Petrology

Study of rocks, their formation and their transformations.

A rock formed by crystallization of magma or lava. Intrusive bodies crystallize below the surface, while extrusive rocks solidify at or near the surface. Granite is a common intrusive example and basalt a common extrusive example.

An igneous rock composed predominantly of plagioclase. Lunar anorthositic rocks are central to the flotation interpretation of the lunar magma ocean. Their mineral and chemical composition does not alone date the formation of the complete crustal layer; sample context and radiometric dating are needed.

The observable physical and compositional character of rock, including its constituent minerals, texture and grain size. Different lithologies can have distinct spectral reflectance, density and magnetic susceptibility, making geological mapping possible through several independent measurements.

Fine fragmented material produced by a volcanic eruption; dispersal can affect regions far from the vent.

A rock formed through accumulation of crystals separated from magma, often during fractional crystallization.

An intrusive rock containing quartz and predominantly plagioclase rather than alkali feldspar; it is one member of tonalite-trondhjemite-granodiorite.

A felsic volcanic rock broadly corresponding in bulk composition to granite.

Volcanism producing rhyolite or liquids of similar felsic composition.

A silica-rich volcanic rock, compositionally between typical andesite and rhyolite.

A volcanic rock with intermediate composition, common in volcanic arcs.

An ultramafic volcanic rock whose inferred magnesium-rich parental liquids constrain high-temperature mantle melting; mantle plume and ambient mantle interpretations must be distinguished.

A texture with elongate or skeletal crystals, commonly olivine, in some ultramafic lavas. It records crystal growth and cooling conditions and supports interpretation of a lava's crystallization history. It is not alone a numerical thermometer for ambient mantle potential temperature.

A coarse-grained intrusive rock with intermediate composition, commonly containing plagioclase and amphibole or pyroxene.

Rock transformed by changes in temperature, pressure, deformation or fluid conditions while predominantly remaining solid.

A common descriptive name for low-grade metamorphosed mafic volcanic rock, often containing greenish minerals. A greenstone belt can contain these rocks together with other volcanic and sedimentary rocks; the belt name does not imply uniform composition or density.

A metamorphic rock with compositional banding; its bulk composition can be felsic, intermediate or mafic.

A metamorphic rock formed from a sedimentary rock precursor.

A high-grade metamorphic rock with an assemblage characteristic of granulite facies; both felsic and mafic examples occur.

A mafic rock with a granulite-facies assemblage. Granulite alone specifies a metamorphic facies, not the mafic composition.

A metamorphic rock dominated by amphibole and plagioclase, commonly derived from a mafic precursor.

A coarse-grained intrusive rock containing quartz and feldspar, with more plagioclase relative to alkali feldspar than typical granite.

A coarse-grained felsic intrusive rock, rich in quartz and feldspar.

Rock formed through deposition and subsequent consolidation or precipitation of material at or near a planetary surface.

A sedimentary rock composed predominantly of sand-sized grains. Cementation, porosity, pore fluid and effective stress strongly affect its seismic velocity.

A compositional description for magnesium- and iron-rich silicate material; basalt and gabbro are common mafic rocks.

A compositional description for rocks or magma rich in silica and commonly feldspar and quartz, contrasting with mafic compositions.

A family of felsic plutonic rocks prominent in studies of early continental crust. Melting hydrous mafic sources can generate related compositions, but the rock name alone does not establish subduction.

Metamorphic rock containing characteristic blue amphibole, associated with some low-temperature, high-pressure assemblages.

High-pressure metamorphic rock with a major assemblage of garnet and omphacitic clinopyroxene, generally lacking plagioclase. It can derive from mafic crust by several burial mechanisms.

Rock dominated by serpentine minerals, commonly produced by serpentinization of ultramafic rock.

An ultramafic rock dominated by pyroxenes; it can originate through several magmatic and reactive processes.

An ultramafic rock dominated by olivine with pyroxenes and other phases, abundant in the upper mantle.

An ultramafic rock dominated by olivine and orthopyroxene; strongly melt-depleted mantle commonly has this composition.

A coarse-grained mafic intrusive igneous rock, commonly forming a substantial part of oceanic crust.

A fine-grained mafic igneous rock, commonly formed from erupted mantle-derived magma.

Rock with a composition similar to basalt; the adjective also describes related magma.

Basalt erupted at oceanic intraplate volcanic islands, used to investigate mantle heterogeneity and possible recycled components.

Modification of a magma by incorporation or assimilation of surrounding crust.

Progressive separation of crystals from a melt, changing the composition of the residual liquid.

Melting of only part of a rock; the melt and remaining solid generally have different compositions.

Chemical modification of residual rock by extraction of melt. In mantle peridotite, depletion commonly changes mineral proportions and trace-element contents, and can accompany volatile loss. Compositional buoyancy and water content must be assessed separately from thermal density changes when explaining a long-lived craton.

Concentration of melt transport toward a narrower extraction zone, affecting how melt production is converted into oceanic crust thickness.

The proportion of material present as geological melt, with a mass or volume convention specified.

Melting during ascent when a hot rock parcel crosses its solidus as pressure falls, without requiring external heating of the ascending parcel.

Melting induced by addition of components, especially water, that lower a rock’s solidus.

Changes in a rock’s mineral assemblage, texture or composition under altered pressure, temperature or fluid conditions, conventionally dominated by solid-state processes.

A grouping of mineral assemblages characteristic of a range of temperature and pressure, allowing for the original rock composition.

A high-temperature metamorphic facies, commonly with relatively water-poor assemblages; it does not determine a unique bulk composition.

A range of pressure-temperature conditions characterized by diagnostic hydrous high-pressure mineral assemblages in suitable compositions.

Transformation of suitable rock to an eclogite assemblage. Increased density can affect slab buoyancy, but reaction kinetics can delay transformation.

A thin metamorphic unit structurally beneath an emplaced ophiolite, potentially recording heating and deformation during early convergence or emplacement.

An elongated region characterized by a related metamorphic history or assemblage distribution.

Metamorphism at elevated pressure relative to ordinary shallow-crustal environments; its tectonic meaning depends on the full pressure-temperature-time path.

Metamorphism reaching conditions sufficient for diagnostic high-pressure phases such as coesite in an appropriate composition. Their growth must be distinguished from detrital inheritance or unrelated inclusions.

The inferred history of pressure and temperature experienced by a rock. Dating specific growth domains and linking them to mineral reactions are needed to attach time to the path.

A reaction that consumes carbonate-bearing assemblages and releases carbon dioxide or another carbon-bearing fluid component.

Chemical modification of rock by externally supplied fluid or melt.