Metamorphic rocks have been changed, usually by heat and pressure, from their original condition into rock with new minerals and/or structures. Texture, structure, and mineral content of metamorphic rock depend both on its protolith (parent material) and metamorphic conditions. Presence of some specific minerals in a metamorphic rock can indicate the degree of heat and pressure it endured. Some metamorphic processes (e.g. tectonic collisions) distort large regions of the earth's crust.
Any kind of rock may serve as metamorphic protolith: limestone may metamorphose into marble; granite into gneiss; sandstone into quartzite; shale into slate, phyllite, or schist. Metamorphic rock of one variety may be further altered into a higher- or lower-grade metamorphic type. Metamorphic rock is notoriously difficult to date, partly because the process of metamorphism "resets the clock" of many chemical reactions and nuclear decay sequences. Also, ideas of stratigraphy -- as conceived for sedimentary or igneous rock -- often are difficult to apply to metamorphic rock. Large-scale (regional) metamorphic processes may throughly distort and rearrange strata, corrupting bedding and intrusive relationships on large and small scales. This is gnarly stuff.
Georgia's Piedmont and Blue Ridge regions comprise mostly metamorphic rock with large igneous intrusions. Georgia's metamorphic rock incorporates remains of marine sediments, volcanics, and crust from the Iapetus Ocean (ancestor of the Atlantic Ocean). From Cambrian through Pennsylvanian time (550-300 million years ago, more-or-less), plate tectonic processes closed the Iapetus basin, driving Proto-Africa and Proto-America (and Proto-Eurasia) together to form the supercontinent Pangea. Most sediment and ocean crust between these continental masses were consumed by subduction zones. Some marine material, along with microcontinents and/or island arcs, was bent, crushed, sheared, and melted in the collision to form metamorphic rock of eastern Appalachian mountains and foothills.
| Common Metamorphic Rocks | ||||
|---|---|---|---|---|
| Description | Major Minerals | Protolith | Metamorphic Conditions | |
| Marble | Non-foliated; coarse grain | Calcite | Limestone or dolomite | Moderate heat and pressure |
| Quartzite | Non-foliated; coarse grain | Quartz | Sandstone | Moderate heat and pressure |
| Hornfels | Non-foliated; very fine grain | Depends on protolith | Various | High heat, moderate pressure; typically forms halo in country rock around igneous intrusions |
| Migmatite | Foliated; dense; light and dark minerals in distorted bands or lenses; transitional between gneiss and granite | Feldspars, quartz, micas, amphiboles, ... | Various; often granitic rocks | High pressure and very high heat (near melting point) |
| Gneiss | Foliated; dense; usually medium-coarse grain; light and dark minerals in bands or lenses; commonly distorted | Feldspars, quartz, micas, amphiboles, ... | Various; often granitic rocks | High heat and pressure, often accompanying regional (tectonic) deformation |
| Schist | Foliated; coarse grain; layered, flaky, or platy texture; visible flat grains of mica or other platelike minerals | Quartz, micas, amphiboles; often garnets, kyanite, or other aluminosilicates | Various; often shales | Moderate heat; shearing pressure accompanying regional (tectonic) deformation |
| Phyllite | Foliated; fine grain; silky sheen; thinly layered | Microscopic micas | Various; often shales | Low-moderate heat; directed pressure |
| Slate | Foliated; very fine grain; splits cleanly on flat, parallel cleavage planes | Clay minerals | Various; often shales | Low heat; directed pressure |
| Amphibolite | Granular or foliated | Feldspars; ferromagnesians (amphiboles) | Mafic; often basalt | Moderate-high heat and pressure |
| Soapstone (Talc) | Granular or fibrous; slippery texture | Ferromagnesians | Ultramafic, e.g. serpentinite | Low-moderate heat[?]; moderate-high pressure; tectonically emplaced fragment of lowest ocean crust or mantle |
| Meta-igneous | Often retains structure/texture of protolith (usually granular) | Depends on protolith | Felsic to mafic | Insufficient heat and pressure to destroy protolith structure; examples: meta-granite, meta-gabbro, meta-tuff |