Natural History of Stone Mountain
Stone Mountain sits on the western edge of a large belt of Lithonia Gneiss granite although the younger intrusive granite that comprises the mountain is entirely different from Lithonia granite. Rising to a height of 1,683 feet above sea level (roughly 650-750 feet above the surrounding Georgia Piedmont, depending on where it is measured), it is visible from Kennesaw Mountain to the west, Amicalola Falls State Park to the north and Mount Yonah to the northeast.
Technically known as a pluton, the mountain was formed during the complex folding and faulting that created the Blue Ridge Mountains, although Stone Mountain is not part of that range. The magma that created Stone Mountain was formed deep inside the earth, then forced its way out of the earth's molten center. Before the molten rock hit the air it stopped, initially forming the west side of the "pluton." Successive attempts at eruption (breaking through to the surface of the earth) also failed, but added to the size of the dome from west to east. Once the pluton was formed it began to cool. This occurred during the Alleghenian Orogeny, a massive collision of tectonic plates perhaps 350 million years ago.
Geologists, however, are stumped as to how this enormous rock (the largest known granite formation) became exposed. Some believe that the entire Georgia Piedmont region was higher than the mountain and over time, erosion simply wore the dirt and metamorphic stone surrounding the mountain away. Others believe that the area was flooded after the formation of the mountain and the water eroded the surrounding material. A third theory includes post-formation geological events (primarily earthquakes). Analysis of the rock reveals that the magma that created the pluton was comprised of quartz, feldspar, microcline and muscovite with smaller amounts of biotite and tourmaline. Embedded in the granite are pieces of biotite gneiss and amphibolite.