A geologic process known as jointing leads to volcanic rocks with geometric fractures, usually hexagonal in columnar jointing, as in the photos below.

Columnar jointing. (upper left) Devil’s Tower, Wyoming (image credit: National Park Service). (upper right) Sheepeater Cliff, Yellowstone National Park, Wyoming (image credit: Randy Korotev). (bottom) Devils Postpile National Monument, Nevada (image credits: Randy Korotev). On the left is the approach, with columns tilted; on the right is the top of the “postpile.” Most of the columns are 6-sided, but with diligence one can find 5- and 7-sided columns.

(left) The finder of this rock insisted that it was a lunar meteorite. But right there on the side pointing toward us is a hexagonal fracture pattern typical of columnar jointing. So far, no meteorite, certainly no lunar basalt, has shown a linear, preterrestrial fracture pattern like this. (right) I do not know whether the trapezoidal shape of this rock does, in fact, result from jointing, but this would be an un usual shape for a meteorite.

Columnar jointing has been identified on Mars, however. “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.” Image and caption credit: NASA/JPL/UArizona