THURSDAY'S THEMIS IMAGE: Got a little behind the clock there with the previous post...Summit Caldera of Pavonis Mons:

This image shows part of the summit caldera of Pavonis Mons. Pavonis is the middle of three Tharsis volcanos that form a line southeast of Olympus Mons and northwest of Vallis Marineris. On Earth volcanic calderas usually form when a massive eruption has emptied out the magma chamber and the "roof" of the chamber collapses into the resultant space. It is likely that summit calderas on Martian volcanoes form in a similar manner.