Mount_St_Helens

Mount St Helens is a stratovolcano, or as they are also called a composite volcano. They occur at subduction zones. It is in the Cascade mountain range in Washington State, U.S.A, and is on the boundary of the North American Plate and the Juan de Fuca plate. When the oceanic plate (Juan de Fuca) subducts, dewatering occurs. Water is released from the subducting plate and rises. This water causes the melting point of the mantle rock to decrease. This causes melting and the partially melted rock rises because it is less dense the mantle rock. The magma rises through the crust, adding silica rich rock, and pools into a magma chamber beneath or within the volcano. The pressure is lower here and the gases dissolved in the magma come out of solution and suddenly explode up through the volcano similar to the CO2 escaping from a shaken coke bottle. Mount St Helens has gone through many eruptive stages. The current “stage” it is in now is called the Spirit Lake Stage and all stages before this are known as ancestral stages. The first eruptive stage on record is the “Ape Canyon Stage” from 40,000 to 35,000 years ago. The Ape Canyon eruptive period ended around 35,000 years ago and was followed by 17,000 years of relative quiet. The second eruptive period, the Cougar Stage, started 20,000 years ago and lasted for 2,000 years. Another 5,000 years of dormancy followed, but then came the Swift Creek eruptive period. Swift Creek ended 8,000 years ago. A dormancy of about 4,000 years was broken around 2500 BC with the start of the Smith Creek eruptive period, around 1900 BC there was the largest known eruption from St. Helens ever recorded, judging by its effect on the environment. This eruptive period lasted until about 1600 BC and left 18 inches deep deposits of material 50 miles away in what is now Mt. Rainier National Park. Deposits have been found as far northeast as Banff National Park in Alberta, and as far southeast as eastern Oregon. It is believed there may have been up to 2.5 cubic miles of material ejected in this cycle. Some 400 years of dormancy followed. The Pine Creek eruptive period lasted from 1200 BC to 800 BC. In 1480, a huge eruption, several times larger than the 1980 eruption marked the start of the Kalama period. Another eruption similar in size to 1980 occurred in 1482 but it did not cause as much destruction. By the end of the Kalama period in 1647 Mount St Helens was at its highest peak. The Goat Rocks period from 1800-1857 was the first time there was written evidence of eruption there The eruption in May 1980 was the most economically destructive eruption in U.S history. 57 people and thousands of animals were killed along with hundreds of square miles of woodland being destroyed. The eruption was preceded by a two-month series of earthquakes and steam-venting episodes, caused by an injection of magma at shallow depth below the volcano that created a huge bulge on the North Slope. USGS scientists convinced the authorities to close Mount St. Helens to the general public and to maintain the closure despite pressure to re-open it; their work saved thousands of lives. At 8:32 am May 18th, 1980 an earthquake caused the entire north face to collapse, one of the largest landslides in history, travelling at 110 to 155 miles per hour. This exposed the partly molten gas rich rock to low pressure; as a result it exploded violently, so fast the lava actually overtook the avalanche. A column of ash rose over 80,000 feet into the atmosphere and deposited ash in over 11 U.S states, reaching as far north as Edmonton, Alberta, Canada. During the nine hours of vigorous eruptive activity, about 540 million tons of ash fell over an area of more than 22,000 square miles. By around 5:30 p.m. on May 18, the vertical ash column decline, but less severe outbursts continued through the next several days. Fifty-seven people were killed, 200 houses, 27 bridges, 15 miles of railways and 185 miles of highway were destroyed. It was the most deadly and economically damaging eruption in the history of the United States. The eruption ejected more than 1 cubic mile of material. The removal of the north side of the mountain (13% of the cone's volume) reduced St. Helens' height by about 1,313 feet and left a crater 1 to 2 miles wide and 2,100 feet deep with its north end open in a huge breach. Over 4 billion board feet of timber was destroyed after the blast. In areas of thick ash, many crops, such as wheat, apples, potatoes and alfalfa, were destroyed. As many as 1,500 elk and 5,000 deer were killed. In all, Mount St. Helens released 24 megatons of thermal energy. This is equivalent to 1,600 times the size of the atomic bomb dropped on Hiroshima. It is estimated that the disaster cost over 2 billion dollars. Funding for mental health programs was also necessary because many locals were reporting stress and depression as a result of the crisis. Between 1980 and 1986, activity continued at Mount St. Helens, with a new lava dome forming in the crater. Numerous small explosions and dome-building eruptions occurred. From December 7, 1989, to January 6, 1990, and from November 5, 1990, to February 14, 1991, the mountain erupted with sometimes huge clouds of ash. Since the eruption there has been a series of minor eruptions that have more or less been rebuilding the dome below. There was a lot of activity between 2004 and 2008, a few times causing a large ash plume. The mountain was reopened in 1986 for mountain climbers but was closed again in 2004 for safety reasons.