I. Topographic mapsweb.mit.edu/12.001/Archive2010/Lectures/Lect14_topography_notes.… · that is,...
Transcript of I. Topographic mapsweb.mit.edu/12.001/Archive2010/Lectures/Lect14_topography_notes.… · that is,...
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TopographyT.Perron–12.001–April2,2010
We’llspendalargefractionofthesecondhalfofthecoursediscussingEarth’ssurface.Todaywe’lldotwothings:First,we’lldiscussthewaystopographyiscommonlyrepresentedinmaps,andhowitinteractswithgeologicstructuresatdepth.Thiswillhelpprepareyoufortoday’slab,aswellasthegeologicmappinglabyouwillbeginnextweek.Second,we’lldiscussfirst‐ordercontrolsontheshapeofEarth’ssurface,startingatthelargestscales.I.TopographicmapsThisismostlycommonsenseandgeometry,butitcanbetrickytovisualize.1.Mapprojections:waysofrepresentingasphericalsurfacein2D
• Geographic/“PlateCarree”/SimpleCylindrical:convenient(x=lon,y=lat),butgeometricallyunfortunate[PPT]
• MosttopomapsofsmallregionswillbeinUniversalTransverseMercator(UTM),orsomevariationthereon.
o Mercator:projectionontocylinderparalleltorotationaxiso UTM:projectionontocylinderperpendiculartorotationaxis,rotated
aroundEarthin60incrementstomake60slices(“zones”)[PPT]2.ContourMaps
• Contourlinesonmapso Needtoplot3Ddataon2Dsurface.Contoursarelinesindicating
constantvalueof“z”coordinate–canbeelevation,oranythingelseo Howdocontoursshow:
Slope:contourspacingreflectsgradientofz Valleys:contoursmakeVspointingupslope(upvalley) Ridges:contoursmakeVspointingdownslope(downridge) Peaks:concentricclosedcontours,withcontourvalues
increasinginward Sinks(localminima):concentricclosedcontours,withcontour
valuesdecreasinginward.Usuallymarkedwithhatches.(Whythespecialnotation,i.e.,whyaresinksuncommon?)
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• Drainageo Ingeneral,waterflowsdownslope,soflowpathsusuallyarenormal
tocontourso Riversgenerallyfollowvalleys.Riverpathscanbeidentifiedby
followingthechainofVsthatpointupslopeo Watersheds
Regionsthatalldraintoacommonpoint. Everypointonthemapfallswithinawatershed Drainagedividesaretheboundariesbetweenwatersheds.
Theyfollowridgelines,wheretheflowdiverges.SodrainagedividescanbeidentifiedbyfollowingthechainofVsthatpointdownslope,startingattheoutletofawatershedandmovingupslopeuntilthedividereachesapeak(localmaximum).
• Cross‐sectionso Canbeconstructedbydrawingatransectacrossacontourmapand
measuringhorizontalpositionwheretransectintersectseachcontour.o Thenwecandrawaplotofz(elevation)vs.x(horizontaldistance)o Verticalexaggeration
Theratioofthelengthofagivenunitonthezscaletothelengthofthesameunitonthexscale.
Topographyissubtle,soverticalexaggeration>1isoftenused,especiallyoverlongdistances.
Usuallyreportedas,e.g.,2:13.Geologiccross‐sections
• Howcanweinfertheorientationsofplanesfromthegeometryatwhichtheyintersectthesurface?
• RuleofVso Horizontalbedsfollowcontourlines[PPT:GrandCanyon]o Ifdippingbedintersectsvalley,itssurfaceoutcropwillmakeaVin
mapviewthatpointsinthedirectionofdip.[PPT:Blockmodels]o ThesharpertheV,theshallowerthedip(verticalbedsmakenoV)o Onlyexception:ifabedisveryshallowlydippingatananglelessthan
thevalleyslope,itwillmakeaVupstream,likeahorizontalbedwould• Apparentthicknessofbeds[PPT:Blockmodels]
o Themorenearlyparallelthebedistothesurface,thethickerthebedappears
o Themoreperpendicular,thethinnerthebedappears4.Howdowemeasuretopography?
• Levelingsurveys• Stereophotos• RADAR
o Altimetryo Interferometric
• Laseraltimetry
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II.Largescaletopography0.Tozerothorder,Earthisspherical.Why?
• Earthisheldtogetherbyitsowngravity• Overlongtimescales,Earthbehavesasafluid.Fluidsseektheirownlevel;
thatis,theytendtoevolvetowardhydrostaticequilibriumunlessactedonbyotherforces.
• Considerperturbationsintheshapeofaspherical,non‐rotatingbody.Theseperturbationswillinducepressuregradients,whichwilldriveflowthatrestoresthesphericalshape.
• Whethersphericalshapeisachieveddependsonpressuregradient,viscosity,
andtime.[PPT:Earthvs.Hyperion]o ViscositydependsonP,Tlargerbodiesdeformfasterevenfor
samepressuregradiento LargestirregularlyshapedmoonisHyperion(Saturn),R~150kmo Smallerones(Deimos,Phobos,asteroids,someKBOs)allirregular
1.Earthisalmostspherical.
• Oblatespheroidwithflattening(a‐c)/a=1/298.26• Ifa~6370km,thena‐c~21km• LargesttopofeaturesonEarth:
o Mt.Everest=8.8kmo MarianaTrench=‐11km
• SoEarth’sflatteningisitslargesttopographicfeature• WhyisEarthoblate?
o Theansweronceagainishydrostaticequilibrium,andthecontributionofrotationtointeriorpressures
o Atanypointintheinterior,Pnet=Pgravitation–Pcentrifugalo BecausePcentrifugalissmallerathigherlatitudes,arotatingsphere
developsapressuregradientthatdrivesflowtowardtheequator,andplanetsdeveloparotationalbulge.
o Thefirst‐ordermodelofEarth’sshapeisthereforeanellipsoid.[SKETCH:XSofellipsoid,showingequatorialradiusa>polarradiusc]
2.Earthisalumpyellipsoid
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• Anotherwaytothinkaboutthisellipsoidalsurfaceisthatitisasurfaceofapproximatelyequalgravitationalpotential.Gravitationalpotentialisafieldwhosegradientisthegravitationalaccelerationvector,g(g=‐grad(potential)=GM/r2r_hat).
• Thisellipsoidofapproximatelyequalgravitationalpotentialisthe“referencegeoid”.
• IfwemeasureactualgravitationalpotentialovertheEarth’ssurface,therearedeviationsfromthereferencegeoid(“anomalies”)ofasmuchas100m.[PPT:Geoidanomalymap].Someoftheseanomalieshaveaknownexplanation:
o SubductingslabscreatepositiveanomaliesoverAndes,Indonesiao Remnantdepressionsfromicesheetscreatesnegativeanomalyover
HudsonBayo Someanomalies(negativeoverSouthernIndia)unexplained
• Anomaliesingravitationalacceleration(thegradientofthegeoid)highlightshort‐wavelengthfeaturesevenbetter.[PPT:GravityanomaliesfromGRACE]
3.Globaldistributionofelevations
• Earth’ssurfaceelevationsarebimodal[PPT:Shadedrelief,thenPDFandCDFofelevations].Thesedistributionsareknownasthe“hypsometry”.
• Meanelevations:o Continents:+0.8km(oftencalled“freeboard”)o Oceanbasins:‐3.8kmo Whatcontrolstheseelevations?Isostaticbalance:
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o So,tofirst,order,bimodalelevationsreflectbimodalityof
compositionandcrustalthickness4.Isostasyandtopography
• Whateffectwillerosionofcontinentalcrusthaveonelevations?
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• Lesson:non‐uniformerosioncancauseuplift• ThismaybeoneofthereasonswhymarginsofTibetanPlateauarehigher
thaninterior[PPT:Satelliteimageofplateau]5.Topographiceffectsofmantleprocesses
• Isostaticrebound:time‐dependentisostaticreponse.Mantleisviscoelastic.o Upliftinresponsetoremovaloficeloads[PPT:HudsonBay
shorelines]o Ongoingsubsidenceinresponsetosedimentdepositioninlargeriver
deltas,despiterecentchangesinload(e.g.Mississippi)• Flexure:Lithosphererespondstoloadsasanelasticshell
o Troughandforebulgearoundoceanislandso Differential“warping”ofglaciallakeshorelinesfollowingremovalof
icesheetloads• Dynamictopography:deflectionofEarth’ssurfaceduetoflowinupper
mantle(=topography–isostasy).Amplitudecanbeasmuchas1km–aslargeasfreeboard!
o Mitrovica:HudsonBayupliftislessthanexpectedforpostglacialrebound,presumablyb/cofdownwellingbeneathCanada
o UpwellingbeneatheastcoastofNorthAmericamayexplainwhyAppalachiansarestillhigh,despitelackoforogeniceventsforhundredsofMyr.