Columnar joints are fractures produced as hot rock cools and contracts. At Yellowstone’s Sheepeater Cliff they divide a basalt flow into conspicuous columns. Polygonal shapes are common, but not every column has six sides.
Look for repeated boundaries through one rock body, then distinguish them from separate layers stacked above each other. Columnar jointing also occurs in other igneous rocks, so columns alone do not prove basalt. This feature turns a striking texture into evidence of cooling.
Source / photo: NPS / Diane Renkin · Public domain. Resized. No claim to original U.S. Government works.