Emplacement of long lava flows on planetary surfaces

Three long lava flows on Mars, Venus, and the Moon were examined in order to evaluate their possible emplacement rate and condition. On the Moon, flows o f the lay1 (phase 111) effusion within the Imbrium impact basin were examined using Apollo photography. The longest phase 111 flow can be followed for 250 km, terminating -400 km from the probable source vent. This flow has a width of 10 t o 25 km, thickness of 10 to 30 m, and a medial channel preserved in its proximal reach, and it was emplaced on a regional slope of -0.3". In the Tharsis region of Mars, a well-defined set of lava flows extends north from the topographic saddle between Ascraeus and Pavunis Montes. Viking Orbiter images show one flow h a t can b e triaced Tor 480 km, with a width ranging from 5 to 50 km, thickness of 3 0 to 100 m, and a prominent medial channel in its proximal reach, and was emplaced on a regional slope of -0.5' to -O.lO. The Strenia Fluctus area on Venus consists of an m a y of intermixed radar-bright and radar-dark lobate flows, one of which can he traced for 180 km, with a width of 5 to 20 km, and an unknown thickness (hut inferred to be -30 m), and was emplaced on the lowland plains where the regional slope is only -0.03'. When viewed at the full Magellan resolution, this flow contains several flow margins, indicating its compound nature. Effusion rates were calculated for the simple lunar and Martian flows using published empirical and theoretical relationships, resulting in a broad range of 500 to 108 (Moon) and 600 to 2 x 108 (Mars) m3/s, with most likely values of -5 X lU4 t o -lo5 for both flows. The compound Venus flow would have required 494 years for emplacement at the typical Kilauea rate of -5 m3/s, but the thermal balance of planetary tube systems could also be consistent with a rate at least an order of magnitude larger. The distinction between simple and compound flows is important to any evaluation of flow emplacement based solely on remote sensing data.