Montana Earth Science Picture of the Week

Layered Igneous Complex

This photo shows the cliffs of Crown Butte a flat-topped butte located 20 miles west of Great Falls. Both Crown Butte and its larger neighbor, Square Butte, are formations called "laccoliths." Laccoliths are formed when magma is injected between layers of sedimentary rock beneath the surface. The magma, which came from an ancient volcano centered 10 miles south of Cascade, worked its way through cracks in the bedrock to get here. Eventually the magma hardened, forming a very durable type of rock that has survived millions of years of erosion. In the meantime the softer sedimentary rocks (sandstones, etc.) that once covered the laccolith have been eroded away, exposing the laccoliths as buttes that can be seen throughout central Montana.

Although layers are usually associated with sedimentary rock, the igneous rock of the butte is made up of very distinct layers. Evidently, the magma filled the laccolith in "pulses" with each new pulse forming another layer. Closer examination reveals a thin lighter-colored layer between each of the thicker, darker layers. This separation within each pulse may have happened as a result of differences in the densities and/or freezing points of various minerals in the magma. Another theory is that the thin light-colored layers formed as a result of water soaking in from the sandstone above before the next layer of magma was injected.

The Nature Conservancy purchased Crown Butte in order to preserve the natural grassland ecosystem located on top of the butte. Except for an occasional hiker, the ecosystem sits undisturbed about 1,000 feet above the surrounding prairie.

To see more photos of Crown Butte and learn more about the volcanic past of this area, check out the Crown Butte Virtual Field trip (see Hot Link below). The web site includes diagrams, maps an aerial photo, and photos that highlight several fascinating geologic features associated with Crown Butte.


1. David W. Baker Ph.D., Consulting Geologist & Technophysicist: Little Belt Consulting Services, Monarch, MT

2. Alt, David. "Charlie Russell's Square Butte" Profiles of Montana Geology; published by the Montana Bureau of Mines and Geology in cooperation with the Montana Magazine 1984

Terms: laccolith, intrusive formation


*Virtual Field Trip to Crown Butte
Read more about Montanas geologic past.
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By Rod Benson
Earth Science Teacher at Helena High School

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