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R I FT VA L L E Y
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AriftvalleyisalowlandregionthatformswhereEarth’stectonicplatesmove
apart,orrift.Riftvalleysarefoundbothonlandandatthebottomoftheocean,
wheretheyarecreatedbytheprocessofseafloorspreading.Riftvalleysdiffer
fromrivervalleysandglacialvalleysinthattheyarecreatedbytectonic
activityandnottheprocessoferosion.
Tectonicplatesarehuge,rockyslabsofEarth'slithosphere—itscrustand
uppermantle.Tectonicplatesareconstantlyinmotion—shiftingagainsteach
otherinfaultzones,fallingbeneathoneanotherinaprocesscalledsubduction,
crashingagainstoneanotheratconvergentplateboundaries,andtearingapart
fromeachotheratdivergentplateboundaries.
Manyriftvalleysarepartof“triplejunctions,”atypeofdivergentboundary
wherethreetectonicplatesmeetatabout120°angles.Twoarmsofthetriple
junctioncansplittoformanentireocean.Thethird,“failedrift”oraulacogen,
maybecomeariftvalley.TheAtlanticOcean,forinstance,isaresultofatriple
junctionthatstartedinwhatisnowtheGulfofGuineaonthewestcoastof
Africa.TwoarmsofatriplejunctiononthesupercontinentPangaea“opened”
theocean,whiletheaulacogenformedtheriftvalleyknownastheBenue
TroughthroughwhatisnowsouthernNigeria.
Riftvalleyscanalsoformattransformfaults,wheretectonicplatesaregrinding
pasteachother.TheSaltonTrough,whichstretchesthroughthestatesof
California(UnitedStates)andBajaCalifornia(Mexico),isariftvalleycreatedin
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partbytheSanAndreasFault.TheSanAndreasisatransformfaultthatmarks
theroughlynorthwardmovementofthePacificplateandtheroughlysouthern
movementoftheNorthAmericanplate.
Mid-OceanRidges
ManyofEarth’sdeepestriftvalleysarefoundunderwater,dividinglong
mountainrangescalledmid-oceanridges.Astectonicplatesmoveawayfrom
oneanotheratmid-oceanridges,moltenrockfromthemantlemaywellupand
hardenasitcontactsthefrigidsea,formingnewoceaniccrustatthebottomof
theriftvalley.
InthenorthernMid-AtlanticRidge,theNorthAmericanplateandtheEurasian
platearesplittingapartatarateofabout2.5centimeters(1inch)peryear.
Overmillionsofyears,theMid-AtlanticRidgehasformedriftvalleysaswideas
15kilometers(9miles).
InthePacificOcean,theEastPacificRisehascreatedriftvalleyswherethe
PacificplateisseparatingfromtheNorthAmericanplate,Cocosplate,Nazca
plate,andAntarcticplate.Likemanyunderwaterriftvalleys,theEastPacific
Riseisdottedwithhydrothermalvents.Geologicactivitybeneaththe
underwaterriftvalleycreatesthesevents,whichspewsuper-heatedwaterand
ventfluidsintotheocean.
ContinentalRiftValleys
Veryfewactiveriftvalleysarefoundoncontinentallithosphere.TheEast
AfricanRift,theBaikalRiftValley,theWestAntarcticRift,andtheRioGrande
RiftareEarth’smajoractivecontinentalriftvalleys.TheEastAfricanRiftispart
ofthe“GreatRiftValley”systemdiscussedbelow.
TheBaikalRiftValley(sometimescalledtheBaikalRiftZone)cutsthrough2,000
kilometers(1,200miles)ofSiberia,ineasternRussia.TheBaikalRiftValleyis
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formedbyadivergentplateboundary,wheretheAmurplateisslowlytearing
itselfawayfromtheEurasianplate,andhasbeendoingsoforabout25million
years.TheAmurplateismovingeastwardatarateofabout4to5millimeters
(.16to.2inch)ayear.
TheWestAntarcticRiftisaseriesofsmallerriftsthatroughlyseparatethetwo
regionsofEarth’ssouthernmostcontinent,WestAntarcticaandEastAntarctica.
TheWestAntarcticRiftisoneofthemostdifficultriftvalleystostudy,becauseit
liesbeneaththemassiveAntarcticIceSheet,whichcanbemorethan2
kilometers(1.2miles)thick.
TheRioGrandeRiftisaseriesofriftvalleysalongfaultsintheSouthwestern
UnitedStates.TheRioGrandeRiftseparatestheColoradoPlateau,whichis
generallymovinginaclockwisedirection,fromtheolderpart(craton)ofthe
NorthAmericanplate.TheRioGrandeRiftstretchesfromcentralColoradoto
theMexicanstateofChihuahua.
GreatRiftValley
Themostwell-knownriftvalleyonEarthisprobablytheso-called"GreatRift
ValleySystem"whichstretchesfromtheMiddleEastinthenorthto
Mozambiqueinthesouth.Theareaisgeologicallyactive,and
featuresvolcanoes,hotsprings,geysers,andfrequentearthquakes.
Today,however,theGreatRiftValleyexistsasaculturalconcept,nota
scientificone.Alloftheriftvalleysinthe“system”areconnected,butnotpart
ofasingleunit.
ThenorthernpartofthesystemistheJordanRiftValley.TheJordanRiftValley
stretchesfromtheGolanHeights,nearIsrael’sborderwithSyriaandLebanon,
totheDeadSea,totheGulfofAqaba—aninletoftheRedSeathatseparates
theSinaiPeninsulafromtheArabianPeninsula.
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AssociatedwiththeJordanRiftValleytothesouthistheRedSeaRift.Millionsof
yearsago,theArabianPeninsulawasconnectedtoAfrica.Seafloorspreading
causedtheArabianandAfricanplatestoriftapart.TheIndianOceanflooded
theriftvalleybetweenthecontinents,creatingtheRedSea.Today,Africaand
AsiaareconnectedbythetriangleoftheSinaiPeninsula.Eventually,theRed
SeaRiftwillseparateAfricaandAsiaentirelyandconnecttheMediterranean
andRedSeas.
EastAfricanRift
SouthoftheRedSeaRiftliesthemassive,complexEastAfricanRift.
ThroughouttheEastAfricanRift,thecontinentofAfricaissplittingintwo.The
Africanplate,sometimescalledtheNubianplate,carriesmostofthecontinent,
whilethesmallerSomaliplatecarriesHornofAfrica.
ThetwomajorriftvalleysystemsoftheEastAfricanRiftaretheGregoryRift
andtheWesternRift.Theseriftvalleysaredottedbyvolcanoes:ErtaAle,
Ethiopia;MountKenya,Kenya(anextinctstratovolcano);OlDoinyoLengai,
Tanzania;MountKilimanjaro,Tanzania(adormantstratovolcano);andMount
Nyiragongo,DemocraticRepublicofCongo.
TheGregoryRiftstretchesfromtheRedSeaandtheArabianSeatoasfar
southasMountKilimanjaro.OneofthemostimportantfeaturesoftheGregory
RiftistheAfarTripleJunction,foundwheretheHornofAfricastraddlestheRed
SeaandtheGulfofAdenintheArabianSea.AttheAfarTripleJunction,the
Arabianplate,NubianplateandSomaliplatearealltearingawayfromeach
other.
TwoarmsoftheAfarTripleJunctioncontinuetowidenintheprocessofseafloor
spreading—thearmextendingintotheRedSeaandthearmextendingintothe
GulfofAden.Astheseriftscontinue,thenarrowvalleycreatedbytheGregory
Rift(thearmoftheAfarTripleJunctionlocatedabovesealevel)maysinklow
enoughthattheArabianSeawillfloodit.SeparatedfromAfricabythisnew
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strait,HornofAfrica(sittingontheSomaliplate)wouldbecomeacontinental
island,likeMadagascarorNewZealand.
TheWesternRift,alsocalledtheAlbertineRift,includesmanyoftheAfrican
GreatLakes.TheWesternRiftisoneofthemostbiodiverseregionsinAfrica,
featuringanarrowcorridorofhighlandforests,snow-cappedmountains,
savannas,andchainsoflakesandwetlands.
RiftLakes
Riftlakes,formedasfreshwaterfloodsriftvalleys,oftenmarkriftvalley
systems.Morethanabillionyearsago,forinstance,theNorthAmericanplate
beganariftingprocess.Atriplejunctionformedinthemiddleoftheyoung
continent,anddeepriftvalleydeveloped.Freshwaterdrainedandcollectedin
thisriftvalley,creatingalake.Aftermillionsofyears,however,theriftfailed.
Thecontinentremainedintactandtherift’sarmsfailedtoopenupanew
ocean.Today,theremainsofthatancientriftlake,LakeSuperior,restatopone
oftheoldestanddeepestriftvalleysintheworld.
LakeBaikal,theriftlakeovertheBaikalRiftValleyinSiberia,isthedeepestand
oldestfreshwaterlakeintheworld.ThedeepestpartsofLakeBaikalare1,642
meters(5,387feet),andaregettingdeepereveryyear.Inaddition,overthe
past25millionyears,layersofsoftsedimenthaveaccumulatedonthelakebed.
Theactualflooroftheriftvalleyismorethan5kilometers(3miles)deep.Lake
Baikalalsohasthelargestvolumeofliquidfreshwaterintheworld—a
staggering23,615cubickilometers(5,700cubicmiles).
TheDeadSeaisariftlakeintheJordanRiftValley.AlthoughtheDeadSeaisnot
theworld'sdeepestlake,thedeepJordanRiftmakesitthelowestland
elevationonEarth.ThesurfaceoftheDeadSeais429meters(1,407feet)
belowsealevel,andthelake’sdepthisanother304meters(997feet).Unlike
LakeBaikal,however,theDeadSeaisnotatrueriftlakeasitwasnotformed
entirelybytheriftbeneathit.Theso-calledDeadSeaTransformisa
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geologicallycomplexarea,wheretectonicplatesinteractinmanyways.
Themostfamousriftlakesintheworldmaybetheseriesofnarrow,deeprift
valleysintheEastAfricanRiftknownsimplyastheRiftValleylakes.TheRift
Valleylakes,stretchingfromEthiopiatoMalawi,aresitesofamazing
biodiversity.Theyincludefreshwaterlakes,similartoLakeBaikal,aswellas
saltwater“sodalakes”similartotheDeadSea.
LakeTanganyika,whoselongshoresaresharedbyBurundi,Democratic
RepublicofCongo,Tanzania,andZambia,isthelargestoftheRiftValleylakes.
LakeTanganyikaistheworld’ssecond-deepestandsecond-biggest(byvolume
offreshwater)lakeintheworld.OnlyLakeBaikalisdeeperandholdsmore
water.LikemanyfreshwaterRiftValleylakes,LakeTanganyikaishometo
hundredsofendemicspeciesofcichlidfish.
LakeNatron,Tanzania,isoneoftheshallow,alkali-richsodalakesoftheEast
AfricanRift.Itsdazzlingredcolorisnotaproductoftheregion’srockygeology,
butthepinksalt-lovingbacteriathatliveinthebrinywater.
Vocabulary
Term
accumulate
Partof
Definition
Speech
verb
togatherorcollect.
systemoflakesinandaroundtheGreatRiftValley:Lake
AfricanGreat
Lakes
noun
Albert,LakeEdward,LakeKivu,LakeMalawi,Lake
Tanganyika,LakeTurkana,andLakeVictoria.Also
calledtheRiftValleyLakesandtheEastAfricanLakes.
chemicalcompoundoftenfoundasmineralsaltinsoils
alkali
noun
ancient
adjectiveveryold.
AntarcticIce
Sheet
noun
andbodiesofwater.
thickglaciercoveringmostofAntarctica.
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Term
aulacogen
bacteria
Partof
Definition
Speech
noun
failedorstuntedarmofatriplejunctioninaplate
tectonicsriftsystem.
plural
(singular:bacterium)single-celledorganismsfoundin
noun
everyecosystemonEarth.
allthedifferentkindsoflivingorganismswithinagiven
biodiversity
noun
border
noun
briny
adjectivesalty.
cichlid
noun
complex
adjectivecomplicated.
concept
noun
idea.
continent
noun
oneofthesevenmainlandmassesonEarth.
continental
island
convergent
plateboundary
noun
noun
corridor
noun
craton
noun
crust
noun
divergent
boundary
dormant
area.
naturalorartificiallineseparatingtwopiecesofland.
spiny-finnedfreshwaterfish.
landonceconnectedtoacontinentbutbrokenoffby
shiftingtectonicplates.
areawheretwoormoretectonicplatesbumpintoeach
other.Alsocalledacollisionzone.
hallway,orconnectingpassageofland.
old,stablepartofcontinentalcrust,madeupofshields
andplatforms.
rockyoutermostlayerofEarthorotherplanet.
areawheretwoormoretectonicplatesaremoving
noun
awayfromeachother.Alsocalledanextensional
boundary.
adjectivestateofminimalgrowthoractivity.
thesuddenshakingofEarth'scrustcausedbythe
earthquake
noun
releaseofenergyalongfaultlinesorfromvolcanic
activity.
elevation
noun
heightaboveorbelowsealevel.
endemic
adjectivenativetoaspecificgeographicspace.
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Term
Partof
Definition
Speech
actinwhichearthiswornaway,oftenbywater,wind,or
erosion
noun
extinct
adjectivenolongerexisting.
fault
noun
forest
noun
ecosystemfilledwithtreesandunderbrush.
freshwater
noun
waterthatisnotsalty.
frigid
adjectiveverycold.
geologic
adjectivehavingtodowiththephysicalformationsoftheEarth.
geyser
noun
glacialvalley
noun
depressionintheearthcreatedbyamovingglacier.
graben
noun
smallriftvalley.
GreatRift
Valleysystem
noun
ice.
acrackintheEarth'scrustwheretherehasbeen
movement.
naturalhotspringthatsometimeseruptswithwateror
steam.
seriesoffaultsandothersitesoftectonicactivity
stretchingfromsouthwesternAsiatotheHornofAfrica.
largepeninsulainnortheastAfrica,includingthe
HornofAfrica noun
countriesofSomalia,Djibouti,Eritrea,andEthiopia.Also
calledtheSomaliPeninsula.
smallflowofwaterflowingnaturallyfroman
hotspring
noun
undergroundwatersourceheatedbyhotormolten
rock.
relatedtohotwater,especiallywaterheatedbythe
hydrothermal adjective
Earth'sinternaltemperature.
inlet
noun
smallindentationinashoreline.
lake
noun
bodyofwatersurroundedbyland.
lakebed
noun
bottomofalake.
lithosphere
noun
mantle
noun
massive
adjectiveverylargeorheavy.
outer,solidportionoftheEarth.Alsocalledthe
geosphere.
middlelayeroftheEarth,madeofmostlysolidrock.
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Term
MiddleEast
mid-ocean
ridge
molten
Partof
noun
regionofsouthwestAsiaandnortheastAfrica.
noun
underwatermountainrange.
adjectivesolidmaterialturnedtoliquidbyheat.
oceaniccrust noun
Pangaea
noun
peninsula
noun
plateau
noun
platetectonicsnoun
rift
noun
riftlake
noun
riftvalley
noun
rivervalley
noun
savanna
noun
seafloor
spreading
Definition
Speech
noun
thinlayeroftheEarththatsitsbeneathoceanbasins.
supercontinentofalltheEarth'slandmassthatexisted
about250millionyearsago.
pieceoflandjuttingintoabodyofwater.
largeregionthatishigherthanthesurroundingarea
andrelativelyflat.
movementandinteractionoftheEarth'splates.
breakintheEarth'scrustcreatedbyitspreadingor
splittingapart.
lakeformedasaresultofplatetectonicsseparatingthe
Earth'scrustandwatercollectinginthedepression.
depressioninthegroundcausedbytheEarth'scrust
spreadingapart.
depressionintheearthcausedbyarivererodingthe
surroundingsoil.
typeoftropicalgrasslandwithscatteredtrees.
riftinunderwatermountainrangewherenewoceanic
crustisformed.
baselevelformeasuringelevations.Sealevelis
sealevel
noun
determinedbymeasurementstakenovera19-year
cycle.
sediment
noun
shore
noun
Siberia
noun
solidmaterialtransportedanddepositedbywater,ice,
andwind.
coast.
regionoflandstretchingacrossRussiafromtheUral
MountainstothePacificOcean.
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Term
Partof
Definition
Speech
lakewithahighconcentrationofcarbonatesalts.Also
sodalake
noun
staggering
adjectiveoverwhelmingorshocking.
strait
noun
stratovolcano noun
subduction
noun
supercontinentnoun
tectonic
activity
noun
calledanalkalinelake.
narrowpassageofwaterthatconnectstwolarger
bodiesofwater.
steepvolcanomadeofhardenedlava,rock,andash.
Alsoknownasacompositevolcano.
processofonetectonicplatemelting,sliding,orfalling
beneathanother.
ancient,giantlandmassthatsplitaparttoformallthe
continentsweknowtoday.
movementoftectonicplatesresultingingeologic
activitysuchasvolcaniceruptionsandearthquakes.
massiveslabofsolidrockmadeupofEarth's
tectonicplate noun
lithosphere(crustanduppermantle).Alsocalled
lithosphericplate.
boundarybetweentwotectonicplates,wheretheplates
transformfaultnoun
aremovinghorizontallyorverticallyinopposite
directions,notagainstorawayfromeachother.Also
calledaconservativeplateboundary.
triplejunction noun
regionwheretheboundariesofthreetectonicplates
meetandinteract.
valley
noun
depressionintheEarthbetweenhills.
ventfluid
noun
chemicalsejectedbyhydrothermalvents.
volcano
noun
volume
noun
spaceanobjectoccupies.
wellup
verb
toswellorbuildup.
wetland
noun
anopeningintheEarth'scrust,throughwhichlava,ash,
andgaseserupt,andalsotheconebuiltbyeruptions.
areaoflandcoveredbyshallowwaterorsaturatedby
water.
Articles&Profiles
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AmericanAssociationofPetroleumGeologists:DeadSeaGeologyPromise
Unmet
Websites
NationalGeographicScience:Valleys—NaturalDepressions
NationalGeographicScience:PlateTectonics
USGS:EastAfricaVolcanoes
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