Mount St. Helens
Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States. It lies northeast of Portland, Oregon, and south of Seattle. Mount St. Helens takes its English name from that of the British diplomat Alleyne FitzHerbert, 1st Baron St Helens, a friend of explorer George Vancouver who surveyed the area in the late 18th century. The volcano is part of the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire.
The Mount St. Helens major eruption of May 18, 1980, is currently the most economically destructive volcanic event in U.S. history. 57 people were killed and 200 homes, 47 bridges, of railways, and of highway were destroyed. A massive debris avalanche, triggered by a magnitude 5.1 earthquake, caused a lateral eruption that reduced the elevation of the mountain's summit from, leaving a horseshoe-shaped crater. The debris avalanche was in volume. The 1980 eruption disrupted terrestrial ecosystems near the volcano. By contrast, aquatic ecosystems in the area greatly benefited from the amounts of ash, allowing life to multiply rapidly. Six years after the eruption, most lakes in the area had returned to their normal state.
After its 1980 eruption, the volcano experienced continuous volcanic activity until 2008. Geologists predict that future eruptions will be more destructive, as the configuration of the lava domes requires more pressure to erupt. However, Mount St. Helens is a popular hiking spot and it is climbed year-round. In 1982, the Mount St. Helens National Volcanic Monument was established by Congress.
Geographic setting and description
General
Mount St. Helens is west of Mount Adams, in the western part of the Cascade Range. Considered "brother and sister" mountains, the two volcanoes are approximately from Mount Rainier, the highest of the Cascade volcanoes. Mount Hood, the nearest major volcanic peak in Oregon, is southeast of Mount St. Helens.Mount St. Helens is geologically young compared with the other major Cascade volcanoes. It formed only within the past 40,000 years, and the summit cone present before its 1980 eruption began rising about 2,200 years ago. The volcano is considered the most active in the Cascades within the Holocene epoch, which encompasses roughly the last 10,000 years.
Prior to the 1980 eruption, Mount St. Helens was the fifth-highest peak in Washington. It stood out prominently from surrounding hills because of the symmetry and extensive snow and ice cover of the pre-1980 summit cone, earning it the nickname, by some, "Fujiyama of America". Its ice cover just prior to the 1980 eruption included eleven named glaciers: Wishbone, Loowit, Leschi, Forsyth, Nelson, Ape,
Shoestring, Swift, Dryer, Toutle, and Talus. Of these eleven, only the Shoestring Glacier revived somewhat post-eruption. The peak rose more than above its base, where the lower flanks merge with adjacent ridges. The mountain is across at its base, which is at an elevation of on the northeastern side and elsewhere. At the pre-eruption tree line, the width of the cone was.
Streams that originate on the volcano enter three main river systems: The Toutle River on the north and northwest, the Kalama River on the west, and the Lewis River on the south and east. The streams are fed by abundant rain and snow. The average annual rainfall is, and the snowpack on the mountain's upper slopes can reach. The Lewis River is impounded by three dams for hydroelectric power generation. The southern and eastern sides of the volcano drain into an upstream impoundment, the Swift Reservoir, which is directly south of the volcano's peak.
Although Mount St. Helens is in Skamania County, Washington, access routes to the mountain run through Cowlitz County to the west, and Lewis County to the north. State Route 504, locally known as the Spirit Lake Memorial Highway, connects with Interstate 5 at Exit 49, to the west of the mountain. That north–south highway skirts the low-lying cities of Castle Rock, Longview and Kelso along the Cowlitz River, and passes through the Vancouver, Washington–Portland, Oregon metropolitan area less than to the southwest. The community nearest the volcano is Cougar, Washington, in the Lewis River valley south-southwest of the peak. Gifford Pinchot National Forest surrounds Mount St. Helens.
Crater Glacier and other new rock glaciers
During the winter of 1980–1981, a new glacier appeared. Now officially named Crater Glacier, it was formerly known as the Tulutson Glacier. Shadowed by the crater walls and fed by heavy snowfall and repeated snow avalanches, it grew rapidly. By 2004, it covered about, and was divided by the dome into a western and eastern lobe. Typically, by late summer, the glacier looks dark from rockfall from the crater walls and ash from eruptions. As of 2006, the ice had an average thickness of and a maximum of, nearly as deep as the much older and larger Carbon Glacier of Mount Rainier. The ice is all post-1980, making the glacier very young geologically. However, the volume of the new glacier is about the same as all the pre-1980 glaciers combined.From 2004, volcanic activity pushed aside the glacier lobes and upward by the growth of new volcanic domes. The surface of the glacier, once mostly without crevasses, turned into a chaotic jumble of icefalls heavily criss-crossed with crevasses and seracs caused by movement of the crater floor. The new domes have almost separated the Crater Glacier into an eastern and western lobe. Despite the volcanic activity, the termini of the glacier have still advanced, with a slight advance on the western lobe and a more considerable advance on the more shaded eastern lobe. Due to the advance, two lobes of the glacier joined in late May 2008 and thus the glacier completely surrounds the lava domes. In addition, since 2004, new glaciers have formed on the crater wall above Crater Glacier feeding rock and ice onto its surface below; there are two rock glaciers to the north of the eastern lobe of Crater Glacier.
Climate
Mount St. Helens has an alpine tundra climate.Geology
Mount St. Helens is part of the Cascades Volcanic Province, an arc-shaped band extending from southwestern British Columbia to Northern California, roughly parallel to the Pacific coastline. Beneath the Cascade Volcanic Province, a dense oceanic plate sinks beneath the North American Plate; a process known as subduction in geology. As the oceanic slab sinks deeper into the Earth's interior beneath the continental plate, high temperatures and pressures allow water molecules locked in the minerals of solid rock to escape. The supercritical water rises into the pliable mantle above the subducting plate, causing some of the mantle to melt. This newly formed magma ascends upward through the crust along a path of least resistance, both by way of fractures and faults as well as by melting wall rocks. The addition of melted crust changes the geochemical composition. Some of the melt rises toward the Earth's surface to erupt, forming the Cascade Volcanic Arc above the subduction zone.The magma from the mantle has accumulated in two chambers below the volcano: one approximately below the surface, the other about. The lower chamber may be shared with Mount Adams and the Indian Heaven volcanic field.
Ancestral stages of eruptive activity
The early eruptive stages of Mount St. Helens are known as the "Ape Canyon Stage", the "Cougar Stage", and the "Swift Creek Stage". The modern period, since about 2500 BC, is called the "Spirit Lake Stage". Collectively, the pre–Spirit Lake stages are known as the "ancestral stages". The ancestral and modern stages differ primarily in the composition of the erupted lavas; ancestral lavas consisted of a characteristic mixture of dacite and andesite, while modern lava is very diverse.St. Helens started its growth in the Pleistocene 37,600 years ago, during the Ape Canyon stage, with dacite and andesite eruptions of hot pumice and ash. Thirty-six thousand years ago a large mudflow cascaded down the volcano; mudflows were significant forces in all of St. Helens' eruptive cycles. The Ape Canyon eruptive period ended around 35,000 years ago and was followed by 17,000 years of relative quiet. Parts of this ancestral cone were fragmented and transported by glaciers 14,000–18,000 years ago during the last glacial period of the current ice age.
The second eruptive period, the Cougar Stage, started 20,000 years ago and lasted for 2,000 years. Pyroclastic flows of hot pumice and ash along with dome growth occurred during this period. Another 5,000 years of dormancy followed, only to be upset by the beginning of the Swift Creek eruptive period, typified by pyroclastic flows, dome growth and blanketing of the countryside with tephra. Swift Creek ended 8,000 years ago.
Smith Creek and Pine Creek eruptive periods
A dormancy of about 4,000 years was broken around 2500 BC with the start of the Smith Creek eruptive period, when eruptions of large amounts of ash and yellowish-brown pumice covered thousands of square miles. An eruption in 1900 BC was the largest known eruption from St. Helens during the Holocene epoch, depositing the Yn tephra. This eruptive period lasted until about 1600 BC and left deep deposits of material distant in what is now Mount Rainier National Park. Trace deposits have been found as far northeast as Banff National Park in Alberta, and as far southeast as eastern Oregon. All told there may have been up to of material ejected in this cycle. Some 400 years of dormancy followed.St. Helens came alive again around 1200 BC—the Pine Creek eruptive period. This lasted until about 800 BC and was characterized by smaller-volume eruptions. Numerous dense, nearly red hot pyroclastic flows sped down St. Helens's flanks and came to rest in nearby valleys. A large mudflow partly filled of the Lewis River valley sometime between 1000 BC and 500 BC.