Editing Mount St. Helens (section)

===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>