)oJ
ISOSTASY
In Encyclopedia of Coastal Science, (M.L. Schwartz, Ed.), Springer, The Netherlands, 2005.
ISOSTASY
Numerous observations point to a complex and changìng rela-tionship
between land and sea surfaces throughout geological time ln some
localities elevatedcoral reefs, wave-cut rock platforms. and molluscs
embedded in their original marine sediments attest to past sea levels
having been higher thãn present. At other sites, drowned forests and
submJrged siteõ of human occupation point to sea levelshaving been
locally lower than present. These observations representa measureol
iilative sea-levelchìnge which can involve a land-movement signal-as
well as an ocean-volulmesignal. The indicators of submergedor elevated coastlines therefore póint to one of three occurrences:land has
moved up or down, ocean volumes havechanged,or both haveoccurred
sinrultaneously.
Tectonic piocess operating within the earth have.caused uplift-and
subsidencetirroughout the Eãrth's history, resulting in relativesealevel
change on a widé range of spatial and temporal scales They'include
uplifiand subsidencea-[conveigent plate margins where,therelativesealeiel change is usually episodic anã abrupt but cumulative over long
periods of"time resultíngìn, for example, the^marinem.olluscbeds high
in the Andes of South America thai were first described by Charles
Danvin. The tectonic processesalso include slower and longer-duration
eventssuch as the initiation of continental rifting and seafloor spreading rvith the concomitant changes in the displacement of water by the
ãe"uelopingocean ridge syst.m.'Long-term thermal contraction of the
.ãoiinê oi,., layers ãl áewl-v creatãd ocean crust at the ocean ridge
resulrs-in sea-flóor subsidenóe. creating basins into which sediments
u..urnulu,., thereby magnifying the subiidence' Larg€ volcanic edifices
stressthe earth andcausè mórelocal subsidenceand deformation of the
e a r t h ' ss u r f â c ei n t h e v i c i n i t y o f t h e l o a d
At the same time that thi tectonics events shape the earth's surface
and shift the relative positions of land and sea surfaces,oceanvolumes
oito .ttong., largely becauseol climate-driven changesin the extent of
g l a c i a t i o r i o i t h õ p i a n e t . D u r i n g e x t e n d e dc o l d p e r i o d sl a r g ei c e s h e e t s
i b r m . e x t r a c t i n gù a t e r t i o n r t l i J o c e a n s a n d l o w e r i n g s e al e v e l sA s t h e
climate rvarms úp sutEcientl-vto melt the ice sheetssea levelsagain-rise'
Such glacial cycies have occirrred at intervals throughout much of the
earth'; history but they have been most signiÊcant during recent ttmes'
the Qu¿terna;y perioá. for which the recõrd has not yet been wholly
overprinted by the subsequent tectonic and Iand-shapingevents'
Tire combiied result oi the tectonics and glacial cyclesis a sea-level
signal thar has varie<lsignificantly in time as wèll as being-seographically
va-riable.The record of this variability is, however, far from complete,
and to be able to model and predict the migration of coastlines,an
understanding and separation'of the underlying causesof sea-level
ctrange ls essãntial.Isóstatic processesare key elementsin this undeistanding and separation.
t':;: :il:"
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ISOSTASY
566
by an elasticlayerover
casesthe isostaticmodelsareusuallyrepresented
halfspaceo¡ in the caseof globalproblems,by
a viscousor viscoelastic
of theearth'scrustand lithosphere-theupper, sphericalshellmodelsof an elasticlithosphereovera-viscoelastic
manIsostasyis the tendency
effectivily elasticlayerof the earth-to adjustits verticalposition when tie and fluid co¡e.Both lithosphereand mantle may be representedby
loadedaf its surfaceby, for example,ice,water,volcanos,or sediments. somedegreeof layeringin physicalproperties(elasticmoduli- r'iscosity,
to a goodapproximation and density).Formulaiion-ofthesesphericalresponse
modelsare well
For this purpose.theearthcanbe represented
as a spheriiallys¡mmetricbody with a fluid core of about 3'400km developed-and
solutionsfor the surfacedeformationunder complex
radius.The upperlayeris calledthe lithosphereand includesthe crust.
Its thicknessiJtt'picallybetween50 and 150km,varyingwith the tecb1'being
tonic history of the region.The litliosphereis characterized
Distance(km)
loâd stresses
whensubjected-to
relativelycold and to behaveelastically
2000
lO00
belowa criticalfailurelimit. The nrantle,betweenthe lithosphereand
core,is at a temperaturethat is relativelycloseto the melting point of
I ¡ooo
with characAs a resultthemantleflowsviscously,
terrest¡ialmaterials.
.ã¡ 2000
years,
when
to
non-hydrosubject
104-105
times
of
relaxation
teristic
staticstress.It is this zonationof a "rigid" lithosphereover "r'iscous"
I 1000
mantlethat givesvalidityto the isostaticmodels.
0
models
of isostasyisby "local" response
The simplistrepresentation
-200
a load of heights/r.denB
of Archin'redes'principle:
whichareótatement
of the undersitv o. placedon theearth'ssurfaceresultsin a subsidence
lviiÅ sürfaceof ô = /iplpn,,wherep,n,the densityof themantle.exceeds
0
(seeFigure122(A)).This modelassumes
the ãensityof thelithosphere
(or
has
failed
shear
strength
has
no
that the crust or the lithosphere
?, zoo
underthe load)and ove¡liesa fluid mantle.This model,whileunrealisusefulfo¡ estimatingmagnitudes.of
is nevertheless
tic in many respects,
crustaldehectionbeneathloads.For exanrple,under a 3km thick ice
400
:
sheetthe crustis predictedto deflectby about I km. A morereasonable
''elastic"
the
both
by
supported
the
load
is
l¡-ri'hich
is
s¡g
model
600
strengthof the crust-lithophereand by the buoyancyfo.rcesat the base
of thé layer(FigureI22(B)).In this n'rodel,the mantlealsobehavesasa
to^
descriptionof the earth's.response
fluid and it providesa reasonable
800
loadswith time constantsthat are longerthan the relaxationtimesof
the mantle. Thesemodelshavebeenextensivelyused to representthe
160
c
loadsor to volcanicloads.They are
of the earthto sedinrent
response
120
models.
isostatic
regional
to
as
referred
usüally
yearsanyload-generWhenthe loaddurationis of the order 103-105
â 8 0
that havepropagatedinto the mantlewill not haverelaxed
ated stresses
=
9 ¿ n
and the viscositl'of the n'rantlemust be takeninto account.In these
The isostaticprocess
;
E -¿o
!- -to
.2
o -t2o
-160
€
eoo
õ
ã
400
5
200
õ
ã.
0
1000
2000
Dist¡nce(km)
Fisure123 (A) Radialcrosssectionof axisymmetricice sheet.(B)
DËformation
of the earth'ssurfaceunderthe ice that hasloadedthe
earthfor 20,000years(curve12+). At 12,000yearsa8othe load-)is
The initial responseis elastic(curve12
removedinstantaneouslv.
creep,the surfacebeingjhown
andthis is followedby úiscoelastic
at 10,000vears(10), Ó,000yuatt, and 5,000yearsago. (C)The
as the deflection
attractionof thé ice load, represented
eraviiationãl
ðf the eeoid(i).and the chansein geiodfrom the changein the
planetísraviwdue to the defórmationof earthunderthe load
fi¡).rheiesulú areshownfor a periodbeforeunloadingstarts.
(D)The relativesea-levelchangè,due to the combinationof crustal
In (A) the
Fieure122 Models(A) local isostasy(B)regionalisostasy.
volume
loãd is supportedby the buoyancyforceaithe baseofthe crustor lith- deiormation,chaneein sravitalionalattraction,and ocean
chaneelone aftertle loãd hasbeen removed'The sealevel is
osphere,whereasin (B)the load is alsosupportedby the elastic
ñith respectto its presentposition.lf the a coastline
the
exorðssed
createdin this layer.As the load diameterin (B)increases
stresses
folmednearthe cdnterof theiload soonafterthe ice melted,it would
that of local
at the óenterof the load approaches
isostaticresponse
now be at nearlv800 m elevation.
isostasy.
ì:.t+a
:, :i v'¿
:: vþ;
;. -.?'t
ISOSTASY
exist.FigureI23 illustratesan exampleof sursurfaceload geometries
face deformation where a large-diameteraxi-symmetricice sheethas
beeninstantaneouslyremoved.The rheology(viscositystructure)of the
planetis realistic(seeFigure 127,below)and theresultsindicatethat the
crustalreadjustmentcontinuesfor thousandsof yearsafter the unloading is completed.
In addition to the surfacedeformation,the gravityfield of the planet
also changesunder the load: the shapeof the envelopecontainingthe
massis modifledby the deformationand materialis redistributedwithin
this envelope.At the sametime thereis a redistributionof the material
on the surface:sedimentsare transportedfrom mountainsinto basins,
or the meltwater from land-basedice sheetsflows into the oceans.
Surlacesof constant gravitational potential-surfaces on which the
perpendicular-therefore,changewith time
gravityvectoris everyuvhere
is the
evolve.Onesuchequipotential
astheloadand planetaryresponse
geoid,the shapeof the ocean.(If theoceanis not an equipotential
surface then the gravity vector has a component along the surfaceand
oceancurrentsresult until an equilibrium stateis reached;thus in the
absenceof winds and other perturbing forces,the oceanwill be an
the
surface.This is calledthegeoid.)Figure123illustrates
equipotential
changein the equipotential surfaceresulting from the unloading. It
includesa contribution from the surlaceload itself-the ice "attracts"
the oceanwâterand pulls the oceansurfaceup aroundit (curveli)-and
a contributionfrom theeartht deformation(curvei). Theillustrationis
for the period while the ice is intact and whenmeltingstartsboth curves
will evolvewith time.
The examplein Figure I23 illustratesthat relativesealevelchange
The
resultingfrom the removalof the icesheetcontainsseveralelements.
crust is displacedradially, the oceansurfaceis deformedby the redistribution of surfaceand internal mass.and water is addedto the ocean.
The reboundresulting lrom the melting (or growth) of the ice sheetis
The water addedto (or withdrawnfrom)
referredto as glacio-isostasy.
the oceanshasits own isostaticeffectand is ¡eferredto ashydro-isostasy.
processes
a¡e of globalextent.
The combinedglacio-hydro-isostatic
The melting of an ice sheetin one location modiûessealevelglobally,
of
notjust by changingthe amountof waterin the oceanbut because
the planet'sisostaticresponseto the changingsurfaceload of ice and
suchas by sedimentsor volcanicloads,
water.Other loading processes,
Also, thesetectonic
are usually more local in their consequences.
processgenerallyoccur on longer time scalesso that the mantle
responsecan usually be approximatedas a fluid, and the local or
regionalisostaticmodelsaremostlyappropriate.
Glacio-isostasy
Ice sheetsrepresentsurface loads that reach radii in excessof l,000km
and thickness approaching 3 km. These loads are lar-ee enough to
deform the earth and to produce substantial changesin sealevel as illustrated in Figure I23. Glacio-isostasy is the major cause of sealevel
change in areas of former glaciation. When a large ice sheetmelts the
rebound of the crust is of lar-eer amplitude than the rise in sea level
resulting from the addition of the meltwater to the oceans (typically
120-130 m. seeFigure I29 below) from all of the ice sheets.If  ( is the
changein volume of ice on land and lo the areâ of the ocean, then this
seconoslgnal ls
- 11l-4-9lrr¡¿¡
P"J
A o ( r )d t
where p;, p,..are the densities of ice and water, respectively,and both lo
andÅVi are functions of time. This contribution is referred to as the iceequivalent sea-levelchange.
Becauseof the viscosity of the mantle, the crust continues to rise
long after the ice has vanished and sea level appearsto have fallen since
deglaciation.This is seenin the Gulf of Bothnia and northern shoresof
the Baltic Sea, as well as in the Hudson Bay area of northern Canada.
For theselocations near former centersof glaciation the rebound signal
dominates and the observed sealevel curves are characteristic relaxation curves (although only the post-glacial part of the change is
recorded) (Figure I24 (Angermanälven)). Near the ice margins the
rebound is reduced in magnitude and may become comparable to the
rise resultin-ef¡om the increasein ocean volume. Now the time dependence of the sea-levelchange becomesmore complex, with its character
depending on the rel¿rtiveimportance of the two contributions. In
Figure l24(Andøya), lor a site just within the ice-sheetmargin, the
rebound initially dominates but later. becauseof the melting of other
and distant ice sheets, the ocean volume increase becomes the dominant
factor and sea level rises until a time when all ice sheetshave melted. The
remaining signal is a late stage of the relaxation process and sea levels
continue to fall up to the present.
t;
--,þ":
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The rate and magnitudeof the seaJevelchangeis a function of the
earth'sviscosityand the ice history: of the duration of the ice load, of
its areal extend,and of its thickness.The importanceof the rebound
phenomenonis that it providesa meansof estimatingthe earth'srheology: if climate modelsand geomorphologicalobservationsconstrain
the ice geometrythrough time, then observationsof seaJevelchange
providea constrainton the mantle viscosity.If the ice modelsare not
sufficientlywell-known then it becomespossibleto learn something
about the ice sheetsas well. Figure I25 illust¡atesobservationalresults
for sealevelchangeacrossScandinavia.Here, the ice sheetreachedits
maximumat about20,000yearsago and most melting occuredbetween
about 16,000and 10,000yearsago. As the ice retreated,coastlines
descriptionof
formed on the emergingland providing a comprehensive
the ¡eboundacrossnorthern Europe.The rebounddid not ceaseat the
time meltingceasedand coastlineshavecontinuedto retreatin formerly
glaciatedregionsup to the present.This can be seenin tide gauge
recordsacrossthe Baltic, with presentsea-levelfalling locally at rates
approachingI cm/yr in the northern part of the Gulf of Bothina.
Figure 126illustratesthe rate of crustalreboundand to obtain relative
sealevelchangethesevaluesmust be increasedby about l-1.5 mm/yr.
Coastlines
herecontinueto retreatdespiteother factorsthat may contribute to an increasein global oceanvolume.
the
Glacio-isostasy
doesnot ceaseat the ice sheetmargins.Because
by the changingsurfaceload is constrainedwithin
mantleflow generated
undera growingload
a deformableshell,whensomeareasaredepressed
othersare uolifted.The latter areasform a broad zoneor swellaround
on the sizeof
the area of glaciationof amplitude that may,dependin-e
the ice sheet,reacha few tens of mete¡s.When the ice sheetmelts this
peripheralswellsubsidesand for islandor coastlineson it sealevelwill be
seento be risingat a ratethat is overand abovethe ice-volumeequivalent
relicice
contribution(FigureI24(StoreBælt)).Beyondthe Scandinavian
marginsthis occursin areasof the North Seaand as far awayas the
westernand centralMediterraneanand here the sealevelcontinuesto
rise evenwhen all melting has ceased.Beyondthe North Arnericanice
sheetthis zoneof recentcrustalsubsidenceand marine floodingoccurs
asfar awayas the southemUSA and Caribbean.
Observationsof sealevel within and beyondthe former ice margins
A typprovidethe principalsourceof informationon mantleviscosity.
Europeis illust¡atedin FigureI27wherethe
icalresultfor northwestern
providesa goodconstrainton theviscosity
of the
reboundphenomenon
uppermantle.Themain featuresof theviscosityprofileincludea lithosphereof thickness
65*75km, a relativelylow valuefor theviscosityof
viscosity
and increasing
the mantleimmediately
belowthe lithosphere,
with depth,particularlyat a depth of about700km. Analysesfor differentregionsproducecomparableresultsalthoughactualvaluesfo¡
of thepossimay differbecause
theviscosityand lithosphericthickness
of such
bility that the rheologyis laterallyvariable.The determination
areasin glacio-isostasy.
variabilityis oneof the importantresearch
oneof thekey
Whiletheglacio-isostatic
modelsarewellunde¡stood.
of the forknowled-ee
limitationsof their applicationis the inadequate
mer ìce sheets.The ice margins at the time of the Last Glacial
by geomorMaxinrum,some20,000yearsago,areusuallywell-deflned
phologicalmarkersbut the timing of their formation is not always
known. This occursparticularly where the ice marginsstood offshore
and left few databletracesof both the time of their formationand of
their retreat.Also, the ice thicknesscannot usuallybe inferredfrom
evidence
aloneand is inferredinsteadfrom glaciological
observational
help
can nevertheless
and climatemodels.The sealevelobservations
constrainthe icemodelsin importantways.Thus,the total icevolumes
in the modelsfor all the major ice sheetsmust yield a globalsealevel
curvethat is consistentwith the changesobservedfar from the icesheefs
(seehydro-isostasy).
Also, detailsin the ice modelscan alsobe derived
from the sealeveldatafrom siteswithin and nearthe formericemargins.
The shapeof the sea-levelcurye from a near-marginsite (Figu¡e I24)
quiterapidlywìth distancefrom the formericemargin,with the
changes
signalevolvingfrom that for a central-loadsiteto that for a siteon the
peripheralswell,and observations
acrossthe margincanconstrainthe
region.Oneof themore
forme¡icedistributionwithin the ice-marginal
is the useof this sea-level
recentresearch
di¡ectionsin glacio-isostasy
durand c¡ustal-rebound
evidenceto improvemodelsof the icesheets
phase.
ing the lastdeglaciation
Hydro-isostasy
As ice sheetsmelt, the additionalwater enteringthe world'soceans
throughtheelasticlitharepropagated
loadsthe seafloor,load stresses
osphereinto the mantle, the newly stressedmantle material flows
The shapeand
towardunstressed
regionsand the seafloor subsides.
holding-capacity
of the oceanbasinis therebymodifredand the ocean
wateris redistributed.
changinssealevel.This adiustmentof the earth
; rli
¡i; =.1;
ISOSTASY
568
2
0
1
5 1
ûme
0
5
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i
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lr
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n
lr¡
t.r
southwesternFinlmd
f
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variabilityintheresponse.Theicesheet
changeforsitesinScandinaviaillustratingsomeofthespatial
Fieurel24 Observedrelativesea-level
and spreadonto ihe Cerman,Polish,and Russianplains.Retreatstartedat about 18,000yearsa8o.and the tinal
all of Scandinavia
co"vered
to the radiocarbontime scalewhich
of ice occurredai about i 0,000 yearsago.Thetime scaleusedin theseplotscorresponds
disappearance
- 1.1-l .15 calendaryear).
d¡ffdrifrom a calendartime scaleby about1O-i5%foithis interval(1 Crayears
,:.,; i:!:
;:. n?i
l:.; ;.y:,
;; 77r
569
ISOSTASY
"
300
200
t00
0
-100
200
Fisure126 Presentratesof crustaluplift (in mm/yr)of Scandinavia
on reboundmodelsand on observedratesfrom tide gauges
ba"sed
of
acrossthe region(from Lambecket al., 1998;with permission
BlackwellPuålishins).
150
r00
o
a
o
o
o
r000
J
2000
l0le
1020
l02l
rcn
ß23
viscosity
tisure 127 Profileof mantleviscosity(in unitsof Pas) inferredfrom
data(C Kaufmann,
sea-level
glãcialreboundanalysis
of European
with permission).
theadditionalload hasbeensufficientto modifythe shapeof theearth'
This is a result of the long wavelengthnatureof the waterload Loads
of dimensionslessthan the thicknessof the lithosphereare supported
Time(x 1,000yr ar)
mainly by the strength of the lithosphereand the resulting surface
defo¡mationis small.But large-dimensionloadseffectivelyseethrough
the
glaciofrom
change
to sea-level
contributions
Figure125 Schematic
the lithosphereand are supportedby the much more ductile mantle
from
in oceanvolume
drivencrustalreboundand increase
isõstatically
which flows evenunder smali changesin the stressfreld.
meltwater.(A) Fora locationneara formercenterof glaciationwhere
deformationof theearth's
At continentalmarginsthehydro-isostatic
the rebound(i) exceedsthe risein sealevel(ii) from ihe addedmeltsurfacedescribesquite complexpatternsbecauseof the geometryof the
water,(iii) ¡s the total change.(B)Fora locationnearthe formerice
load. The lithosphereactsas a continuouselasticlayeror shelland the
but continentalmarþinis draggeddown by the subsidingoceanlithosphere
magnitude
areof comparable
marginwherethetwo contlibutions
of oóposites¡gn.(C)Fora locationbeyondthe ice marginwherethe
but, becauseof the asymmetryof the load,not by the sameamountas
The effectof the waterload
crusiåluplitt Ë replacedby subsidencê.
therefore,the subsidence
in mid-ocean.At the continentalcoastlines,
(i).
(ii) now is importäntaswéll asthe ice-loadeffect The meltwater
At thesametime,someof the
will belessthan it wouldbein mid-ocean.
(iv).
contributionis givenby (ii)and thetotalchangeby
mantle material flowing away from the stressedoceanicmantle flows
beneaththe continentallithosphere,causingminor uplift of the interior.
The net effect of the oceanvólume increaseis a seawardtilting of the
under the time-dependentwater load and the concomitant sealevel continental margin which will be seenas a vaúable sea-levelsignal
change is referred to as hydro-isostasy.Since the onset of the last acrossthe shelf.ihis éffect is clearly seenfor tectonicallystablecontideglaiiation, sealevelshaverisen on averageby about 120-130m and nents that lie far from former ice sheets,as in the caseof Australia.
2
;;: :r,:é,
0
1
5
1
0
5
0
ISOSTASY
570
While the ice sheetsare still melting the donrinant sealevel signal here is
from the increasein ocean volumè and the glacio- and hydro-isostatic
àffects are secondorder. But when melting ceasesthe on-going isostatic
effects come into their own. Now sea level appears to be falling at the
coastal site as the oceanwaters recedeto fill the still-deepeningocean' In
consequence,small sealevel highstan'I are left.behind with peak am-pìitudes åf l-3 m occurring at the time global melting ceased(Figure I28)'
Such highstandsa.e co-mon features along many continental margins
ãnd maiifest themselvesas relic shorelines or fossil corals above the
oresent formation level or habitat. If the coast is deeply indented. sites
ät the heads of gulfs, being furthest away from the water load. -experience sreatestupiift while õffshore islands experienceleâst uplift This
differãntial rnouetnentprovides a direct meâsure of the viscositl'of the
mântle acrossthe continental malgin.
Like the glacio-isostaticeffect, the water lo¿d does not only deform the
surfaceof tñe eanh, it also resultsin a redistribution of massand a change
in sravity and in the shape of equipotential surfaces.The total sea-level
cliäges-associatedwith-the hydro-isostasyinclude, therefore,both the
crus!ãl radial deflection and the associated geoid change.Also, the glacioand hydro-isosratic effects are closely linlced when^ their cause is the
deelaciation of the last ice sheets. Near the edge of the ice sheets, for
exãmple.the water is pulled up (Figure I23) and the waterloadis increased
above wiiat would résult from a uniform distribution of the meltg'ater
over the entire ocean. Here the hydro-isostatic signal is a function of the
mamitude of the glacio-isostatið effects. Elsewhere. the broad zone of
iruital rebound suirounding a large ice sheet may occu¡ in an oceanic
environment. Then, when the ice sheetmelts this swell subsides,increasing
the volume of the ocean basin, water is u'ithdrawn from other parts of the
ocean, and a further global adjustment of sea level occurs Thus' the
treatment of hydro-glãcio isostasy requires a global and consistent formulation that ensurei that thesevarious interactions are included'
The hydro-isostatic signal is an on-going one even-when major meltins of the world's ice shèetsceasedabout 6,000-7'000 years ago' Thus
seãlevel change today will contain a small but not insignificant component of hydro--isostaticorigin (cf. Figure I28 fo¡ the Austraiian region)'
This signil must. of course be superimposed upon -any other changes,
includi-ngpossibleglobal warming signals. The resultsindicate that sea
levels aiound the Australian margin are slowly falling under the
combined glacio-hydro-isostatic response to th-eÌast melting of the
larse ice sñeets (with the possible exception of Tasmania where the
slaãio-isostaticeffect of Antarctic ice volume changesbecomessignifrõant, canceling out the hydro-isostatic signal such that little overall
change now occurs). Similar isostaticeffectswill be present:ìt all coastline, increasing in magnitude as the locality approaches the regions of
former glaciation.
The i;portance of the sealevel observations far fronr the ice margins
is that beiause the glacio-hydro isostatic effects are relatn'ely small
(10-15% of the total-signal) ihey provide an estimate of the change-in
ìolume of the oceanJwhen corlected for the isostatic effects: the
observedsea level, less the isostatic correction yields the ice-equivalent
sea level defined above and hence an estimate of the change in ocean
volume Atrl¡.Several long records, extending-back to th.eLast Glacial
change exist which provide er-idencefor
Maximum,'of local sea-l-evel
ihe changein ice volume since this time. They-indicate (Fìgure I29) that
maximuir ice volumes globally were (50-55)xl0('km'grÈater than
today but, they do not indicate necessartlywhere thls extra lce was
,tor.d. To'r.toÍue that issuerecourseto the study of the glacio-isostatic
processlrom formerly glaciated regions is necessary'
Sedimentand volcanicloading
of sedimentoccur along many of the continental
Large accumulations
a thicknessof 10kn¡ or more The rate
maftns reaching,in someinstances.
oi aäumulatlotiisusuallyslowand continuous,occun'ingover periodsof
of sedinentscomingfrom contiænsof millionsof yearswith the sources
havecauseduplift and erosion
nentalinteriorsv/heretectonicprocessed
to the sea.Examplesincludethe Bay
havecarriedthe sediments
processes
ãi nengut,the northwesternnrargins9f E¡¡9qe, the easternmargin of
Ñoi*t Ã*.ti.u, and the Gulf of fuexico. Thick accumulationsof sediof the lithosphereunderthe
of the subsidence
mentsareþossiblebecause
an ocean
srowingsédimentload.With the abovemodel of local isostas.v
leadto a nraximum
sedimentsupply,
Ëasinoî depthd" can,with adequate
This
subsidence'of
4plpr-p.) whðrep. is the densityof sediments'
:
=
ttratttte'bäsinii uitimatelyfilled to sealevel'For do 4 km, p,
assumes
that canbe
:
of
sediment
thickness
g
maximum
cm3the
3'5
Z.Sg
P.
"-t, ít ätout iO*km.However,in this casethe deepersedimentswill
=.4 km, u'hereasthe
"ttiin.¿
tt"ui ¡..n depositedin water depthsinitially o1.do
of the faunapresérvedin the basinsedimentsusuallyindicharacteristics
cate that depositioninvariably occurred-in- relatively shallow waters'
but it
iio*uty alon'e,therefore,cannotproducethick sedimentsequences
that is the resultof other processes:
doesact asan amplifierof subsidènce
in this casemosfly the thermal contraction of oceanlithosphereas it
cools from an iniiially hot layer formed at the oceanridges and then
movesawayfrom the heatsource.
sedimentloadingcanleadto substantialcoastal
On shoritimescales,
subsidence.This may occur in conjunction with deglaciation cycles
wheresedimentsareerodedfrom the continentsduring the deglaciation
stageand deliveredto coastalenvironmentsat somelater stageAn.iuäpt. of suchsubsidenceoccursalong the US coast of the Gulf of
M"*iôo, particularlyfor the Mississippidelta.Here,coastalsubsidence
o."ur, áitut.t appioaching10mm/yr and are attributedin part to the
but.alsoin part to.the
deliveredsediments,
isostaticresponsèìo.ecently
eìtriction of flui¿t from thê sedimentsand the associatedcompaction'
Here,asin most isostaticproblems,severalfactorswill contribute to the
signal.
observed
of the crustprovidesanotherexampleof isostasyat
Volcanic-loading
work. Large volcaãiccomplexesform on the seafloor, and elsewhere'
becauseoiupwelling conväctioncurrentsin the mantle that lead to an
i"¡.iti* of magmaiîto the crustand ultimatelyonto the surfaceasvolcanos.The manllesourceregionsfor the magmaappearto be longJived^
;;ã;. th. lithospheremovei overthe earth'ssurfaceunder the forcesof
The Hawaiian
flate tectonics,á trail of volcanosis left on the surface'
ähain providesthe type example.Orher examples.includethe Society
Island chainwhosecurrent centerof volcanicactivity liesto the eastof
Tahiti. The subsidenceof the lithospherebeneaththe volcano is adeouatelvdescribedbv the regionalisõstaticmodel in which the load is
suppoitedby the elásticstreiseswithin the lithosphereand by the buoyof the elastic
ancyforcea[ the baseof the layer(Figu¡el22(b.))'.Because
oroíerties of the lithosphereimall pèripheralbulges,concentricabout
ih.i.nt.t of loading,developand any islandslocatedin this zoneat
the time of volcanoãeveiopmentare uplifted by sometens of meters'
Án example of this is piovided bv 1tr9 u.pf¡gd atoll that forms
l
4
æ
1500
130e
110p
t2@
ü"nde.soir island,southeâstof Pitcairn island' This smallisland about
200km from the volcanic island of Pitcairn, appear to have been
margin
Australian
years
the
around
ago
Fieure128 Sealevelat 6,000
upiift.a some 20-30 m at the time of Pitcairn's formation, Plihap9
margins
the
tilting
of
as a
illistratinethe effectof hydro-isostasy
Zö0,000y""tt ago. The location of the zone of maximum.peripheral
Sealevelsare presentrelativeto presentmeansea
of the conitinents.
upíft próvidesi-tasut" of the flexuralwavelengthof the lithosphere,
rateof change
level.Contourintervalsare 1.2 rir.The present-day
u'pu.á.n"t.t that characterÞesthe physical responseof the layer to
in mm/yrgivenapproximatelyby dividing the contourvaluesby
loåding. With time, some relaxation of the loading stressescan be
most
value
for
negative
resulting
the
sign,
the
ihañeine
6 and
expectãdto occur within this layer suchthat the isostaticstateevolves
alone.
from
isostasy
level
in
sea
a-fall
in-dicãting
locations
; ;-j1:
"J):: !!,.)
t|,a:. V..ta:,
ä:;-: :i'kt
ISOSTASY
iltüÌË
Bibliography
*
Lambeck, K., and Johnston, P., 2000. Responseto "What about
asthenosphereviscosity?By W Fjeldskaar" GeophysicalJournal
International,142:277-281.
Lambeck,K., 1988.Geophysical
Geodesy:TheslowDeþrmationsof the
Earth. New York: Oxford University Press.
Lambeck,K., Smither,C., and Ekman, M., 1998.Theseof glacial
rebound modelsfor Fenoscandinaviabasedon instrumentedseaand lakelevelrecords.Geophysical
Journal,135:375-387.
Peltier,WR., 1998.Postglacialvariationsin the levelof the sea:impliReviewsin
cationsfor climatedynamicsand solid-earthgeophysics.
36: 603-689.
Geophysícs,
Watts,4.8.,2001.Isostasyand Flexureof the Lithosphere
Cambridge
UniversityPress.
t
ufit
+
Eo r o
ä
o
¡I
-60
hlf
.r+
tf
!)
-80
!
P
#
o -tm
+
Cross-references
ChangingSeaLevels
CoastalChanges,Gradual
CoastalChanges,Rapid
CoastalSubsidence
Endogenicand ExogenicFactors
Eustasy
Geodesy
GlaciatedCoasts
Ingression,
Regression,
and Transgression
Paleocoastlines
Sea-Level
ChangeDuring the Last Millennium
Sea-LevelRise,Effect
Submerged
Coasts
Tidal Datums
Uplift Coasts
-t20
0q
o
o
e -2o
o
4
o
q
571
o
-40
o
o
)
¿Jm
o
ó
Ë -80
¡i -r00
zci'
a
F
o
ò
54
l)
lime (x 1,000yr er)
Figure129 Sea-levelchangefor the past20,000years.(A)A record
of-observedlocal relativeiea-levelchangefrom Barbadosand other
Caribbeansites,and (B) isostatically
corrãctedsealevelfrom
globallyand combinedintoa single
a numberof sitesdistributed
ice-equivalentsea-levelcurve-.Scalêon the right handsidegivésthe
corresponding
changein volumeof ice on lañdand groundðd
on
snailowseailoor.
slowly from regional to local isostasyand that the volcanoslowlysubsides.Thus Tahiti, a relativeyoungvolcanicload of about 1-2 million
years,may be subsidingat a rateof about0.2mm,/yror less.
Theseexamplesof verticalmovementsdriven by sedimentor volcanicIoadingillustratethe interactionthat occurbetweenthe various
isostaticcontributionsto sea-level
chanse.To estimaterâtesof tectonic
uplift or subsidence,
heightsof identifia6le
coastlines
aremeasured
with
respectto presentsealevel.Thus,the fluctuationsin sealevelof glacioisostaticorigin must be known,but thesefluctuationsareinferredfrom
the same observationalevidence.An important researcharea is to
developmethodsfor separating
out theseehects,throughobservational
rmprovements and through improved modeling of the physical
processes.
Suggested
furtherreadingon this subjectmay befoundin thefollowing bibliography.
Kurt Lambeck
;:. v.Yx
!,:i
V.1?.r
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lrri
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