Formation of Hrad Vallis (Mars) by low viscosity lava flows

Hrad Vallis is a Martian outflow channel previously interpreted as a product of aqueous outbursts from the subsurface, possibly involving mudflows associated with lahar-like events. However, an alternative volcanic hypothesis for the development of the system is worthy of consideration on the basis of (1) the nature of landforms preserved along component channels and adjacent uplands and (2) similarities between the basic properties of this system and large volcanic channels of the inner solar system. Hrad Vallis commences on the distal flanks of the Elysium Mons shield volcano, terminates within extensive volcanic plains, is associated with landforms typical of large volcanic channels, and shows evidence for having been a conduit for large volumes of lava. The properties of this system are consistent with incision by low viscosity lava. Crude thermal estimates suggest that this system could have formed through effusion of as little as ~ 10,900 km3 of magma to the surface, or ~ 6% of the volume of the terrestrial Columbia River Basalt Group. Incision rates of up to several meters per day are estimated for mechanical and thermal processes involving lava flows with depths of 5–20 m and dynamic viscosities of ~ 1 Pa s. Flow of lava within the Hrad Vallis system is predicted to have been fully turbulent and characterized by discharges as great as ~ 865,000 m3/s. Predicted flow conditions are consistent with those previously determined for Athabasca Valles, which also formed as a result of the expulsion of flows from structures associated with Elysium Mons.