Category: Geology Dictionary

Cleavage : The tendency of a rock to break along preferred planes of weakness, caused by the development of a planar fabric as a result of deformation.

Clastic rocks : Mechanically redeposited remains of eroded older rocks; rocks formed of fragments, or clasts, of pre-existing rocks.

Clast : Any individual constituent grain or fragment of a sediment or rock produced by mechanical weathering of a rock mass.

Chlorite : Any of a set of allied minerals which may be regarded as hydrated silicates of aluminium, iron, and magnesium. They crystallise in the monoclinic system and are green in colour. They occur as alteration products of such minerals as biotite and hornblende, and also in schistose rocks.

Chert : A fine-grained, silica-rich, microcrystalline, cryptocrystalline or microfibrous sedimentary rock that sometimes contains small fossils.

Chalk : A soft, white, porous sedimentary rock, a form of limestone composed of calcite coccolith plates.

Cenozoic Era : The most recent of the geological eras, which followed the Mesozoic Era.

Carbonate hardgrounds : Surfaces of synsedimentarily-cemented carbonate layers that have been exposed on the seafloor.

Carbonate : A salt or ester of carbonic acid.

Carbon film : A type of fossil or preservation.

Cambrian : The earliest geologic period of the Paleozoic Era, lasting from 541.0 ± 1.0 to 485.4 ± 1.9 million years ago and succeeded by the Ordovician.

Caldera : A volcanic feature formed by the collapse of land following a volcanic eruption.

Calcrete : 1.  A conglomerate of surficial sand and gravel cemented by calcium carbonate precipitated from solution.2.  A calcareous duricrust.

Calcite sea : A body of water in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate.

Calcite : A mineral that is the crystalline form of calcium carbonate (CaCO₃), showing trigonal symmetry and a great variety of mineral habits. It is one of the commonest of minerals in association with both igneous and sedimentary rocks.

Calcareous : Formed from or containing a high proportion of calcium carbonate in the form of calcite or aragonite, used of a sediment, sedimentary rock, or soil type.

Buckling : A failure mode of a rock subjected to high compressive stresses, where the actual compressive stress at the point of failure is less than the ultimate compressive stresses that the material is capable of withstanding. Typically, folding is thought to occur by simple buckling of a planar surface and its confining volume. The volume change is accommodated by layer parallel shortening the volume, which grows in thickness.

Breccia : A coarse-grained clastic rock consisting largely of angular fragments of existing rocks.

Breadcrust bomb : A rounded, smooth-surfaced volcanic bomb with a cracked surface resembling a cracked crust of bread, hence the name.

Brackish : Water with a salinity higher than freshwater but lower than seawater.

Bowen’s reaction series : The sequence in which minerals crystallize from a cooling basaltic magma.

Boudin : A structure formed by extension, in which a rigid tabular body such as a bed of sandstone is stretched and deformed amidst less competent beds. See also boudinage.

Blueschist : A rock that forms by the metamorphism of basalt or rocks of similar composition at high pressures and low temperatures, approximately corresponding to a depth of to and a temperature of to.

Bioturbation : The displacement and mixing of sediment particles by benthic fauna (animals) or flora (plants).

Biotite : A form of black mica widely distributed in igneous rocks (particularly in granites) as lustrous black crystals, with a singularly perfect cleavage. In composition it is a complex silicate, chiefly of iron and magnesium, together with potassium and hydroxyl.

Biostratinomy : The study of the processes that take place after an organism dies but before its final burial.

Biostratigraphy : A branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.

Bioerosion : The erosion of hard ocean substrates by living organisms through various biological mechanisms.

Bedrock : Native consolidated rock underlying the loose rock or soil surface of the Earth.

Batholith : A large discordant pluton with an outcropping area greater than 100 square kilometres (39 sq mi).

Basin and Range Province : A particular topography covering much of the southwestern United States and northwestern Mexico that is typified by elongate north-south trending arid valleys bounded by mountain ranges which also bound adjacent valleys.

Basin : A landform scooped out by water erosion.

Basic rock : Igneous rock with low silica content (<54%). The groups ultrabasic, basic, intermediate and acid constitute a series with progressively increasing SiO₂ content.

Basement rock : The thick foundation of ancient, and oldest metamorphic and igneous rock that forms the crust of continents, often in the form of granite.

Basalt : A fine-grained, mafic igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar.

Banded iron formation : A distinctive type of rock often found in primordial sedimentary rocks.

Autochthon : A fossil, sediment, or rock that was formed or produced in the location where it is now found. The term is widely applied to a coal or peat that originated at the place where the plants comprising it grew and decayed and to rocks that have not been displaced by overthrust faulting. An object of this type is referred to as autochthonous. Contrast allochthon.

Aureole : A zone surrounding an igneous intrusion in which country rock shows effects of contact metamorphism.

Augite : A complex aluminous silicate of calcium, iron and magnesium, crystallising in the monoclinic system, and occurring in many igneous rocks, particularly those of basic composition. It is an essential component of basalt, dolerite and gabbro.

Asthenosphere: A region of the Earth’s outer shell beneath the lithosphere. The asthenosphere is of indeterminate thickness and behaves plastically.

Assembled gem : Also called a composite gem.

Asphalt : A sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits.

Ash : Fragments less than 2 mm (0.079 in) in diameter of pulverized rock, minerals and volcanic glass, created during volcanic eruptions.

Argillaceous : Sedimentary rocks of the clay grade, i.e. composed of minute mineral fragments and crystals less than 0.005 mm in diameter, as well as large amounts of colloidal material. Apart from finely divided detrital matter, they consist of the so-called clay minerals, such as montmorillonite, kaolinite, gibbsite and diaspore. Siltstones, mudstones, shales, clays, etc. may all be referred to as argillaceous.

Arkose : An arenaceous sedimentary rock. Like sandstone in its general character but containing feldspar to at least 10%. Formed by the disintegration of the acid igneous rocks and gneisses.

Arenitic : Pertaining to, having the quality of, or resembling sandstone.

Arenite : 1.  General term for consolidated sedimentary rock composed of sand sized fragments.2.  “Clean” sandstone, well-sorted, less than 10% argillaceous matrix. Opposite to Wacke.

Arenaceous : Sediments consisting essentially of sand grains, that is, of quartz and rock fragments down to 0.005mm in size. Conglomerates, sandstones, grits and siltstones fall into this category Particle size 2mm to 1/16mm.

Archipelago : A chain or cluster of islands.

Archean Eon : The oldest eon of the Earth’s history.

Aquifer : A body of saturated rock or sediment through which water can move readily.

Aplogranite : A light-coloured rock of granitic texture consisting mainly of alkali feldspar and quartz, with subordinate biotite; muscovite  may be present.

Aphanitic : Said of the texture of igneous rock in which the crystalline components are not distinguishable by the naked eye. Both microcrystalline and cryptocrystalline textures are included.

Aphanic : Having the texture of carbonate sedimentary rocks characterized by individual crystals or clastic grains less than 0.01 mm in diameter.

Anticline : An arched fold in which the layers usually dip away from the fold axis. Contrast syncline.

Anorthite : Also called Indianite.

A mineral from the lime-rich end of the plagioclase group of minerals. Anorthites are usually silicates of calcium and aluminium occurring in some basic igneous rocks, typically those produced by the contact metamorphism of impure calcareous sediments.

Angular unconformity : An unconformity in which younger strata overlie an erosion surface on tilted of folded layered rock.

Andesite : Fine-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar with the rest being ferromagnesian minerals.

Andalusite : One of several crystalline forms of aluminium silicate; a characteristic product if the contact metamorphism of argillaceous rocks.

Anatexis : Melting of pre-existing rock. Compare metatexis, diatexis, and syntexis.

Amygdaloidal : Amygdules or amygdales form when the gas bubbles or vesicles in volcanic lava (or other extrusive igneous rocks) are infilled with a secondary mineral such as calcite, quartz, chlorite or one of the zeolites. Rocks containing amygdules can be described as amygdaloidal.

Amphibolite : A crystalline, coarse-grained rock, containing amphibole as an essential constituent, together with feldspar and frequently garnet. Like hornblende schist, amphibolite is formed by regional metamorphism of basic igneous rocks, but is not foliated.

Amphibole : An important group of dark-coloured, rock-forming silicate minerals, including hornblende, the commonest.

Amber: Fossilized resin or tree sap that is appreciated for its vivid colour, usually reddish-orange to gold or yellow.

Alluvium : Soil or sediments deposited by a river or other running water.

Alluvial fan : A fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain.

Allochthon : A fossil, sediment, or rock that was formed elsewhere and later transported into the location where it is presently found, usually by low angle thrust faulting. An object of this type is referred to as allochthonous. Contrast autochthon.

Alkaline rock : A type of rock characterized by a high content of Na₂O and K₂O relative to the other oxides. They occur throughout the range from ultrabasic to acid, but have their strongest expression in the acid-intermediate part of the range.

Alkaline : A highly basic substance that dissolves in water.

Albite : The end member of the plagioclase group of minerals, ideally consisting of silicates of sodium and aluminium, but commonly containing small quantities of potash and lime in addition. Compare barbierite.

Aggregate : A mass consisting of rock or mineral fragments.

Agglomerate : An indurated rock built of large angular rock fragments embedded in an ashy matrix and resulting from explosive volcanic activity. Occurs typically in volcanic vents.

Aftershock : A small earthquake that follows a main shock.

Acid rock : The groups ultrabasic, basic, intermediate and acid constitute a series with progressively increasing SiO₂ content.

Achnelith : A small, glassy volcanic bomb, sphere, dumbbell and droplet shapes resulting from very liquid magma.

Accretion : A process by which material is added to a tectonic plate or landmass.

Accident : A sudden discontinuity of ground, such as fault of great thickness, bed or lentil of unstable ground.

Absolute dating : The process of determining a specific date (in years or some other unit of time) for an archaeological, geological or paleontological site or artifact.

Abyssal plain : Flat or very gently sloping areas of the deep ocean basin floor.