 1980 The eruption of 1980 lowered the height of Mount St. Helens by 1,200 feet. The eruption on May 18 began with an earthquake which in turn started an avalanche. While we think of an avalanche of snow, an avalanche can also happen when rocks and dirt slide violently down the side of a mountain, which is what happened at Mount St. Helens. In places below the mountain, the avalanche debris was 180 feet deep. Once the avalanche opened the side of the mountain, the volcanic material inside was released and superheated water instantly turned to steam. The blast came out of the volcano at 200 miles per hour, carrying rocks and chunks of mountain up to the size of a small house. The combination of steam and rock material traveling at 200 miles per hour was called a stone wind by geologists. This material flattened trees for 150 square miles. The wind emptied nearby Spirit Lake, filled it with fallen trees and the water slid back into the lake. Trees outside the blast zone that were not toppled were instead scorched and killed by the heat.

2 After the eruption, Mount St. Helens spewed out ash and gases that extended miles upward into the sky. The wind carried this ash to the east and 80 miles away into the city of Yakima, Washington. It became so dark that the nighttime sensors on the streetlights turned them on at noon. Next the volcano sent out plumes of pumice along with ash. Pumice is a form of fine powdery volcanic rock that forms under the heat of a volcanic eruption. At the next stage of the devastation, mudflows streamed down the mountain instead of lava flows. The ice and snow on the mountain was melted in the heat and this liquefied the dirt and rocks loosened by the avalanche. Ash, pumice and ground up rocks fell into the mixture which flowed down the mountain like a fast river. The mudslide acted like flood waters, picking up large boulders and even cars and trucks as well as toppling homes and steel bridges as it moved through the countryside. There were 10 inches of ash ten miles from the mountain and one inch of ash sixty miles away. There was a small amount of measurable ash covering 22,000 square miles around the volcano. Washington State was not the only area impacted by the eruption. Evidence of the ash spread across the United States within three days of the eruption and circled the earth in fifteen days. Since 1980 Mount St. Helens remained somewhat active from the 1980 eruption to Lava emerged from the center of the volcano and remained in the crater that was formed by the eruption. There were hundreds of small explosions that occurred, but these did not change the mountain much or impact the surrounding area. Between 1986 and 1999, the mountain experienced many periods of minor earthquake activity and this pattern continues today. It is possible that Mount St. Helens will remain quiet for the next one hundred years but the U.S. Geologic Survey (USGS) continuously monitors it. In 1982, the U.S. Congress created the Mount St. Helens National Volcanic Monument. Congress preserved 110,000 acres that include the volcano and its surrounding land and it has been left undisturbed since then. Today these preserved grounds still resemble a moonscape, barren and grey. Only a few fir trees are returning to the lower slopes of Mount St. Helens, but over time the area will heal and become forest once again. Timber companies own much of the land in the Cascade Mountains

3 around Mount St. Helens. They began replanting their lands after the eruption and now there are 30 year old trees in the area.

4 Circle True or False after analyzing each of the following statements. 1. True False The Cascade Mountains in Washington State are part of the Ring of Fire, a series of land volcanoes that follow the coastline of the Pacific Ocean. 2. True False Besides Mount St. Helens, other volcanic mountains in Washington State include Mount Rainier and Mount Hood. 3. True False The avalanche caused by the 1980 eruption of Mount St. Helens did not have much impact because there wasn t much snow on top of the mountain at the time. 4. True False The steam wind as it was called by geologists blasted from Mount St. Helens at 200 miles per hour. 5. True False After the eruption of Mount St. Helens the wind carried ash all around the world. 6. True False The mudflow from the Mount St. Helens eruption acted like flood waters, carrying away vehicles, homes and bridges. 7. True False Mount St. Helens has not had a major eruption since 1980 and scientists expect that the volcano will not experience another major eruption for the next 100 years. 8. True False The area around Mount St. Helens is now protected as a National Volcanic Monument and the protected area is filled with large trees. 9. True False The U.S. Government Services (USGS) division of the Environmental Protection Agency (EPA) continuously monitors Mount St. Helens for volcanic activity. 10. True False Much of the area around Mount St. Helens has returned to normal forest due to the efforts of timber companies.

5 Answers 1. True 2. False 3. False 4. False 5. True 6. True 7. True 8. False 9. False 10. True

Debris Avalanches. Debris avalanche deposits on a volcano in Chile. All of the area in the foreground is buried by a thick debris avalanche.

Debris Avalanches. Debris avalanche deposits on a volcano in Chile. All of the area in the foreground is buried by a thick debris avalanche. Debris Avalanches Volcanoes are not very stable structures. From time to time, they collapse producing large rock and ash avalanches that travel at high speeds down valleys. Collapse maybe caused by an