EVALUATION OF LAVA FLOW EMPLACEMENT CONDITIONS ON THE TERRESTRIAL PLANETS

Initial investigation of the morphometric aspects of lava flow margins has been undertaken through a grant from the Planetary Geology and Geophysics program of NASA. The objective of this work is to improve our understanding of the mechanisms that produce distinctive attributes of lava flow margins. Three complimentary lines of effort are underway: 1) accurate mapping of flow margins in plan view (at various scales), 2) quantitative documentation of flow margin thickness, looking for terracing as revealed by distinct levels on the margins of inflated basaltic flows, and 3) development of a model for processes active at flow margins that can account for the observed morphometric attributes. Preliminary results will be presented from field work carried out at the Carrizozo and McCartys flows in New Mexico, the 1907 Mauna Loa flow and alkalic flows from the Laupahohoe volcanics on Mauna Kea in Hawaii, and analysis of THEMIS data for a 500-km-long flow in the Tharsis region of Mars. The New Mexico flows display distinctive topographic terracing at their margins, interpreted to be related to progressive release of fluid lava from fractured edges of these inflated flows. Similar terraced margins are present on some lava flows on Mars, suggesting similar inflation and release processes. The two Hawaiian flows have very different thicknesses, although both have similar planforms and leveed central channels. Flow thickness, traditionally the most difficult physical dimension to measure remotely, is thus a major key to making inferences about the rheology and composition of flows that can only be observed remotely. The Tharsis region of Mars has numerous long lava flows that radiate from three large shield volcanoes that comprise the Tharsis Montes. New imaging data from the Thermal Emission Imaging System (THEMIS) on the Mars Odyssey spacecraft provide important new insights into the Tharsis flows, and we will present initial observations and results for one particular flow that wraps around the western side of Ascraeus Mons, the northernmost of the Tharsis Montes volcanoes.