Please note – I no longer maintain Glossaries by alphabet; A, B, C… etc. All items on these pages have been moved to subject-specific glossaries such as Volcanology, Sedimentary facies and processes, and so on. The list of subject-based glossaries can be viewed in the drop-down menu on the navigation bar. These glossaries are continually updated.

 

Calcite compensation depth (CCD): As ocean water depths increase, the partial pressure of CO₂ increases and the temperature decreases – in both cases CaCO₃ becomes increasingly soluble. An important consequence of this convergence is a decrease in CaCO₃ saturation to the point where calcite and aragonite begin to dissolve. For calcite, the depths range from about 4.6 to 5.1 km. Aragonite is more soluble and the ACD depths are about 3 km. This means that the sea floor at or below these depth limits will tend to be devoid of calcareous sediment (particularly microfossils like foraminifera and coccoliths).

 

Calcite divide (geochemistry):  The stage during evaporation of brines where calcite precipitation determines the succession of minerals in waters subsequently depleted in Ca²⁺ and CO₃^2-. It determines whether the brine subsequently evolves as HCO₃ rich or  HCO₃ poor.

 

Caldera: A large volcanic collapse basin resulting from withdrawal and eruption of large volumes of magma or explosive pyroclastics. Basin walls are initially steep but may become degraded over time. Many become lakes post-eruption. Calderas are the sites of some of the largest known eruptions (e.g. Yellowstone, Krakatoa, Taupo).

 

Caliche: Also called calcrete. Soil horizons in which carbonate precipitation results in a hardened crust. They develop in regions in which evaporation exceed precipitation, where periods of wetting alternate with drying. Thus, carbonate textures commonly show evidence of dissolution and reprecipitation. A common product is vadose pisoids that also show evidence of multiple episodes of dissolution and precipitation. They can develop in alluvial-lacustrine and intertidal-supratidal settings.

 

Capillary zone:  In hydrogeology, also called the capillary fringe.  It is a relatively narrow interval above the watertable where surface tension forces on aquifer materials cause water to rise and partly fill pore spaces. The capillary fringe is part of the unsaturated, or vadose zone.

 

Carbonate factory: A concept based on the recognition of geologically and geographically recurring facies and associated biotic and abiotic production systems. Definition of a factory is based on the kind of carbonate production. Four primary factories are: Tropical, where photosynthetic autotrophs are a critical energy source for heterotrophic frameworks (such as reefs); Cool-water dominated by hydrodynamically distributed heterotrophs; mud mounds dominated by biotic and abiotic predipitation of carbonate mud, either directly or indirectly by algae, bacteria, and cyanobacteria; and planktic where the primary producers are phytoplankton and zooplankton.

 

 

Carbonate mudstone: Dunham’s (1962) limestone classification, reviewed and modified by Lockier and Junaibi (2016).

>90% mud-supported framework; <10% clasts larger than 2 mm (i.e. granule and larger).  The equivalent Folk designation is micrite.

 

Carbonates: The most diverse group of sediments and sedimentary rocks, usually presented as limestones and dolostones. Carbonate precipitation (and dissolution) is based on the chemical equilibria involving CO₂, HCO₃^–, CO₃^2-, and H₂CO₃. Their primary mineralogy includes calcite and aragonite polymorphs (CaCO₃), and dolomite (Ca.Mg [CO₃]₂). Carbonate formation at Earth’s surface is intimately associated with biological production where precipitation is either induced directly by organisms, or indirectly promoted by the activity and metabolism of organisms. Organisms involved in carbonate production range from microbial to large invertebrates.

 

Carbonate platform: Also called carbonate shelf. Thick successions of carbonate rock, that occupy shelf-like structures attached to continental landmasses, or as stand alone, isolated platforms surrounded by relatively deep ocean basins; also called carbonate banks. Heterotrophs and autotrophs contribute to carbonate production. Evaporites may form part of the stratigraphic succession in arid climates. The proximity to landmasses will determine the degree of mixing with siliciclastic sediment. Islands, banks and bars, and reefs generate significant relief across a platform. Platform-margin reefs mark the transition to slope and deep ocean basins.

 

Carbonate ramp: A platform-like region of carbonate accumulation that slopes gently seaward to relatively deep basin. There are no significant margin builds such as reefs or mud mounds.

 

Carbonic acid: A weak acid that forms naturally from the reaction:

CO₂ + H₂O = H₂CO₃

It is the primary cause of slight acidity of rain (pH 5.5 to 5.8). It is an important component in the series of carbonate equilibria, particularly for pH buffering.

 

Carlsbad twins: Common twins in plagioclase and some potassium feldspars. It is an penetration twin with a plane that separates two crystal segments.

 

Cataclastite: Fine ground-up bedrock produced by grinding during faulting. The grain size range is >0.1 mm and <10 mm. Clasts are angular. There is no preferred orientation. The difference between a cataclastite and fault breccia is mainly in the degree of induration in the former.

 

Catastrophism: The principle that interpreted Earth landscapes and processes as the product of catastrophies. It had its origin in the Biblical Noachian deluge, and garnered support from events like volcanic eruptions and earthquakes. It was also compatible with Bishop Ussher’s estimate of age for the Earth computed from Biblical genealogy at about 6000 years. The principle was stood on its head in the late 18^th century by James Hutton’s principle of Uniformity, later reinforced by Charles Lyell.

 

Celestial pole: An imaginary line drawn along Earth’s axis of rotation to the Pole Star, Polaris.  Because Polaris is very close to this axis, it appears to be stationary in the night sky, whereas all other stars appear to move from east to west. However, even this pole moves slowly with precession of the equinoxes, completing a complete cycle about every 25,000 years. Cf. geographic pole, magnetic pole.

 

Cement: Precipitation of pore-filling minerals, such as quartz, calcite, aragonite, high-magnesium calcite, dolomite, clays, and gypsum, is an important process during sediment lithification. Crystal growth begins at grain boundaries, gradually filling the available pore space. Cementation can begin at the sea floor, particularly by aragonite and calcite, and continue during burial. Cementation gradually occludes effective porosity.

 

 

Chalcedony: A fibrous form of microcrystalline quartz, or chert. It commonly form radial clusters. Under crossed polars, extinction patterns are sweeping or radial.

 

Chemical equilibria: Chemical reactions normally written with the reactants on the left and products on the right. The two are separated by either:

• An equal sign indicating equilibrium, where forward reactions (to the right) equal reverse reactions, or
• By two opposing arrows that indicate forward and reverse reactions.

Equilibria should be charge and mass balanced. The quantities of reactants and products are written as concentrations or activities.

 

Chemical equilibrium: At equilibrium there is no net gain or loss of reactants (by convention, the left side of the equation) or products and no net change in energy. Note that this does not mean the system is static – even at equilibrium there are still collisions between ions (all reactions in solution involve collisions), but collisions on the left equal those on the right side of the equation.

 

Chemical facies: (hydrogeology) This is a useful concept to demonstrate the chemistry of groundwater in relation to aquifer rock-sediment composition, and the evolution of groundwater chemistry as it flows from one rock type to another. For example, flow from sandstone to limestone aquifers will be accompanied by a change in HCO₃^– and pH, plus the concentrations of cations like calcium and magnesium.

 

Chemical kinetics: Also called Reaction kinetics. This is the study of reaction rates and reaction pathways, and hence is distinct from thermodynamics that deals with energy transfer during reactions and is independent of rate. Kinetics is a measure of the rate of change (in concentration or activity) of both reactants and products, in reversible and irreversible reactions. It is particularly important in reactions that are slow relative to mass/solute transport. A good example if these conditions is the precipitation of dolomite under surface conditions – the reaction is thermodynamically favoured, but kinetically is very slow. Kinetics is related to thermodynamics in terms of equilibrium constants, the activation energy of reactions (i.e. Gibbs free energy), and temperature. As a general rule, the rate of 1st-order reactions doubles for every 10º increase in temperature.

 

Chemical stability (of sedimentary grains): The ability to resist dissolution or chemical change during sediment transport and burial.  Quartz tends to be chemically stable, compared with feldspar that may react, particularly during burial diagenesis to form new minerals such as clays. Minerals like zircon are extremely stable and can survive several sedimentary cycles.

 

Chemocline: A boundary within a water column at which there is a fairly abrupt change in chemical gradient. Examples include the boundary between fresh water and seawater, or changes in REDOX conditions, from oxidation to reducing.

 

Chemotroph: Organisms that obtain their metabolic energy and synthesize biomass (such as carbohydrates) from reduced elements like sulpur, sulphide, and ferrous iron, instead of sunlight.

 

Chert: SiO₂. The general name given to micro- and cryptocrystalline quartz that precipitates in sediment, as biogenic products, in under volcanic and hydrothermal conditions. It may be bedded or laminated – a common occurrence in some carbonates associated with microbialites, or as siliceous oozes on the deep-sea floor and composed of diatoms or radiolarians. Sponges provide another source of fine particulate silica.  Nodular cherts tend to be diagenetic; classic examples are the nodules in Cretaceous chalks of southern England and Europe. Chalcedony is a fibrous form if microcrystalline quartz that under crossed polars shows sweeping or radial extinction. Recrystallization is common, in part because of the high surface area (and therefore reactivity) afforded by crystal size.

 

Chronostratigraphy: The part of stratigraphy that evaluates time relationships of rock units, whether as relative time like that determined from fossils or observing stratigraphic succession, or from numerical values of time measured by geochronology.

 

Chute cutoff: Erosion through the inner or accretionary part of a river bend, that eventually forms a new channel. In meandering river systems the chute develops across the point bar. The former meander bend is abandoned and may eventually form an oxbow lake.

 

Clast-supported framework: This term applies to granular rocks where clasts are mostly in contact with one another. It usually refers to lithologies containing clasts that are sand sized and larger; it does not apply to mudstones or siltstones because it is difficult or impossible to distinguish framework from matrix. This textural property applies to siliciclastics and carbonates. Cf. matrix-supported framework.

 

Clathrate: A general term for gas molecules that become trapped in an ice crystal cage. There are no chemical bonds between the gas and water ice and the gas can be released upon melting. Also called gas hydrates. Vast amounts of methane are trapped this way beneath the sea floor and in permafrost.

 

Clay: This term has two meanings: (1) as a layered or sheet-like silicate mineral such as kaolinite and illite, and (2) as sediment with grain size less than 4 microns. See also Mud which consists of a clay-silt mix.

 

Cleavage (crystallography): A plane of weakness within a crystal that will break with relative ease. It is a function of weak bonds between certain planes of atoms within a crystal lattice; the pattern of weakness repeats regularly through a crystal. Some minerals have poor or no cleavage (e.g. quartz, olivine); others have good cleavage along several lattice planes (e.g. calcite, feldspar). Cleavage can be a defining characteristic of a mineral, particularly in thin section.

 

Cleavage (structural): A regularly spaced planar to curviplanar foliation, that is a plane of weakness in metamorphic or strained rock, caused by the growth and alignment of platy minerals like muscovite. It is a penetrative fabric, pervasive at macro and micro-scales. In folded rock the intersection of cleavage with bedding is a lineation that parallels the fold axis.

 

Climbing ripples: Also called ripple drift. These structures are indicative of deposition from suspension load combined with bedload formation of ripples. In profile cross-section views, successive ripples climb over the stoss face of their nearest neighbours. They tend to form when there is an abrupt decrease in flow velocity, as at the mouth of a river, or in a waning turbidity current.

 

 

Clinoform: John Rich (1951) originally defined clinoforms as the depositional surface from wave base to the base of slope, including a shoreward undaform and a deep water fondoform. The latter two terms have been discarded. Clinoforms are now defined as the sinusoidal, chronostratigraphic surface extending from a shoreline across the adjacent shelf or platform to the slope and deep basin beyond. Clinoforms are important components of modern stratigraphic sequences.

 

Clints: Fracture networks in limestones formed by surface (meteoric) dissolution. They are common karst landscapes and occur sympathetically with grykes.

 

Coccoliths: Marine phytoplankton that secrete calcium carbonate skeletons; they are one of the main constituents in natural chalk. Coccospheres are algal cells surrounded by coccoliths arranged into spheres tubes and cup-shaped bodies, up to 100 microns in diameter. They are  one of the culprits responsible for marine algal blooms.

 

Codiacean algae: A group of green algae that precipitate aragonite needles 2-3µm long. Two common species are Halimeda and Penicillus that, across carbonate platforms and reefs, produce large volumes of aragonite mud. Cf. coralline algae.

 

Cognate epiclasts: Also called Accessory Pyroclasts. Pyroclasts derived from earlier-formed and co-magmatic volcanic rocks at the same volcano. Cf. Accidental pyroclasts, Juvenile pyroclasts.

 

Cohesionless grains: Grains (usually sand or silt) that do not stick together. This property is necessary for most sandy bedforms to form. Cohesion in finer grained particles prevents the formation of sediment bedload and saltation load movement.

 

Colluvium: Sedimentary particles of any size that accumulate near the base of, or on lower slopes, by continuous or discontinuous surface runoff, sheet flood, soil and rock creep, and solifluction. Cf. Alluvium.

 

Colonnade jointing: Columnar ‘organ-pipe’ like cooling joints oriented at right angles to magma body margins. In lava flows, colonnades may be tiered, with a larger cross-section columns at the base and smaller columns that intersect the lava surface (and cooled more quickly). Cf. entablature.

 

Column collapse: Plinian and Vulcanian eruptions produce columns of hot, turbulent mixtures of juvenile fragmentals, gas and air. Gravity-induced collapse of the column produces hot pyroclastic flows and surges. This is the most common mechanism of PDC generation.

 

Columnar jointing: Regular arrays of joints formed during cooling and contraction of magma. They can occur in lava and hot ignimbrite flows, and intrusive dykes and sills. Cooling begins from the outer surfaces and progresses towards the centre of the magma body where joints are oriented normal to the outer surface. They form as straight to slightly curved columns with 4 to 8 sided polygonal cross-sections. Cf. colonnade, entablature, fracture porosity.

 

Compaction:  The process where sediment particles, once deposited, are pushed closer together to form a more tightly knit framework. Compaction begins almost immediately following deposition and continues during sediment burial. The normal compressive stress in this case is applied by the overlying sediment. Because porosity is also reduced, an additional requirement for compaction to take place is the release of interstitial water through aquifers. If fluid cannot escape (for example because of permeability barriers) then the rock body will not compact, and internal fluid pressures will rise – this is called overpressure. Mudrocks can compact to less than a tenth their depositional thickness. More rigid frameworks like sandstones compact far less. See also pressure solution, lithic fragments.

 

Compensation depth: Applies to models of isostasy where, at equilibrium there is a common depth at which lithostatic pressures are equal across all components of the lithosphere.

 

Composite terrane: See Superterrane.

 

Compressibility: The ability of a fluid or rock to change its volume in concert with changing stress, for example changing lithostatic pressures during sediment burial. It is usually expressed as the ratio of relative volume change (V) with pressure (P):

β = 1/V. (δV/δP)

Water has very low compressibility – at 6000 psi (41.4 MPa) (equivalent to 3.2 km water depth) the change in volume is 1.8%. Mudstone is highly compressible; halite is not. Compression results in a loss of porosity and permeability.

 

Compression: The application of stress that results in shortening of a rock body, or the reduction in volume of sediment, rock, or fluid. Shortening my occur through the entire body, or along faults.  It is the opposite of extension. Compression can be represented as an axis, or axes on a strain ellipse.

 

 

Concordia plot: (Geochronology). Devised by George Wetherill in 1956, For the Pb/U decay system (commonly used for zircon geochronology), the curve plots age against the three Pb/U ratios for the two U-Pb decay systems, assuming an ideal closed system (i.e. no loss of any isotope during a crystal’s lifetime). The measured isotope ratios for any crystal or batch of crystals are compared with this ideal curve; if the age from each decay system is the same and they lie on the curve, i.e. they are concordant, then that is the true age of the sample.

Discordance ages arise from Pb loss, inherited Pb from the time of crystallization, from the analysis of several crystals of different ages in a single sample, or from single zoned crystals where ages from the inner to outer most zones become progressively younger. A straight line discord will plot below the concordia and intersect it at two points – the oldest will be close to the age of crystallization.

 

Condensed section: (Stratigraphic condensation). Basically, very thin stratigraphic units that represent long periods of slow and non-deposition. They are characterised by: one or more biozone (depending on duration – 10⁵ to 10⁷ years); contain internal, non-depositional or erosional discordances, including omission surfaces; abundant authigenic minerals like carbonate, phosphate, chert, glauconite); commonly have hardgrounds or nodules of carbonate, phosphate, iron-manganese.

 

Conduction:  This is a diffusive process where heat is transferred via molecular vibrations. Conduction does not involve the transfer of mass, cf. convection, advection. It is a less efficient mechanism of heat transfer than convection.

 

Confined aquifer: see Aquifer-confined.

 

Conglomerate: Sedimentary rock where the framework consists of clasts coarser than 2 mm (granule). Clasts show variable degrees of rounding and shape. Sorting tends to be poor. The term gravel is used for modern sediments. They typically represent high energy conditions like those found in braided rivers, alluvial fans, and gravel beaches. Cf. breccia, pebbly mudstone.

 

Conjugate Riedel shears: See Riedel shears.

Conjugate faults: Fault pairs where the fault planes intersect at (commonly) 60^o such that the direction of minimum extension bisects this angle, and the direction of maximum extension bisects the obtuse angle (~120^o). Conjugate faults occur in dip-slip and strike-slip structural domains.

 

Contaminant: A chemical or substance that we would rather not be present in our environment, food, air, etc., but is present because of either natural occurrences and processes, or human-induced processes. For example, heavy metals like lead, mercury and arsenic can occur naturally concentrated in ore bodies, and released by natural weathering, or by mining, into local surface and groundwaters. Cf. pollutant.

 

Continental rifts:  Basins formed by lithospheric stretching and thinning, where the primary subsidence mechanism is faulting. Rifts commonly have high heat flow, manifested as volcanism, resulting from shallow mantle plumes (in part an isostatic response to crustal thinning). Sediment fill early in rift history tends to be terrestrial and coarse grained.

 

Continental rise: The bathymetric transition from continental slope to abyssal plain. Gradients are less then those of continental slope, merging with the deep basin beyond. Water depths are commonly >3000 m. Much of the rise are is made up of submarine fans that are fed by submarine canyons and gullies on the adjacent slope. Mass transport deposits derived from the slope generally move across the rise.

 

Continental shelf: The submarine extension of a continent. Shelf inclinations are generally <1^o averaging about 0.1^o . Water depths range from about 60 m to 200 m. Shelves and their environments are sensitive to sea level fluctuations. During low sea levels (e.g. during glaciations) the shorelines migrate seawards and the shelf thus exposed is subjected to weathering and fluvial erosion . A significant change in slope at their seaward margin is called the slope break – it marks the bathymetric transition to continental slope. It also corresponds to the transition from continental to oceanic crust.

 

Continental slope: The bathymetric region beyond the shelf and shelf break, extending from about 100m to 3000 m, with gradients between 2^o – 5^o . Slopes are commonly transected by gullies and submarine canyons that focus sediment transport, some of which remains on the slope (finer-grained sediment), and some bypassing the slope on its way to the basin beyond; in this case sediment transport is commonly via turbidity currents and other types of sediment gravity flow. Gravitational failure also shapes the slope. Hemipelagic sediment is important to slope accumulations.

 

Convection:  The flow of fluids en masse resulting from temperature and buoyancy gradients. Convection is the primary mechanism for transferring heat from Earth’s mantle to the lithosphere. Cf. conduction, advection.

 

Convoluted laminae: Laminae that are initially parallel or crossbedded, will become folded and pulled apart during the early stages of compaction (soon after deposition) and dewatering. They are characteristic of turbidites where dewatering is hindered by muddy permeability barriers, such that local fluid pressures are elevated. They are also common in fluvial and other channelised sediments (here called ball and pillow structures).

 

Cool-water limestone: Predominantly bioclastic limestones typically made up of bryozoa, various molluscs, brachiopods, calcareous algae, barnacles, and echinoderms. Isopachous, micritic, and pore-filling cements are mostly calcite; aragonite cement is uncommon.

 

Coquina: A limestone made up of shells, shell fragments and other bioclasts, with a degree of sorting that indicates relatively high depositional energy. Where the fragments are mostly sand-sized, the Dunham limestone classification equivalent is grainstone.

 

Coralline algae: Calcite and high magnesium calcite precipitating red algae, that build upon substrates such as bioclasts and rock surfaces and other algae. All begin life as encrusters, but grow to different forms such as articulated branches, or nodular clusters around shells or pebbles (e.g. Lithothamnion). They are an important contributor to cool-water bioclastic limestones.

 

Correlative conformity: A surface marking the end of sea level fall (regression) that is correlative with the subaerial unconformity at the lowest shoreline. Its extension basinward takes it across the top of the lowstand deposits. The use of correlative conformities in sequence stratigraphy has been the subject of considerable debate.

 

Cosmogenic isotopes: Relatively rare isotopes formed on Earth surface materials (soils, rocks) and asteroid surfaces, by the interaction of cosmic rays and certain elements, such as beryllium (Be-7, Be-10), and chlorine (Cl-36). Half-lives are as short as 34 minutes (Ci-34) and as long as 15.7 million years (I-129). They can be used for dating of ice, groundwater, and exposure times at the surface.

 

Cosmopolitan taxa: Species that are distributed globally according to the appropriate environments in which they live. They are important for biostratigraphic correlations between sedimentary basins. Cf. endemic taxa.

 

Crater lakes: Water that accumulates in volcanic craters, extinct or active. On active volcanoes, eruption through a crater lake may have a strong phreatic or phreatomagmatic  imprint (depending on whether there is new magma) until all the water has been vapourised.

 

Craton: The stable interior of continents, usually made up of very old geological provinces including Precambrian shields and distinguished from more marginal mobile belts or collisional orogens. Note however that Cratonic rocks may be structurally involved during orogenic deformation.

 

Crevasse splay: A crudely fan-shaped body of sediment deposited on the flood plain when a river in flood breaks through its levee. The sediment is mostly fine sand and silt. Ripples and climbing ripples tend to form close to the levee breach where flow velocities are highest; erosional discordances are also common. Flow competence wanes rapidly as the flood waters splay across the floodplain, depositing progressively finer-grained sediment.

 

Critical flow: Also called Tranquil flow. The flow conditions for a Froude number of 1 , at some critical flow velocity and flow depth, where any surface wave will remain stationary (it will not move upstream or downstream). Surface waves will usually be in-phase with their bedforms, for example antidunes. See also subcritical and supercritical flows.

 

Critical taper theory: A mechanical theory used to explain the formation of wedge-shaped fold-thrust belts and accretionary prisms. During compression the slope, and therefore the angle of the wedge taper reaches a critical point depending on the strength of the materials, the frictional forces along the décollement, and the slope of that surface. As the critical slope or taper angle is approached, the materials within the wedge will deform – once the critical taper is reached, the entire mass slides along the décollement and there is little subsequent internal deformation of materials.

 

Crossbed: Refers to the dipping cross stratification, or foresets of bedforms like ripples and sandwaves. Foresets dip in the direction of flow (air, water).

 

Crossed polars (nicols: The upper polarizer, between the objective lenses and the Bertrand lens, filters out all remaining frequencies present in plain polarized light (PPL). The nicols can be moved in and out of the light path. If you look through the oculars when the polarizer is in the light path (i.e., crossed nicols) then no light will reach the oculars – all will be black. However, if a thin section is placed between the lower and upper polarizers, most minerals will reorient the PPL such that some of this light will pass through the upper polarizer; this light will contain slow (ordinary) and fast (extraordinary) vibration directions that will arrive at the eye pieces at slightly different times. The resulting interference produces the kaleidoscope of colour among all the minerals present. Minerals that permit the passage of light are called anisotropic; those that do not are isotropic.

 

Crust:  The outer layer of Earth. The boundary between crust and mantle is defined by the Moho. Oceanic crust is produced at oceanic spreading ridges. It averages 6 km in thickness, and consists primarily of basalt, gabbro, peridotite and ultramafic rock types. Continental crust averages 35 km thick but ranges to 70 km beneath some Cratonic blocks. Continental crust tends to be more felsic than oceanic crust. The crust is underlain by the mantle lithosphere.

 

Cryptalgal laminates: A general term for laminated mats composed primarily of cyanobacteria, but like includes other microbes. The laminates may be flat and uniform, or tufted, pustulose, or polygonal, resulting from desiccation or, in arid environments, evaporite precipitation. In the rock record they are commonly found with stromatolites. The term microbialite is generally used in modern examples because there are several groups of microbes including bacteria, cycanobacteria, and red and green algae.

 

Cryptocrystalline: Materials like chert and chalcedony where visual resolution of individual quartz crystals require a very high powered microscope, or Scanning Electron Microscope.

 

Crystal symmetry: Symmetry describes the shape of an object and can be represented both mathematically and visually. In crystallography, the two most useful forms of symmetry are:

1. Axes of rotation (crystallographic axes) where a particular crystal face will be repeated during rotation through 360^o. The number of repetitions for a 360^o rotation can be 2, 3, 4, or 6, that are referred to as two-fold, three-fold, four-fold, and six-fold (axial) symmetry respectively.
2. Planes of symmetry where two parts of a crystal are mirror images. For an analogy, think of this concept in terms of the common bilateral symmetry in many living organisms, such as people, and many classes of mollusc. Note that planes of symmetry are NOT the same as twin planes.

Crystal systems: There are 6 crystal systems based on combination of the elements of symmetry; a seventh system – trigonal – is usually considered a subclass of the hexagonal system. The defining criteria are axial lengths, the angles between axes, and axial symmetry (the number of repetitions about an axis). The systems are: cubic (isometric – the most symmetrical), tetragonal, hexagonal, orthorhombic, monoclinic, triclinic (the least symmetrical).

 

Crystallographic axes: Three or four axes about which a crystal can be rotated through 360^o.  The axes intersect at a single point (the centre of symmetry). They are labelled according to their lengths. If axes are the same length, then they are referred to as a₁, a₂, a₃ etc. If they have different lengths, they are labelled a, b, and c. Thus, in the cubic (isometric) crystal system they are labelled a₁, a₂, a₃, and in the tetragonal system a₁, a₂, c. The hexagonal system is the only one with four axes. Angles between axes are labelled α, β, γ.

 

Crystal zoning: Zoning commonly displays as concentrically arranged crystal growths, where the composition changes outwards from the crystal interior. The zones maintain the same crystallographic and optic axis. The changes in composition involve substitution of certain cations, for example in calcite Fe²⁺ and Mn²⁺ substitute for Ca²⁺, and in plagioclase sodium may substitute for calcium such that the inner core is a calcium anorthite and the outer zone is a sodium albite. Zoning indicates changing fluid or magma crystallization conditions. Zoned crystals may also be twinned.

 

Cut bank: An outside river bank subjected to erosion. In meandering fluvial channels, cut banks are located opposite point bars (the inside channel margin on which deposition occurs).  Channels tend to be deepest along the cut bank margin.

 

Cyanobacteria: Microscopic, single cell or colonial, prokaryotic organisms that today are aquatic and photosynthetic. They are likely the first known photosynthetic organisms on Earth, and were the primary builders of stromatolites and cryptalgal  laminates (or microbialites) the oldest being about 3.4 Ga; as such they were responsible for producing free (molecular) oxygen in Earth’s ancient atmosphere. Precambrian fossil microbes, best preserved in cherts, are an assortment of filaments and coccoid colonies.

 

Cycles: The regular, periodic repetition of events. Measurement of cycle periodicities allows us to predict past and future events. In Earth sciences we recognise cycles at all scales of  time and space: daily ocean tides, revolutions around the sun, sea level rise and fall, Milankovitch orbitals, and perhaps the grandest cycle – Wilson cycles in the life and death of tectonic plates and sedimentary basins.

 

Cycle hierarchies: We recognise several orders of stratigraphic cyclicity that are usually inferred to have a causal relationship with cycles of relative sea level fluctuation. High order cycles are commonly nested, or superposed on lower order cycles:

• 1^st order cycles – about 50-100 Ma; Allogenic, depending on plate tectonic interactions.
• 2^nd  order cycles – about 5-50 Ma; Allogenic, depending on plate tectonic interactions.
• 3^rd order cycles – about 0.2-5 Ma;  Allogenic and autogenic processes.
• 4^th order cycles – about 100-200 thousand years (ka); Allogenic and autogenic processes.
• 5^th order cycles – 10 years -100 ka; Allogenic (e.g. Milankovitch orbitals) and autogenic processes.

 

Cyclothem: Cyclothems are the stratigraphic record of cycles. They consist of repetitive successions of marine sandstone, shale or limestone overlain by non-marine deposits such as coal, sandstone, and paleosols. Each cyclothem records a cycle of transgression and regression. The term was originally defined by European explorers for coal who recognised the repetitive nature of the sandstone-mudstone-coal successions. Harold Wanless (1932) extended it to include shale-limestone- paleosols cycles in the Pennsylvanian of central and eastern USA.

 

Cylindrical folds: Most folds can be described as cylindrical (imagine the cross-section of a soup can), or they contain segments that can be inscribed by (imaginary) cylindrical curves of different diameters. The concept is very useful for stereonet analysis because the dips and strikes on each limb (plotted as great circles) will intersect at a point corresponding to the fold axis bearing and plunge. All cylindrical folds have (straight) fold axes and (flat) axial planes.