Mt. St. Helens History MountStHelens.com Information Resource Center History - Mt. St. Helens Mount St. Helens, located in southwestern Washington about 50 miles northeast of Portland, Oregon, is one of several lofty volcanic peaks that dominate the Cascade Range of the Pacific Northwest; the range extends from Mount Garibaldi in British Columbia, Canada, to Lassen Peak in northern California. Geologists call Mount St. Helens a composite volcano (or stratovolcano), a term for steepsided, often symmetrical cones constructed of alternating layers of lava flows, ash, and other volcanic debris. Composite volcanoes tend to erupt explosively and pose considerable danger to nearby life and property. In contrast, the gently sloping shield volcanoes, such as those in Hawaii, typically erupt nonexplosively, producing fluid lavas that can flow great distances from the active vents. Although Hawaiian-type eruptions may destroy property, they rarely cause death or injury. Before 1980, snow-capped, gracefully symmetrical Mount St. Helens was known as the "Fujiyama of America." Mount St.

Helens, other active Cascade volcanoes, and those of Alaska form the North American segment of the circum-Pacific "Ring of Fire," a notorious zone that produces frequent, often destructive, earthquake and volcanic activity. Some Indians of the Pacific Northwest variously called Mount St. Helens "Louwala-Clough," or "smoking mountain." The modern name, Mount St. Helens, was given to the volcanic peak in 1792 by Captain George Vancouver of the British Royal Navy, a seafarer and explorer. He named it in honor of a fellow countryman, Alleyne Fitzherbert, who held the title Baron St. Helens and who was at the time the British Ambassador to Spain. Vancouver also named three other volcanoes in the Cascades--Mounts Baker, Hood, and Rainier--for British naval officers. Indians on the Cowlitz River watching an eruption of Mount St. Helens, as painted by Canadian artist Paul Kane following a visit to the volcano in 1847 (Photograph courtesy of the Royal Ontario Museum). Indian Legend of Mount St. Helens Eruption Before Mt. St.

Helens blew its top is was a beautifully symmetric rounded snow-capped mountain that stood between two powerfully jagged peaks Mt. Hood ( which Indians called Wy'east) and Mt. Adams ( which Indians called Klickitat). According to one Indian legend, the mountain was once a beautiful maiden, "Loowit". When two sons of the Great Spirit "Sahale" fell in love with her, she could not choose between them. The two braves, Wyeast and Klickitat fought over her, burying villages and forests in the process ( hurling rocks as they erupted?). Sahale was furious. He smote the three lovers and erected a mighty mountain peak where each fell. Because Loowit was beautiful, her mountain (Mount St. Helens) was a beautiful, symmetrical cone of dazzling white. Wyeast (Mount Hood) lifts his head in pride, but Klickitat (Mount Adams) wept to see the beautiful maiden wrapped in snow, so he bends his head as he gazes on St. Helens. This is one of many indian legends involving Mount St. Helens. See Bridge of the Gods also. Witnessed Eruptions of Mount St.

Helens The local Indians and early settlers in the then sparsely populated region witnessed the occasional violent outbursts of Mount St. Helens. The volcano was particularly restless in the mid-19th century, when it was intermittently active for at least a 26-year span from 1831 to 1857. Some scientists suspect that Mount St. Helens also was active sporadically during the three decades before 1831, including a major explosive eruption in 1800. Although minor steam explosions may have occurred in 1898, 1903, and 1921, the mountain gave little or no evidence of being a volcanic hazard for more than a century after 1857. Consequently, the majority of 20th-century residents and visitors thought of Mount St. Helens not as a menace, but as a serene, beautiful mountain playground teeming with wildlife and available for leisure activities throughout the year. At the base of the volcano's northern flank, Spirit Lake, with its clear, refreshing water and wooded shores, was especially popular as a recreational area for hiking, camping, fishing, swimming and boating.

The tranquility of the Mount St. Helens region was shattered in the spring of 1980, however, when the volcano stirred from its long repose, shook, swelled, and exploded back to life. The local people rediscovered that they had an active volcano in their midst, and millions of people in North America were reminded that the active--and potentially dangerous--volcanoes of the United States are not restricted to Alaska and Hawaii. Previous Eruptive History The story of Mount St. Helens is woven from geologic evidence gathered during studies that began with Lieutenant Charles Wilkes' U.S. Exploring Expedition in 1841. Many geologists have studied Mount St. Helens, but the work of Dwight R. Crandell, Donal R. Mullineaux, Clifford P. Hopson, and their associates, who began their studies in the late 1950's, has particularly advanced knowledge of Mount St. Helens.

Their systematic studies of the volcanic deposits, laboratory investigations of rock and ash samples, and radiocarbon (carbon-l4) dating of plant remains buried in or beneath the ash layers and other volcanic products enabled them to reconstruct a remarkably complete record of the prehistoric eruptive behavior of Mount St. Helens. Ancestral Mount St. Helens began to grow before the last major glaciation of the Ice Age had ended about 10,000 years ago. The oldest ash deposits were erupted at least 40,000 years ago onto an eroded surface of still older volcanic and sedimentary rocks. Intermittent volcanism continued after the glaciers disappeared, and nine main pulses of pre-1980 volcanic activity have been recognized. These periods lasted from about 5,000 years to less than 100 years each and were separated by dormant intervals of about 15,000 years to only 200 years. A forerunner of Spirit Lake was born about 3,500 years ago, or possibly earlier, when eruption debris formed a natural dam across the valley of the North Fork of the Toutle River.

The most recent of the pre-1980 eruptive periods began about A.D. 1800 with an explosive eruption, followed by several additional minor explosions and extrusions of lava, and ended with the formation of the Goat Rocks lava dome by 1857. The post-A.D. 1400 segment of the 50,000-year eruptive history of Mount St. Helens (after USGS Bulletin 1383-C). Mount St. Helens is the youngest of the major Cascade volcanoes, in the sense that its visible cone was entirely formed during the past 2,200 years, well after the melting of the last of the Ice Age glaciers about 10,000 years ago. Mount St. Helens' smooth, symmetrical slopes are little affected by erosion as compared with its older, more glacially scarred neighbors--Mount Rainier and Mount Adams in Washington, and Mount Hood in Oregon. As geologic studies progressed and the eruptive history of Mount St. Helens became better known, scientists became increasingly concerned about possible renewed eruptions. The late William T.

Pecora, a former Director of the USGS, was quoted in a May 10, 1968, newspaper article in the Christian Science Monitor as being "especially worried about snow-covered Mt. St. Helens." On the basis of its youth and its high frequency of eruptions over the past 4,000 years, Crandell, Mullineaux, and their colleague Meyer Rubin published in February 1975 that Mount St. Helens was the one volcano in the conterminous United States most likely to reawaken and to erupt "perhaps before the end of this century." This prophetic conclusion was followed in 1978 by a more detailed report, in which Crandell and Mullineaux elaborated their earlier conclusion and analyzed, with maps and scenarios, the kinds, magnitudes, and areal extents of potential volcanic hazards that might be expected from future eruptions of Mount St. Helens. Collectively, these two publications contain one of the most accurate forecasts of a violent geologic event. Reawakening and Initial Activity A view to the north of the "two-tone" mountain--an appearance produced by prevailing easterly winds during the initial activity of Mount St.

Helens. Mount Rainier is visible in background (Photograph by C. Dan Miller). A magnitude 4.2 (Richter Scale) earthquake on March 20, 1980, at 3:47 p.m. Pacific Standard Time (PST), preceded by several much smaller earthquakes beginning as early as March 16, was the first substantial indication of Mount St. Helens' awakening from its 123-year sleep. Earthquake activity increased during the following week, gradually at first and then rather dramatically at about noon on March 25. The number of earthquakes recorded daily reached peak levels in the next 2 days, during which 174 shocks with magnitudes greater than 2.6 were recorded. Many hundreds of smaller earthquakes accompanied these larger events, the largest of which were felt by people living close to the volcano. Aerial observations of Mount St. Helens during the week of seismic buildup revealed small earthquake- induced avalanches of snow and ice, but no sign of an eruption. With a thunderous explosion, or possibly two nearly simultaneous ones, widely heard in the region at about 12:36 p.m. PST on March 27, Mount St.

Helens began to spew ash and steam, marking the first significant eruption in the conterminous United States since that of Lassen Peak, California, from 1914 to 1917. The crown of the ash column rose to about 6,000 feet above the volcano. The initial explosions formed a 250-foot-wide crater within the larger, preexisting snow- and ice-filled summit crater, and new fractures broke across the summit area. View of the "bulge" on the north face of Mount St. Helens, from a measurement site about 2 miles to the northeast (Photograph by Peter Lipman). The drawing above the photograph illustrates, in a highy exaggerated fashion, the nearly horizontal movement--about 85 feet in 20 days--of one of the measured points on the "bulge." Through April 21, Mount St. Helens intermittently ejected ash and steam in bursts lasting from a few seconds to several tens of minutes. The first crater was joined on the west by a second, slightly larger crater, and as the activity continued, both craters enlarged and ultimately merged.

Several avalanches of snow and ice, darkened by ash, formed prominent streaks down the mountain's slopes. The effect of the prevailing easterly wind was striking during the March-April eruptive activity, transforming the snow-covered Mount St. Helens into a "two-tone" mountain. The ash blown out between March 27 and May 18 was derived entirely from the 350-year-old summit dome, shattered and pulverized by phreatic (steam-blast) processes driven by the explosively expanding, high-temperature steam and other gases. No magma (molten rock and contained gases) was tapped during the initial eruptions. Intense earthquake activity persisted at the volcano during and between visible eruptive activity. As early as March 31, seismographs also began recording occasional spasms of volcanic tremor, a type of continuous, rhythmic ground shaking different from the discrete sharp jolts characteristic of earthquakes. Such continuous ground vibrations, commonly associated with eruptions at volcanoes in Hawaii, Iceland, Japan, and elsewhere, are interpreted to reflect subsurface movement of fluids, either gas or magma. The combination of sustained strong earthquake activity and harmonic tremor at Mount St.

Helens suggested to scientists that magma and associated gases were on the move within the volcano, thereby increasing the probability of magma eruption. Visible eruptive activity ceased temporarily in late April and early May. Small steam-blast eruptions resumed on May 7, continued intermittently for the next several days, and ceased again by May 16. During this interval, the forceful intrusion of magma into the volcano continued with no respite, as was shown by intense seismic activity and visible swelling and cracking of the volcano. The swelling was easily measurable and affected a large area on the north face of Mount St. Helens; this area became known as the "bulge," the initial growth of which probably began during the first eruption (March 27) or perhaps even a few days before. Through mid-May about 10,000 earthquakes were recorded. The earthquake activity was concentrated in a small zone less than 1.6 miles directly beneath the bulge on the north flank of Mount St. Helens.

A comparison of aerial photographs taken in the summer of 1979 with those taken during and after April 1980 showed that by May 12 certain parts of the bulge near the summit were more than 450 feet higher than before the magma intrusion began. Repeated measurements begun in late April with precise electronic instruments that shoot a laser beam to reflector targets placed on and around the bulge showed that it was growing northward at an astonishing rate of about 5 feet per day. The movement was predominantly horizontal--clear evidence that the bulge was not simply slipping down the volcano's steep slope. As the bulge moved northward, the summit area behind it progressively sank, forming a complex down-dropped block called a graben. These changes in the volcano's shape were related to the overall deformation that increased the volume of the mountain by 0.03 cubic mile by mid-May. This volume increase presumably corresponded to the volume of magma that pushed into the volcano and deformed its surface. Because the intruded magma remained below ground and was not directly visible, it was called a cryptodome, in contrast to a true volcanic dome exposed at the surface.

In summary, during late March to mid-May 1980, Mount St. Helens was shaken by hundreds of earthquakes, intermittently erupted ash and debris derived by steam blast reaming out of its preexisting summit dome, and experienced extremely large and rapid deformation caused by magma intrusion. The hot intruding magma provided the thermal energy to heat groundwater, which explosively flashed to generate and sustain the observed steam-blast eruptions. For 2 months the volcano was literally being wedged apart, creating a highly unstable and dangerous situation. The eventual collapse of the bulge on the north flank triggered the chain of catastrophic events that took place on May 18, 1980. The Climactic Eruption of May 18, 1980 The climactic eruption in full fury in the late morning of May 18, 1980 (Photograph by Joseph Rosenbaum). May 18, a Sunday, dawned bright and clear. At 7 a.m. Pacific Daylight Time (PDT), USGS volcanologist David A.

Johnston, who had Saturday-night duty at an observation post about 6 miles north of the volcano, radioed in the results of some laser-beam measurements he had made moments earlier that morning. Even considering these measurements, the status of Mount St. Helens' activity that day showed no change from the pattern of the preceding month. Volcano-monitoring data--seismic, rate of bulge movement, sulfur-dioxide gas emission, and ground temperature--revealed no unusual changes that could be taken as warning signals for the catastrophe that would strike about an hour and a half later. About 20 seconds after 8:32 a.m. PDT, apparently in response to a magnitude 5.1 earthquake about 1 mile beneath the volcano, the bulged, unstable north flank of Mount St. Helens suddenly began to collapse, triggering a rapid and tragic train of events that resulted in widespread devastation and the loss of 57 people, including volcanologist Johnston. Debris avalanche Aerial views of the volcano at the moment the summit collapse (see text) triggered the debris avalanche and associated catastrophic eruption (Photographs selected from the copyrighted sequence taken by Keith and Dorothy Stoffel).

The tail of the plane can be seen in the upper right-hand corner of the lower picture, as the Stoffels took a final backward look while escaping. Although the triggering earthquake was of slightly greater magnitude than any of the shocks recorded earlier at the volcano, it was not unusual in any other way. What happened within the next few seconds was described by geologists Keith and Dorothy Stoffel, who at the time were in a small plane over the volcano's summit. Among the events they witnessed, they "noticed landsliding of rock and ice debris inward into the crater . . . the south-facing wall of the north side of the main crater was especially active. Within a matter of seconds, perhaps 15 seconds, the whole north side of the summit crater began to move instantaneously. . . . The nature of movement was eerie. . . . The entire mass began to ripple and churn up, without moving laterally. Then the entire north side of the summit began sliding to the north along a deep-seated slide plane. I [Keith Stoffel] was amazed and excited with the realization that we were watching this landslide of unbelievable proportions. . . .

We took pictures of this slide sequence occurring, but before we could snap off more than a few pictures, a huge explosion blasted out of the detachment plane. We neither felt nor heard a thing, even though we were just east of the summit at this time." Realizing their dangerous situation, the pilot put the plane into a steep dive to gain speed, and thus was able to outrun the rapidly mushrooming eruption cloud that threatened to engulf them. The Stoffels were fortunate to escape, and other scientists were fortunate to have their eyewitness account to help unscramble the sequence and timing of the quick succession of events that initiated the May 18 eruption. The collapse of the north flank produced the largest landslide-debris avalanche recorded in historic time. Detailed analysis of photographs and other data shows that an estimated 7-20 seconds (about 10 seconds seems most reasonable) elapsed between the triggering earthquake and the onset of the flank collapse. During the next 15 seconds, first one large block slid away, then another large block began to move, only to be followed by still another block.

The series of slide blocks merged downslope into a gigantic debris avalanche, which moved northward at speeds of 110 to 155 miles an hour. Part of the avalanche surged into and across Spirit Lake, but most of it flowed westward into the upper reaches of the North Fork of the Toutle River. At one location, about 4 miles north of the summit, the advancing front of the avalanche still had sufficient momentum to flow over a ridge more than 1,150 feet high. The resulting hummocky avalanche deposit consisted of intermixed volcanic debris, glacial ice, and, possibly, water displaced from Spirit Lake. Covering an area of about 24 square miles, the debris avalanche advanced more than 13 miles down the North Fork of the Toutle River and filled the valley to an average depth of about 150 feet; the total volume of the deposit was about 0.7 cubic mile. The dumping of avalanche debris into Spirit Lake raised its bottom by about 295 feet and its water level by about 200 feet. Volcanoes : Mount St. Helens Mt St. Helens Introduction Mount St.

Helens is a volcanic mountain in the Cascade Range in Western North America, in the state of Washington. It is, perhaps, the most famous mountain in the range, and certainly is the most active volcano in the area today. In fact, it has probably been the most active of the Cascade volcanoes for over 4000 years. If the eruption of 18th May 1980 follows the pattern of previous eruptions the mountain will remain very active for perhaps ten or twenty years before beginning to quieten down again. The 1980 eruption was the first time that it was possible to use modern technology to carefully monitor a volcano as it built up towards a violent eruption. Scientists gathered vast quantities of data about the mountain in the months that led up to the eruption, and were able to record events as they happened. Despite this, the precise moment of eruption was impossible to determine beforehand, and caught the researchers by surprise. The prediction of volcanic eruptions is difficult, although most scientists studying them, volcanologists , would say that it can be easier than predicting earthquakes. The location of Mt St Helens and key places around it are shown in the satellite image below.

History The Cascade mountains lie along the western coast of the United States. This area is one subjected to frequent earth movements and has a long history of volcanic activity. The mountains have formed as a result of seismic activity. The American coast is part of the North American Plate; the Pacific Ocean to the west overlies the Juan de Fuca plate. The Juan de Fuca plate is being subducted below the North American plate by processes explained by the theory of Plate Tectonics. As the descending Juan de Fuca plate is subjected to increasing pressure it becomes hotter and begins to partially melt. The molten rock, called magma begins to rise towards the surface. When it reaches the surface it erupts and a volcano is formed. Mt St. Helens is the youngest of the volcanoes in the region, being a mere 2,500 years old, but the area has been volcanically active for almost 40,000 years, hence the large number of volcanic mountains forming the Cascade Range. The Build Up To The 1980 Eruption Mt St. Helens had been quiet, or dormant , for 123 years.

On 20th March 1980 a series of small earthquakes were detected in the region and volcanologists started to become interested in the mountain. During the following few days the magnitude and frequency of the shock waves began to increase, then the first small eruptions began. These consisted of releases of steam and ash. This was the cue for the scientists to begin regarding the mountain as a serious potential hazard. The United States Geological Survey began issuing warnings to people living in the area, and briefed the Forest Service. They, in turn, began to close access roads into the mountains and to clear the surrounding area of tourists and 'non-essential' personnel. These early actions were a calculated risk. As always there was a balance to be made between warning and protecting people, and causing panic or issuing a false alarm. Undoubtedly though, thousands of lives were saved by the evacuation and closure of the area. Geologists installed highly sophisticated equipment on the mountain. They constantly monitored the chemical composition of groundwater and escaping gasses, they used laser surveying techniques to look for any expansion of the ground, and recorded the constant seismic activity. The side of the mountain was clearly beginning to expand as pressure began to increase.

By late April there was a bulge 1000m wide and over 2000m long. It was rising at a steady rate of about 1.5m every day as the mountain began to fill with magma being forced up from below. The gas eruptions were also being very carefully monitored. Erupting volcanoes give off large quantities of sulphur dioxide gas, SO2 ( the one that smells of rotten eggs ), so any increases in the emission of this gas might have suggested an impending eruption. The Eruption The volcano seemed to quieten down during mid May. Saturday,17th May saw only 18 recorded earthquakes in the region, less than half the daily value of the previous days. Sunday morning was very quiet, hardly any seismic activity, and reduced gas eruptions. At about 8.30am a research scientist, David Johnston, was working at an observation station about 8km away from the main crater. He had been monitoring gas emissions and watching the mountain whilst in contact with his base,Vancouver, via a two way radio. He transmitted a short message "Vancouver!,Vancouver!,This is it".

Moments later he died as over 4km³ of rock, gas and steam exploded from the northern side of the mountain. The eruption has been started by an earthquake with a magnitude of 5 on the Richter Scale . The surface of the northern side began to move downhill. As it moved, the pressurised magma was able to explode outward through the weakened area. The blast was equivalent to 500 times the power of the atomic bomb that flattened Hiroshima. As it moved outwards it flattened an area of mature forest roughly 35km by 20km. Trees were pushed flat pointing away from the source of the blast. Such was the speed of the moving air and debris that it overtook cars as people tried to escape. The initial explosion was followed by further problems as ice and snow on the mountain was converted to superheated steam. This combined with ash to produce fast flowing mudflows which swept down the mountain into the Toutle River, which at one stage was heated to 90°C. The mudflows moved through houses picking up cars, trucks, trees and even whole buildings. The collected debris was carried downstream where it smashed into river defences and bridges.

As the mud settled in the Columbia river it reduced the depth in places from over 12m to neared 4m, trapping large boats upstream of the shallows. Other ash, not cooled by water, moved away at speeds up to 130km per hour. These ash clouds, at 500°C, burnt everything in their path, before settling out around Spirit Lake. As the deposits built up they blocked the exit from the lake causing the water level to rise by a massive 60m and trigger fears of another disaster waiting to happen. Ash which was thrown vertically into the atmosphere was found over 15km above ground level. As time passed the ash spread out and by the 5th June had traveled right around the Earth. The image below, taken from the Space Shuttle, shows the devastation as it looked in 2002, twenty years after the eruption. Despite the magnitude of the disaster, in true American style, a local business saw a unique marketing opportunity. The ash which fell on the local towns was gathered up and heated in kilns before being molded into 'Genuine Mt St. Helens Ash Trays'!

Today, the area still shows scars of the explosion, but vegetation is returning to the area. As you can see from the photography below, by 2003 there were still vast areas of bare rock debris. Replanting by the Forest Service has helped mixed vegetation re-establish itself in selected areas, but steeper slopes remain bare. We value your ideas and suggestions. Please contact the maintainer of this site. This page can be found at: Last update to this statement was on: May 3, 2007 Mount Saint Helens | mountain, Washington, United States | Britannica.com Mount Saint Helens Mount Saint Helens, volcanic peak in the Cascade Range , southwestern Washington , U.S. Its eruption on May 18, 1980, was one of the greatest volcanic explosions ever recorded in North America . The north face of Mount St. Helens in June 1970. R.W. Decker Eruption of Mount St. Helens on May 18, 1980. U.S. Geological Survey The lava dome of Mount St. Helens in May 1984. Courtesy of the U.S. Geological Survey; photograph, Lyn Topinka Mount St.

Helens, named by the English navigator George Vancouver for a British ambassador, had been dormant since 1857. An explosive steam eruption on March 27, 1980, was followed by alternating periods of quiescence and minor eruption. Pressure from rising magma within the volcano caused extensive fissures and the growth of a bulge on the north flank of the peak. On the morning of May 18, an earthquake with a magnitude of 5.1 on the Richter scale triggered a gigantic landslide on the mountain’s north face. The north slope fell away in an avalanche that was followed and overtaken by a lateral air blast, which carried a high-velocity cloud of superheated ash and stone outward some 15 miles (25 km) from the volcano’s summit; the blast reached temperatures of 660 °F (350 °C) and speeds of at least 300 miles (500 km) per hour. The avalanche and lateral blast were followed by mudflows, pyroclastic flows, and floods that buried the river valleys around Mount St. Helens in deep layers of mud and debris as far as 17 miles (27 km) away.

Meanwhile, simultaneously with the blast, a vertical eruption of gas and ash formed a column some 16 miles (26 km) high that produced ash falls as far east as central Montana . Complete darkness occurred in Spokane , Washington, about 250 miles (400 km) northeast of the volcano. On May 18, 1980, as geologists watched in awe, Mount Saint Helens erupted in a gigantic explosion. Encyclopædia Britannica, Inc. Paricutín A total of 57 people and thousands of animals were killed in the May 18 event, and trees over an area of some 200 square miles (500 square km) were blown down by the lateral air blast. At the event’s end, Mount St. Helens’s volcanic cone had been completely blasted away; in place of its 9,677-foot (2,950-metre) peak was a horseshoe-shaped crater with a rim reaching an elevation of 8,363 feet (2,549 metres). Further eruptions occurred until 1986, and a dome of lava grew intermittently in the crater.

Seismic activity occurred again between 1989 and 1991 (including some small explosions) as well as in 1995 and 1998. Statue of a man planting trees, dedicated to those who replanted the area around Mount Saint … © Michael Hynes In 1982, 172 square miles (445 square km) of land surrounding the volcano was designated Mount St. Helens National Volcanic Monument, administered by the U.S. Forest Service as part of Gifford Pinchot National Forest. The monument provides a unique opportunity for scientific study of the dynamics of an active composite volcano and for research on how ecosystems respond to cataclysmic disturbances. The monument also presents many recreational and educational opportunities. Visitors can view the crater, lava dome , pumice plain, and effects of the landslide from Johnston Ridge Observatory on the monument’s west side, less than 5 miles (8 km) from the volcano. The west side also provides opportunities to observe animals and plants that have recolonized the blast zone and lakes that have formed as a result of the eruption.

The edge of the blast zone, marked by standing dead trees, lies in the eastern part of the monument where old-growth forests, undamaged by the blast, still stand. On the south side are lava formations of various ages, including the longest continuous lava tube in the 48 conterminous U.S. states, which formed during an eruption about 2,000 years ago. Mount Rainier National Park is to the northeast. Mount Saint Helens, Wash. The Volcanoes of Lewis and Clark - Mount St. Helens - Summary [an error occurred while processing this directive] Mount St. Helens, Washington - Summary Mount St. Helens, Washington: Mount St. Helens (8,364 feet, 9,677 feet before May 18, 1980) is located in southwestern Washington about 50 miles northeast of Portland, Oregon, and is one of several lofty volcanic peaks that dominate the Cascade Range of the Pacific Northwest. Geologists call Mount St. Helens a composite volcano (or stratovolcano), a term for steep-sided, often symmetrical cones constructed of alternating layers of lava flows, ash, and other volcanic debris.

Composite volcanoes tend to erupt explosively and pose considerable danger to nearby life and property. In contrast, the gently sloping shield volcanoes, such as those in Hawaii, typically erupt nonexplosively, producing fluid lavas that can flow great distances from the active vents. Although Hawaiian-type eruptions may destroy property, they rarely cause death or injury. "Fujiyama of America": Before 1980, snow-capped, gracefully symmetrical Mount St. Helens was known as the "Fujiyama of America." Mount St. Helens, other active Cascade volcanoes, and those of Alaska comprise the North American segment of the circum-Pacific "Ring of Fire," a notorious zone that produces frequent, often destructive, earthquake volcanic activity. Ancestral Mount St. Helens: Ancestral Mount St. Helens began to grow before the last major glaciation of the Ice Age had ended about 10,000 years ago. The oldest ash deposits were erupted at least 40,000 years ago onto an eroded surface of still older volcanic and sedimentary rocks.

Intermittent volcanism continued after the glaciers disappeared, and nine main pulses of pre-1980 volcanic activity have been recognized. These pulses lasted from about 5,000 years to less than 100 years each and were separated by dormant intervals of about 15,000 years to only 200 years. A forerunner of Spirit Lake was born about 3,500 years ago, or possibly earlier, when eruption debris formed a natural dam across the valley of the North Fork of the Toutle River. Recent History: Mount St. Helens is the youngest of the major Cascade volcanoes, in the sense that its visible cone was entirely formed during the past 2,200 years, well after the melting of the last of the Ice Age glaciers about 10,000 years ago. Mount St. Helens' smooth, symmetrical slopes are little affected by erosion as compared with its older, more glacially scarred neighbors - Mount Rainier and Mount Adams in Washington, and Mount Hood in Oregon. The local Indians and early settlers in the then sparsely populated region witnessed the occasional violent outbursts of Mount St. Helens.

The most recent of the pre-1980 eruptive activity began in A.D. 1800 with an explosive eruption, followed by several additional minor explosions and extrusions of lava, and ended with the formation of the Goat Rocks lava dome by 1857. May 18, 1980: On May 18, 1980, the volcano was transformed. The catastrophic eruption, was preceded by 2 months of intense activity that included more than 10,000 earthquakes, hundreds of small phreatic (steam-blast) explosions, and the outward growth of the volcano's entire north flank by more than 260 feet. A magnitude 5.1 earthquake struck beneath the volcano at 8:32 a.m. on May 18, setting in motion the devastating eruption. Within seconds of the earthquake, the volcano's bulging north flank slid away in the largest landslide in recorded history, triggering a destructive, lethal lateral blast of hot gas, steam, and rock debris that swept across the landscape as fast as 680 miles per hour. Temperatures within the blast reached as high as 570 degrees Fahrenheit (300 degrees Celsius).

Snow and ice on the volcano melted, forming torrents of water and rock debris that swept down river valleys leading from the volcano. Within minutes, a massive plume of ash thrust 15 miles into the sky, where the prevailing wind carried about 520 million tons of ash across 22,000 square miles of the Western United States. 1980 Devastation: On May 18, 1980, the bulge on Mount St. Helens north flank slide away in a huge landslide which covered 23 square miles north and west of the volcano, and buried 14 miles of the North Fork Toutle River Valley to an average depth of 150 feet, with a maximum depth of 600 feet. The landslide depressurized the volcano's magma system, and triggered powerful explosions. Rocks, ash, volcanic gas, and steam were blasted upward and outward to the north, reached 17 miles northward from the volcano, and covered an area of 230 square miles, felling and singing trees. Over 4 billion board feet of timber - enough to build about 300,000 two-bedroom homes - was destroyed.

The lateral blast produced an eruption column of ash and gas which rose more than 15 miles into the atmosphere in only 15 minutes. Over the course of the day, prevailing winds blew 520 million tons of ash eastward across the United States and caused complete darkness in Spokane, Washington, 250 miles from the volcano. The plume spread across the United States in 3 days and circled the earth in 15. Beginning just after noon, pyroclastic flows - swift avalanches of hot ash, pumice, and gas - poured out of the crater at 50 to 80 miles per hour and spread as far as 5 miles to the north. Based on the eruption rate of these pyroclastic flows, scientists estimate that the eruption reached its peak between 3:00 and 5:00 p.m. Melting snow and ice which had covered the volcano melted sending surges of water that eroded and mixed with loose rock debris to form volcanic mudflows (lahars). Several lahars poured down the volcano into river valleys, ripping trees from their roots and destroying 27 bridges and over 200 homes.

The lahars with the blast destroyed more than 185 miles of highways and roads, and 15 miles of railway. Fifty-seven people were killed. Following 1980: Following the 1980 explosive eruption, more than a dozen extrusions of thick, pasty lava built a mound-shaped lava dome in the new crater. The dome is about 3,600 feet in diameter and 820 feet tall. In 2004: After 18 years of relative quiescence, Mount St. Helens volcano once again captured the world?s attention when it showed signs of reawakening in September 2004. On September 23 a swarm of small, shallow earthquakes began in and beneath the 1980-1986 lava dome. Initially this swarm resembled previous swarms that had not led to eruption. By September 24 many hundreds of earthquakes had occurred. Earthquake rates peaked that day at about 2 to 3 per minute then gradually declined, and it appeared that the swarm was ending. But on September 25 shallow earthquakes increased again in both size and rate.

This reinvigorated level of seismicity prompted scientists to issue a notice of volcanic unrest for the first time since 1986. The release of energy by earthquakes increased dramatically during the next several days, and by October 1 earthquakes occurred nearly continuously. Peak sizes reached M3.5, and earthquakes larger than M2.5 occurred about once per minute. Concurrently, parts of the glacier south of the lava dome rose and cracked noticeably, creating a broad welt. The accelerated size and frequency of earthquakes and noticeable deformation of the glacier indicated that molten rock (magma) was rising toward the surface. On October 1, 2004, an explosion lofted steam and ash several thousand feet above a vent blasted through the fractured glacier, and hurled rock fragments at least ? mile across the western half of the glacier and 1980-1986 lava dome. Four more steam and ash explosions occurred through October 5. Following this brief series of steam-and-ash explosions, the volcano settled into a quieter phase during which the welt continued to grow and a new lava dome appeared. This process continued into 2005 and currently (October 2005) shows no signs of stopping.

Recorded History: Mount St. Helens was recognized as a volcano at least as early as 1835; the first geologist apparently viewed the volcano 6 years later. James Dwight Dana of Yale University, while sailing with the Charles Wilkes U.S. Exploring Expedition, saw the peak (then quiescent) from off the mouth of the Columbia River in 1841. Another member of the expedition later described "cellular basaltic lavas" at the mountain's base. Baron St. Helens: Mount St. Helens was named for British diplomat Alleyne Fitzherbert, whose title was Baron St. Helens. The mountain was named by Commander George Vancouver while surveying the northern Pacific coast from 1792 to 1794. Map, "Lewis and Clark Volcano Sitings" 1978 Map, Mount St. Helens and Vicinity, and its major river drainages. (Click to enlarge). Swift Creek, Pine Creek, and the Muddy River drain the southern flanks of Mount St. Helens and drain into the Lewis River. The Kalama River drains west into the Columbia River.

The North and South Fork Toutle Rivers drain to the north west and enter the Cowlitz (a tributary from Mount Rainier), which drains into the Columbia River south of Longview, Washington. Map modified from Crandell and Mullineaux, 1978, USGS Bulletin 1383-C. 1795 Map, Pacific Northwest and the Columbia River (section of original). (Click to enlarge). Includes the Columbia River ("River Oregan"), Mount Rainier, Mount St. Helens, and Mount Hood, and Point Vancouver. Original Map: A map exhibiting all the new discoveries in the interior parts of North America / inscribed by permission to the honorable governor and company of adventurers of England trading into Hudsons Bay in testimony of their liberal communications to their most obedient and very humble servant A. Arrowsmith, January 1st 1795. Published: London, 1802. Notes: Relief shown by hachures. Shows "Rivers added east of the Rocky Mountains, 900 L. 450 Lat.," from Tooley's Printed maps of America. In lower margin: London: Published Jan. 1, 1795 by A.

Arrowsmith, No. 24 Rathbone Place. Includes notes. Reference: Tooley. Printed maps of America 137. Scale [ca. 1:3,800,000] (W 1600--W 400/N 700--N 250). Library of Congress American Memory Archives #G3300 1802 A7 Vault Casetop. -- U.S. Library of Congress Archives, 2004, "American Memory" 1798 Map, Columbia River of George Vancouver (section of original). (Click to enlarge). Includes Columbia River, Cape Disappointment, Point Adams, Point Vancouver, and Mount St. Helens. Original Map: George Vancouver's "A Chart Shewing Part of the Coast of N.W. America." In A Voyage of discovery to the North Pacific ocean, and Round the World. London, 1798. University of Virginia Special Collection "Lewis & Clark, The Maps of Exploration 1507-1814".

-- University of Virginia Library Archives Website, 2004 1804 Map, Pacific Northwest and the Columbia River (section of original). (Click to enlarge). Includes the Columbia River, Mount Baker, Mount Rainier, Mount St. Helens, and Mount Hood. Original Map: "North America". Year: 1804. Author: Robert Wilkinson and E. Bourne. Notes: Scale, ca. 1:9,000,000; London, R. Wilkinson, 12th of Aug. 1804; 1 map, hand col.; 51 x 61 cm.; Relief shown by hachures; Prime meridians: Greenwich, Ferro. -- Henry Stevens and Roland Tree, Comparative cartography, exemplified in an analytical & bibliographical description of nearly one hundred maps and charts of the American continent published in Great Britain during the years 1600 to 1850, no. 62a. In the Collection of Map & Geography Library, University of Illinois at Urbana-Champaign, Champaign, Illinois, Call Number: G3300 1804 .W5.

-- Univerisity of Illinois at Urbana-Champaign Website, 2004. 1805 Map, Pacific Northwest and the Columbia River (section of original). (Click to enlarge). Includes the Columbia River, Mount Rainier, Mount St. Helens, and Mount Hood. Original Map: A map of part of the continent of North America : between the 35th and 51st degrees of north latitude, and extending from 89o degrees of west longitude to the Pacific Ocean / compiled from the authorities of the best informed travellers by M. Lewis ; copied by Nicholas King, 1805. Notes: Map covers western North America, from Lake Michigan and Lake Superior to the Pacific Ocean, including southern Canada and northern Mexico. Pen-and-ink and watercolor. Note from map folder: Map was copied from a sketch by William Clark, not one by Meriwether Lewis as indicated in the title. Copy sectioned and mounted on cloth. Library of Congress American Memory Archives #G3300 1805 .C5 Vault Oversize. -- U.S.

Library of Congress Archives, 2004, "American Memory" 1814 Map, Lewis and Clark's map of the Columbia River (section of original). (Click to enlarge.) Includes three of the five volcanoes Lewis and Clark saw and commented on. While the journals mention the expedition seeing Mount Adams, it does not appear on their map. Mount Jefferson is to the south (bottom) and off the map. From the "Nicholas Biddle/Paul Allen" 1814 publication. Original Map: "A Map of Lewis and Clark's Track, Across the Western Portion of North America, From the Mississippi to the Pacific Ocean". From: History of the expedition under the command of Captains Lewis and Clark : to the sources of the Missouri, thence across the Rocky Mountains and down the river Columbia to the Pacific Ocean : performed during the years 1804-5-6 : by order of the government of the United States / prepared for the press by Paul Allen. Philadelphia : Bradford and Insskeep, 1814. Harold B. Lee Library, Brigham Young University #upbover maps37. -- Brigham Young University, Harold B.

Lee Library Website, 2004. 1814 Map, Lewis and Clark's map of the Columbia River (section of original). (Click to enlarge.) Map includes three of the five volcanoes Lewis and Clark saw and commented on. While the journals mention the expedition seeing Mount Adams, it does not appear on their map. Mount Jefferson is just visible to the south (bottom) and Mount Rainier is to the north but off the map. From the "Nicholas Biddle/Paul Allen" 1814 publication. Original Map: "A Map of Lewis and Clark's Track, Across the Western Portion of North America, From the Mississippi to the Pacific Ocean". From: History of the expedition under the command of Captains Lewis and Clark : to the sources of the Missouri, thence across the Rocky Mountains and down the river Columbia to the Pacific Ocean : performed during the years 1804-5-6 : by order of the government of the United States / prepared for the press by Paul Allen. Philadelphia : Bradford and Insskeep, 1814. Harold B. Lee Library, Brigham Young University #upbover maps37. -- Brigham Young University, Harold B.

Lee Library Website, 2004. 1833 Map (section of original), Columbia River. (Click to enlarge). Includes Mount Hood and Mount Jefferson. Note: Mount Baker is depicted (upper middle) but Mounts Adams, Rainier and St. Helens are missing. The Columbia River is shown as "Oregon River" at its mouth and "Columbia or Oregon R." further inland. "Wappatoo Valley" is labeled. Also shows Fort Clatsop ("F. Clatsop or F. George"), the Willamette River ("Multnomah R."), Sandy River ("Quicksand R."), John Day River ("R.La Page"), Walla Walla River ("Wallwullah R."), Snake River ("Lewis R."), and the Yakima River ("Tapete R."). Original Map: Oregon Territory, 1833. Creator: Illman & Pilbrow, published by Illman & Pilbrow, New York. Comments: Illman & Pilbrow is the engraving firm which copyrighted and published this map, the actual artist is unknown. Washington State University Digital Maps Collection #WSU323. University of Washington Digital Maps Collection #UW104.

-- Washington State University Early Washington Maps Digital Collection Website, 2004 1841 Map (section of original), Columbia River, west. (Click to enlarge). Includes Mount Rainier, Mount St. Helens, and Mount Hood. Mount Adams is depicted but not named. From Wilkes, 1841, the U.S. Exploring Expedition. -- Washington State University Early Washington Maps Digital Collection Website, 2004 1841 Map (section of original), Columbia River, east. (Click to enlarge). Includes Mount St. Helens, Mount Hood, and Mount Jefferson. Mount Adams is depicted but not named. From Wilkes, 1841, the U.S. Exploring Expedition. -- Washington State University Early Washington Maps Digital Collection Website, 2004 1849 Map (section of original), Alexander Ross's Columbia River. (Click to enlarge). Original Map: Map of the Columbia to illustrate Ross's adventures. Author: Alexander Ross; Publication Date: 1849; Publisher: London, Smith, Elder and Co., 1849. Washington State University Archives #WSU478.

-- Washington State University Early Washington Maps Digital Collection Website, 2004 1853 Map, Washington and Oregon and the Columbia River, from the Clearwater River to the Snake River and down the Columbia River to the Pacific Ocean (section of original). (Click to enlarge). Includes: Clearwater River (Kooskooski), Lapwai Creek (Lapwai R.), Snake River (Saptin or Lewis R.), Columbia River (Columbia R.), Yakima River (Yakima R.), Walla Walla River (Wallawalla R.), Umatilla River (Umatilla R.), Willow Creek (Quesnells R.), John Day River (John Day's R.), Deschutes River (Fall R.), Willamette River (Willammette R.), and Cowlitz River (Cowlitz R.). Original Map: "Map of California, Oregon, Washington, Utah, and New Mexico (1853)", by Thomas Cowperthwait & Co. Washington State University Archives #WSU22. -- Washington State University Library Collections Website, 2003 1855 Map, Columbia River from Vancouver to the Pacific (section of original).

(Click to enlarge). Includes Mount Rainier, Mount St. Helens, Mount Adams, Mount Hood (depicted but not named), and Mount Jefferson. Original Map: "Map of Oregon and Washington Territories: showing the proposed Northern Railroad route to the Pacific Ocean, by John Disturnell, 1855. University of Washington Archives #UW155. -- University of Washington Library Collections Website, 2002 1855 Map, Columbia River from Cascade Range to Walla Walla (section of original). (Click to enlarge). Includes Mount Adams, Mount St. Helens, Mount Hood (depicted but not named), and Mount Jefferson. Original Map: "Map of Oregon and Washington Territories: showing the proposed Northern Railroad route to the Pacific Ocean, by John Disturnell, 1855. University of Washington Archives #UW155. -- University of Washington Library Collections Website, 2002 1860 Map, Columbia River, Washington State, and Oregon (section of original). (Click to enlarge). Original Map: Map of Oregon, Washington, and British Columbia, (1860).

This map dates between March 2nd, 1861 (when the Dakota Territory was formed) and March 4th, 1863 (when the Idaho Territory was formed from eastern Washington and western Dakota) Nearing retirement from a thirty year long and rather successful career, S. Augustus Mitchell printed this map showcasing Oregon, the Territory of Washington, and British Columbia. Washington became a territory in 1853, arguing that distances to Willamette Valley kept them from obtaining a voice in the Oregon territorial government. As this map shows, when it split from Oregon proper the Washington territory included parts of Wyoming and Montana and all of Idaho. Territorial government for Idaho would not be approved until 1863. When Mitchell retired he left the business for his son to manage. Washington State University Archives #WSU7. -- Washington State University Archives, 2004 1919 Map of Mount St. Helens (section of original), from Mount St. Helens 1:125,000 topographic quadrangle. (Click to enlarge).

Original map surveyed in 1913-1914 and 1916, contour interval of 100 feet. Map published in 1919. -- University of Washington Library Collections Website, 2002 ca.1853, Engraving. Mouth of the Columbia River (section of original). (Click to enlarge). Engraving depicts the Mouth of the Columbia River, Point Ellice, Mount St. Helens, and Tongue Point. Original also depicts Cape Disappointment and Point Adams. From: NOAA Photo Archives, America's Coastline Collection #line2075. -- NOAA Photo Archvies Website, 2002 1879, Detail of engraving of Portland, Oregon and Mount Rainier and Mount St. Helens, Washington. (Click to enlarge). The Columbia River is middleground and the Willamette River is in the foreground. Created by E.S. Glover. Published 1879, San Francisco. "Bird's-eye-view", looking east to the Cascade Mountains. Original lithograph shows Mount Rainier, Mount St.

Helens, Mount Adams, and Mount Hood, and also the Columbia River and the Willamette River. Reference #LC Panoramic Maps #722. -- Library of Congress American Memories Website, 2002 1889, Engraving/Sketch. Mouth of the Columbia River (section of original). (Click to enlarge). Engraving depicts the Mouth of the Columbia River, Point Ellice, Mount St. Helens, and Tongue Point. Original also depicts Scarborough Hill. From: NOAA Library, Pacific Coast Coast Pilot of California, Oregon, and Washington, 1889 -- NOAA Photo Archives Website, 2004 1890, Detail from engraving of Portland, Oregon, with Mount St. Helens. (Click to enlarge). Created by Clohessy & Strengele. Published 1890, San Francisco. "Bird's-eye-view", Portland, Oregon, 1890. Original lithograph shows Mount St. Helens, Mount Adams, and Mount Hood, with the Columbia River and the Willamette River. Reference #75694939.

-- Library of Congress American Memories Website, 2002 1978, View of Mount St. Helens, before the eruption of May 18, 1980. (Click to enlarge). U.S. Army Corps of Engineers Photograph #Sce0234. Photograph Date: 1978. Photographer: unknown. From: U.S. Army Corps of Engineers Photo Archives. -- U.S. Army Corps of Engineers Website, 2002 1982, USGS Photo showing Mount St. Helens after the May 18, 1980 eruption. The volcano is reflected in Spirit Lake. (Click to enlarge). Photographer: Lyn Topinka, Date: May 19, 1982. -- USGS Cascades Volcano Observatory Photo Archives, 2004 2003, Mount St. Helens, Washington, as seen from Blurock Landing, Washington. (Click to enlarge). Blurock Landing is across the Columbia River from the mouth of the Willamette River, Oregon. From this location Lewis and Clark spotted five volcanoes.

Copyright © 2003 Lyn Topinka, private archives, used with permission. 2004, Mount Adams (left) and Mount St. Helens, Washington, as seen from Point Ellice, Washington, 14 miles upstream from the mouth of the Columbia River. (Click to enlarge). Copyright © 2004 Lyn Topinka, private archives, used with permission. 2004, Mount St. Helens, Washington, and the Lewis and Clark Bridge, as seen from near Rainier, Oregon. (Click to enlarge). The Lewis and Clark Bridge connects Longview, Washington, with Rainier, Oregon. Copyright © 2004 Lyn Topinka, private archives, used with permission. Excerpts from: Brantley, 1994, Brantley and Myers, 2000, Foxworthy and Hill, 1982, Major, et.al., 2005, and Tilling, et.al., 1990. vlcnos.html MOUNT St. HELENS ERUPTION, May 18, 1980 May 17, 1980, before the eruption.

(Courtesy USGS, photo by Harry Glicken) Click here or on image for view of Mt. St. Helens after the eruption. Mount St. Helens, located in southwest Washington State, is a typical stratovolcano. The volcano is one of several that line-up to form the Cascade Range, which extends from northern-central California northward into the Province of British Columbia in Canada. Six of the volcanoes are shown below in relationship to the convergent plate boundary separating the Juan de Fuca and North American plates. 2.1 Approximately how far is Mount St. Helens from the convergent plate boundary separating the Juan de Fuca and North American plates? Mount St. Helens, like the other volcanoes of the Cascades, is composed of andesitic and rhyolitic pyroclastic materials. Some of the pyroclastic material has been remobilized as landslides and lahars. As explained at the beginning of this lab exercise, eruptions of stratovolcanoes can be catastrophically explosive. In May, 1980, Mount St. Helens did just that.

The images at the top of this page show the state of the mountain before and after the May, 1980 eruption. Let's take a closer look at the relationship between andesitic/rhyolitic volcanism and a convergent plate boundary. Below you can examine a larger scale map of the area, and an east-west cross-section (A-A') from the plate boundary to Mount St. Helens. For the purpose of comparison, the scale is the same as the scale used on the map and cross-section of the Island of Hawaii. Stratovolcanoes are typically circular in plan, about the size of the circle shown on map. The radius of Mount St. Helens is about 6 km at the base, and the elevation of the base is about 1 km above sea level. Before the eruption of May, 1980, the elevation at the top of Mount St. Helens was about 3 km. 2.2 Using this information, and modeling the stratovolcano as a simple cone-shape, estimate the volume of volcanic material in Mount St. Helens, in cubic kilometers (km3).

To remind you, the volume of a cone is given by the following formula, volume = (1/3)*pi*r2*h, where pi ~ 3.14, r = radius of the cone, and h = height of the cone. 2.3 How does this compare with the volume of a shield volcano, as exemplified by the Island of Hawaii? The cross-section (above the map) shows the relationship between the convergent plate boundary and the volcanism. In the case of Mount St. Helens, like the other stratovolcanoes of the Cascades, the production of the andesitic/rhyolitic magma is ultimately related to the subduction of the Juan de Fuca plate beneath the western edge of the North American plate. Oceanic crust of the Juan de Fuca plate and sedimentary material (light green) are being shoved downward, toward the east, underneath North America. This down-going material undergoes metamorphism as it becomes exposed to progressively higher pressures and temperatures in the interior of the earth beneath North America. Earthquakes generated near the top surface of the down-going Juan de Fuca plate originate at a depth of about 75 km beneath Mount St. Helens.

That's about where the top of the Juan de Fuca plate is located directly beneath Mount St. Helens. Temperatures at these depths can be as high as 1200 C, hot enough to cause some partial melting of the metamorphosed basalt of the subducting oceanic crust. The metamorphism also releases volatile (gas) components such as H2O, CO2, and SO2. The relatively small amounts of basaltic magma and rather large amounts of volatile gasses migrate upward, ultimately encountering the base of the overlying continental crust. Here, the basaltic magma and volatile gasses contribute to partial melting of the base of the continental crust. The magma thus formed is andesitic and rhyolitic, reflecting the composition of the lower part of the continental crust. Generally, only very small amounts of the basaltic magma ever reach the surface. The greater part of the basaltic magma becomes mixed, sort of homogenized, with the andesitic/rhyolitic magma. This andesitic/rhyolitic magma readily dissolves, quite literally soaks up, the volatile components.

Under the very high pressures at such depths near the base of the continental crust these dissolved gasses are effectively trapped in the magma. However, the buoyant, gas-charged magma tends to work it's way upward toward the surface (intrusion). Most of the andesitic/rhyolitic magma cools and crystallizes before it ever reaches the surface, forming sometimes vast plutonic (intrusive) bodies of granodiorite/granite. Only comparitively small amounts of the gas-charged andesitic/rhyolotic magma ever reach the surface, but when this happens - LOOK OUT! The rapid formation of gas bubbles in the magma and their explosive expansion can have catastrophic effects. See the "before eruption" and "after eruption" images of Mount St. Helens, at the top of this page. 2.4 How might these "vast plutonic bodies of granodiorite and granite" ever become exposed at the surface? (Hint: Rock Cycle) Now, let's take a closer look at the physiography (shape) of a stratovolcano. In the case of Mount St. Helens, this makes for an interesting "before" and "after" study of the effects of a eruption.

Below, you can examine "before" and "after" topographic maps of Mount St. Helens. Before the May 18, 1980 eruption, Mount St. Helens was a simple cone-shaped stratovolcano, typical of very many stratovocanoes around the world. The elevation at the top was 9677 ft (~3000 m). Oops, I apologize here for getting away from the metric system in these maps and some you will see later. The unit of measure is "feet" in all of the existing maps from which these were constructed. For what you will have to do, this won't make any difference. So you will be doing your work in "feet." The slope of the flanks of a stratovolcano are pretty much controlled by the angle of repose of the coarser pyroclastic material making up the greater part of the volcano. 2.5 What is the slope and angle of the slope of the surface of Mount St. Helens along the red bar in the "before" map above? The length of the red bar is 10,000 ft.

2.6 How does this compare with the slope and angle of slope of flanks of a shield volcano? One way to examine the effects of the eruption is to construct "before" and "after" topographic profiles of the volcano. The diagram below shows how this is done for a topographic profile along the segment X-X' (marked on the maps above) for Mount St. Helens before the May 18, 1980 eruption. Basically, the topographic profile shows how the elevation changes along a specified line on the map, here X-X'. It's just a graph of elevation (vertical axis) versus horizontal distance (horizontal axis). It should come as no surprise to you that this just makes a side-view of the mountain. O.K., so now you know what Mount St. Helens looked like before the eruption, when viewed from the east. Wouldn't you like to know what Mount St. Helens looked like just after the eruption, from exactly the same vantage point? 2.7 Well, get to work. Construct the post-eruption profile on the hand-out of the map shown above. The pre-eruption profile is shown for comparison.

2.8 On your copy of the post-eruption profile, mark the following regions, (1) where there has been no change, (2) where material has been removed, and (3) where material has been added (deposited). 2.9 As nearly as you can estimate, what is the thickness of pyroclastic material (mostly ash) that has been deposited at the spot marked X'? The best way to do this is to estimate the elevation at X' on the "before" and "after" maps, then subtract. The amount of material that was removed from Mount St. Helens has been estimated at 6.5 km3 (cubic km). Most of this material was old volcanic material that was part of the volcano before the eruption. Some of this pulverized pyroclastic material was carried aloft and to the east by the wind, where it settled on the ground in noticeable amounts as far away as Montana. In Idaha, some 500 km east of Mount St. Helens, volcanic ash blanketed the landscape in a layer up to 5 mm thick. 2.10 The amount of material (6.5 km3) that was removed from Mount St.

Helens represents what percentage of the original volcano? (Help: You need your answer to item 2.2.) There is another way to examine some of these changes, by superimposing the "before" and "after" topographic maps. Take a look. In these superimposed topographic maps you can identify basically three kinds of regions, (1) where the contour lines superimpose nearly exactly, (2) where the contour lines don't coincide and the "after" elevations are higher than the "before" elevations, and (3) where the contour lines don't coincide and the "after" elevations" are lower than the "before" elevations. 2.11 What does it mean if the "after" elevations are higher than the "before" elevations? (HINT: no change, material added, material removed.) 2.12 What does it mean if the "after" elevations are lower than the "before" elevations? (HINT: no change, material added, material removed.) 2.13 On your copy of this map, shade in the three regions. If you have color pencils or markers, use the following scheme for shading in the different regions.

Regions of type 1, "no difference," leave blank. Regions of type 2, green. Regions of type 3, red. Based on this kind of analysis, it becomes obvious that the explosion of the Mount St. Helens and dispersal of pyroclastic material was not the same in all directions. 2.14 In what direction was the explosion directed? What is your reasoning? NOW YOU HAVE COMPLETED THE SECOND PART OF THE LAB EXERCISE. USGS: Volcano Hazards Program CVO Mount St. Helens 2004-2008 Activity Geology and History Summary for Mount St. Helens Mount St. Helens, located in Washington State, is the most active volcano in the Cascade Range, and it is the most likely of the contiguous U.S. volcanoes to erupt in the future. The volcano is almost 53 km (33 mi) due west of Mount Adams and approximately 80 km (50 mi) northeast of the Vancouver, Washington—Portland, Oregon metropolitan area. Volcanism occurs at Mount St.

Helens and other volcanoes in the Cascades arc due to subduction of the Juan de Fuca plate off the western coast of North America. Over its rich and complex 275,000-year history, Mount St. Helens has produced both violent explosive eruptions of volcanic tephra and relatively quiet outpourings of lava . In the beginning stages of eruptive activity, the volcano mostly consisted of a cluster of domes that was surrounded by an apron of tephra and debris fans of fragmented volcanic rocks. It was only during the past few thousand years that the volcano grew to its pre-1980 elevation of 2,950 m (9,677 ft), making it the, then, fifth highest peak in Washington. Starting about 3,000 years ago, substantial amounts of basalt and andesite began to erupt as lava flows between phases of dacite eruptive activity. These lava flows buried large parts of a central cluster of dacite domes and flanking fans, which started the cone building in earnest. Based upon detailed chemical analysis of the eruptive products from each stage of Mount St.

Helens volcanism, scientists deduce that the volcano's magmatic system has evolved from relatively simple to more complex as the volcano matured. The evidence indicates that interaction between more silicic ( dacite ) magma batches with more mafic ( basalt to andesite ) magma increased from the earliest to most recent stages of Mount St. Helens volcanism.

Question: In which American state is mount St.Helens?
Answer: Washington