The subglacial birth of Olympus Mons and its aureoles

Abstract

The vast volcanic plains and shields of Mars, together with the thermal, spectrographic, and morphological evidence for water ice at the poles, for several percent water in Viking soil samples, for ground ice or permafrost over much of the planet, and for the existence of surface water at some time in the past, suggest that magma and water or ice may have interacted during evolution of the planet's landscape. Relatively small mesas and buttes, with and without summit craters, are remarkably similar to the table mountains of Iceland that formed by subglacial eruption during the late Quaternary period. Table mountains typically comprise foundations of pillow lava and palagonitized tuff breccias (móberg), overlain by subaerial lava flows that commonly culminated in a typical shield volcano; some table mountains, however, failed to reach the subaerial stage and thus lack the cap rock. Subglacial fissure eruptions produced ridges composed of pillow lava and móberg. Conical knobs on steep‐sided Martian plateaus are reminiscent of the small Icelandic shield volcanoes atop móberg pedestals. Candidate table mountains on Mars are especially numerous in the region between latitude 40°N and the margin of the north polar cap, and interaction of lava with a formerly more extensive ice cap may have occurred. More significant is the possibility that Olympus Mons and the broad lobate aureole deposits around its base may have had similar subglacial beginnings. This hypothesis requires an ice cap several kilometers thick in the vicinity of this enormous shield during its initial stages of eruption. The amounts of water ice required do not appear excessive, given the limits to present knowledge of the water budget throughout the planet's history. A mechanism for localizing such ice, however, is required.