Stone Mountain is located in the central part of the Piedmont
physiographic province of Georgia. The Piedmont is composed of
hard
igneous and metamorphic rocks derived from the recrystallization
of
ancient (300 to 600 million year old) sediments that were once
deeply
buried and subjected to high temperatures and pressures. They
were
re-exposed during a collision between the African and North
American
Continents about 250 to 300 million years ago.In the period that followed this collision, low density fluids, characterized by an abundance of silicon dioxide, collected into hot molten magmas and migrated upward through the surrounding denser rock. About 230 million years ago, the Stone Mountain magma ceased to migrate and cooled approximately 6 miles (or 10 km) below the land surface. Erosion by wind and water have exposed the Stone Mountain granite at the present land surface. The mountain's prominence above the local landscape is simply a function of its resistance to erosion relative to the surrounding older rocks.
The domed shape of Stone
Mountain is the result of a process called
exfoliation. As
the 6 miles of overlying rock was eroded away, the pressure on
the
granite was reduced and the rock began to expand slowly. As the
rock
expanded, cracks or joints developed parallel to the surface of
the
mountain and slabs of rock broke off. Exfoliation continues
today, as
Stone Mountain continues to expand and new joints form.
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