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Lunar and Planetary Science XXXII (2001) 1483.pdf

MARTIAN VOLCANOES SEEN IN CROSS-SECTION IN ORBITER CAMERA IMAGES. P. J. Mouginis-Mark, Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Manoa, Honolulu HI 96822, U.S.A. (e-mail [email protected])

1. Introduction Mars Orbiter Camera (MOC) images provide an opportunity to observe parts of many volcanoes on Mars in cross-section. By virtue of the high spatial resolution (2.78 - 7.75 m/pixel for images used here), the angle of repose of rock faces, and sensor viewing geometry, we can investigate some of the 3-dimensional structure of these volcanoes to see if the surficial deposits are consistent with the (i.e., subsurface) structure of the volcano. Preliminary observations of several constructs are presented here to evaluate the potential contribution of MOC images in better understanding the long-term history of volcanoes on Mars.

2. Lava flow sections The 3 km-high caldera wall on (Fig. 1a) reveals numerous layers close to the rim that are interpreted to be lava flows. However, each of these layers is only contiguous for a few hundred meters before a new sequence is observed. Erosion of numerous narrow (<1 km wide), relatively thin (a few tens of meters) lava flows is believed to cause this variability, and implies that there were no caldera-wide eruptions that filled the caldera, and then over-spilled to cover the upper flanks [1]. In contrast, the former lava lake that now forms the floor of Olympus Mons caldera [2] shows no layering in a vertical section more than 500 m high (Fig. 1b). The simplest explanation here is that the entire floor of the caldera was flooded by a late-stage eruption of lava flows. If this is indeed the correct interpretation, then large intra-caldera eruptions must have covered the caldera floor. Taking an estimate of 750 m [2] for the Fig. 1: MOC images of the walls of volcanoes that have entire section would imply a minimum volume of 13.5 lava flows on their flanks, or sections through lava km3 for this eruption, given a radius of 33 km for this plains adjacent to volcanoes. a) Southern caldera wall of part of the caldera. Olympus Mons (frame M0402248); b) Section through Numerous flows are also observed in sections of a main collapse crater within the caldera of Olympus lava channel located to the east of Hecates (Fig. Mons. Caldera floor (top) has been cut by a series of 1c). These flows may be relevant to the mode of graben perpendicular to the scarp (frame M0402248). c) formation of the lava channel; by virtue of the varied Western rim of a sinuous rille just a few kilometers to the wall stratigraphy extending down the long-axis of the east of (frame M0705930); d) Northern channel it appears most likely that the channel was caldera wall of Tholus (frame SP2-43405). Scale carved into the pre-existing lava plains, rather than in each image is in meters. formed as the top-most unit through constructional processes. The caldera rim of Tharsis Tholus (Fig. 1d) offers an 2. Sections through ash cones? intriguing comparison to Olympus Mons. Tharsis Several studies [4 - 8] have drawn attention to valley Tholus is unusual in that it has the deepest caldera on networks on the flanks of Hecates and Ceraunius Tholi, Mars [3] and appears to have experienced large-scale and on Tyrrhena and Hadriaca Paterae. These studies have flank collapse. From the MOC images, no layering proposed that the valleys are evidence that the main characteristic of lava flows is evident in the caldera cones comprise easily eroded ash deposits. MOC images walls, despite several flows being visible on the flanks. have now been obtained for several of these volcanoes, Instead, extensive spur-and-gully erosion can be so that it is possible to see whether they have a similar observed, suggestive of easily erodable materials. This (or different?) type of stratigraphy as the lava shields. may indicate that the materials from which Tharsis Recognizing ash deposits may be difficult, however, as Tholus is constructed are not competent lava flows, but air-fall deposits and/or pyroclastic flows may be rather ash deposits and/or weathered units where the extensive and yet too thin to be resolved even at the stratigraphy has been destroyed. In either case, this may highest spatial resolution of the MOC data. explain why the volcano has experienced sector- collapse. Lunar and Planetary Science XXXII (2001) 1483.pdf

MARTIAN VOLCANOES IN 3-D: P. J. Mouginis-Mark

The southern flank of Hadriaca Patera is cut by Dao Vallis, which has been imaged several times by MOC. These images reveal very uniform layers over hundreds of kilometers of the rim of Dao Vallis. Only one example is shown in Fig. 2a, but other MOC data obtained at varying radial distances from the summit of the volcano reveal the extensive nature of this unit. Hecates Tholus is often taken to be the clearest example of a Martian volcano that has experienced explosive eruptions by virtue of the and the spatial distribution of small impact craters on the flanks [5]. Fig. 2b reveals that further evidence for thin, uniform, layers on the flanks can be seen in a cliff section located on the lower western flank of the volcano. As seen at Hadriaca Patera, units exposed within Hecates Tholus appear to be relatively uniform in thickness and spatially extensive. In contrast, the northern caldera wall of (Fig. 2c) displays no evidence of layering. Extensive talus deposits and recent (i.e., low-albedo) landslide scars can be found in both the upper and lower segments of the caldera. Unlike other calderas, the rim of Ceraunius Tholus also appears to be heavily degraded rather than a sharp boundary. While potentially different from the units seen at Hadriaca Patera and Hecates Tholus, these observations at Ceraunius Tholus are also consistent with the existence of unconsolidated layers of ash [9].

4. Conclusions MOC data enable the third dimension of Martian volcanoes to be studied in detail. Although no definitive observations have been made to unambiguously distinguish lava flows from layers of ash deposits, the general morphology of the caldera walls of the volcanoes described here are consistent with the styles of volcanism inferred from Viking Orbiter images. No major changes in the style of activity over time can be inferred from the walls of Olympus Mons caldera, which are strikingly different from the calderas of Tharsis and Ceraunius Tholi. The lack of stratigraphy in the faulted floor of Olympus Mons implies that late-stage caldera in filling may have been accomplished by a few large eruptions, rather than many smaller events. Spatially extensive deposits in the walls of Dao Vallis support the idea [8, 9] that Hadriaca Patera had explosive eruptions.

References [1] Mouginis-Mark, P. J. & S. K. Rowland (2001) Geomorphology, in press. [2] Mouginis-Mark, P. J. & M. S. Robinson (1992) Bull. Volc. 54: 347 - 360. [3] Robinson M. S. & S. K. Rowland (1994) in abstracts Fig. 2: MOC images of the walls of volcanoes that lack presented at the Conference on Volcano Instability on lava flows on their flanks. a) A section through the Earth and Other Planets, Geol. Soc. London, p. 44. [4] northern rim of Dao Vallis reveals layering within Reimers, C. E. & P. D. Komar (1979) Icarus 39: 88 - 110. Hadriaca Patera. Note the unusual “lobate” fingers of [5] Mouginis-Mark P. J. et al. (1982) JGR 87: 9890 - material in the lower part of this image, which appear to 9904. [6] Gulick V. C. & V. R. Baker (1990) JGR 95: be sediment that has adhered to the side of the valley and 14,325 - 14,344. [7] R. & D. A. Crown (1990) then scoured away at some localities (frame M1101601); JGR 95: 7133 - 7149. [8] Crown, D. A. & R. Greeley b) Section through the western lower flanks of Hecates (1993) JGR 98: 3431 - 3451. [9] Plescia, J. B. (2000) Tholus (frame M0301763; c) Northern caldera wall of Icarus 143: 376 - 396. Ceraunius Tholus, showing two levels of caldera collapse (frame M0806047). Scale in each image is in meters.