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 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. 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 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 St. Helens: 1980, 1831-11857 Mount Adams: 3,000-4,000 years ago 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 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. 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 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 over three feet of mud was left behind. The mudflows not only destroyed all the trees and plants, but also killed thousands of animals including deer, elk and beers. Different sources say that 57 or 62 people were killed form the eruption of MSH including the volcanologist David Johnston, a member of the U. S. Geological Survey team at MSH. The lateral blasts, avalanche, mudflows and flooding caused huge amounts of damage to the land and city. All buildings and structures in the area of Spirit Lake were buried. More than 200 houses were destroyed, leaving many people homeless.
Tens of thousands of acres of forest, recreational areas, bridges, roads and trails were destroyed. Wildlife suffered huge losses. The Washington state Department of Game estimated that 7,000 big game animals (beer, elk and deer) died in the area, as did all birds and small mammals. Volcanoes can be very destructive but in the long run they are helpful. Because the ash s rich in potassium and phosphorus it helps to grow very healthy plants and crops. Volcanic heat energizes most of the world’s thermal areas and hot springs. Volcanoes are also responsible for creating much of the mineral wealth on earth.
The hot water underground removes minerals from the magma dn deposits them in the earth’s crust. They form veins of silver, gold, zinc, copper and lead. Because of the plate movements, some minerals that were deposited on ancient sea floors are now accessible to mining. Diamonds are also created by volcanic activity and are fund in eroded volcanic pipes or tubes. The May 18 eruption wiped out most large forms of life on the north side of the mountain. Life began to reemerge in the form of bacteria, fungi, weeds, seeds, insects, spiders and pocket gophers.
Bt the end of summer 1981, life was returning to the volcano. Volcanic eruptions are important to life on Earth because they add gasses to the atmosphere and water to the ocean. Volcanoes also build land mass. Floods of lava have built large area of continents like the Columbia Plateau in the Pacific Northwest. Volcanoes also make soil as the ash, pumice and lava break down. This mixed with plant and animal remains makes very rich soil. In 1980, MSH erupted six more time, usually blowing ash into the air and causing pumice to slide down the side of the mountain.
In June and August, the lava that came out from the top was thick enough to finally form a new dome. This dome was destroyed only to be formed again in October. After this, the eruptions only added to the size of the dome making it larger. The erupting volcano sent out material and debris of all sizes. The term of all of this is tephra. the material that was blown out of the volcano was classified by its size. 0. 1-2. 5 inches= Lapillo or little stones 2. 5 inches and larger = Bombs and blocks Bombs are rocks that are soft when they are ejected while blocks are solid.
Both bombs and blocks can land miles away and fly at speeds of 1250 miles per hour. The blocks destroy whatever they land on while the bombs explode and burst into red hot liquid when they land. The earth has many volcanoes, the majority of them circling the Pacific Ocean. These volcanoes are known as the Ring of Fire. Scientists know that he volcanoes are in this formation because of the movement of the earth’s crust. There are over 400 active volcanoes in the world. They are found in weak places, or faults, in the Earth’s crust, where two of the Earth’s plates meet. Volcanoes and earthquakes occur in the same general area.
Volcanoes seem to relieve some of the pressure of the Earth’s mantle, so the earthquakes near them are milder. The band of faults around the edge of the Pacific Ocean has most of the worlds active volcanoes. In the Ring of Fire, many of the volcanoes are underwater and as they grow they become islands. The Cascade Range that MSH is a part of was formed by the small Juan de Fuca plate colliding with the bigger plate that carries North America. When one plate collides with another plate, one slides under the other into the Earth’s mantle causing rock to melt, magma to rise and volcanoes to erupt.
This is what happens all around the rim of the Pacific Ocean. Earthquakes are the biggest clue that a volcano that has been dormant is coming back to life. Tremors under the volcano and in nearby areas are sometimes caused by magma rising and moving rock around. The scientists who study volcanoes, volcanologists, can not predict when the eruption will happen, but they can measure the gases seeping from the volcanoes and the changes in the temperature of the earth. These are clues that let them know that activity is increasing. They use an instrument called a tiltmeter.
For weeks before MSH erupted, volcanologists were able to warn the people in the area that the rock around the volcano was active. The volcanologists measured the bulge of the volcano and the changes in the surface of the mountain. This helped to save many lives since they were able to let people evacuate the area. Current plate movement can be tracked directly by using ground-based or space-based geodetic measurements. Geodesy is the science of the size and shape of the earth. Ground-based measurements are taken with very precise surveying techniques using laser electronic instruments.
Because plate motions are global in scale, they are measured by satellite based methods. Among the techniques are VLBL (very long baseline interferometry) SLR (satellite laser ranging, and GPS (Global Positioning Systems) and are all based on technology developed for the military, aerospace, radio astronomy and satellite tracking. By monitoring the interaction of the Pacific Plate and the surrounding continental plates, scientists hope to learn more about the events leading up to earthquakes and volcanic eruptions in the Pacific Ring of Fire. Not all volcanoes are the same shape even though they all begin as a crack or hole in the ground.
As the volcanoes erupt, lava and ash build up forming the cone of the volcano. If running lava from the volcano forms a gently sloping shape it is called a shield volcano. This lava can flow for many miles before it becomes cold. The eruption from this type of volcano is not very violent. If the lava is sticky and hardens quickly, the volcano will have steep sides. This is a composite cone or stratovolcano. MSH is a stratovolcano. This type of eruption is the most violent. Stratovolcanoes are the most common type on earth. They are composed of almost equal amounts of ash and thick lava flows.
This layering of ash and lava gave it the name of strato. The magma is extremely powerful and can form steep slopes of up to 35 degrees. These volcanoes are not usually very big and occur along the subduction zone, mostly around the Pacific Ocean. Subduction is the process of one plate of the earth’s crust moving under another plate. MSH is also called a composite volcano because it erupts both lava and ash. If a volcano shows no signs of life for thousands of years it is thought to be extinct. If a volcano shows activity, even slight movement, it is dormant.
Volcanoes that have violent eruptions between long periods of no activity are usually stratovolcanoes. Volcanoes that are inactive and not expected to erupt in the near future are called dormant volcanoes. Volcanoes that are never expected to erupt again are called extinct. All of the volcanoes in the Cascade Range where MSH is found are dormant volcanoes. Approximately 50 or so volcanoes erupt every year. The International Association of Volcanology defines active volcanoes as one that has erupted in historic times. Historic can mean 200 years in Hawaii or 3,000 years in the Mediterranean depending if there are historical records.