GeoGlossary
Search Index
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
Calcite, aragonite
Calcite (CaCo3) is an exceptionally common mineral, forming many rocktypes, particularly limestones, marbles, chalks and travertines. It occurs in two forms, aragonite, a less stable polymorph found in many seashells and as a secondary mineral in cavities and veins in volcanic rocks, and as calcite, a more stable polymorph, which is the principle constituent of limestones, marbles and chalks, as well as a common secondary mineral in cavities, veins, cements and evaporites. Given time, crystals of aragonite will revert to the more stable crystal structure of calcite.
Calcrete
A hard layer of soil, sand or gravel formed by precipitation of calcite from ground or surface waters into pore spaces. Frequently occurs in arid or semi arid regions where evaporation increases salt concentrations in waters and promotes precipitation. Such hard layers are also called duricrusts or caliche. Nodules form first and then layers develop.
Caldera
A round, steep walled depression (often anywhere from 1 to 100km in diameter) formed in many rhyolitic volcanoes by the post eruption collapse of the overlying rocks into the roof of the magma chamber. Lake Taupo is one of several calderas in the Taupo Volcanic Zone.
Carbon dating
A method of dating relatively young (< 70,000 years) biological material and sub fossil material through measuring the rate of the decay of radioactive carbon-14 to carbon-12.
Cataclasis, cataclasite
Cataclasis is the metamorphic process of shearing in a fault zone whereby rocks are torn apart and minerals are ground down, and reformed into a stretched, granular rock called cataclasite. Cataclasites are associated with major fault lines (like the Alpine Fault) and rocktypes like mylonite.
Cataclastic metamorphism
Download image ( PDF 13 KB)
Metamorphism that involves high shear stress and tearing apart and grinding up rocks, in shear or fault zones. This metamorphism involves high pressure and shear stress, but does not require high temperatures. Rocks produced range from fault breccias and cataclasites to mylonite depending on the amount of shear stress, the depth of the shearing (temperature) and the length of time that shearing has occurred (also total movement across the shear zone).
Catastrophism
An early theory supported by many 19th Century geologists, Catastrophism explained the geological record by postulating a series of immense catastrophic events.
Cementation
A diagenetic process, where the particles that make up a sedimentary rock are cemented together by new minerals after deposition. Cements are precipitated from mineral rich water moving through any cavities or pore spaces between the grains of sediment.
Chalk
A very fine grained and porous variety of limestone composed of the tiny calcite skeletons of foraminifera and coccoliths. Chalk deposits are common in the Upper Cretaceous of Europe.
Channel, channel fill
Download image ( PDF 105 KB)
A channel is a linear route of least resistance along which surface water flows. Channels are usually concave in cross section, and they may follow straight, sinuous, anastomosing or dendritic paths. A narrow sea-way may also be called a channel. Channel fill sediments may be from sediment deposited in the channel by the water flowing through it, or from infilling of an abandoned channel. The type of sediment within a channel fill deposit may provide information on the type of channel environment, such as a floodplain channel, submarine channel, braided stream channel, etc.
Chert
Fine grained or cryptocrystalline silica (sio2) deposits lacking developed crystal form, (also 'chalcedony' or 'agate'). Chert may occur as nodules, lenses or beds within other rock types, and may formed by either biogenic, diagenetic or volcanogenetic processes.
Chilled margin
The fine grained, often glassy margin found at the edge of an intrusive magma body, particularly dikes and sills. The magma at the edge of the intrusion is chilled or 'supercooled' against the country rock, and crystallises quickly, producing fine grained rock with many small crystals or, if chilled enough, glass.
Chondrite, chondrite model
Chrondrites are the most common meteorites, and are of basaltic compositions with olivine, pyroxene and plagioclase feldspar, together with iron and iron-nickel minerals. They are characterised by the presence of small round glassy spheres called chrondules, thought to be derived from sudden shock melting during impacts. The chrondrite model proposes that the bulk composition of the Earth is similar to that of the high volatile carbonaceous chrondrites.
Clast, clastic rocks
A clast is a fragement of rock or mineral produced by the disintegration of a larger rock or rock mass. Clastic rocks are sedimentary rocks composed primarily of clasts derived from erosion of older rocks.
Clay
Clay can either mean any clast of any composition that is smaller than 1/256 mm (4 microns), or a sediment composed of clay sized particles, or a mineral belonging to a complex group of water bearing sheet silicate minerals, for example kaolin, illite, smectite, bentonite, and montmorillionite. Clay minerals typically have the ability to absorb water and ions onto their surface, due to their layered structures. Different groups have different capabilities, and are used as filters, for cleaning up contaminants etc. Some groups absorb so much water that they swell (so-called ‘swelling clays’), and this creates problems for slope and building stability.
Claystone
Sedimentary rock composed of clay-sized particles (clay that has been lithified).
Cleavage
Cleavage can refer to the breaking of a mineral along planes of weakness corresponding to crystallographic planes, or the similar breaking of a rock along planes of weakness caused by the alignment of minerals (usually platy minerals, clays or micas). Slates, phyllites, schists and gneisses all possess cleavage, created during regional metamorphism when increased pressure and temperature causes existing minerals to realign themselves and new micaceous minerals to grow in a particular orientation.
Coal
A rock composed of more than 50% carbonaceous material, formed by compaction and lithification of plant remains. Forming coal requires significant thicknesses of plant debris, so swamp conditions and peat formation are generally required. The coal is classified based on its volatile matter and water composition, and the type of carbonaceous matter present, and the amount of impurities (ash) present. Carbonaceous matter types vary depending on the original flora and the amount of decomposition that occurred prior to burial.
Colluvium
Loose and unconsolidated deposits that have not been transported by wind or water. Usually applied to material lying at the base of a slope that has been transported there by gravity.
Columnar jointing
Joints or fractures that are regular and parallel and create columns with a polygonal cross section in lava flows and other igneous rocks due to contraction of the rock during cooling.
Conchoidal fracture
A smoothly curved fracture surface, characteristic of glassy minerals and rocks, such as quartz, olivine (minerals) and obsidian (rock).
Concretion
An irregular to subsperical shaped mass of cemented material, formed by precipitation of a mineral (usually calcite, also siderite, silica, hematite, pyrite) from solution around a nucleus (usually a shell, bone, grain), in a sedimentary or pyroclastic rock. The grains within the concretion are identical to those outside, but the cement is different. Concretions often occur in layers, where there is a concentration of suitable nuclei, or where water has flowed more easily.
Conformable
Where there are no breaks in sedimentation and beds are deposited one on top of the other without break or changing orientation of the bedding surfaces. Opposite of unconformable.
Conglomerate
A clastic sedimentary rock where the majority of clasts are larger than 2 mm in diameter (granules, pebbles, cobbles, boulders) and rounded or subrounded. Clasts are set in a finer grained matrix, and can be supported by the matrix or by clast to clast contact. Conglomerates are the lithified equivalents of gravel.
Contact
The boundary between two beds, layers, units, rock types, intrusions and country rock, metamorphic zones etc.
Contact metamorphism
Metamorphism caused by the intrusion of a body of hot igneous rock into cooler older sedimentary, igneous or metamorphic rock. The heat from the intrusion causes new minerals to develop in a zone around the intrusion called an aureole. Also called thermal metamorphism.
Continental crust
The crust that underlies the continents and ranges in thickness from about 35 km to as much as 60 km under high mountain ranges. The crust is relatively silica rich compared to mantle and ocean crust compositions and has a density in the upper part of ~2.7g/cm3. It is derived by recycling of older crust into sedimentary rocks, deformation of rocks, and intrusion of silica rich magmas. The resistance of quartz to weathering accounts for much of the dominance of silica in continental crust.
Continental drift
The original theory proposed by Alfred Wegener in 1912 to explain the good fit of African and American coastlines and the distribution of similar flora and fauna across Gondwanan continents. The theory is that large plates of continental crust moved freely across ocean crust like ice drifts through water. The theory has been largely superseded by plate tectonics.
Continental margins
The margins of continents where continental crust grades into oceanic crust. Continental margins come in two sorts, passive continental margins (e.g. The West Coast of NZ) where there is no plate boundary at the margin and the continental crust beneath the continental shelf passes into oceanic crust beyond the continental slope, and active continental margins where the edge of the continent is coincident with a plate boundary, either convergent (e.g. North Island of New Zealand) or divergent (e.g. California).
Convection
Convection is density or heat driven movement of fluids, and occurs in hydrothermal systems (heat driven movement of water adjacent of igneous intrusions), in the atmosphere (density and heat driven), in the ocean (density and heat driven), and possibly in the mantle (density and heat driven movement of mantle material (very slowly) causing movement of the crustal plates above).
Convergent margin
The margin of a tectonic plate where the adjacent plates are converging towards each other. Convergence usually causes subduction of the denser (older, cooler or oceanic) plate beneath the other plate; however where plates are of similar densities convergence can result in uplift and mountain building (e.g. Himalayas, Alps, Southern Alps).
Coprolite
Fossil crap… excrement from large vertebrates (reptiles, dinosaurs, mammals) which has been buried and usually replaced by other minerals such as calcium phosphate.
Core
The centre of the earth, from about 2900 km depth, it is dividable into an inner and outer core. The outer core may be liquid and the inner core solid. It is likely composed of iron-nickel alloy. The word core is also used to refer to cylindrical section of rock obtained by drilling, and as the central part of folds.
Coriolis force
The apparent force caused by the earth’s rotation which causes objects to move to the right in the northern hemisphere and to the left in the southern hemisphere.
Country rock
An older rock that is intruded by an igneous intrusion or mineral deposit.
Crater
Craters are round pits or depressions, rimmed or not, formed either by volcanic eruptions at the top or on the flanks of a volcanic cone, or by meteorite impacts or explosions at the earth’s surface.
Craton
A craton is a part of the earths surface or crust that has been stable and undeformed for a long time (100’s of millions of years).
Cross-bedding
Also cross-stratification and cross-lamination, where laminae or beds within a bed or layer are formed at an angle to the bedding surfaces. This occurs where ripples or dunes develop, and sediment is deposited on the downstream, lee face of the ripples or dunes at an angle to the stream/sea/land surface. Variations include herringbone cross-bedding where successive beds have different directions of cross-beds, indicating tidal flows; hummocky cross-stratification where the laminae form hummocks and swales caused by wave action during storms; low-angle cross-bedding formed on dunes by aeolian action; and linsen bedding or lenticular bedding where sand ripples are isolated in a muddy sediment (the opposite is flaser bedding where mud ripple trough fills are preserved in sand beds) all formed in shallow tidal flow regimes or deeper episodic currents.
Crust
The outermost layer of the earth above the Mohorovicic discontinuity. The crust is subdivided into continental and oceanic crust, which form the land surfaces and the ocean floors and have different compositions and densities reflecting their different origins.
Cryptocrystalline
Where the crystals are too small too be seen under a microscope.
Crystal
A solid body of a particular chemical element of compound of elements with a regular repeated atomic arrangement and homogenous composition. The atomic arrangement is reflected in crystal faces, and the arrangement is used to classify crystals based on the symmetry.
Crystalline
Said of rocks that are composed of interlocking crystals of the same
or different compositions.
Search Index
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z