Introduction:WelcometoMars!
JamesR.Zimbelmana,*
,DavidA.Crownb,W.BrentGarryc, andJacobE.Bleacherc a SMITHSONIANINSTITUTION,W ASHINGTON,DC,UNITEDSTATES b PLANETARYSCIENCEINSTITUTE,TUCSON,AZ,UNITEDSTATES c NASAGODDARDSPACEFLIGHTCENTER ,GREENBELT,MD,UNITEDSTATES * CORRESPONDINGAUTHOR.E- MAILADDRESS:ZIMBELMANJ @SI.EDU
1.1Introduction
Peoplehavewatchedared“wandering”objectinthenightskyformillennia,wondering whatitcouldbe.Itsdistinctiveorange-red(ochre)color(Fig.1.1)mademanycultures associatethismoving“star”withwarfare,andMarsisnamedaftertheRomangodof war.Today,weknowthatallofthese“wandering”starsareplanetsorbitingtheSunjust asEarthdoes,butMarscontinuestobetheplanetthatmostoftencapturesourattention andourimagination(asinthewell-knownstoriesbyH.G.Wells,E.R.Burroughs,andR. Bradburyorincountlesssciencefictionmoviessincethe1930s).Increasinglysophisticatedspacecrafthavebecomehumanity’sroboticemissariestothe“RedPlanet,”taking ourfascinationwithMarsoutoftherealmofsciencefictionintothatofsciencefact.These spacecraftdatahaverevealedabundantevidencethatMarsishometosomeofthemost dramaticandamazingvolcanoesinoursolarsystem,thesubjectofthisbook.
HowdidaplanethalfthesizeoftheEarthproduceenormousvolcanicmountainslike OlympusMons (Fig.1.2),somethingmanytimesthesizeofthelargestvolcanoesonEarth? WhyaretheMartianvolcanoeslocatedwheretheyare?Dovolcanoesincloseproximityhave thesameeruptivehistoriesandweretheyactiveatthesametime,orweretheredifferenteruptionstylesinthesameregionindifferentgeologicepochs?Questionssuchastheseareexamplesofthemanyissuescurrentlybeinginvestigatedunderthebroadumbrellarepresentedby theterm comparativeplanetology.Today,wehavesomeunderstandingofalloftheplanetsin thesolarsystem,thankstothemanyspacecraft missionslaunchedfromEarthduringthelast halfcentury.Theseexplorationshavediscoveredthatvolcanismisaubiquitousgeologicprocessacrossthe terrestrial (rocky)planetsandeventoanextremeonthebizarremoonofJupiternamedIo.Intheoutersolarsystem,watertakestheplaceofmoltenrock,aprocesscalled cryovolcanism.However,amongallofthesevolcanicworlds,therelativelydiminutiveplanet Marshassomeofthelargestvolcanoestobeseenanywhere.Throughthisbook,wewilltake you,thereader,onafantasticjourneyofexplorationtothemanyvolcanoesofMars.
Thejourneybeginswithabriefreviewofhowscientistsandengineershavesteadilyobtained increasinglydetailedinformationaboutMars.Subsequentchapterswillfocusonthevolcanic
FIG.1.2OlympusMonsvolcano.ShadedreliefrenditionsofOlympusMonsonMars(NASAMarsOribterLaser Altimeterdata)andtheBigIslandofHawai’i(upperleft;NASAShuttleRadarTopographyMissiondata).Both imagesareshownatthesamescale.
historyoftheRedPlanetbydiscussingseveraldistinctvolcanicprovinces,emphasizingboth familiaranduniqueaspectsofeachregion.The goalisforthiscompilationofinformation toprovideacurrentsynthesisofourknowledge ofMartianvolcanoesandtoallowthereader tocompareandcontrastMartianvolcanoeswith themanyvolcanoesthathavebeenstudiedin greatdetailhereonEarth,aswellastovolcanoesnowknownthroughoutthesolarsystem.
1.2LearningaboutMars
Theancientswerekeenobserversofthenightsky.Over2500yearsago,BabylonianastronomersregularlyrecordedhowMarsmovedamongtheseemingly“fixed”stars,andChineseastronomersdocumentedthatMarsoccasionallymovedina retrograde direction (thereverseofitsnormalmotion)forweeksatatimebeforereturningtoitsmoreregular motion(Bakich,2000,pp.169–171).Exoticideasweredevelopedtoexplainthisperplexing behavior,whichbothJupiterandSaturnalsoexhibited,buttoalesserdegreethanthat demonstratedbyMars.CarefulmeasurementsofMarsbyTychoBraheallowedJohannes Keplertodevisehisfamousthree“laws”ofplanetarymotionin1600,thefirstofwhich statesthatplanetsfollow elliptical (noncircular)orbitswiththesunatonefocusofthe ellipse,thefirstmathematicaldescriptionofaplanetaryorbit. ScientificinvestigationofMarsbeganinearnestfollowingGalileo’s1610publication thatlettheworldknowthatthetelescopewasawonderfulnewtoolforexploringthe heavens.TelescopessoonrevealedthepresenceoflighteranddarkerregionsonMars, butperhapsevenmoreimportant,Marsdidnotexhibitphasessimilartothoseseen
monthlyforEarth’sMoon,unlikewhatGalileo’stelescopealsorevealedforVenus.These earlytelescopicobservationsprovidedobservationalsupportforCopernicus’modelof thesun-centeredsolarsystem,withVenusclosertotheSunandMarsfurtherfromthe SunthanwastheEarth.Astelescopesbecameevermorepowerful,Marsshowedvariations initssurfacefeaturesthatrepeatedduringthenearly2EarthyearsittakesforMarstomake onerevolutionaroundtheSun.Eventually,brightpolarcapsweredetectedontheplanet, includingpartsthatremainedyear-round,whileotherpolardepositsgrewandshrank throughouttheMartianyear.Inthe1780sSirWilliamHerschel(theastronomerwhodiscoveredtheplanetUranus)usedsuchobservationstosuggestthatMarsexperiencedseasonssimilartothoseofEarth(Bakich,2000,p.183).OccasionallythewholeglobeofMars becameauniformochrecolorwithnosurfacedetaildiscernable;thiswaseventually attributedtomassiveduststormsthatattimesobscuredtheentiresurfaceformanyweeks.
TelescopicobservationsofMarsarebestobtainedaboutevery26Earthmonths,when Marsisat opposition (directlyoppositefromtheSunasviewedfromtheEarth),butthe apparentsizeofMarsattheseoppositionsvariessystematicallybecausetheorbitofMars ismoreellipticalthantheorbitofEarth.The1877oppositionwasaparticularlygoodone, andGiovanniSchiaparellimadeadetailedmapofMarsthatincludednumerousstraight darklinesacrossthebrightregions.Hismapwaspublishedin1890withthelineslabeled “canali”(meaninganaturalchannelorgrooveinItalian),butthiswordwaslooselytranslatedintoEnglishas“canals,”whichimpliedfeaturesconstructedbyintelligentbeings (Bakich,2000,p.183).PercivalLowellexpandedontheconceptofMartiancanalsin his1895booktitled Mars,championingtheideathatMartiansgloballyengineeredthe planettobringwaterfromthepolarregionstoparchedequatorialdeserts(Fig.1.3).
FIG.1.3LowellMarsglobe.Marsglobe(500 diameter)withhand-drawnobservationsrecordedbyPercivalLowell in1901.GlobewasonloanfromLowellObservatorywhileondisplayattheNationalAirandSpaceMuseum.
Untilhisdeathin1916,LowellusedhispersonalobservatoryinFlagstaff,Arizona(which remainsanactiveresearchcentertoday),tomakemapsoftheextensiveMartiancanal system,andhepublishedmorebookstopopularizehisinterpretationthatadvancedintelligentlifeexistedonMars.Thecanalsremainedunseenbymostothertelescopic observers,butLowellwasundeterred.ThepossibilityofadvancedlifeonMarsremained popularuntilthefirstspacecrafttoflypastMars(Mariner4,in1965)returned22imagesof amostlycrateredsurfacereminiscentofEarth’sMoon.
VolcanoesenteredtheMarsstoryin1971when Mariner9 becamethefirstspacecraftto orbitanotherplanet.ThespacecraftarrivedatMarsduringthemostintenseglobaldust stormindecades,butcommandsfromEarthkeptitfromstartingitsglobalmappingmissionuntilthedustbegantoclear.AsthedustpallgraduallysettledoutofthethinMartian atmosphere,fourdarkspotsappearedinMarinerimagestakentomonitortheprogressof theduststorm(Fig.1.4).Withcontinueddustsettling,thespotssoonresolvedintoelevatedregionseachwithcomplexcratersattheirsummits.Itdidnottakescientistslong todeducethattallmountainswithcratersattheirsummitsweremostlikelyvolcanoes. Oncetheatmospherefullycleared, Mariner9 mappedtheentireMartiansurfaceataspatialresolutionfarexceedingwhatwaspossiblewiththelargesttelescopesonEarth,giving humanitythefirstdetailedlookatthescopeofthegeologyofMars.Thisglobalmapping effortrevealedthatthefour“spots”werethesummitsofthelargestvolcanoesthenknown, aswellasfindingmanyothervolcaniccentersscatteredacrosstheplanet(Mutchetal., 1976,pp.36–39).SubsequentspacecraftorbitingandlandingonMarshaveprovided increasinglydetailedinformationabouttheMartiansurface;thisincrediblewealthofdata formsthebasisformuchofwhatisdescribedinthisbook.
FIG.1.4Mars’volcanoesrevealed.Four“darkspots” (arrowed) werethefirstsurfacefeaturesseeninMariner9 imagesastheglobalduststormof1971begantodissipate.Thespotsarethesummitsoffourenormous volcanoes.At upperleft isOlympusMons(see Fig.1.2);thethreealigneddarkspotsaretheTharsisMontes. Extremecontraststretchingoftheseimagescausedthewhite“echoes”aboveandbeloweachdarkspot.Subtle dustcloudstructuresareevidentthroughoutthisimagemosaic. NASAhttps://www.hq.nasa.gov/office/pao/History/ SP-4212/ch9-4.html.
1.3Geology
Interestinvolcanoesandvolcanismhasalonghistorybecausemanycultureswantedaway toexplainwhyriversofmoltenrockoccasionallyappearedfromtheEarth(Macdonald, 1972,pp.26–41).OneofthebetterknownlegendsinvolvestheHawai’iangoddessoffire, Pele,whotraveledfromislandtoisland(startingatNi’ihauandmovingsoutheast),eventuallysettlingintotheHalemaumaucrateratthetopofKilaueavolcanoontheBigIslandof Hawai’i(Beckwith,1970;Cashman,2004;Westervelt,1916;Roberts,2018).Thedirectionof Pele’sislandmigrationisconsistentwithmoderndatingofvolcanicrocksonthedifferent islands;today,weexplainthisobservationthroughthemotionofEarth’srocky lithosphere aboveadeep-seated“hotspot”(see Section1.7).However,beforedelvingintomodernconceptsofvolcanism,weshouldfirstconsiderseveraldifferenttypesofrocksthatareimportanttounderstandingthestorybehindvolcanoes.
Geology isthescienceoftheEarth,arelativenewcomertogeneralscienceslike physics,chemistry,andbiology.Foralongperiodoftime,thecollectionofrockswasconsideredtofallwithintherealmofthehobbyist.In1669NicolasStenoformulatedtheprincipleof superposition,whichstatedthatrockswereemplacedinatemporalsequence withtheolderrocksbeneaththeyoungerones(PressandSiever,1974,p.46).James Hutton,andlaterCharlesLyell,usedtheobservedsequenceofemplacementinferred fromobservationsofwhichrockslieontopofotherrockstodeducethatgeologicevents occurred“uniformly”throughtime,whichLyellpublicizedastheprincipleof uniformitarianism (PressandSiever,1974,pp.61–62).Thisrelationshipbecameinadequatewhen itwasrecognizedthatsomelayeredrocks,assumedtohaveoriginallyformedinahorizontalorientation,weretodaytiltedtodifferentdegrees,eventothepointthatsomerocks wereturnedcompletelyupsidedown.
Whenfossilswererecognizedtoberemnantsofpastlifepreservedintherocks,they becameacrucialtoolfordefiningstratigraphicsequencesofrocks.Fossil-bearingstrata areasubsetofthemoregeneral sedimentary rocktype.Sediments(fineparticles)are depositedaftersettlingoutofeitherwaterorair,bothmediumsthatcantransport sedimentslongdistancesfromtheirsources.Sedimentaryrockscoverabout75%ofthesurfaceofthecontinentsontheEarth(HamblinandChristiansen,1998,p.106),sotheyare likelytherocksthatmostpeoplethinkoffirst(whentheythinkaboutrocksatall,a situationthatwehopewillbemuchencouragedbyreadingthisbook).TheGrandCanyon (Arizona)isoneofthebest-knownexposuresofsedimentaryrocksonEarth,wherethe upper800mofthecanyonexposesastratigraphicsequencerepresentingmorethan300 millionyearsofEarth’shistoryandthelowerpartofthecanyonextendstimebacknearly 2billionyears,althoughmanyofthoselowerrocksarenotsedimentaryrocks.
TwoimportantsystemsaffecttheEarthtodepositorchangetherocksnearitssurface: the hydrologic system(acomplexcyclethroughwhichwatermovesfromtheoceansto theatmospheretothelandandbacktotheoceans)andthe tectonic system(themovementofsolidrockneartheEarth’ssurface)(HamblinandChristiansen,1998,pp.32–42). Sedimentaryrocksresultfromseveraldifferentmechanismsworkingwithinthe
hydrologicsystem,andthetilting,folding,andfaultingofsedimentarystrataaretheresult offorcesactingwithinthetectonicsystem.Sometectonicforcescanburyrockstovarious depthswithinthecrustwhereincreasedheatandpressure,alongwithchangesinthe compositionoffluidsthatmaymovethroughthoserocks,alterthemineralsintheoriginal rocktogenerate metamorphic rocks.Thethirdmajorrocktype, igneous,formsfrom magma (amoltenmixtureofliquidrockmaterial,gas,andsolidcrystals);ifmagmasolidifieswhilebeneaththesurface,itformsa plutonic (intrusive)rock;ifthemagmareaches thesurface,itbecomesa volcanic (extrusive)rock,theprimaryfocusofthisbook.Tectonic forcescanopencracksandfissureswithinthecrustthroughwhichmagmareachesthe surfacetoproducevolcanicrock.Igneousrocksrepresentafundamentalcomponent oftheEarth’scrustasthevolcanicoriginofmostoftheoceanfloorrocksbecameknown. Whensubjectedtoweatheringanderosion,igneousrockscontributeparticlesthatsubsequentlybecomeincludedinbothsedimentaryandmetamorphicrocks.
1.4Volcanism
WhenrockwithinEarth’sinteriorishotterthanthemeltingtemperatureofitscomponents,thisliquidrockbecomesthesourcematerialforigneousrocks(magma).Magma tendstorisewithinthecrustbecauseitislessdense(morebuoyant)incomparisonwith thesurroundingrock.ChangingtemperatureandpressureconditionsbeneaththeEarth’s surfacecanalterthechemistryofmagmaasbothsolids(crystalsthatsolidifyoutofthe coolingmelt)andgases(volatilesoriginallydissolvedintheliquid)escapefromtheevolvingliquid.Asequenceofspecificmineralsformsasthetemperatureofthemagmadrops, withmineralsheavier(moredense)thanthemagmasettlingtothebottomofthemagma poolandmineralslighter(lessdense)thanthemagmarisingtothetopofthemagmapool. Thedepartingmineralsremoveelementsfromthemagmathroughtheprocessof fractionalcrystallization,thebasicmechanismforchangingthechemistryofthemagma. Asfractionalcrystallizationprogresses,itproducesdifferentkindsofigneousrocks.
Themajorrocktypesgeneratedfromevolvingmagmathroughthisfractionationprocessare,inorderofdecreasingtemperature,volcanicrocksthatrangefrom komatiite, basalt,andesite,dacite, to rhyolite andtheirintrusiveequivalentsrangefrom peridotite, gabbro,diorite,granodiorite, to granite (see Section8.2).Thedominantmineralswithin eachvolcanictype,indecreasingorderofabundance,areolivineandpyroxeneinkomatiite;plagioclase,pyroxene,andolivineinbasalt;plagioclase,pyroxene,andamphibolein andesite;andpotassiumfeldspar,plagioclase,quartz,andbiotiteindaciteandrhyolite (see Fig.8.2).Variationsintheorderofthecrystallizationandtherelativeabundance ofmineralcomponentsoccurwithintheintrusiveequivalentsofeachvolcanicrock,as prolongedconditionsatdepthallowfordiversechemicalseparationstotakeplace.The aforementionedisagreatlysimplifiedrenderingofacomplexsequenceofevents;interestedreadersarereferredtoHamblinandChristiansen(1998,pp.77–100)foraveryreadableelaborationonthegenerationofvolcanicrocks.Seismicstudieshaveshownthatthe uppermostpartoftheEarthisdividedbetweenanoutercrust(consistingofbothdense
FIG.1.5LavatexturesonHawai’ianbasaltflows.(A)Smooth,glassypahoehoe,witha50-cm-widesheetflow extrudingbeneaththecooledcrustofanearlierflow.Thenewlyexposedlavarapidlychills,formingagrowing glassycrust.PortionofthePKKflowonKilauea,Feb.20,2005;USGS/HVOphoto20050220-0584_CCH.(B)Clinkery ’a’aflowmarginemplacedonanearlierpahoehoeflow,withthehotinteriorcoreexposed.Heatingofthe atmospheredistortstheimagefocusabovetheflow.EastbranchofthePKKflowonKilaueaatPulamaPali, scenewidth8m,Feb.25,2005;USGS/HVOphoto20050225-0786_TO.
oceanic[basaltic]andlightercontinental[granitic]rock),alloverlyingtheupperpartof Earth’spartiallymoltenmantle(PressandSiever,1974,p.24).
Whenvolcanicrocksareeruptedontothesurface,thateruptioncantakeplaceeither effusivelyorexplosively.Effusiveeruptionsform lava,withdifferentvolcaniclandforms resultingfromthedifferingchemistryandtherelated viscosity (the“stickiness”ofthe flowingliquid)ofthesourcemagma.Withinlavaflows,thesolidifiedrockprovidesclues totheconditionofthemagmawhenitwaserupted.InHawai’i,itispossibletowatch activelavaflowsduringtheiremplacement.Consequently,Hawai’ianwordsdescribe thetwodominantflowsurfacetexturesusedinthevolcanicliterature. Pahoehoe lava hasasmooth,glassycrustproducedbylow-viscositymoltenrockthatisslowlyextruded ontothesurface(Fig.1.5A).’A‘a lavahasaroughsurfaceproducedbycountless“clinkers,” eachwithfineglassspinesorshardscoveringtheirexteriors(thissharpglassrapidly chewsuphikingboots!);attheirfronts,’a‘a’flows(Fig.1.5B)movefasterthanpahoehoe flows.Differencesbetween’a‘aandpahoehoederivefromtherateoferuptionofthelava, withpahoehoeassociatedwithlowvolumeperseconderuptionsanda‘ahighervolume persecond(RowlandandWalker,1990).ThetwotexturetypesrepresentedbytheHawai’iantermshavetheirequivalentsinothercultureslivingonvolcanicterrain(e.g.,inIceland “helluhraun”and“apalhraun”aretheequivalentofpahoehoeanda‘a,respectively; Gudmundsson,1996).Flowsofmoreviscousandesiticorrhyoliticlavascanform blocky flows,wherethelavaisbrokenintoangularblocksrangingfrommanytensofcentimeters tometersinsizewiththickerflowsandamoredomicalflowshapeduetothehigherviscosity.Aswiththechemistrysummarizedearlier,therearemanyvariationsonthebasic flowtexturesjustdescribed;theinterestedreaderisreferredto Macdonald(1972,pp. 71–98)and Gregg(2017) formoredetail.
FIG.1.6Volcanicconstructs.Profilesofacompositevolcano(stratovolcano),a(small)calderaonavolcano,ashield volcano,andfourexamplesofpyroclasticcones,allshownat2 verticalexaggeration. Modifiedfrom Siebert,L.,Simkin,T.,Kimberly,P.,2010.VolcanoesoftheWorld,3rded.UniversityofCaliforniaPress.
Lavaflowsbuildupintoconstructsaroundtheirsourcevent,someofwhichcanattain enormousdimensions.KomatiitesarequiterareonEarth,butwheretheyarefound,their productswereextremelyfluid,forminglongthinflowsratherthanlargenear-ventconstructs.BasaltsarethemostabundantvolcanicrockontheEarth(oceanfloorsareprimarilybasalt,coveredbymud),forminglongtopographicridgesonoceanfloors,aswellas basalticlavaflowsandmanyvolcanoesonEarth’scontinents.Whenbasaltseruptat thesurfacefromalong-activesourcevent,theycanproduceabroadmountainaround thecentralventwithflankslopesgenerally <5° andwithanoverallshapesimilartothat ofanoldVikingshield,hencethename shield volcano(Fig.1.6).TheHawai’ianandGalapagosIslandsformedfromcoalescingshieldvolcanoes,makingthemamongthelargest volcanoesonEarth,butbothislandcomplexesaredwarfedbytheenormousbulkofthe OlympusMonsshieldvolcanoonMars(Fig.1.2).
Shieldvolcanoesrepresentonetypeoflargevolcanicconstruct(onethatiswell expressedonMars),butthereareothervolcanoesthatalsoenterintothediscussionof thevolcanoesonMars.OnEarth,iftheeruptinglavaismorechemicallyevolvedthan basalt(acompositioncalledandesite)andtheflowsareintermixedwithpyroclastic deposits,thevolcanohassteepslopes(around10°),steeperthantheslopesonashield volcano,producingtheconicalshapethatmostpeopleassociatewithvolcanoes.Thistype ofconstructresultsfrommoreviscousextrudedlavaflowscombinedwithexplosiveeruptionsthatgenerateanabundanceofvolcanicparticlesofvarioussizes(includingfinegrained ash);theresultisa composite volcano,alsocalleda stratovolcano (Fig.1.6).While smallerinvolumethanshieldvolcanoes,someofthebest-knownvolcanoesonEarth, suchasFujiinJapanandVesuviusinItaly,arecompositevolcanoes.Inthefollowing, wewillseethatthesevolcanoesoccuratgeologicsettingsthatarenotcommononMars. Ifthemagmachamberfeedingeruptionsatthesummitofthevolcanobecomessufficientlyemptied,thesurfaceofthevolcanocancollapsetoformalarge,generallycircular depressioncalleda caldera (Fig.1.6).Largecalderascanalsobeproducedbecauseofvery largeexplosiveeruptions,oftenfrommagmasmoresilicicthanthesourceofbasaltflows; see Francis(1993,pp.291–321)formoredetailaboutthecomplexitiesofcalderasandgeneralinformationaboutexplosivevolcanism.Whenthemagmabecomeschemically
evolved(andalsocontainsmorevolatiles)beyondwhatproducesanandesiticcomposite volcano,massiveexplosiveeruptionscanproducethicksheetsofrhyolitic pyroclastic (see nextparagraph)depositsaroundlarge(>10kmdiameter)calderas,depositsthatareso largethattheydonothavemuchreliefoutsideofthecaldera.Suchexplosiveeruptions causesomeofthemostvoluminousvolcanicdepositsonEarth,likethoseassociatedwith theTobaeruptioninSumatra,Indonesia(Zielinskietal.,1996).Thereiscontinuingdebate astotherolethatsuchlarge-volumeexplosiveeruptionsplayedonMars(see Section7.4). BeforetheadventofspacecraftmissionstoMars,someresearchersusedtelescopicobservationstosuggestthattheshapesandseasonalchangestothedark(lowalbedo)regionsof Marsweretheresultofwindblownvolcanicash(McLaughlin,1955).Ifandesiteorrhyolite lavadoesnoteruptinlargeexplosions,thenavolcanic dome composedofthick sequencesofhigh-viscositylavacanresult.
Whenthevolumeoferuptedmaterialislessthanthatassociatedwithshieldorcompositevolcanoes(whichformoveramultitudeoferuptivecycles),smallvolcanicconstructsare generated.Smallvolcanicconstructscanoccurinisolatedsettings,oncalderafloors,on volcanoflanks,oringroupsassociatedwithlavaflows.Theproductofexplosiveeruptions, regardlessofthecompositionofthesourcemagma,iscalleda pyroclastic (“fire-broken”) deposit;thesecanbeemplacedasacoherentflowoverthesurfaceorviaballisticemplacementfromorthroughsettlingofparticlesoutoftheatmosphere.
Themostcommontypeofsmallvolcanoisa scoriacone (Fig.1.6),(alsoreferredtoasa cindercone)whereasingleeruptionspreadsvolcanic scoria (typicallyrangingfrom graveltocobblesize)aroundtheeruptivevent;theeruptedscoriafollowsballistictrajectorieswhileflyingthroughtheairandafterlandingpilesupalongaslopeclosetotheangle ofrepose(theangleabovewhichgranularparticlescascadedownslope).Consolidated pyroclasticdepositsarecalled tuff (Macdonald,1972,p.134).Wheneruptinglavainteractswithnear-surfacegroundwaterwithoutexcavatingintothebedrock,acindercone-like tuffcone orabroadlow-profile tuffring results(Fig.1.6),dependingonhow muchwatergetsmixedinwiththeeruptinglava(Francis,1993,pp.342–345).Ifsuch aneruptionoccursalongacoastline,whereoceanwaterinteractswiththeeruptinglava, theresultisa littoralcone.Whenlavainteractswithgroundwater,theresultingsteam builtupgeneratesalocalizedvolcanicexplosionringcalleda maar,whichoftenexcavates intotherockunderlyingtheexplosivedeposit(Fig.1.6)(Francis,1993,pp.341–347).When lavaflowsoverwetground,suchasaroundthemarginofalake,a pseudocrater canresult, alow-profile“rootlessvent,”sonamedbecausetheexplosionsoccurwherethesteamis generatedbeneaththeflowratherthanattheventwherelavareachedthesurface(Francis, 1993,pp.151–152).
1.5Platetectonics
OurunderstandingofEarthhistoryunderwentahuge“paradigmshift”inthe1960swhen geologicalandgeophysicalinformationfrommanydifferentsourcescouldfinallybe placedwithinabroadconceptualframeworkthattodayisknownasthetheoryof plate
tectonics (PressandSiever,1974,pp.24–31).Thedevelopmentofthistheoryisacomplex story(see HamblinandChristiansen,1998,pp.442–469,fordetails),butitemergedfrom anearlierconceptthatmetwithgreatresistancefromthescientificestablishment,anidea termed“continentaldrift.”Almostassoonasmappingtechniquesbecamepreciseenough toaccuratelyshowtheoutlineoftheworld’scoastlines,manyearlynaturalhistorians(the scienceofgeologydidnotyetexist)notedthattheAtlanticcoastsofAfricaandSouth Americahadverysimilarshapes.Afewwentsofarastosuggestthatthesetwocontinents werejoinedatsomepointinthepast.
TheGermanmeteorologistAlfredWegenerpublished(in1915)anexhaustivecollectionofdatatosupporttheideathatcontinentswerepreviouslyjoined,includingseveral inadditiontoAfricaandSouthAmerica,butnobody(includingWegener)proposedaviablemechanismtoexplainhowthecontinentscouldbemoved.Thissituationchanged quitesuddenlywhenabundantevidencefrommultipledisciplines,includingpaleontology(thestudyofancientlife),geology(thedistributionofrocktypesandstructures),glaciology(depositsfrommultipleepisodesofcontinentalglaciation),paleoclimatology(the recordofpastclimatespreservedinrocksandsediments),seismology(thestructureof Earth’sinteriorobtainedfromearthquakerecords),oceanography(thefirstsystematic mappingoftheoceanfloors),andpaleomagnetism(orientationsofEarth’smagneticfield preservedinrocksofdiverseages),couldbestbeexplainedbythemovementofbroad sectionsofEarth’scrustascoherentpackagescalled plates,consistingofboththechemicallydistinctcrustandtherigidupperportionofthemantle(togethercalledthe lithosphere).Themechanismbehindthiscrustalmovementfinallycouldbeexplainedas theinteractionbetweentheslowcirculationwithinthepartiallymoltenmantleandthe lithosphereridingalongontopofthesebroadinternalcirculationpatterns.
Crucialnewevidenceforthemovementoflargecrustalplatescamefrommappingthe patternof polarity (indicatedbythedirectiontowardmagneticnorth)preservedinthe rocksonbothsidesofenormousmountainridgesdiscoveredonthefloorofseveralocean basins,includingthelongest mid-oceanridge locatedintheAtlanticOceanbasin.Careful mappingofthemagneticpolaritypreservedinrocksectionsfromseveralcontinents clearlydemonstratedthatEarth’smagneticfieldreverseditspolaritymanytimesintemporallyvariablebutgeographicallyconsistentways;thissamepolaritypatternwaspreservedsymmetricallyonbothsidesofmid-oceanridges.Theagesoftheoceanfloor rockswerealsoshowntosteadilyincreasesymmetricallyawayfromtheridgesonboth sides.Themostreasonableexplanationforalltheseobservedpatternsisthatnewcrust formedatthemid-oceanridgesandthenprogressivelymovedawayfromthem.
Ifnewcrustwasbeingformedatmid-oceanridges,crusthadtodisappearsomewhere elsetopreserveEarth’smassandvolume.Deep oceantrenches werediscoverednearthe marginsofseveralplates,withcompositevolcanoesoftenfound60–100kmawayfromthe trenches,onthesideofthetrenchawayfromthenearestmid-oceanridge.Theareasnear thetrenchesbecameknownas convergent marginswhereacrustalplatedisappearedinto themantleata subduction zone,whilethemid-oceanridgespreadingcenterswerecalled divergent margins.Insomeplacestheplatesslippedpasteachotherwithinzonesof
FIG.1.7Volcanoesandplatetectonics.Volcanoesresultingfromdifferingplatetectonicsettings. ModifiedfromaUS GeologicalSurveydiagraminSimkin,T.,Tilling,R.I.,Vogt,P.R.,Kirby,S.H.,Kimberly,P.,Stewart,D.B.,2006.This dynamicplanet:worldmapofvolcanoes,earthquakes,impactcraters,andplatetectonics.U.S.Geol.Surv.Map I-2800,scale1:30,000,000.https://volcano.si.edu/learn_dynamicplanet.cfm.
enhancedseismicactivity,suchasalongtheSanAndreasfaultinsouthernCalifornia; theselocationswerecalled transcurrent (orstrike-slip)faultmarginswhereneithercrust growthnorcrustdestructionwastakingplace.Themarginsofthecrustalplatescorrespondcloselytothemajorityofseismicallyactivezonesidentifiedaroundtheplanet, asdomostoftheworld’sactive(orrecentlyactive)volcanoes.Platetectonicscantherefore explainmuchofwhathadpreviouslyseemedtobeunrelatedgeologicfeaturesscattered aroundtheplanet,particularlytheassociationofmanyvolcanoeswithplatetectonic settings(Fig.1.7).
Neithersubductionzonesnorspreadingcenterridgesareconfinedtotheedgesofcontinents.Wheretwooceanicplates(lackingcontinentalcrust)collide,asubductionzone occursnearanarcofvolcanicislands,leadingtothename islandarc.Thevolcanoesassociatedwithislandarcs,suchastheAleutianIslandssouthwestofAlaska,aretypically andesitecompositecones,muchlikethechainofactivevolcanoescomprisingtheCascadesinwesternNorthAmericaandtheAndesalongthewesternedgeofSouthAmerica. Spreadingcenterscansometimesoccuroncontinents,suchasalongtheEastAfricanRift Valley,wherevolcanismisabundant.
Notallactivevolcanoesoccuralongplatemargins.Inparticular,somevolcaniccenters showaclearageprogressionalongonedirection.Withincreasedprecisioninthetracking ofplatemotions,theseseeminglyisolatedvolcaniccenterswereshowntobeexpressions oftheplatesmovingabovea hotspot whoselocationwasstablerelativetothedeepinterioroftheplanet(Fig.1.7).BoththeHawai’ianIslands(withtheassociatedHawai’ianEmperorseamountchain)andtheGalapagosIslandsareexamplesofhotspotvolcanic systemswherethemostrecentvolcanicactivityoccursclosesttothedeep-seatedsource ofthehotspot(Poland,2014).Hotspotvolcanoesalsocanoccuroncontinents,suchas theprogressivelyyoungervolcanismleadingtotheYellowstonevolcaniccenter,withits
abundantgeothermalgeysers.Icelandrepresentsauniquesituationwhereahotspot happenstobelocatedbeneathamid-oceanridge;itistheonlyplaceweknowofwhere mid-oceanridgevolcanismoccursabovesealevelandcanbeeasilydocumented (Gudmundsson,1996).
PlatetectonicsformsaunifyingtheoryforEarth,butdoesitrelatetovolcanismon Mars?EvidenceforplateboundarieswassearchedforasMarsgeologywasrevealed throughsteadilyimprovingglobalimaging,butnocompellingcasecouldbemadefor widespreadplatetectonicshavingtakenplaceonMars.AnexceptionistheartfulinterpretationofvolcanismandtectonisminandaroundthelownorthernplainsofMars, whichwasinterpretedtoindicateoldplatetectonicprocesses(Sleep,1994),butthis hypothesishasnotbeenvalidatedbysubsequentresearchers.Gravitymeasurements forMars,obtainedfromorbitingspacecraftwithsteadilyimprovedradiotrackingcapabilities,providedarobustindicationofthecrustalthicknessacrosstheplanet;thenorthernlowlandsdohavesomeofthethinnestcrustonMars,butitisstilltensofkilometers thickthere,andcrustalthicknesselsewhereis >80km(Zuber,2001; Neumannetal.,2004).
AthalfthediameterofEarth,Marslostitsheatmuchfasterthandidourhomeplanet; unlikeMars,Earthhadsufficientinternalheatresourcestosupportactiveplatetectonics forbillionsofyears.Marscanbeviewedasa“singleplateplanet,”withageophysicalsettingthatisfardifferentfromthatoftheactiveEarth.Theongoing InSight missiontoMars seekstoidentifywhetherMarsisstilltectonicallyactiveviatheplacementofanextremely sensitiveseismometerontothesurfaceofMars(Banerdt,2020).
1.6SamplesfromMars
Mostpeopledonotrealizethatwehave >260samplesfromMars,noneofwhichwere obtainedasaresultofaspacecraftmission(see Section8.6).Auniquegroupofmeteorites isnamedafterthreeindividualmeteoritefallsthatrepresentdistinctchemicalandtexturalsubsetsofthegroup: Shergotty, Nakhla,and Chassigny;hencetheterm SNC is appliedtotheentiregroup(McSween,1994).HowdoweknowthattheSNCmeteorites camefromMars?Innovativemeasurementsmadeononeoftheserocks,EETA79001,a shergottitecollectedfromAntarcticain1979(Fig.1.8),demonstratedthatgasestrapped withinglassyportionsoftherockwereunlikeanythingobtainedfromothermeteorites, norliketheatmosphereofEarth,norsimilartoanygasesderivedfromrocksorsoilscollectedontheMoon,buttheliberatedgasesfromthemeteoritewereexactlylikewhatthe VikinglandersmeasuredintheatmosphereofMars(BeckerandPepin,1984).
TheshergottitemembersoftheSNCsarebasalticinchemistryandvolcanicintexture, similartosomethingyoumightfindonalavaflowinHawai’i(HartmannandNeukum, 2001),exceptthattheSNCsmostlyrangeinagefrom0.17to1.4Ga,lessthanathird theageofessentiallyallothermeteorites(McSween,1994,2008;Nyquistetal.,2001).This wasakeypieceofevidencealongwiththetrappedgases,becausemostotherpotential sourcesformeteoriteswouldnothavebeenvolcanicallyactiveatthis“recent”timein thehistoryofthesolarsystem.TheSNCsalsolacksomethingpresentinmanyother
FIG.1.8Martianmeteorite.Sawedfaceofabasalticshergottitemeteorite,thefirstmeteoriteidentifiedhaving Martianatmospheretrappedinsidetheglassyportionsofthemeteorite.1cmcube,at lowerleft.Recoveredfrom ElephantMoraineinAntarcticaduringthe1979collectingseason. NASAphotoS80-37631.
commonmeteorites:smallroundfeaturescalled chondrules,amongtheoldestmaterials availablefromtheearlysolarsystem(Norton,2002,pp.166–174).TheuniquetrappedvolatilechemistryconvincedthesciencecommunitythattheSNCsdidcomefromMars,but wedonotknowforsurewhereontheplanettheycamefrom.
PossibleimpactcratersaspotentialsourcesforsomeoftheSNCgrouphavebeenidentifiedusingorbitaldata(e.g., Tornabeneetal.,2006;Werneretal.,2014),butthesedata cannotyetconfirmalinktospecificmeteorites(see Section3.8).The Opportunity rover studiedonerock(BounceRock)thatischemicallyverysimilartothelithologyBpartof EETA79001aswellastoQUE94201,bothcollectedfromAntarctica(Zipfeletal.,2011). Unfortunately,BounceRockisnotinplace(i.e.,itwasejectedfromsomewhereelseon Mars);19-km-diametercraterBopoluis75kmsoutheastofwhereBounceRockwasexamined,butdefinitiveconnectionofBopolupropertiestobothBounceRockandtothe Martianmeteoritessimilartoitremainselusive(Zipfeletal.,2011).
OneSNCisdistinctfromtheothersinthegroupanddeservesbriefdiscussion.ALH 84001isanorthopyroxenitecumulaterock,meaningthatitiscomposedofcrystalsthat accumulatedinamagmabodythatsolidifiedwithinthenear-surfacecrust.Itistheoldest oftheSNCmeteoritesat4.5Ga,makingitasamplefromtheancientMartiancrust (Nyquistetal.,2001).Thisrockgainedworldwidenotorietywiththepublicationofapaper sayingthatthemeteoritecontainedpossiblefossilizedevidenceofmicrobiallifeonMars (McKayetal.,1996).Thisconclusionwascontroversialformanyyears;today,most researchersdonotconsidertheevidencesupportiveofancientMartianlife(Sawyer, 2006;Treiman,2004).Inspiteofthis,carbonateveinsinALH84001,wheretheputative microbialfeatureswerefound,aredefinitiveevidencethatliquidwaterflowedthrough cracksintheancientMartiancrust.
1.7Chronology
Howdoesonedeterminethe“age”ofarock?OnEarth,fossilsintherocksthemselves,as wellasinsurroundingrocklayers,providearobustmeanstoconstraintheageofrocks thatweredepositedsincethetimethatmulticellularlifeformsleftmacroscopicevidence oftheirexistence,butthisisnotpossibleforMartianrocks(asfarasweknow). Relative age canbedeterminedfromhowadjacentrockunitsareincontact;iflocalverticalcanbe establishedforwhentherockswereemplaced,thensuperpositionindicatesyounger rocksontopofolderones;wherefaultsarepresent,crosscuttingrelationshipscanoften bedetermined,revealingarelativeagesequence;erosionaldegradationcanprovidean indicationofhowlongtherockshavebeenexposedtotheerodingenvironment.None oftheserelativeagestellusaboutthe absoluteage,thequantifiedtimesinceformation (inyears).Thenucleusofradioactiveelementsemitsparticleslikeprotonsandneutrons, alteringtheatomicweightandthusthecompositionofthehostatoms,withdifferenttime scalesforthevariousdecaypaths.Precisemeasurementoftheabundanceofparentand offspringmaterialsallowsarocktobedatedthroughthisprocess,somethingcalled geochronology (PressandSiever,1974,pp.68–77).Makinguseofgoodrockagesrequiresthat weknowpreciselywherethesampledrockcamefrom,andsofar,wedonothavethat informationforanyavailableMartiansamples.
Lackingradiometricagesfromwell-documentedsamples,scientistsconstrainagesby countingimpactcratersonplanetarysurfaces;thelongerthesurfacehasbeenexposed, themorecraters(perunitarea)arepresent(Mutchetal.,1976,pp.123–138).Modelsfor therateandsizedistributionofimpactingobjectshittingMarsallowcraterrecordstobe relatedtoabsoluteages(HartmannandNeukum,2001; Neukumetal.,2001),butevenso, suchageswillremain“modelages”untildocumentedsamplesfromMarscancalibrate themodelcrateringcurves.Afirststeptowardthatgoaloccurredwhenthe Curiosity rover useditsmassspectrometertodeterminetheradiometricage(4.21 0.35Ga)ofamudstone(sedimentary)rockthattheroversampledonthefloorofGalecrater(Farley etal.,2014).Martianagescitedthroughoutthisbookshouldbetakenasthebestestimate currentlyavailableuntilmanyradiometricagesfordocumentedMartiansamplescan calibratetheMartiancrateringrecord.
1.8Outlineofthebook
TheremainderofthisbookwillleadthereaderthroughadiscussionofourcurrentunderstandingofthevolcanichistoryofMars.Eachchaptercanstandonitsown,buteach includesmanyreferencestosectionsinotherchapters.Thevolcanoesaredescribedusing aregionalapproachtoplaceindividualvolcanoeswithintheregionalcontextofeachvolcanicprovince;thisapproachdiffersfromamoretraditionaltreatmentofvolcanoesbased onagroupingofconstructtype,buttheuniquesettingofeachMartianvolcanicprovince ledtotheorganizationalplanadoptedhere.Followingarebriefdescriptionsofthesubsequentchapterstoshowthereaderwhereweareheaded:
Chapter2:Areography. ThegeographyofMars(replacing“geo”forEarthwith“areo” forMars),withparticularemphasisondescribingtheregionalsettingforthevolcanic provinces.
Chapter3:TheTharsisProvince. Theprovincecoveringthegreatestareaandhaving thelargestcentralvolcanoes,alongwithmanysmallervolcanicconstructs.
Chapter4:TheElysiumProvince. ThesecondlargestvolcanicprovinceonMars, whichincludessomeconstructssteeperthantheTharsisvolcanoes,andthuslikely withdifferentcompositionoreruptivehistory.
Chapter5:TheCircum-HellasProvince. Theoldestvolcanicprovince,withvolcanoes verydifferentinshape,andthereforeemplacementconditions,fromthosefoundin TharsisandElysium.
Chapter6:TheSyrtisMajor/HighlandsProvince. Oneofthelargestvolcanoesby surfacearea(andintheregionoftheMars2020landingsite),plusisolatedvolcanic ventsscatteredthroughouttheMartianhighlands.
Chapter7:TheMedusaeFossaeformation. Anenormousdepositsubjecttointense winderosion,potentiallyaresultofvoluminouspyroclasticeruptions(althoughthis hypothesisremainsunconfirmed).
Chapter8:IgneousComposition. InformationaboutMartianrocksobtainedfromthe roversandthestudyofMartianmeteorites,withongoingmodelingeffortstoputthis informationintoaglobalcontext.
Chapter9:Volcanic“Cousins.” ComparisonofMartianvolcanoeswithvolcanic featuresobservedonotherplanetsandmoonsthroughoutthesolarsystem.
Chapter10:Thefuture. What’snext?Implicationsofwhatwehavelearnedsofarfor whatweshouldlearninthenearfuturefromvariousmissionstoMars.
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