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.
Thanks.
Aa flows: Massive lava flows that grade upward to a chaotic jumble of blocky and clinker lava fragments. Aa flows tend to advance more slowly than pahoehoe flows, where broken lava blocks tumble down the flow front and are overridden by the oncoming mass. (cf. pahoehoe budding).
A/S ratio (Accommodation/supply): The ratio between the rate of change of accommodation and sediment supply rate. The concept recognises that the expected change in accommodation with relative sea level rise or fall can be offset by sediment supply. For example during rising sea level, high rates of sediment supply can produce progradation instead of retrogradation during transgression.
Abiotic: Physical and chemical conditions not directly associated with life forms, but interact with biotic conditions to form ecosystems. For example, salinity, pH, temperature, precipitation. The term includes organic compounds present in abiotic conditions such as comets. Cf. prebiotic.
Ablation: The removal of ice and snow by melting, evaporation, wind erosion, sublimation (solid to vapour phase without an intervening liquid water phase), calving (glacial). Melting occurs in more temperate climates. Sublimation in cold, arid climates. Any rocky material dispersed in the ice/snow will concentrate on an ablation surface.
Abrasion: The mechanical wear and tear on sedimentary particles, commonly developed during transport where grain-to-grain impacts are common. Abrasion reduces particle grain size. It is an important mechanism that produces new and smaller sedimentary particles.
Absolute age: A term that should be abandoned. There are no absolute ages in geology, only relative ages or radiometric ages. Radiometric ages depend on isotope half life and blocking temperature; any measured age has errors.
Accidental pyroclasts: Fragmental debris derived from basement rocks during an explosive eruption. May occur with Juvenile and Cognate pyroclasts.
Accommodation: In depositional systems, it is broadly defined as the space available for sediment to potentially fill. It is usually referenced to baselevel that in marine systems is sea level. This definition does not imply the mechanisms that create accommodation space. Accommodation space can increase during relative sea level rise or decrease during sea level fall. Such changes are caused by allogenic and autogenic processes. The concept of accommodation has evolved into one that also incorporates sediment supply.
Accretionary aggregates: The aggregation of fine ash into pellets, a few millimetres in diameter, within turbulent, wet ash columns and plumes derived by explosive phreatic and phreatomagmatic eruptions. Electrostatic charges in the turbulent plume play an important role. Experimental evidence also indicates that cementation by sulphates and other minerals can occur rapidly in the plume – this increases their preservation potential. Pellet cores may contain fine ash, or fine lapilli. Pellets may be completely unstructured, or consist of concentrically layered fine ash. Aggregates with multiple concentric layers constitute the well know accretionary lapilli. They may flatten in impact with the ground. They range from about 5 – 25 mm diameter. There is some evidence they have formed on Mars.
Accretionary lapilli: Accretionary aggregates of fine ash surrounded by multiple, concentric layers (onion like) that form within turbulent, wet ash columns and plumes during explosive phreatic and phreatomagmatic eruptions.
Accretionary prism: Accretionary prisms are wedge-shaped stacks of oceanic sediment and some volcanic rock scraped from the top of the subducting lithosphere and plastered over the trench slope. Older autochthonous deposits on the upper plate may also be incorporated. Each slice of sediment is separated by landward-dipping thrusts (i.e. verging towards the trench). Accretion begins at the frontal taper. Landward stacking of thrust panels occurs above a décollement, where the oldest panels are farthest from the trench.
Acid: A substance that releases or donates a proton when dissolved in water. The proton is a hydrogen ion that in solution associates with an H20 molecule to form H30+ , but is usually written as H+ . Acids react with bases (bases contain hydroxyl ions – OH– ). Solutions with excess H+ are acidic, such that pH < 7.
Active rifting: Extension and stretching of the lithosphere, and development of rift basins promoted by a rising mantle plume. Cf. Passive rifting.
Activity (geochemical): Sometimes referred to as effective concentration. The activity of an ion is the ratio of its concentration versus some standard concentration and is therefore dimensionless (unlike concentration). The ratio is calculated using an activity coefficient. It is used in equilibria because it expresses the amount of an anion or cation that is available for reaction; compare concentration that measures the total amount of an ion. In a solution like sea water there are many different cations and anions, all reacting to collisions of various kinds. For example, the CO32- anion may collide with cations other than Ca2+ (Na+, Mg2+, K+ and so on), such that the amount of CO32- available to react with Ca2+ is less than the measured concentration. In other words, the amount of CO32- available in real solutions depends not just on its overall concentration, but also on its environment. For this reason, it is preferable to use activities in thermodynamic calculations, such as equilibrium constants. The activity of solids is usually taken as 1.
Activity coefficient: The activity coefficient (γ) for a specific ion species is related to the degree of ionic interaction with other species in solution. For dilute solutions γ approaches 1 because there are few ion interactions (γ is dimensionless). Thus, the γ value for HCO3– in fresh river water averages about 0.95, but in sea water is much lower (0.57) because of ionic interactions. Activity (a) is calculated for specific ions from the relationship:
a = γ m where m is concentration.
Actualistic models: Models based on the principle that natural processes and laws we witness today have acted in the past. This does not mean that the products of such processes, for example some environmental condition, will be the same today and in the distant past, but that the laws governing such processes will be the same. cf. Uniformitarianism
Advective fluid flow: The flow of fluids through a porous medium; in this case only the fluids move. Advective flow via aquifers is the most efficient mechanism for mass transfer of dissolved solids in the shallow crust. cf. convective flow, groundwater flow.
Aeolianite: Dune sands cemented by calcite are an example of shallow meteoric-vadose zone diagenesis. Dune sand mineralogy may be siliciclastic or bioclastic, or a mix of both. Most common in subtropical to tropical coastal dunes.
Aerosol: Small droplets of liquid of solid particles suspended in air, mainly by air turbulence. Liquid aerosols commonly have dissolved compounds like sulphuric and hydrochloric acid derived from volcanic eruptions. They are important in Earth’s upper atmosphere because can they reflect incoming solar energy, resulting in cooling, or absorb heat that raises atmospheric temperatures.
Afar depression: Also called the Afar Triangle. The northernmost sector of the East African Rift System, characterised by alkaline volcanism and hypersaline-hyperacidic lakes. It is the most advance sector of continental rifting that appears to be transitional to incipient production of oceanic crust and sea floor spreading. It borders Ethiopia, Eritrea, and Djibouti. Its shores are washed by Red Sea and Gulf of Aden. In a plate tectonic context It is considered to be a triple junction – the confluence of EARS, and the Red Sea – Gulf of Aden sea floor spreading.
Aggradation: The vertical accretion of strata when sediment supply greatly exceeds the generation of accommodation either at the beginning or end of sea level rise. In a sequence stratigraphic context, it occurs during normal regression. The shoreline trajectory has a significant vertical component.
Airfall ash/tephra: Volcanic ejecta (ash to block sizes) that falls to the surface from an eruption column. Deposits tend to mantle topography but may be reworking by precipitation runoff. Deposits may be size-sorted because of gravitational settling; individual beds become finer and thinner with distance from source.
Air sparging: A method of groundwater remediation that uses air forced down a borehole into an aquifer, to volatilize hydrocarbon contaminants. The produced vapour phase is extracted and scrubbed to remove the offending compounds.
Albite twins: Common twinning in plagioclases and potassium feldspars, presented as multiple, parallel lamellae that traverse the entire crystal section. The width of twin segments decreases and the number of lamellae increases in more calcic plagioclases.
Alizarin Red-S: This is a soluble organic acid that reacts with calcium. Distinguish between calcite (stains pink-red) and dolomite (no stain) can be easily done using this stain, on rock slabs or thin sections.
Alkalinity: Alkalinity is a measure of the amount of acid that can be added to an aqueous solution without causing significant changes to the pH; also referred to as the acid neutralizing capacity or buffering capacity. The total alkalinity of seawater is primarily determined by the major anions:
mHCO3– + 2mCO32- + minor constituents like borate, phosphate, and silicate anions.
Allochem: Framework components of granular or rudaceous limestones that show some evidence of transport or movement; i.e. they have not formed in situ. Common examples are ooids, oncoids, pellets, fossils, and intraclasts.
Allochthonous: In geology this means a sediment, rock, lithospheric block, or fluid body that has moved from the place where it formed to the place where it is now found. Cf. Autochthonous.
Allochthonous terrane: See Terrane.
Allodapic limestone: Slope and deeper basin limestones deposited by turbidity currents.
Allogenic processes: (allocyclic processes). Control of stratigraphic architectures and sea level by processes acting outside a depositional system. Typically, this includes regional subsidence, tectonics in sediment source areas (e.g. mountain building and erosion), climate, and glacio-eustatic sea level fluctuations. Cf. Autogenic https://www.geological-digressions.com/autogenic-or-allogenic-dynamics-in-stratigraphy/
Alluvial fan: Coarse-grained sediment bodies that are linked to elevated terrain where the rate of sediment supply and aggradation are controlled by tectonics, climate, and the size of the drainage basin, have broadly radial geometry with longitudinal and lateral extents measured in 100s of metres to a few kilometres, have high depositional slopes (several degrees), where sediment is delivered via a single, commonly canyon-like channel at the fan apex, and where sediment supply is episodic.
Alluvium: Sediment (clay to boulder size particles) deposited or reworked by water in a terrestrial setting; the most common forms are fluvial, alluvial fan, and lacustrine environment. Cf. Colluvium.
Amygdaloids: Vesicles that are filled with mineral precipitates (commonly calcite, zeolite, chlorite). Precipitation occurs after the magma has cooled. cf. Spherulites.
Anastomosing river: A river in which the channels are confined by heavily vegetated banks and floodplains, and within-channel islands also vegetated. The river may contain 2 or 3 sinuous channels but the overall sinuosity of the river is low. Bedload is commonly sandy, forming bars of tabular crossbeds and ripples. Cf. Meandering, braided rivers.
Angle of repose: The natural slope of loose, cohesionless sedimentary particles (sand, gravel) under static conditions, as a function of gravity and friction forces. In dry sand the angle is 34°. In water saturated sand where friction is reduced, the angle is 15° to 30°.
Angular unconformity: A stratigraphic surface that separates two bodies of strata having different orientations, the underlying rocks being much older than those overlying. Iconic examples are Hutton’s unconformities at Lochranza and Siccar Point that record long hiatuses between periods of deposition and mountain building. Cf. disconformity.
Anhedral: Refers to crystal form where the original crystal faces have been removed (by abrasion or dissolution) or are not recognizable because of crystal intergrowth. Neomorphic crystals are commonly anhedral.
Anisotropic minerals: Minerals in thin section reorient plain polarized light, resolving it into two vibration directions that will pass through the upper polarizer when nicols are crossed. One direction contains a fast light ray (also called the extraordinary ray), the other a slow ray (ordinary ray); the fast and slow rays are perpendicular to each other. Anisotropic minerals are further divided into uniaxial and biaxial based on the presence of one or two optic axes.
Anisotropy: An aquifer or aquitard is considered anisotropic if its permeability or hydraulic conductivity is not the same in all directions; usually specified along three principal orthogonal axes. Most porous aquifer media are anisotropic because of sedimentary bedding, sedimentary structures like crossbedding, fracture and joint networks, or tectonically induced structures like cleavage, folds or faults. Cf. isotropy.
Anthropogenic: Processes and products produced by human activity that impact natural conditions and environments. There is frequently an emphasis on negative impacts, such as environmental degradation, loss of biodiversity, reduction of the gene-pool, and pollutants.
Antidunes: Ripple-like bedforms that develop in Upper Flow Regime conditions. The corresponding standing (surface) waves are in-phase with the bedforms. Unlike ripples, the accreting bedform face grows upstream – antidunes appear to migrate upstream, against the flow. When flow conditions wane, they tend to wash out – therefore, preservation potential is low.
Anticline: Convex upward or outward folds, where layers are stratigraphically younger in the convex direction. Cf. antiform, syncline.
Antiform: Convex upward folds; the name is reserved for folds where stratigraphic younging, or facing directions are unknown. If younging direction is known, the name anticline is used. Cf. Synform.
Antithetic faults: Subsidiary or minor faults that have a sense of displacement opposite that of a master fault or principal deformation zone. They occur in extensional regimes (such as accommodation in the hanging wall of listric faults), compressional regimes as back-thrusts, and strike-slip faults. Opposing subsidiary structures are synthetic where the displacement has the same shear sense as the master fault. Antithetic and synthetic faults commonly occur in parallel arrays. Cf. Riedel shears.
Antoine Lavoisier: (1743-1794) published one of the first explanations of transgression and regression, and the relationship of grain size in marine environments that would later become important for the development of facies concepts.
Aphanitic: Used to describe fine-grained volcanic and intrusive rocks where individual crystals cannot be observed without a microscope. Cf. Phaneritic
Aquifer: A porous and permeable medium beneath the surface that permits groundwater flow. In hydrogeology, the definition has a very pragmatic value, where the amount of groundwater flow is usable (as in extraction); everything else is an aquitard.
Aquifer – confined: This term applies to aquifers that are bound above, below, and laterally by aquitards. Confined aquifers are always saturated. Their hydraulic potential is defined by a potentiometric surface.
Aquifer mining: Excess removal of groundwater from a confined aquifer will cause irreversible changes to the structure of the porous medium (commonly sand grains), causing the grains to pack more densely. Not only does this reduce porosity, permeability and therefore water production, it also causes a reduction in the solid volume of the aquifer. Excessive mining can eventually cause land subsidence.
Aquifer – unconfined: The upper boundary of unconfined aquifers is at Earth’s surface. They contain a watertable, above which is an unsaturated zone where pore spaces are air-filled at atmospheric pressures, and a saturated zone below. Drainage of an unconfined aquifer is by gravity alone. Common examples include fluvial and alluvial gravels and sands.
Aquiclude: An aquiclude prevents any kind of groundwater flow. Examples include granite-like lithologies, and thick sequences of halite (although even these lithologies have permeability, albeit extremely low. Other aquicludes involve artificial barriers designed to prevent or deflect contaminated groundwater flow.
Aquitard: Any rock or sediment unit that retards groundwater flow. Common examples include mudstones and other mud-prone lithologies such as glacial diamictites. An important property of aquitards is their ability to release water by vertical seepage to confined aquifers.
Arctic circle: Currently at latitude 66°33′46.9″N, it is the southern limit of continuous 24 hour daylight (summer) or night (winter) – actually measured to the centre of the Sun. It is moving north at about 15m/year because the earth’s tilt moves about 3o over a 41,000 year cycle known as Obliquity. There is an corresponding polar circle in the southern hemisphere.
Arenite: Almost synonymous with sandstone, although its definition is a bit more precise: a rock composed mainly of sand-sized grains and having less than 15% matrix. Greater than 15% matrix and the rock is a wacke.
Argillite: The general name for highly indurated mudstone. They tend to have a greenish hue, in part because of chlorite cements, in addition to illite plus or minus calcite. In many argillites there is a subtle transition from burial diagenesis to incipient metamorphism with the alteration of illite clays to white micas, and the appearance of prehnite or pumpellyite.
Arkose: An arenite (sandstone) that has 25% and more feldspar grains.
Armillary sphere: A device with moveable, concentric spheres designed to map the heavens in three dimensions, mostly with Earth at the center; They were probably invented in China about the 3rd to 1st centuries BC, or ancient Greece, or both. They became popular adornments in the 15th and 16th centuries. Their construction required remarkable craftsmanship.
Armoured mudball: A soft, cohesive ball of mud (mud rip-up) that picks up and embeds smaller clasts of rock as it is rolled along a channel floor.
Artesian spring: Flow from an aquifer is spontaneous if the watertable (unconfined aquifer) or potentiometric surface (confined aquifer) lies above the local ground surface. Spring flow is usually focused along faults, fractures, or bedding planes.
Artesian condition: The water level in a borehole drilled into a confined aquifer, will rise to a level that reflects the hydraulic potential of the aquifer at that location. If the water level lies above the top of the aquifer, the aquifer is said to be artesian. This is the most common condition for shallow, confined aquifers. See also potentiometric surface, hydraulic head.
Aspect ratio: In basin analysis this applies to the ratio of areal extent represented as along-strike length, to down-dip width. It is more commonly used in description of strike-slip basins that tend to be long and narrow.
Asthenosphere: The mechanically weak, upper mantle layer underlying the lithosphere, where deformation is primarily by viscous creep. The lithosphere-asthenosphere boundary is defined by the solidus – an isotherm at 1100o – 1300oC.
Asymmetric ripples: The most common ripple form in unidirectional flow. Bedforms are asymmetric in profile views, with the lee face the steepest and facing downstream.
Atoll: Small oceanic islands and a lagoon protected by fringing coral reefs that sit atop an extinct submarine volcano, or sea mount. The volcanic edifice subsides during cooling of the sub-crustal rocks. Growth of the coral reef keeps pace with subsidence, but the atoll will eventually founder beneath the waves.
Aulacogen: Failed rifts oriented at high angles to plate boundaries. They may represent rifting at triple junctions where one ‘arm’ of the rift system failed to progress to the drift, or sea floor spreading stage.
Autobreccia (volcanicliastic): The non-explosive brecciation of cooling lava as it flows can take place at the flow top, base, and margins. Breccia fragments can be incorporated into the bulk of the flow and, depending on their temperature, may bend or weld to other fragments. They are common in viscous, slow moving felsic magmas (rhyolites, dacites), but can occur in basaltic lavas.
Autochthonous: In geology this means a sediment, rock, or fluid body forming in the place where it is now found. Cf. Allochthonous.
Autogenic processes: (autocyclic). In stratigraphy, this refers to processes acting within a depositional system, that produce changes stratigraphic architecture and relative sea level. They can produce depositional patterns similar to those associated with allogenic processes like climate, tectonic, or sea level changes. Cf. Allogenic.
Autotroph: An organism that produces its own energy and food without the assistance of other organisms. Important groups include photoautotrophs that photosynthesize organic production, and chemoautotrophs that use inorganic chemical compounds. The group includes microbes like algae, bacteria and fungi, plants, and phytoplankton. Cf. Heterotroph.
Avulsion (fluvial geomorphology): The rapid abandoning of a channel at one location and formation of a new channel at another location. Avulsion may be forced by geomorphic factors like gradient advantage, floods, seismic events, or abrupt changes in baselevel. Cf. gradual channel migration.
Axial plane: An imaginary plane that connects the fold axes for every layer in a fold. It is always flat. All cylindrical folds have an axial plane. Cf. Axial surface.
Axial surface: An imaginary plane formed by connecting the hinge lines of all layers in a fold. It may be flat or curved. If it is flat it is called an axial plane.
