Devils Tower

The magnificent American monument with mysterious origins.

Among the pine forests of Crook County, Wyoming, stands an enormous lump of rock reaching high up into the sky. Known as Devils Tower, it is so awe-inspiring that in 1906, President Theodore Roosevelt established it as the United States’ first national monument, but no one quite knows how it formed.

What we do know is that it is made from phonolite porphyry, an igneous rock that is formed when magma cools and crystallises. In this case, as the magma cooled, it also contracted, cracking the rock into the polygonal columns that now make up the Tower. Most geologists agree that the rock formed when magma rose up into the surrounding sedimentary rock, but there are three possible theories for how this happened.

The formation theories:

Theory 1 – Volcanic plug. The rock is the neck of an extinct volcano or a plug that lay beneath it. Although there is no evidence of volcanic activity, such as ash or lava flows, this material could have simply eroded away.


Theory 2 – Laccolith. The Devils Tower is a laccolith – a large, mushroom-shaped mass of igneous rock, which spreads between the layers of sedimentary rocks beneath the Earth’s surface. The rounded bulge on top has eroded away.


Theory 3 – Stock. Magma benath the Earth’s surface cooled and crystallised to form the lump of rock you can see today. Over time it was exposed by erosion wearing away the rock above it.


We know Devils Tower formed underground and the soft rock above it eroded away. But the details of this event remain unknown.

Did you know? Devils Tower is officially missing an apostrophe, as it was omitted in a proclamation signed by Roosevelt.

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