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Home , Alphonsus (crater), Arsia Mons, Ascraeus Mons, Carmichael (crater), Lava, Lava lake

Proc. Lunar Planer. Sci., 128 (1981), p. 1431-1447. Printed in the United States of America

Late-stage summit activity of shield volcanoes

Peter J. Mouginis-Mark

Department of Geological Sciences, University, Providence, Rhode Island 02912

Abstract-The preservation of morphologically fresh flows which pre-date the most recent episodes of collapse at the summits of Ascraeus, Arsia and Olympus Montes indicates that explosive eruptions were not associated with this stage of shield . The existence of resurfaced floor segments, complex wrinkle , and lava terraces within the summit craters suggests that lava comprised the dominant form of the intra-caldera activity. Multiple collapse episodes on Ascraeus and Olympus Montes are indicated by the nested summit craters. The most plausible cause of caldera collapse appears to be large-scale sub-terminal effusive activity, which is corroborated by the previously recognized existence of large lava flows on the flanks of these volcanoes. Due to the implied sequence of large-scale explosive () volcanism followed by effusive (basaltic) activity, it appears highly unlikely that or other forms of pyroclastic flows (previously proposed as possible deposits within the aureole material) were ever erupted from the .

INTRODUCTION Of fundamental importance to the current understanding of martian geological evolution was the recognition of the numerous large Tharsis and Elysium volcanoes from 9 and Viking Orbiter images (Masursky, 1973; Carr, 1973; Carr et al., 1977 ; and Spudis, 1981). In particular, the Tharsis volcanoes received early attention due to their size and similarity (and hence interpretable morphology) to terrestrial shield volcanoes such as , (e.g., Greeley, 1973; Carr and Greeley, 1980). Numerous lava flows were shown to characterize the summit and flanks of each Tharsis shield (Carr et al., 1977; Schaber et al., 1978), in many instances, however, the morphology and evolution of each martian were treated in general, and sometimes conflicting, terms. An assortment of diverse eruptive styles were suggested by authors such as King and Riehle (1974), Hodges and Moore (1979), Morris (1980) and Scott and Tanaka (1980) in order to explain, for example, the aureole and surrounding Olympus Mons. Alternatively, analyses of volcano morphology concentrated on the regional and global significance of shield evolution for the entire geological province and I or the martian (Carr, l974a; Blasius and Cutts, 1976; Comer et al., 1980). Considerable theoretical and quantitative data now exist for lunar (e.g., McGetchin and Ullrich, 1973; Head and Wilson, 1979; Wilson and Head, 1981a) and terrestrial (e.g., Wilson et al., 1980; and references contained therein) volcanic eruptions, so that reason­ able extrapolations to the martian environment can be made (Blackburn, 1977; Wilson and Head, 1981b). The acquisition of high resolution (15-40 meters per pixel) Viking Orbiter images of the summit areas of Ascraeus, Arsia and Olympus Montes permits some of these theoretical predictions to be tested. In addition, the eruptive sequence and style of activity for each martian volcano can be recognized. Specifically, high resolution Viking images provide the capability to identify the spatial relationships between summit fracturing, caldera collapse and the eruption of lava flows associated with the shield volcanoes. Such attributes have been shown by Nordlie (1973) to be intimately related to different evolutionary stages of Galapagos volcanoes, and it appears reasonable to expect that similar information can be derived for the martian volcanoes. Furthermore, if plinian and sub-plinian eruptions (Walker, 1973) were to occur on (Wilson and

1431 1432 P. J. Mouginis-Mark

Head, 198lb), the caldera rim may show evidence of extensive deposits identifiable on Viking images: major differences in volcanic activity (effusive versus explosive) should therefore be discernible. This paper presents observational data on three of the Tharsis Montes ( is excluded due to the absence of high resolution Viking images). The caldera morphology is treated here as an indicator of the late-stage summit activity on each volcano, thereby permitting the style(s) of volcanism to be determined. Interpretations of the evolutionary sequences of terrestrial intra-plate shield volcanoes are employed to infer the individual characteristics of each martian volcano investigated here.

OBSERVATIONS Detailed descriptions of the evolution of terrestrial intra-plate volcanoes have been given by a number of , and it is assumed here that their observations would also be applicable to Mars were the same styles of activity to occur there. For example, Simkin and Howard (1970) described the 1968 caldera collapse of Fernandina volcano, Galapagos, and noted that rapid collapse permitted large segments of the wall to slump en masse, while slow collapse was associated with block fragmentation. Thus, on a general basis, caldera collapses on Mars could be subdivided into those produced by rapid chamber evacuation and those generated by a slower removal of magma. Relationships between volcano growth, fracturing, and caldera collapse have also been proposed by Nordlie (1973}, Guest (1973) and Wadge (1977). Radial fractures with associated lava flows are believed to be related to volcano swelling ("tumescence") as magma rises within the volcano, while concentric fracturing and circumferential fissure eruptions are com­ monly associated with caldera formation (Nordlie, 1973). In this paper the term "caldera" is applied to the entire collapse structure at the summit of a volcano, while individual pit craters are referred to as "volcanic craters". The production of a caldera in itself appears to be an indicator of the magnitude of each eruption, and the relatively shallow depth of the (Macdonald, 1972; and McBirney, 1979). To a first approximation, equivalent volumes of erupted material (lava flows, ash and pyroclastics) should initiate comparable collapse episodes. Thus, the volumetric evolution of magma chambers on Mars should be evident from the size progression of the nested craters. The presence of caldera structures in which the largest crater was the first to form would suggest that the magma chamber decreased in size as each volcano evolved. Conversely, small craters truncated by a final large crater would indicate an increase in magma chamber volume. Wilson and Head (1981b) provide the theoretical basis for recognizing ash deposits associated with the creation of a caldera and predicting the distribution of these deposits around martian volcanoes. Explosive eruptions on terrestrial basaltic shield volcanoes typically occur when magma withdrawal permits ground to enter the vent (Jaggar and Finch, 1924 ; Simkin and Howard, 1970 ; Tazieff, 1976-7). Because ground ice-volcano interactions have been proposed for Mars (Allen, 1979; Hodges and Moore, 1979}, deposits from phreatic eruptions may also exist. In addition, martian basaltic plinian eruptions, with their associated mantle deposits around the vents, might also be observed (Wilson and Head, 1981 b). This section consequently documents the characteristics of Ascraeus, Arsia and Olympus Montes to serve as primary constraints for subsequent analyses.

Ascraeus Mons: A total of eight collapse features are prominent at the summit of (Fig. 1), and are interpreted to represent volcanic craters in various stages of preservation. Table 1 lists the individual dimensions of these craters, which range in size from about 7 to 40 km in diameter. A chronology for these collapse events can be derived from the cross-cutting sequence of the crater wall segments. Using this method, crater 8 (Fig. lb) appears to be the oldest, while craters 6 and 7 were the next to form. Craters 5 and then 4 were produced prior to a major slump episode on the northern wall of the caldera, approximately contemporaneous with a period of circumferential Martian shield volcanoes 1433

(a)

CRATER CHRONOLOGY

20 km

(b)

Fig. I. (a) Medium resolution image (60 meters per pixel) of the summit of Ascraeus Mons, showing the locations of Figs. 2 and 3. Viking frame 90A50. (b) Schematic diagram of area shown in Fig. I A, giving the crater numbers utilized in the text. The block diagram gives the inferred sequences of crater formation (I youngest), based on cross-cutting relationships of the crater walls. 1434 P. 1. Mouginis-Mark

Table I. Dimensions of Ascraeus Mons craters.

3 Crater No. Diameter (km)' Depth (km)* Approx. Volume (km )

I 40 X 38 3.15 3760 2 14X 10(14) 0.48 75 3 21 X 13 (19) 0.98 325 4 26 x II (17) ? ? 5 32 X 10 (27) 0.75 530 6 7 X 4 (9) ? ? 7 17 X 5 (25) ? ? 8 15 X (14) ? ?

' Dimension in parentheses gives an estimate of the original size of the crater. *Crater depths derived in this analysis from shadow measurements.

formation (also evident in craters 6 and 7). The small craters 2 and 3 were formed on the eastern rim after this graben-forming event was complete. The most recent modification of the summit area resulted in the formation of crater I, which now occupies the center of the caldera complex. Based purely on the size and superpositioning of each , it appears that several intermediate-sized (20-30 km) individual craters charac­ terized the original summit. These craters were then modified by the formation of smaller (7-15 km) craters before the final eruption produced the single large (40 km) crater. More detailed information on the eruption characteristics of Ascreaus Mons can be derived from Viking Orbiter images acquired on rev. 401 of Orbiter 2. Four episodes of crater formation are evident on the southern caldera wall and floor (Fig. 2). The maximum depth of the caldera at this point (from the rim to the floor of crater I) is approximately 3.1 km (based on shadow measurements), with craters 2, 3 and 5 perched respectively 1.7 2.2 and 2.4 km above the floor of crater I. Prominent along the entire length of the caldera rim are numerous lava flows and a few sinuous channels which flow radially away from the caldera. These lava flows are typically less than I km wide and 10-20 km in length, while the largest channel measures 200 meters wide and 18 km long. It is clear that the lava flows on the rim are traceable all the way to the caldera rim crest but lack any obvious source vents. It therefore appears that each flow has been truncated by the collapse events, rather than preserving its original length. No evidence of the same flows can be identified on the lower "slump blocks", however, suggesting that each new crater floor within Ascraeus caldera experienced a resurfacing event after the collapse episode. Numerous small wrinkle ridges upon the caldera rim are also evident (Fig. 2). These ridges appear analagous to small-scale mare ridges on the , and are generally orientated down the inferred maximum slope of the volcano close to the rim of crater 5. Spacing of the ridges is remarkably constant for adjacent ridges, and varies between 0.9 to 2.6 km. It is inferred here that these ridges have a tectonic origin, perhaps formed during post-tumescence contraction of the summit. Many narrow ridges less than 3 km in length also occur on the floor of crater I. The origin of these floor ridges is less clear, but their uniform spacing (600-800 meters) and preferential orientation (east-west) suggest that they too may be tectonic in , although image resolution is inadequate to preclude, for example, an eolian origin. Sinuous channels are located on the northwest wall of crater 5 (Fig. 3). These features are interpreted as sinuous lava channels created by turbulent flow in a similar manner to lunar sinuous (Hulme, 1973; Carr 1974b; Wilson and Head, 1981a). Despite the initiation of circumferential fracturing of the rim area, some of these channels can still be identified on fractured segments of the caldera rim. Apparently, at least at this locality, rim slumping was not associated with any process capable of destroying (or burying) the previously produced small surface features on the rim, implying that major explosive activity (such as plinian eruptions) can be discounted. : Arsia Mons caldera measures 113 x 128 km in diameter and has been '~f·~1 ~ ' ..- ... ..~ ..

~~ranli"I!J--. - '.II,-

s: "';::<.. ss· ;:::

;:::-""' ;;;· ss::

<::: 0 ;:;- "';::: ,_ 0 \ ' ""'"" Fig. 2. High resolution (22 meters per pixel) mosaic of the southeastern rim of Ascraeus Mons caldera. Craters 2 and 3 (left), I (center) and 5 (right) are illustrated. "F" denotes the position of truncated lava flows on the caldera rim, "S" is a sinuous channel interpreted as a lava channel, and "R" marks the location of numerous wrinkle ridges: see text for discussion. Viking frames 401817-20. ....,+:>. l..ll 1436 P. J. Mouginis-Mark

Fig. 3. Two sinuous chann els (arrowed) on th e southwestern rim of crater 5 on Ascraeus Mons are shown here to be cut by subsequent circumferential graben. The existence of these preserved pre-coll apse features indicate th at cald era enlargement was not associated with explosiv e eruptions that generated large ai r-fall deposits. Viking e 40 I B 17. Im age resolution 22 meters per pixel.

estimated (using photogrammetry; Wu, 1980) to be approximately 900-1100 meters deep. Crumpler and Aubele (1978) interpreted the volcano to be more evolved in comparison to Ascraeus Mons. Arsia Mons differs from Ascraeus Mons in terms of the degree of subsidence of the summit, the presence of a pronounced set of concentric rim fractures, and the post-subsidence flooding of the floor by younger lava flow s (Fig. 4) . Prominent on the caldera rim, and present up to distances of 60 km from the rim (Fig. 4) , are numerous graben. Spacing between the major graben varies from 5-12 km, while several smaller examples are located within I km of each other. Transected by many of these fractures is a series of small lava flow s which extend downslope toward the northwest. Several dozen individual flow s can be recognized, and each measures approximately 0.5-1.3 km in width and 10-14 km in length. The thicknesses of the lava fl ows in this area have been estimated by Schaber et al. (1978) to be less than 10 meters. The primary source for most of these flow s is a major graben located on the rim approximately 12.5 km from the edge of the current caldera floor. Nearly all of the lava flow s appear to have been emplaced prior to the fracturing of the caldera rim; they are cut by the fractures and none of them pond against any of the slump blocks (Fig. 4) . One exception is a flow labelled "A" in Fig. 4b. Emanating from the same system of fi ss ure vents which produced the other flow s, this lava " fan" differs from the Martian shield volcanoes 1437

other eruptives in that the flow direction was toward the caldera floor and its time of emplacement was after the last episode of graben formation. A small measuring 10 x 16 km has been constructed by this sequence of eruptions. Image resolution (40 meters per pixel) is not, however, adequate to say confidently if the flows extend across the caldera floor, or have been buried by subsequent floor deposits. Lobate outlines on the floor suggest that individual lava flows exist within the caldera, but none can be positively identified. Several small, gently sloping elongated hills with summit craters ("B" in Fig. 4b) have been observed on the caldera floor (Carr eta/., 1977) and these may be example s of martian cinder cones or hornitoes (Moore and Hodges, 1980).

Olympus Mons: Like Ascraeus Mons, the summit area of Olympus Mons (Fig. 5) is characterized by a complex nested caldera. Six coalescing volcanic craters exist, which range in diameter from 20 to 65 km (Table 2). The caldera of Olympus Mons differs from that of Ascraeus Mons in terms of the apparent sequence of the collapse events. Superposition relationships 'indicate that the largest crater (number 6 in Fig. 5) formed first. Successive collapse episodes subsequently produced the intermediate-sized (30- 40 km) craters 3 and 4. Crater 2 represents a relatively small collapse event, and was probably associated with the flooding of the floor of crater 3. Based on the degree of preservation of craters I and 5, these two 20 km craters are probably the most recently formed summit craters, although the relative age of crater 5 can only be confidently placed as younger than crater 6. The Viking Survey Mission produced extensive high resolution ( - 15 meters per pixel) coverage of the southeastern portion of the Olympus Mons caldera (Fig. 6). A complex sequence of wrinkle ridges exists on the floors of craters 2 and 3, while circumferential graben are located on the floor of crater .6. No obvious surface features exist on the floor of crater I. Although Greeley and Spudis (1981) urge caution in the interpretation of the caldera ridges (Figs. 6 and 7), an analysis of ridges (Sharpton and Head, 1980) would suggest a tectonic origin for these martian examples. These Olympus Mons ridges are much larger than ones seen within Ascraeus Mons caldera (Fig. 2): individual examples measure 0.1-3.0 km in width and arches may be as much as 6 km in length. A narrow (200 meter) sinuous ridge also extends around the base of the wall within crater 3. As can be seen from Fig. 7, several very thin ridges within craters 2 and 3 possess central depressions (making them appear more like collapsed lava tubes than "mare" ridges). Other ridges possess overlapping field relationships with earlier examples, imply­ ing a multiphased mode of formation. Olympus Mons apparently has no individual lava flows on the caldera floor; the high resolution Viking images fail to show any evidence of intra-caldera activity similar to that observed on terrestrial volcanoes (Macdonald, 1972). A semicircular depression ("V" in Fig. 6) may mark the location of a small vent at the junction of ·craters I and 2, but no mantle deposits or lava flows can be seen around this feature. Few lava flows are apparent on the volcano's near summit flanks (Fig. 8), producing a rim morphology that is appreciably different from that of Ascraeus Mons (Figs. 2 and 3). One explanation for this may be the very low relief in this area on Olympus Mons (Wu, 1981), which has precluded individual lava flows from forming and extending downslope. Additional features of Olympus Mons caldera are also noteworthy. Within crater 6, the floor fractures appear to vary in width as a function of their location, being widest close to the crater edge and progressively narrower toward the crater center (Fig. 6). Together with the wrinkle ridges on the caldera floor, it is likely that these fractures are tectonic in origin and are probably related to the subsidence of the central portion of the caldera floor. This subsidence may have been due either to the solidification and contraction of the lava , or to the withdrawal of support from an underlying magma chamber. Evidently these floor fractures exerted little structural control over the evolution of the caldera, however, since the shape of crater 3 has been almost unaffected by this fracture pattern. A landslide is evident on the southern wall of crater I (Figs. 6 and 8), with a small lobe of material extending 2 km across the crater floor. This landslide may indicate that w-1:>. 00

~ :-

~ :;::: 1)0 ;:;· c;:;·

~ s:::> *

1 20km , --~~

Fig. 4. (a) Northwestern rim of Arsia Mons caldera. Viking frames 422A30-35, image resolution 40 meters per pixel. oO

0 0

3::: 0 :::. iS" ;:,; , "':::;- ;;;· ~ c<:: ;:;- 0 ;:,; CALDERA FLOOR 0 I 20 KM I "'

~ w Fig. 4. (b) Schematic diagram of the area of Arsia Mons caldera shown in Fig. 4A. Illustrated are the lava fan ("A"),Iow hills ("B") and circumferential \0 graben which cut the radial lava flows (solid outlines) on the flanks of the volcano. See text for discussion. 1440 P. J. Mouginis-Mark

(a)

N~

CRATER CHRONOLOGY 1 2 5 314 6 30 km

(b)

Fig. 5. (a) Medium resolution image (140 meters per pixel) of the summit caldera of Olympus Mons. The locations of Figs. 6 and 7 are also shown. Viking frame 46831. (b) Schematic diagram of area shown in Fig. 5A, giving the crater numbers discussed in the text. Block diagram derived in the same manner as Fig. I B. Martian shield volcanoes 1441

Table 2. Dimensions of Olympus Mons craters.

3 Crater No. Diameter (km)' Depth (km)* Approx. Volume (km )

I 24 X 20 3.28 1245 2 36 X (30) 0.65 555 3 (40) X 25 1.37 1135 4 (38) X 6 (32) 1.42 1365 5 20 x 23 2.85 1184 6 66 x 66 1.65 5145

' Dimension in parentheses gives an estimate of the original size of the crater. *Crater depths derived in this analysis from shadow measurements. the wall materials of Olympus Mons are more fragmented than those of Ascraeus Mons (where no landslides are observed), and therefore more susceptible to slumping. ­ natively, the place where the landslide occurred is the closest point (within 17 km) on the caldera rim to a very recent 8 km . This impact crater post-dates the formation of crater I, as evidenced by the secondary craters on the caldera floor. Thus, the landslide may be an anomaly induced by the subsequent disruption of the rim during the impact cratering event.

THEORETICAL CONSTRAINTS ON VOLCANIC ACTIVITY In addition to the extrapolation of field observations of terrestrial volcanoes, numerous theoretical analyses of volcanic eruptions permit specific features of martian volcanism to be predicted and, hence, sought for on the Tharsis Montes. Interpretations of lunar and terrestrial eruptions (cf. Wilson et al., 1980; Wilson and Head, 1981a) indicate that gravity, atmospheric pressure, and volatile species are likely to be the factors which will most influence martian volcanism (Wilson and Head, 1981b). Adapting the numerical models to martian conditions should therefore permit the morphology and morphometry

Fig. 6. High resolution mosaic (17 meters per pixel) of the southeastern floor of Olympus Mons caldera. Prominent are craters 6 (left), 3 (center) and I (right). A small depression, possibly a vent, is denoted by "V", while the arrow points toward a small landslide within crater I. Viking survey mission frames from rev. 474S; part of JPL mosaic 211-5930. 1442 P. J. Mouginis-Mark

Fig. 7. High resolution image (16 meters per pixel) of the wrinkle ridges on the floor of crater 2 within Olympus Mons caldera. Numerous episodes of deformation are indicated by one set of ridges atop a second set. Several of the narrow ridges display central depressions, which may indicate that these are collapsed lava tubes. Viking frame 473S21. of pyroclastic flows and sinuous rilles, the vent size, and the areal distribution of air-fall deposits to be predicted for different types of eruptions. From the numerical models of Wilson and Head (1981b), sinuous rilles are predicted to form during high effusion rate martian hawaiian-style activity, while the low atmospheric pressure on Mars should favor the formation of pyroclastic flows during vulcanian eruptions due to the ease of eruption cloud collapse. Strombolian eruptions on Mars would also be influenced by the low atmospheric pressure, with fine particles ejected to a significant fraction of their vacuum ballistic range (McGetchin and Ullrich, 1973). Strombolian activity would therefore create a subdued deposit, recognizable as an two to four kilometers in diameter around the caldera rim. Convective eruption clouds (should they occur) would also be more susceptible to collapse than their terrestrial counterparts, increasing the likelihood of lag-fall deposits close to the vent and sheets surrounding the volcano. Large-scale, coarse deposits would develop close to the vent during plinian eruptions, while even very low mass eruption rates for such events 3 (100m / sec dense equivalent) would produce vents in excess of 200 meters in diameter. Consequently, for each style of volcanic activity on Mars, the related surface features should be easily visible in Viking images with a nominal resolution of 30-100 meters per line pair.

INFERRED STYLES OF THARSIS MONTES VOLCANISM The combination of observational data and the theoretical constraints permits the styles of volcanic activity on the three Tharsis Montes to be deduced. Of primary importance in Martian shield volcanoes 1443

Fig. 8. Unlike the rim of Ascraeus Mons (Fig. 2), the southern rim of Olympus Mons lacks prominent lava flows . While some examples can be found (at the positions labelled "F"), this portion of the volcano has a paucity of flows and circumferential graben. The small landslide (arrowed) on the floor of crater I may have been initiated by the creation of the 8 km impact crater shown at top right. Viking frame 475S15, image resolution 23 meters per pixel.

distinguishing between explosive and effusive activity is the identification of the lava flows close to the caldera rim on both Ascraeus (Fig. 2) and Arsia (Fig. 4) Montes. These lava flows indicate that effusive eruptions extended to the highest points on the volcano summits, rather than being confined purely to lateral and subterminal activity. Sinuous rilles on the flanks of Ascraeus Mons (Figs. 2 and 3) imply that at least some of these eruptions involved high effusion rates of turbulent lava (Hulme, 1973; Wilson and Head, 1981a) corresponding to hawaiian-style volcanism. The absence of preserved vents (e.g., on the rim of crater 3 of Ascraeus Mons; Fig. 2) means that the areas of the summit which subsequently collapsed were no doubt similar in morphology to the preserved rim segments. Due to the retention of these summit lava flows, a strong case can be made for saying that almost all the preserved summit activity on the three montes was non-explosive in character. Evidently lava flows on the caldera rims of Ascraeus and Arsia Montes (Figs. 3 and 4) pre-date the last collapse events (and, hence, the last major summit eruption on each volcano). These lava flows are estimated to be very thin ( ~ 10 meters; Schaber et al., 1978), and so would have been quickly buried if air-fall deposits associated with plinian eruptions had been produced (Walker, 1973; Wilson, 1976; Wilson and Head, 1981b). Further evidence against air-fall deposits is that infilling of pre-existing volcanic craters by subsequent eruptions is not observed in Ascraeus Mons (Fig. 2) or Olympus Mons (Fig. 6) caldera. Because of this absence of near-rim air-fall deposits, not only can plinian 1444 P. J. Mouginis-Mark

eruptions be excluded for the Tharsis shields, but also the volcanic products associated with eruption cloud collapse (ignimbrites and other pyroclastic flows ; Sparks and Wilson, 1976; Sparks et al., 1978) can be discounted. Implicit in this observation (i.e., the exclusion of explosive eruptions) is the fact that volcanic materials previously identified around Olympus Mons as possible ignimbrites (King and Riehle, 1974; Scott and Tanaka, 1980) or other pyroclastic flows (Morris, 1980) could not have been produced during an eruptive phase consistent with the deduced summit activity. In order to generate either deposit around Olympus Mons, the ac­ companying near-vent materials (Wilson, 1976; Sparks and Wilson, 1976; and Walker, 1981) would have to be formed, but these are not observed at the summit. Also absent are any pit craters within the caldera that are large enough to be the source vents for a plinian or ignimbrite-forming eruption. Although significant temporal changes in the eruption characteristics might be invoked to explain former explosive activity in the Tharsis region, the implied sequence of large-scale explosive (silicic) volcanism followed by voluminous basaltic eruptions is very unusual on ( et al., 1974), and presumably this would also be true for Mars. Several lines of evidence suggest that molten lava lakes once covered much of each caldera floor. The absence of truncated lava flows formerly on the caldera rim demon­ strates that some resurfacing has occurred. The retention of fine-scale rim morphology is considered to preclude the presence of a thick ash mantle upon the floor as a result of explosive eruptions, so that floor resurfacing by lava flows /lava lakes is the most likely mechanism. Wrinkle ridges within Olympus Mons caldera (Figs. 6 and 7) are suggestive of a molten which has subsequently cooled, contracted, and been tectonically modified in a manner comparable to the mode of formation of lunar mare ridges (Lucchitta, 1976; Sharpton and Head, 1980). Perched lava terraces and the absence of prominent small lava flows on the caldera floor of each volcano also support this lava lake hypothesis. During the final summit eruption, each crater floor was totally resur­ faced, rather than being modified in a piecemeal manner. Intra-caldera activity was probably of minor importance on Mars in comparison to that observed upon terrestrial shields such as Kilauea (Carr and Greeley, 1980), the Galapagos volcanoes (Nordlie, 1973; Swanson et al., 1974), and Piton de Ia Fournaise, Reunion Island (Ludden, 1977). Very few individual vents within each martian caldera can be seen, with the major exceptions being the low domes within Arsia Mons caldera. More significantly, there is a total absence of any extensive constructional volcanism within the three martian . Typically, central cones several hundred meters high are sub­ sequently constructed within calderas on Earth (such as Crater Dolomieu on Piton de Ia Fournaise; Upton and Wadsworth, 1966), but only crater "V" within Olympus Mons caldera (Fig. 6) may be a candidate for comparable resurgent activity. Even in this instance, however, no evidence can be found for constructional growth at the summit after final caldera collapse.

MODE OF CALDERA FORMATION Excluding the possibility of explosive activity on the Tharsis volcanoes nevertheless requires that an alternative explanation for summit collapse has to be found. There appears to be good evidence from terrestrial examples that volcano magma chambers are full at the onset of activity (Blake, 1981), so that only during (or immediately after) an eruption can any void space be created to permit caldera formation or enlargement. Thus, each martian volcanic crater must have been associated with a withdrawal of magma from the chamber either as a result of effusive activity or the intrusion of dikes within the volcanic pile. Distinguishing between the relative importance of summit versus subter­ minal or lateral eruptions as instigators of summit collapse does, however, depend upon the relative volumes of magma/lava involved and the timing of the collapse episodes. Numerous very large flows in excess of 600 km in length can be seen upon the flanks of all three volcanoes (Carr et al., 1977; Schaber et al., 1978). Existing data preclude the Martian shield volcanoes 1445

Table 3. Lava flow s needed for crater collapse A: Observed flow dimensions on Arsia Mons

Flow Altitude Flow Flow Flow Location of vent Length (km)1 Width (km)1 Thickness (m)*

Summit > 18km 10 - 15 0.5 - 1.3 7 High Flanks 12- 18 km so - 100 3.0 - 4.0 20 Low Flanks < IOkm 100 - 600 4.0 - 7.0 35

B: Number of flows for observed collapse events

Flow Arsia Mons Olympus Mons Ascraeus Mons Location Caldera Crater I Crater I

5 4 4 3 5 4 Summit 3.6 X !0 - 9.2 X 10 3.5 x i0 - 9. t x i0 1.1 X 10 - 2.7 X 10 3 3 3 High Flanks 4.2 X 10 - 1.6 X 10 415 - 155 1.2 X 10 - 470 Low Flanks 896 - 85 89 - 8 268 - 26

'Data from Carr et al. (1977) and this analysis. *Average of data given by Schaber et al. (1978).

direct correlation of individual flows with each collapse event, and even for terrestrial volcanoes this can be a very difficult task. Obviously, the number of flows required to account for the displaced caldera volume would decrease as more of the volume is accounted for by these large flank eruptions. Taking the simple case where all eruptions at a given altitude are of comparable size, the number and dimensions of flows at several elevations that are needed to initiate the observed collapse events are summarized in Table 3. Assuming that none of the magma displaced prior to collapse was distributed throughout the volcano in the form of dikes (probably an oversimplification if martian shield s are similar to terrestrial examples: Simkin and Howard, 1970; Macdonald, 1972), an unrealistic number of very small summit eruptions (more than 27000 for crater I of Ascraeus Mons) would be needed to initiate each collapse. Plausible numbers (25-270) of very large lava flows (such as those seen at elevations of 8-12 km on Arsia Mons) could, however, account for the observed displaced volumes. Flank eruptions of this type are also consistent with activity recognized on terrestrial volcanoes, where voluminous subterminal and lateral eruptions occur due to the of the volcano (Wadge, 1977; Ludden, 1977). Thus the existence of the calderas on Mars can be adequately explained by the observed effusive activity, with no explosive eruptions required to initiate summit collapse.

CONCLUSIONS The variety of surface features identified within the calderas and on the summit flanks suggests that the simple interpretation of the Tharsis volcanoes as the martian equivalents of the young hawaiian volcanoes (e.g., Mauna Loa and Kilauea) is no longer adequate to describe these planetary volcanoes. The absence of lag-fall deposits within the cald eras and the preservation of small lava flow s on the crater rims does, however, indicate that only effusive activity characterized Tharsis Montes volcanism. This observation limits the possible terrestrial analogues to the general class of shield volcanoes, which is parti­ cularly important for the interpretation of surface materials surrounding Olympus Mons. By comparison with both field observations and theoretical models of terrestrial vol­ canoes, it appears that an ignimbritic (or other ) origin for the aureole materials can be rejected. Were these materials to be the products of explosive vol­ canism, this would imply an earlier period of large-scale silicic activity, which would give 1446 P. J. Mouginis-Mark

the Olympus Mons magma chamber a chemical evolutionary trend that is extremely unusual for terrestrial volcanoes. The presence of nested summit craters, wrinkle ridges on the crater floors and rims, circumferentia l graben, and low domes within Arsia Mons indicates that the summit of each volcano evolved over a period of time. Volcanism within each caldera was most likely restricted to the formation of lava lakes, while the large flank eruptions were the principle cause of caldera collapse. Evidently the system of conduits within each volcano permitted the magma to find repeated egress from the source chamber to both the summit area and the distal flanks. The absence of any intra-caldera eruptions after the formation of the lava lakes requires that a mechanism for terminating summit activity after each collapse has to be found. The cause of the different trends in the size-evolution of the magma chambers of Ascraeus and Olympus Montes is also unknown, but it appears that each volcano (or, more specifically, the effective volume of each magma chamber) was capable of changing in a variety of ways. In order to interpret these differences, it consequently seems that future analyses of the martian shields would benefit from additional comparisons with more evolved terrestrial volcanoes. The older hawaiian volcanoes and other shields located in, for example, Galapagos, Reunion and Iceland would appear to be prime candidates for such a comparison. Only in this manner may it be possible to interpret adequately the summit morphology of the Tharsis Montes a nd provide additional constraints for the interpretation of their flank deposits.

Acknowledgments-The author wishes to thank Lionel Wilson for extensive discussions during the preparation of this manuscript, and for stimulating support over the last decade in the analys is of planetary volcanoes. Richard Grieve gave valuable comments on an early version of the paper, while Chuck Wood performed admirable editorial support. Reviews were provided by Tom Simkin and an anonymous person. The talents of Sam Merrell (photography) and Sally Bosworth (typing) were appreciated during manuscript preparation. NSSDC supplied the negatives used in preparing Figs 2, 3, 7 and 8. This research was supported by NASA Grant NER 40-002-088 of the Program.

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