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What geologic process caused this strange rock formation


This is columnar basalt. It started as a flow of lava. Like most substances, as the lava cools, it contracts. As the lava contracts, stress builds up, causing vertical fractures that produce the columns.

The basalt in this photo is Sheepeater Cliff in Yellowstone National Park (one of my favorite spots in Yellowstone), but columnar basalt can also be found at Devils Tower, Devil's Postpile, and the Giants Causeway, as well as many other sites.