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Earth Sense: Stone Mountain a pluton, not an old, extinct volcano
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Just 15 miles northeast of Atlanta, a round granite formation stands 800 feet tall. Stone Mountain generates a lot of questions, but also misconceptions.

One of the latter is that Stone Mountain is the remnant of an old volcano. Not so, since the Georgia Piedmont isn’t volcanic. After all, the last thing we’d need in the metro Atlanta area would be a volcanic eruption at Interstate 285 during rush hour.

Stone Mountain is a “pluton.” It formed when magma, or molten rock, pushed upward into the solid crust from very great depth. It didn’t make it to the surface. Over a period of several thousand years, it cooled and solidified into granite.

Because its crystals had lots of time to grow, they became large and gave the granite the rough appearance that you see today. In the long time that followed, the work of erosion removed the overlaying material and gradually exposed the pluton.

Now, with almost no soil on top of it that would hold water and acids from vegetation, the rock dissolves much more slowly than the surrounding landscapes.

Another myth: “It’s one of the world’s oldest rocks.” Also not true. Analysis shows it to be about 300 million years old, which places it in the Paleozoic Era. That’s less than 10 percent of Earth’s total age, but another 200 million years would still pass before dinosaurs populated the world.

Farther north, near Lafayette and Ringgold, limestone ridges had already formed earlier, to develop into the spectacular plateaus and caves of Walker County while the Stone Mountain pluton was gradually emerging in DeKalb County.

Georgia’s Stone Mountain isn’t the only such feature in the U.S. South; 315 miles away is Elkin, N.C., with another Stone Mountain Park. The mountain there looks almost like a carbon copy of the one in Georgia.

A 120-mile trip from Gainesville to the Brevard area takes you near Looking Glass Rock, another of the whaleback-shaped features. It too shares the geologic history of the Stone Mountains. An enjoyable hiking trail from Road 475B (off N.C. 276) lets you get all the way to the top.

 But unlike Stone Mountain, Ga., Looking Glass Rock has no fence or guardrails at the summit, and great caution is advised not to advance past the end of the trail. There is no “edge” to the rock, and its steepness increases to danger level very quickly.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.

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