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[2007]

MASSWASTING:TheWorkOf Gravity

OmondiFelixMark 19932003OpulitheCorporation 2007January14th

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MASSWASTING:THEWORKOFGRAVITYEarthssurfaceisneverperfectlyflatbutinsteadconsistsofslopes.Somearesteepand precipitous;othersaremoderateorgentle.Somearelongandgradual;othersareshortand abrupt.Slopescanbemantledwithsoilandcoveredbyvegetationorconsistsofbarrenrock andrubble.Takentogether,slopesarethemostcommonelementinourphysicallandscape. Althoughmostslopesmayappeartobestableandunchanging,theforceofgravitycauses materialtomovedownslope.Atoneextreme,themovementmaybegradualandpractically imperceptible.Attheotherextreme,itmayconsistofathunderingrockfalloravalanche. Occasionally,newsmediareporttheterrifyingandoftengrimdetailsoflandslides.Forexample, onMay312006,agiganticlandslideburiedmorethantenhomesinMalavadivisionKakamega district.Therewaslittlewarningoftheimplementingdisaster;itbeganandendedinjusta matteroffewminutes.Thelandslidestartedafewmetersfromthevillagehills,nearthe summitof6700meter,theloftiestpeakinMalava.Triggeredbyheavysupersaturatedground fromastrongandhugerainfall,alargemassofsoilandwaterbrokefreefromtheprecipitous northfaceofthehills.Afterplungingnearlyafewmeters,thematerialpulverizedonimpact andimmediatelybeganrushingdownthehillside,madefluidbytrappedairandwater. Aswithmanygeologichazards,thetragicsoilavalancheinMalavadivisionwastriggeredbya naturaleventinthiscase,asupersaturatedground.Infact,mostmasswastingevents,whether spectacularorsubtle,aretheresultofcircumstancesthatarecompletelydependentonhuman activities.Ifthetreesonthehillshadnotbeenremovedthisdisasterwouldhavebeen controlled.Inplaceswheremasswastingisarecognizedthreat,stepscanoftenbetakento controldownslopemovementsorlimitthedamagesthatsuchmovementscancause.Ifthe potentialformassWastinggoesunrecognizedorisignored,theresultscanbecostlyand dangerous.Itshouldalsobepointedoutthat,althoughmostdownslopemovementsOccur Whetherpeoplearepresentornot,manyoccurrenceseachyearareaggravatedoreven triggeredbynaturaleventinmostcases,earthquake.

Mass WastingandlandformDevelopmentLandslidesarespectacularexamplesofacommongeologicprocesscalledmasswasting.Mass wastingreferstothedownslopemovementofrock,regolith,andsoilunderthedirectinfluence ofgravity.Itisdistinctfromtheerosionalprocessesthatareexaminedinsubsequentchapters becausemasswastingdoesnotrequireatransportingmedium. Intheevolutionofmostlandforms,masswastingisthestepthatfollowsweathering.Byitself weatheringdoesnotproducesignificantlandforms.Rather,landformsdevelopastheproducts of weathering are removed from the places where they originate. Once weathering weakens andbreaksrockapart,masswastingtransfersthedebrisdownslope,whereastream,actingas aconveyorbelt,usuallycarriesitaway.Althoughtheremaybemanyintermediatestopsalong theway,thesedimentiseventuallytransportedtoitsultimatedestination,thesea. The combined effects of mass wasting and running water produce stream valleys, which are

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themostcommonandconspicuousofEarth'slandforms.Ifstreamsalonewereresponsiblefor creating the valleys in which they flow, valleys would be very narrow features. However, the fact that most river valleys are much wider than they are deep is a strong indication of the significance of masswasting processes in supplying material to streams. This is illustrated by theGrandCanyon(Figure8.2)andtheGreatHellsGate,Naivasha(Figure8.3).Thewallsofthe canyon extend far from the Colorado River Owing to the transfer of weathered debris downslopetotheriveranditstributariesbymasswastingprocesses.Inthismanner,streams andmasswastingcombinetomodifyandsculpturethesurface.Ofcourse,glaciers,groundwa ter, waves, and wind are also important agents in shaping landforms and developing landscapes.

FIGURE 8.2: The walls of the Grand Canyon extend far from the channel of the Colorado River. This results primarily from the transfer of weathered debris downslope to the river and its tributaries by masswastingprocesses.(PhotobyTomTill)

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FIGURE 8.3: The walls of the Great Hells Gate. This results primarily from the transfer of weathered debris downslope to the river and its tributaries by masswasting processes of an ancient River (KamasianAge).(PhotobyOmondiFM)

ControlsandTriggersofMassWastingGravity is the Controlling force of mass wasting, but several factors play an important role in overcominginertiaandtriggeringdownslopemovements.Amongthesefactorsaresaturation ofmaterialwithwater,oversteepeningofslopes,removalofanchoringvegetation,andground vibrationsfromearthquakes. TheRoleofWater When the pores in sediment become filled with water, the cohesion among particles is destroyed,allowingthemtoslidepastoneanotherwithrelativeease.Forexample,whensand is slightly moist, it sticks together quite well. However, if enough water is added to fill the openingsbetweenthegrains,thesandwilloozeoutinalldirections.Thus,saturationreduces theinternalresistanceofmaterials,whicharetheneasilysetinmotionbytheforceofgravity. Whenclayiswetted,itbecomesveryslickanotherexampleofthe"lubricating"effectofWater.19932003OpulitheCorporation.Allrightsreserved

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Wateralsoaddsconsiderableweighttoamassofmaterial.Theaddedweightinitselfmaybe enoughtocausethematerialtoslideorflowdownslope. Malavadisasterisagoodexampleoftheroleofwaterinmasswasting. OversteepenedSlopes Oversteepeningofslopesisanothercauseofmanymassmovements.Therearemany situationsinnaturewhereoversteepeningtakesplace.Astreamundercuttingavalleywalland Wavespoundingagainstthebaseofacliffarebuttwofamiliarexamples.Furthermore,through theiractivities,peopleoftencreateoversteepenedandunstableslopesthatbecomeprimesites formasswasting. Unconsolidated,granularparticles(sandsizedorcoarser)assumeastableslopecalledthe angleofrepose.Thisisthesteepestangleatwhichmaterialremainsstable.Dependingonthe sizeandshapeoftheparticles,theanglevariesfrom25to40degrees.Thelarger,moreangular particlesmaintainthesteepestslopes.Iftheangleisincreased,therockdebriswilladjustby movingdownslope. Oversteepeningisnotjustimportantbecauseittriggersmovementsofunconsolidatedgranular materials. Oversteepening also produces unstable slopes and mass movements in cohesive soils, regolith, and bedrock. The response will not be immediate, as with loose, granular material, but sooner or later, one or more masswasting processes will eliminate the oversteepeningandrestorestabilitytotheslope. Vegetation Plants protect against erosion and contribute to the stability of slopes because their root systems bind soil and regolith together. Where plants are lacking, mass wasting is enhanced, especiallyifslopesaresteepandwaterisplentiful.Whenanchoringvegetationisremovedby forest fires or by people (for timber, farming, or development), surface materials frequently movedownslope. AnunusualexampleoccurredseveraldecadesagoonsteepslopesnearMountKenya,Kikuyu region.Farmersreplacedindigenoustrees,whichhavedeeproots,withamoreprofitable,but shallowrootedcrop,carnation.Whenthelessstableslopefailed,thelandslidetook11lives. EarthquakesasTriggers Conditions that favor mass wasting may exist in an area for a long time without movement occurring. An additional factor is sometimes necessary to trigger the movement. Among the moreimportantanddramatictriggersareearthquakes.Anearthquakeanditsaftershockscan dislodge enormous volumes of rock and unconsolidated material. The event in the China describedinthemediaisonetragicexample.Inmanyareasthatarejoltedbyearthquakes,itis not ground vibrations directly, but landslides and ground subsidence triggered by vibrations19932003OpulitheCorporation.Allrightsreserved

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thatcausethegreatestdamage.

ClassificationofMassWastingProcessesThereisabroadarrayofdifferentprocessesthatgeologistscallmasswasting.Fourprocesses are illustrated in Figure 8.4. Generally, the different typesareclassified based on the type of materialinvolved,thekindofmotiondisplayed,andthevelocityofthemovement. TypeofMaterial The classification of masswasting processes on the basis of the material involved in the movementdependsuponwhetherthedescendingmassbeganasunconsolidatedmaterialoras bedrock.Ifsoilandregolithdominate,termssuchas"debris,""mud,"or"earth"areusedin thedescription.Incontrast,whenamassofbedrockbreakslooseandmovesdownslope,the term"rock"maybepartofthedescription. TypeofMotion Inadditiontocharacterizingthetypeofmaterialinvolvedinamasswastingevent,thewayin whichthematerialmovesmayalsobeimportant.Generally,thekindofmotionisdescribedas eitherafall,aslide,oraflow. Fall.Whenthemovementinvolvesthefreefallofdetachedindividualpiecesofanysize,itis termed a fall. Fall is a common form of movement on slopes that are so steep that loose material cannot remain on the surface. The rock may fall directly to the base of the slope or moveinaseriesofleapsandboundsoverotherrocksalongtheway.Manyfallsresultwhen freeze and thaw cycles and/or the action of plant roots loosen rock to the point that gravity takesover.Althoughsignsalongbedrockcutsonhighwayswarnoffallingrockfewofushave actuallywitnessedsuchanevent.However,asFigure8.4illustrates,theydoindeedoccur.In fact, this is the primary way in which talus slopes are built and maintained (see Figure 5.3).Sometimesfallsmaytriggerotherformsofdownslopemovement. Slide. Many masswasting processes are described as slides. Slides occur whenever material remains fairly coherent and moves along a welldefined surface. Sometimes the surface is a joint,afault,orbeddingplanethatisapproximatelyparalleltotheslope.However,inthecase ofthemovementcalledslump,thedescendingmaterialmovesenmassealongacurvedsurface ofrupture. A note of clarification is appropriate at this point. Sometimes the word slide is used as a synonymforthewordlandslide.Itshouldbepointedoutthatalthoughmanypeople,including geologists, use the term; the word landslide has no specific definition in geology. Rather, it shouldbeconsideredasapopularnontechnicaltermusedtodescribeallperceptibleformsof masswasting,includingthoseinwhichslidingdoesnotoccur. For example, recall that the Malava disaster described at the beginning of the chapter was initiatedbyamassoffreefallingmaterialthatbrokefromthenearlyverticalsummitofthehill.

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Flow. The third type of movement common to masswasting processes is termed flow. Flow occurswhenmaterialmovesdownslopeasaviscousfluid.Mostflowsaresaturatedwithwater andtypicallymoveaslobesortongues.

A. Slump

D. Earthflow B. Rockslides

Figure8.4:Thefour processesillustratedhereareallconsideredtoberelativelyrapidformsofmass wasting.BecausematerialinslumpsA.androckslidesB.movealongwelldefinedsurfaces,theyaresaid tomovebysliding.Bycontrast,whenmaterialmovesdownslopeasaviscousfluid,themovementis describedasaflow.DebrisflowC.andearthflowD.advancedownslopeinthismanner.

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FIGURE8.5:RockfallblockingMontanaHighway2eastofPipestonePass,May1998.(AP/WideWorld Photo)

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FIGURE8.6:RockslidefromFishersTowerinHellsGateNationalParkNaivasha,July2006.(OmondiFM Photo)

RateofMovement Theeventdescribedatthebeginningofthischapterclearlyinvolvedrapidmovement.Therock and debris moved downslope at speeds well in excess of 200 kilometers per hour. This most rapidtypeofmassmovementistermedadebrisavalanche.Manyresearchersbelievethatrock avalanches,suchastheonethatproducedthesceneinFigure8.5,mustliterally"floatonair" as they move downslope. That is, high velocities result when air becomes trapped and compressedbeneaththefallingmassofdebris,allowingittomoveasabuoyant,flexiblesheet19932003OpulitheCorporation.Allrightsreserved

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acrossthesurface. Mostmassmovements,however,donotmovewiththespeedofarockavalanche.Infact,a great deal of mass wasting is imperceptibly slow. One process that we will examine later, termed creep, results in particle movements that are usually measured in millimeters or centimetersperyear.Thus,asyoucansee,ratesofmovementcanbespectacularlysuddenor exceptionally gradual. Although various types of mass wasting are often classified as either rapid or slow, such a distinction is highly subjective because there is a wide range of rates between the two extremes. Even the velocity of a single process at a particular site can vary considerably.

FIGURE8.7:This4kilometerlongtongueofrubblewasdepositedatopAlaska'sShermanGlacierbya rockavalanche.TheeventwastriggeredbyatremendousearthquakeinMarch1964.(PhotobyAustin Post,u.S.GeologicalSurvey)

SLUMPSlumpreferstothedownwardslidingofamassofrockorunconsolidatedmaterialmovingasa unit along a curved surface (Figure 8.4A). Usually the slumped material does not travel spectacularly fast nor very far. This is a common form of mass wasting, especially in thick accumulations of cohesive materials such as clay. The rupture surface is characteristically spoonshaped and concave upward or outward. As the movement occurs, a crescentshaped scarp is created at the head and the block's upper surface is sometimes tilted backwards. Although slump may involve a single mass, it often consists of multiple blocks. Sometimes water collects between the base of the scarp and the top of the tilted block. As this water percolatesdownwardalongthesurfaceofrupture,itmaypromotefurtherinstabilityandad19932003OpulitheCorporation.Allrightsreserved

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ditionalmovement. Slumpcommonlyoccursbecauseaslopehasbeenoversteepened.Thematerialontheupper portionofaslopeisheldinplacebythematerialatthebottomoftheslope.Asthisanchoring materialatthebaseisremoved,thematerialaboveismadeunstableandreactstothepullof gravity. One relatively common example is a valley wall that becomes oversteepened by a meanderingriver.Anotherisacoastalcliffthathasbeenundercutbywaveactionatitsbase. Slumping may also occur when a slope is overloaded, causing internal stress on the material below. This type of slump often occurs where weak; clayrich material underlies layers of stronger, more resistant rock such as sandstone. The seepage of water through the upper layersreducesthestrengthoftheclaybelowandslopefailureresults.

ROCKSLIDERockslides frequently occur in high mountain areas such as the Andes, Alps, and Canadian Rockies. They are sudden and rapid movements that happen when detached segments of bedrockbreaklooseandslidedownslope(Figure8.4).Asthemovingmassthundersalongthe surface, it breaks into many smaller pieces. Such events are among the fastest and most destructivemassmovements. Rockslidesusuallytakeplacewherethereisaninclinedsurfaceofweakness.Suchsurfacestend toformwherestrataaretiltedorwherejointsandfracturesexistparalleltotheslope.When rockinsuchasettingisundercutatthebaseoftheslope,itlosessupportandeventuallygives way.Sometimestherockslideistriggeredwhenrainormeltingsnowlubricatestheunderlying surfacetothepointthatfrictionisnolongersufficienttoholdtherockunitinplace.Asaresult, rockslidestendtobemorecommonduringthespring,whenheavyrainsandmeltingsnoware mostprevalent. Earthquakes can trigger rockslides and other mass movements. The 1811 earthquake at New Madrid,Missouri,forexample,causedslidesinanareaofmorethan13,000squarekilometers (5000 square miles) along the Mississippi River valley. A more recent example occurred on August 17, 1959, when a severe earthquake west of Yellowstone National Park triggered a massive slide in the canyon of the Madison River in southwestern Montana. In a matter of momentsanestimated27millioncubicmetersofrock,soil,andtreesslidintothecanyon.The debrisdammedtheriverandburiedacampgroundandhighway.Morethan20unsuspecting campersperished. NotfarfromthesiteoftheMadisonCanyonslide,theclassicGrosVentrerockslideoccurred34 yearsearlier.TheGrosVentreRiverflowswestfromthenorthernmostpartoftheWindRiver Range in northwestern Wyoming, through the Grand Teton National Park, and eventually emptiesintotheSnakeRiver,OnJune23,1925,amassiverockslidetookplaceinitsvalley,just east of thesmall townof Kelly. In the span of just a few minutes a great mass of sandstone, shale,andsoilcrasheddownthesouthsideofthevalley,carryingwithitadensepineforest. Thevolumeofdebris,estimatedat38millioncubicmeters(50millioncubicyards),createda 70meterhigh dam on the Gros Ventre River (Figure 8.8). Because the river was completely19932003OpulitheCorporation.Allrightsreserved

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blocked,alakewascreated.Itfilledsoquicklythatahousethathadbeen18meters(60feet) above the river was floated off its foundation 18 hours after the slide. In 1927, the lake overflowed the dam, partially draining the lake and resulting in a devastating flood downstream. WhydidtheGrosVentrerockslidetakeplace?Figure8.7showsadiagrammaticcrosssectional viewofthegeologyofthevalley.Noticethefollowingpoints:(1)Thesedimentarystratainthis areadip(tilt)15to21degrees;(2)underlyingthebedofsandstoneisarelativelythinlayerof clay;and(3)atthebottomofthevalleytheriverhadcutthroughmuchofthesandstonelayer. During the spring of 1925, water from heavy rains and melting snow seeped through the sandstone, saturating the clay below. Because much of the sandstone layer had been cut throughbytheGrasVentreRiver,thelayerhadvirtuallynosupportatthebottomoftheslope. Eventually the sandstone could no longer hold its position on the wetted clay, and gravity pulledthemassdownthesideofthevalley.Thecircumstancesatthislocationweresuchthat theeventwasinevitable.

FIGURE8.8: Although the Gros Ventre rockslide occurred in1925, the scar left on the side of Sheep Mountain is still aprominent feature. (Photo byStephen Trimble)

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FIGURE8.9 Cross-sectional view of the Gros Ventre rockslide. The slide occurred when the tilted and undercut sandstone bed could no longer maintain its position atop the saturated bed of clay. (After W. c.Alden, "landslide and Flood at Gros Ventre, Wyoming:' Transactions (AIME) 76 (1928):348)

DEBRISFLOWDebris flow is a relativelyrapid type of masswasting that involves aflow of soil and regolith containingalargeamountofwater(Figure8.3C).Debrisflows,whicharealsocalledmudflows, aremostcharacteristicofsemiaridmountainousregionsandarealsocommonontheslopesof somevolcanoes.Becauseoftheirfluidproperties,debrisflowsfrequentlyfollowcanyonsand stream channels. In populated areas, debris flows can pose a significant hazard to life and property. DebrisFlowsinSemiaridRegions When a cloudburst or rapidly melting mountain snows create a sudden flood in a semiarid region, large quantities of soil and regolith are washed into nearby stream channels because there is usually little vegetation to anchor the surface material. The end product is a flowing tongueofwellmixedmud,soil,rock,andwater.Itsconsistencycanrangefromthatofwetcon crete to a soupy mixture not much thicker than muddy water. The rate of flow therefore depends not only on the slope but also on the water content. When dense, debris flows are capableofcarryingorpushinglargeboulders,trees,andevenhouseswithrelativeease. Debrisflowsposeaserioushazardtodevelopmentinrelativelydrymountainousareassuchas southernMauforestregion.Hereconstructionofhomesoncanyonhillsidesandtheremovalof nativevegetationbycharcoalburnersandothermeanshaveincreasedthefrequencyofthese destructiveevents.Moreover,whenadebrisflowreachestheendofasteep,narrowcanyon,it spreadsout,coveringtheareabeyondthemouthofthecanyonwithamixtureofwetdebris. Thismaterialcontributestothebuildupoffanlikedepositsatcanyonmouths.Thesestructures arecalledalluvialfansandwillbediscussedingreaterdetailinthenextChapters.Thefansare relativelyeasytobuildon,oftenhaveniceviews,andareclosetothemountains;infact,like thenearbycanyons,manyhavebecomepreferredsitesfordevelopment.Becausedebrisflows19932003OpulitheCorporation.Allrightsreserved

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occuronlysporadically,thepublicisoftenunawareofthepotentialhazardofsuchsites. Lahars Debrisflowsarealsocommonontheslopesofsomevolcanoes,inwhichcasetheyaretermed lahars.ThewordoriginatedinIndonesia,avolcanicregionthathasexperiencedmanyofthese oftendestructiveevents.Laharsresultwhenhighlyunstablelayersofashanddebrisbecome saturatedwithwaterandflowdownsteepvolcanicslopes.Theseflowsgenerallyfollowexisting streamchannels.Heavyrainfallserodingvolcanicdepositsoftentriggertheseflows.Othersare initiated when large volumes of ice and snow are suddenly melted by heat flowing to the surfacefromwithinthevolcanoorbythehotgasesandnearmoltendebrisemittedduringa violenteruption. When Mount St. Helens erupted in May 1980, several lahars were created. The flows and accompanyingfloodsraceddownthevalleysofthenorthandsouthforksoftheToutleRiverat speedsthatwereofteninexcessof30kilometersperhour.Fortunately,theaffectedareawas notdenselysettled.Nevertheless,morethan200homesweredestroyedorseverelydamaged. Mostbridgesmetasimilarfate. InNovember1985,laharswereproducedduringtheeruptionofNevadodelRuiz,a5300meter (17,400foot) volcano in the Andes Mountains of Colombia. The eruption melted much of the snow and ice that capped the uppermost 600 meters of the peak, producing torrents of hot, thickmud,ash,anddebris.Thelaharsmovedoutwardfromthevolcano,followingthevalleys ofthreerainswollenriversthatradiatefromthepeak.Theflowthatmoveddownthevalleyof theLagunillaRiverwasthemostdestructive.ItdevastatedthetownofArmero,48kilometers fromthemountain.Mostofthemorethan25,000deathscausedbytheeventoccurredinthis oncethrivingagriculturalcommunity. Deathandpropertydamageduetothelaharsalsooccurredin13othervillageswithinthe180 squarekilometer disaster area. Although a great deal of pyroclastic material was explosively ejectedfromNevadodelRuiz,itwasthelaharstriggeredbythiseruptionthatmadethissucha devastatingnaturaldisaster.Infact,itwastheworstvolcanicdisastersince28,000peopledied followingthe1902eruptionofMountPeleeontheCaribbeanislandofMartinique.

EARTHFLOWWe have seen that debris flows are frequently confined to channels in semiarid regions. In contrast, earthflows most often form on hillsides in humid areas during times of heavy precipitation or snowmelt (see Figure 8.4D), when water saturates the soil and regolith on a hillside, the material may break away, leaving a scar on the slope and forming a tongue or teardropshapedmassthatflowsdownslope(Figure8.10). The materials most commonly involved are rich in clay and silt and contain only small proportionsof sandandcoarserparticles.Earthflowsrangeinsizefrombodies afewmeters long, a few meters wide, and less than a meter deep to masses more than a kilometer long, severalhundredmeterswide,andmorethan10metersdeep.19932003OpulitheCorporation.Allrightsreserved

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Becauseearthflowsarequiteviscous,theygenerallymoveatslowerratesthanthemorefluid debrisflowsdescribedintheprecedingsection.Theyarecharacterizedbyaslowandpersistent movement and may remain active for periods ranging from days to years. Depending on the steepnessoftheslopeandthematerial'sconsistency,measuredvelocitiesrangefromlessthan 1 millimeter per day up to several meters per day. Over the time span that earthflows are active,movementistypicallyfasterduringwetperiodsthanduringdriertimes.Inadditionto occurringasisolatedhillsidephenomena,earthflowscommonlytakeplaceinassociationwith largeslumps.Inthissituation,theymaybeseenastonguelikeflowsatthebaseoftheslump block. Aspecialtypeofearthflow,knownasliquefaction,sometimesoccursinassociationwithearth quakes. Porous clay to sandsized sediments that are saturated with water are most vulnerable.Whenshakensuddenly,thegrainslosecohesionandthegroundflows.Liquefaction cancausebuildingstosinkortipontheirsidesandundergroundstoragetanksandsewerlines tofloatupward.Tosaytheleast,damagecanbesubstantial.

FIGURE8.10: Thissmall,tongueshapedearthflowoccurredonanewlyformedslopealongarecently constructedhighway.Itformedinclayrichmaterialfollowingaperiodofheavyrain.Noticethesmall slumpattheheadoftheearthflow.(PhotobyE.J.Tarbuck)

SLOWMOVEMENTSMovementssuchasrockslides,rockavalanches,andlaharsarecertainlythemostspectacular and catastrophic forms of mass wasting. These dangerous events deserve intensive study to enablemoreeffectiveprediction,timelywarnings,andbettercontrolstosavelives.However, because of their large size and spectacular nature, they give us a false impression of their importanceasamasswastingprocess.Indeed,suddenmovementsareresponsibleformoving19932003OpulitheCorporation.Allrightsreserved

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lessmaterialthantheslowerandfarmoresubtleactionofcreep.Whereasrapidtypesofmass wasting are characteristic of mountains and steep hillsides, creep can take place on gentle slopesandisthusmuchmorewidespread. Creep Creep is a type of mass wasting that involves the gradual downhill movement of soil and regolith. One of the primary causes of creep is the alternate expansion and contraction of surfacematerialcausedbyfreezingandthawingorwettinganddrying.AsshowninFigure8.10, freezingorwettingliftsparticlesatrightanglestotheslope,andthawingordryingallowsthe particlestofallbacktoaslightlylowerlevel.Eachcyclethereforemovesthematerialashort distancedownhill. Creepcanalsobeinitiatedifthegroundbecomessaturatedwithwater.Followingaheavyrain or snowmelt, a waterlogged soil may lose its internal cohesion, allowing gravity to pull the material downslope. Because creep is imperceptibly slow, the process cannot be observed in action. What can be observed, however, are the effects of creep. Creep causes fences and utilitypolestotiltandretainingwallstobedisplaced.

FIGURE8.11:Therepeatedexpansionandcontractionofthesurfacematerialcausesanetdownslope migrationofrockparticlesaprocesscalledcreep.

Solifluction Solifluction is a form of mass wasting that is common in regions underlain by permafrost. Permafrost refers to the permanently frozen ground that occurs in association with Earth's harshtundraandicecapclimates(Figure8.12).Solifluctioncanberegardedasaformofcreep in which unconsolidated, water saturated material gradually moves downslope. Solifluction occurs in a zone above the permafrost called the active layer, which thaws in summer and refreezes in winter. During the summer season, water is unable to percolate into the imperviouspermafrostlayerbelow.Asaresult,theactivelayerbecomessaturatedandslowly flows.Theprocesscanoccuronslopesasgentleas2to3degrees.Wherethereisawellde velopedmatofvegetation,asolifluctionsheetmaymoveinaseriesofwelldefinedlobesoras aseriesofpartiallyoverridingfolds(Figure8.13).19932003OpulitheCorporation.Allrightsreserved

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FIGURE8.12:Thisbuilding,locatedsouthof Fairbanks,Alaska,subsidedbecauseofthawingpermafrost. Noticethattherightside,whichwasheated,settledmuchmorethantheunheatedporchontheleft.

FIGURE8.13:SolifluctionlobesnortheastofFairbanks,Alaska.Solifluctionoccurswhentheactivelayer thawsinsummer.(PhotobyJamesE.Patterson)

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