Editing Mount St. Helens (section)

==Geology==
{{see also|Geology of the Pacific Northwest}}
[[File:Cascade Range plate tectonics-en.svg|thumb|alt=Map of the west coast of United States showing subduction zones in the ocean and location of Cascade Volcanoes.|[[Plate tectonics]] of the [[Cascade Range]]]]
Mount St.&nbsp;Helens is part of the [[Cascade Volcanic Arc|Cascades Volcanic Province]], an arc-shaped band extending from southwestern [[British Columbia]] to [[Northern California]], roughly parallel to the Pacific coastline.<ref name="cascade1">{{cite web|url=http://geomaps.wr.usgs.gov/parks/province/cascade1.html|title=Pacific – Cascades Volcanic Province|archive-url=https://web.archive.org/web/20060923012715/http://geomaps.wr.usgs.gov/parks/province/cascade1.html|archive-date=2006-09-23|access-date=2021-03-13}} {{PD-notice}}</ref> Beneath the Cascade Volcanic Province, a dense oceanic plate sinks beneath the [[North American Plate]]; a process known as [[subduction (geology)|subduction]] in geology. As the [[oceanic plate|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 fluid|supercritical]] water rises into the pliable [[mantle (geology)|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 [[fault (geology)|faults]] as well as by melting wall rocks. The addition of melted crust changes the [[geochemistry|geochemical]] composition. Some of the melt rises toward the Earth's surface to erupt, forming the [[Cascade Volcanic Arc]] above the subduction zone.<ref name="cascade2">{{cite web|url=http://geomaps.wr.usgs.gov/parks/province/cascade2.html|title=Pacific – Cascades Volcanic Province|archive-url=https://web.archive.org/web/20060924090635/http://geomaps.wr.usgs.gov/parks/province/cascade2.html|archive-date=2006-09-24}} {{PD-notice}}</ref>

The magma from the mantle has accumulated in two chambers below the volcano: one approximately {{convert|5|-|12|km|mi|0|order=flip}} below the surface, the other about {{convert|12|-|40|km|mi|0|order=flip}}.<ref name=chambers>{{cite journal|title=Deep magma chambers seen beneath Mount St. Helens|first=Eric|last=Hand|date=2015-11-04|journal=Science|doi=10.1126/science.aad7392|url=https://www.science.org/content/article/deep-magma-chambers-seen-beneath-mount-st-helens}}</ref> The lower chamber may be shared with [[Mount Adams (Washington)|Mount Adams]] and the [[Indian Heaven]] volcanic field.<ref>{{cite web|url=https://gizmodo.com/mount-saint-helens-may-share-magma-with-an-entire-field-1741775204|title=Mount Saint Helens May Share Magma with an Entire Field of Volcanoes|first=Mika|last=McKinnon|date=2015-11-10|website=Gizmodo}}</ref>

===Ancestral stages of eruptive activity===
The early eruptive stages of Mount St.&nbsp;Helens are known as the "Ape Canyon Stage" (around 40,000–35,000&nbsp;years ago), the "Cougar Stage" (ca. 20,000–18,000&nbsp;years ago), and the "Swift Creek Stage" (roughly 13,000–8,000 years ago).<ref name="USDAHistory">{{cite web|url=http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/volcanic-history-summary.shtml|publisher=USDA Forest Service |archive-url=https://web.archive.org/web/20081011012508/http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/volcanic-history-summary.shtml |archive-date=2008-10-11 |title=Mount St.&nbsp;Helens – Summary of Volcanic History}}</ref> The modern period, since about 2500&nbsp;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 (ranging from [[olivine]] [[basalt]] to andesite and dacite).<ref name=Harris1988/>{{rp|page=214}}

St.&nbsp;Helens started its growth in the [[Pleistocene]] 37,600&nbsp;years ago, during the Ape Canyon stage, with dacite and andesite eruptions of hot pumice and ash.<ref name=Harris1988/>{{rp|page=214}} Thirty-six thousand years ago a large [[lahar|mudflow]] cascaded down the volcano;<ref name=Harris1988/>{{rp|page=214}} mudflows were significant forces in all of St.&nbsp;Helens' eruptive cycles. The Ape Canyon eruptive period ended around 35,000&nbsp;years ago and was followed by 17,000&nbsp;years of relative quiet. Parts of this ancestral cone were fragmented and transported by [[glacier]]s 14,000–18,000&nbsp;years ago during the last glacial period of the [[Quaternary glaciation|current ice age]].<ref name=Harris1988/>{{rp|page=214}}

The second eruptive period, the Cougar Stage, started 20,000&nbsp;years ago and lasted for 2,000&nbsp;years.<ref name=Harris1988/>{{rp|page=214}} [[Pyroclastic flow]]s of hot pumice and ash along with [[Lava dome|dome]] growth occurred during this period. Another 5,000&nbsp;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&nbsp;years ago.

===Smith Creek and Pine Creek eruptive periods===
A dormancy of about 4,000&nbsp;years was broken around 2500&nbsp;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.<ref name=Harris1988/>{{rp|page=215}} An eruption in 1900&nbsp;BC was the largest known eruption from St.&nbsp;Helens during the [[Holocene]] epoch, depositing the [[Yn tephra]].<ref name=Harris1988/>{{rp|page=215}}<ref>{{cite journal |last1=Pallister |first1=John S. |last2=Clynne |first2=Michael A. |last3=Wright|first3=Heather M. |last4=Van Eaton |first4=Alexa R. |last5=Vallance |first5=James W. |last6=Sherrod |first6=David R. |last7=Kokelaar |first7=B. Peter |title=Field-Trip Guide to Mount St.&nbsp;Helens, Washington: An overview of the eruptive history and petrology, tephra deposits, 1980 proclastic density current deposits, and the crater / Eruptive history |journal=Scientific Investigations Report |page=11 |publisher=[[United States Department of the Interior]] |year=2017 |issn=2328-0328}}</ref> This eruptive period lasted until about 1600&nbsp;BC and left {{convert|18|in|cm}} deep deposits of material {{convert|50|mi|km|-1}} 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]].<ref name=Harris1988/>{{rp|page=215}} All told there may have been up to {{convert|2.5|mi3|km3}} of material ejected in this cycle.<ref name=Harris1988/>{{rp|page=215}} Some 400 years of dormancy followed.

St.&nbsp;Helens came alive again around 1200&nbsp;BC—the Pine Creek eruptive period.<ref name=Harris1988/>{{rp|page=215}} This lasted until about 800&nbsp;BC and was characterized by smaller-volume eruptions. Numerous dense, nearly red hot pyroclastic flows sped down St.&nbsp;Helens's flanks and came to rest in nearby valleys. A large mudflow partly filled {{convert|40|mi|km}} of the Lewis River valley sometime between 1000&nbsp;BC and 500&nbsp;BC.

===Castle Creek and Sugar Bowl eruptive periods===
[[File:East Dome Mount St. Helens.jpg|thumb|East Dome on the east flank of Mount St. Helens in 2013.]]
The next eruptive period, the Castle Creek period, began about 400&nbsp;BC, and is characterized by a change in the composition of St.&nbsp;Helens' lava, with the addition of [[olivine]] and [[basalt]].<ref name=Harris1988/>{{rp|page=216}} 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&nbsp;BC that traveled all the way into the Lewis and Kalama river valleys.<ref name=Harris1988/>{{rp|page=216}} Others, such as Cave Basalt (known for its system of [[lava tube]]s), flowed up to {{convert|9|mi|km}} from their vents.<ref name=Harris1988/>{{rp|page=216}} During the first century, mudflows moved {{convert|30|mi|km|-1}} down the Toutle and Kalama river valleys and may have reached the [[Columbia River]]. Another 400&nbsp;years of [[dormancy]] ensued.

The Sugar Bowl eruptive period was short and markedly different from other periods in Mount St.&nbsp;Helens history. It produced the only unequivocal laterally directed blast known from Mount St.&nbsp;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"/> East Dome, a small hypersthene-homblende dacite dome on the east slope of the volcano, was likely formed around the Sugar Bowl period.<ref>{{Cite web |date=2017-06-02 |title=The Eruptive History of Mount St. Helens |url=https://volcano.oregonstate.edu/eruptive-history-mount-st-helens |access-date=2023-03-02 |website=Volcano World |language=en}}</ref> Formation of East Dome was preceded by an explosive eruption.<ref>{{Cite web |title=Global Volcanism Program {{!}} Image GVP-02835 |url=https://volcano.si.edu/gallery/ShowImage.cfm?photo=GVP-02835 |access-date=2023-03-02 |website=volcano.si.edu |language=en}}</ref>

===Kalama and Goat Rocks eruptive periods===
[[File:1890 Clohessy and Strengele engraving of Mount St Helens (cropped).jpg|thumb|upright=1.15|alt=Painting of a rolling landscape with a conical mountain in background.|The symmetrical appearance of St.&nbsp;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&nbsp;years of dormancy were broken in about 1480, when large amounts of pale gray dacite pumice and ash started to erupt, beginning the Kalama period. The 1480 eruption was several times larger than that of May 18, 1980.<ref name="USGS-EruptiveHistory">{{cite report |url=https://pubs.er.usgs.gov/publication/pp1563 |title=Pre-1980 tephra-fall deposits erupted from Mount St. Helens, Washington |website=United States Geological Survey |id=Professional Paper 1563|first=Donal R.|last=Mullineaux|year=1996 |access-date=2006-11-15}}</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 {{convert|6|mi|km}} northeast of the volcano to a thickness of {{convert|3|ft|m|1}}; {{convert|50|mi|km|-1}} away, the ash was {{convert|2|in|cm|0}} deep. Large pyroclastic flows and mudflows subsequently rushed down St.&nbsp;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=Harris1988/>{{rp|page=216}} Blocky andesite lava then flowed from St.&nbsp;Helens' summit crater down the volcano's southeast flank.<ref name=Harris1988/>{{rp|page=216}} 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 (~200&nbsp;m) high at the volcano's summit, which filled and overtopped an explosion crater already at the summit.<ref name=Harris1988/>{{rp|page=217}} 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.&nbsp;Helens reached its greatest height and achieved its highly symmetrical form by the time the Kalama eruptive cycle ended, in about 1647.<ref name=Harris1988/>{{rp|page=217}} The volcano remained quiet for the next 150&nbsp;years.

The 57-year eruptive period that started in 1800 was named after the Goat Rocks dome and is the first period for which both oral and written records exist.<ref name=Harris1988/>{{rp|page=217}} As with 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 rivaled 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 (state)|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 1831 eruption is likely what tinted the sun bluish-green in [[Southampton County, Virginia]] on the afternoon of August 13—which [[Nat Turner]] interpreted as a final signal to launch [[Nat Turner's slave rebellion|the United States' largest slave rebellion]].<ref>{{cite book |last1=Breen |first1=Patrick H. |title=Nat Turner's revolt: rebellion and response in Southampton County, Virginia |date=2005 |url=https://esploro.libs.uga.edu/esploro/outputs/doctoral/Nat-Turners-revolt-rebellion-and-response-in-Southampton-County-Virginia/9949332806802959 |access-date=21 November 2021}}</ref>) The vent was apparently at or near Goat Rocks on the northeast flank.<ref name=Harris1988/>{{rp|page=217}} Goat Rocks dome was near 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 {{convert|1300|ft|m}} of the mountain.

===Modern eruptive period===
[[File:MSH-10B Mount St. Helens Erupts, Portland View, May 18, 1980 (22636191326).jpg|thumb|225px|Mount St. Helens erupting on [[1980 eruption of Mount St. Helens|May 18, 1980]] as viewed from [[Portland, Oregon]]. The [[Fremont Bridge (Portland, Oregon)|Fremont Bridge]] is visible in the bottom left corner.]]
[[File:MSH80 eruption mount st helens 05-18-80.jpg|thumb|225px|upright|alt=Ash cloud erupting from volcano|Mount St.&nbsp;Helens erupted on May 18, 1980, at 08:32 [[Pacific Daylight Time|PDT]].]]

====1980 to 2001 activity====
{{Main|1980 eruption of Mount St. Helens}}
On March 20, 1980, Mount St.&nbsp;Helens experienced a [[Richter magnitude scale|magnitude]]&nbsp;4.2 [[earthquake]],<ref name="USDA" /> and on March 27, steam venting started.<ref name="USDAMay18">{{cite web|url=http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/pre-eruption-0322.shtml|publisher=USDA Forest Service |archive-url=https://web.archive.org/web/20071113092736/http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/pre-eruption-0322.shtml |archive-date=2007-11-13 |title=Summary of Events Leading Up to the May 18, 1980, eruption of Mount St.&nbsp;Helens: March 22–28}}</ref> By the end of April, the north side of the mountain had started to bulge.<ref name="USDAMay18_2">{{cite web |url=http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/pre-eruption-0426.shtml |publisher=USDA Forest Service |archive-url=https://web.archive.org/web/20071113092741/http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/pre-eruption-0426.shtml |archive-date=2007-11-13 |title=Summary of Events Leading Up to the May 18, 1980 Eruption of Mount St.&nbsp;Helens: April 26–May 2}}</ref> 
On May 18, at 8:32 am, a second earthquake, of magnitude 5.1, triggered a massive collapse of the north face of the mountain. It was the largest known [[debris avalanche]] in recorded history. The [[magma]] in St.&nbsp;Helens burst forth into a large-scale [[pyroclastic flow]] that flattened vegetation and buildings over an area of {{convert|230|sqmi|km2|-1}}. More than 1.5&nbsp;million metric tons of [[sulfur dioxide]] were released into the atmosphere.<ref>{{cite web |url=http://volcanoes.usgs.gov/activity/methods/gas/msh1980-88.php |title=Emission of sulfur dioxide gas from Mount St.&nbsp;Helens, 1980–1988 |publisher=United States Geological Survey |date=2008-09-25 |access-date=2009-03-25 |archive-date=13 April 2009 |archive-url=https://web.archive.org/web/20090413190247/http://volcanoes.usgs.gov/activity/methods/gas/msh1980-88.php |url-status=dead }}</ref> On the [[Volcanic Explosivity Index]] scale, the eruption was rated a 5, and categorized as a [[Plinian eruption]].

The collapse of the northern flank of St.&nbsp;Helens mixed with ice, snow, and water to create [[lahar]]s (volcanic mudflows). The lahars flowed many miles down the [[Toutle River|Toutle]] and [[Cowlitz River]]s, destroying bridges and [[lumber camp]]s. A total of {{convert|3900000|yd3|m3|-5}} of material was transported {{convert|17|mi|km}} south into the [[Columbia River]] by the mudflows.<ref name=Harris1988/>{{rp|page=209}}

For more than nine hours, a vigorous plume of ash erupted, eventually reaching {{convert|12|to|16|mi|km}} above sea level.<ref>{{cite book|title=Geology of U.S. parklands|first1=Eugene P.|last1=Kiver|first2=David V.|last2=Harris|year=1999|edition=5th|page=149|isbn=978-0-471-33218-3|publisher=Wiley}}</ref> The plume moved eastward at an average speed of {{convert|60|mph|km/h|-1}} with ash reaching [[Idaho]] by noon. Ashes from the eruption were found on top of cars and roofs the next morning as far away as [[Edmonton]], Alberta, Canada.

By about 5:30&nbsp;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.&nbsp;Helens May&nbsp;18 eruption released 24&nbsp;megatons of thermal energy<ref name="USGSFrom1980"/><ref>{{cite web |url=http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/volcanic-eruption-summary.shtml |archive-url=https://web.archive.org/web/20090529034823/http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/volcanic-eruption-summary.shtml |archive-date=2009-05-29 |title=Mount St.&nbsp;Helens National Volcanic Monument |publisher=United States Forest Service |quote=24 megatons thermal energy}}</ref> and ejected more than {{convert|0.67|mi3|km3|2}} of material.<ref name="USGSFrom1980">{{cite web |url=http://pubs.usgs.gov/fs/2000/fs036-00/ |title=Mount St.&nbsp;Helens – From the 1980 eruption to 2000 |id=Fact Sheet 036-00 |publisher=United States Geological Survey |access-date=2006-11-12|year=2000}}</ref> The removal of the north side of the mountain reduced St.&nbsp;Helens' height by about {{convert|1300|ft|m|-2}} and left a crater {{convert|1.2 to 1.8|mi|km|0}} wide and {{convert|2,084|ft|m}} deep, with its north end open in a huge breach. The eruption killed 57&nbsp;people, nearly 7,000&nbsp;big-game animals ([[deer]], [[elk]], and [[bear]]), and an estimated 12&nbsp;million fish from a hatchery.<ref name="Tilling1990impact"/> It destroyed or extensively damaged more than 200&nbsp;homes, {{convert|185|mi|km}} of [[highway]], and {{convert|15|mi|km|0}} of [[railway]]s.<ref name="Tilling1990impact"/>

Between 1980 and 1986, activity continued at Mount St.&nbsp;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, sometimes huge clouds of ash.<ref>{{cite journal |url=http://vulcan.wr.usgs.gov/Volcanoes/MSH/Publications/EV23/small_expl.html |first=Bobbie |last=Myers |year=1992 |title=Small explosions interrupt 3&nbsp;year quiescence at Mount St.&nbsp;Helens, Washington |journal=Earthquakes and Volcanoes |volume=23 |issue=2 |pages=58–73 |bibcode=1992EVUSG..23...58M |access-date=2006-11-26 |via=vulcan.wr.usgs.gov |archive-url=https://web.archive.org/web/20061230141620/http://vulcan.wr.usgs.gov/Volcanoes/MSH/Publications/EV23/small_expl.html |archive-date=2006-12-30}}</ref>

====2004 to 2008 activity====
{{Main|2004–2008 volcanic activity of Mount St. Helens}}
[[File:Whaleback, Mount St Helens volcanic crater (February 22 2005).jpg|thumb|alt=Large fairly smooth rock structure inside a crater|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 a [[lava spine]] nicknamed the "whaleback", which comprised long shafts of solidified magma being extruded by the pressure of magma beneath. These features were fragile and broke down soon after they were 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>{{cite web |title=Before and after images |publisher=USGS |url=http://vulcan.wr.usgs.gov/Imgs/Jpg/MSH/MSH05/MSH05_dome_from_sugarbowl_tip_spine_collapse_July2005_med.jpg |archive-url=https://web.archive.org/web/20050903224652/http://vulcan.wr.usgs.gov/Imgs/Jpg/MSH/MSH05/MSH05_dome_from_sugarbowl_tip_spine_collapse_July2005_med.jpg |archive-date=2005-09-03}}</ref>

Mount St.&nbsp;Helens showed significant activity on March 8, 2005, when a {{convert|36000|ft|m|adj=on}} plume of steam and ash emerged—visible from [[Seattle]].<ref>{{cite web |archive-url=https://web.archive.org/web/20050311015440/http://vulcan.wr.usgs.gov/Volcanoes/MSH/Eruption04/Monitoring/plume_in_the_evening_8march05.html |url=http://vulcan.wr.usgs.gov/Volcanoes/MSH/Eruption04/Monitoring/plume_in_the_evening_8march05.html |archive-date=2005-03-11 |title=Mount St.&nbsp;Helens, Washington – 'Plume in the Evening' |date=March 8, 2005 |publisher=USGS |access-date=2006-11-15}}</ref> This relatively minor eruption was a release of pressure consistent with ongoing dome building. The release was accompanied by a magnitude&nbsp;2.5 earthquake.

Another feature to emerge from the dome was 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 {{convert|6|ft|m|0|abbr=on}} per day.<ref name=kgw>{{cite news |title=New slab growing in Mount St.&nbsp;Helens dome |url=http://www.foxnews.com/story/0,2933,194342,00.html |access-date=2010-12-06 |newspaper=Fox News |archive-url=https://web.archive.org/web/20121026095220/http://www.foxnews.com/story/0,2933,194342,00.html |archive-date=2012-10-26 }}</ref><ref>{{cite web |title=Rock Slab Growing at Mt. St.&nbsp;Helens Volcano |date=May 9, 2006 |url=http://apod.nasa.gov/apod/ap060509.html |website=Astronomy Picture of the Day}}</ref> In mid-June 2006, the slab was crumbling in frequent rockfalls, although it was still being extruded. The height of the dome was {{convert|7550|ft|m}}, still below the height reached in July 2005 when the whaleback collapsed.

On October 22, 2006, at 3:13&nbsp;p.m. PST, a magnitude&nbsp;3.5 earthquake broke loose Spine&nbsp;7. The collapse and avalanche of the lava dome sent an [[ash plume]] {{convert|2000|ft|m|-2}} over the western rim of the crater; the ash plume then rapidly dissipated.

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 did not mention any significant ash plume.<ref>{{cite web |url=http://vulcan.wr.usgs.gov/ |title=In the News |website=Cascades Volcano Observatory |archive-url=https://web.archive.org/web/20070107023839/http://vulcan.wr.usgs.gov/ |archive-date=2007-01-07 |access-date=2007-01-04}}</ref> The volcano was in continuous eruption from October&nbsp;2004, but this eruption consisted in large part of a gradual extrusion of lava forming a dome in the crater.

On January 16, 2008, steam began seeping from a fracture on top of the lava dome. Associated seismic activity was the most noteworthy since 2004. Scientists suspended activities in the crater and the mountain flanks, but the risk of a major eruption was deemed low.<ref>{{cite news |url=https://www.usatoday.com/tech/science/2008-01-17-3582451403_x.htm |title=Small quake reported at Mount St.&nbsp;Helens |newspaper=USA Today |access-date=2010-12-06 |date=January 17, 2008}}</ref> By the end of January, the eruption paused; no more lava was being extruded from the lava dome. On July 10, 2008, it was determined that the eruption had ended, after more than six months of no volcanic activity.<ref>{{cite web |url=http://vulcan.wr.usgs.gov/Volcanoes/MSH/Eruption04/framework.html |title=Mount St.&nbsp;Helens, Washington – Eruption 2004 to current |publisher=USGS |access-date=2008-10-06 |archive-url=https://web.archive.org/web/20081006111613/http://vulcan.wr.usgs.gov/Volcanoes/MSH/Eruption04/framework.html |archive-date=2008-10-06}}</ref>

===Future hazards===
[[File:Mount Saint Helens from the air, with Mount Adams in background 02.jpg|thumb|Aerial view with [[Mount Adams (Washington)|Mount Adams]] in background]]
Future eruptions of Mount St.&nbsp;Helens will likely be even larger than the 1980 eruption.<ref name=Harris2005/>{{rp|296}} A large [[lahar]] flow is likely on branches of the [[Toutle River]], possibly causing destruction in inhabited areas along the [[Interstate 5|I-5]] corridor.<ref>{{cite web | url=https://www.usgs.gov/volcanoes/mount-st-helens/volcanic-hazards-mount-st-helens | archive-url=https://web.archive.org/web/20210128124052/https://www.usgs.gov/volcanoes/mount-st-helens/volcanic-hazards-mount-st-helens | archive-date=28 January 2021 | title=Volcanic Hazards at Mount St. Helens }}</ref>

The volcano is considered "very high threat" by the [[United States Geological Survey]] and is closely monitored by the [[Cascades Volcano Observatory]].<ref>{{Cite web |date=2023-11-08 |title=Hazards {{!}} U.S. Geological Survey |url=https://www.usgs.gov/volcanoes/mount-st.-helens/science/hazards |access-date=2025-02-16 |website=www.usgs.gov |language=en}}</ref>