#11 - The Cliffs of Crown Butte

Click on photo to enlarge.

Volcanic past . . .
The photo above 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.

Layered igneous complex . . .
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.

Publc access . . .
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. CLICK HERE to access my blog and photo tour - Crown Butte is one of my favorite places!

Terms: laccolith, intrusive formation