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.