Like the other major shield volcanoes on Mars, Arsia Mons has a caldera (large volcanic crater) at its summit.
Calderas form when magma (molten rock) is removed from the magma chamber in the volcano, and the roof of the magma chamber collapses into the resulting void. In the case of Arsia Mons, there are relatively young lava flows that overtop the northeast rim of the caldera.
This HiRISE image samples some of these lava flows. The long elliptical depression is the summit crater of a small shield volcano that fed some of these lava flows. At HiRISE resolution, we see that even these younger lavas are covered by a thick layer of dust. The small dark-rayed crater in the southwest edge of the image shows that the rock under the dust is dark, as expected of lava.
Written by: Laszlo P. Keszthelyi (10 March 2010)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_002157_1715
Image: NASA/JPL/University of Arizona