"Geologists agree that the igneous material intruded and then cooled as phonolite porphyry, a light to dark-gray or greenish-gray igneous trachyte rock with conspicuous crystals of white feldspar. As the lava cooled, hexagonal (and sometimes 4-, 5-, and 7-sided) columns formed. As the rock continued to cool, the vertical columns shrank horizontally in volume and cracks began to occur at 120 degree angles, generally forming compact 6-sided columns. Superficially similar, but with typically 2 feet (0.61 m) diameter columns, Devils Postpile National Monument and Giant's Causeway are columnar basalt." -Wikipedia
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