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Terrestrial analogs and thermal models for Martian flood lavas
Citation for published version:
Keszthelyi, L, McEwen, AS & Thordarson, T 2000, 'Terrestrial analogs and thermal models for Martian flood
lavas' Journal of Geophysical Research, vol 105, no. E6, pp. 15027-15049. DOI: 10.1029/1999JE001191
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JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 105, NO. E6, PAGES 15,027-15,049, JUNE 25, 2000
Terrestrial analogs and thermal models for Martian flood
lavas
L. KeszthelyiandA. S. McEwen
LunarandPlanetaryLaboratory,Universityof Arizona,Tucson
T. Thordarson
Divisionof ExplorationandMining, CSIRO MagmaticOre DepositsGroup,Floreat,WesternAustralia
Abstract. The recentflood lavason Mars appearto havea characteristic
"platy-ridged"
surfacemorphologydifferentfrom thatinferredfor mostterrestrialcontinentalfloodbasalt
flows. The closestanalogwe havefoundis a portionof the 1783-1784Laki lava flow in
Icelandthat hasa surfacethatwasbrokenup andtransported
on top of movinglava during
majorsurgesin the eruptionrate. We suggestthata similarprocessformedthe Martianflood
lava surfacesandattemptto placeconstraints
on the eruptionparameters
usingthermal
modeling. Our conclusions
from thismodelingare (1) in orderto produceflows> 1000 km
longwith flow thicknesses
of a few tensof meters,thethermophysical
propertiesof the lava
shouldbe similarto fluid basalt,and(2) the averageeruptionrateswereprobablyof the order
of 104m3/s,
withtheflood-like
surges
having
flowrates
oftheorder
of 10s- 106m3/s.
We
alsosuggestthatthesehigheruptionratesshouldhaveformedhugevolumesof pyroclastic
depositswhich may be preservedin the MedusaeFossaeFormation,the radar"stealth"
region,or eventhe polarlayeredterrains.
1. Introduction
The objectiveof this paper is to provide initial qualitative
and quantitativeinterpretationsof new data on Martian flood
lavas. The Mars Orbital Camera (MOC) on board the Mars
Global Surveyor (MGS) spacecrafthas returnedspectacular
new images of Mars [Malin et al., 1998]. We were
particularlyinterestedin imagesof the Martian flood lavasin
order to comparethe emplacementof Martian and terrestrial
flood volcanism.
Flood volcanism is one of the most significant crustforming processesidentified on Mars [e.g., Greeley et al.,
2000; Greeleyand Spudis,1981], and the extremepaucityof
cratersin someof the imagesof the flood lavassuggests
that
this style of volcanismmight plausiblypersistto this day on
Mars [Hartmann, 1999; Hartmann et al., 1999; Hartmann
and Berman, this issue]. Understandingthe eruptionsthat
formedtheseflood lavasis a criticalstepin understanding
the
evolutionof the surface,interior,andatmosphere
of Mars.
Before proceeding,we shouldnote that we use the term
"flood lava" to denotea packageof sheet-likelava flows that
inundatesa large region, producinga smoothplain without
•-100 m tall constructs.
This use of the word is in line with
that presentedby Greeleyand King [1977] and its application
to terrestrial continental flood basalts [Bates and d•ackson,
1984] andis essentiallyidenticalto the useof the term "mare"
on the Moon (but withoutthe requiredlow albedo). However,
someother workershave used"flood lava" to simply refer to
long lava flows with no clear channels or tubes [e.g.,
Cartermole,1990].
Copyright2000 by theAmericanGeophysical
Union
Papernumber1999JE001191.
0148-0227/00/1999JE001191509.00
Some of the most recentand bestpreservedflood lavas lie
in eastern Elysium Planitia (the Cerberus Formation of
Plescia [1990]) and in a portion of Amazonis Planitia
centeredat 30ø N, 160ø W [McEwen et al., 1999b]. These
two regions are connectedby Marte Vailis, which is also
partially filled by flood lavas. The surfaceof the Cerberus
plainsrecordsa complexhistoryof volcanic,fluvial, impact,
tectonic,aeolian,and possiblylacustrineand glacialprocesses
[e.g., Mouginis-Mark et al., 1984; Tanaka and Scott, 1986;
Plescia, 1990; Scott and Chapman, 1991; McEwen et al.,
1998, 1999c]. In this paper we focus solely on the best
preserved(andpresumablyyoungest)volcanicfeatures.
Well-preservedflood lavasof similarappearance
(although
probably older) are also presenthigh on the TharsisBulge,
near TharsisTholus. Layers exposedin the walls of Valles
Marineris are also likely to be yet older flood lavas [McEwen
et al., 1999a]. Figure 1 showsthe locationof thesefloodlava
exposureswith respect to other major features on Mars.
Crater countingand modelssuggestagesof about 10 Ma for
the youngestof the flows in the CerberusPlains [Hartmann
and Berman,this issue]. However, MOC imagesalso reveal
that there are locationswhere the lava is eroding out from
beneath the Medusae Fossae Formation, which may have
protectedthe lava from small impact craters. While many
processescan complicate the age estimates from crater
counts, the crisp, undegradedmorphology of the flows
supportsthe idea that theseflood lavasare geologicallyvery
young. Together,the recent flood lavas appearto cover an
areaofabout107km2,roughly
thesizeofCanada
[McEwen
et
al., 1998]. The minimum thicknessof the Cerberusdeposits
is estimated to be in the hundreds of meters, and the new
MOC images supportthe estimate by Plescia [1990] that
individualflow frontsare only of the orderof 10 m thick.
15,027
15,028
KESZTHELYI
ET AL.' MARTIAN
These flood lavas have a distinctivesurfacemorphology
dominated by plates and ridges [McEwen et al., 1999b]
(Figures2-9). The platy-ridgedmaterial is interpretedto be
lava becauseit is boundedby lobatemarginscharacteristic
of
lava flows (Figure 4). At high spatial resolution(2 to 20
m/pixel), theseflows exhibit a complexsurfacemorphology
interpreted to include rare channels (Figures 3 and 4),
common rafted crustal slabs(Figures 2, 3, 5, 6, 7, and 9),
ubiquitous pressure ridges (Figures 2-9), some ponded
surfaces(Figures 5-7), and possiblesqueeze-ups(Figures2
and 5-7). Coherentslabsof crustappearto rangein sizefrom
a few hundred meters to a few kilometers.' Ridges are
typically only tens of meterswide but up to kilometersin
length. Most ridges are interpretedto have formed at the
edgesof plates as they are rammed into each other during
emplacement(i.e., pressureridges). Other ridges may have
formedby liquid lava oozingup throughextensionalcracksin
the plates(i.e., squeeze-ups).
These flows are remarkably flat, with about 400 m of
vertical elevation over 1500 km of lateral distance(0.027%
slope) [Smith et al., 1999]. However, the region is radar
bright, implying a rough surface on the decimeter scale
[Harmon et al., 1992, 1999].
Vents in the flood lava regions are difficult to identify,
probablyas a result of burial by the flows, their small size,
and/or rapid removal of the relatively fragile vent structures.
However, the CerberusRupes fracturesand nine low shields
on the western side of the Cerberus Plains were identified
as
likely ventsfrom Viking data[Plescia,1990;Edgettand Rice,
1995]. The most recentMOC imagesshow small lava flows
emanatingfrom both sides of some of the CerberusRupes
fissures,confirmingthat they indeedservedas volcanicvents
[McEwen et al., 1999c]. However, there is still no conclusive
proof that most of the large flood lavas were fed from
CerberusRupes. Also, the CerberusRupesfracturescut all
the lava flows, indicatingthat it has been tectonicallyactive
morerecentlythanit hasbeena volcanicvent.
One of the ongoinglimitationsin workingwith the Martian
flood lavas is our inability to trace individual flows. The
Viking datado not have the resolutionto allow flow margins
to be confidently traced. The MOC data cover only a tiny
fraction of the Cerberus Plains, allowing only extremely
discontinuoussampling. Our bestestimatefor the extentof a
singleflow is shownin the lightestshadingin Figure 1. This
area correspondsto the brightestradar returnsreportedby
Harmon et al. [1999]. It also follows the local topographic
gradient away from the presumed vents along Cerberus
Rupes. It is relatively dark in the Viking images,and the
freshestlooking lavas in MOC imagesfall within this area.
Also, the radar bright area terminatesat the samearea as the
flow terminusseenin Figure4.
On a smallerscalethe flows appearto have 100- to 1000m-size lobesat the margins,but the largersheetsare generally
not capturedin any singleMOC frame(3-10 km wide). From
the small areassampledby MOC and the subtlemottling in
the Viking images,we tentativelyinfer that both the widths
and lengthsof the individualsheetsare of the orderof tensof
kilometers. Only a few sheet-likelobescan be seento be less
than a kilometerwide (e.g., Figure 4). However, if previous
suggestions
for vent locationsare correct,the lavasmusthave
generallyspreadfor >500 km and may have exceeded2000
km in somecases. This impliesthat eachflow is composedof
a greatnumberof smallersheet-likelobes.
FLOOD LAVAS
..
.,
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KESZTHELYI
ET AL.: MARTIAN
FLOOD
LAVAS
15,029
Figure 2. MOC imageSP1-21905andViking contextimagefrom ElysiumPlanitiacenteredat 5.7øN,209.0øW. In this andthe
followingMOC images,northis up, andthe Viking contextis froma basemapmosaic[Davies,et al., 1992]. Note raftedplatesof
lavathatcanbejigsaw fit in a patternconsistent
with flow to theNW or SE. The wakebehindan obstaclein theNE cornerof the
image(arrow)showsthatthe flow directionwasactuallyto the NW. Larger,curvingridgesin the platesare probablypressure
ridgesformedduringthe collisionof plates. Thin, involuteridgeswithin the brightermaterialbetweenthe platesare probably
squeeze-ups
of lavaformedafterthelavasurfacestagnated.
2. Terrestrial Analogs
Without well-studied terrestrial analogsto provide a
qualitativemodelfor the Martianfloodlavas,it is impossible
to pin downthe physicalprocesses
to includein quantitative
lava flow models. Currentlava flow modelsrequiresome
basicassumptions
abouthow the flow advances(e.g., tubefed versus channel-fed). Even empirical relationships
betweeneruption parametersand flow dimensions[e.g.,
Walker,1973; Pieri and Baloga, 1986;Kilburn and Lopes,
1991] are criticallyaffectedby the styleof emplacement.In
this sectionwe will showthatportionsof the 1783-1784Laki
lava flow in Icelandmay be the bestavailableanalogto the
Martian flood lavas.
2.1. Continental
Flood Basalts
The Martian flood lavas derive their name from their
obvious similarities to terrestrial continental flood basalts
(CFBs). Continentalflood basalt provincestypically cover
morethan106km2 witha lavapilekilometers
thick[e.g.,
Macdougall, 1988]. When not deeplyeroded,they are also
characterizedby flat, plateau-like,topography[e.g., Greeley
and King, 1977]. The best preservedand most extensively
15,030
KESZTHELYI
ET AL.: MARTIAN
studiedCFB provinceis the ColumbiaRiver Basalt Group
FLOOD LAVAS
lavas have radiometricagesbetween 17 and 6 Ma, but the
(CRBG). TheCRBGconsists
of 174,300ñ31,000km3 of majorityof the lava was eruptedbetween16.5 and 15.3 Ma
dominantly basaltic-andesitelava covering 163,700 ñ5000
[Tolan et al., 1989]. Comparisonto other CFB provinces
km2of area.These
lavas
aredivided
intoapproximately
40 showsthat most CRBG lava flows are similar to the majority
stratigraphicunitsthat contain>300 individual"flows". The
of lava flows found in other flood basalt provinces [e.g.,
Hooper,1982;Selfet al., 1998].
Although continentalflood basalt provinceshave been
studiedextensively,their flows are not sufficientlywell
preserved
or exposedfor the originalsurfacemorphology
to
be directlyobserved.Instead,the surfacemorphologymust
be inferred from observationsmade in crosssectionsalong
natural and artificial
cliffs.
Such examination
of the CRBG
lava flows indicatesthat they were primarily emplacedas
inflated pahoehoe lava flows [Self et al., 1997, 1998].
However, it is importantto note that only one unit, the Roza
Member of the WanapumFormation,has the combinationof
exposureand preservationthat has allowed the detailsof an
entire flow field to be described[e.g., Shaw and Swanson,
1970; Swansonet aL, 1975; Martin, 1989; Thordarson, 1995;
Thordarsonand Self, 1998]. Most other CRBG flow fields
are either too poorly exposedor too extensivelyerodedfor
suchdetailedstudy. Thus there is a continuingdebateabout
the role of inflation in the emplacementof otherCRBG lava
flows [e.g., Reidel, 1998]. Other terrestrial flood basalt
provinces (including the Deccan, Entendeka, Kerguelen,
Parana,and Keewena)alsoappearto be dominatedby inflated
pahoehoe [Self et al., 1997, 1998; Jerram et al., 1998;
Keszthelyi et al., 1999; Frey et al., 2000]. However, a
substantial
fraction
of flows
in the CRBG
and other flood
basaltprovinceshave brecciatedflow tops that do not fit in
the aa versuspahoehoecategorization[Macdonald,1953;Self
et al., 1998;Frey et aL, 2000].
Based on the surface morphology of large inflated
pahoehoe flows in Hawaii, New Mexico, Australia, and
elsewhere[cf., Theilig and Greeley, 1986; Keszthelyiand
Pieri, 1993;Hon et al., 1994;Stephenson
et al., 1998;Self et
al., 1998], we infer that a substantialportionof the surfaceof
CFB lavas consistsof a convoluted set of lobes, tumuli, and
plateaus,all inflatedto aboutthe samelevel. Sucha surface
has a distinctive mottled appearance at 1-10 m/pixel,
especiallywhen partially infilled by aeoliandeposits.Figure
10 shows this surface morphology of terrestrial inflated
pahoehoe lava flows in a variety of settings. This
morphology is diagnostic of inflation; no other style of
eraplacement
produces
thismorphology.
Do we expect inflated flows to be common on Mars?
Inflated flows requirethe presenceof a coherentuppercrust
Figure 3. MOC imageSP2-38804and Viking contextfrom
Matte Vailis centeredat 6.9øN, 183.0øW. Note platesin the
lava flow consistentwith the centerof the a channelwinding
North. The parallel ridgesin the top of the imageappear
more similarto compressional
pressureridgesformedby the
collisionof solid platesthan to leveesformedby overbank
depositsof liquid lava. The streamlinedfeature SE of the
center of the image appearsto be sheetsof crustpiling up
againsta topographicobstacle,leavinga wake in the crust.
This type of wake is seenon a muchsmallerscalein Hawaii
whenchannelizedlava piles againsta tree. The groovesseen
in the Laki flow field in Iceland are more similar in scale(see
Figures12 and 13).
KESZTHELYI
ET AL.: MARTIAN
FLOOD LAVAS
15,031
Figure 4. Cutoutwindowsfrom MOC imageSP2-40703andViking context. Imagecenteredat 19.2øN,174.6øWinsideMarte
Vailis. Lower (left) cutoutwindow showsthe terminusof a flow lobe that was confinedto preexistingchannelspresumedto be
fluvial in origin. The marginsof the flow aretypicalfor basalticlava flows. The albedomarkingswithinthe flow are suggestive
of multiple channelsthat did not have the opportunityto drain. The flow marginsand surfaceappearto be only minimally
modifiedby postemplacement
geologicprocesses.The upper(right) cutoutwindowshowsthe marginof a flood lava flow with
smoothsheet-likechanneloverbankdepositsand disruptedchannelsurfaceconsistentwith flow to the NE. Also of interestare
the small fluvial channelsthat the lava embays. The marginof the flow is partiallycoveredby fluvial and/oraeoliansediments.
Numberson contextimageare latitudeandlongitude.
and conditions under which it is easier for the lava to raise the
crustthanto advanceforward. The major differencebetween
the Earth and Mars is the lower gravitationalaccelerationon
Mars. This decreases
both the force drivingthe advanceof
the flow and the force holding the crust down. Thus the
reducedgravity shouldsignificantlyenhancethe tendencyof
lava flows to inflate [Keszthelyiand Self, 1998a]. The other
major difference between the Earth and Mars is the •100
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Figure 5. Cutoutwindowsfrom MOC imageSP1-24203andViking contextfrom AmazonisPlanitia. Imagecenteredat 24.6øN,
163.6øW. In the upper(right) cutoutwindow,notethe ridgeson the plateson the lava surfaceat the top of the image. Our
interpretationof the formationof this platy-ridgedsurfaceis the following:First,the ridgesformedas compressional
hummocks
perpendicular
to flow direction. Theseridgesdo not appearto be inflationfeatures(seeFigure 10) and insteadare similarto
compressional
pressureridgesformedin brecciatedflow tops(slabpahoehoe,
aa, etc.). We suggest
that belowthe brecciatedflow
top, the lava solidifiedinto a coherentslab. A later surgein lava flux then raftedpiecesof this crust,translatingand rotating
blockswith ridgeson their surface(see Figure 15). In the lower (left) cutoutwindow the sameflow exhibitsa much smoother
appearance.The randomorientationof the web of thin convolutedridgeson the smoothlava surfacesuggeststhat they are
squeeze-ups
of moltenlava comingup betweencracksin the cruston a pondedarea. Pressure
ridgeswouldbe expectedto showa
clearpreferredorientationperpendicular
to flow.
KESZTHELYI ET AL.' MARTIAN FLOOD LAVAS
15,033
.:
ß
...
Figure 6. MOC imageSP2-42604andVikingcontextfromAmazonis
Planitiacentered
at 25.7øN,167.0øW.Groupsof parallel
ridgesare probablycompressional
pressureridgesof disruptedcrust,and more sinuous,isolatedridgesare probablylarge
squeeze-ups
in the flat pondedsurfaces.Localflow directionis ambiguous,
but motionto the SW is mostconsistent
with the
ridgeorientations.
times lower atmosphericpressureon Mars. This shouldlead
to reducedconvectivecoolingof the lava surfaceand slower
crustgrowthon a lava flow. However,the initial formationof
a crust is governedby thermal radiation,which will be very
similar on Earth and Mars [e.g., Wilson and Head, 1994].
Overall, the conditionson Mars appearto be very conducive
the resolutionneeded(<<10 cm/pixel) to identify diagnostic
features (tumuli, inflation clefts, lobe boundaries,vesicle
distribution,etc.) in the Martian cliffs. Instead,we makethe
assumption
that mostof the Martianflood lavasthroughtime
have
had
surfaces
like
the
recent
flood
lavas.
Our
examinationof the MOC imagesof the exposedsurfacesof
for the formation of inflated flows. What remains to be seen
the well-preservedMartian flood lavas suggeststhat only
is if the eruptionson Mars were alsoin the regimethat forms isolated segments of their margins might be inflated
inflated flows.
pahoehoe. The raftedplatesthat characterize
the Martian
Locallyhigh strainrates(relativeto the lavaviscosity)lead flood lavasare not seenon inflatedterrestrialpahoehoeflows
to tearingand disruptionof the crust[Petersonand Tilling, and insteadsuggesta disruptedcrustthat translateslaterally
1980]. In Hawaii, aa flows with disruptedcrustsbegin to during emplacement. Thus, while the scale and gross
format effusion
ratesaboveabout15 m3/s[Rowland
and physiography
of theterrestrialandMartianfloodlavasappear
Walker, 1990]. However, the Roza Member of the CRBG is similar, the more detailed flow morphologyrevealedby the
an inflated pahoehoe flow that was emplaced at MOC images suggeststhat they formed by significantly
approximately
2000m3/s[Thordarson,
1995;Thordarson
and differentprocesses.Thereforewe look for otherterrestrial
Self, 1998]. In the caseof the Roza eruption,the lava front analogsto providethe neededinsightinto the emplacement
was tens of kilometerswide, so the muchhighervolumetric processes
of Martianfloodlavas.
effusion rate did not translateinto rapid flow rates or high
strain rates. Thus high volumetric effusion rates, by
2.2. The 1783-1784Laki Eruption
themselves,do not rule out emplacementas inflatedpahoehoe
sheet flows.
The eruptionin 1783-1784 at Laki, Iceland, produceda
The layereddepositsseenin the walls of Vailes Marineris largeflow field thathasmanysimilaritiesto CFB lavaflows,
appearsimilar to the layersof lava flows commonlyseenin but alsohasareaswith a morphologythat is strikinglysimilar
CFB provinces[McEwen et al., 1999a]. However, we lack to the Martian flood lavas. The Laki eruptionis the largest
15,034
KESZTHELYI
ET AL.: MARTIAN
FLOOD LAVAS
..
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....
Figure7. MOC imageSP2-53504
andVikingcontext
fromAmazonis
Planitia,
centered
at 23.9øN,164.3øW.Roughridged
areasandthemostprominent
linearridges
appear
tobepressure
ridges
of brecciated
material.Smoothareasappear
to beponded
lava.In general,
theroughareas
matchthedarkmottles
in theVikingcontext
images.Themargin
of theflow,whereit abuts
the
hillsto thenorth,is notmorphologically
distinctfromtherestof thelava. Dunesalongthe flowmarginandthe sparse
craters
suggest
thatthisflowmightbeslightly
olderthansomeoftheElysium
Planitia
flows.Flowdirection
isambiguous,
withnoclear
interaction between the lava flow and the older hills.
KESZTHELYI
ET AL.: MARTIAN
FLOOD LAVAS
15,035
Figure 8. MOC image SP2-54304 and Viking contextfrom lava plains near TharsisTholus, centeredat 17.0øN, 84.1øW,
showingolder lava surfacewith ridgesand grooves,similarto thoseseenon the recentflood lavasin Elysiumand Amazonis.
Despitesignificantaeoliancover, including>100-m-longdunes(arrows),theseridgesare readily recognized. However, the
marginsof platesaregenerallyobscured.
historicaleruptionand the detailedwritten recordsof the
effusionratesappearto have reachedvaluesas high as 8700
eyewitness observationsof this eruption (e.g., J. m3/sforshort
periods
during
theearlypartof theeruption
Steingrimsson,
writtencommunications,
July 4, 1783 and [Thordarsonand Self, 1993].
November24, 1788;O. Stephensen,
writtencommunication, Ten eruptiveepisodesfed fissuresegments1.6-5.1 km in
August15, 1783)areparticularly
helpfulin understanding
the length along a fissure system with a 27 km total length
eruptive
historyandbehavior
of thelava. (Writteneyewitness [Thordarsonand Self, 1993]. Generally, only one fissure
reportsare publishedin Icelandicby Einarssonet al., 1984 segmentwas active at any given time, but for a shortperiod
and the accountof Jon Steingrimssonis translatedinto sectionsof three fissure segmentswere active over a total
English[Steingrirnsson,
1998]). Also, while the Laki flow distance of-10 km [Thordarson and Self 1993]. The
has been carefullyexaminedby severalIcelandicworkers eruptionreleased350 Mt of CO2, 240 Mt of H20, 120 Mt of
[e.g.,Jakobsson,1979;Oskarsson
et al., 1982;Thordarson, SO:, 15 Mt of HF, and7 Mt of HCI [ Thordarson
et al., 1996].
1990;Thordarson
and Self, 1993],little is published
on the The fluorine in particularwas deadlyto the local livestock
flowmorphology.
Muchof thefollowingdescription
is based (primarily sheep),which led to a massivefamine that killed
onpreviously
unpublished
fieldworkby T. Thordarson.
approximately 20% of the population of Iceland
The eruption startedon June 8, 1783, and lasted until [Thorarinsson, 1979]. The tops of the buoyanteruption
February
7, 1784,erupting
15.1km3 of basalt(dense
rock columnspenetratedthe tropopause,and SO2was distributed
equivalent (DRE)) that advanced>60 km from the vent acrossthe Northern Hemisphere[Woods, 1993]. The Laki
[Thordarson
andSelf,1993;Thordarson
et al., 1996](Figure eruptionwas associatedwith an unusuallycold winter, with
11). About10%of the mass(and>50% of thevolume)was the Danube River freezing over at Vienna and rafted ice
deposited
aspyroclastics.
Of the lavaflows, 60% (by mass filling the Mississippi River at New Orleans [Thordarson,
andvolume)was eruptedin the first 1.5 monthsand 90% in 1995; Thordarsonet al., 1996].
thefirst5 months
[Thordarson
andSelf,1993]. Theaverage The lavasflowedawayfromthe fissuresystemandpassed
effusion
ratefortheentire
eruption
was730m3/s[Thordarson
throughtwo river gorgesbeforefanningout on the coastal
and Self, 1993], not muchhigherthan historicalMaunaLoa plains(Figure 11). Field mappingandexaminationof aerial
eruptions[Rowlandand Walker,1990]. However,basedon photographsshow that the majority of the lava formed
theeyewitness
accounts
of therateof advance
of lavasurges, inflatedpahoehoe. However, one sectionof the distalend of
15,036
KESZTHELYI
ET AL.: MARTIAN
FLOOD LAVAS
Figure 9. MOC image SP2-54303 and Viking contextfrom lava plains near TharsisTholus, centeredat 14.2øN, 83.7øW,
showingolder, degradedexampleof flood lava with similar ridged morphologyas the more recent Elysium and Amazonis
examples. Groovesin the crustsuggestflow to the NW during the initial emplacement.However,the crackindicatesopeningto
the NE, suggestingthat the flood-like breakoutfrom this lobe movedin that direction. Note that in theseimages,there is no
correlationbetweenthe lavamorphologyandthe albedomottlingin the Viking image.
the flow field is composedof arcuatepressureridges,rafted
crustalplates,and longitudinalgrooves(Figures12 and 13).
The lava in this areais coveredby a few centimeters
of moss,
but the underlyingflow top can be seen to be brecciated,
forming a lava surfaceunlike any commonexamplesin
Hawaii. The closestHawaiiananalogto thisdisrupted
crustis
slabpahoehoe,a form of lava transitionalbetweenpahoehoe
andaa [Macdonald,1953].
We inferthatthe raftedcrustalplatesinitiallyformedon a
relativelystagnant
flow andweredisrupted
andtransported
by
a latersurgein the flow rate. This is directlyanalogous
to the
manner in which the ice cover on a partially frozen river
breaksup during springfloods ("undanhlaups"
in Icelandic).
Recordedobservationsof the flows advancingthroughthe
areasshownin Figures 12 and 13 includereportsof the lava
surgingforwardat 2-5 timesthe averagerate for severalhours
[Steingrirnsson,
1998].
Based
on field
work
and
examination
of the aerial
photographs(Figures 12 and 13), we conclude that the
groovesin the flow surfaceare wakes left in the brecciated
crust as it translatedpast isolated obstacles. The arcuate
ridgesappearto be locationswhere the brecciatedcrustwas
compressedas it ponded before overtoppingtopographic
ridges. This part of the Laki flow field is underlain by
KESZTHELYI
ET AL.' MARTIAN
FLOOD LAVAS
15,037
(a)
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;•::•,:•:•:½"•"f•:•
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'"';½•;::•.:L.½•:•:-•;;;;::;•;.;:½.
::•%•:4:..•:•;;•:J-•
' ::"";"•
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•4:•
...........
• .......
........
,.. :.•.•
............
•:.....•:::::,:..•.:
.:•.•:•:.j;:•:.:....•.>•..:.•:•;•4•(.•
'.'.:•;---:-½.
;.-'•:?;..
-.- .:.•..• ._•:•-•..:•..•.½•"
-.:-•,:-•-*:%
...:;.;.;-.•-.'
•
'•
......
Figure 10. Examplesof terrestrialinflated pahoehoeflow surfacemorphology. (a) Aerial photographof SE corner of the
Macarthy'sflow field, New Mexico. Note largeinflationplateauin centerof imagewith inflationpits. Smallerinflationfeatures
on hummockypahoehoesurfacesappearas mottling at this resolution. (b) Aerial photographof SE portion of Carrizozo flow
field, New Mexico. Note the largeinflationplateaualongthe flow marginwith inflationpits andthe moremottledappearance
of
the hummockyportions. Dark delta-shaped
featuresare smallpatchesof slabpahoehoebreakingout from the inflatedpahoehoe
lobes. (c) Aerial photographof SE portionof the Laki flow field, Iceland. Note the largeinflationplateauin the centerof this
lobe andthe more hummockytextures. Highway 1 crossesthe lava in this image. (d) Obliqueaerialphotographlookingsouth
over the west end of the Pisgahflow field, California. Note mottled patternof the hummockypahoehoesurfacemade more
visibleby partialfilling with aeoliansands.Truckson Interstate
40 areshownfor scale.Photocourtesyof BruceC. Murray.
rootlessconesin the older (934 A.D.) Eldgja flow field.
These cones occur both in lines and in isolated clusters of
cones, making them the only likely candidate for the
topographicobstaclesthat the Laki lava encountered. The
raised edgesand streamlinedshapeof the groovessuggest
that the brecciatedtop of the lava had a significantyield
strength,while pahoehoebreakoutsshow that the interior
remained fluid.
KESZTHELYI ET AL.: MARTIAN FLOOD LAVAS
15,038
...
.
.
...
-1'.:
.km:
. -.--.:....-:
>..m.,-.::..:-...:-:-.--.-....::.............
- .:.-
-. :--
.-
Figure 10. (continued)
Thissurging
andautobrecciating
modeof emplacement
has beforenarrow channelsor tubesevolvedwithin the flow).
not beendescribedin Hawaii,but may applyto the -•20%of
the flows in the Columbia River flood basalt provincethat
Thesenear-ventflowsarecharacterized
by highlocaleffusion
ratesandhot,gas-rich
lavaswithrelatively
lowviscosities.
haverubblyflowtopsandsmooth
pahoehoe-like
flowbases Where the flows have ponded,their surt•tcesare also
by veryshallowslopes
(<<1ø).
[Selfet al., 1998]. It mayalsohelpexplaintheenigmatic characterized
brecciatedflows seenon the KerguelenPlateau[Freyet al.
20001.
In Hawaii, longitudinalgroovesand pressureridgesare
3. Modeling
The observations
from Iceland,andto a lesserextent,from
largelyabsent
withinlavaflows. However,
we havefound
Hawaii
and
continental
flood basalt provinces,strongly
somepiecesof raftedcrustthataretensof metersin scale.
suggest
that
the
Martian
flood
lavaswereemplaced
assheet
Raftedpiecesof crustarevisiblealongthe Southwest
Rift
flowswithintermittent,
surging
advances.
In thissection
we
Zone of Kilauea and in the summit caldera of Mauna Loa
thisqualitative
emplacement
modelin moredetail
(Figure14). The raftsaremostlyconfined
to thenear-vent describe
to placequantitative
constraints
oneruption
faciesof lavaflows(i.e., thebroadsheets
of highlyvesicular andthenattempt
lava within a few hundred meters of the vent that formed
parameters.
Surging
emplacement
is inherently
difficult
to
KESZTHELYI
ETAL.'MARTIANFLOODLAVAS
15,039
Figure 10. (continued)
model
andwelackcritical
measurements
such
asthelength crust.Thesecond
involved
opensheetflowwitha disrupted
andwidth
ofindividual
outpourings.
Therefore
it iscurrentlyandmobilecrust.Thethermalmodels
fromKeszthelyi
and
impossible
to produce
a quantitative
modelreplicating
the Self [1998b]are readilymodifiedto Martianconditionsand
eraplacement
ofa specific
Martian
floodlavaflow.However, candescribe
eachof these
twodistinct
flowregimes.
it is possible
to showhowtheflowswerenotemplaced. 3.2.1. Insulated sheet flow. The thermal model for
Elimination
ofthephysically
impossible
provides
surprisingly
insulated
sheet
flowspresented
byKeszthelyi
andSerf[1998b]
usefulconstraints
on the formationof the flows.
is a simplemodificationof the thermalmodelfor lavatubes
presented
by Keszthelyi
[1995]. Bothmodels
involvesteady
3.1. Qualitative Model
stateconductive
heatlossthroughthe uppercrustandheat
The brokenraftsof crustandthe observations
of the Laki generationvia steadystate viscousdissipation. Flow
usingthe
flowfieldsuggest
thattheMartian
flood
lavas
have
disruptedvelocitieswithinthe sheetflow are calculated
equationfor flow betweentwo parallelplatesand the
flowtops.Theobserved
raftsof crust
arehighly
suggestive
of laminarflow, a Newtonianrheology,and
ofsurges
traveling
through
theflow,inflating
it tothepoint assumptions
shallow
slopes.
Theresulting
setof equations
is givenby
thata sudden
breakout
carries
away
large
pieces
ofpreviously
solidified
crust(Figure15). Thecrustridingthefloodis KeszthelyiandSelf[ 1998b]as
broken
upasit issheared
along
themargins
oftheflowandas
theflowmoves
between
andaround
topographic
obstacles.<v>=pg0H2/ 8q,
Asthesurge
wanes,
thebase
ofthedisrupted
upper
crust
may
become
coherent
viasolidification
of theliquidlavamatrix,
Qvi• = p g H <v> 0,
Qcond
= k (T-Ta)/ Hc,
allowinginflationto resume.Whilethereis someevidence •r/•x = (Q,•- Q•o•d)
/ (<v>p Cp*H),
(1)
(2)
(3)
(4)
fortheformation
of marginal
levees,
based
ontheimageswhere<v> is theaverageflow velocity,p is thebulklava
availableto date,the flow doesnot seemto be restricted
to
density,
g isthegravitational
acceleration,
0 isslope,
H isthe
narrowchannels.
Thedisrupted
crustontheseopensheet
thickness
of theliquidlava,q is viscosity,
Qv•,½
is theheat
flowsshould
haveexperienced
a steady
process
ofproduction
by viscous
dissipation,
Q½o,a
is theheatlossby
anddestruction
as it is churned
by the flowpastbends, generated
conduction,
k isthermal
conductivity,
T isthetemperature
of
constrictions,
andothertypesof obstacles.
theliquidlava,T•isthetemperature
offlowsurface
(assumed
to beambient),
Hc isthethickness
of thesolidcrust,OT/Ox
is
3.2. Quantitative Thermal Model
thecooling
perunitdistance
offlow,andCp*isheatcapacity
The surgingflow model impliesthat the lava was including
the effectof latentheat(dominated
by the
transported
in two very differentregimes.The first flow crystallization
of olivine). Thismodelis verycrude,and
regime
involved
flowundera thick,stationary,
insulatingresultsshouldbe consideredaccurateto no betterthana factor
15,040
KESZTHELYI
ET AL.: MARTIAN FLOOD LAVAS
August
....
"Laki
fissures
64ON
7 August
,.
.,
..
9 August
•:-•'-.......
18June
20 J,uly
1.8øW
Figure11. Mapof Lakiflowfield,Iceland.Positions
of theflowfrontat knowndatesareshown.Large,flood-like
breakout
withridgedsurface
andraftedplates
(Figure13)isindicated
bylocation
1. Smaller
flood-like
breakout
withraftedplates
(Figure
12)isshown
by location
2. Classic
inflated
pahoehoe
structures
(Figure10c)areshown
by location
3.
KESZTHELYI
ET AL.: MARTIAN
FLOOD LAVAS
15,041
.;.........
::.::...'"•;:'½?.:'.';'?..••:•- -'"'•'"?-::;.:-:;.:?.
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:;::•.•:.•.•:;
.,
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.•:;•
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..
•..:.:;•:•::•::::..
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::::::::::::::::::::::
.;.:..::•..;.....:...:..:
.::..::•.:::::
....::;..:.:•::
:.....:..•:::.•.:•:..::.:..•;:.
:.:..::
....... :..:
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....
....:..
.........
:....:::::.
:........:.;.:..:..
:.•:.::..:.:....:•.•:•.:•..:•:::
:.....:•:....•
.....:::..
:•.....:
...: ::.....:.:...........
:........:
........
. ... : . ...:%.½:?.
z lkm
Figure 12. Aerialphotograph
of portionof theLaki flow field,Iceland(labeled2 in Figure11). (a) Comextshowingthe Skafia
River, Highway 1, andfarmsas well as the locationsof Figures12band 12c. (b) Close-upof the northernchannelwhichwas
activeon July 17, 1783,showing100-m-scale
raftsof lavacrusttransported
in an openchannel.This channelbrokeout from a
lava frontthathad stagnated
aroundJuly 12, 1783. (c) Close-upof thisearlierlava,showingthat it formeda broadsheetwith
pressure
ridges,plates,andgrooves.The ridgesarepilesof breccia,theplatesarepiecesof moreiraactcrust,andthe groovesare
"wakes"left in the brecciated
topasit movedpasttopographic
obstacles.
15,042
KESZTHELYI
ET AL.: MARTIAN
FLOOD
LAVAS
Figure 13. Aerial photograph
of portionof the Laki flow field, Iceland(labeled1 in Figure11), showingpart of the largefloodlike breakoutthat formedin mid-June,1783. Note the largeraftsof ridgedcrusttom from the earlierformedflow top, as well as
long groovesin the ridged lava. The ridged lava surfaceconsistsof pressureridgesin the brecciatedflow .top,not tumuli as
suggested
by ThedigandGreeley[ 1986]. The groovesformedasthisbrecciated
flow toptranslated
overtopographic
obstacles.
of 2. However, more sophisticated
numericalmodels[e.g.,
Sakimoto and Zuber, 1998] do not produce significantly
differentresults,only morepreciseones.
The major modifications required to adjust the model
presentedby Keszthelyiand Se/f[1998b] to the Martian flood
lavas are the reductionin both slopeand gravity. Ambient
temperatureis also reducedto-30øC, but this parameterhas
very little effect on the modelresults. Also importantis that
the Martian flood lavasappearto have advancedas muchas
2000 km from the presumedvent areas,while Keszthelyiand
Self [ 1998b] only examinedthe requirementsfor a flow to
extend •100
km.
In examining the Martian flood lavas, we chose to
investigatethe effectof lava viscosityandthe thicknessof the
insulating crust on the minimum flow thicknessand flow
velocities
needed
for a flow
to travel
1000
km without
roughly correspondingto typical values for ultramafic,
basaltic,and basaltic-andesiteflows.
Severalpointsin Table I requiresomediscussion.First,
irrespectiveof the chosenviscosity,a flow that can extend
1000 km under a l-m-thick
crust will be turbulent because of
the highflow ratesrequired.Becauseit is difficultto envision
a stationaryinsulatingcrustsurvivingover a turbulentflow,
we regard this scenarioas unlikely. In the case of the
ultramafic(10 Pa s) lavas, the crust must be •20 m thick
beforeflow velocitiesdropinto the laminarregime. While
the •25 m total thicknessrequiredby this scenariomay be
compatiblewith someobservedflow thicknesses,
it suggests
thatit mightnot be possibleto produceinsulatingsheetflows
with extremely fluid lavas, even with low gravity and
remarkablyshallowslopes. This arguesagainstultramafic
compositions
for the Martianflood lavas. However,we note
that recent studiessuggestthat mildly turbulentterrestrial
freezing. As in the work of Keszthelyiand Self[ 1998b] we
assumethat a temperature drop of 50øC within the lava komatiite lava flows may have been emplacedas insulated
transportsystemwould lead to significantcrystallizationand pahoehoesheetflows [Hill et al., 1995].
For a viscosityof 1000 Pa s, the minimummodel flow
the flow stopping.Thusthe maximumcoolingrate allowable
are of the orderof 60 m. This alreadyexceedsthe
for a 1000-km-longflow would be 0.05øC/km. Table 1 lists thicknesses
the model runs and results for the Mars Observer Laser
measuredflow thicknesses,so we can confidently rule out
Altimeter (MOLA) derived slopeof 0.025%. Viscositiesof more evolvedand viscouscompositions(e.g., andesites)for
10, 100, and 1000 Pa s for the fluid lava are considered, these flood lavas. We conclude that physical properties
KESZTHELYI
ET AL.: MARTIAN
FLOOD
LAVAS
15,043
(a)
Figure 14. Aerial photographs
of platy lava in Hawaii. (a) Pondedflows at the northernend of Mauna Loa'ssummitcaldera
(Mokuaweoweo)and in North Pit. In this case,thereis no cleardirectionof flow recordedin the orientationsand shapesof the
crustalrafts. (b) The 1974 KilaueaSouthwestRift Zone openchannelflow. Lava pondedagainstthe cliff face and formeda solid
crustbeforethe flow surgedonward. Boththeseareasarecharacterized
by highflow ratesandshallowslopes.
similarto terrestrialbasaltsare probablythe mostappropriate
for furthermodelingof the Martianfloodlavas.
Since the model constrainsboth flow velocity and flow
thickness,we can also make quite robustconclusionsabout
minimum flow ratesper unit width of flow. This mustbe at
least1-5m2/sforthelavatobetransported
>1000kmwithout
freezing. The available images suggestthat the sheetsare
generally on the order of 10 km wide, requiring average
effusion
ratesontheorderof l-5xl04m3/sorgreater.
Thisis
approximatelyI orderof magnitudehigherthanestimatedfor
15,044
KESZTHELYI
ET AL.: MARTIAN
FLOOD
LAVAS
a
Figure 15. Cartoonof formationof raftedplateson Martian flood lavas. (a) Lava entersthe area,most likely as a rapid flood.
(b) The flow front stagnates
anda largevolumeof liquid lava is storedwithin the confinesof a thickening,insulatingcrust. Lava
is fed into the areaundera thick insulatingcrust. (c) The flow frontfails and lava breaksout in a runawayflood,raftingaway
piecesof crust.The sequence
is repeatedasthefloodwanesandslows,andan insulatingcrustbeginsto growagain.
Table 1. Results From Insulated Sheet Flow Model
Viscosity
(Composition)
Crust
TotalFlow
Thickness Thickness
Pa s
m
10
(ultramafic)
1
5
(turbulent)
(turbulent)
10
(turbulent)
20
100
(basaltic)
1000
(basalticandesite)
I
5
m
24
(turbulent)
27
terrestrialflood basalteruptions[Thordarson,1995;Self et
al., 1997, 1998;Thordarsonand Self, 1998].
Average
As an aside,the Keszthelyi[ 1995] thermalmodel for lava
FlowVelocity
tubes
indicatesthat 1000 km long flows could be formedon
m/s
theMartian
plains
withaneffusion
rateof only33m3/sanda
tube diameter of 22.7 m. However, since we have not found
0.25
0.75
10
26
0.39
20
32
0.21
1
5
(turbulent)
70
10
56
0.33
20
53
0.18
Insulatedsheet flow not consideredphysicallypossiblewith
turbulent flow.
any evidencefor narrowlavatubesin the Martian flood lavas,
and becausesuchlava tubesare not expectedon theseshallow
slopes[Keszthelyiand Self, 1998b], we find it very unlikely
that the Martian flood lavasseenin the recentMOC images
wereemplacedat low effusionrates.
3.2.2. Open sheet flow. In order to quantify the more
rapid, surging,stageof our hypothesizedemplacementmodel
for the Martian flood lavas, we require a different set of
thermalmodels. Crisp and Baloga [1994] developeda model
for the open channel aa flows during the 1984 Mauna Loa
eruptionthat is generallyapplicableto the open sheetflows
we postulate. The Crisp and Baloga [1994] modelcalculates
the coolingwithin the well-mixed core of the flow, including
KESZTHELYI
ET AL.: MARTIAN
FLOOD LAVAS
15,045
the effectsof reductionin radiativecoolingby a disrupted
While laminar,averageflow velocity for Newtoniansheet
crust,mixing of cold crustback into the flow, and the latent flow is givenby [e.g.,Bird et al., 1960]
heat of crystallization. Keszthelyi and Self [1998a,b]
(10)
presenteda minor modification of the Crisp and Baloga <v>= pg 0 H2/ 3 fl.
[1994] model to convertthe inputsto parametersthat are
There are many differentformulationsfor turbulentflow, but
somewhat easier to constrain for flows that are not observed
while active. In particular, the increasein crystallinity is Keszthelyiand Self[ 1998b] explainedwhy the one presented
calculatedfrom the coolingratherthanusingpetrologicaldata by Goncharov [1964] appearsto be the most applicableto
from samplescollectedfromthe activechannel.Atmospheric lava flows. Thusfor moderatelyturbulentflow we use
convectivecoolingand viscousheatingwere alsoadded,with
=gH0 / C•
(11a)
flow ratescalculatedassuminga Newtonianrheologywhile <v>2
Cf
=
•,/2,
(1
lb)
the flow is laminar and using the formulaspresentedby
),,
=
{4
1og•o[6.15((Re'+800)/4
1
)0.92]}-2,
(11
c)
Goncharov[1964] whenthe flow is turbulent.Equations(5)(11d)
(10) describethe Keszthelyiand Self [ 1998a,b] modification Re'= 2Re,
of the Crisp and Baloga [ 1994] model:
whereCf is a frictionfactorand•, is an empiricallyderivedfit
to experimentsof fluid flow that are moderatelyturbulent
(5)
c3r/c3x
= {Qvisc-Qrad-Qatm-Qentr}
/ {<v> 19Cp,S},
[Goncharov,1964].
Combining these equations, the Keszthelyi and Self
where T is the core temperature,x is the down flow direction,
[1998a,b]
modelcalculatesthe flow rate and coolingrate for
Qvisc
is theviscousheating,Qradis radiativeheatlosses,Qatm
is
the
core
of
a sheetor wide channelflow with a disrupted
atmosphericheat loss, Qentris the heat lost from the core of
the flow via entrainmentof the crust,<v> is the averageflow crust. However, it must be noted that the model does have
velocity,p is lava density,Cp* is heat capacityincluding several critical shortcomingswhen applied to the Martian
latent heat, and H is the flow depth. Viscous heating is flood lavas. First,the pulsatingnatureof the proposedmodel
describedby equation(2), and radiative heat lossescan be is not incorporatedinto this simple model. However, by
examiningthe two flow regimesseparately,we are able to
expressed
as
make some reasonable
inferences
about the overall
flow
Qrad
= •;*f(74- ra4),
(6) emplacement. Second, even if the liquid lava is
approximatelyNewtonJan,a thick brecciatedcrustcanimpart
the
bulk flow with a significantyield strength[e.g.,Boothand
where e is emissivity (-0.95), , is the Stefan-Boltzmann
factor,f is the fractionof coreexposed,and T, is the ambient Self, 1973; Hulme, 1974; Fink and Zimbelman, 1986].
temperature[Crisp and Baloga, 1990]. The atmosphericheat However,inputtinga reasonableviscosityof 500 Pa s for the
Laki lava, our flow modelproducesflow velocities(1-4 m/s)
losshasa similarexpression,
anderuption
rates(1500-9000
m3/s)thatareconsistent
with
the eyewitnessobservations. This suggeststhat the crust
Qatm-'h f(T- Ta),
(7)
riding on this type of flood might have a relatively minor
where h is the atmosphericheat transfercoefficient,which effect on the bulk flow rheology. Third, the modelcannotbe
hasan approximate
valueof 70 W m'2K'l on the Earth used to examine the temporal and spatial evolution of the
[Keszthelyiand Denlinger, 1996]. This value shouldbe flow. Still, the successof the original Crisp and Baloga
directlyproportional
to atmospheric
densityandheatcapacity [1994] model suggeststhat this modified version shouldbe
[e.g., Arya, 1988], suggestinga value of-1 for Mars. Heat more than adequate to provide broad constraintson the
transfer due to entrainment of the crust is
eruption parameters. More rigorous modeling of the flow
behavior,as in the work by Glaze and Baloga [1998], will
becomeapplicableonly when individualflows and breakouts
(8)
Qentr
= 19re, Hc (T- T0 / z,
canbe mappedout.
where Hc is the thicknessof the crust, T• is the average
The key inputs into this model are the thermophysical
temperature
of the crust,andz is the averagetime a pieceof propertiesof the lava (initial temperature,bulk viscosity,heat
crustsurvivesbeforebeingentrainedbackintothe coreof the capacityincludinglatentheat and bulk density)and the crust
flow.
(average temperature, survival timescale, and thickness).
Slopeis the singlemostimportantfactor[Keszthelyiand Self,
Reynoldsnumber (Re) is less than 500. We note that the 1998a],but is well constrainedfor the mostrecentflood lavas
valueof Re markingthe transitionto turbulentflow depends by the new MOLA data at about0.025% [Smithet al., 1999].
on both the exact versionof the Reynoldsnumberusedand The thermo-physicalpropertiesare related to the assumed
the geometryof the flow. The variouspermutations
of Re and compositionof the lavas. We have determinedthat the open
the critical Re that permeatethe publishedliteraturewere sheet flow model does not help further constrain the
discussed
by Keszthelyiand Self[1998b]. In this paperwe thermophysicalpropertiesof the lava and thus only discuss
use
runsusingthe basalticcompositionsuggested
by the insulated
The
sheet flow
is assumed to be laminar
while
the
sheet model.
Re = p rh<v> / rl,
(9)
The propertiesof the crust are poorly constrained. The
only published observational constraints on the crust
whererhis the lengthscaleof thelargestphysicalperturbation parametersare from Crisp and Baloga [1994] for the 1984
that canpropagatedownthe lengthof the flow (i.e., the flow MaunaLoa openchannelaa flow. After examininga rangeof
thickness
for a sheet),<v> is the averagevelocity,andrl is possiblecrustal parameters,we have chosento reportjust
viscosity.
three exampleswhich illustrate the main results. We call
15,046
KESZTHELYI
ET AL.: MARTIAN
Table2. CrustalProperties
forOpenSheetFlowModel
CrustType
Thin
Medium
Thick
'c,s
FLOOD LAVAS
extend more than a few tens of kilometersbefore cooling
>50øC, the flow must be >20 m thick. At such a flow
f, %
Hc, m
100
1000
50
10
0.05
1
100
500
thickness the calculated flow velocities will be of the order of
100,000
1
10
700
m2/sperunit flow width. For sheets
-10 km widethis
T-Tc, øC
a few metersper second,andthe localflow rateswill be >40
translated
to minimumlocalvolumetric
flowratesof-4 x 10s
m3/s. If individualflood-likebreakoutstraveledcloserto 100
thesethree cases"thin" crust, "medium"crust, and "thick"
crust. The parametersusedfor eachcaseare listedin Table 2
but requiresomejustification. In general,we assumethat a
km,thevolumetric
flowratesarelikelyto be-106m3/sand
The values listed in Table 2 for the thin crust case are for a
the recent Martian flood lavas are broad sheet flows that were
flow thicknesseshad to be >40 m. However,it is likely that
the final flow thicknessdroppedas the flood stageof the
waned,explainingwhy >40-m-tallflow margins
thinnercrustwill be bothhotterandmorerapidlydestroyed. emplacement
In the "thin"crustcase,we examinethe possibilitythat the havenot beenobservedin thisregion.
crust is only centimetersthick. Such a thin crustwould be
expectedto be disruptedandreincorporated
into the flow in a 4. Conclusions and Discussion
matterof minutes. The extensivelydisruptedcrustwouldalso
Based on comparisonwith the morphology of large
exposea relatively large portionof the interiorof the flow. terrestriallava flows andthermalmodeling,we concludethat
crustmarginallythinner,hotter,andmoredisrupted
thanthat emplacedby alternatingbetweentwo flow regimes:insulated
reportedby Crisp and Baloga[1994] for the earlypartof the sheet flow and flood-like breakouts. The closest terrestrial
1984 Mauna Loa eruption. The "medium" crust case analogwe have identifiedis part of the 1783-1784Laki lava
examinesparameters
relatedto a crustapproximately
a meter flow in Iceland that was subjectedto large surgesin lava
thick. Crustalblocksmight survivefor tens or hundredsof influx.
minutesand would be substantially
coolerthan the liquid
Modeling results suggest that the flows should have
interior of the flow. The "thick" crust is taken to be about 10 thermo-physicalpropertiesclose to basalts. More evolved
m thick with blockssurvivingfor roughlya day. The thick compositions(i.e., andesites)would requireflow thicknesses
crustcaseis likely to be the bestrepresentation
of the rafting greaterthan thoseobserved.Ultramafic lavasare not likely,
lavaon Mars,wherepiecesof crustarestrongenoughto form because they would require the formation of a stable,
rafts kilometers across. While these three cases do not cover
insulatingcrustovera turbulentflow.
the full rangeof plausiblecrustproperties,
they do provide
Average eruption rates for basaltic Martian flood lavas
importantinsightintothedynamics
of thesurgingfloodlavas. wereprobably--•10
4 m3/s,about1 orderof magnitude
higher
Despite the shortcomingsof the model noted earlier, than those estimated for typical terrestrial flood basalt
severalpointsare clearlydemonstrated
by the modelresults eruptions.However,the large surgesthat raftedaway pieces
shownin Table3. First,we canruleoutcertaintypesof flow of crust are likely to have involved short-livedvolumetric
behaviorwith a highdegreeof confidence.If the disrupted flowrates1 to2 orders
ofmagnitude
higher
(10s- 106m3/s).
crustis vigorouslybrokenup and mixed into the interiorof This style of emplacementis significantlydifferentfrom that
the flow, the flow will be remarkablythermallyinefficient.In inferredfor the majorityof terrestrialfloodbasalts.However,
fact, the "medium"thicknesscrust, with a survival timescale
of 15 minutes,coolsalmostan orderof magnitudefasterthan
the "thin" crustthat losesheat via essentiallyuninhibited
thermal radiation. This counterintuitiveresult is actually
easilyexplained. The crustridingthe flood-likebreakoutis
the productof daysor weeksof cooling. When this material
is mixed into the interior of the flow, the cumulativeheat loss
from the many daysof coolingis suddenlytransported
into
if typical
Martian
floodlavaeruptions
produce
103-104
km3of
lavaat-104m3/s,
theeruption
durations
areexpected
tobeof
the orderof a year to a decade,very similarto thoseestimated
for someterrestrialflood basalt eruptions[e.g., Thordarson,
1995; Thordarsonand Self, 1998].
Theseconclusions
naturallyraisea hostof new questions.
Perhapsthe mostobviousis, Why are the stylesof eruptionof
flood lavas different on the Earth and Mars? We speculate
the previouslyinsulatedflow interior. Not even wholesale
radiative heat loss from the surface of the flow can match this
rateof cooling.
The modeling suggeststhat in this most thermally
inefficient mode, the flood should not have been able to
advancemore than 1-12 km before losing>50øC from the
interior of the flow and being brought to a halt by the
resultingcrystallization.The fact that we seethe sheetlobes
with rafted piecesof crust extendingfor tens of kilometers
strongly indicatesthat this thermally inefficient style of
emplacementwas not the norm for the Martian flood lavas.
This rapid mixing of the disruptedcrustand freezingof the
lava would be mostlikely if the flow was turbulent,again
arguingagainstvery fluid ultramaficlavas.
The model runs with the "thick" crust are our current best
attemptto mimic the undanhlaup-like
phaseof the Martian
flood lavas. Even thoughthe thick crustcasehasa crustthat
is stableon the timescaleof a day,the occasional
entrainment
of blocksof crustleadsto substantialcooling. In fact, to
Table3. Results
FromOpenSheetFlowModel
Flow
Thickness
m
5
Flow
Velocity
m/s
0.19
Flow
Rate
m2/s
9.5
Crust
Type
Cooling
Rate
øC/km
"thin"
71
"medium"
520
"thick"
73
"thin"
8.8
10
0.78
7.8
"medium"
"thick"
65
9.1
"thin"
1.8
20
1.8
36
"medium"
"thick"
14
1.9
"thin"
0.57
40
3.0
120
"medium"
4.2
"thick"
0.58
KESZTHELYI
ET AL.: MARTIAN
thatthismaybe theresultof thethickerlithosphere
on Mars.
Wilsonand Head [1983, 1994] have shownthat larger
volumesof magmamustaccumulate
to breakthroughthe
thick lithosphere
on Mars. The resultingdikesshouldbe
widerandproduce
highereruption
rates.Thegasphasein the
propagating
dikeis expected
to be concentrated
in theupper
partof thedike[e.g.,Spence
et al., 1987],leadingto aninitial
vigorousfountaining
phasefollowedquicklyby a waning
phaseas the volatilesare rapidlyexpended.Eachphaseof
extending
the fissuresystemcouldleadto a pulsein the flux
of lavatravelingthroughthe flow,asin theearlypartof the
FLOOD LAVAS
15,047
Another location where the predictedrecent pyroclastic
depositsmight be preservedis in the polar regions. Again,
detailed in situ observationswill probably be needed to
distinguishash fall layersformedby an eruptionfrom dustrich layers causedby dust storms. Such observationscoult
enable
us to observe
the
interaction
between
volcanic
eruptionsand climate,in the samemanneras the terrestrialice
cores[Zielinskiet al., 1994].
Acknowledgments.This researchwas partially fundedby grant
NAG5-8308 from the NASA Mars Data AnalysisProgram. We
Laki eruption. Suchcyclesare commonfor terrestrialbasaltic thank David Pieri (JPL) and Bruce Murray (Caltech) for aerial
photographs
of the CarrizozoandPisgahflow fields. We alsowould
eruptions[Wadge, 1981], but might have beeneven more like to thankTracyGreggandDavid Crownfor veryhelpfulreviews.
dramatic on Mars.
We thankMike Malin, Ken Edgett,Elsa Jensenand othersat Malin
The hypothesis
thatthe Martianfloodlavaswerefed by SpaceScienceSystemsfor the excellentimages.
fissureeruptions
withrepeated
veryvigorous
phases
leadsto
predictionsthat may be testable with current and future References
missions
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styleof eruptionshould
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307 pp.,Academic,
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