Devils Tower Formation

Devils Tower

Devils Tower is 867 feet (264 meters) tall (base to summit) and is 1000 feet (305 meters) in diameter at its base. The area at its summit is 1.5 acres. It is an igneous intrusion that formed about 60 million years ago into sedimentary rock that has eroded away over millions of years to expose the Tower. It is the first national monument established in 1906 by Theodore Roosevelt under the Antiquities Act. It was made famous in the 1977 movie Close Encounters of the Third Kind. I looked carefully---there is no alien base behind the Tower or even a runway for the spaceships to fly down like that shown in the movie. Sigh!

The Kiowa legend for the formation of Devils Tower is given in the graphic below:

Kiowa legend for Devils Tower

What follows are geological explanations of the formation of Devils Tower. Two main points agreed to by all geologists (from the visitor center sign):

1) The Tower is composed of igneous (previously molten) rock called phonolite, which formed approximately 1.5 miles below the earth's surface when magma (molten rock) pushed up through the sedimentary rock layers about 50 million years ago. [monument brochure says 60 million years]

2) Over millions of years, erosion stripped away the softer layers of sedimentary rock, exposing the Tower as we now see it.

After that the geologists have offered three basic interpretations of the Tower's geologic history given in the graphic below:

Devils Tower geologic history

A close-up view shows that the grooves in the Tower are actually many columns, mostly hexagonal in cross-section:

Close-up of sides of Devils Tower

Here is how the columns formed:

Formation of columns in Devils Tower

Compare this with the Sawn Rocks feature at Mt Kaputar National Park outside of Narrabri in New South Wales, Australia (link appears in a new window). The hexagonal columns of Devils Tower are much larger in diameter than those at Sawn Rocks.

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last updated: January 1, 2009

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel