Download Absence of Large Shield Volcanoes and

GEOPHYSICAL RESEARCH LETTERS, VOL. 18, NO. 11, PAGES 2121-2124, NOVEMBER 1991
ABSENCE
OF LARGE SHIELD VOLCANOES
AND CALDERAS
ON THE MOON:
CONSEQUENCE OF MAGMA TRANSPORT PHENOMENA?
James W. Head III
Departmentof GeologicalSciences,
BrownUniversity,Providence,RI
Lionel Wilson
EnvironmentalScienceDivision,LancasterUniversity,Lancaster,U.K.
Abstract.Large shieldvolcanoesandcalderasare not ob-
km diameter and are similar to those seen on the Earth in
served on the Moon. Theoretical assessments show that melt
Iceland and Hawai'i (Head and Gifford, 1980). However,
ascendingfrom the mantlewill stallat rheologicaltrapsor at
the shallowerdensitytrapat the baseof thelow-densitylunar
crust.Magma will then only reachthe surfacealongdikes
thereare no knownexamplesof shieldvolcanoescomparable
to the large shieldsfound on the Earth, Mars, and Venus
(Guestand Murray, 1976; Head andGifford, 1980;Head et
propagated
fromthesedeepreservoirs,
andsucheventswill be
infrequentandcharacterized
by very large-volumeeruptions.
Theseconditionsfavor the formationof sinuousrilles, single
long flows, and widespreadflow units,ratherthan abundant
al., 1981; Wilhelms, 1987). The small lunar shieldvolcanoes
(commonly
calleddomes)havesummitpitsin the 1-3 km
diameterrange(HeadandGifford,1980),butthereis little
evidencefor the presenceof large circular caldera-like
smalleruptionsfrom a centraledifice.The absenceof large
structuressimilar to those seenon the Earth, Mars and Venus.
shieldvolcanoesandcalderason the Moon is interpretedto be
related to the fact that shallowneutral buoyancyzonesand
magmareservoirsarenotfavoredon theMoon.
Shield volcanoeswith basal diametersin excessof 50 km
are commonon the Earth (Petersonand Moore, 1987), Mars
Whatis theexplanation
forthisobservation?
Largenumbers
of flowsemanating
froma centralsource
(Cart, 1974)andVenus(RobertsandHead, 1990;Schaber,
1991; Pike, 1978) (Figure 1). On Earth, thesefeaturesare
generally
builtfroma largenumber
of small-volume
basaltic
eruptions
andtheirassociated
dikes.Theyarecharacterized
by
a broadshield-shaped
topographic
profilewith low flanking
slopes,summitcalderas,and associated
flows, and they
commonly
haveflankingrift zones,fromwhichadditional
or associated
rift zone,andtheirintrusivecomponents
(dikes),
are the basiccomponents
thatbuild up shieldvolcanoeson
Earth(Figure2). Individualflowvolumes
aresmall,andflow
lengths
average
lessthanthebasaldiameter
of theedifice.Associatedsummitcalderasreflectthepresenceof shallowmag-
mabodieswheremagmahasreached
neutralbuoyancy
to form
a magmareservoir(Ryan,1987;WilsonandHead,1990).
Summitand flank eruptionsand calderaformation(Ryan,
EARTH
flows emanate. Summit calderasalso are common on large
Edifice
volcanic edifices on Mars and Venus. On the Moon, about
Magma stalls at
neutral buoyancy
Evolution of reservoir
causes lateral and
zone; dike eraplacement
vertical eraplacement
to produce
reservoir.
eruptions.
fiftysmallshield
volcanoes
areobserved
in thesizerange3-20
frequency hi_ahenough
o• 300
Growth
and numerous low-volume
•1
km
•Oceanic
Crust
/ •.^•.^
•::'i•-•'•••l•...uO.y.a.n.c.y:
k••"
...... i I• ('d
•: :-.'.............................
Venus Shield
t.,•<^<^<•,<^
I::/
• ))/zone a• t ))'••••••.t: iOY:.:::
I'.•
MOON
Lunar
Mare
Highestvolumedikesrea.ch surf.
ace to produce
•
'-,'
Terrestrial
Shield
•
Martian
•]
Tobias
Mayer
Lunar
Mare
Domes
Shield
Fig. 1. Scale comparisonof edifices and depositson the
Earth, Mars, Venus and the Moon. Modified from Gifford and
Head (1980).
Copyright1991by the AmericanGeophysical
Union.
,,/
•,
v
v
•, Dikes
propagate
•
/...
A
A
v
Crust v
L. Vtoward
^surface....•
/x
I I /--^L ^ A
,.
................
;;;;.... _ :. , ß.....
Fig. 2. Structureof a Hawaiian-typeshieldvolcanoon the
Earth,comparedto the characteristics
of crustalstructureand
magmaascenton the Moon.
Papernumber91GL02536
0094-8534/91/91GL-02536503.00
2121
2122
Head andWilson:Magma Transporton theMoon
1987) resultfrom changesin the volumeandexcesspressure
in the reservoir.Continuedreplenishmentof the magmareservoir at the neutralbuoyancyzone,andthe subsequent
lateral
andverticalemplacement
of smallvolumesof magmafromthe
reservoirinto and onto the volcano,build up the shielduntil
the magmasupplyceases.Therefore,the presenceof shield
volcanoesandcalderason a planetarysurfacestronglyimplies
the presenceof shallowneutralbuoyancyzonesandmagma
reservoirs.The presenceof thesereservoirsinvolvesthe stalling and evolutionof magmathere,the productionof numerous eruptionsof relatively small volume and duration, and
associated
shallowmagmamigrationto causecalderacollapse.
A wide variety of volcaniclandformshasbeenobservedin
Key factorsin the formationof sinuousrilles by thermal
erosionare: 1) turbulentflow, requiringhigh effusionrates
andaidedby low viscosityandyieldstrength;
and2) sustained
flow (implying very long durationeruptionsand thusvery
higheruptionvolumes)to causethecontinued
downcutting
to
the observedrille depths.Eruptionsthat causedmany of the
largesinuous
rillesfoundon theMoonswere
thereforeapparently characterizedby rapid effusionof low yield strength
lavasfor prolongedperiodsproducingflowsof extremelyhigh
volumes(in therange300-1200km3).Thesecharacteristics
are in strongcontrastto typicaleruptionvolumesfor shieldrelated flows on Earth, which are much less than one km3
(Petersonand Moore, 1987), and to the volume of the largest
and associated with the lunar maria, and these are illustrated
and discussed in detail elsewhere (Head, 1976; Wilhelms,
historiclavaflow (Laki,Iceland;about12 km3).
1987). Lava flow unitshave beendistinguished
on the basisof
topographic,albedo,andcolor boundaries(Wilhelms, 1987),
but flow units are usuallybroad (not linear) and their sources
are obscure.No examplesare known where many individual
flows radiatefrom a centralsource.One of the few examples
of volcanic features on the Moon. (Guest, 1971; Guest and
Murray, 1976; Whitford-Stark and Head, 1977) and these
could be candidatesfor shallowmagma reservoirs.The most
where
individual
linear
flows
to a common
significantof theseconcentrations
occurin OceanusProcellarum and includethe Marius Hills area(35,000 km2), which
displays20 sinuousrilles and over 100 domesand cones,and
where Eratosthen-
theAristarchus
Plateau/Rima
Prinzregion(40,000km2)which
can be traced
source area is in southwest Mare Imbrium
There are several areas that show unusual concentrations
Jan-agedflows originatefrom the basinedgeand extendinto
the basin interior in three phases1200, 600, and 400 km in
length.The characteristics
of theseflow unitsimply higheffusionratesand large-volumeeruptions,comparableto someof
those observed in the Columbia River basalts, and the units
appearto be separatedin time by hundredsof millionsof years
(Schaber, 1973).
Meanderingchannelsknown as sinuousrilles occurprimarily in the lunar maria, andpreferentiallyalongthe edgesof
the maria (Schubertet al., 1970; Murray, 1971). Rille widths
rangeup to about3 km and are commonlyrelativelyconstant
alongtheir course;lengthsrangefrom a few km to more than
300 km. In somecases,sinuousrilles originatein circularor
elongatedepressionsand theseare sometimeslocatedin the
highands adjacent to the maria. These depressionsdo not
appearto have associatedflows other than the sinuousrille,
and they are not characterizedby topographicbuilduptypical
of a shield. Although the general morphologic similarity
between sinuous rilles and terrestrial lava channels and tubes
led to the interpretation that they were of similar origin,
sinuousrilles are generallyan order of magnitudelarger and
oftenmuchmore highly sinuousthanterrestriallava channels.
The characteristicsof lunar sinuousrilles unexplainedby
simple lava channel/tubemodels can be accountedfor by
associatedthermal erosion (Hulme, 1973; Carr, 1973; Hulme,
1982). Thermal erosion processesseem to best explain the
majority of characteristicsof larger sinuousrilles, although
many lava channelsand collapsedtubes do occur and are
clearlylinkedgenetically.
Eruption conditionscharacteristicof large sinuousrilles
can be inferredfrom their length,width, depth,andthe nature
of their sourceregions.Hulme (1973) calculatedan effusion
rateof 4 x 104m3/s,an eruptiondurationof aboutoneyear,
anda totalvolumeof about1200km3for a 50 km longrille in
the Marius Hills. Wilson and Head (1980) and Head and
Wilson (1980) usedthe characteristics
of sourcedepressions
of sinuousrilles to developindependentevidencefor extremely high effusion rate eruptions of long duration. In their
model, sourcedepressions
are accountedfor by lava fountaining, very high accumulationrates,and thermalerosionof the
near-ventarea duringa high-volume,long-durationeruption.
is dominatedby 36 sinuousrilles (Whitford-Stark and Head,
1977). Although small domes and conesoccur, there is no
evidencefor the centralizedoccurrenceof flows reflectingthe
presenceandevolutionof a shallowmagmareservoir.Rather,
the high concentrationof sinuousrilles suggeststhat these
complexesare the sitesof multiple eruptionswith high effusionratesandlargevolumes,andthat thesecentersmay be the
sourceof muchof the lava in OceanusProcellarumemplaced
duringthe Imbrian andEratosthenian
periods(Whitford-Stark
and Head, 1977; 1980). The large-volume eruptionsapparently spreadtheir lava depositsextensivelyinto the surrounding maria,ratherthanbuildinga largelocaledifice.The lack of
associatedcalderas,collapseddepressions,or sagssuggests
that the sourcesof these high-volume eruptions must have
beenat significantdepth.Somecratersin the sizerange20-40
km (smooth-rimmedcraters;Wilhelms and McCauley, 1971)
havebeeninterpretedasvolcanicin origin(DeHon, 1971),but
these types of features appear more likely to be linked to
modified impact craters than to shield-like structuresor
calderas(Pike, 1980; Wilhelms, 1987, p. 32-33).
Considerationof the total volume of volcanic deposits
emplacedduring the period of mare volcanismindicatesthat
averageglobaleffusionratesduringthepeakperiodwerevery
low (about 10-2 km3/a), comparableto the presentlocal
averageeffusionratesfor individual terrestrialvolcanoessuch
as Hawai'i and Etna (Head and Wilson, 1991). However, the
characteristics
of volcaniclandformsand depositsassociated
with the lunarmariasuggestthatemplacement
did not occuras
a continuousseriesof low-volumeeruptions,but that largevolumeeruptionswith very higheffusionrateswererelatively
common (e.g., sinuousrilles and the Imbrium basin Eratosthenjan-agedflows), and separatedin time. Eruptionswere
oftenlocalizedalongbasinmarginsandin volcaniccomplexes,
particularly in OceanusProcellarum, and may have been
separatedin time by significantintervals,particularlyin the
later stagesof mare emplacement.In addition,there is little
evidencefor shallow magma reservoirsand repeatedsmallvolumeeruptionsthat wouldbuild up largeshieldvolcanoes.
Takentogether,thesecharacteristics
appearto favora styleof
emplacementinvolving large batchesof magmaeruptedover
shortperiodsof time from relatively deep sources,and separ-
Head andWilson: Magma Transporton the Moon
2123
atedin time and spacefrom one another.Thesecharacteristics
do not favor the conditions that produce shallow neutral
buoyancy zones, shallow magma reservoirs,calderas, and
vertical extent which is approximately double the crustal
thickness,so that the positive buoyancy of the melt in the
shield volcanoes.
negativebuoyancyof thepart of the dikein thecrust.
If the lower end of a dike staysconnectedto its melt
source,two factorsact to allow magmawhich is negatively
buoyantin the crustto reach the surface.The first (Solomon,
1975)involvesconsidering
thenetbuoyancyof themeltin the
The basic physical principles of lunar basaltic magma
ascent and eruption (Wilson and Head, 1981; Head and
Wilson, 1991) permit a better understanding of the
implicationsof mare landformsand depositsfor the rangeof
eruptionconditionsin existenceduring emplacementof the
lunar maria. The key factorcontrollingwhetheror not surface
mare basalteruptionscan take placeon the Moon is that the
densityof the anorthositic
crust(2.8 g/cm3)is significantly
lessthanthedensityof themantle(3.4 g/cm3).Basalticpartial
meltsproducedin the lunar mantle,havingdensitiesof about
3.2 g/cm3,are positivelybuoyantthroughoutthe mantlebut
negativelybuoyantthroughoutthe crust.This meansthat the
crust/mantle boundary acts as a physical (density) trap
preventing the further rise of basaltic magmas as long as
buoyancyalonecontrolstheir ascent(Solomon,1975).
Buoyancy is certainly the major factor affecting the
separationof partial meltsfrom their sourcerocks.However,
partial melting is itself most commonly the result of the
buoyantrise of the sourcerocksrelativeto their surroundings
and their consequentadiabatic decompression.Various
alternativeshavebeenproposedfor thedegreeof separation
of
melt and solidresiduumduringthe meltingprocess,but it is
clearthat meltsrisingfrom greatenoughdepthsin the mantle
are likely to do so aspartsof partiallymoltendiapiricbodies
with sizesranging from hundredsof metersto kilometers.A
diapirrisesat a ratedeterminedby its densitycontrastwith the
surrounding
rocksandthe temperature-dependent
rheological
propertiesof thoserocks.Heat is transferredfrom the diapir
into the surroundingrocks, reducing their viscosity and
increasingthe rise ratesof the diapir;however,this process
commonlyleadsto the freezingof the melt componentof the
diapirbeforeit approaches
the surface(Marsh, 1984).The fact
that the temperatureof the country rocks decreaseswith
increasingproximityto the surfacemeansthatthe abilityof a
diapir to rise will be rapidly reducedat somelevel in the
mantle,leadingto theconceptof a rheological
trapfor diapirs.
Melt may still be able to reachthe surfacefrom a diapir
stalledat a rheologicaltrap providedstressesaccumulatein
sucha way that a dike can beginto propagateinto the overlying rocks(Sleep, 1988). This is essentiallyaccomplished
by
the presencein the melt of a pressurein excessof the local
lithostaticpressure.This pressurearisesfrom expansionon
meltingandthe differencebetweenthe compressibilities
of the
melt and the surroundingsolid rocks. Failure of the rocks
overlying a melt body will occurwhen the excesspressureis
approximately
onehalf of thetensilestrengthof therocks(Tait
et al., 1989). The tensilestrength(which may have a value of
order 30-50 MPa, but in general is a function of the local
temperatureandpressure,andpossiblyalsothe appliedstrain
rate) thereforeacts to limit the largestpressuredifferential
whichcanbuild up to a valueof theorderof 15-25 MPa.
Dikes can propagatefrom a melt body at depth in two
ways: 1) the dike growsuntil it extendscontinuously
from the
melt source to the surface; 2) the lower end of the dike
disconnectsfrom the melt body beforethe upperend reaches
the surface.In the lattercasethe dike propagatesas a separate
structurewithin the countryrocks, and can only continueto
rise as long as the melt within it has a net positivebuoyancy;
suchdikescannotpenetratethe lunarcrustunlesstheyhavea
mantle
section of the dike more than counterbalances
the
dike, mentioned above in connection with disconnected dikes.
If thepressure
dueto theweightof thecolumnof magmain an
open dike is lessthan the pressuredue to the weight of the
surroundingcountry rocks overlying the sourcebody, the
pressuredifferentialrepresentsan excesspressurewhich is
availableto overcomethe wall friction involvedin driving
magmathroughthedike,anda surfaceeruptioncantakeplace
(Wilson and Head, 1981). This analysisassumesthat stresses
in the rocks surroundingthe magma sourcebody can be
approximated
by lithostaticpressures,
butthisis acceptable
at
thedepthswithin thelithosphere
of interesthere.
There is, however, a second factor involved in the
existanceof an open dike. The pattern of stresseswhich
controlsdike geometryis suchthat a finite excesspressure
mustexistin the magmasourcebodyto hold the dike open
againstelastic forces (Pollard, 1987). The size of this excess
pressuredependson whether or not the melt in the dike is
buoyant, and for the negatively buoyant case (Rubin and
Pollard, 1987) it is always the case that a larger excess
pressureis required to keep the dike open than the excess
pressurewhich arisesfrom the magmaweightconsiderations
(HeadandWilson, 1991).Thus,it is the availabilityof a high
enoughstressto keep a dike open to the surfacethat is the
limitingfactorin allowingsurfaceeruptions
to takeplace,not
theneedto supporttheweightof themagmain thedike.
When the abovestresses
are evaluatedfor magmasource
bodiesstalledat the densitytrap at the baseof the crustor at
greater depthsin the mantle (Head and Wilson, 1991), it is
foundthatstresses
greaterthanthe 15-25MPa limit imposed
by thecountryrocktensilestrengtharerequiredif themagma
sourceis deeperthan about150 km. Thus, surfaceeruptions
cannotbe achievedunlessmelt canpenetrateto within 150km
of the surface.As it happens,
thisdepthis comparable
to twice
the thicknessof the lunarcrust,implyingthatdikeswhichare
disconnected from melt sources will be no more common than
connected
dikes.Modelsof thethree-dimensional
geometryof
dikes(Lister,1990)canbe usedto calculatethe approximate
widthsandthicknesses
of dikesextendingfromdepthsof 60150 km and yield melt volumesin the range200-600 km3
(Head and Wilson, 1991). Thus, unlessa dike happensto
penetratejust as far as the surface(a statisticallyunlikely
circumstance),
thevolumeof magmaeruptedthrougha dikeof
this size is likely to be much greaterthan the volume of the
dike itself. Dikes growingfrom magmabodiesstalledat the
baseof the crustwill havevolumesof about20-60 km3, about
10 timeslessthan the valuescited above,but still far in excess
of eruptionstypicalof terrestrialshieldvolcanoes.
Largeshieldvolcanoes
andcalderas
areapparently
absent
on the Moon. The characteristics of lunar mare landforms and
deposits
indicatethattheaverage
globaleffusionratewasvery
low,butthatmanyindividualeruptions
werecharacterized
by
high effusionratesand relativelylong durations,and were
separated
in timefromoneanotherby relativelylongperiods.
Thesecharacteristics
areconsistent
with theoretical
analyses
of
the ascentanderuptionof magmaon theMoon, whichindicate
2124
HeadandWilson:MagmaTransport
ontheMoon
that the low density of the lunar crust and its thickening
lithosphere
createbuoyancy
andrheological
trapswhichcause
ascending
magmadiapirsto stallat significantdepths(at the
baseof thecrustor deeper)(Figure2). Analysisof thefactors
requiredto propagatedikesfrom thesesources
to the surface
indicatesthat surfaceeruptions
will be infrequentandof very
Pike, R. J., Geometricinterpretation
of lunarcraters,U.S. G.
S. Prof. Paper 1046-C, C1-C177, 1980.
Pollard,D. D., Elementaryfracturemechanicsappliedto the
structuralinterpretationof dykes,in Mafic Dyke Swarms,
editedby H. C. Halls and W. F. Fahrig,Geol. Assn.of
Canada,Sp. Paper 34, 5-24, 1987.
highvolume,similarto thosethoughtto beresponsible
for the
Roberts, K. M. and Head, J. W., Lakshmi Planum, Venus:
Characteristicsand modelsof origin, Earth, Moon and
Planets 50/51,193-250, 1990.
formation of sinuousrilles. Shallow neutral buoyancyzones
and the resulting magma reservoirs,from which smallvolume,high-frequency
eruptions
canoccurto buildanedifice
surrounding
a centralvent,arevery difficultto produceunder
these conditions.
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