Mt Saint Helans Essays - Volcanology, Cascade Volcanoes,

Mt Saint Helans Mount St. Helens Location: Washington, United States Latitude: 46.20 N Longitude: 122.18 W height: 2,549 meters or 8,364 feet - 9,677 feet before May 18, 1980 Type: Stratovolcano Number of eruptions in past 200 years: 2-3 Latest Eruptions: Between 1660-1700, around 1800-1802, 1831, 1835, 1842-1844, 1847-1854, 1857, 1980-? Present thermal activity: strong steaming Nickname: Mount Fuji of the West Remarks: continuous intermittent activity since 1980 with occasional eruptions of steam and ash; occasional pyroclastic flows; intermittent dome forming. MSH is considered a young volcano that developed over the last 40,000 years and is one of the most active volcanoes in the Cascade Range. Geologists predicted that the volcano would erupt before the year 2000. The May 18, 1980 eruption of Mount SH was the most destructive in the history of the United States. In a matter of hours, MSH caused loss of lives and widespread destruction of valuable property because of the avalanche, lateral blast and mudflows. On March 20, 1980, starting with an earthquake that was followed by many others, MSH became active again after a quiet period of 123 years. On March 27, 1980, there was a huge explosion and MSH began blowing ash and steam. This lasted until May 14, 1980. The explosion in March opened up two craters that quickly became one huge crater. While this was happening, an enormous bulge on the north side of the mountain top appeared. It grew about six feet each day. Geologist kept measuring the bulge, recording the earthquakes and sampling the ash and gases. By May, the bulge was 300 feet wide and more than one mile in length. On May 18 at 8:32 in the morning, Mount St.. Helens erupted taking the top 1,200 feet off the volcano. The eruption went on until nightfall. The area of destruction was 230 square miles and was one of the largest landslides ever recorded in history. The blast was preceded by two months of intense activity that included over 10,000 earthquakes, hundreds of small phreatic (steam blasts) , explosions and the north side bulge. A magnitude 5.1 earthquake below the volcano at 8:32 am started the eruption. MSH is still a potentially dangerous and active volcano even though it has been quiet since 1995. In the last 515 years th ere have been four major eruptions and dozens of lesser eruptions. Two of the eruptions were only two years apart. In 1480, the eruption was about five times larger than the one in May 1980. There have been even larger eruptions during MSH' 50,000 year lifetime. After the May 18, 1980 eruption, there have been five smaller explosive eruptions over a five month period. Since then, there have been 16 dome building eruptions through October 1986 when the new dome in the crater was formed. As the mountain was torn open, the pressure in inside was suddenly relieved. The rock shattered inside the mountain was exploded out the top at speeds over 200 miles per hour. The blast was so strong that it leveled whole forest of fir trees. Geologist call this a stone wind since the winds carried the rocks form the blast with them. The rocks gave the winds extra force that let them flatten the trees. 150 square miles of land was leveled. The edges of this area also lost their forested areas from the heat of the blast and the fires it caused. The original blast of the volcano only lasted 10 to 15 minutes. It quickly started up again. A dark cloud of ash and gases went up for miles into the sky and spread for miles in every direction, but mostly eastward. Forest fires broke out everywhere. After abut four hours, the color of the ash became much lighter since the volcano was now throwing out new magma instead of old rock. The temperature of the volcanic flow was approximately 1000 degrees and was traveling extremely fast....about 100 miles per hour. The volcanic flows went on until late in the afternoon. These flows triggered an avalanche. The avalanche poured rocks, tress and dirt into nearby Spirit Lake and then downward to the valley of the North Fork of Toutle River. The ice and snow caps that melted caused mudflows. The mud traveled down the same path. It was incredibly destructive.