petrology, branch of geology specifically concerned with the origin, composition, structure, and properties of rocks , primarily igneous and metamorphic, and secondarily sedimentary. It includes petrography, the systematic description and classification of rocks using microscopic examination of rock in thin sections; and petrogenesis, which deals with the origin and formation of the various kinds of rock. Petrology is also concerned with the laboratory simulation of rock-forming processes and the application of principles of physical chemistry to natural environments. Petrologic analyses of oceanic rocks have given insights into plate tectonic processes, especially rock from mid-oceanic ridges, which may be formed from magma derived from deep in the mantle . Lunar rocks returned by Apollo astronauts were studied with petrographic techniques providing a wealth of information on the makeup and origin of the moon.
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