the vent, and the volume of materials ejected from it. MSH was rated a VEI-5. Like a Richter scale in measuring earthquakes, the VEI rates volcanoes. Only once in a decade do VEI-5 events like MSH happen. The volcano devastated hundreds of square miles and created a local catastrophe where most volcanoes of this size are much larger in their scale of destruction. Mount St. Helens was a Plinian eruption which is the least common type of eruption. Plinian eruptions have the shortest outburst and cause the most damage and destruction, Plinian eruptions that send out ash that covers hundreds of square miles are called ultra Plinian. There have been 19 Plinian eruptions in history. Plinian Eruptions The plume height is more that 25 km or more than 15.5 miles high. The Volcanic Explosivity Index rating is VEI 5 and the violence of eruption is categorized as Paroxysmal. The volume is 668,900 cubic miles. Plinian eruptions are named after Pliny the Elder who watched the Mount Vesuvius eruption in A.D. 79. Plinian eruptions are sometimes called Vesuvian eruptions. They blast out tons of materials in a blast that is the most powerful force on earth. The explosion tears apart the mountain and the hot gas filled clouds that form set off lightening and thunderstorms. So much ash is released in Plinian eruptions that it is carried by the wind around the entire planet. The ash forms a layer around the Earth, blocking out some of the suns energy and brings cooler temperatures to the Earth. Mudflows Mudflows are very dangerous. As they melt the ice and snowcap on the mountain, they rush down the mountain's side taking everything with them. They become very thick and cement like and bury everything in their pathway. The mudflows from MSH raced down the north face of the mountain at ninety miles per hour. At the bottom of the mountain it slowed down too thirty miles per hour. The mudflows was forty- four to sixty-six feet deep. When the water drained away...
