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Mt Saint Helans

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.

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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 there 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. The mudflows tore down houses, steel bridges and blocked the Columbia River with its debris. The next day showed a very different MSH.

The mountain had lost more that three quarter of a cubic mile of rock and was now 1,200 feet shorter. What used to be a lush green slope was now a gray wasteland that looked like the surface of the moon. Mount St. Helens was built by many eruptions over thousands of years. With each eruption, hot rock from inside the earth forced its way to the surface.

This type of rock is called magma. Once the magma reaches the surface of the earth it is called lava. With some eruptions, the magma was liquid so the lava flowed out of the volcano and hardened. With others, the magma was thick so it burst violently with sprays of molten rock. It rained down as tiny bits of rock, (ash) and as rocks puffed up by gases (pumice).

The two types of lava have Hawaiian names. Aa is a sharp stone that cools down to a surface that is hard to walk on. Aa occurs from high lava fountains. The lava chunks cool in the air and cannot form into flows when they land. Pahoehoe is a much smoother stone and dries into tubes that are sometimes hollow. Pahoehoe happens when eruptions are at high temperatures and low viscosity.

The low viscosity lets the lava flow easily and a skin is formed on top. Lava that hardens is called pumice. The speed of both types of flows are hard to tell apart, but Aa is faster than Pahoehoe. A Pahoehoe flow moves around one yard a minute but with a slope it can move up to 400 yards per minute or 14 miles per hour. Aa flows are usually 61/2 to 16 feet thick, and Pahoehoe flows are about one foot thick.

The width of both types of flows is usually around 100 yards wide. Because lava moves so slowly it is seldom dangerous to people. It moves about two to three miles per hour. NAMED for: Mount St. Helens was named for the British diplomat Alleyne Fitzherbert (1753-1839) whose title was Baron St. Helens.

The mountain was named by Commander George Vancouver and the officers of the H.M.S. Discovery while they were surveying the northern Pacific coast form 1792- 1794 Location Mt. St. H is part of the Cascade Range which is a chain of volcanoes that runs from Northern California northward to British Columbia. MSH is about 95 miles south of Seattle, Washington MSH was 9,677 ft high before its eruption in 1980.

It is now 8,364 feet high and about six miles wide at its base. The eruptions from this volcano were the first to happen in the continental United States except for Alaska since 1921 when Lassen Peak last erupted. MSH is 34 miles west of Mount Adams in the eastern part of the Cascade range. These brother and sister volcanic mountains are bout 50 miles from Mount Rainier. Mount Hood, the nearest major volcanic peak in Oregon, is about 60 miles southeast of MSH. The other volcanoes in the Cascade range are Mount Rainier, Mount Baker, Mount Adams, Mount Hood, Mount Jefferson, Three Sisters, Newbury, Mount Mazama, Mount Shasta, Medicine Lake Highland and Lassen Peak.

MSH has erupted many times in the past 4,500 years, but was inactive from 1857 until its eruption in 1980. The volcanic eruptions in the Cascade Range have been: Mount Baker: 1870 Mount Rainier Mount St. Helens: 1980, 1831-11857 Mount Adams: 3,000-4,000 years ago Mount Hood: 1865 Mount Jefferson: 1030 years ago Three Sisters: 2500 years ago Newbury: 1400 years ago Mount Mazama: 6600 years ago Mount Shasta: 1840, 1786 Lassen Peak: 1914-1921 Medicine Lake Highland: 1910 The Volcanic Explosivity Index, or VEI describes the size of volcanoes based on three observations. A volcano is rated VEI -2, VEI -3 etc. on its violence of eruption, height of the plume that shoots out of 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 …


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