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.

Scientific study of rocks, including their composition, texture, and structure; occurrence and distribution; and conditions of origin. Petrology is concerned with all three major types of rocks: igneous, metamorphic, and sedimentary. The subdiscipline of experimental petrology involves synthesizing rocks in the laboratory to ascertain the physical and chemical conditions under which they form. The subdiscipline of petrography is concerned primarily with the systematic study and description of rocks using a petrographic microscope.

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In geology, petrology (from Greek: πέτρα, petra, rock; and λόγος, logos, knowledge) is the study of rocks and the conditions on which they form. Lithology once was approximately synonymous with petrography, but in current usage, lithology is a subdivision of petrology focusing on macroscopic hand-sample or outcrop-scale description of rocks, while petrography is the speciality that deals with microscopic details.

In the oil industry, lithology, or more specifically mud logging, is the graphic representation of geological formations being drilled through, and drawn on a log called a mud log. As the cuttings are circulated out of the borehole they are sampled, examined (typically under a 10x microscope) and tested chemically when needed.

Methodology

Petrology utilizes the classical fields of mineralogy, petrography, optical mineralogy, and chemical analyses to describe the composition and texture of rocks. Modern petrologists also include the principles of geochemistry and geophysics through the studies of geochemical trends and cycles and the use of thermodynamic data and experiments to better understand the origins of rocks.

Branches of petrology

There are three branches of petrology, corresponding to the three types of rocks: igneous, metamorphic, and sedimentary, and another dealing with experimental techniques:

• Igneous petrology focuses on the composition and texture of igneous rocks (rocks such as granite or basalt which have crystallized from molten rock or magma). Igneous rocks include volcanic and plutonic rocks.
• Sedimentary petrology focuses on the composition and texture of sedimentary rocks (rocks such as sandstone, shale, or limestone which consist of pieces or particles derived from other rocks or biological or chemical deposits, and are usually bound together in a matrix of finer material).
• Metamorphic petrology focuses on the composition and texture of metamorphic rocks (rocks such as slate, marble, gneiss, or schist which started out as sedimentary or igneous rocks but which have undergone chemical, mineralogical or textural changes due to extremes of pressure, temperature or both)
• Experimental petrology employs high-pressure, high-temperature apparatus to investigate the geochemistry and phase relations of natural or synthetic materials at elevated pressures and temperatures. Experiments are particularly useful for investigating rocks of the lower crust and upper mantle that rarely survive the journey to the surface in pristine condition. The work of experimental petrologists has laid a foundation on which modern understanding of igneous and metamorphic processes has been built.

Terminology

Acid test: using acid to test rock or soil for carbonatesSuite: a collection of rock specimens from a given area, or it can refer to the succession of closely associated sedimentary strata or layers, especially a repeated sequence of layers.

See also

• Important publications in petrology

References

• Bayly, M. Brian (1968) Introduction to petrology Prentice-Hall, Englewood Cliffs, NJ;
• Blatt, Harvey and Tracy, Robert J. (1996) Petrology: igneous, sedimentary, and metamorphic W.H. Freeman, New York, ISBN 0716724383 ;
• Dietrich, Richard Vincent and Skinner, Brian J. (1979) Rocks and rock minerals Wiley, New York, ISBN 0471029343 ;
• Fei, Yingwei; Bertka, Constance M. and Mysen, Bjorn O. (eds.) (1999) Mantle petrology: field observations and high-pressure experimentation The Geochemical Society, Houston, TX, ISBN 0941809056 ;
• Stanton, R. L. (1972) Ore petrology McGraw-Hill, New York, ISBN 0070608431 ;

External links

• Atlas of Igneous and metamorphic rocks, minerals, and textures Geology Department, University of North Carolina;
• Metamorphic Petrology Database (MetPetDB) Department of Geology and Geophysics, Louisiana State University;
• Petrological Database of the Ocean Floor (PETDB) Center for International Earth Science Information Network (CIESIN), Columbia University;