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The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core sample, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. (1984). 9780385181013, American Geological Institute. ISBN 9780385181013 Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.
Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of Geological map and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard .
Igneous rocks are further broken into three broad categories. Igneous rock composed of broken rock fragments created directly by Volcanism processes (tephra) are classified as pyroclastic rock. Pyroclastic rocks are further classified by average fragment (Clastic rock) size and whether the fragments are mostly individual mineral Crystal, particles of volcanic glass, or rock fragments. Further classifications, such as by chemical composition, may also be applied. (1984). 9783540127567, Springer-Verlag. ISBN 9783540127567 (2025). 9783540436508, Springer. ISBN 9783540436508 Igneous rocks that have visible mineral grains (Phanerite rocks) are classified as Intrusive rock, while those that are glassy or very fine-grained (Aphanite) are classified as extrusive rock. Intrusive igneous rocks are usually classified using the QAPF diagram, which is based on the relative content of quartz, alkali feldspar, plagioclase, and feldspathoid. Special classifications exist for igneous rock of unusual compositions, such as ultramafic rock or . Where possible, extrusive igneous rocks are also classified by mineral content using the extrusive QAPF classification, but when determining the mineral composition is impractical, they may be classified chemically using the TAS classification. This is based on the total content of Silicon dioxide and alkali metal oxides and other chemical criteria. (2025). 9780521880060, Cambridge University Press. ISBN 9780521880060
Sedimentary rocks are further classified by whether they are siliciclastic or carbonate rock. Siliciclastic sedimentary rocks are then subcategorized based on their grain size distribution and the relative proportions of quartz, feldspar, and lithic (rock) fragments. (1996). 9780716724384, W.H. Freeman. ISBN 9780716724384 Carbonate rocks are classified with the Dunham or Folk classification schemes according to the constituents of the carbonate rock. (2025). 9780131547285, Pearson Prentice Hall. ISBN 9780131547285
Metamorphic rock naming can be based on protolith, mineral composition, texture, or metamorphic facies. Naming based on texture and a pelite (e.g., shale, mudrock) protolith can be used to define slate and phyllite. Texture-based names are schist and gneiss. These textures, from slate to gneiss, define a continually-increasing extent of metamorphism. (1989). 9780582300965, Longman Scientific & Technical. ISBN 9780582300965 Metamorphic facies are defined by the pressure-temperature fields in which particular minerals form. Additional metamorphic rock names exist, such as greenschist (metamorphosed basalt and other extrusive igneous rock) or quartzite (metamorphosed quartz sand).
In clastic sedimentary rocks, grain size is the diameter of the grains and/or clasts that constitute the rock. These are used to determine which rock naming system to use (e.g., a conglomerate, sandstone, or mudstone). In the case of sandstones and conglomerates, which cover a wide range of grain sizes, a word describing the grain size range is added to the rock name. Examples are "pebble conglomerate" and "fine quartz arenite".
The mineralogical composition of a rock is one of the major ways in which it is classified. Igneous rocks are classified by their mineral content whenever practical, using the QAPF classification or special ultramafic or carbonatite classifications. Likewise metamorphic facies, which show the degree to which a rock has been exposed to heat and pressure and are therefore important in classifying metamorphic rocks, are determined by observing the mineral phases that are present in a sample.
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