English: Columnar jointing of basalt in Marte Vallis, Mars. The image shows layers of solidified lava flows exposed on the rim of a 16-kilometer-diamter crater in the Marte Vallis region on Mars. This image from the High Resolution Imaging Science Experiment HiRISE instrument aboard NASA’s Mars Reconnaissance Orbiter shows the first columnar joints positively identified on a planet other than Earth. Columns between 30–40 meters (100–130 feet) tall and about 2 meters (6.6 feet) wide comprise the lowest of the visible layers in the Mars image. (The exposed columns run from lower left to upper right in this image.) Beneath the columns, talus (eroded debris) slopes towards the crater floor. The crater rim is visible in the lower right corner. The Sun is lighting the scene from the lower left.

Jointed columns occur around the entire circumference of the crater, indicating that the lava flows cover a huge area: at least 200 square kilometers (77 square miles). The widespread lava flows stacked on top of one another appear similar to the terrestrial flood basalts of the Columbia River Basin in Idaho, Oregon, and Washington, where instances of columnar jointing are common. It is likely that runny lava from a distant source formed layers of basalt in Marte Vallis, and they were uncovered when an asteroid punched through the Martian surface.

Mars image courtesy High Resolution Imaging Science Experiment, Arizona State University.