Lithology

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy.

[1][2][3] Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock.

[4] Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas.

Sedimentary rock is formed from mineral or organic particles that collect at the Earth's surface and become lithified.

Intrusive igneous rocks are usually classified using the QAPF classification, which is based on the relative content of quartz, alkali feldspar, plagioclase, and feldspathoid.

Naming based on texture and a pelite (e.g., shale, mudrock) protolith can be used to define slate and phyllite.

[19] In rocks in which mineral grains are large enough to be identified using a hand lens, the visible mineralogy is included as part of the description.

[15] The colour of a rock or its component parts is a distinctive characteristic of some rocks and is always recorded, sometimes against standard colour charts, such as that produced by the Rock-Color Chart Committee of the Geological Society of America based on the Munsell color system.

Igneous rocks may also have fabrics as a result of flow or the settling out of particular mineral phases during crystallisation, forming cumulates.

Sedimentary textures include the degree of sorting, grading, shape and roundness of the clasts.

[27] In igneous rocks, small-scale structures are mostly observed in lavas such as pahoehoe versus ʻAʻā basaltic flows,[28] and pillows showing eruption within a body of water or beneath ice.

Surficial lithologies can be given to lacustrine, coastal, fluvial, aeolian, glacial, and recent volcanic deposits, among others.