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Topic:ChangesinRelativeSeaLevelandAssociatedFeatures 3.1.3.3CoastalLandscapeDevelopment Whatyouneedtoknow Eustatic,isostaticandtectonicrelativesealevelchanges Landscapesassociatedwithemergentcoastlines Landscapesassociatedwithsubmergentcoastlines Introduction: Therelativepositionoflandtoseaisrarelyconsistent.Thelandcanriseorfallover: • veryshorttimescales(tectonicupliftordownthrust) • overthemediumtermmeasuredinthousandsofyears(isostaticchange)and • overmillionsofyears(continentalseparationandcollision) Similarly,sealevelcanriseorfall: • relativelyrapidly(thermalexpansion) • overthemediumterm(globalclimatechangeandmeltingice)and • overtheverylongtermofmillionsofyears(tectonicplatemovement) Asdifferentregionsofcoastlinemaybesubjecttodifferentprocessestheconsequence isthatwhilesomecoastlinesmaybe‘emerging’fromthesea,othersmaybe ‘submerging’whileothersarechanginginbalancewithanysea-levelchangesothat thereisnoapparentalterationinthelevelofthesea. Tectonicfactors(shortterm) Acoastalregionexperiencingseismicitymayexperiencelandbeingshiftedupwards (uplift)ordownwards(downthrust)asgeologicalpressuresarereleasedandadjacent rocksadjustatfaultlines.Asaresult,therecanbearapidchangeinrelativesea-level dependingonwhetherlandhasbeenshiftedupordown.Somenotableland/sealevel changesare: • TheGreatAlaskanearthquakeof1964withamagnitudeof9.2(thesecondmost powerfulearthquakeeverrecorded)hadatiltingeffectsuchthatthecoastline200 milessouth-westofAnchoragewasleft9.1m.higher,whilesouth-eastofthecitythe coastlinedroppedby2.4m. • Over200milesoftheeasterncoastlineofHonshu,northernJapan,droppedby0.6 m.asaresultoftheTohokuearthquakeof2011,knownforitsdestructivetsunami. Isostaticchanges(mediumterm) Isostasyisthestateofcrustalmaterialfloatingontopofmorepliablemantlematerial beneath.Thenaturalbuoyancyofcrustalrocksmayseethemdepressedbytheweight ofaccumulatingiceontop,orreleasediflong-existingsurfaceicesubsequentlymelts. Isostaticsealevelchangemostcommonlyarisesfrommovementofthelandrelativeto thesea. tutor2uAQAALevelGeographyCourseCompanion(Edition1) Specification7037ForFirstTeachingfromSeptember2016 Topic:ChangesinRelativeSeaLevelandAssociatedFeatures 3.1.3.3CoastalLandscapeDevelopment • Submergingland:duringperiodsoficeadvancelandmassesinhigherlatitudes havebeenweighteddown,sinkingdeeperintothemantle,aprocesscalled compression.(Whilethismightbeexpectedtoraisesealevelrelativetotheland, somuchwaterwaslockedinicesheetsthatsealeveldroppedatafasterrate thanthelandwasdepressedduringtheglacialmaximum). • Emergingland:atthebeginningofthecurrentwarmerinterglacialicesheets startedtorapidlymeltafter18,000BCEandlandmassesbeganrisingina processknownasdecompression.Thisisthoughttotakeplaceintwophases: o Increasedbuoyancy:theearlystagesofisostaticupliftoccurastheweight oficeissuddenlyreleasedand,likeacorkthathasbeenpusheddown,it ‘bouncesbackup’. o Mantlereadjustment:amoreextendedphaseofuplift(thatisstillgoing oninnorthernScotlandasitcontinuestoriseafterthelasticeadvance) asviscousmantlethathadbeenforcedawayfrombeneaththedepressed crustveryslowlyflowsbacktoitsformerlocation. Eustaticchanges(short,mediumandverylong-term) Theseprocessesinfluencetheleveloftheseaitselfandareoftentheresultofeitherthe volumeand/ormassofseawaterchangingduetoclimaticfactors,oragivenvolume beingredistributedoverachangingsurfaceareaasaresultoftectonicplate movements. • Thermalexpansion/contraction:thebiggestsinglefactorresponsibleforsealevelriseinthelast300yearsisthoughttobetheexpansionofseawaterasit becomeswarmerduetoglobalclimatechangesincethestartoftheIndustrial Revolution.Warmerwateroccupiesalargervolume(despiteanunchanged mass). • Meltingice:thesecondlargestfactor(whichmaybecomethemostsignificantin thenextcenturyortwo)istheincreasingmass(andalsovolume)oftheworld’s oceansaswaterisredistributedfrompolaricesheetsandcontinentalglaciersas theymeltatanincreasinglyrapidrate. • Oceanbasinexpansion:ascontinentsshiftapartopeningoceanbasins,suchas theexpansionbetweenAfricaandSouthAmerica,thelargerareatobeoccupied byafixedmass/volumeofseawatermaycausesealeveltofall.(Effectively,itis spreadoutthinner.)If,however,submarinevolcanoesandoceanridge expansionoccupiesagreaterarea,sealevelrisestoreflectthelargerlithological masswithinit.(Thesameashappenstoyourbathwaterwhenyougetinit). Keypoint:isostaticfactorsarespecifictoparticularregionsofcoastlinewhileeustatic factorsoperategloballyacrosstheworld’soceans.Asaconsequence,somecoastlines tutor2uAQAALevelGeographyCourseCompanion(Edition1) Specification7037ForFirstTeachingfromSeptember2016 Topic:ChangesinRelativeSeaLevelandAssociatedFeatures 3.1.3.3CoastalLandscapeDevelopment maybeemergingwhileatthesametimesomearesubmerging,andothersachievinga stateofdynamicequilibriumwithachangingsealevel. Historicsealevelchanges • 18000BCEtheglobalsealevelwasaround120mbelowcurrentlevelsasso muchwaterwaslockedupinicesheetsandglaciers. • Sealevelsroseveryrapidlyasicesheetsmeltedwithasmuchasa10mrisein just500yearsafter17,000BCE.(equivalentto20mmperyearonaverage) • Aslowergeneralrisewaspunctuatedbypulsesofmuchfastermeltingandsea levelrise.Thepulseof6000BCEisthoughttohaveadded6.5minlessthan140 yearsandfinallyseparatedtheBritishIslesfromtheEuropeancontinent. • Sealevelstabilisedaround1000BCEuntilthe19thcentury.Sincethenithas beenrising,withanaveragerateof1.8mmperyearduringthe20thcentury. • Projectionsoflikelysealevelriseby2100rangefromalowestimateof0.28mto ahighof0.98mon1990baselinelevel,buildingonthecurrent3.4mm./yrrise. Featuresofemergentcoastlines. Feature Description Whereseen Raised Abeachthatwasatsealevelbutis Commononthewestcoastof beach nowwellaboveit. Scotlandasaresultofisostatic recovery Elevated Awavecutplatformthatnowexists (asabove) marine asanextensiveflatareainfrontofa platform relictcliffaboveactivewaveaction. Seacave and/or arch Featuresofpastcoastalerosionthat arenowwellabovetheactivecoastal zone. (asabove) tutor2uAQAALevelGeographyCourseCompanion(Edition1) Specification7037ForFirstTeachingfromSeptember2016 Topic:ChangesinRelativeSeaLevelandAssociatedFeatures 3.1.3.3CoastalLandscapeDevelopment Featuresofsubmergentcoastlines Feature Description Whereseen Ria Arivervalleyatthecoastthathas SouthcoastofDevon beeninundatedbytheseaasrelative (Dartmouth)andCornwall sealevelhasrisen,resultingina (Falmouth) ‘drownedvalley’. Fjord Aglaciatedvalleyatthecoastthathas WestcoastofNorway. beeninundatedbythesea.Has steeperrisingwalls,astraighterlong profileandisdeeperthanariadueto glaciersover-deepeningthevalley. Dalmation Aseriesoflongitudinalrivervalleys AlongthecoastofCroatiainthe coastline nexttothecoastthathavebeen north-easternMediterranean. inundatedleavingparallelrangesof watershedridgesasislands. Humanusesofthesefeatures: • Riasandfjordsoftenoffershelteredestuariesofrelativelydeepwaterinwhich ships,fishingvesselsandpleasure-craftmayfindsafeharbour. • Evidenceofpastrelativesealevelchangessothatcurrentratesofchangemaybe understoodandmeasured. Coastalmanagementissues • Relativelyrapidshort-termrisesinsealevelasaresultofthermalexpansionand ice-sheetmeltarelikelytocausemorefrequentfloodingoflow-lyingcoastalareas. Futurecoastalplanninghastoconsidertheimpactsthismayhaveandwhether adaptationpoliciesarerequired. • Asdifferentcoastlinesmaybeaffectedbyrelativeeustaticsealevelriseatdifferent ratesaccordingtotheirisostaticcontext,coastalmanagementpoliciesneedto assesstherateoflikelyfutureriseonacoastalregion-by-regionbasis.Aroundthe coastsofmembercountriesoftheEUrelativesealevelrisetakesplaceatdifferent ratessopolicyisgearedtoidentifyingcoastlinesingreatestneedofprotectionand prioritisingactionforthoseareas. tutor2uAQAALevelGeographyCourseCompanion(Edition1) Specification7037ForFirstTeachingfromSeptember2016