The Subglacial Birth of Olympus Mons and Its Aureoles

Home , Elysium Planitia, Geology of Mars, Martian surface, Olympus Mons

VOL. 84, NO. BI4 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER 30, 1979

CARROLL ANN HODGES AND HENRY J. MOORE

U.S. GeologicalSurvey, Menlo Park, California 94025

The vast volcanic plains and shieldsof Mars, togetherwith the thermal, spectrographic,and morphological evidencefor water ice at the poles,for severalpercent water in Viking soil samples,for ground ice or permafrostover much of the planet, and for the existenceof surfacewater at some time in the past, suggestthat magma and water or ice may have interactedduring evolutionof the planet'slandscape. Rel- atively small mesas and buttes, with and without summit craters, are remarkably similar to the table mountainsof Iceland that formed by subglacialeruption during the late Quaternary period. Table mountains typicallycomprise foundations of pillow lava and palagonitizedtuff breccias(m6berg), overlain by subaeriallava flows that commonly culminated in a typical shield volcano;some table mountains,however, failed to reachthe subaerialstage and thus lack the cap rock. Subglacialfissure eruptions produced ridgescomposed of pillow lava and m6berg. Conical knobs on steep-sidedMartian plateausare reminis- cent of the small Icelandic shield volcanoesatop m6berg pedestals.Candidate table mountains on Mars are especiallynumerous in the region betweenlatitude 40øN and the margin of the north polar cap, and interaction of lava with a formerly more extensiveice cap may have occurred.More significantis the possibility that Olympus Mons and the broad lobate aureole depositsaround its base may have had similar subglacialbeginnings. This hypothesisrequires an ice cap severalkilometers thick in the vicinity of this enormous shield during its initial stagesof eruption. The amounts of water ice required do not appear excessive,given the limits to presentknowledge of the water budget throughout the planet's history. A mechanismfor localizing suchice, however, is required.

INTRODUCTION [1960, 1966a,b, 1967], Sigvaldason[1968], and Preusser[1976]. The morphologiesof numerous small landforms on Mars These distinctivelandforms may be roughly circular but more are not readily interpretable in terms of conventionalimpact, commonly are subrectangular,with abrupt edges and steep flanks that slopeas much as 35ø or more (Figures 5a and 5b). volcanic, or erosionalprocesses. Such featuresinclude circular to rectangularmesas and buttes, with and without summit The top is flat or gently convex, with or without a crater at the craters(Figures 1 and 2), and 'two-story'structures consisting summit, and is composedof aa and pahoehoe flows which in of conicalor irregularknobs atop broad, flat, steep-sidedped- most cases developed into a small subaerial shield volcano estals(Figure 2a). Somelandforms exhibit morphologies con- [Kjartansson,1960]. Underlying the subaerialflows is a pedes- sistentwith either volcanic or impact origins, so their genetic tal-like foundation of 'm6berg,' the Icelandic term for pala- classificationis not obvious.These types of features are espe- gonitized tuffs and breccias[Kjartansson, 1960]. The m6berg cially abundant in the 'northern plains,' betweenabout lati- mass in turn overlies and is partly intercalated with a basal tude 40øN and the margin of the polar ice cap. Features that core of pillow lava. Some table mountains lack the subaerial are clearly volcanicalso occurin parts of this region, specifi- basalt cap (Figure 6). cally, north of the Tharsis and Elysium volcanicprovinces. The pillow lava and m6berg pedestalsindicate that water Well-defined lava flows on the flanks of Alba Patera are inter- was abundant at the site of extrusion. The prevailing genetic mingled with conical cratered domes a few kilometers across hypothesisis diagramedLn Figure 7 [Einarsson,1968]. Basaltic that resemble small volcanic cones; clusters of such features magma was extruded subglacially into water-filled caverns have been noted elsewhereas well (Figure 2b). melted into or through a late Quaternary ice sheet.The initial If terrestrialanalogs are to be found for theseMartian land- extrusions of pillow lava were emplaced beneath meltwater forms,they are mostprofitably sought where geologic and cli- and ice. As the pile of pillow lava grew upward, confining matic conditions are similar to past or present environments pressuresof the overlying meltwater and ice decreased,and on Mars; Iceland, with its combination of active volcanism the consequentincreased vesiculation of the magma, aug- and glaciation,is a promisinglocale to explore [Allen, 1977, mented by influx of water into the vent, initiated phreatomag- 1978, 1979;Hodges, 1977; Hodges and Moore, 1978a, b; Wil- matic explosions [Jones, 1970]. Sideromelane tuff breccias, liams, 1978].The table mountainsof Iceland (Figures3a and subsequentlyaltered to palagonite,were then depositedatop 3b), whichformed as a resultof subglacialvolcanic extrusion, the pillow lava. If the accumulatingpile reachedthe surfaceof are our major topic of interest here, augmentedby reference the meltwater lake, subaerialflows were emplaced,commonly to oceanic shield volcanoes such as Mauna Loa, which have capped by a shield volcano. As the subaerial flows advanced subaqueouseruptive historiesparallel with the subglacial into the meltwater at the margin of the pile, explosive dis- stagesof the Icelandic table mountains[Kjartansson, 1966a; ruption occurred,and tuff brecciaswere depositedas steeply Moore and Fiske, 1969]. dipping sediments,much like the foreset beds of a delta, but in this caseoverridden by topsetbeds of lava [Jones,1969]. ICELAND'S TABLE MOUNTAINS With subsequentwithdrawal of the ice, the subaerial flows The relatively small, isolated plateaustermed table moun- formed a resistantcap overlying the less resistantm6berg tains (Figure 4) have been describedin detail by Van Bernrne- foundation. The thicknessof the ice sheet, indicated by the len and Rutten [1955], Jones [1966, 1969, 1970], Kjartansson height of the m6berg pedestals,was apparently 500-1000 m during the Pleistocenestage of table mountain development. Copyright ¸ 1979 by the American GeophysicalUnion. Crateredhills or mesasof m6bergthat are not cappedby lava Paper number 9B 1125. 8061 0148-0227/79/009B- 1125501.00 8062 HODGES AND MOORE: SECOND MARS COLLOQUIUM

a b Fig. 1. The small circular plateau with summit crater (arrow) is one of the best candidates for a Martian table mountain. It has a morphology and dimensions that are similar to the table mountains in c d Iceland which are formed by subglacial volcanic eruption (compare with Figure 3). Terraces and ridgesoccur near the base.Smaller, similar featuresare nearby. Horizontal and approximate vertical scalesof --- 6 km profile are same scaleas photograph.(Height estimatedfrom shadow length.) Sun at left. (Latitude 49øN, longitude 225ø; Viking photograph 009B 15.) Fig. 2a. Rectangular to subcircular Martian mesas that resemble Icelandic table mountains with (arrow a) and without (arrow b) obvious summit craters.Two-story structureand typical impact craters in- (Figure 6) and m6berg ridges (formed by subglacialfissure dicated by arrows c and d. Diagrammatic profiles drawn at ap- eruptions) presumably representextrusions that did not reach proximately twice the scale of photograph;no vertical exaggeration. (Heights estimatedfrom shadowlengths.) Sun at left. (Latitude 45øN, the surfaceof the ice or meltwater [Kjartansson,1960]. longitude 20ø; Viking photograph026A30.) These Icelandic features are relatively small, generally a few kilometers across and a few hundred meters high; the highest,Herdubreid (Figure 5), has over 1000 m of total relief [Van Bemmelenand Rutten, 1955]. The contrast in morphology between strictly subaerial shieldsand table mountains in . . Iceland [Jones, 1969] is readily apparent on topographic maps of the two types of structures(Figure 8). The tuyas of British Columbia also have been interpreted as ..i•' ;•;•i;..;:•::::::::::::::::::::::::::::::::::::::: productsof subglacialPleistocene eruptions [Mathews, 1948], ... ß:'*:i.'*" "'"'::• as have similar volcanic rock successionsin Antarctica [Le Masurier, 1972] and Alaska [Hoare and Coanrad, 1978]. Anal-

...:. yses of subglacial shield-type eruptions and resulting land- "':.•i""•*' *<":' n..... ":'-};:. forms are applicable to the submarine development of larger oceanic shield volcanoes [Kjartansson, 1966a, 1967; Jones, 1966; Moore and Fiske, 1969].

OCEANIC SHIELD VOLCANOES

Moore and Fiske [1969] documented a threefold structure analogousto that of the table mountains in the shield volcanoes on the island of Hawaii (Figure 9): (1) pillow lava and fragmentserupted from deepwatervents, overlain by (2) vitric explosiondebris, littoral cone ash and breccia erupted from shallow water vents or at the interface of subaerial lava and water, and (3) the subaerial shield that forms the super- structure of the volcano. As in the table mountains, the con- tact between the'subaerial shield and the submarine pedestal is marked by a distinct break in slope (becauseof the suscepti- bility to masswasting and erosionof the brecciasand pillow Fig. 2b. Small,possibly volcanic, cratered domes (arrow). Dia- lava), but the lower flank is not as steepas in the subglacial grammaticprofile drawn at scalelarger than that of photograph;no case. Kjartansson [1966a] inferred that the same stages and verticalexaggeration. (Height estimated from shadowlength.) Sun at productsof eruption characterizeddevelopment of the Surtsey left. (Latitude40øN, longitude 15ø; Viking photograph 035A62.) HODGESAND MOORE: SECONDMARS COLLOQUIUM 8063

Fig. 3a. Landsatview of three table mountainsin northern Ice- land: (a) Baejarfjall;(b) Kviholafj611;(c) Gaesafj611.GaesaO611 (c) has a cap of subaeriallava; the othertwo tablemountainsapparently con- sist entirely of m6berg. All three have summit craters. Compare a with probableMartian counterpartin Figure 1. North at top; sun at right. (Landsatimage 2094-11403, band 7.)

Island shield,off the south coastof Iceland, but recent drilling showedno evidenceof pillow lava. The subaqueoussection at Surtseyconsists entirely of tuffs, down to the preexistingsea Fig. 4. Aerial photographof GaesaQ611with summit crater (arrow a) and cap of subaerialflows (b) that are superposedon m6berg floor at about 180 m (J. G. Moore, personalcommunication, exposedon steepslopes (c). Postglacialflows indicated by d. North at 1979). top; sun at lower right. Profileat approximatelysame scale as photograph; no vertical exaggeration[Van Bemmelenand Rutten, 1955, LANDFORMS ON THE NORTHERN PLAINS OF MARS Plate XIX].

A preliminarysurvey of Viking photographshas yielded the heightsmentioned here and shownin the illustrationpro- numerous examplesof landforms that resemblerather strik- files were estimated on the basis of shadow lengths and are ingly the Icelandic table mountainsand m6berg hills, or pos- thus subjectto revision;both sun angle and image-processing sible variantsthereof (Figure 10). Thesestructures, like those techniquesaffect suchestimates.) The featuresare commonly in Iceland, are a few kilometers acrossand of the order of a associated with other landforms more convincingly inter- few hundredmeters high. (We mustemphasize, however, that preted as volcanic. For example, the isolated flat-topped mesasof Figure 1la are adjacentto two crateredcones that resembleterrestrial subaerial volcanoes(Figures 11b and 11c). In some plains areas (notably, westernElysium Planitia and southeasternAcidalium Planitia), domesand coneswith summit craters are numerous; their morphology is different from that of typical impact craters of similar size that appear equallyfresh (Figure 2b). The configurationof the plainssur- face in the Elysium area is irregular and pitted (Figure 12), much like the surfaces of terrestrial lava flows. The geomorphicevidence that volcanismoccurred locally in the northern plains is persuasive;we proposethat some of the unusualmorphologies may have resultedfrom interaction -,,,,-2,5'km of magmawith ice in a processanalogous to thatof the sub- glacialeruptions that formedtable mountainsin Iceland.This explanationwould require surfaceice severalhundred meters thick to have existed beyond the present north polar cap of Mars, a condition that could perhapshave occurredwith relatively slight modificationsin temperatureor pressureand/or Fig. 3b. Verticalaerial photographof table mountainsa and b water content of the Martian atmosphere.A comprehensive shownin Figure3a, with third vent and m6berg lobe visible south- analysis (in progress)of the spatial distribution of thesekinds westof juncture.North at top;sun at lowerleft. E-W profileacross circulartable mountain atapproximately samescale asphotograph; offeatures isnecessary inorder to determine therelation, if novertical exaggeration. (U.S.Air Force photograph AF-55-AM-3, any, between their occurrence and latitude on the planet. Our roll 121,frame 13394.) preliminaryresults are consistentwith thoseof Allen [1979]. 8064 HODGES AND MOORE: SECOND MAR•-•COLLOQUIUM

Fig. 5a. Herdubreid, highestof the Icelandic table mountainswith over 1000 m of relief, as viewed from the northeast. Snow-coveredshield volcano is superposedon subaerial flows at top of steepcliff; intercalated palagonite tuff breccia and columnar basalt form the pedestalfoundation. (Photographby C. A. Wood.)

OLYMPUS MONS AND ASSOCIATED FEATURES of superposedarcuate lobes,differing in degreeof degradation and burial. The distal margin of one of the oldest lobes is The enormousshield volcano Olympus Mons, 26 km high about 600-800 km north and west of Olympus Mens; younger and 600 km acrossat its base, may also have had subglacial lobes are closerto the shield (Figures 13a and 13b). A gravity beginnings. The shield is at an elevation of 2-3 km on the anomaly requiring excessmass at depth has been resolved northwest flank of the 'Tharsis bulge,' a northeast trending over the northwest lobe of the aureole by line-of-sight track- topographicridge 10 km above Mars datum [U.S. Geological ing of a Viking orbiter [Phillipsand Bills, 1979;Sjogren, 1979]. Survey, 1976]. The bulge is surmounted by a chain of three Smooth plains materials [Scott and ½arr, 1978] embay and shield volcanoes, each of which is somewhat smaller than partly bury thesedeposits in an annular belt at the baseof the Olympus Mens, but like the larger shield, all attain elevations shield. The exposedarea of the aureoles,with some allowance of 26 km. Olympus Mens is distinctivenot only for its size but for overlap,is about 1.3 x 106km 2. also for the prominent escarpment,up to 6 km high [Blasius, The prominent escarpmentencircling the base of Olympus 1976], that rings its base. In addition, it is nearly surrounded Mens is perhaps its most perplexing feature, for there is no by peculiar grooved terrain informally termed aureole depos- apparent counterpart around subaerial terrestrial volcanoes. its [Carr, 1973], which appear to consistof a sequentialseries The scarp does, however, suggestanalogy with the subglacial O' /-/e rdo re t d

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Fig. 5b. Diagrammatic crosssection through Herdubreid (Figure 5a); intercalated columnar basaltsand palagonite tuff brecciasform the base, overlain by subaerial top basaltsand surmountedby shield volcano [ Van Bernrnelenand Rut- ten, 1955]. HODGESAND MOORE: SECONDMARS COLLOQUIUM 8065

Fig. 6. Aerial photograph of BaejarOall (arrow a) and Kviholaf- j/S11(b) (also shown in Figure 3), which are composedalmost entirely of m6berg. These two table mountains either formed subglacially without subaerial venting and thus were not capped by subaerial flows or were eroded after emplacement so that basaltic cap rocks were removed. Both have summit depressions.Low sun angle en- chancestopographic irregularities; compare with Figure 3b. North at iliiiiiiiiiii::: ':::ii::::'::t' top; sun at lower right [ Van Bemmelenand Rutten, 1955,Plate XXV]. .... : ...... ,i;!ii!iill table mountain pedestalsand with the subaqueousflanks of oceanicshields. Such an analogy implies that (1) ice existed in the Olympus Mons vicinity when eruptions began, (2) the vent or vents eventually surfaced above the ice, and (3) the enormoussubaerial shield was emplaced atop the subglacially confined platform. Lava flows cascading over the scarp indicate that Olympus Mons continued to grow long after the postulated ice receded;such flows are evidencethat the scarp existed during late growth of the shield and is not simply a postvolcanicerosional product. Similar relations occur in Ice- land. The grooved, eroded appearance of the aureole lobes and their specificassociation with Olympus Mons lead us to sug- gest further that these depositsalso may be products of volcanic eruptions beneath or into an ice cap, but like the m6- berg ridges in Iceland these extrusionsfailed to surface above the ice or meltwater lakes. The age of the lobes is uncertain becauseof the difficulty in recognizingcraters within this rug- ged terrain. In a preliminary effort, however, we classifiedde- pressionsand circular features (>2.5 km across)as definite craters,probable craters, and possiblecraters, and our analy- ses showed that the aureoles appear to,have preceded development of Olympus Mons; additionally, they are probably as old as or older than the Chryse area [Dial, 1978] and cratered plains [Scott and Carr, 1978]. The aureole materials bear some resemblance to the Icelan- dic m6berg hills that are not capped by lava, one of which is portrayed in the low-sun photographof Figure 6. The m6berg depositsof Iceland are highly susceptibleto erosion by wind Bedrock Glacier Meltwater Pillow lava and subsurfacewater and commonly exhibit irregular gro- and dikes tesque shapesand closed depressionswithout surficial drain- age [Kjartansson,1960, Preusser,1976]. Several ragged mesas that appear to be remnants,or outliers,of the aureoledeposits are perhaps the best Martian analogs for table mountains; they are about 250 m high (as inferred from shadow length Tuff and Foreset bed- Lava Scree measurements),with craterlike depressions,and occur 200- breccia ded pillow- 300 km southof the baseof Olympus Mons (Figure 14). breccia If the aureole lobes representsequential volcanic deposits, Fig. 7. Development of an Icelandic table mountain from sub- the eruption centers appear to have migrated within a rela- glacial birth (I, bottom)to subaerialerosion (IV, top). (Modified after tively confined area before eventually being sustainedat the Einarsson[ 1968].) 8066 HODGES AND MOORE: SECOND MARS COLLOQUIUM

19o30 ' Kilometers 0 10 20 30 4O 50 •.-J--!64o50' A VERTICALEXAGGERATION X2 6,000' SEA! 2,000'LEVEL •

B SEA LEVEL ..... 12,000'6,000' • "•

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SEA6,000'LEVEL •

6,000' D' SEA LEVEL ' 6,000' ! 2,000' ! 4,000'

Pillowlava Clasticrocks, Subaeriallava flows Intrusive rocks includingphreatic (M•in shield) t explosion ash Fig. 9. Geologic sectionsthrough Kilauea and Mauna Kea volcanoes, Hawaii, showing sequentialstages of developmentfrom initial emplacementof pillow lavas (A-A') through constructionof subaerial shield (D-D') [Moore and Fiske, 1969].

SHIELD

TABLEMOUNTAIN

Fig. 8. Contrastingtopography of subglaciallyerupted shield volcano that formed table mountain (bottom) and subaerialshield (top) in Iceland; grid is 1 km, contour interval is 20 m, north at top. Profiles at approximatelysame scaleas map; no vertical exaggeration.(From AMS SeriesC762, sheet 5721 III, Hrutafell, Iceland, 1:50,000, May 1951.)

site of Olympus Mons. According to this model the peculiar aureole terrain representserosional remnants of thick (of the order of a kilometer in places)m6berg, which was not capped by subaerial lava. The cratered mesasof Figure 14 appear to be vestigesof the initial m6berg surface,vulnerable to erosion by wind and meltwater becauseof the poorly consolidated, brecciated, and tuffaceousnature of the deposits.In addition to explaining the peculiar dissectedfabric of these deposits, which are apparently unique on Mars, this hypothesisalso is compatible with the gravity data [Phillipsand Bills, 1979]; the aureoles represent volcanic centers and, as is characteristicof the table mountains, there should be numerous dikes and sills at depth.

Alternative Models for Origin of the Aureoles and Basal Scarp Fig. 10. Martian cratered mesa amid irregular, hummocky pat- Alternative explanations thus far suggestedfor either the terned ground; narrow terraceoccurs near baseof flanks. Ratio of cra- basal scarp or the aureole deposits of Olympus Mons pose ter diameter to mesa diameter is small in comparisonto ratios of cra- various difficulties. Several hypotheses involve pyroclastic ter diameter and surrounding ejecta diameters of impact craters (see Figure 2d). Diagrammatic profile drawn approximately at scale of flows, landslides, gravity sliding, gravity thrust faults, debris photograph, no vertical exaggeration.(Height estimatedfrom shadow flows, or lava flows originating from the vicinity of Olympus length.) Sun at left. (Latitude 47øN, longitude 230ø; Viking photo- Mons; some of these addressthe scarp problem but do not ac- graph 009B 10.) HODGES AND MOORE: SECOND MARS COLLOQUIUM 8067

c snovv Fig. 11. (continued)

count for the gravity anomaly. Those involving preexisting volcanoes or domical plutons account for the gravity but are improbable in other respects and have no relation to the scarp. None adequately accounts for both the scarp and the gravity results.

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Fig. 11. Martian mesas (a) that resemble Icelandic table mountains with subaerial lava caps; these features are closely associated with (b) a crateredcone and (c) a crateredpeak that stronglyresemble terrestrial volcanic cones.Unlike impact craters,length of slope from center to rim of cratered cone (Figure I lb) is equal to the length of slope from rim to edge of flank, and crater flanks appear smooth. If cratered features of Figures I lb and 1l c are volcanic, then mesas 6 km (Figure 1la) also may be volcanic, possiblyoriginating as subglacial eruptions subsequentlycapped by subaerial flows. White patcheson flanks of all three landforms may be 'snow' localized on north facing Fig. 12. Complex terrain west of Viking 2 landing site that ap- slopes.North is approximately at top in all photographs.Diagram- pearsto be at leastpartly volcanic.Dark irregularpatch (a) is prob- matic profiles not to scale (no shadows from which to estimate ably a lava flow, and the irregularsurrounding surface (b) resembles heights). (a) Latitude 77øN, longitude 68 ø, sun at lower fight, Viking that of manyterrestrial flows (see Figure 4, unit d). Crateredmesa (c) photograph070B27. (b) Latitude 78øN, longitude 66ø, sun at bottom, may be a table mountain.Diagrammatic profile drawn approximately Viking photograph 070B29. (c) Latitude 79øN, longitude 75 ø, sun at at scaleof photograph.(Height estimated from shadowlength.) (Lati- lower right, Viking photograph 070B09. tude 44øN, longitude238ø; Viking photograph009B22.) 8068 HODGESAND MOORE: SECOND MARS COLLOQUIUM

1.50o-w •i;•40• N

0 lOO I•_:::...... L:...... :..... J t5•N SCALEINKILOMETERS 250N- Fig.13a. OlympusMons and surrounding lobate aureole deposits. Lobes appear to have been emplaced sequentially (asnumbered in figure13b), and the deposit at the farthest periphery on the northwest (a) appearsto beamong the oldest. Aureoleb apparentlyissuperposed onaureoles at a andc. Only arcuate remnants are exposed on the south and east. Cra- teredplateaus ofFigure 14 are at d. Base of Olympus Mons at the scarp is about 600 km across. North-south profile along longitude133.25øW isapproximately toscale as shown (larger than that of photographmosaic); no vertical exaggeration. Compositeof Mariner 9 photographs. HODGES AND MOORE: SECOND MARS COLLOQUIUM 8069

145øW

35øN

Fig. 13b. Outlinesof OlympusMens shield and surrounding aureole lobes, indicating a possiblenine separate eruptive centers.Numbered from presumedoldest (1) to youngest.Outline of OlympusMens is about 600 k_rnacross.

King and Riehle [1974] proposed that the scarp resulted Carr [1975] suggestedthat great massesof the volcano from extensivewind erosionand slumpingof a pyroclasticash could have been dislodgedand emplacedas landslide deposits flow or ignimbrite foundation beneath the shield. The founda- hundreds of kilometers away; but had the shield originally tions of terrestrial shield volcanoes, however, are not known continued its gentle slope outward, merging with the sur- to include substantialpyroclastic phases except during late rounding plain, there is no obvious reason for the accumu- submarinegrowth; -subaerial pyroclastic activity usually oc- lated flows to have become uniformly unstable at the low, out- curs as essentiallythe last gasp in shield development,as ex- ermost margin of the flanks. Terrestrial landslides occur on emplified at Mauna Kea and Haleakala in Hawaii. Extensive preexistingscarps or steepslopes. Furthermore, presenttopog- ash flows are commonly associatedwith more silicic strato- raphy would have required uphill sliding on the southeast volcanoes,not basalticshields. Also the dependenceon wind [U.S. GeologicalSurvey, 1976]. to erodethrough lava flowsat the baseof a shieldthat initially An alternative, somewhat similar origin for the aureoles, slopedgradually to the surroundingplains appears to require suggestedby J. G. Moore (oral communication, 1978), in- an intense 'sandblasting' symmetrically on all sides of the volves subaqueousdebris flows. If the vent were surrounded shieldin order to removethe resistantbasalt and exposethe by a body of meltwater, then subaerial flows from the shield ash flows; such a processmust have occurred within a rela- would tend to decrepitateupon entering the water, and, aug- tively restricted time before later lava flows cascadedover the mented by the aqueous medium, substantial flow breccias scarp.King and Riehle [1974] do not addressthe origin of the could perhapshave moved over the great distancesrequired, aureoles,but E. C. Morris (oral communication,1979) has more or lessas density currents. The resulting depositsmight proposedthat thesedeposits also are a seriesof ash flows. be similar to m6berg without the pillow lava. A recent investi- 8070 HODGESAND MOORE: SECONDMARS COLLOQUIUM

pose instability on the crust by loading at the volcanic pile and consequentlifting and sliding of the outermost flanks of the volcano. Harris [1977] proposed that the aureoles were a series of gravity thrust faults in the plains materials caused by emplacementof and crustalloading by the hugevolcanic shield. Thrusting of surrounding ground may indeed have occurred; in fact, the offset in the outer margin of the northwest segment of lobe b (Figure 13a) resemblesrather strikingly the map pattern of the Appalachian Pine Mountain overthrust [Thorn- berry, 1965]. However, the distinctive outlines of the lobes that define discretebodies of material and can be traced nearly to the volcano'sbase (Figure 13b), the elevated but virtually ac- cordant surfaceof the deposits,and, particularly, the development of the shield'sbasal scarp seem inadequately explained by this means. Neither of the gravity-induced mechanismsde- scribedabove would accountfor the gravity anomaly. If the depositswere the eroded base of a larger Olympus Fig. 14. Table mountains (a) southwestof Olympus Mons (Fig- Mons or of similar but separateshields [Carr, 1973], the ero- ure 13a), probably remnant outliers of ridged-and-groovedaureole terrain (b), here interpreted as m6berg deposits(palagonitized tuffs sional agent or processrequired to remove successivelyseveral and breccias) and pillow lavas. Texture of aureole terrain (b) resem- structureson the scale of Olympus Mons, with destruction of bles that of m6berg hills in Figure 6 (featuresa and b). Profile drawn each before the next is built, is difficult to envision. A similar at scale larger than that of photograph; no vertical exaggeration. problem is raisedby the hypothesisof Head et al. [ 1976],who (Height estimated from shadow length.) (Latitude 10øN, longitude attributed the scarp to erosion by unknown agent or process 142ø;Viking photograph044B 13.) of the prevolcano cratered terrain in virtually the entire northern hemisphere of Mars. The absenceof scarpsaround other gation (J. G. Moore, personal communication, 1978) of a older shieldsis not explained, however, nor is the origin of the landslide off the island of Hawaii, however, revealed that in a aureoles addressed. It would appear that Olympus Mons is subaqueousenvironment the coefficientof friction as inferred considerablyyounger than developmentof the hemispheredi- from the ratio of length of runout to height of fall [Howard, chotomy on Mars, inasmuch as the plains at the base of 1973] is about 17, essentiallyequivalent to that of comparable Olympus Mons, now surfacedby young lava flows partly from subaerialslides, whereas similar computationfor the aureoles the shield, are topographically continuouswith the older mot- suggeststhat ratiosas large as 117would be required. tled plains to the north [Scott and Carr, 1978]. The age of the All landslidehypotheses present some difficulty in explain- older plains constrainsthe time of the northern hemisphere ing the location of the most extensive volume of aureole de- plains development. The lava flows cascadingover the basal posits on the northwest side, where the volcano's flank is scarpof the volcano indicate that the scarpexisted well before broadest. The scarp appears to be a nearly constant height volcanism ceased.Erosion of domical plutons (K. R. Blasius, aroundthe entire shield, irrespective of variations in apparent unpublished manuscript, 1974), although consistentwith the volume and location of aureole lobes. None of the above slide gravity data [Phillips and Bills, 1979; Sjogren, 1979], does not or debris flow models accountsfor the gravity anomaly re- explain apparent age differencesand superpositionof the au- quiring excessmass beneath the aureole lobes northwest of reole lobes becausean extensiveerosion period would plane Olympus Mons. such coalescingplutons to a relatively continuous surface, ir- Steep fault scarpsof limited extent occur on the subaerial respectiveof ages of intrusion. If the aureolesßare simply the flanks of Hawaiian volcanoes,exemplified by the Hilina Pali, deeply eroded remnantsof old subaeriallava flows [McCauley more than 300 m high, on the southeast flank of Kilauea. et al., 1972; Morris and Dwornik, 1978], such lavas must have Analogy with the continuous scarp at the base of Olympus been entirely different physically and/or chemically from the Mons, however, seems inappropriate because the Hawaiian long, narrow flows that now cover the surrounding plain; scarpsform in the subaerialshield and possiblyare dependent these were clearly derived from the shield volcanoes and re- on the existenceof the incompetent material in the submarine semble morphologically typical terrestrial basalt flows from baseof the shield as well as on the break in slopeat sea level. central vents [Greeley, 1973; Carr et al., 1977]. Further, the continuity of the Martian scarp is not duplicated The volcanic explanation for the aureoles that at present by the fault scarps in Hawaii, which have a discontinuous seemsto us most compatible with their configurationand dis- scallopedpattern (J. G. Moore, oral communication,1978). tribution as well as with the gravity data is subglacialerup- Similar kinds of slump scars can be identified inboard from tion. The pillow lavas and tuffaceous breccias thus accumu- the main scarp at Olympus Mons (Figure 13b), but it is post- lated would be analogous in origin, though clearly not in ulatedhere that their formationwas dependenton the pre- scale, to those Icelandic table mountains on which subaerial existingand unrelated basal scarp. basaltseither were removedby erosionor were not emplaced. Gravity sliding on a subsurfaceplane has also been sug- The cratered mesas of Figure 14,, and the massive aureole gested as an explanation for the scarp and aureole (M. H. lobesas well, probably are not subaeriallavas, nor were they Carr, oral communication, 1978). In this model the plane of likely capped by such lava, but they may represent instead lubrication is the interface between permafrost and a pre- thick subglacially developed m6berg that has been modified sumed underlying zone with liquid pore water. Although this by erosion. A complex network of intrusionsand dikes would model has not been examined in detail, it evidently would im- thus be present beneath the aureoles to produce the excess HODGES AND MOORE: SECOND MARS COLLOQUIUM 8071

massat depth requiredby the gravity data. The gravity high the disc-widespectrum of Mars. Palagoniteis also a reason- northwestof' Olympus Mons is currentlyinterpreted by Phil- able petrologicinterpretation of samplesanalyzed by both Vi- lipsand Bills [1979] as evidencefor magmaticsource rocks be- king landers;major constituentsof the Martian fines, as of the neath the aureoles, and this analysis is consistentwith the Icelandic palagonites,are iron-rich clays. Toulmin et al. [1977] model proposedhere. postulated that explosive interaction of iron-rich basaltic An obviousdifficulty with this explanation for the aureoles magma and subterranean ice could readily account for the is the mechanismthat permittedsuch extensive eruptions be- large quantitiesof clay-rich material apparently presentat the neath ice and meltwaterwith little or no subaerialventing. Martian surface. Subaqueouspillow lavasgenerally tend to pile up becauseof The interaction of magma with water and ground ice or rapid chilling [Sigvaldason,1968]. As in subaerial flows on permafrostappears inescapable if the climatic regime of Mars earth, however, a high rate of effusion [Shaw and Swanson, during volcanic epochswas similar to its current state. Land- 1969;Walker, 1973]apparently permits extensive spreading of forms similar in size and morphology to the Icelandic table submarine extrusionsas sheet flows [Holcomb, 1978; Ballard mountains, however, suggestthat large expansesof surfaceice et aL, 1979].The total volumeof aureolematerial may be of may have existedover someparts of the planet during various the orderof 1-4 x 106km 3, and someindividual lobes prob- times in the past. On the basis of shadow lengths the heights ably contain as much as 5 x 105 km3. An ice sheet 1-2 km of the pedestalsdescribed here are generallyof the order of a thick might not have been breached if such volumes were few hundred meters, and therfore the requisite thicknessof erupted from different vents at different times and at very ice, at least in the northern latitudes, could probably have rapid rates. Walker and Blake [1966]described a m6bergridge existedin the past with relatively slightmodifications in atmo- in Iceland that formed as lava flowed 22-35 km beneath a val- spheric temperature of pressure and/or an increase in the ley glacier at the relatively low gradient of about 0.018, and amount of water. they speculatedthat an enormous'j6kulhlaup' (catastrophic A more demanding challenge to current models of volatile flood),caused by the eruption,must have formeda subglacial budget and planetary history, however,is engenderedby our conduit down which the lava subsequentlyflowed. Subaerial interpretation of Olympus Mons and adjacent features,which Martian lavas have flowed on gradients as low as 0.002 calls for extensiveice in the Tharsis region of Mars. Assuming [Moore et aL, 1978],so that subglacialflows on similar gradi- that the top of the talus on the Olympus Mons scarp is ap- entsare possible.Olympus Mons is on the flank of the Tharsis proximatelyat the baseof the subaerialflows, the thicknessof bulge,and a low westwardgradient of about 0.002 likely ex- ice required to form the m6berg foundationwould be perhaps isted when eruptions began; subglacialconfinement and the 3-4 km. low gradient unrestrictedby valley walls may have induced The m6berg pedestal on which Olympus Mons rests, ac- broadlateral spreadingor pondingfrom eachof the 8-10 pos- cording to this hypothesis,would, like the scarpsof terrestrial tulatedsubglacial vents (Figure 13b). table mountains, readily retreat by mass wasting and wind Alternatively,if the magmareservoirs were initially shallow erosion,but the initial relief of the scarpcould be explained as within the crustand becameprogressively deeper as the crust a primary feature, an inherent consequenceof the volcano's thickened,then low pressureheads may have existedduring subglacial beginnings. Furthermore, like the Pacific guyots the initial aureole eruptions,permitting little vertical growth. and shields[Moore and Fiske, 1969], the huge Olympus Mons As the planet cooled and magma reservoirsdeepened, suf- edifice probably subsidedsomewhat, causing the annular de- ficient pressurehead must have been attained to permit the pressionaround its base. Subsequentdeposition of lava, eo- vertical growth of Olympus Mons [Carr, 1976a, b] and the lian debris, and mass-wasted materials in this circumferential three large shieldsnear the crestof the Tharsisridge, each of depressionobscured aureole depositsthat probably existedat which also reaches 26 km in elevation. the base of the scarp, as suggestedby the outcrop pattern. If As magma contactedice, vast meltwater lakes probably the broad aureoles do consistof m6berglike materials, they formed, although surfaces could have remained frozen. The would supply an extensivesource for the palagonitic debris existenceof liquid water at the surface would of course re- that appearsto be widely distributedover the planet. quire atmosphericconditions different from thoseof the present, a difficulty encounteredas well in attemptsto explain Ice Volume and the Volatile Budget Martin channel configurations. The morphology of the Olympus Mons complex seems rather well explained by the table mountain analogy, but ac- The Role of Ice countingfor an ice accumulationof perhaps7-10 x 106 km3 If the Icelandicanalogy is to be drawn,the former presence posessome difficulty. Some current estimatesof the volatile of more extensive surface ice on Mars must be established. budget, based on argon isotope ratios, permit a maximum The pervasiveevidence for a substantialthickness of ground amount of water-outgassedequivalent to a planet-wide thick- ice has been described elsewhere [Carr and Schaber, 1977; nessof only about 9.5 m [Andersand Owen, 1977]. However, Masurskyet eL, 1977;Soderblom and Wenner,1978]. A largely the amount of H20 intrinsic to Mars is a matter of some dis- water-ice compositionof the north polar cap appearsconclu- pute, and it appears that the total volatile inventory of the sive, and its present thicknesshas been estimated at as much planet is not yet well defined. Bogard and Gibson[1978] enu- as 1 or 2 km [Farmeret eL, 1976;Kieffer et eL, 1976].Water in merated the difficulties of using noble gas ratios and com- Viking samplesmeasured about 0.3-2% [Biemannet eL, 1977]. parisonswith chondritesand stated that presentdata are in- Geochemicaland spectraldata from the Martian surfacefur- adequate for determining the relative abundancesof volatile ther indicate that magma may have interacted with H20 to elements originally supplied to Mars. Lewis [1974] proposed form sideromelane,the precursorof palagonite. Soderblom that Mars had an initial water content 6 times (per unit mass) and Wenner[1978] observed that the 0.3- to 2.5-/.tinspectrum that of earth, basedon an equilibrium condensationmodel of of antarcticpalagonite samples is the closestapproximation to solar systemformation. McElroy et al. [1977],using a nitrogen 8072 HODGES AND MOORE: SECOND MARS COLLOQUIUM

isotope analysis, suggestedthat H20 could have been abun- at lower elevations.The maximum thicknessof the ice cap on dant enough to form a planet-wide surfaceice layer 200 m Greenland is currently 3.4 km [Flint, 1971]; even thicker thick. If this last amount were concentratedin a 30ø spherical sheetsmay have existedon earth during the Pleistocene. cap of ice centeredon Olympus Mons, a 6-km-thick ice sheet A possiblealternative to the extensivecoverage by glacial could result. Assuming adequate amounts of water, the tem- ice is the interaction of magma with more readily explicable perature of the planetary surfacewould dictate that ice be the ground ice. If such interaction occurred,tuffaceous flows and dominant phase, unless atmospheric pressure increased, as breccias may have erupted until the conduit was insulated may have occurredduring times of extensivevolcanism. from further entry by meltwater. But under approximately The ice coveragecalled for here is probably best derived as these conditions on earth, small maars and tuff cones are the a localized cap. The possibility that the rotational axis may predictablelandforms. On the SewardPeninsula (Alaska), an- have wandered during the courseof the planet's history has gular blocksof loessare incorporatedin the ejectaaround sev- been proposedby Murray and Malin [1973] and by Ward et al. eral maars, indicating (along with other evidence) that the [1979],who cite as evidencethe positionof the Tharsisbulge sedimentswere frozen at time of eruption (D. M. Hopkins, with its excessmass essentially at the equator. They consider oral communication, 1979). Only the subglacial and sub- it more probablethat the bulge and volcanicshields grew else- marine eruption style appearsto producemorphologies com- where (i.e., when the rotational axis was differently located) parable to those of the table mountains and the Olympus and caused a shift of the poles, such that the excessmass Mons shield and associated features on Mars. would be repositionednearer the equator.This postulateis SUMMARY further supportedby the large basin (~800 km across)and resulting massdeficiency near the presentsouth pole. Conceiv- The morphologiesof small Martian buttesand mesas,espe- ably, the hypothesizedglacial stageoccurred if and when the cially concentrated in latitudes north of 40 ø, are similar to Tharsis region was nearer the pole before adjustmentof the thoseof Icelandic table mountainsthat are presumedto have planet to the excessmass. formed by subglacialeruption. The geologicand climatic re- An alternative and perhaps more appealing causeof a lo- gimesof Mars may be or have been analogousto the environ- calized ice cap is the relief of the Tharsis bulge itself, regard- ment of Iceland, where volcanism and glaciation have inter- less of its latitudinal position. Under favorable atmospheric acted throughout much of recent geologic history. In the conditions,ice may have accumulatedat high altitude and northern latitudes of Mars the candidate table mountain been sustainedbelow 10 kin, even at equatorial latitudes. structuresare associatedwith landformsand terrainsthat ap- Thus both ice and volcanism would have followed the defor- pear more obviouslyvolcanic, and thus it seemsa reasonable mation that produced the bulge and, presumably,the exten- postulatethat a relatively slightmodification in positionor ex- sive fracturing of the old cratered plains. Stratigraphic diffi- tent of the polar cap could have allowed volcanic extrusion culties(M. H. Carr, oral communication,1978) that arisewith into or throughice. the polar wander hypothesisare thereby eliminated. More problematical is the localization of a sufficientthick- The means by which such ice could be preferentially con- ness of ice in the Thatsis region to account for our inter- centrated as a result of altitude changesdepends on atmo- pretation of Olympus Mons and its surroundingaureoles by spheric conditions in addition to water concentration.The initially subglacial eruptions. The basal scarp around the present atmospheredecreases in temperaturewith altitude enormousshield is readily explained if the shield were built from about 240øK at the surface to about 100øK at 200 kin, atop a pedestalof pillow lava and palagonitizedtuff breccias, with about a 50ø decreasein the first 20 km. Higher temper- or m6berg. The position, apparent sequence,and grooved, atures must have occurred at the surface when and if fluvial serrate texturesof the aureole lobes are compatiblewith their action occurred to form or modify channels. According to interpretation as separate subglacial eruptives that did not Masurskyet al. [1977] fluvial channel agesvary from 3.5 to 0.5 reach the surface and remained as expansesof m6berg un- b.y. During periodswhen liquid water existedat low altitude, capped by subaerial lava flows. Massive palagonite would ice may have accumulatedcontemporaneously at higher alti- thushave been subjectedto wind erosionand transport,partly tudeson the flanks of the Thatsis ridge. accounting for the widespread distribution of such material An ice cap could have been sustained throughout inter- implied by spectral and Viking geochemicaldata. Initial lo- mittent volcanism, as is the case in Iceland today. Williams calization of the ice could perhapsbe attributed to the 10-kin [1978] has suggestedthat the blocky, conspicuouslyridged ter- elevation of the Tharsis bulge. rain at the northwestbase of Arsia Mons is bestexplained as a The total water budget may indeed be an obstacleto this sequenceof recessionalmoraines from a glacier that devel- hypothesis,but there appearsto be at presenta broad rangeof oped on the volcano or existed prior to eruption and was volume estimates,dependent on analytical technique, and maintained as the volcano grew. Similar depositsoccur in an little knowledgeof the possiblevariations of near-surfacevol- apronlike lobe at the northwestbase of PavonisMons. How- atile concentrationsthroughout Martian history. Photogeo- ever, there are no such deposits around Ascraeus Mons, logic interpretationspresented here suggestthe former exis- youngest of the large Tharsis shields [½arr, 1976a; Wood, tence of more extensiveglacial ice than is currentlymeasured 1976], so that the 'Ice Age' may have vanishedby the time the or inferred; the feasibility of such an ice age dependsulti- AscraeusMons eruptionsbegan. Although the presentsurface mately on developmentof a compatible and defensiblemodel of the Olympus Mons shield appearsto be younger than As- for atmosphericevolution, and further geologicinvestigations craeusMons [Wise et al., 1979], eruptionscould have begun at of Mars may require sucha model. the larger volcano during an earlier glacial stage.If a thick ice Acknowledgments.We are indebted to a number of reviewerswho cap existedon the Tharsis bulge, it probably was in a dynamic offered critical comments. Especially helpful were Robert B. Har- state, with ice accumulating rapidly at highest elevations as graves,who brought to our attention the possibleanalogy between the massflowed outward and downward into regionsof thaw Icelandic landforms and those on Mars, and Richard S. Williams, HODGES AND MOORE: SECOND MARS COLLOQUIUM 8073 who kindly sharedhis extensiveknowledge of Icelandicgeology and lithospheric spreading centers (abstract), Eos Trans. AGU, 59, whosethinking about Martian featureshad earlier progressedalong 1104-1105, 1978. lines similar to ours. We benefited greatly from several discussions Howard, K. A., Avalanche mode of motion: Implications from lunar with JamesG. Moore, who providedconsiderable background for our examples,Science, 180, 1052-1055, 1973. comparisonswith Hawaiian volcanicshields. In additionto detailed Jones, J. 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