This HiRISE image shows a collapse pit in Tractus Fossae, a region of large ridges and troughs created by tectonic activity.
The fossae occur on the Tharsis volcanic rise, a giant region of enhanced volcanic activity that includes the three large volcanoes Ascraeus Mons, Pavonis Mons and Arsia Mons.
The pit in this image has very steep walls, and so only a narrow arc is illuminated by sunlight. The rest of the pit is in dark shadow. However, a stretched version of the image shows details of the pit floor, due to a small amount of scattered sunlight.
Pits like this form by collapse into underground voids, such as those left by propagating magma-filled dikes. They may sometimes have overhanging walls, although in this case the walls can be seen and appear nearly vertical. Some similar features are found on Earth: Devil’s Throat, in Hawaii, is one example. Other similar examples have been imaged on Mars as well.
Written by: Colin Dundas (9 March 2009)
This is a stereo pair with ESP_011531_2065 .
More info and image formats at http://hirise.lpl.arizona.edu/ESP_011386_2065
Image: NASA/JPL/University of Arizona