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.
Back-arc basins: Basins behind magmatic arcs, floored by oceanic or continental crust. Oceanic back-arc basins are commonly formed by sea floor spreading; their continental cousins by rifted continental crust. In continental varieties, high heat flow is manifested as volcanic centres, and fault-controlled geothermal activity.
Back-bulge: A shallow basin between a flexural forebulge and the adjacent continent. It is regarded as a separate depozone in foreland basin systems.
Backflow: Flow on the lee side of bedforms that is opposite the overall direction of flow. Backflow may be strong enough to form small-scale beforms (commonly ripples) that migrate up the main bedform lee face or upstream.
Backstripping: A numerical geohistory analysis that calculates the contribution to total subsidence by tectonics – the method was developed by Watts and Ryan, 1976. The tectonic subsidence component is determined by subtracting the isostatic effect of sediment load. It is usually done in conjunction with decompaction and bathymetric corrections.
Back thrust: A thrust that has vergence opposite the dominant trend of a thrust system. In many cases back thrust vergence will be towards the hinterland, i.e. thrust plane dip is towards the foreland.
Backwash: Water that completes its run-up across a beach (swash) and returns to the wave-surf zone. Flow velocities are determined primarily by the gravity component imposed by the beach gradient.
BAF: The acronym for block and ash flows.
Ballistics (volcaniclastic): Blocks and bombs ejected by powerful explosive volcanic eruptions that follow a parabolic trajectory to be deposited as tephra.
Ball and pillow structure: Ball, or spheroidal shaped structures formed by soft sediment deformation of sandstone crossbeds during early differential compaction, that is promoted by sediment dewatering. The original cross laminations are folded, oversteepened, and even overturned. They are common in fluvial deposits.
Base: A base is a substance that gains a proton in aqueous solution. This can be written in a generalized way as H+ + OH– = H20. Water can act as a base or an acid. Solutions with excess OH– are basic with pH > 7.
Baseflow: (Hydrogeology) Baseflow is the subsurface discharge to streams from the watertable. The amount of discharge depends on the hydraulic gradient of the watertable with respect to the stream surface. During dry periods, baseflow may be the only source of water to maintain stream flow.
Baselevel: It is an imaginary or theoretical plane to which geological, geomorphic and geodetic measurements are referenced. The commonly accepted datum is sea level, although it is also recognized that this too changes with time. The choice is based on common sense and a recognition that shorelines are a natural boundary between marine and nonmarine realms. Other baselevels may be useful depending on the problem being investigated, for example the margin of endorheic lakes, or some arbitrary position on a deep-water submarine fan.
Base surge: Synonymous with pyroclastic surge. The term base surge was first used to describe turbulent, bottom hugging flows generated by nuclear test detonations in the late 1940s and early 1950s.
Basic (igneous petrology): Volcanic and intrusive rocks poor in free silica (quartz), but enriched in feldspars and alkali earth (calcium, magnesium) ferromagnesian minerals like amphiboles, pyroxenes and olivines. Typical examples are basalt, gabbro, and diorite. Cf. felsic.
Basin axis: Applies to elongate basins usually corresponding to the deepest part of a basin. The axis usually parallels orogenic belts (e.g. foreland basins) or volcanic arcs (forearc basins), rifted plate boundaries as in passive margins, or parallels fault displacement as in strike-slip basins. A basin axis may migrate laterally with changes in tectonic and/or sediment loads.
Basin-wide evaporites: Accumulate on subsiding basin floor and as a consequence can reach several 100 m thick. Associated facies range from shallow marine shoreface to deeper slope and base-of-slope environments. Precipitation takes place either within the water column with crystals sinking to the basin floor (meromict), or at the sea floor (holomict). The particular mode of precipitation depends on the degree of brine stratification and evaporitic drawdown.
Beach: Obvious to most what this looks like. But from a sedimentological perspective it is the part of the coast, marine or lacustrine, where wave wash and backwash sorts sand and gravel according to the hydraulic potential of the waves, and where invertebrate and vertebrates have adapted to saline conditions and regular periodic exposure. It provides a stratigraphic datum for sea level change and shoreline excursions over geological time frames.
Beachrock: Rapidly cemented carbonate and siliciclastic sand-gravel across a beach face; cementation occurs at or just beneath the surface. Cementation rates are measured in months. Once lithified, they can be eroded by storms into boulder deposits, that can then be re-cemented. Rapid lithification of beach sand-gravel changes the habitat for local benthic organisms. Cements are mostly aragonite and high magnesium calcite.
Bedform: Sedimentary structures produced by bedload transport of loose, non-cohesive sediment. Typically manifested as ripple and dune-like structures.
Bedload: Loose or non-cohesive sediment particles (silt, sand, gravel – sizes) at the sediment-water or sediment-air interface, that will move along the bed if fluid flow velocities exceed the threshold velocity. The bedload consists of a traction carpet, and a suspension load.
Benioff zones: Identified in 1930s (before plate tectonics) as a zone of earthquakes associated with oceanic trenches, some as deep as 650 km.. Benioff (1949) suggested that they originated on large, continentward-dipping reverse faults where the direction of slip placed oceanic crust beneath continental crust. The Zone is now recognised as a subducting slab of oceanic crust.
Benthic: (adjective) An ecological term applied to organisms that live on a sediment-water interface, or within sediment. It includes invertebrates, vertebrates, and plants (particularly algae and cyanobacteria). The most prolific benthic zones are located within the photic zone that constrains the limits of photosynthesis.
Benthos: (noun) An assemblage of benthic organisms.
Bernoulli equation: Named after Daniel Bernoulli who in 1738 expressed the conservation of energy in a flowing fluid as:
Total energy E = ½ ρv2 + ρgz + P
Where ρ = fluid density, v = velocity, g = gravity constant, z = elevation with respect to a datum, P = fluid pressure.
The first term ½ ρv2 is kinetic energy; the term ρgh is potential energy; P is fluid pressure, or force per unit area. Because groundwater generally moves very slowly, the kinetic energy term is ignored. The equation allows us to express the potential energy, or hydraulic potential for groundwater flow, commonly referred to as total hydraulic head, in terms of two components – a pressure head, and an elevation head, relative to a datum. Thus hydraulic head can be expressed in terms of some height, or elevation (e.g. metres, feet etc.).
Biaxial minerals: Anisotropic minerals where plain polarized light entering at any angle, other than along two optic axes, is resolved into two planes of polarized light; these two planes each contain the fast and slow rays. The resulting colour depends on the different in the refractive indices of these two light paths – i.e., the birefringence. Minerals may be positively or negatively biaxial, depending on the orientation of fast and slow rays.
Bindstone: Consists of organically bound frameworks (not transported), such as encrusting algae or bryozoa, that bind some pre-existing substrate.
This term was introduced by Embry and Klovan (1971) as a modification of Dunham’s (1962) limestone classification scheme; see review and modification by Lockier and Junaibi (2016).
Bioclast: Any clast derived from vertebrate or invertebrate organisms. Bioclasts may represent an entire organism, such as a bivalve shell or foraminifera, or fragments thereof. They are common framework constituents of siliciclastics and carbonates.
Biomould: Also biomold. The impression of an organism left in a rock following dissolution of the original skeletal mineral – commonly calcite and aragonite. A cast of this impression is formed if the mould is filled with a new precipitate or sediment.
Bioremediation: The use of living organisms to help clean up contaminated sites or aquifers, primarily using naturally available or introduced microbes. For example, certain bacteria will break oil down into manageable compounds like carbon dioxide or methane.
Biostratigraphy: The chronological ordering of strata based superposition of strata and the observed stratigraphic variations in fossils and fossil assemblages. The principle of faunal succession is based primarily on the appearance of specific organisms in certain strata that, in progressively younger rocks (deemed younger because they occur higher in the stratal succession), evolve into different, but related organisms.
Bioturbation: The general term for the activity of organisms that live on and within sediment. During the course of scavenging, grazing and burrowing for food, constructing a home, travelling from one place to another, or escaping predation or burial, these critters produce traces that reflect the type of sediment and the behavioural activity of the organisms. Intense bioturbation may destroy primary sedimentary structures like and bedforms.
Birefringence: Plain polarized light that passes through a mineral is resolved into mutually perpendicular fast and slow rays that will each have different indices of refraction as (i.e., their refraction paths and velocities will be different). Birefringence is the maximum difference between these two index values. Under crossed nicols, the difference is manifested in the ‘intensity’ of interference colours.
Blind thrust: A thrust that does not breach the surface at the time of its formation. Blind thrust tip points (tip lines) typically contain fault propagation fold pairs.
Block and ash flow: Ground-hugging, concentrated PDCs characterised by a vast range of clast sizes, including blocks having dimensions measured in metres. They are usually derived from collapsing lava domes. They are commonly associated with pyroclastic surges. Deposits consist of a mix of ash and blocks; they are poorly sorted, usually matrix-supported, and poorly- or ungraded. Block angularity is highly variable.
Blocks/bombs (volcaniclastics): Both terms are used as textural descriptions for primary volcaniclastics (regardless of their origins). Clast sizes are 64 mm and coarser.
Boil over: PDCs can be generated by fire fountains that eject large volumes of fragmented lava over a crater rim. PDCs formed in this way are concentrated in lapilli, splatter, and flattened or aerodynamically shaped bombs.
Bombs – ballistics (volcaniclastic): Ejected lava fragments, or bombs (particularly in Hawaiian and Strombolian fire fountains), can be shaped aerodynamically into spindle-like ballistics while being flung through the air. If the lava is still molten when it lands it will spatter and cool in a variety of shapes (e.g. cow-pat, bombs, bread crust bombs).
Bomb sags (Volcaniclastic): Large, ballistic blocks and incandescent fragments of magma ejected during an eruption, may land on earlier deposited tephra causing the beds to sag. The bedding deformation may be accentuated during compaction.
Bottom simulating reflector (BSR): In some oceanic regions, methane hydrate layers (methane clathrate) just below the sea floor have much lower density than the overlying and underlying sediment. They present as a prominent reflector on seismic profiles.
Botryoidal cement: In limestones, this cement form is presented as radial clusters of fibrous or bladed calcite or aragonite that precipitate in more cavernous porosity. Common examples are found in reef frameworks, and fenestrae that form by mineral dissolution, gas bubbles, and crystal expansion (e.g. halite-gypsum crystal growth in sabkhas). Fenestrae are common in some cryptalgal laminates and mud mounds containing Stromatactis.
Bouma sequence: Named after Arnold Bouma, one of the first to recognise the repetitive sedimentological organisation of turbidites. Bouma sequences represent individual turbidity current flow units, whether the sequence is complete or truncated. A complete sequence contains 5 divisions, becoming progressively finer-grained towards the top; some divisions may not develop:
- Massive muddy sandstone, with or without a scoured base.
- Graded and laminated muddy sandstone.
- Laminated with ripples and climbing ripples, commonly convoluted by soft sediment deformation.
- Graded, laminated siltstone-mudstone.
- A mix of turbidity current mud and hemipelagic mud, that are deposited from suspension.
Boundstone: A kind of fall-back term for limestone description where the mode of binding is not readily identifiable. This term replaces Embry and Klovan’s Bafflestone in which the mode of binding and identification of the organisms responsible was equivocal.
This term is introduced by Lockier and Junaibi (2016). in their review and modification of Dunham’s (1962) limestone classification.
Bowen reaction series: A predictable order of mineral crystallization in a cooling magma, after the early 20th Century geologist Norman Bowen. One of the first minerals to crystallize from magma is olivine (from about 1300o to 1200oC). Feldspar, the most common rock-forming mineral, begins to form below temperatures of about 1000oC, and one of the last to appear, quartz at about 800oC. Bowen’s discovery revolutionised the way we think about the evolution of igneous rocks.
Braided river: Low sinuosity braided rivers contain mostly sand and gravel bedload, and have multiple channels and bars that present a braided pattern. The bars contain a mix of tabular and trough crossbeds from beforms that migrate downstream during flood stages. The bar tops become dissected by chutes and rills during falling stage and low water.
Branch point: Locations along a thrust where branching or fault splays are generated.
Breakup unconformity: A significant, commonly angular stratigraphic discordance between syn-rift deposits that are confined to fault-bound grabens and half grabens, and the base of more widespread post-rift strata that signal the change to a passive margin. In a plate tectonic context it signifies the transition from rifting to sea floor spreading (rift-drift).
Breccia: Consists predominantly of angular clasts larger than 2mm. Like conglomerates they are poorly sorted, clast-supported frameworks. The degree of clast angularity indicates little or no reworking.
Brines: Generally used for natural waters more saline than seawater. The main dissolved salt is sodium chloride (NaCl), but calcium and magnesium sulphates are also important constituents, and there are several important trace elements, such as lithium. The primary mechanism for brine concentration in ocean basins and saline lakes is evaporation. The saturation level for NaCl is about 357 ppt (normal seawater is 32 ppt).
Brittle behaviour (rheology): Most Earth materials behave elastically up to their elastic limit, beyond which deformation is irreversible. If the strain rate is high, this deformation will take place as sudden fracturing (e.g. broken glass). Brittle deformation is also enhanced by low confining pressures and low temperatures – these are the conditions that lead to faulting and fracturing during an earthquake. Cf. Ductile flow.
Bubble texture (volcaniclastic): A texture characteristic of volcanic ash presented as highly arcuate apophyses in shard walls, or as complete bubble outlines within shards. They commonly form during explosive eruptions, from the introduction of superheated steam when magma is in contact with water (as in phreatomagmatic eruptions), or from degassing of volatiles within the magma.
Buoyancy: Buoyancy is the result of fluid forces acting on a body immersed in a fluid. If the resultant force is greater than the gravitational force acting on the body (that itself is a function of its density), then the body will rise (positive buoyancy – negative buoyancy is the opposite). Buoyancy plays an important role in many processes – the rise of mantle plumes and magmas, diapirism, density and temperature stratification in the oceans, the support of clasts in sediment gravity flows and pyroclastic flows.
Buoyant plume: A turbulent mix of gas, air and fine particles that is less dense than air. It develops above the main body of a pyroclastic flow or sediment gravity flow by elutriation of particles from the main flow. The plume dissipates as the particles settle gravitationally.