The igneous material that forms the Tower is a phonolite porphyry intruded about 40.5 million years ago, a light to dark-gray or greenish-gray igneous rock with conspicuous crystals of white feldspar. As the magma cooled, hexagonal (and sometimes 4-, 5-, and 7-sided) columns formed, each about six feet in diameter. As the rock continued to cool, the vertical columns shrank in width and cracks began to occur at 120-degree angles, generally forming compact 6-sided columns. The nearby Missouri Buttes, to the northwest of Devils Tower, are also composed of columnar phonolite of the same age. Devils Postpile National Monument in California and Giant's Causeway in Northern Ireland are also columnar basalt, which are superficially similar, but with columns typically in diameter.