of the week, featuring Devil’s Tower!
If you are wondering how this massive pillar formed, you are in good company. Scientists are still debating the exact geologic process that formed Devils Tower. What we do know is, about 200 million years ago, an ancient sea that covered much of the central and western United States formed layers of sedimentary rocks. Some 150 million years later, molten rock, or magma, pushed up through these older rocks, creating what is known as an “igneous intrusion.” This particular intrusion might be a remnant of an extinct volcano or a laccolith – a mushroom-shaped mass of igneous rock. Or it might be a magma plug known as a “stock.” Whichever of these things it was, its molten rock eventually cooled and hardened. As it did, it contracted into the giant, vertical columns you see now. The columns are mostly hexagonal in shape, with 6 sides, although some are 4-, 5-, or 7-sided and can be 600 feet tall and 15 feet in diameter.
At the time of formation, the intrusion was still underground. Eventually the relentless forces of erosion gradually wore away the soft sandstones and shales above and around it, unveiling the harder igneous rock underneath – and that's what you see today!
Erosion continues to expose more and more of Devils Tower. However, the Tower itself – that hard igneous rock -- is eroding as well. The boulders, broken columns and piles of rubble you see lying at its base have broken off of its steep walls. In the very distant future, this amazing landmark will eventually disappear altogether!
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