Crown Butte Laccolith, Great Falls, MT

February 06, 2001


Provided by: Rod Benson, Helena High School Science Dept.
Summary authors & editors: Rod Benson

This photo shows the cliffs of Crown Butte, an exposed laccolith located 20 miles west of Great Falls, Montana. A laccolith is a blister-like sill of igneous rock injected beneath the surface. 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.

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