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Igneous Rocks

Igneous rocks are very rare in surface exposures in Kentucky. They are formed by the cooling and solidification of molten rock (magma) that originated deep within the earth. The two main types of igneous rocks are intrusive, which formed when magma cooled slowly and hardened beneath the earth's surface, and extrusive, which formed when magma solidified after it reached the surface.

picture of Peridotite with yellowish weathering rind
Peridotite with yellowish weathering rind

In Kentucky, a dark-colored igneous rock, peridotite, occurs in sills and dikes (intrusions) in Elliott County in northeastern Kentucky and Caldwell, Crittenden, and Livingston Counties in western Kentucky. Peridotites formed very deep in the earth near the mantle under high temperature and pressure, and were thrust explosively toward the surface where they intruded into the host rocks. Peridotite consists in part of the minerals olivine and pyroxene.

The Elliott County peridotite is a special type of peridotite called kimberlite. It is dark green to black, and is composed of various grain sizes (porphyritic texture) in a fine- to medium-grained groundmass called a matrix. Numerous phenocrysts (individual crystals embedded in a finer grained groundmass) average from 2 to 10 mm in diameter, but some individual phenocrysts may reach 25 mm. Olivine is the most abundant mineral in the dikes and accounts for the greenish color of the rock. Most of the calcite in these igneous rocks is primary, meaning it is of magmatic origin.

Kimberlites are famous for containing diamonds. The dikes in Elliott County are geologically similar to the diamond-bearing kimberlites in Kimberly, South Africa, and kimberlites in Arkansas, where diamonds have been mined extensively, but no diamonds have ever been authenticated from the Elliott County deposits. Twenty-eight known peridotite dikes occur in Livingston and Crittenden Counties in the Western Kentucky Fluorspar District. These dikes vary in mineralogical content because of differences in the original parental magma.

Other igneous rocks such as granites, basalts, gneisses, and amphibolites occur in the deep subsurface of Kentucky. Most of these rocks occur at depths of 5,000 to 15,000 feet below the surface, and samples of these rocks are encountered when oil or mineral well tests are drilled. Igneous rocks have also been found in glacial deposits in northern Kentucky.