PLATY LAVA FLOWS: CONTRIBUTIONS OF COOLING AND FLOW DYNAMICS TO SURFACE PLATE MORPHOLOGIES

Platy surface textures at scales of cm to km are found on a variety of geologic flows composed of diverse materials ranging from lava through mud/debris to ice. When platy morphologies are observed in remote sensing images, the processes responsible for the platy textures may not be readily apparent. For terrestrial images, the combination of extensive regional imaging, knowledge of the regional geology, and ground-truth studies will mitigate uncertainty in interpreting probable origins. For planetary images, few if any of these aids may be available. Recent data from several martian instruments, including Mars Orbiter Camera (MOC), Thermal Emission Imaging System (THEMIS), and the High Resolution Stereo Camera (HRSC) have revealed enigmatic platy flow textures variously interpreted as evidence of an ice pack on a shallow frozen sea, the surface of lava flows, or evidence for mud/debris flows. Many of the observed platy martian flows are in areas thought to be tens to a few hundred millions of years old, and thus some of Mars' youngest regions. Thus, the ambiguity in determining ice versus debris or lava origins has major implications for regional geology as well as global climatic conditions. One significant factor in the debate of “ice versus lava” is an incomplete knowledge of the range of terrestrial platy features in ice and lava materials. This study presents observations and models of a range of platy textures in lava flows, and documents field and remote image evidence for extensional as well as compressional conditions in flow surfaces for a range of thermal conditions. A few types of platy lava flow features—such as extensional breakup of a thin crust— have been discussed in the planetary geology community. However, other types—such as plates bounded by compressional rubble ridges—have received little or no mention, and may provide additional alternative volcanic explanations for features previously considered to be evidence for shallow sea ice or debris flows.