The weight of two miles or so of rocks and earth overhead was sufficient to contain the tremendous pressure of the molten flow, so the upper crust literally floated on a hot liquid base.
Something had to give as liquid rock thrust into an area where there was no space. Forty miles or so to the northwest is a chain of mountains, of which Kennesaw is the tallest, formed by pressure from the side which buckled underlying rocks up like a steep roof, or folded layers over each other. Some of that pressure may have been applied by Stone Mountain. This admittedly is theory—upper layers which held much of the factual story have long since washed away.
Since the intruding material was contained in its original prison cell and held under constant pressure, it cooled gradually, a process which took perhaps a hundred million years. By cooling slowly, the molecules formed compact, uniform crystals.
Meanwhile the older, softer granite overhead was weathering and turning to soil and eroding away. Some went to extend the coastal area of southeast Georgia and some to help build up the rich black belt of South Alabama.
In the two hundred million years since the intrusion, the two-mile-thick overlay has eroded down to its present level, leaving the hard core of Stone Mountain standing up like a great gray egg. The surface of the mountain wears very slowly—scuffing feet of millions of visitors have left barely discernible marks along the western trail. Meanwhile the original crust is still wearing away at a rapid rate, so Stone Mountain is continuing to grow taller in reference to its base.
Around the base have been noted fingers of Stone Mountain granite extending outward into the old rock, or sometimes soil, where the molten material was forced into crevices during the lateral movement of underground strata.
The mountain is a natural target for lightning. Thunderclouds bombard it with their heaviest artillery. A bolt of lightning behaves very much like the thermo-jet torch. Its extreme heat converts moisture in underlying molecules to steam and literally blasts off the surface crystals, making a slight saucer-shaped depression four to six inches across. Heat fuses the bottom of the depression, leaving a slick, glassy surface.
Every lightning bolt for many years has left its mark. It is noticeable that they are thickest not on the highest points, but in depressions. Meteorologists say that is where the first drops from a shower soak into the granite and therefore make the best ground to attract the lightning.
From the time Gutzon Borglum began carving in 1923, stone rubble piled up at the base of the mountain below the monument. Hardly a man alive could remember what lay under it. After nearly fifty years, when the rubble was removed, there was revealed a low hill of the original granite gneiss peeping out from under the mountain, or more accurately, pushing into its side. The old rock clearly shows how it was twisted, turned and tortured by the great pressures of two hundred million years ago. Unable to shove aside this lot of rock, the molten mass tried to engulf and digest it.