Download Topic: Changes in Relative Sea Level and Associated Features 3.1

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