Geology

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Devil’s Tower is a national monument that is surrounded by grassland like a rock protector. It was named our first national monument by President Theodore Roosevelt in 1906. This landscape is surrounded by sedimentary rocks, which are rocks that are formed from by the deposition of materials at the Earth’s surface. The rock layer that is visible in the Devil’s Towner is called the Spearfish Formation. It is a reddish color because of the dark red sandstone and the maroon siltstone. The oxidation of iron in this causes the redness. These rocks were the oldest rocks that covered most of the western and central United States in Triassic Park that were visible in the Devil’s Tower that was laid in the inland sea. The Gypsum Springs formation is a thin band that is located above the Spearfish formation. The mineral gypsum is an important resource used to make drywall.

There was an ongoing debate between geologists. They agree that the Devil’s Tower was formed by magma making its way into or between rock formations. But what they did not agree on was whether or not magma reached the land surface. Devil’s Tower was studied in the late 1800s by geologist Carpenter and Russell. They came to the conclusion that the Tower was formed because of igneous intrusion.

There is no research on the Devil’s Tower forming because of volcanoes, but ideas have suggested that it is a volcanic plug that have eroded away. The only explanation that seemed simple enough was that the Devil’s Tower is a stock. A stock is a body of magma that is cooled underground and later eroded. The magma that formed the Tower is a dark grey or greenish-grey igneous rock that is called phonolite porphyry. When the magma is hot, it becomes less dense and it occupies more volume than cool hardened rock. Columns separated by cracks are created when the rock is cooled.
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