Past Eruptive BehaviorMount Hood is more than 500,000 years

old. The volcano has grown in fits and starts,with decades to centuries of frequent eruptionsseparated by quiet periods lasting fromcenturies to more than 10,000 years. In therecent past, Mount Hood has had twosignificant eruptive periods, one about 1,500years ago and the other about 200 years ago.

Unlike its neighbor to the north, Mount St.Helens, Mount Hood does not have a history ofviolent explosive eruptions. Instead,(see inside pages for definitions of bold terms),rarely traveling more than 6 to 8 miles fromtheir source, have built up the flanks of thevolcano one sector at a time. Sometimes,instead of flowing slowly downhill, lava piles

up over its vent forming a manyhundreds of feet high. On the steep upperslopes of Mount Hood, growing lava domeshave repeatedly collapsed to form extremelyhot, fast-moving . Few ofthese pyroclastic flows have traveled more than8 miles. But because they are extremely hot,such flows can melt significant quantities ofsnow and ice to produce that flow downriver valleys, often far beyond the flanks of thevolcano. Over the past 30,000 years, growthand collapse of lava domes and generation oflahars have dominated Mount Hood's eruptiveactivity.

lava flows

lava dome

pyroclastic flows

lahars

Throughout Mount Hood's history, rapidlandslides, called , ofvarious sizes have occurred. The largest onesremoved the summit and sizable parts of thevolcano's flanks and formed lahars that flowedto the Columbia River. Large debris avalanchesoccur infrequently and are usually triggered byeruptive activity. But small ones not associatedwith eruptive activity occur more frequently.Small avalanches can occur when rocks,altered and weakened by acidic volcanic fluidsor by weathering, such as freezing andthawing, fail spontaneously.

debris avalanches

U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOLCANO HAZARDS

Mount Hood's last majoreruption occurred in the

1790's not long before Lewis andClark's expedition to the PacificNorthwest. In the mid-1800's, localresidents reported minor explosiveactivity, but since that time thevolcano has been quiet. Someday,however, Mount Hood will eruptagain. What will those eruptions belike and how will they affect us?Scientists from the U.S. GeologicalSurvey (USGS) are studying thevolcano's past eruptive behavior tobetter anticipate and prepare forfuture eruptive activity.

Mount Hood from Portland, Oregon. When Mount Hood next erupts, Portland could be affected by light ashfallssimilar to those it experienced during the 1980 eruptions of Mount St. Helens. The city will not be directlyaffected by lava flows, pyroclastic flows, or lahars, but regional transportation and water supplies could bedisrupted. (Photo by David Wieprecht, USGS)

U.S. Department of the InteriorU.S. Geological Survey

USGS Fact Sheet 060-00August 2000

Mount Hood story and Hazards of Oregon’s MostRecently Active Volcano

—Hi

Eruptions at Mount HoodDuring the Past 30,000 Years

Mid-1800’sSmall steam and ashexplosions

Multiple episodes of lavadome growth, pyroclasticflows, lava flows, lahars,and tephra fall; valleys onall flanks affected

30,000 to 15,000 years ago

About 1,500 years agoDebris avalanche fromupper south flank; lavadome near Crater Rock,pyroclastic flows, lahars insouth and west valleys;substantial tephra fallsnear volcano

About 200 years agoLava dome at Crater Rock;pyroclastic flows, lahars insouth and west valleys, andminor tephra falls

Subduction of the Juan de Fuca Plate under theNorth American Plate controls the distribution ofearthquakes and volcanoes in the Pacific Northwest.Mount Hood is just one of several recently active,major volcanic centers in the Cascade Range.

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Are You at Risk from the Next Eruption of Mount Hood?

The Next Eruption of MountHood

Planning for the Future

Scientists expect the next eruption toconsist of small explosions and the growthand collapse of lava domes, generatingpyroclastic flows, ash clouds, and lahars.Lahars pose the greatest hazard becausemore people live downstream in lahar-prone river valleys than live on thevolcano's flanks. Thus, it is important toknow where one lives, works, and plays inrelation to Mount Hood's hazard zones.

Hazardous Areas

Tephra Hazards

Hazard zones shown on the map weredetermined on the basis of distance fromthe volcano, vent location, and type ofhazardous evens. Proximal hazard zones( ) are areas subject to rapidly movingdebris avalanches, pyroclastic flows, andlahars that can reach the hazard boundaryin less than 30 minutes, as well as to slow-moving lava flows. Areas within proximalhazard zones should be evacuated before aneruption begins, because there is little timeto get people out of harm's way once aneruption starts. Most pyroclastic flows, lavaflows, and debris avalanches will stop

within the proximal hazard zone, but laharscan travel much farther.

Distal hazard zones ( ) are areasadjacent to rivers that are pathways forlahars. Estimated travel time for lahars toreach these zones is more than 30 minutes,which may allow individuals time to moveto higher ground and greater safety if givenwarning. Shown are inundation areas forlahars of a size similar to lahars that sweptthrough the Sandy River 1,500 year ago.Smaller or larger lahars will affect smalleror larger areas, respectively. Lahars couldaffect transportation corridors by damagingor destroying bridges and roads. Waterfrom the Bull Run Watershed, vital toPortland, is transported in pipes that crossdistal hazard zones along the Sandy River.

Proximal and distal hazard zones aresubdivided into zones and on the basisof the vent location during the nexteruption. During the past two eruptiveepisodes, the vent was located near CraterRock. Scientists anticipate that the vent forthe next eruption will most likely be in thesame area. Thus, areas within hazard zones

and have a higher probability ofbeing affected during the next eruption thando areas within hazard zones and ,

which reflect a vent located on thevolcano's west, north, or east flank.

During and after an eruption, largeamounts of sediment could be carried byrivers and discharged into the ColumbiaRiver. This sediment could narrow theColumbia's channel, forcing it to the northand potentially causing bank erosion alongthe river's north bank.

Mount Hood does not have a history oflarge explosive eruptions. Therefore, it isunlikely that communities downwind(typically east of the volcano) will receivetephra accumulations thick enough tocollapse roofs. However, even minoramounts of tephra can damage machineryand electronic equipment or make drivinghazardous. Ash clouds from smallexplosions or from pyroclastic flows canreach tens of thousands of feet in altitude,endangering jet aircraft by causing enginefailure. Minor amounts of tephra can makebreathing difficult for those withrespiratory problems, the elderly, andinfants. For most residents, however, tephrawill be a nuisance and can be mitigated by

such actions as shutting down and coveringequipment, frequently replacing air filtersin machinery, wearing dust masks, andavoiding unnecessary travel.

Scientists do not know when MountHood will erupt again or whether it willerupt in our lifetimes, but, as Mount St.Helens taught us, it is best to be prepared.Scientists continuously monitor MountHood for signs of unrest and are incommunication with responsible local,State, and Federal agencies. Individuals toocan prepare by doing the following:

about the volcano hazards thatcould affect your community anddetermine whether you live, work, orgo to school in a volcano hazard zone.

what you and your family will doif a hazardous volcanic event occurs.

in helping your communitybe prepared.

A few moments spent now could helpkeep the next eruption of Mount Hood frombecoming a disaster for you, your family,and your community.

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House partly buried by lahars from the 1980 eruption of Mount St. Helens. Note the high mud line on thehouse and large amounts of woody debris carried by the lahars. (Photo by Lyn Topinka, USGS)

Simplified volcano-hazards-zonation map for Mount Hood, Oregon. For moreinformation on hazard zones see text or for a more detailed map and description seeScott and others, 1997, Volcano Hazards in the Mount Hood Region, Oregon, U.S.Geological Survey Open-File Report 97-89.

PA PB

Major valleys

Proximal Hazard Zones (P)(lava flow, pyroclastic flow,debris avalanche, lahar)

DA DB

Bank Erosion

Distal Hazard Zones (D)(lahar)

Mount Hood’s lastmajor eruptionoccurred 200 yearsago and consistedof growth andcollapse of a lavadome that sentnumerouspyroclastic flowsdown the south

and west flanks. Crater Rock (above) is theremnant of that dome. The left photo shows apyroclastic flow during the August 7, 1980,eruption of Mount St. Helens. A billowing ashcloud rises from the pyroclastic flow. Futurepyroclastic flows from Mount Hood coulddevastate areas on the flanks of the volcano, andfallout of ash from the ash cloud could disrupttransportation and prove a nuisance to residentsdownwind. (Photos by Peter Lipman and

, USGS)David

Wieprecht

Explosive eruptions blast lavafragments ( ) and gas into the

air. Tephra can also be carriedaloft in billowing ash clouds

above pyroclastic flows. Largefragments fall to the groundclose to the volcano, but

smaller fragments ( ) cantravel hundreds to thousands

of miles downwind.

tephra

ash

Debris avalanches are rapid landslides of rock,soil and overlying vegetation, snow, or ice. Theycan bury or smash objects in their path. All orsome portion of debris avalanches can transforminto lahars. are fast-moving slurries ofrock, mud, and water that move down rivervalleys. They can bury, move, or smash objects

in their path. Lahars form when pyroclasticflows melt snow or ice, or by the

transformation of debrisavalanches, or by mobilization

of loose debris on theflanks of

volcanoes.

Lahars

Volcano Hazards

AshCloud

Lava Flows and Domes

LavaLava flows

Lava domes

is molten rock that flows onto the Earth'ssurface. move downslope away froma vent and bury or burn everything in their paths.

form when lava piles up over a vent.

Debris Avalanches and Lahars

Pyroclastic Flows

Pyroclastic flows are high-speed avalanches of hot rock,gas, and ash that are formedby the collapse of lava domesor eruption columns. Theycan move up to 100 miles

per hour and havetemperatures to 1500 F.They are lethal, burning,

burying, or asphyxiatingall in their paths.

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PyroclasticFlow

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Ash Cloud

Crater Rock

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Minimum timefor lahar toreach an area,in hours:minutes

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Hazards Zonation Map

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A River TransformedIn 1806, Meriwether Lewis andWilliam Clark named a river on thesouth side of the Columbia River gorgethe “Quicksand River.” Their descriptionof a wide, shallow with a bed “formedentirely of quicksand,” bears little resemblanceto the narrow, moderately deep river we call todaythe Sandy River. What happened? The answer lay50 miles away at Mount Hood. An eruption in the1790's caused a tremendous amount of volcanicrock and sand to enter the Sandy River drainage.That sediment was still being flushed downstreamwhen Lewis and Clark saw and named the river.Since 1806, the river has removed the excesssediment from its channel. The Toutle River insouthwest Washington was similarly affected by the1980 eruptions of Mount St. Helens. A sediment-retention dam was built in the late 1980's to trapsediment and ease flooding in the lower ToutleRiver below the dam.

river

Hazards Can Occur Even WithoutEruptive Activity

Past Catastropic Events

Mount Hood Today

Monitoring for the Next Eruption

Lahars are often associated with eruptiveactivity, but they can also be generated byrapid erosion of loose rock during heavy rainsor by sudden outbursts of glacial water. OnChristmas Day 1980, an intense rainstormrapidly melted snow and triggered a smalllandslide in fragmental debris in upper PolallieCreek. The resulting lahar moved downvalleyat 25 to 35 miles per hour. At the mouth ofPolallie Creek, the lahar spread out, killing acamper and temporarily damming the EastFork Hood River. Flooding after failure of thistemporary dam destroyed 5 miles of highway,three bridges, and a state park—at a cost of atleast $13 million. Small lahars such as thisoccur every few years at Mount Hood, but fewhave been as destructive.

Two past eruptions at Mount Hood provideperspective on the impact of future largeevents. Both were associated with eruptiveactivity that triggered debris avalanches andwere accompanied by lava extrusion,pyroclastic flows, and lahars. One represents atruly catastrophic event.

About 100,000 years ago, a large portion ofthe volcano's north flank and summitcollapsed. The resulting debris avalanchetransformed into a lahar that swept down theHood River valley. At the river's mouth, wherethe town of Hood River now stands, the laharwas 400 feet deep. The lahar crossed theColumbia River and surged up the WhiteSalmon River valley on the Washington side.Since that time lava has filled in the scar leftby the debris avalanche.

On the south side of the volcano, the scarfrom a 1,500-year-old debris avalanche is stillvisible, forming the amphitheater aroundCrater Rock. A lahar formed by this eventtraveled the length of the Sandy River valley,

depositing boulders as large as 8 feet indiameter, 30 feet above present river levelwhere the towns of Wemme and Wildwoodnow stand. The lahar spread out over the deltaat the mouth of the Sandy River and pushedthe Columbia River to the north. This event,although large by Mount Hood's standards,was only about one-tenth the size of the100,000-year-old event.

Mount Hood's next eruption is much morelikely to be smaller than or similar in size tothe 1,500-year-old event. An eruption similarin size to the 100,000-year-old event, althoughpossible, is much less likely.

Today Mount Hood shows no signs ofimminent volcanic activity, but hot steamvents, or fumaroles, near Crater Rock attest toheat below. On clear, cold, windless days, asteam plume is often seen rising from thefumaroles. Visitors to Mount Hood frequentlysmell the "rotten egg" odor of the fumarolegas, whose composition indicates that magmalies a few miles below the summit.

Renewed activity at most volcanoes beginswith increasing numbers of earthquakesbeneath the volcano as magma moves towardsthe surface. Since 1977 the University ofWashington's Geophysics Program and theUSGS have continuously monitoredearthquakes at Mount Hood. Typically, one tothree small earthquake swarms (tens to morethan one hundred earthquakes lasting 2 to 5days) occur every year. What scientists arelooking for as a sign of renewed activity is fora swarm to persist, for the number ofearthquakes to increase dramatically, or for thedepths of earthquakes to become shallower.Such signs of reawakening might also beaccompanied by changes in composition ortemperature of fumarole gases, or bydeformation of the volcano's flanks.

Scientists can generally detect when avolcano becomes "restless," thereby providingsome warning to officials and the public. Butthey cannot say precisely when an eruptionwill begin, how big it will be, or how long itwill last. Thus, we will all have to confrontmany uncertainties when Mount Hoodreawakens. Recent eruptions around the worldreveal that lava-dome eruptions, like thosetypical of Mount Hood, can begin after weeksto months of restlessness, last for time periodsof months to years, and generate tens tohundreds of pyroclastic flows and lahars ofvarying size. Unfortunately, the end of aneruption doesn't always mean the end oferuption-related problems. New deposits ofrock debris on the volcano's slopes and in rivervalleys can be reworked to form lahars formany years after an eruption ends.

For more information contact:

Volcano Hazards in the Mount Hood Region,Oregon

What are Volcano Hazards?

U.S. Geological SurveyCascades Volcano Observatory

5400 MacArthur Blvd., Vancouver, WA 98661Tel: (360) 993-8900, Fax: (360) 993-8980

http://vulcan.wr.usgs.gov/or

USGS Volcano Hazards Programhttp://volcanoes.usgs.gov/

See also(USGS Open-File Report 97-89) and

(USGS Fact Sheet 002-97)

COOPERATING ORGANIZATIONSU.S. Department of Agriculture, Forest ServiceUniversity of Washington, Geophysics Program

Geologist examining100,000-year-old lahardeposit exposed alongUnderwood Hill Road,near the mouth of theWhite Salmon River inWashington. The lahar,derived from a largedebris avalanche off thenorth side of MountHood, flowed down theHood River, crossed theColumbia River, andtraveled several milesup the White SalmonRiver before stopping.Here, the deposit isabout 40 feet thick andabout 300 feet abovepresent river level.(Photo by William Scott,USGS)

The two faces of Mount Hood. In the photo of the south flank taken from Trillium Lake, the dashed line marksthe steep scarp above Crater Rock formed by a landslide 1,500 years ago. Dome growth and collapse fromCrater Rock during the last two eruptive episodes (1,500 and 200 years ago) sent numerous pyroclastic flowsdown the south flank, resulting in the smooth south-facing slope. On the north flank, lack of recent volcanicactivity has preserved the deeply carved glacial landscape on this side of the mountain. In its long history,Mount Hood has experienced numerous ice ages, each lasting for thousands of years, when glaciers weremore extensive than today. During the last one, which ended about 15,000 years ago, glaciers extended 4 to 5miles beyond their present limits. (Photos by William Scott, USGS)

Crater Rock

North FlankSouth Flank

Cynthia A. Gardner, William E. Scott,Jon J. Major, and Thomas C. Pierson

Graphics and Design by

Bobbie Myers, Lisa Faust, and Christine Janda