New data reveal the presence of a system of parallel dune-like ridges extending in a band for up to 25--30 km around the eastern margin of the source trough for the Mangala Valles outflow channel system. We interpret these ridges to be formed during initial phreatomagmatic activity caused by dike emplacement, cryospheric cracking, magma-groundwater mixing, and explosive eruption to the surface. The erupted material consisted of fragmented magma, steam, and country rock which expanded from a choked state at the surface vent to form a near-ballistic, Io-like eruption plume. Outward flow of the plume at velocities of ~430 m/s from a ~30--60 km-long section of the graben is interpreted to have formed the system of dune-like ridges. Subsequent outpouring of groundwater formed the Mangala Valles outflow channel system.