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A Destructive Plate boundary - Mt St Helens

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Introduction

A Destructive Plate boundary Mt St Helens The eruption in May 1980 of Mount St. Helens, Washington State, astounded the world with its violence. A gigantic explosion tore much of the volcano's summit to fragments; the energy released was equal to that of 500 nuclear bombs. The event occurred along the boundary of two of the moving plates that make up the earth's crust. They meet at the junction of the North American continent and the Pacific Ocean. One edge of the continental North American plate over-rides the oceanic Juan de Fuca micro-plate, producing the volcanic Cascade Range that includes Mounts Baker, Rainier and Hood, and Lassen Peak as well as Mount St. Helens. According to geological evidence found by the United States Geological Survey, there had been two major eruptions of Mount St. ...read more.

Middle

On the 18th May at 8:32 a Nuee Ardente erupted from Mt St Helens leaving the mountain 1300 feet shorter than it had been when its growth began. The Nuee Ardente was unstoppable, moving at over 200mph. Eventually viscous lava, accompanied by the burning daddy clouds of ash and gas, welled out of the volcano's new crater, and from lesser vents and cracks in its flanks. 61 deaths were reported during the course of the eruption. A Constructive Plate Boundary Galapagos Island The volcano on Isabel, the largest of the Galapagos Islands, is composed of five smaller volcanoes known as the Five Hills. The Galapagos Islands have been built entirely by volcanism, and about 60 previous eruptions have been recorded over the last 200 years, so it is not surprising that another eruption has occurred. ...read more.

Conclusion

It is not totally understood why the slopes are so steep, but one theory is lava flows mantling together pyroclastic cones, which were formed at an earlier stage of activity the Galapagos are formed from basalt, the most basic of all types of lava. Basalt has a very different chemical composition from the lavas that erupt from continental volcanoes, and is much more fluid. Consequently, as the lava flows build up to produce a volcanic cone, the island cones have a much shallower slope than those on the mainland. These shallow-sloped volcanoes are called shield volcanoes and in the Galapagos, they are often compared to over-turned soup bowls. Such shield volcanoes can clearly be seen in the younger western islands of Isabela and Ferdina to the east, the volcanoes are lower and more eroded. ...read more.

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