July 16. Stone Mountain, Georgia

The southeastern part of the United States bore the brunt of the collision with Gondwana. What is now the westernmost part of Africa, around Senegal and Mauritania, collided with the Carolinas and Georgia. This was a true continent-continent collision, similar to that between India and Eurasia, and the mountain uplift squeezed between the continents was a huge one. 

When continents collide, everything doesn’t just go up. Plenty of material is forced down, too. Far enough down for temperatures and pressures to change the rock, to metamorphose the rock into radically different forms. A sandstone with quartz and feldspar and iron oxide cement and other grains can be changed into a banded rock called gneiss, with the chemicals in the original rock reorganized into entirely different minerals, often arranged in thin layers whose geometry is related to the orientation of the pressure regime.

Under such conditions, melting can also happen, and molten rock, magma, is typically associated with collision zones. Because of the chemistry of the rocks involved, it’s actually pretty easy to see the differences between oceanic subduction, with a slab of oceanic crust diving down beneath a continent and giving rise to a volcanic mountain range like the Andes in western South America, and the melting of complex continental crust which tends to be more silica rich, more granitic in composition.

Photo by Kyleandmelissa22 via Wikipedia, public domain.

In Georgia, Stone Mountain outside Atlanta is a solidified granitic body that formed as a result of the collision of Gondwana and North America about 300 million years ago, near the end of the Pennsylvanian Period. Technically the rock is called quartz monzonite, but it amounts to granite with a lower percentage of quartz than most granite. Petrologists have dozens of terms for rocks to give them a way to talk about specific compositions more clearly.

The granitic rock at Stone Mountain solidified well within the earth’s crust, probably 5 to 10 miles down. The difference between chemically similar deep-seated or plutonic rocks (from the realm of Pluto) and those that solidify on the surface, such as lava flows, is their grain size. At the surface, molten rock cools and solidifies quickly, so individual crystals don’t have time to grow very large. Insulated within the earth, granites and other plutonic rocks solidify over many thousands or even millions of years, so their crystal grains are relatively large, sometimes several centimeters across.

Over time, with more uplift and more erosion, the solid granitic rock was brought to the earth’s surface. It is more resistant than the surrounding rock, so it eventually eroded into the prominent dome-like feature that stands outside Atlanta today. There is a huge carving of Confederate Generals on one side of the mountain.

—Richard I. Gibson

Reference:
Age and origin of the Stone Mountain Granite, Lithonia district, Georgia (Whitney, Jones, and Walker, 1976)


Photo by Kyleandmelissa22 via Wikipedia, public domain.