Thelithology of arock unit is a description of its physical characteristics visible atoutcrop, in hand orcore samples, or with low magnification microscopy. Physical characteristics include colour, texture,grain size, and composition.^[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. Examples of lithologies in the second sense includesandstone,slate,basalt, orlimestone.^[4]

Lithology is the basis of subdividing rock sequences into individuallithostratigraphic units for the purposes ofmapping and correlation between areas. In certain applications, such assite investigations, lithology is described using a standard terminology such as in the European geotechnical standardEurocode 7.

The naming of a lithology is based on therock type. The three major rock types areigneous,sedimentary, andmetamorphic. Igneous rocks are formed directly frommagma, which is a mixture of molten rock, dissolved gases, and solid crystals. Sedimentary rock is formed from mineral or organic particles that collect at the Earth's surface and becomelithified. Metamorphic rock forms by recrystallization of existing solid rock under conditions of great heat or pressure.^[5]

Igneous rocks are further broken into three broad categories. Igneous rock composed of broken rock fragments created directly byvolcanic processes (tephra) are classified aspyroclastic rock. Pyroclastic rocks are further classified by average fragment (clast) size and whether the fragments are mostly individual mineralcrystals, particles ofvolcanic glass, or rock fragments.^[6] Further classifications, such as bychemical composition, may also be applied.^[7][8] Igneous rocks that have visible mineral grains (phaneritic rocks) are classified asintrusive, while those that are glassy or very fine-grained (aphanitic) are classified asextrusive rock. Intrusive igneous rocks are usually classified using theQAPF classification, which is based on the relative content ofquartz,alkali feldspar,plagioclase, andfeldspathoid. Special classifications exist for igneous rock of unusual compositions, such asultramafic rock orcarbonatites. 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 theTAS classification. This is based on the total content ofsilica andalkali metal oxides and other chemical criteria.^[9][10][11]

Sedimentary rocks are further classified by whether they aresiliciclastic orcarbonate. Siliciclastic sedimentary rocks are then subcategorized based on their grain size distribution and the relative proportions of quartz, feldspar, and lithic (rock) fragments.^[12] Carbonate rocks are classified with theDunham orFolk classification schemes according to the constituents of the carbonate rock.^[13]

Metamorphic rock naming can be based onprotolith, mineral composition, texture, ormetamorphic facies. Naming based on texture and apelite (e.g.,shale,mudrock) protolith can be used to defineslate andphyllite. Texture-based names areschist andgneiss. These textures, from slate to gneiss, define a continually-increasing extent of metamorphism.^[14] Metamorphic facies are defined by the pressure-temperature fields in which particular minerals form.^[15] Additional metamorphic rock names exist, such asgreenschist (metamorphosedbasalt and other extrusive igneous rock) orquartzite (metamorphosed quartz sand).^[16]

In igneous and metamorphic rocks,grain size is a measure of the sizes of the crystals in the rock. In igneous rock, this is used to determine the rate at which the material cooled: large crystals typically indicate intrusive igneous rock, while small crystals indicate that the rock was extrusive.^[17] Metamorphism of rock composed of mostly a single mineral, such as quartzite ormarble, may increase grain size (grain growth), while metamorphism of sheared rock may decrease grain size (syntectonicrecrystallization).^[18]

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., aconglomerate,sandstone, ormudstone). 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 quartzarenite".^[19]

In rocks in which mineral grains are large enough to be identified using a hand lens, the visiblemineralogy is included as part of the description. In the case of sequences possibly includingcarbonates,calcite-cemented rocks or those with possible calcite veins, it is normal to test for the presence of calcite (or other forms ofcalcium carbonate) using dilutehydrochloric acid and looking foreffervescence.^[20]

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.^[9][10][11] 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.^[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 theGeological Society of America based on theMunsell color system.^[21]

Thefabric of a rock describes the spatial and geometric configuration of all the elements that make it up. In sedimentary rocks the main visible fabric is normallybedding, and the scale and degree of development of the bedding is normally recorded as part of the description. Metamorphic rocks (apart from those created bycontact metamorphism), are characterised by well-developed planar and linear fabrics. Igneous rocks may also have fabrics as a result of flow or the settling out of particular mineral phases during crystallisation, formingcumulates.

Thetexture of a rock describes the relationship between the individual grains or clasts that make up the rock. Sedimentary textures include the degree ofsorting,grading, shape and roundness of the clasts.^[22] Metamorphic textures include those referring to the timing of growth of large metamorphic minerals relative to a phase of deformation—before deformationporphyroclast—after deformationporphyroblast.^[23]Igneous textures include such properties as grain shape, which varies from crystals with ideal crystal shapes (euhedral) to irregular crystals (anhedral), whether the rock shows highly nonuniform crystal sizes (isporphyritic), or whether grains are aligned (which is described astrachytic texture).^[24]

Rocks often contain small-scale structures (smaller than the scale of an individual outcrop). In sedimentary rocks this may includesole markings,ripple marks,mudcracks andcross-bedding. These are recorded as they are generally characteristic of a particulardepositional environment and may provide information on paleocurrent directions.^[25] In metamorphic rocks associated with the deeper levels offault zones, small scale structures such as asymmetricboudins^[26] andmicrofolds are used to determine the sense of displacement across the zone.^[27] In igneous rocks, small-scale structures are mostly observed inlavas such as pahoehoe versus ʻAʻā basaltic flows,^[28] andpillows showing eruption within a body of water or beneath ice.^[29][30][31]

Unconsolidated surficial materials may also be given a lithology. This is defined by grain size and composition and is often attached to an interpretation of how the unit formed. Surficial lithologies can be given tolacustrine, coastal,fluvial,aeolian,glacial, and recent volcanic deposits, among others. Examples of surficial lithology classifications used by theU.S. Geological Survey are, "Glacial Till,Loamy", "Saline Lake Sediment", and "Eolian Sediment, Coarse-Textured (SandDunes)".^[32]