Editing Mount St. Helens (section)

====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>