Igneous Rocks

Igneous rocks form when magma (molten rock) solidifies. Magma hardening in intrusive bodies -- beneath the earth's surface -- cools slowy to form large mineral crystals. Magma erupting at the earth's surface cools quickly to form extrusive deposits containing microscopic mineral crystals. Magma ejected above the earth's surface may harden fully or partially before returning to earth, where it forms volcaniclastic deposits (tuffs).

Color and density of igneous rock depend on its mineral composition. Felsic (light-colored and low-density) igneous rocks include large proportions of quartz and feldspar. Mafic (dark and dense) igneous rocks incorporate minerals containing iron and magnesium. Ultramafic igneous rocks contain iron- and magnesium-bearing minerals that can form only under deep subterranean pressures, with relatively little silica.

Georgia's Piedmont region comprises mostly metamorphic rock, but it includes many granitic intrusive bodies. The best known exposure is Stone Mountain, east of Atlanta, which rises 825 feet (250 meters) above the surrounding surface. Many granite bodies are quarried for monument and construction stone. Granitic plutons may also underlie Blue Ridge domal features such as Brasstown Bald and the Tallulah Falls Dome. Blue Ridge and Piedmont Georgia also exhibits many northwest-oriented diabase dikes. These dikes are believed to have formed by basaltic magma filling cracks when Pangea split to form the Atlantic basin about 200 million years ago. You might consider these dikes as Triassic/Jurassic continental stretch marks.

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