Comment on 'Aureole Deposits of the Martian Volcano Olympus Mons'

It is true that according to our analogy with table mountains in Iceland, the initial height of the Olympus Mons scarp up to the base of the subaerial lava flows would indicate the maximum thickness of ice under which eruptions occurred. However, because of the scarp's susceptibility to failure by mass movement, as we explicitly acknowledged, the height of the scarp would increase with time as erosion progressed upslope on the flanks of the shield. Therefore, the thickness of our postulated ice cap is most nearly approximated by the minimum height of the present scarp (about 3 km on the south flank), not its maximum, as implied by Morris. Indeed, the highest sections of the scarp [U.S. Geological Survey, 1981] are on the northwest (9 to 11 km), where debris deposited by processes of mass movement is most apparent. Moreover, extensive deformation apparently occurred on the northwest flank, where the slope of the shield decreases from >3.5 ø in its uppermost two thirds to about 1.5 ø, forming a broad bench bounded by two radial faults adjacent to this highest section of the basal scarp; along narrow ridges at the top edge of the northwest scarp, slopes are reversed inward toward the volcano's center. The entire perimeter escarpment has been modified substantially, both by mass wasting and by lava cascades from the shield, so that its initial height is problematical. However, it is the present minimum height that constrains the maximum thickness of ice required by our hypothesis. We agree with Morris' interpretation that the textural patterns of the aureoles suggest eruptions from several distinct centers within the lobes rather than derivation by landsliding from the scarp. We also agree that the large positive gravity anomaly coincident with the aureole deposits further supports that interpretation and thus is compatible with both our hypothesis and that of Morris. We are skeptical, however, that 'very fluid ash flows' would develop, by either primary or secondary processes, the distinctive textures exhibited by the aureoles. Lack of any adequate terrestrial counterpart makes interpretation speculative at best, but we believe that the erosional patterns developed on the primary textures of moberg deposits in Iceland more closely approximate the rugged textures of the aureoles than do any thus far observed on terrestrial ignimbrites. Scott and Tanaka [1982, p. 1184, Figure 4] pointed out that pressure ridges (as described by Morris on the aureoles) are 'common on lava flows but absent on ash flows.' We wish to emphasize that regardless of other deficiencies in our subglacial-eruption hypothesis for the origin of the Olympus Mons pedestal, it does not require an ice cap '> 10 km thick' and cannot be so easily discarded. We stand by our estimate of 3 to 4 km of ice at most, and we continue to maintain that the table mountains of Iceland are a reasonable candidate in the ongoing effort to discover an analog for the unique morphology of Olympus Mons.