The lithology of a rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 unit is a description of its physical characteristics visible at outcrop
An outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. -Features:Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial deposits are covered by a mantle of soil and vegetation and cannot be...

, in hand or core sample
Core sample
A core sample is a cylindrical section of a naturally occurring substance. Most core samples are obtained by drilling with special drills into the substance, for example sediment or rock, with a hollow steel tube called a core drill. The hole made for the core sample is called the "core hole". A...

s or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of the gross physical character of a rock. It is the basis of subdividing rock sequences into individual lithostratigraphic
Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology...

 units for the purposes of mapping
Geologic map
A geologic map or geological map is a special-purpose map made to show geological features. Rock units or geologic strata are shown by color or symbols to indicate where they are exposed at the surface...

 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 Eurocode 7
EN 1997
EN 1997 - Eurocode 7: Geotechnical design, is the European Standard for the design of geotechnical structures, using the limit state design philosophy. It is published in two parts; "General rules" and "Ground investigation and testing". It was approved by the European Committee for Standardization...


Rock type

The naming of a lithology is based on the rock type. The three major rock types are sedimentary, igneous, metamorphic.

Sedimentary rocks are further classified by whether they are siliciclastic
Siliciclastic rocks are clastic noncarbonate sedimentary rocks that are almost exclusively silica-bearing, either as forms of quartz or other silicate minerals. All siliciclastic rocks are formed by inorganic processes, or deposited through some mechanical process, such as stream deposits that are...

 or carbonate
Carbonate rock
Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite and dolostone, which is composed of the mineral dolomite .Calcite can be either dissolved by groundwater or precipitated by...

. Siliciclastic sedimentary rocks are then subcategorized based on their grain size distribution and the relative proportions of quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

, feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

, and lithic (rock) fragments. Carbonate rocks are classified with the Dunham
Dunham classification
The Dunham classification system for carbonate sedimentary rocks was devised by Robert J. Dunham in 1964, and refined by Embry and Klovan in 1971 to include sediments that were organically bound during deposition.-History:...

 or Folk classification schemes according to the constituents of the carbonate rock.

The name of an igneous rock requires information on crystal size and mineralogy. This classification can often be performed with a QAPF diagram
QAPF diagram
A QAPF diagram is a double triangle diagram which is used to classify igneous rocks based on mineralogic composition. The acronym, QAPF, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid ". These are the mineral groups used for classification in QAPF diagram...


Metamorphic rock naming can be based on texture, protolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...

, metamorphic facies
Metamorphic facies
The metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space...

, and/or the locations in which they are found. Naming based on texture and a pelite
Pelite is old and currently not widely used field terminology for a clayey fine-grained clastic sediment or sedimentary rock, i.e. mud or mudstone. It is equivalent to the Latin-derived term lutite. More commonly, metamorphic geologists currently use pelite for a metamorphosed fine-grained...

 (e.g., shale, mudrock) protolith can be used to define slate
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...

 and phyllite
Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the...

. Texture-based names are schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...

 and gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...

. These textures, from slate to gneiss, define a continually-increasing extent of metamorphism. Metamorphic facies are defined by the pressure-temperature fields in which particular minerals form. Additional metamorphic rock names exist: greenstone
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....

 (metamorphosed basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

 and other extrusive igneous rock) is a classification based on composition and being located Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

 terranes, while quartzite
Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...

 is based only on composition, as quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 is too stable and homogeneous to change phase at typical metamorphic temperatures and pressures.

Grain/clast size

In igneous
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

 and metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...

 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
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...

, while small crystals indicate that the rock was extrusive
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...

. As metamorphic reactions progress, the grains in metamorphic rocks can often be broken down into smaller grains.

In clastic sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s, grain size
Particle size (grain size)
Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which is the size of a single crystal inside the...

 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
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...

, sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

, or mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...

 one). 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".


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. In the case of sequences possibly including carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

s, calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

Cementation may refer to:*Cementation , the process of deposition of dissolved mineral components in the interstices of sediments*Cementation , a small deposit of calcium, similar to a cyst...

 rocks or those with possible calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

 veins, it is normal to test for the presence of calcite (or other forms of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

) using dilute hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

 by looking for effervescence
Effervescence (chemistry)
Effervescence is the escape of gas from an aqueous solution and the foaming or fizzing that results from a release of the gas. The word effervescence is derived from the Latin verb fervere preceded by the adverb ex, which means to boil...


The mineralogical composition of a rock is one of the major ways in which it is classified. In general, igneous rocks can be categorized by increasing silica content as ultramafic, mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...

, intermediate
Intermediate composition
In igneous petrology an intermediate composition refers to the chemical composition of a rock that has 52-63 wt% SiO2 being an intermediate between felsic and mafic compositions. Typical intermediate rocks include andesite, dacite and trachyandesite among volcanic rocks and diorite and granodiorite...

, or felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....

, though more mineral-specific classifications also exist. Likewise, metamorphic facies
Metamorphic facies
The metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space...

, 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.


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
Geological Society of America
The Geological Society of America is a nonprofit organization dedicated to the advancement of the geosciences. The society was founded in New York in 1888 by Alexander Winchell, John J. Stevenson, Charles H. Hitchcock, John R. Proctor and Edward Orton and has been headquartered at 3300 Penrose...

 based on the Munsell color system
Munsell color system
In colorimetry, the Munsell color system is a color space that specifies colors based on three color dimensions: hue, value , and chroma . It was created by Professor Albert H...



The fabric
Fabric (geology)
In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up.-Types of fabric:* Primary fabric — a fabric created during the original formation of the rock, e.g...

 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 normally bedding
Bedding refers to the materials laid above the mattress of a bed for hygiene, warmth, to protect the mattress, and for decorative effect. Bedding is the removable and washable portion of a human sleeping environment. It is more easily and economically replaced than the bed itself...

 and the scale and degree of development of the bedding is normally recorded as part of the description. Metamorphic rocks (apart from those created by contact metamorphism), are characterised by well-developed planar and linear fabrics. Igneous rocks may also have fabrics due either to flow or to the settling out of particular mineral phases during crystallisation, forming cumulates.


The texture
Texture (geology)
Texture in geology refers to the physical appearance or character of a rock, such as grain size, shape, arrangement, and pattern at both the megascopic or microscopic surface feature level. This includes the geometric aspects and relations amongst the component particles or crystals which is called...

 of a rock describes the relationship between the individual grains or clasts that make up the rock. Sedimentary textures include the degree of sorting
Sorting (sediment)
Sorting indicates the distribution of grain size of sediments, either in unconsolidated deposits or in sedimentary rocks. Poorly sorted indicates that the sediment sizes are mixed ; whereas well sorted indicates that the sediment sizes are similar .The degree of sorting may also indicate the energy...

, grading
Graded bedding
In geology, a graded bed is one characterized by a systematic change in grain or clast size from the base of the bed to the top. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones...

, shape and roundness of the clasts. Metamorphic textures include those referring to the timing of growth of large metamorphic minerals relative to a phase of deformation – before deformation porphyroclast
thumb|350px|right|A [[mylonite]] showing a number of porphyroclasts: a clear red [[garnet]] left in the picture while smaller white [[feldspar]] porphyroclasts can be found all over...

 – after deformation porphyroblast
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape....

. Igneous textures
Igneous textures
Igneous textures include the rock textures occurring in igneous rocks. Igneous textures are used by geologists in determining the mode of origin igneous rocks and are used in rock classification...

 include those that refer to the size of the mineral grains; vitreous
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 - glassy, without crystals, aphanitic - grains not visible, phaneritic
Phaneritic is a term usually used to refer to igneous rock grain size. It means that the size of matrix grains in the rock are large enough to be distinguished with the unaided eye as opposed to aphanitic . This texture forms by slow cooling of magma deep underground in the plutonic environment...

 - grains clearly visible, and porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...

 - large grains in a finer matrix.

Small-scale structures

Rocks often contain small-scale structures (smaller than the scale of an individual outcrop). In sedimentary rocks
Sedimentary structures
Sedimentary structures are those structures formed during sediment deposition.Sedimentary structures such as cross bedding, graded bedding and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terrains and understand the depositional...

 this may include sole markings, ripple marks, mudcrack
Mudcracks are sedimentary structures formed as muddy sediment dries and contracts.- Formation :...

s and cross-bedding
In geology, the sedimentary structures known as cross-bedding refer to horizontal units that are internally composed of inclined layers. This is a case in geology in which the original depositional layering is tilted, and the tilting is not a result of post-depositional deformation...

. These are recorded as they are generally characteristic of a particular depositional environment
Sedimentary depositional environment
In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...

 and may provide information on paleocurrent directions. In metamorphic rocks associated with the deeper levels of fault zones, small scale structures such as asymmetric boudins
thumb|Boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia.Boudinage is a geological term for structures formed by extension, where a rigid tabular body such as a bed of sandstone, is stretched and deformed amidst less competent surroundings...

 and microfolds are used to determine the sense of displacement across the zone. In igneous rocks, small-scale structures are mostly observed in lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

s such as pahoehoe versus [[ʻAʻā]] basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

ic flows, and pillows
Pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in...

 showing eruption within a body of water or beneath ice.

Surficial lithology

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 to lacustrine
Lacustrine means "of a lake" or "relating to a lake".Specifically, it may refer to:*Lacustrine plain*Lacustrine delta-Fish:*Lacustrine goby , a type of small fish found in Philippine waters belonging to the Gobiidae family, known in Tagalog as dulong-See also:*Fluvial - of or relating a...

, coastal, fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...

, aeolian
Aeolian or Eolian may refer to:* things related to Aeolus, the Greek God of wind or the patriarch of Greeks of Aeolia* Aeolian harp, a harp that is played by the wind* Aeolian processes, wind generated geologic processes...

, glacial, and recent volcanic deposits, among others. Examples of surficial lithology classifications used by the US Geological Survey are, "Glacial Till
thumb|right|Closeup of glacial till. Note that the larger grains in the till are completely surrounded by the matrix of finer material , and this characteristic, known as matrix support, is diagnostic of till....

, Loam
Loam is soil composed of sand, silt, and clay in relatively even concentration . Loam soils generally contain more nutrients and humus than sandy soils, have better infiltration and drainage than silty soils, and are easier to till than clay soils...

y", "Saline Lake Sediment", and "Eolian Sediment, Coarse-Textured (Sand Dunes)".
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