How Was Devils Tower Formed?

Devils Tower lies within the Bear Lodge Mountains near the Black Hills.
••• John_Brueske/iStock/Getty Images

The Kiowa and Cheyenne say an outsized grizzly bear raked the spire of northeastern Wyoming’s Devils Tower -- Tree Rock to the Kiowa, the Bear’s Lodge to the Cheyenne -- while people huddled on top. It’s a more vivid origin story than the ones geologists propose, which nonetheless has the drama of molten rock and deep time.

Devils Tower as Igneous Intrusion

Many scientists suspect Devils Tower represents a fist of magma, or molten rock, that “intruded” overlying sedimentary layers but didn’t attain the surface: either a laccolith or a stock. The sedimentary deposits, which include sandstone, shale and beds of gypsum, were laid down in the Mesozoic era, when the region often lay drowned under inland seaways. The Devils Tower magma billowed up 50 million to 60 million years ago, coinciding with the uplift of the Black Hills.

Forming the Tower

The underground magma cooled into phonolite porphyry, an igneous rock, fracturing as it did so to form the iconic hexagonal columns of today's pillar. Subsequent erosion has removed the sedimentary strata once surrounding the intrusion, which -- tougher in consistency -- better resisted the gnawing force of water. Stolid as Devils Tower looks, it’s nevertheless being dismantled by erosion and weathering, as the rubble at its base reveals.

Alternate Theories

Not all geologists have agreed that the seed magma of Devils Tower formed as a laccolith or stock. Earlier theories represented the butte as the neck of a mostly demolished volcano. A paper presented at a 2011 session of the American Geophysical Union, meanwhile, proposed the tower might be the ruins of a crater-pooled lava lake.

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