Petrology (from
Ancient Greekπέτρος (pétros) 'rock', and -λογία (-logía) 'study of') is the branch of
geology that studies
rocks, their mineralogy, composition, texture, structure and the conditions under which they form.[1] Petrology has three subdivisions:
igneous,
metamorphic, and
sedimentary petrology.[2] Igneous and metamorphic petrology are commonly taught together because both make heavy use of
chemistry, chemical methods, and phase diagrams. Sedimentary petrology is commonly taught together with
stratigraphy because it deals with the processes that form
sedimentary rock.[3] Modern sedimentary petrology is making increasing use of chemistry.
Background
Lithology was once approximately synonymous with
petrography, 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 the
petroleum 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 10× microscope) and tested chemically when needed.
Methodology
Petrology utilizes the fields of
mineralogy, petrography,
optical mineralogy, and chemical analysis to describe the composition and texture of rocks. Petrologists also include the principles of
geochemistry and
geophysics through the study of geochemical trends and cycles and the use of
thermodynamic data and experiments in order to better understand the origins of rocks.
Branches
There are three branches of petrology, corresponding to the three types of rocks:
igneous,
metamorphic, and
sedimentary, and another dealing with experimental techniques:
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 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 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. They are also one of the prime sources of information about completely inaccessible rocks, such as those in the Earth's
lower mantle and in the mantles of the other
terrestrial planets and the
Moon. 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.