Mount St Helens Volcano 1980

2135 WordsNov 16, 20149 Pages
Mount St Helens Volcano 1980 The Eruption Mount St. Helens is an active volcano in the Cascade Range (see Figure 1) . It is situated in southwestern Washington about 70 km northeast of Portland, Oregon and was formed during four eruptive stages beginning about 275,000 years ago (Crandell, 1987). Prior to Figure 1: http://news.bbc.co.uk/2/hi/americas/3721266.st 1980, the last time it erupted is believed to have been in 1857. The first indications of renewed activity in 1980 were observed in the middle of March when earthquakes were detected beneath the volcano. There were many quakes each day and the largest ones were able to be felt by people in the nearby area. At 8:32 am PDT on May 18, 1980, Mount St. Helens erupted…show more content…
Intense earthquake activity persisted during this time and aerial observations indicated that the earthquakes had induced small avalanches of snow and ice. As early as March 31st, volcanic tremors had begun to be observed on seismographs. Volcanic tremor is a continuous, rhythmic ground shaking that is generally longer in duration and more continuous than that produced by an earthquake of similar amplitude. As a result of these data, scientists believed that a magma eruption from Mount St. Helens was very likely (Mounsthelens.com, 2014). However, no eruptive activity was observed between late April and early May at which time small steam blast eruptions began again. These ceased on May 16th. During this time, the north face of the volcano was becoming distended and cracks were appearing. The "bulge" grew at a rate of about 1.5 metres per day to more than 135m higher than before the magma intrusion. The bulge eventually collapsed and triggered the eruption and subsequent events of May 18, 1980. See Figure 3 for a diagrammatic representation of the sequence of events. Figure 2 "The Bulge" - Photo by Peter Lipman, USGS According to Zeilinga de Boer & Sanders (2002) the north side of the volcano slid down in 3 blocks. The blocks then broke up and formed into an avalanche that raced down the volcano at more than 200 km/h. The avalanche of rock, glacial ice, snow and debris relieved pressure on the
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