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Friday, 14 September 2012

Excellent Study on the Causes of The Mount St Helen's eruption

J.Stroomer has produced this excellent study on the causes of the eruption at Mt St helens in 1980:

Mt. St. Helens is a composite volcano within the Cascade Range in southern Washington, approximately 50 miles northwest of Portland, Oregon. Though Mt. St. Helens is approximately 40,000-years old, it is considered a relatively young, active volcano. The Juan de Fuca plate is converging with the North American Plate. The lava forms as the oceanic crust is subducted under the continental crust at the convergent plate boundary.

The eruption on May 18th 1980 is the biggest that it has ever experienced. The build-up to the eruption was a two-month series of earthquakes and steam-venting episodes, caused by an inflowing of magma at shallow depth below the volcano that created a huge bulge on Mount St. Helens' north slope. This s called a cryptodome and can be seen on the images below, it makes a striking impact on the side of the volcano. The scientists and geologists studying the area say
that in the weeks leading up to the eruption it was expanding at a rate of 6ft a day in some places of the dome.

An earthquake, measuring at 4.2 on the Richter scale, at 8:32:17 a.m. caused the entire weakened north face to slide away, exposing the partly molten, gas rich rock in the volcano to lower pressure. This resulted in a landslide where more than half a cubic mile of material was released. This can be seen in the diagram below.

A lot of pressure had built up as this was the first eruption for 123 years. Although it was not without warning, on March 25th the seismographs started to climax at about noon reaching peak levels on the next two days, including one earthquake that measured 5.1 on the Richter scale. A total of 174 shocks, of magnitude 2.6 or greater, in the following two days. This then continued through April and May with 5 earthquakes above magnitude 4 or greater per day in April. This then increased to 8 per day the week before May 18th. There is a before and after shot below to show the impact on the landscape.

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