2. What's a laccolith?

The East Side of Crown Butte

Crown Butte is one of several laccoliths scattered throughout central Montana. Here are some others:

• Box Elder Butte south of Havre, and Snake Butte south of Harlem
• Round Butte and another Square Butte 50 miles west of Great Falls
• Square Butte, Shaw Butte, and Cascade Butte 15-25 miles west of Great Falls

The diagrams above are from a special publication of the Montana Bureau of Mines and Geology called Profiles of Montana Geology (publication #89). Permission was granted to use it. The publication includes a collection of articles by David D. Alt that were previously published in Montana Magazine. Although it depicts a butte located 50 miles east of Great Falls, the formation of Crown Butte was very similar.

Laccoliths are a type of intrusive igneous formation . . . . also called "plutonic formations" or "igneous intrusions". This means that they were formed as magma cooled beneath the surface. With laccoliths the magma moves into an area beneath the ground, causing the overlying rock layers to dome upward. After the magma becomes rock, it is exposed as the less durable rock above erodes away as shown in the diagrams.

Below: This photo, taken by Bob Rumney, shows Crown Butte as viewed from the southwest. Birdtail Butte (left) and Haystack Butte (right) are also intrusive igneous formation. However, they are not laccoliths.

Photo Courtesy of Bob Rumney, Cascade Area Rancher