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{{for|the mountain in California|Mount Saint Helena}}
{{Infobox Mountain
| Name = Mount St. Helens
| Photo = MSH82_st_helens_plume_from_harrys_ridge_05-19-82.jpg
| Caption = <small>3,000 ft (1&nbsp;km) steam plume on [[May 19]], [[1982]]</small>
| Elevation = {{Convert|8365|ft|m|0}}
| Location = [[Washington]], [[United States|USA]]
| Range = [[Cascade Range]]
| Prominence = {{Convert|4605|ft|m|0}}
| Coordinates = {{coord|46|12|00.17|N|122|11|21.13|W|type:mountain_region:US|display=inline,title}}
| Topographic map = [[USGS]] Mount St. Helens
| Type = Active [[stratovolcano]]
| Volcanic_Arc/Belt = [[Cascade Volcanoes|Cascade Volcanic Arc]]
| Tectonic setting = [[Subduction zone]]
| Age = &lt; 40,000 yrs
| Last eruption = 2004–2008 (ongoing)
| First ascent = 1853 by [[Thomas J. Dryer]]
| Easiest route = Hike via south slope of volcano (closest area near eruption site)
}}
'''Mount St. Helens''' is an active [[stratovolcano]] located in [[Skamania County, Washington|Skamania County]], [[Washington]], in the [[Pacific Northwest]] region of the [[United States]]. It is 96 miles (154&nbsp;km) south of [[Seattle, Washington|Seattle]] and 53 miles (85&nbsp;km) northeast of [[Portland, Oregon]]. Mount St. Helens takes its [[English language|English]] name from the [[United Kingdom|British]] diplomat [[Alleyne Fitzherbert, 1st Baron St Helens|Lord St Helens]], a friend of explorer [[George Vancouver]] who made a survey of the area in the late 18th century. The mountain is located in the [[Cascade Range]] and is part of the [[Cascade Volcanoes|Cascade Volcanic Arc]], a segment of the [[Pacific Ring of Fire]] that includes over 160 active [[volcano]]es. This volcano is well known for its [[volcanic ash|ash]] explosions and [[pyroclastic flow]]s.

Mount St. Helens is most famous for [[1980 eruption of Mount St. Helens|its catastrophic eruption]] on [[May 18]], [[1980]],<ref name="USDA">{{cite web|publisher=USDA Forest Service |url=http://www.fs.fed.us/gpnf/mshnvm/ |title=Mount St. Helens National Volcanic Monument}} (accessed 26 November 2006)</ref> which was the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 15 miles (24&nbsp;km) of railways, and 185 miles (300&nbsp;km) of highway were destroyed. The eruption caused a massive [[debris avalanche]], reducing the elevation of the mountain's summit from {{Convert|9677|ft|m|0}} to {{Convert|8365|ft|m|0}} and replacing it with a mile-wide (1.5&nbsp;km-wide) [[horseshoe]]-shaped crater.<ref name="USDATeacherCorner">{{cite web|publisher=USDA Forest Service |url=http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/volcanic-eruption-summary.shtml |title=May 18, 1980 Eruption of Mount St. Helens|accessdate=2007-08-11}}</ref> The debris avalanche was up to 0.7 cubic miles (2.9 km³) in volume. The [[Mount St. Helens National Volcanic Monument]] was created to preserve the volcano and allow for its aftermath to be scientifically studied.

As with most other volcanoes in the Cascade Range, Mount St. Helens is a large eruptive cone consisting of [[lava]] rock interlayered with ash, [[pumice]], and other deposits. The mountain includes layers of [[basalt]] and [[andesite]] through which several [[Lava dome|dome]]s of [[dacite]] lava have erupted. The largest of the dacite domes formed the previous summit, and off its northern flank sat the smaller Goat Rocks dome. Both were destroyed in the 1980 eruption.
[[Image:sthelens1.jpg|thumb|right|300px|Mount St. Helens the day before the 1980 eruption, which removed much of the northern face of the mountain, leaving a large crater ([[caldera]]).]]
[[Image:Mt St Helens.JPG|thumb|right|Mt. St. Helens in 2007. Viewed from Johnston Ridge Observatory.]]

==Geographic setting and description==
===General===
[[Image:Mt St Helens from climbers biviouc.JPG|thumb|left|The view in 2002 from Climbers Bivouac]]
Mount St. Helens is 45 miles (72&nbsp;km) west of [[Mount Adams (Washington)|Mount Adams]], in the western part of the Cascade Range. These "sister and brother" volcanic mountains are approximately 50 miles (80&nbsp;km) from [[Mount Rainier]], the highest of Cascade volcanoes. [[Mount Hood]], the nearest major volcanic peak in [[Oregon]], is 60 miles (96&nbsp;km) southeast of Mount St. Helens.

Mount St. Helens is geologically young compared to the other major Cascade volcanoes. It formed only within the past 40,000 years, and the pre-1980 summit cone began rising about 2,200 years ago.<ref>Mullineaux, The Eruptive History of Mount St. Helens, USGS Professional Paper 1250, page 3</ref> The volcano is considered the most active in the Cascades within the [[Holocene]] epoch (the last 10,000 or so years).<ref name="USGS-Description">[http://vulcan.wr.usgs.gov/Volcanoes/MSH/description_msh.html USGS Description of Mount St. Helens], USGS.gov (accessed 15 Nov 2006)</ref>

[[Image:St Helens and nearby area from space.jpg|thumb|left|A view of St. Helens and the nearby area from space.]]
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 "Fuji-san of America" ("[[Mount Fuji]] of America").<ref name="FireMountains201">Harris, ''Fire Mountains of the West'', 1st edition, page 201</ref> The peak rose more than 5,000 feet (1,525 m) above its base, where the lower flanks merge with adjacent ridges. The mountain is six miles (9.6&nbsp;km) across at its base, which is at an altitude of 4,400 feet (1,340 m) on the northeastern side and 4,000 feet (1,220 m) elsewhere. At the pre-eruption [[tree line]], the width of the cone was four miles (6.4&nbsp;km).

[[Stream]]s 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 (Washington)|Lewis River]] on the south and east. The streams are fed by abundant rain and snow. The average annual rainfall is 140 inches (3.6 m), and the snowpack on the mountain's upper slopes can reach 16 feet (4.9 m).<ref name="Tilling1990"/> The Lewis River is impounded by three [[dam]]s 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.

[[Image:Phreatic.jpg|thumb|right|250px|Phreatic eruption at the summit of Mount St. Helens, Washington in the spring of 1980.]]

Although Mount St. Helens is in Skamania County, Washington, the best access routes to the mountain run through [[Cowlitz County, Washington|Cowlitz County]] to the west. [[Washington State Route 504|State Route 504]], locally known as the [[Spirit Lake Memorial Highway]], connects with the heavily traveled [[Interstate 5 (Washington)|Interstate 5]] at Exit 49, 34 miles (55&nbsp;km) to the west of the mountain. That major north-south [[highway]] skirts the low-lying cities of [[Castle Rock, Washington|Castle Rock]], [[Longview, Washington|Longview]] and [[Kelso, Washington|Kelso]] along the [[Cowlitz River]], and passes through the [[Vancouver, Washington]]-[[Portland, Oregon]] [[metropolitan area]] less than 50 miles (80&nbsp;km) to the southwest. The community nearest the volcano is [[Cougar, Washington]], in the Lewis River valley 11 miles (18&nbsp;km) south-southwest of the peak. [[Gifford Pinchot National Forest]] surrounds Mount St. Helens.

=== Crater Glacier and other new rock glaciers ===
{{main|Crater Glacier}}
During the winter of 1980&ndash;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 (46 feet or 14 m/year in thickness). By 2004, it covered about 0.36 square mile (0.93 km²), 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 328 feet (100 m) and a maximum of 656 feet (200 m), nearly as deep as the much older and larger [[Carbon Glacier]] of Mount Rainier. The ice is all post&ndash;1980, making the glacier very young geologically. However, the volume of the new glacier is about the same as all the pre&ndash;1980 glaciers combined.<ref name="Brugman81">{{cite web | last = Brugman | first = Melinda M. | coauthors = [[Austin Post]] | title = USGS Circular 850-D: Effects of Volcanism on the Glaciers of Mount St. Helens | url= http://pubs.er.usgs.gov/usgspubs/cir/cir850D | date = 1981 | accessdate = 2007-03-07 }}</ref><ref name="Wiggins02" >{{cite journal | last = Wiggins | first = Tracy B. | coauthors = Hansen, Jon D.; Clark, Douglas H. | title = Growth and flow of a new glacier in Mt. St. Helens Crater | journal = Abstracts with Programs - Geological Society of America | volume = 34 | issue = 5 | pages = 91 | publisher = | date = 2002 | url = }}</ref><ref name="Schilling04">{{cite journal | last = Schilling | first = Steve P. | coauthors = Paul E. Carrara, Ren A. Thompson, and Eugene Y. Iwatsubo | title = Posteruption glacier development within the crater of Mount St. Helens, Washington, USA | journal = Quaternary Research | volume =61 | issue =3 | pages =325–329 | publisher = Elsevier Science (USA) | date = 2004 | url = }}</ref><ref>{{cite journal | last = McCandless | first = Melanie | coauthors = Plummer, Mitchell; Clark, Douglas | title = Predictions of the growth and steady-state form of the Mount St. Helens Crater Glacier using a 2-D glacier model | journal = Abstracts with Programs - Geological Society of America | volume = 37 | issue = 7 | pages = 354 | publisher = | date = 2005 | url = }}</ref><ref>{{cite web | last = Schilling | first = Steve P. | coauthors = David W. Ramsey, James A. Messerich, and Ren A. Thompson | title = USGS Scientific Investigations Map 2928: Rebuilding Mount St. Helens | url= http://pubs.usgs.gov/sim/2006/2928/ | date = [[2006-08-08]] | accessdate = 2007-03-07 }}</ref>

With the recent volcanic activity starting in 2004, the glacier lobes were pushed aside and upward by the growth of new volcanic domes. The surface of the glacier, once mostly uncrevassed, turned into a chaotic jumble of [[icefall]]s heavily criss-crossed with [[crevasse]]s and [[serac]]s caused by movement of the crater floor. The appearance of the glacier changes rapidly from constant uplift of the crater and heavy snowfall. The new domes have almost separated the Crater Glacier into an eastern and western lobe. Despite the ongoing 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. Since 2004, new glaciers have formed on the crater wall above Crater Glacier feeding rock and ice onto its surface below. In addition, there are two rock glaciers to the north of the eastern lobe of Crater Glacier. Several patches of permanent snow have been observed on the volcano. These may evolve into glaciers in the future.

==Human history==
===Importance to Native Americans===
Traces of ancient campsites have been found in the [[Gifford Pinchot National Forest]], which surrounds the monument. Dating of these sites reveals that people have lived in this area for at least 6,500 years.<ref name="USGSNA">[http://volcano.und.edu/vwdocs/msh/p/pb/pbna.html Native Americans], USGS (accessed 12 Nov 2006)</ref> Throughout human history, Mount St. Helens eruptions have had a dramatic effect on the lives of local inhabitants. Work by archaeologists has shown that a massive eruption 3,500 years ago buried native settlements with a thick layer of [[pumice]]. As a result, people abandoned the area for nearly 2,000 years.<ref name="USGSNA"/> Later, members of the [[Cowlitz (tribe)|Cowlitz]], [[Taidnapam]], [[Klickitat tribe|Klickitat]], [[Chinookan|Upper Chinook]], and [[Yakama]] tribes moved seasonally over the land, harvesting [[huckleberry|huckleberries]] and hunting [[salmon]], [[elk]], and [[deer]].<ref name="USGSNA"/>

[[Image:St Helens before 1980 eruption.jpg|thumb|left|Indigenous American legends were inspired by the volcano's beauty.]]

====Local legends====

[[Indigenous peoples of the Americas|American Indian]] lore contains numerous legends to explain the eruptions of Mount St. Helens and other Cascade volcanoes. The most famous of these is the [[Bridge of the Gods (geologic event)|Bridge of the Gods]] legend told by the Klickitats. In their tale, the chief of all the gods, [[Tyhee Saghalie]] and his two sons, Pahto (also called Klickitat) and Wy'east, traveled down the [[Columbia River]] from the Far North in search for a suitable area to settle.<ref name="CountyRoads">Archie Satterfield, ''Country Roads of Washington'' (Backinprint.com: 2003) ISBN 0-595-26863-3, page 82</ref>

They came upon an area that is now called [[The Dalles]] and thought they had never seen a land so beautiful. The sons quarreled over the land, so to solve the dispute their father shot two arrows from his mighty bow&mdash;one to the north and the other to the south. Pahto followed the arrow to the north and settled there while Wy'east did the same for the arrow to the south. Saghalie then built [[Tanmahawis]], the Bridge of the Gods, so his family could meet periodically.<ref name="CountyRoads"/>

When the two sons of the Saghalie fell in love with a beautiful maiden named Loowit, she could not choose between them. The two young chiefs fought over her, burying villages and forests in the process. The area was devastated and the earth shook so violently that the huge bridge fell into the river, creating the [[cascade]]s of the [[Columbia River Gorge]].<ref>[http://www.theoutlaws.com/indians4.htm The Bridge of the Gods], theoutlaws.com (accessed 26 November 2006)</ref>

For punishment, Saghalie struck down each of the lovers and transformed them into great mountains where they fell. Wy'east, with his head lifted in pride, became the volcano known today as Mount Hood. Pahto, with his head bent toward his fallen love, was turned into Mount Adams. The fair Loowit became Mount St. Helens, known to the Klickitats as Louwala-Clough, which means "smoking or fire mountain" in their language (the [[Sahaptin people|Sahaptin]] called the mountain Loowit).<ref name=crvn>{{cite web|title=Volcanoes and History: Cascade Range Volcano Names|url=http://vulcan.wr.usgs.gov/LivingWith/Historical/volcano_names.html|author=USGS|accessdate=2006-10-20}}</ref>

===Exploration by Europeans===
[[Royal Navy]] Commander [[George Vancouver]] and the officers of [[HMS Discovery (1789)|HMS ''Discovery'']] made the Europeans' first recorded sighting of Mount St. Helens on [[May 19]], [[1792]], while surveying the northern [[Pacific Ocean]] coast. Vancouver named the mountain for British diplomat [[Alleyne Fitzherbert, 1st Baron St Helens]] on [[October 20]], [[1792]],<ref name="crvn"/> as it came into view when the ''Discovery'' passed into the mouth of the Columbia River.

Years later, explorers, traders, and missionaries heard reports of an erupting volcano in the area. Geologists and historians determined much later that the eruption took place in 1800, marking the beginning of the 57-year-long Goat Rocks Eruptive Period (see [[#Geologic history|geology section]]).<ref name="FireMountains217">Harris, ''Fire Mountains of the West'', 1st edition, page 217</ref> Alarmed by the "dry snow," the Nespelem tribe of northeastern Washington danced and prayed rather than collecting food and suffered during that winter from starvation.<ref name="FireMountains217"/>

In late 1805 and early 1806, members of the [[Lewis and Clark Expedition]] spotted Mount St. Helens from the Columbia River but did not report either an ongoing eruption or recent evidence of one.<ref>Pringle, ''Roadside Geology of Mount St. Helens National Volcanic Monument and Vicinity''</ref> They did however report the presence of [[quicksand]] and clogged channel conditions at the mouth of the [[Sandy River (Oregon)|Sandy River]] near Portland, suggesting an eruption by [[Mount Hood]] sometime in the previous decades.

===European settlement and use of the area===

[[Image:Fur trapper in Mount St Helen area.jpg|thumb|left|19th century photo of a [[fur trapper]] working in the Mount St. Helens area.]]

The area's first non-aboriginal inhabitants were European [[fur trade]]rs and trappers. Most of these men worked for the fur trading enterprise of the British-owned [[Hudson's Bay Company]].<ref name="ExplorersSettlers">[http://volcano.und.edu/vwdocs/msh/p/pb/pbes.html Explorers and Settlers], USGS.gov (accessed 12 Nov 2006)</ref> In the early 1890s, Ole' Peterson set up housekeeping at Cougar Flats, on the Upper Lewis River.<ref name="ExplorersSettlers"/> He was a true [[hermit]]—preferring to keep to himself, and enjoying the quiet solitude of nature.

Also in the early 1890s, a 156-square mile (404&nbsp;km²) mining district north of [[Spirit Lake (Washington)|Spirit Lake]] was established. By 1911, over 400 mining claims had been filed.<ref name="ExplorersSettlers"/> However, the minerals were never found in profitable quantities, and though much effort was spent in attempting to build a road or railroad into the district, by 1911, it was clear that there were no veins of precious minerals rich enough to offset the high transportation costs.<ref name="ExplorersSettlers"/>

[[Image:James Dwight Dana.jpg|thumb|[[James Dwight Dana]] was among the first geologists to view the volcano.]]

The first authenticated eyewitness report of a volcanic eruption was made in March 1835 by Dr. [[Meredith Gairdner]], while working for the Hudson's Bay Company stationed at [[Fort Vancouver]].<ref name="FireMountains219">Harris, ''Fire Mountains of the West'', 1st edition, page 219</ref> He sent an account to the ''Edinburgh New Philosophical Journal'', which published his letter in January 1836. [[James Dwight Dana]] of [[Yale University]], while sailing with the [[United States Exploring Expedition]], saw the quiescent peak 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.<ref name="USGS-LewisClark">[http://vulcan.wr.usgs.gov/LivingWith/Historical/LewisClark/Info/summary_mount_st_helens.html The Volcanoes of Lewis and Clark], USGS.gov (accessed 15 Nov 2006)</ref>

In late fall or early winter of 1842, nearby settlers and missionaries witnessed the so-called "Great Eruption." This small-volume outburst created large ash clouds, and mild explosions followed for 15 years.<ref>Harris, ''Fire Mountains of the West'', 1st edition, pages 220–221</ref> The eruptions of this period were likely [[phreatic eruption|phreatic]] ([[steam]] explosions). The Reverend [[Josiah L. Parrish|Josiah Parrish]] in [[Champoeg, Oregon]], witnessed Mount St. Helens in eruption on [[November 22]], [[1842]]. Ash from this eruption may have reached [[The Dalles, Oregon]], 48 miles (80&nbsp;km) southeast of the volcano.<ref name="USGS-Description"/>

[[Image:Mount St Helens erupting at night by Paul Kane.jpg|left|thumb|Mount St. Helens erupting at night. Painting by [[Paul Kane]] after his 1847 visit to the area.]]

In October 1843, future [[California]] governor [[Peter Hardeman Burnett|Peter H. Burnett]] recounted a story of an aboriginal American man who badly burned his foot and leg in lava or hot ash while hunting for deer. The likely apocryphal story went that the injured man sought treatment at Fort Vancouver, but the contemporary fort commissary steward, Napolean McGilvery, disclaimed knowledge of the incident.<ref>Harris, ''Fire Mountains of the West'', 1st edition, page 224</ref> British lieutenant Henry J. Warre sketched the eruption in 1845, and two years later [[Canada|Canadian]] painter [[Paul Kane]] created watercolors of the gently smoking mountain. Warre's work showed erupting material from a vent about a third of the way down from the summit on the mountain's west or northwest side (possibly at Goat Rocks), and one of Kane's field sketches shows smoke emanating from about the same location.<ref>Harris, ''Fire Mountains of the West'', 1st edition, page 225, 227</ref>

On [[April 17]], [[1857]], the ''Republican,'' a [[Steilacoom, Washington]] newspaper, reported that "Mount St. Helens, or some other mount to the southward, is seen ... to be in a state of eruption".<ref name="FireMountains228">Harris, ''Fire Mountains of the West'', 1st edition, page 228</ref> The lack of a significant ash layer associated with this event indicates that it was a small eruption. This was the first reported volcanic activity since 1854.<ref name="FireMountains228"/>

[[Image:Boating in Spirit Lake-pre 1980.jpg|thumb|Boats in [[Spirit Lake (Washington)|Spirit Lake]] sometime before the 1980 eruption.]]

Before the 1980 eruption, [[Spirit Lake (Washington)|Spirit Lake]] offered year-round recreational activities. In the summer there was [[boating]], [[swimming]], and [[camping]], while in the winter there was [[skiing]].

===Human impact from the 1980 eruption===

[[Image:1980 Mount st helens ash distribution.svg|left|thumb|Ash from the eruption disrupted life for millions of people. Fifty-seven people died, and the economic cost was [[US$]]1 billion.]]

St. Helens catastrophically erupted on [[May 18]], [[1980]]. After many months of lead-up activity, including the growth of a huge bulge on the north part of the mountain, a moderate earthquake caused the entire north flank of the mountain to slide away in the largest landslide in recorded history.<ref name="USGSFrom1980">[http://pubs.usgs.gov/fs/2000/fs036-00/ Mount St. Helens – From the 1980 Eruption to 2000], USGS Fact Sheet 036-00 (accessed 12 Nov 2006)</ref> The newly-exposed hot and pressurized rock in the volcano responded by producing the largest historic volcanic eruption in the 48 contiguous U.S. states.<ref name="USGSFrom1980"/> (See the [[#Modern eruptive period|Geology section]] for more detail.)

During the lead-up to the [[1980 eruption of Mount St. Helens]], 83-year-old [[Harry Truman (volcano victim)|Harry R. Truman]], who had lived near the mountain for 54 years, became famous when he decided not to evacuate before the impending eruption, despite repeated pleas by local authorities. His body was never found after the eruption. Fifty-seven people were killed or never found. Had the eruption occurred one day later, when loggers would have been at work, rather than on a Sunday, the death toll would almost certainly have been much higher.<ref name="Tilling1990">Tilling et al., [http://vulcan.wr.usgs.gov/Volcanoes/MSH/Publications/MSHPPF/MSH_past_present_future.html Eruptions of Mount St. Helens: Past, Present, and Future], USGS Special Interest Publication, 1990 (accessed 12 Nov 2006)</ref>

[[Image:MSH80 david johnston at camp 05-17-80 med.jpg|thumb|[[David A. Johnston]] hours before he was killed by the eruption.]]

Among the victims of the 1980 eruption was 30-year-old [[volcanologist]] [[David A. Johnston]], who was stationed on the nearby Coldwater Ridge. Moments before his position was hit by the hot ash cloud, Johnston uttered his famous last words: "Vancouver! Vancouver! This is it!"<ref>Scott LaFee. [http://www.signonsandiego.com/news/science/20031203-9999_mz1c3deadsci.html "Perish the thought: A life in science sometimes becomes a death, too."] SignOnSanDiego.com: December 3, 2003. Retrieved October 26, 2006.</ref> Johnston's body was never found.

[[Image:Mtsthelens 7242007.jpg|thumb|left|Mount St. Helens viewed from a regional jet, 7-24-2007]][[President of the United States|U.S. President]] [[Jimmy Carter]] surveyed the damage and said "Someone said this area looked like a moonscape. But the moon looks more like a golf course compared to what's up there."<ref>[http://murray.senate.gov/news.cfm?id=237728 Mount St. Helens: Senator Murray Speaks on the 25th Anniversary of the May 18, 1980 Eruption], Senate.gov (accessed 12 Nov 2006)</ref> A film crew, led by Seattle filmmaker Otto Seiber, was dropped by [[helicopter]] on St. Helens on [[May 23]] to document the destruction. Their [[compass]]es, however, spun in circles and they quickly became lost. A second eruption occurred on [[May 25]], but the crew survived and was rescued two days later by [[United States National Guard|National Guard]] helicopter pilots. Their film, ''The Eruption of Mount St. Helens'', later became a popular documentary.

===Protection and later history===

[[Image:StHelensDecember2004.jpg|thumb|left|A steam plume rises from the mountain in December 2004.]]

In 1982, President [[Ronald Reagan]] and the [[Congress of the United States|U.S. Congress]] established the [[Mount St. Helens National Volcanic Monument]], a 110,000 acre (445 km²) area around the mountain and within the [[Gifford Pinchot National Forest]].<ref>[http://www.fs.fed.us/gpnf/04mshnvm/general/index.shtml Mount St. Helens National Volcanic Monument: General Visitor Information], USDA Forest Service (accessed 12 Nov 2006)</ref>

Following the 1980 eruption, the area was left to gradually return to its natural state. In 1987, the [[National Forest Service]] reopened the mountain to climbing. It remained open until 2004 when [[2004 and later volcanic activity of Mount St. Helens|renewed activity]] caused the closure of the area around the mountain (see [[#2004 to present activity|Geology section]] for more detail).

Most notable was the closure of the Monitor Ridge trail, which previously let up to 100 permitted hikers per day climb to the summit. However, on [[July 21]], [[2006]], the mountain was again opened to climbers.<ref>[http://www.fs.fed.us/gpnf/recreation/current-conditions/special.shtml Climbing Mount St. Helens], USDA Forest Service (accessed 12 Nov 2006)</ref>
<br clear="all"/>

==Geologic history==
===Ancestral stages of eruptive activity===

[[Image:Cascade Range-related plate tectonics.svg|thumb|300px|[[Plate tectonics]] of the [[Cascade Range]].]]

The early eruptive stages of Mount St. Helens are known as the "Ape Canyon Stage" (around 40&ndash;35,000 years ago), the "Cougar Stage" (ca. 20&ndash;18,000 years ago), and the "Swift Creek Stage" (roughly 13&ndash;8,000 years ago).<ref name="USDAHistory">{{cite web|publisher=USDA Forest Service |url=http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/volcanic-history-summary.shtml |title=Mount St. Helens - Summary of Volcanic History}} (accessed 26 November 2006)</ref> The modern period, since about 2500 BCE, 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 (ranging from [[olivine]] [[basalt]] to andesite and dacite).<ref name="FireMountains214">Harris, ''Fire Mountains of the West'', 1st edition, page 214</ref>

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.<ref name="FireMountains214"/> 36,000 years ago a large [[lahar|mudflow]] cascaded down the volcano;<ref name="FireMountains214"/> 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 [[glacier]]s 14,000 to 18,000 years ago during the last glacial period of the current [[ice age]].<ref name="FireMountains214"/>

The second eruptive period, the Cougar Stage, started 20,000 years ago and lasted for 2,000 years.<ref name="FireMountains214"/> [[Pyroclastic flow]]s of hot pumice and ash along with [[Lava dome|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 BCE 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 BCE was the largest known eruption from St. Helens during the [[Holocene]] epoch, judged by the volume of one of the tephra layers from that period. This eruptive period lasted until about 1600 BCE and left 18-in (46-cm) deep deposits of material 50 miles (80&nbsp;km) distant in what is now [[Mt. Rainier National Park]]. Trace deposits have been found as far northeast as [[Banff National Park]] in [[Alberta]], and as far southeast as eastern [[Oregon]].<ref name="FireMountains215">Harris, ''Fire Mountains of the West'', 1st edition, page 215</ref> All told there may have been up to 2.5 cubic miles (10 km³) of material ejected in this cycle.<ref name="FireMountains215"/> Some 400 years of dormancy followed.

St. Helens came alive again around 1200 BCE—the Pine Creek eruptive period.<ref name="FireMountains215"/> This lasted until about 800 BCE and was characterized by smaller-volume eruptions. Numerous dense, nearly red hot pyroclastic flows sped down St. Helens' flanks and came to rest in nearby valleys. A large mudflow partly filled 40 miles (65&nbsp;km) of the Lewis River valley sometime between 1000 BCE and 500 BCE.

===Castle Creek and Sugar Bowl eruptive periods===

The next eruptive period, the Castle Creek period, began about 400 BCE, and is characterized by a change in composition of St. Helens' lava, with the addition of [[olivine]] and [[basalt]].<ref name="FireMountains216">Harris, ''Fire Mountains of the West'', 1st edition, page 216</ref> The pre-1980 summit cone started to form during the Castle Creek period. Significant lava flows in addition to the previously much more common fragmented and pulverized lavas and rocks ([[tephra]]) distinguished this period. Large lava flows of andesite and basalt covered parts of the mountain, including one around the year 100 CE that traveled all the way into the Lewis and Kalama river valleys.<ref name="FireMountains216"/> Others, such as Cave Basalt (known for its system of [[lava tube]]s), flowed up to 9 miles (15&nbsp;km) from their vents.<ref name="FireMountains216"/> During the first century, mudflows moved 30 miles (50&nbsp;km) down the Toutle and Kalama river valleys and may have reached the [[Columbia River]]. Another 400 years of [[dormancy]] ensued.

The Sugar Bowl eruptive period was short and markedly different from other periods in Mount St. Helens history. It produced the only unequivocal laterally directed blast known from Mount St. Helens before the 1980 eruptions.<ref name="USGS-EruptiveHistory"/> During Sugar Bowl time, the volcano first erupted quietly to produce a dome, then erupted violently at least twice producing a small volume of tephra, directed-blast deposits, pyroclastic flows, and lahars.<ref name="USGS-EruptiveHistory"/>

===Kalama and Goat Rocks eruptive periods===

[[Image:1890 Clohessy and Strengele engraving of Mount St Helens.jpg|thumb|220px|left|The symmetrical appearance of St. Helens prior to the 1980 eruption earned it the nickname "[[Mount Fuji]] of America". The once-familiar shape was formed out of the Kalama and Goat Rocks eruptive periods.]]

Roughly 700 years of dormancy were broken about 1480, when large amounts of pale gray dacite pumice and ash started to erupt, beginning the Kalama period. The eruption in 1480 was several times larger than the [[May 18]], [[1980]] eruption.<ref name="USGS-EruptiveHistory">[http://vulcan.wr.usgs.gov/Volcanoes/MSH/Publications/PP1563/eruptive_history.html Mount St. Helens Eruptive History], USGS.gov (accessed 15 Nov 2006)</ref> In 1482, another large eruption rivaling the 1980 eruption in volume is known to have occurred.<ref name="USGS-EruptiveHistory"/> Ash and pumice piled six miles (9.5&nbsp;km) northeast of the volcano to a thickness of three feet (1 m); 50 miles (80&nbsp;km) away, the ash was two inches (5 cm) deep. Large pyroclastic flows and mudflows subsequently rushed down St. Helens' west flanks and into the Kalama River drainage system.

This 150-year period next saw the eruption of less [[silica]]-rich lava in the form of [[andesitic]] ash that formed at least eight alternating light- and dark-colored layers.<ref name="FireMountains216"/> Blocky andesite lava then flowed from St. Helens' summit crater down the volcano's southeast flank.<ref name="FireMountains216"/> Later, pyroclastic flows raced down over the andesite lava and into the Kalama River valley. It ended with the emplacement of a dacite dome several hundred feet (~100 m) high at the volcano's summit, which filled and overtopped an explosion crater already at the summit.<ref name="FireMountains217">Harris, ''Fire Mountains of the West'', 1st edition, page 217</ref> Large parts of the dome's sides broke away and mantled parts of the volcano's cone with [[Scree|talus]]. Lateral explosions excavated a notch in the southeast crater wall. St. Helens reached its greatest height and achieved its highly symmetrical form by the time the Kalama eruptive cycle ended, about 1647.<ref name="FireMountains217"/> 150 years of quiet returned to the volcano.

The 57-year eruptive period that started in 1800 was named after the Goat Rocks dome, and is the first time that both oral and written records exist.<ref name="FireMountains217"/> Like the Kalama period, the Goat Rocks period started with an explosion of [[dacite]] [[tephra]], followed by an andesite lava flow, and culminated with the emplacement of a dacite dome. The 1800 eruption probably rivalled the 1980 eruption in size, although it did not result in massive destruction of the cone. The ash drifted northeast over central and eastern [[Washington]], northern [[Idaho]], and western [[Montana]]. There were at least a dozen reported small eruptions of ash from 1831 to 1857, including a fairly large one in 1842. The vent was apparently at or near Goat Rocks on the northeast flank.<ref name="FireMountains217"/> Goat Rocks dome was the site of the bulge in the 1980 eruption, and it was obliterated in the major eruption event on [[May 18]] [[1980]] that destroyed the entire north face and top 1,300 feet (400 m) of the mountain.

=== Modern eruptive period===
====1980 to 2001 activity====

{{main|1980 eruption of Mount St. Helens}}

[[Image:MSH80 eruption mount st helens 05-18-80.jpg|thumb|150px|left|Mount St. Helens erupted on [[May 18]], [[1980]], at 08:32 a.m. Pacific Daylight Time.]]

On [[March 20]] [[1980]], Mount St. Helens experienced a [[Richter magnitude scale|magnitude]] 4.2 [[earthquake]].<ref name="USDA" /> Steam venting started on [[March 27]].<ref name="USDAMay18">{{cite web|publisher=USDA Forest Service |url=http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/pre-eruption-0322.shtml |title=Summary of Events Leading Up to the May 18, 1980 Eruption of Mount St. Helens: March 22–28.|accessdate=2006-10-28}}</ref> By the end of April, the north side of the mountain started to bulge.<ref name="USDAMay18_2">{{cite web|publisher=USDA Forest Service |url=http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/pre-eruption-0426.shtml |title=Summary of Events Leading Up to the May 18, 1980 Eruption of Mount St. Helens: April 26–May 2.|accessdate=2006-10-28}}</ref>
With little warning, a second earthquake of magnitude 5.1 [[May 18]] triggered a massive collapse of the north face of the mountain. It was the largest known [[debris avalanche]] in recorded history. The [[magma]] inside of St. Helens burst forth into a large-scale [[pyroclastic flow]] that flattened vegetation and buildings over 230 square miles (600 km²). On the [[Volcanic Explosivity Index]] scale, the eruption was rated a five (a [[Plinian eruption]]).

The collapse of the northern flank of St. Helens mixed with ice, snow, and water to create [[lahar]]s (volcanic mudflows). The lahars flowed many miles down the Toutle and Cowlitz Rivers, destroying bridges and lumber camps. A total of 3.9 million [[cubic yard]]s (3.0 million [[m³]]) of material was transported 17 miles (27&nbsp;km) south into the [[Columbia River]] by the mudflows.<ref>Harris, ''Fire Mountains of the West'', 1st edition, page 209</ref>

For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 16 miles (20 to 27&nbsp;km) above sea level.<ref>Kiver and Harris, ''Geology of U.S. Parklands'', 6th edition, page 149</ref> The plume moved eastward at an average speed of 60 miles per hour (95&nbsp;km/h), with ash reaching [[Idaho]] by noon.

By about 5:30 p.m. on [[May 18]], the vertical ash column declined in stature, and less severe outbursts continued through the night and for the next several days. The St. Helens May 18 eruption released 24 megatons of thermal energy;<ref>[http://pubs.usgs.gov/fs/2000/fs036-00/ Mount St. Helens – From the 1980 Eruption to 2000, Fact Sheet 036-00] usgs.gov</ref><ref>[http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/volcanic-eruption-summary.shtml Mount St. Helens National Volcanic Monument] fs.fed.us, '24 megatons thermal energy'</ref><!-- unable to verify: 350 MT<ref>Harris, ''Fire Mountains of the West'', 1st edition, page 211</ref> or seven times more than the strongest atomic bomb ever built and tested, the [[Tsar Bomba]]. --> it ejected more than 0.67 cubic miles (2.8 cubic km) of material.<ref name="USGSFrom1980"/> The removal of the north side of the mountain reduced St. Helens' height by about 1,300 feet (400 m) and left a crater one to two miles (1.6 to 3.2&nbsp;km) wide and 0.5 mile (800 m) deep, with its north end open in a huge breach. The eruption killed 57 people, nearly 7,000 big game animals ([[deer]], [[elk]], and [[bear]]), and an estimated 12 million fish from a hatchery.<ref name="Tilling1990"/> It destroyed or extensively damaged over 200 homes, 185 miles (300&nbsp;km) of [[highway]] and {{convert|15|mi|km|0}} of [[railway]]s.<ref name="Tilling1990"/>

[[Image:Mt st helens dome growth schematic 80-86.svg|thumb|240px|[[Lava dome]] growth profile from 1980–1986.]]

Between 1980 and 1986, activity continued at Mount St. Helens, with a new [[lava dome]] forming in the crater. Numerous small explosions and dome-building eruptions occurred. From [[December 7]], [[1989]] to [[January 6]], [[1990]], and from [[November 5]], [[1990]] to [[February 14]], [[1991]], the mountain erupted with sometimes huge clouds of ash.<ref>[http://vulcan.wr.usgs.gov/Volcanoes/MSH/Publications/EV23/small_expl.html Bobbie Myers, 1992, Small Explosions Interrupt 3-year Quiescence at Mount St. Helens, Washington: IN: Earthquakes and Volcanoes, v.23, n.2, p.58–73] (accessed 26 November 2006)</ref>

====2004 to present activity====

{{main|2004 and later volcanic activity of Mount St. Helens}}

[[Image:Whaleback 2-22-05.jpg|thumb|left|Appearance of the "Whaleback" in February 2005.]]

Magma reached the surface of the volcano about [[October 11]], [[2004]], resulting in the building of a new lava dome on the existing dome's south side. This new dome continued to grow throughout 2005 and into 2006. Several transient features were observed, such as the "whaleback," which comprised long shafts of solidified magma being extruded by the pressure of magma beneath. These features are fragile and break down soon after they are formed. On [[July 2]], [[2005]], the tip of the whaleback broke off, causing a rockfall that sent ash and dust several hundred meters into the air. <ref>see [http://vulcan.wr.usgs.gov/Imgs/Jpg/MSH/MSH05/MSH05_dome_from_sugarbowl_tip_spine_collapse_July2005_med.jpg USGS before and after images]</ref>

Mount St. Helens showed significant activity on [[March 8]], [[2005]], when a 36,000-foot (11,000 m) plume of steam and ash emerged—visible from [[Seattle]].<ref>[http://vulcan.wr.usgs.gov/Volcanoes/MSH/Eruption04/Monitoring/plume_in_the_evening_8march05.html Mount St. Helens, Washington, "Plume in the Evening", [[March 8]] [[2005]]], USGS.gov (accessed 15 Nov 2006)</ref> This relatively minor eruption was a release of pressure consistent with ongoing dome building. The release was accompanied by a magnitude 2.5 earthquake.

Another feature to emerge from the dome is called the "fin" or "slab." Approximately half the size of a football field, the large, cooled volcanic rock was being forced upward as quickly as 6 feet (2 m) per day.<ref name=kgw>{{cite web|title=New slab growing in Mount St. Helens dome|url=http://www.kgw.com/news-local/stories/kgw_050306_env_mt_st_helens_slab.7608185.html|author=Northwest NewsChannel8|accessdate=2006-10-20}}</ref><ref>See [http://antwrp.gsfc.nasa.gov/apod/ap060509.html close-up of the slab.]</ref> In mid-June 2006, the slab was crumbling in frequent rockfalls, although it was still being extruded. The height of the dome was 7,550 feet (2,301 m), still below the height reached in July 2005 when the whaleback collapsed.

On [[October 22]] [[2006]], at 3:13 p.m. PST, a magnitude 3.5 earthquake broke loose Spine 7. The collapse and avalanche of the lava dome sent an ash plume 2,000 feet (610 m) over the western rim of the crater; the ash plume then rapidly dissipated.

[[Image:mtsthelensstirring.jpg|thumb|right|The volcano continuously steams as seen here in late 2007.]]

On [[December 19]] [[2006]], a large white plume of condensing steam was observed, leading some media people to assume there had been a small eruption. However, the Cascades Volcano Observatory of the USGS does not mention any significant ash plume.<ref>[http://vulcan.wr.usgs.gov/ Cascades Volcano Observatory, vulcan.wr.usgs.gov (accessed 4 January 2007)]</ref> The volcano has been in continuous eruption since October 2004, but this eruption has in large part consisted of a gradual extrusion of lava forming a dome in the crater.

On [[January 16]], [[2008]], steam began seeping from a fracture atop the lava dome. Associated seismic activity is the most noteworthy since 2004. Scientists have suspended activities in the crater and the mountain flanks, but the risk of a major eruption is deemed low.<ref>[http://ap.google.com/article/ALeqM5hNI23udU54lmQNReM4lTq55hz3ygD8U7CJ2G0 Small Quake Report- ap.google.com (accessed 16 January 2008)]</ref>

==See also==
* [[Geology of the Pacific Northwest]]
* [[Cascade Volcanoes]]
* [[Silver Lake (Washington)]]

== Notes ==
{{reflist|2}}

==References==
[[Image:Mt st helens Johnston ridge 25 years later.jpg|thumb|View of the hillside at the Johnston Ridge Observatory, 25 years after the eruption.]]
*{{cite book|last=Harris|first=Stephen L.|chapter=Mount St. Helens: A Living Fire Mountain| pages=pp. 201–228|title=Fire Mountains of the West: The Cascade and Mono Lake Volcanoes|year=1988|edition=1st edition|publisher=Mountain Press Publishing Company|location=Missoula, Montana|id=ISBN 0-87842-220-X}}
*Mullineaux, D.R.; Crandell, D.R. (1981). ''[http://vulcan.wr.usgs.gov/Volcanoes/MSH/Publications/PP1250/MullineauxCrandell/eruptive_history.html The Eruptive History of Mount St. Helens]'', USGS Professional Paper 1250. Retrieved on [[October 28]], [[2006]].
*Mullineaux, D.R. (1996). ''[http://vulcan.wr.usgs.gov/Volcanoes/MSH/Publications/PP1563/eruptive_history.html Pre-1980 Tephra-Fall Deposits Erupted From Mount St. Helens]'', USGS Professional Paper 1563. Retrieved on [[October 28]], [[2006]].
*Pringle (1993). ''Roadside Geology of Mount St. Helens National Volcanic Monument and Vicinity'', Washington State Department of Natural Resources, Division of Geology and Earth Resources Information; Circular 88.
* USGS/Cascades Volcano Observatory, Vancouver, Washington. [http://vulcan.wr.usgs.gov/Volcanoes/MSH/description_msh.html DESCRIPTION: Mount St. Helens Volcano, Washington.] Retrieved on [[October 28]], [[2006]].

==External links==
{{Commonscat|Mount St. Helens}}
*{{VNUM|1=1201-05-}}
*[http://www.spiritoftruth.org/mtsthelenswatch.htm Mount St. Helens Watch] links to webcams, live video feeds and news sites to monitor Mount St. Helens
*[http://vulcan.wr.usgs.gov/Volcanoes/MSH/ Mount St. Helens photographs and current conditions] from the [[United States Geological Survey]] website
*[http://www.fs.fed.us/gpnf/volcanocams/msh/ The VolcanoCam] image of Mount St. Helens automatically updated approximately every five minutes from the [[United States Forest Service|U.S. Forest Service]]. It is located at Johnston Ridge and is able to view the new dome, even at night when the glow of new magma may be visible via the camera's infrared capabilities.
*[http://vulcan.wr.usgs.gov/Volcanoes/MSH/EruptiveHistory/framework.html USGS: Mount St. Helens Eruptive History]
*[http://pubs.usgs.gov/fs/2000/fs036-00/ USGS: Mount St. Helens &mdash; From the 1980 Eruption to 2000]
*[http://vulcan.wr.usgs.gov/Volcanoes/MSH/Images/MSH04/ Most recent photos] (most aerial) from the [[United States Geological Survey]]
*[http://www.luscombe-carter.com/mount_st_helens/ Enhanced night time images of Mount St Helens]
*[http://www.fullscreen360.com/st-helens.htm Virtual Tour of Mt St Helens. Comparison of 2003 and 2006 crater]
*[http://content.lib.washington.edu/ University of Washington Libraries: Digital Collections]:
**[http://content.lib.washington.edu/mshchemweb/index.html Mount St. Helens Post-Eruption Chemistry Database] This collection contains photographs of Mount St. Helens, post-eruption, taken over the span of three years to provide a look at both the human and the scientific sides of studying the eruption of a volcano.
**[http://content.lib.washington.edu/mtsthelensweb/index.html Mount St. Helens Succession Collection] This collection consists of 235 photographs in a study of plant habitats following the May 18, 1980 eruption of Mount St. Helens.
*[http://freesound.iua.upf.edu/samplesViewSingle.php?id=21432 Audio recording of the May 18 1980 eruption] Recorded {{convert|140|mi|km|0}} southwest of the mountain. Believed to be the only audio recording of the eruption.

{{Cascade volcanoes}}
{{featured article}}

[[Category:Active volcanoes|St. Helens, Mount]]
[[Category:Cascade Range|Saint Helens, Mount]]
[[Category:Mountains of Washington|Saint Helens, Mount]]
[[Category:Stratovolcanoes|Saint Helens, Mount]]
[[Category:VEI-5 volcanoes|Saint Helens, Mount]]
[[Category:Volcanoes of Washington|Saint Helens, Mount]]
[[Category:Subduction volcanoes|Saint Helens, Mount]]
[[Category:Cascade Volcanoes|Saint Helens, Mount]]

[[af:Mount Saint Helens]]
[[ca:Saint Helens]]
[[cs:Hora Svaté Heleny]]
[[cy:Mynydd St. Helens]]
[[da:Mount St. Helens]]
[[de:Mount St. Helens]]
[[et:Saint Helens]]
[[es:Monte Saint Helens]]
[[eo:Saint Helens]]
[[fr:Mont Saint Helens]]
[[hi:माउण्ट सेण्ट हेलेन्स]]
[[hr:Mount Saint Helens]]
[[id:Gunung St. Helens]]
[[it:Monte Sant'Elena]]
[[he:הר סנט הלנס]]
[[ku:Çiyayê St. Helens]]
[[la:Mons Sanctae Helenae]]
[[lt:Šv. Elenos kalnas]]
[[nl:Mount Saint Helens]]
[[ja:セント・ヘレンズ山]]
[[no:Mount St. Helens]]
[[pl:Mount St. Helens]]
[[pt:Monte Santa Helena]]
[[ro:Mount St. Helens]]
[[ru:Сент-Хеленс (вулкан)]]
[[simple:Mount St. Helens]]
[[sk:Saint Helens]]
[[sl:Sveta Helena (ognjenik)]]
[[fi:St. Helens (vuori)]]
[[sv:Mount Saint Helens]]
[[ta:புனித எலன்சு மலை]]
[[vi:Núi St. Helens]]
[[tr:St. Helens Dağı]]
[[zh:圣海伦火山]]

Revision as of 12:41, 2 June 2008

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