Geology

Stone Mountain is a pluton, a type of igneous intrusion. Primarily composed of granite, the dome of Stone Mountain was formed during the formation of the Blue Ridge Mountains, the eastern edge or front range of the Appalachian Mountains. It formed as a result of the upwelling of magma from within the Earth’s crust. This magma solidified to form granite within the crust below the surface.

The granite is composed of quartz, feldspar, microcline and muscovite, with smaller amounts of biotite and tourmaline. Embedded in the granite are xenoliths or pieces of foreign rocks entrained in the magma. The xenoliths of the Stone Mountain granite are composed of two types of metamorphic rocks; gneiss and amphibolite xenoliths of the country rock torn from the conduit as the granite ascended through the earth’s crust. These xenoliths are generally angular, display a foliation, have feathery black amphibole and have a reaction rim of pale yellow orthoclase around them.

Other xenoliths are composed of restite and are generally rounder, lack the amphibole and reaction rims and have weaker foliation. These are cognate inclusions and were presumably the rock which the granite melted from. The presence of abundant metamorphic xenoliths and restite implies that the granite is an S-type granite formed from melting of sedimentary metamorphic rocks.

The granite displays an east-west foliation and abundant muscovite. The muscovite is probably metamorphic in origin. Late metasomatic veins of black tourmalne, K-feldspar, and amphibole are present through the granite and manifest as pale feldspar-filled fractures, often with large fans of amphibole.

The granite intruded into the metamorphic rocks of the Piedmont region during the last stages of the Alleghenian Orogeny, which was the time when North America and North Africa collided. Over time, erosion eventually exposed the present mountain of more resistant igneous rock, in processes similar to those that have exposed Devils Tower National Monument in Wyoming.