Proc. Lunar Planer. Sci., 128 (1981), p. 1431-1447. Printed in the United States of America Late-stage summit activity of martian shield volcanoes Peter J. Mouginis-Mark Department of Geological Sciences, Brown University, Providence, Rhode Island 02912 Abstract-The preservation of morphologically fresh lava flows which pre-date the most recent episodes of caldera collapse at the summits of Ascraeus, Arsia and Olympus Montes indicates that explosive eruptions were not associated with this stage of Tharsis shield volcanism. The existence of resurfaced floor segments, complex wrinkle ridges, and lava terraces within the summit craters suggests that lava lakes 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 (silicic) volcanism followed by effusive (basaltic) activity, it appears highly unlikely that ignimbrites or other forms of pyroclastic flows (previously proposed as possible deposits within the Olympus Mons aureole material) were ever erupted from the Tharsis Montes. INTRODUCTION Of fundamental importance to the current understanding of martian geological evolution was the recognition of the numerous large Tharsis and Elysium volcanoes from Mariner 9 and Viking Orbiter images (Masursky, 1973; Carr, 1973; Carr et al., 1977 ; Greeley 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 Mauna Loa, Hawaii (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 volcano 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 escarpment 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 lithosphere (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 Mars (Wilson and 1431 1432 P. J. Mouginis-Mark Head, 198lb), the caldera rim may show evidence of extensive mantle 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 (Pavonis Mons 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 volcanologists, 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 magma 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 magma chamber (Macdonald, 1972; Williams 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 water 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 Ascraeus Mons (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 graben 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 volcanic crater, 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.