Mount St. Helens, famous for itsexplosive 1980 eruption, haslong been the most active volcano inthe Pacific Northwest. U.S. GeologicalSurvey (USGS) scientists havedocumented the volcano's 300,000-year geologic history, includingpowerful explosions of ash, outpouringsof lava, and huge landslides andvolcanic mudflows. Understandingthis history helps USGS scientistsevaluate current activity at MountSt. Helens so that timely warnings ofhazards can be issued to the public. |
On May 18, 1980, Mount St. Helens,Washington, exploded in a spectacular anddevastating eruption that brought the volcanoto the attention of the world. Few people realizedthat Mount St. Helens had long been themost active volcano in the Cascade Range ofthe Pacific Northwest. It has a rich and complex300,000-year history and has producedboth violent explosive eruptions of volcanicash and pumice and relatively quiet outpouringsof lava. The volcano's edifice was mostlybuilt by lava domes and flows from numerouseruptions. Using evidence in these lavasand other deposits, U.S. Geological Survey(USGS) scientists have documented dozens ofmajor individual eruptions of the volcano.
An extensive apron of ash and fragmentedvolcanic rocks surrounds Mount St. Helensand mostly fills the valleys draining its slopes.This material was transported by pyroclasticflows (searingly hot flows of ash and volcanicgases), lahars (volcanic mudflows), and debrisavalanches (landslides). Farther away fromthe volcano, pumice and ash that fell duringexplosive eruptions form layers that bury thelandscape to depths of 10 feet or more.
The pre-1980 eruptive history of MountSt. Helens is strongly episodic. Volcanologistshave recognized and named four episodesof volcanic activity, called "stages"--Ape Canyon, Cougar, Swift Creek, and SpiritLake--separated by dormant intervals. Theyoungest stage, Spirit Lake, is further subdividedinto six eruptive periods. Because thepreservation of deposits and other geologicevidence is best for the youngest stages, thefarther scientists look back in time the lessdetail they can infer for the history of volcanismat Mount St. Helens.
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The early history of Mount St. Helens ispoorly known, and a long timespan is coveredby the Ape Canyon Stage. During this stage,lava domes erupted west of the present edificeof the volcano in two distinct periods--onefrom 300 to 250 thousand years ago (ka) anda second from 125 to 35 ka. A number of ashlayers, called the "C" set, are clearly relatedto volcanism during the younger phase of theApe Canyon Stage, and a few ashes found incentral Washington older than the C set arealso from Mount St. Helens. Although someApe Canyon-age lava domes are exposed inthe area of Goat Mountain and Butte Camp,the best record of early Mount St. Helens volcanismis preserved in the Cougar-age debrisavalanche (see below) and in glacial depositsand lahars in the Lewis River Valley.
Many Ape Canyon-age rocks were alteredhydrothermally (by volcanically heatedground water), indicating that an extensivehydrothermal system existed during the latterpart of the stage. Volcanism during theApe Canyon Stage produced a small clusterof lava domes with maximum elevations ofabout 4,000 feet. |
Smith Creek Eruptive Period (3.9 to 3.3 ka)--During this period, Mount St. Helenserupted mostly ash. Two periods of activity,about 3.90 to 3.85 ka and 3.5 to 3.3 ka, depositedset "Y" ashes. The second period wasinitiated with an eruption that produced "Yn"ash. This eruption, possibly the most voluminousin Mount St. Helens' history, was aboutfour times larger than the 1980 eruption. Duringlate Smith Creek time, huge lahars sweptdown the Toutle River, and some probablyreached the Columbia River. The primarilyash-producing eruptions of Mount St. Helensduring Smith Creek time did not significantlychange the volcano's shape.
Pine Creek Eruptive Period (2.9 to 2.55ka)--During the Pine Creek Eruptive Period,Mount St. Helens erupted ash and producedpyroclastic flows and dacite domes, and twosmall debris avalanches occurred on its northflank. Repeated collapse of hot, growing lavadomes produced an extensive and broad fanof volcanic debris as much as 600 feet thickon the south flank of the volcano. Similardeposits on the north flank can still be foundas far downstream as the town of Toutle. PineCreek-age dacite domes exposed in the wallsof the crater left by the 1980 eruption showthat at the end of Pine Creek time, the volcanowas a cluster of lava domes with a maximumelevation of about 7,000 feet.
Castle Creek Eruptive Period (2.55 to1.895 ka)--The Castle Creek Eruptive Periodproduced many lava flows and domes, pyroclasticflows, and ash. Andesite lava flows andash erupted from the summit were emplacedon all flanks of Mount St. Helens between2.55 and about 2.50 ka. A lull of about 300years followed, and volcanism resumed atabout 2.2 ka with eruption of andesite lavaflows on the volcano's north flank. Severalthick dacite lava flows and domes, pyroclasticflows and ash, and lahars were producedat 2.0 ka. Castle Creek activity culminatedwith eruption of three groups of fluid basaltlava flows that poured down all flanks of thevolcano as far as 8 miles. The Cave Basalt,erupted at 1.895 ka, was the most recent ofthese. Castle Creek lavas transformed thePine Creek-age cluster of domes into a classiccone-shaped composite volcano, with a summitelevation of about 8,500 feet.
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Sugar Bowl Eruptive Period (1.2 to 1.15ka [A.D. 850 to 900, corrected radiocarbondates])--During the Sugar Bowl EruptivePeriod, three lava domes were built on theflanks of Mount St. Helens. Explosive eruptionsassociated with growth of the SugarBowl Dome produced two "lateral blasts"that affected an area about one-tenth as largeas that of the lateral blast in the 1980 eruption.Ash layer "D" and lahars were alsoemplaced. The Sugar Bowl period was shortlived, produced a small volume of volcanicmaterials, and did not significantly changethe appearance of the volcano.
Kalama Eruptive Period (A.D. 1479 to1720)--Activity during this period producedlarge-volume dacite ashes, pyroclastic flows,domes, lahars, and andesite lava flows. MountSt. Helens added about 1,000 feet of elevationand attained its pre-1980 form during theKalama Period. The Kalama Eruptive Periodis subdivided into three series of events—theearly, middle, and late Kalama phases.
The early Kalama phase began in 1479with a large pyroclastic eruption that depositeddacite ash layer "Wn." In 1482, a smallereruption produced ash layer "We." Over thenext 10 to 20 years, a number of lava domesgrew in the volcano's crater and were disruptedby explosive eruptions. Lahars andpyroclastic flows associated with early Kalamaeruptions are abundant on the volcano'swest and south flanks.
The middle Kalama phase began about1510 with eruption of andesite as pyroclasticflows (which generated hot lahars), a fewlava flows, and ash set "X." The middle phasepeaked about 1535 with eruption of the manythick andesite lava flows prominent on allflanks of Mount St. Helens, including theWorm Complex flows, and ended by 1570.
The most significant event of the late Kalamaphase was growth of a large dacite domeat the summit (Summit Dome). The SummitDome took nearly 100 years to grow (1620 to1720) and gave Mount St. Helens its pre-1980form. During growth, it shed material as pyroclasticflows and lahars on all flanks of thevolcano. Mount St. Helens acquired its pre-1980 cover of glaciers as a result of growth ofthe Summit Dome.
Goat Rocks Eruptive Period (A.D. 1800–1857)--The Goat Rocks Period was shortand relatively small. An explosive eruptionin 1800 produced ash layer "T" and was followedin 1801 by an andesite lava fl ow, calledthe "Floating Island," on Mount St. Helens'north fl ank. Eruptions observed intermittentlyfrom 1831 to 1857 produced ash and the GoatRocks Dome, whose growth also resulted in asmall fan of volcanic debris and lahars.
The last significant eruption of Mount St.Helens before 1980 is generally consideredto have occurred in 1857. Minor explosionsreported in 1898, 1903, and 1921 were probablysteam-driven and not magmatic (moltenrock) eruptions. Eruptions of the Goat RocksPeriod did not significantly change the appearanceof Mount St. Helens, but theyadded the final pieces to the edifice and setthe stage for the 1980 eruption.
Recent work by scientists with the USGSin cooperation with the U.S. Forest Service isshedding new light on the 300,000-year historyof Mount St. Helens Volcano. The workof these USGS scientists is only part of theUSGS Volcano Hazards Program's ongoingefforts to protect people's lives and property inall of the volcanic regions of the United States,including the Pacifi c Northwest, eastern California,Wyoming, Alaska, and Hawaii.