Monday, September 13, 2010

Types of Eruption

Apart from hotspots over mantle plumes, volcanoes frequent crustal plate boundaries, making a “ring of fire” around the Pacific. The ocean ridge system is a chain of submarine volcanoes.  Where it breaks the surface, in Iceland for example, it coincides with a mantle plume. The volcanoes of the Rift Valley of east Africa represent a new ocean trying to open. Where ocean crust dives beneath a continent, it takes with it water locked in minerals. As the rocks heat and melt, the wet magma rises like uncorked champagne to produce some violent eruptions.
Mt. St. Helens in Washington State is one of many volcanoes above the sub ducting Pacific plate. The wet magma ascends periodically like a pressure cooker letting off steam. Up to May 1980 geologists had monitored 10,000 small earthquakes in the region and had used lasers to measure the growing bulge on the mountains north flank. By May 12 parts of the bulge were 138 m higher than before and very unstable suddenly on May 18, the entire north flank collapsed in three great landslides only seconds apart. The second exposed pressurized molten magma which erupted in a tremendous lateral blast, flattening trees up to 30 Km away, the third block to slide exposed the top of the magma column itself, which erupted upwards sending ash more than 19Km high and coating 50,00Km2 with 540 million tons of ash.