Geology
Stone Mountain is a pluton, a type of igneous intrusion. Primarily composed of quartz monzonite, the dome of Stone Mountain was formed during the formation of the Blue Ridge Mountains, part 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 five to ten miles below the surface.
Stone Mountain granite extends underground 9 miles (14 km) at its longest point into Gwinnett County. Numerous reference books and Georgia literature have dubbed Stone Mountain as “the largest exposed piece of granite in the world". This misnomer is most likely a result of advertisement by granite companies and early park administration. Stone Mountain, though often called a pink granite dome, actually ranges in composition from quartz monzonite to granite and granodiorite.
The granite is composed of quartz, plagioclase feldspar, microcline and muscovite, with smaller amounts of biotite and tourmaline. The tourmaline is mostly black in color, and the majority of it exists as optically continuous skeletal crystals, but much larger, euhedral pegmatitic tourmaline crystals can also be found in the mountain's numerous, cross-cutting felsic dikes. Embedded in the granite are xenoliths or pieces of foreign rocks entrained in the magma.
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 in Wyoming. This vein of granite also gave rise to Arabia Mountain and Panola Mountain in DeKalb County, smaller outcroppings further east of Stone Mountain.
The mountain's composition was described by one political commentator—and used as such as a simile for racial segregation in the 1950s—as "soft, exfoliating rock turns to dust under the hammer."
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