Athin section of a volcanicsand grain seen under the microscope, with plane-polarized light in the upper picture, andcross-polarized light in the lower picture. Scale box is 0.25 mm.
Petrology (from Ancient Greekπέτρος (pétros)'rock' and -λογία (-logía)'study of') is the branch ofgeology that studiesrocks, their mineralogy, composition, texture, structure and the conditions under which they form.[1] Petrology has three subdivisions:igneous,metamorphic, andsedimentary petrology.[2] Igneous and metamorphic petrology are commonly taught together because both make heavy use ofchemistry, chemical methods, and phase diagrams. Sedimentary petrology is commonly taught together withstratigraphy because it deals with the processes that formsedimentary rock.[3] Modern sedimentary petrology is making increasing use of chemistry.
Lithology was once approximately synonymous withpetrography, but in current usage, lithology focuses on macroscopic hand-sample or outcrop-scale description of rocks while petrography is the speciality that deals with microscopic details.
In thepetroleum industry, lithology, or more specificallymud 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 theborehole, they are sampled, examined (typically under a 10× microscope) and tested chemically when needed.
Petrology utilizes the fields ofmineralogy, petrography,optical mineralogy, and chemical analysis to describe the composition and texture of rocks. Petrologists also include the principles ofgeochemistry andgeophysics through the study of geochemical trends and cycles and the use ofthermodynamic data and experiments in order to better understand the origins of rocks.
There are three branches of petrology, corresponding to the three types of rocks:igneous,metamorphic, andsedimentary, and another dealing with experimental techniques:
Sedimentary petrology focuses on the composition and texture ofsedimentary rocks (rocks such assandstone,shale, orlimestone which consist of pieces or particles derived from other rocks or biological or chemical deposits, and are usually bound together in amatrix of finer material).
Metamorphic petrology focuses on the composition and texture ofmetamorphic rocks (rocks such asslate,marble,gneiss, orschist) which have undergone chemical, mineralogical or textural changes due to the effects of pressure, temperature, or both). The original rock, prior to change (called the protolith), may be of any sort.[4]
Experimental petrology employs high-pressure, high-temperature apparatus to investigate the geochemistry andphase relations of natural or synthetic materials at elevated pressures and temperatures. Experiments are particularly useful for investigating rocks of the lowercrust andupper mantle that rarely survive the journey to the surface in pristine condition. They are also one of the prime sources of information about completely inaccessible rocks, such as those in the Earth'slower mantle and in the mantles of the otherterrestrial planets and theMoon. The work of experimental petrologists has laid a foundation on which modern understanding of igneous and metamorphic processes has been built.
^The 22nd edition of the Manual of mineral science. Buch. New York: Wiley. 2002. p. 1.ISBN978-0-471-25177-4.
^Blatt, Harvey; Tracy, Robert J.; Owens, Brent E. (2006).Petrology: igneous, sedimentary and metamorphic (3rd ed.). New York: Freeman.ISBN978-0-7167-3743-8.
^Frost, B. R.; Frost, C. D. (2014).Essentials of Igneous and Metamorphic Petrology.Cambridge University Press.
^abWinter, John D. (2010).Principles of igneous and metamorphic petrology (2nd ed.). New York: Prentice Hall. pp. 467–468.ISBN978-0-321-59257-6.
Fei, Yingwei; Bertka, Constance M.; Mysen, Bjorn O. (eds.) (1999),Mantle Petrology: field observations and high-pressure experimentation (Houston TX:Geochemical Society)ISBN0-941809-05-6.
