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.

Lacuna: Introduced by Harry Wheeler (1964), it encompasses the total time missing at the unconformity, and is divided into an hiatus which is a non-depositional or erosional episode above the unconformity, and an erosional or degradational vacuity below it (i.e. the time represented by rocks removed by erosion). The contact separating these two domains is the base level transit, which defines stratigraphic onlap and offlap geometries. These concepts pre-empted by about 20 years the fundamentals of sequence stratigraphy.

Lagoon: A shallow bay protected from ocean swells, and to some extent storms, by barrier islands, spits, and bars. Extensive tidal flats commonly border the landward margins of lagoons, crossed by estuaries and small tidal channels. In tropical and temperate climates, mangrove swamps provide breeding grounds for all manner of critters. There is regular tidal exchange of ocean seawater through large channels; delta platforms at the inner or seaward channel exits are called flood- and ebb-tidal deltas respectively. There is a continuous exchange of sediment (mostly sand) between the lagoon, barrier and coastal dunes, and the open sea shelf.

Lahar: A terrestrial gravelly mudflow, or debris flow consisting largely of volcanic debris. Most flows are initiated on the flanks of volcanoes. They develop during and after eruptions, initiated by seismic tremors, or periods of high rainfall that saturate soils and reduce their shear strength. Lahars are capable of carrying vehicle-sized blocks and can be very destructive. Flow run-out is commonly several kilometres. Cf. block and ash flow.

Laminar flow: Defined and quantified by Osbourne Reynolds, laminar flow is described conceptually as flow lines that are parallel, or approximately so, and relatively straight. The flow velocity will be the same across each flow line. Expressed in terms of Reynolds numbers (Re), it is the flow condition when Re < 2000. The transition to turbulent flow is usually abrupt.

Lapilli: Primary volcaniclastic particles, derived directly from volcanic eruptions, and ranging in size from 2 mm to 64 mm. See Accretionary lapilli.

Lapout: The geometry of stratigraphic termination of clinoforms relative to some through-going surface. Although these can be observed in some outcrops, seismic reflection profiles provide the best opportunity to identify lapout classes. See Onlap, Downlap, Offlap, Toplap.

Lateral blast: An ground-hugging eruption that is triggered by the collapse of a volcanic flank or lava dome. These blasts produce hot, fast-moving block and ash flows, pyroclastic density currents and pyroclastic surges. Mt. St. Helens eruption in 1980 is an iconic recent example.

Lateral linkages (stromatolites): Laminae that extend from one stromatolite dome or column to its neighbours and covering the intercolumn sediment.

Lateral moraine: The accumulation of rocky debris at the surface of glacier margins, derived by scraping and erosion of adjacent bedrock valley walls. They form as parallel ridges in the ablation zone of glacier margins. As deposits they are characteristically a very poorly sorted mix of rock flour to boulder size, angular clasts.

Lateral Ramps: Fault ramps at right angles or oblique to the strike of a thrust complex that transfer displacement from a lower to higher flat. Fault planes dip 10^o to 30^o. As shown in the diagram below, fault displacement across the ramp is basically strike-slip.

Laths (igneous petrography): Very small needle-like crystals commonly set in very fine or glassy groundmass. Aphanitic volcanic rocks may have abundant feldspar laths; alignment of crystals during lava flow is common.

Laurentia: Laurentia and Gondwana were the two major continental blocks that split from Pangea supercontinent. Laurentia originally was an amalgam of North American, Greenland, and northwest Scotland. Beginning about 175 Ma, the North American-Greenland portion of Laurentia split from Pangea, leaving the Scottish sliver behind.

Lava dome: Hemispherical to spine-shaped extrusion of viscous magma in the craters or upper flanks of volcanoes. Lava domes are inherently unstable: debris derived from lava cooling and cracking may spall and accumulate at the dome base; or domes may collapse under the influence of gravity or from internal pressures. Dome collapse commonly generates block and ash flows, or hot pyroclastic flows and surges. Cf. endogenic and exogenic domes.

Lava dome collapse: Domes of viscous lava that grow on volcano summit craters or flanks are inherently unstable. Their collapse from gravitational instability or internal pressures can generate PDCs, commonly as block and ash flows, or pyroclastic surges.

Lava lake: The accumulation of lava in summit craters. Eruption may be quietly effusive, or as fire fountains from fissures around the crater walls. Crusts that form rapidly on the lake surface are usually broken by lava surges and reworked into the melt. When activity ceases the lava solidifies into a solid plug.

Lava tube: Long, tunnel-like openings through which lava flows; primarily in basaltic pahoehoe lavas. They usually begin life as open lava streams where crust mantles gradually coalesce into a more solid roof. Tubes commonly branch to form networks. When flow ceases, the lava may drain leaving the tube open, or solidify within the tube. The roof of a tube can collapse at any time during or after lava flow.

Lebensspuren: The German word for traces and trace fossils left by animals on or within the sediment.

Lee face (bedforms): The steep, angle of repose face on bedforms, such as ripples and sandwaves, that faces down-flow. Bedload is moved up the stoss face (upstream face) to the bedform crest, and subsequently avalanches down the lee face. Immediately downstream of the lee face is a region of relatively low fluid pressure that is detached from the main flow.

Lenticular crossbedding: Sand ripples that occur as single bedforms within laminated mudstone. They commonly occur with flaser beds. They are commonly found on tidal flats and shallow, low energy subtidal environments such as in lagoons. Together, the bedforms provide evidence for tidal current asymmetry.

Levee: Natural levees are linear, mound-like deposits that accumulate along the banks of many fluvial, delta distributary, tidal, and submarine channels; they act as a partition between an active channel and adjacent floodplain. Deposition occurs during channel flooding. If levee accretion is significant it may prevent regular flood plain inundation. Rippled and laminated fine-grained sand tend to be deposited during rising flood stages, and silt-mud veneers during waning flow stages. In some cases, vegetation will stabilise the levee, and dampen overbank flow to the floodplain. Levees that rim submarine channels accumulate during the passage of turbidity currents. See crevasse splay.

Liquidus: The temperature at which there is complete melting of a solid; it is the maximum temperature at which crystals can coexist with their liquid melt. Note that the liquidus occurs at higher temperatures than the solidus.

Liquefaction:  If water-saturated sediment is disturbed, for example by earthquake ground shaking, the grains begin to separate until they are ‘floating’ in the interstitial water.  At this point, the fluid now consists not only of water but also the floating grains and a consequence of this is that fluid pressures increase.  The sand is now liquefied. It no longer has shear strength and cannot support surface loads. Eventually the grains will settle and at this point the excess water will escape to the surface. Cf. dewatering, fluidization, sand volcanoes.

Listric fault: Listric faults are a product of extension. They have normal dip-slip(or oblique-slip displacements where fault surfaces are concave upward, dipping steeply near the surface and flattening at depth in a detachment that is common to other listric faults. They occur at crustal scales (e.g. continental rifts), and in smaller depositional systems such as deltas and slumps. Faulting is commonly incremental and can take place during sedimentation. Rotation or folding of strata in the hanging wall block results in multiple unconformities. Cf. Growth fault

Lithic (petrology): Also called rock fragments. A general term for clastic material that cannot be pigeonholed into the other main framework components like quartz and feldspar. It usually applies to fine grained lithologies such as mudstone and siltstone, or aphanitic, glassy volcanic rock (with or without small feldspar laths). Chert fragments present something of a problem in sandstone classifications because they consist of micro- and cryptocrystalline quartz; they are sometimes placed in the total quartz category, at other times in the lithics category.

Lithification: The combination of compaction and cementation that produces hard, hammer-ringing rock from loose, uncompacted sediment. Lithification depends on a complex association of physical and chemical processes. Cementation can occur at very shallow depths in the case of carbonates, or at different stages of burial depending on temperature, and rock – fluid chemistry. Compaction begins soon after deposition and continues at depth.

Lithosphere – the compositional version:  Consists of the crust and mantle lithosphere. The lithosphere-asthenosphere boundary is defined by a low seismic velocity discontinuity.

Lithosphere – the rheological version:  The lithosphere – asthenosphere boundary is defined by the solidus, a isotherm at 1100^o-1330^oC, that marks the transition to partial melting of mantle rock. This is manifested as the transition to mechanically weak ductile-viscous behaviour in the asthenosphere. Defined in this way, oceanic lithosphere is about 5 km thick at spreading ridges increasing to 90 km thick in older, colder oceanic lithosphere. Continental lithosphere commonly ranges from 100-250 km thick.

Lithostatic pressure:  At any depth, the pressure exerted by the overlying column of rock and sediment having unit-area cross-section, is calculated from the expression P = ρgh where ρ = density of the rock column, g = gravity constant, and h = depth from some datum, commonly sea level. Note that, assuming a cross-section of unit-area reduces volume to units of depth. Also called overburden pressure. It is analogous to hydrostatic pressure.

Lithostratigraphy: Formal lithostratigraphy is concerned with the description and mapability of rocks, using physical, fossil, and mineralogical attributes. The basic lithostratigraphic unit is the Formation. There is no reference to time.

Littoral zone: The nearshore region of marine and lacustrine environments. In the marine setting it extends from high tide to shallow offshore depths. The term is used primarily to designate ecological environments for diverse marine organisms.

Load casts: Sole marks, or sole structures consisting of bulbous sand or silt structures that protrude into and deform underlying beds – they are most pronounced when protruding into mudrocks. Load casts, or load balls may become disconnected from the parent sand bed. They form during early compaction and are commonly associated with dewatering structures. Soft mud squished between load casts may produce wispy flame structures.

Lobe and cleft: Lobes, billows, and intervening pockets at the head of a PDC where turbulence is generated by frictional forces across the substrate and the contact with air.

Longshore drift: Drift of water masses, sediment, and swimmers occurs when waves approach a beach at an angle. Here, water moving up the beach (wave swash) returns farther along the beach. Longshore drift (or along shore drift) is an important coastal process that contributes to coastline straightening by sand bars, and to the formation of sand spits and barrier islands. See undertow, rip currents.

Lowstand Systems Tract (LST): Forms at the end of sea level fall and the beginning of sea level rise. Depositional systems include submarine fan, base of slope and mass transport deposits. The top of the LST is the maximum regressive surface; the base is the correlative conformity equivalent to the subaerial unconformity (also a sequence boundary).

Lysocline: The ocean water depth where the dissolution of calcite is first observed in sediment. Its identification requires detailed observation of dissolution textures and is somewhat subjective. It lies above the calcite and aragonite compensation depths; the lysocline should, theoretically, be close to the saturation levels for both minerals.

Maar eruptions:  A phreatomagmatic, or hydromagmatic eruption caused by magma intruding shallow groundwater. These highly explosive events produce low-relief craters that extend beneath the local watertable and are capable of bringing deep basement rocks to the surface. They subsequently fill with water and form closed lakes or coastal embayments. Volcaniclastics usually include airfall and pyroclastic surge deposits. Accretionary lapilli are common.  They may be linked to diatremes.

Magmatic arc: Also called volcanic arcs. A chain of volcanoes and associated intrusions that form in the plate above a subduction zone. Arcs generally parallel the deep oceanic trenches. At mantle depths, dewatering of oceanic crust in the subducting slab lowers the melting point of mantle rock. The partial melts rise because of buoyancy.

Magmatic eruption: Explosive eruptions where magma fragmentation is controlled by decompression of magma volatiles such as CO₂ and water. Cf. phreatomagmatic, phreatic, effusive eruptions.

Magnesium calcite: Also called magnesian calcite. In the calcite crystal lattice, magnesium can occupy the position of calcium, up to about 20 mole percent. Two varieties predominate in carbonate sediments and limestones: Low magnesium calcites (LMC) with <4 mole % Mg), and high magnesium calcites (HMC) with 11-19 mole % Mg). HMC commonly recrystallize to LMC during burial diagenesis.

Magnetic field: Earth’s magnetic field is generated by a hot (4000-5000^oC), fluid-like, iron-nickel rich outer core that moves slowly around a solid iron inner core. The field is forced into a tear-drop shape by solar winds, with the head of the ‘drop’ towards the sun (extending about 65,000 km), tapering over 600,000 km away from Earth. The magnetic field protects us from harmful components of the solar spectrum, like cosmic rays. The field has North and South poles that occasionally reverse over geological periods of 10⁴ -10⁵ years. The field intensity also waxes and wanes.

Magnetic poles: The points in a magnetic field where lines of equal magnetic intensity converge. On Earth, these poles are close to, but not coincident with the geographic poles; the magnetic poles also wander. During magnetic reversals the N and S magnetic poles switch places.

Magnetic reversals: Reversal of Earth’s magnetic field has occurred many times, and over the last few million years this has happened about every 200,000 to 300,000 years.  The last reversal took place 780,000 years ago; this is called the Brunhes-Matuyama Reversal. Reversals are recorded by iron-bearing minerals in volcanic and sedimentary rocks where the minerals act as tiny magnets – the direction of polarity (i.e. magnetic N and S) is locked in mineral at the time of lava solidification or sedimentation, and this remnant magnetism can be measured.

Magnetostratigraphy: The formal stratigraphic measure is the magnetostratigraphic polarity unit that is used to subdivide and correlate rock units according to whether polarity is Normal (north pointing) or Reversed for a body of rock.

Mangrove: Flowering shrubs and small trees that are salt tolerant, living on sandy and muddy tidal flats and salt marshes. Most common between the subtropics but do extend beyond these latitudinal limits. They deal with salt uptake by excreting it from their leaves. They have complex root systems that help stability under conditions of shifting sediment and tides. Mangroves provide important habitats and breeding  grounds, and also help protect coasts from storm wave surges and erosion. See also paralic, marsh.

Mantle plumes: A process of mantle convection involving the buoyant rise of asthenosphere; From lab experiments it has been learned that the shape of the plume depends on its viscosity – high viscosity leads to relatively narrow bodies, high viscosity to mushroom-shape bodies with tails. They are important forms of heat transfer and from partial melting, sources of magma.

Marsh: A wetland dominated by herbaceous plants, that is transitional between a lake or sea and terrestrial environments. In paralic settings they form salt marshes that are inundated during spring tides and contain plant species that have adapted to saline conditions, such as the succulent Salicornia. Fresh and salt water marshes are important habitats and breeding grounds for many vertebrate and invertebrate species. Cf. Swamp

Master fault: The primary fault or detachment in a system of faults such as splays, duplexes, imbricate fans, or synthetic and antithetic shears associated with strike-slip faults. Pretty much synonymous with principle deformation zone (PDZ).

Matrix: The fine-grained material among coarser framework components like sand grains or pebbles. Matrix is a depositional product, that accumulates at the same time as the coarser grained sediment. Its fine-grained quality commonly renders it diagenetically more reactive than the associated framework grains. Note the term groundmass is reserved for the finely crystalline to glassy component of volcanic rocks.

Matrix-supported framework: Applies to mud-dominated lithologies where sand-size and larger clasts appear to float in the mud-silt matrix. A classic example includes pebbly mudstones. Cf. Clast-supported framework.

Maunder minimum: Also known as the Little Ice Age. Between 1645 and 1715, Northern Hemisphere temperatures averaged about 1^oC lower than normal. Evidence generally points to changes in solar insolation (e.g. sunspots) as the primary cause.

Maximum flooding surface: The MFS represents the sea floor at the end of transgression, marking the change to normal regression, and where the shoreline trajectory reverses from landward to seaward. It is easily mapped because it commonly overlies a (transgressive) condensed section and is overlain by mudrocks of the succeeding normal regression.

Maximum regressive surface: Also called the Transgressive surface. This surface represents the sea floor at the time when regression ends and transgression begins, and the shoreline trajectory reverses from seaward to landward.

Mean grain size:  The most common statistic used in describing grain size distributions, is the average size in a population of grains; it is one measure of central tendency (see also Median, Mode, and Skewness). The mean of a population should always be quoted with its standard deviation. The Folk-Ward (1957) calculation uses Phi values at the 16^th, 50^th and 84^th percentiles:

Mϕ = [ϕ₁₆ + ϕ₅₀ + ϕ₈₄] /3

Meandering channel: One of the fundamental fluvial channel types, also known as high sinuosity channels. They are generally single channels organised as sinuous loops. Channel thalweg is constantly on the move such that meanders migrate laterally and downstream. Abandoned meanders may be preserved as ox-bow lakes. Deposition takes place in three main settings: the main channel, point bars, and flood plain (that includes swamps, lakes, and vegetated areas).

Mechanical dispersion: In geofluids, this occurs when solute molecules are carried from the source by local eddies around grains or through fractures; this kind of tortusoity takes place at a scale much smaller than the en masse advective flow.  Cf. Molecular diffusion.

Mechanical stability (of detrital grains): The ability to resist abrasion and breakage during sedimentary transport. It depends on grain-mineral hardness, and in the case of crystalline minerals the presence of cleavage or twinning. Thus quartz is mechanically more resistant than feldspar, even though their hardness is similar; both are more resistant than calcite that is softer and has good cleavage.  Cf. Chemical stability.

Median grain size:  One of the three common measures of central tendency, the median is taken as the Phi value at the 50^th percentile. See also Mean, Mode and Skewness.

Mélange: An extensive, mappable body of brecciated and sheared rock and sediment, chaotically mixed. They tend to form in compressive, accretionary tectonic settings and are more common in subduction-related accretionary prisms. However, they are also know from strike-slip and extensional regimes where more local compressive stresses are possible.

Meniscus cement: Cements that accumulate at clast contacts in vadose, or unsaturated conditions, where precipitation (commonly calcite or aragonite) takes place in the fluid meniscus. The meniscus itself is caused by surface tension forces at the boundaries of grains in contact.

Meromict: A stratified lake or enclosed sea where the layers do not mix. Bottom water layers may become anoxic as dissolved oxygen is used up by organisms. In saline waters it applies to salt crystals that precipitate within saturated layers and then sink to the bottom.

Meteoric diagenesis (carbonates): Diagenesis of limestone under fresh-water conditions, both in the vadose (unsaturated) zone, and below the watertable. It is largely controlled by the degree of fresh- water seepage and groundwater flow. Vadose zone diagenesis is dominated by dissolution that, if prolonged, produces caverns, sinkholes (dolines), subterranean streams, and spectacular karst landforms. Dissolved calcium carbonate may reprecipitate as cement and fracture-fill in the saturated zone, and as stalactites-stalagmites in caves.

Meteoric flow: Subsurface flow of water or brine that originates at the surface. Most meteoric groundwater flow is driven by topographic gravitational potential. Cf. topography-driven flow, hydraulic potential.

Micobialite: A recent term for laminated mats composed of microbes including bacteria, cycanobacteria, and red and green algae; it replaces the term cryptalgal laminate that is generally applied to fossil laminates, particularly (but not exclusively) Precambrian.  The laminates may be flat and uniform, or tufted, pustulose, and polygonal, resulting from desiccation or, in arid environments, evaporite precipitation.

Micron: A micro-metre, or millionth of a metre, usually written µm.

Microporosity: Porosity that is 1-2 µm contributes to the total pore volume of a rock or sediment, but in terms of advective fluid flow it is inefficient. Transfer of dissolved mass probably takes place by diffusion.  Common examples are present in pore throats of granular rock, between clay particles in mudrocks, and between pore-filling cements.

Micrite: A contraction of microcrystalline carbonate mud, refers to aragonite or high-magnesium calcite crystals less than 4 microns, that form as cements (usually isopachous cements), or the product of substrate alteration by boring algae, fungi, and larger critters like boring Clionid sponges – a process call micritization. Micrite is highly reactive and susceptible to recrystallization during meteoric and burial diagenesis because the original mineralogy is metastable, and because of the high surface area afforded by the micron-sized crystals.

Microcrystalline: From a practical point of view, crystals too small to see with a hand lens, requiring a microscope to see individual microcrystals. Common examples are chert, flint, and chalcedony composed of microcrystalline quartz. See also Cryptocrystalline.

Mid-Ocean Ridge: The elevated ridge-like bathymetry with a central rift in oceanic crust, created by upwelling mantle plumes, intrusion of basic igneous rocks such as gabbros, and eruption of basaltic flows and pillow lavas during sea floor spreading. The total length of these ridges in all Earth’s oceans is more than 64,000 km. The ridge system was discovered by Marie Tharpe in the early 1950s.

Milankovitch (Milutin): 1879 – 1958. A Serbian mathematician and engineer, best remembered for the eponymous Milankovitch orbital cycles. Earth’s rotation and orbit around the sun is an ellipse that is perturbed by ever-changing wobbles and tilts.  Milankovitch’s mathematical theory outlines three kinds of orbital cycle: Precession, Obliquity and Eccentricity, that are influenced by gravitational interactions between the earth, sun, moon, and to a lesser extent the planets. These astronomical cycles have a direct impact on solar insolation and therefore global climates, on a scale of 100s to 10,000s of years.

Miogeocline (miogeosyncline): The part of a geosyncline that represents the continental margin. It consists of seaward thickening wedge or prism of sediment, commonly more than 10 km thick, that contains fluvial, coastal, and shelf facies. It is analogous to a passive margin succession. According to geosynclinal theory, a eugeosyncline occupies the oceanic realm seaward of the prism. Although the term geosyncline is no longer used, miogeocline does occasionally creep into modern literature.

Modal grain size:  The mode of a grain population is the Phi percentile for the most abundant grain size class on a frequency distribution curve. See also Median, Mode and Skewness.

Mohorovičić (Moho) discontinuity:  The Moho defines the base of the crust that is recorded as an abrupt seismic P-wave velocity discontinuity from an average 6.7–7.2 km/s above, to 7.6–8.6 km/s in the mantle below. The increased seismic velocities are due to an increase in density, from basalt-gabbro lithologies in the lower crust, to peridotites in the uppermost mantle.  See also Lithosphere – asthenosphere.

Molecular diffusion: When a solute gradually mixes with solvent molecules; in geofluids this is primarily water. The process does not involve the physical flow of water, but depends on solvent-solute properties such as polarity and charge, and vibration energies. Cf. Mechanical dispersion.

Monocrystalline quartz: Applies to grains of quartz that consist of a single crystal. It is used in petrographic descriptions of arenite and coarser lithologies to distinguish this variety of quartz from polycrystalline quartz. See also strained quartz, volcanic quartz.

MTD (Mass Transport Deposits): MTD is the acronym given to soft sediment slumps, slides and debris flows, mostly generated on relatively high angle slopes between the shelf or platform margin, and deep-water settings at the base-of-slope and beyond. The term is generally reserved for sediment packages at or close to the sea floor, that move and deform en masse under the influence of gravity, commonly in multiple events.

Mud: A mix of silt- and clay-sized particles. On the Wentworth scale it includes all sizes smaller than 0.0625 mm, or 4 phi. Grain size analysis of unconsolidated mud samples is usually by pipette, or Laser Size Analyser.

Mud cracks: See Desiccation cracks.

Mud-supported: Synonymous with matrix-supported, where large framework clasts appear to float in a mud matrix.

Mud volcano: Small cone-shaped buildups associated with erupting mud, ranging from about a metre to 10s of metres high. Eruptions may be quiet where mud flows, slithers and slides down slope, or more violent, reminiscent of lava fire fountains, shooting mud 10s of metres into the air (or water). If methane is present in the mud, the eruptions can ignite. They form on land and on the sea floor.

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.

Ichnology: The study of trace fossils, the behaviour of the critters that made them, the environment they lived, fed, escaped, and traveled in, and their relationship with other sedimentary facies  and stratigraphic surfaces. See Lebenspurren.

Ignimbrite: A recent definition states ” …the rock or deposit formed from pumice and ash- through to scoria and ash-rich pyroclastic density currents” regardless of thickness, areal extent, volume, composition, crystal content, relationship with topography, or temperature (usually >500°C) . They are usually regarded as concentrated PDCs where grain-to-grain dispersive pressures and/or fluidization maintain flow support. As such they are fundamentally different to pyroclastic surges and block and ash flows.

Imbricate fan: Fan-like splay of thrust panels and thrust faults generated from a single décollement. Unlike duplexes, there is no roof thrust,

Imbrication: The alignment of platy or bladed clasts (usually in pebbles or larger) in relatively strong unidirectional currents. The flat clasts dip upstream, and tend to be stacked one upon the other. Most common in coarse grained fluvial deposits. They are good paleocurrent indicator.

Inboard basin:Basins on the leading edge of a terrane. Includes oceanic basins in the intervening ocean between terrane and autochthon, forearc basins and trenches associated with subduction, or intermontane basins that overlie the incoming terrane. All these basins are deformed during terrane accretion. Cf. outboard basins.

Induration: Refers to the degree of sedimentary rock hardening during compaction and chemical diagenesis, as temperatures, pressures, and fluid compositions evolve during burial.

Inertia: Inertia is generally defined as a force that resists the change in motion of a body; here motion refers to a vector that describes velocity and direction, and ‘body’ refers to anything composed of matter, including a body of fluid. Inertia was codified by Newton in his Laws of Motion – in the 1st Law as the Law of Inertia, and in the 3rd, as the Action-Reaction  law. Inertial forces are central to the quantification of fluid mechanics expressed in Froude and Reynolds numbers.

In sequence thrusts: In a system of thrusts, the most recent fault is at base of the thrust pile and most proximal to the foreland – propagation is towards the foreland. Older thrusts are stacked progressively hinterland-ward. In sequence thrusts place older rocks on younger as the fault propagates up the ramp through progressively younger strata.

Insolation: A measure of solar energy that reaches the Earth’s surface, taking into account the Sun’s output, orbital distance from the Sun, reflection by the upper atmosphere (30%), clouds and ice sheets, and absorption by atmospheric components like CH₄ and CO₂.

Interference figures: These appear as curved isogyres or crosses when a mineral is viewed under crossed nicols at high magnification with the Bertrand lens inserted. There are two basic types:

1. Uniaxial crosses that do not break up or rotate as the stage is rotated.
2. Biaxial isogyres or crosses that rotate and move with the stage; crosses will break into two curved isogyres.

Interference ripples: Across the sediment-water interface, two sets of ripples each set having a different orientation, will cross each other forming an apparent interference pattern. These structures are common on modern intertidal and shallow subtidal flats and platforms. In the rock record, the two sets will exist on the same bedding plane.

Interglacial: The period between glaciations. Periods of warming controlled by Milankovitch orbitals and Solar insolation, changing atmospheric carbon dioxide, albedo, and ocean currents.

Intergranular: Literally, between grains. The term is commonly used to describe matrix, cements, and porosity, and sedimentary fabrics such as preferred orientation.

Intertidal: Literally means between tides. It is the region above mean low tide, and below mean high tide. Its morphology is that of a beach, tidal flat, and tidal or estuarine channel. Seaward is the subtidal zone (rarely exposed) that includes the shoreface; also called the littoral zone. Landward is the supratidal zone.

Intra-arc basins: Basins located within magmatic arcs, between and over the flanks of volcanic edifices. They occur in oceanic and continental crust. Crustal extension may be involved in subsidence, caused by transtension at the convergent margin or the more localised effects of caldera collapse. Loading by growth of volcanic edifices will also contribute to subsidence. There will likely be multiple depocenters along the line of the volcanic arc. Heat flow is high.

Intracratonic basins: Also called intracratonic sags. They occur in continent interiors, generally remote from the direct effects of plate boundary tectonics and heat flow. Subsidence may in part be induced by far-field stresses that can generate long-wavelength buckling, the down-welling of mantle and cooling. Subsidence is aided by sediment loads.

Intragranular: Literally within grains. It is most commonly used to describe textures, cements, and porosity in the chambers and whorls of bioclasts such as corals, bryozoa, gastropods and various microfossils.

Iridium anomaly: Anomalously high concentrations of Ir derived from meteorite impacts incorporated into sediments, particularly mudrocks. It was first recognized at the Cretaceous-Tertiary (K-T) boundary, corresponding to the widespread distribution in aerosols generated by the Chicxulub impact.

Island arcs: Volcanic chains and the associated magmatic rocks beneath, that build on the upper plate above subduction zones. Thus, they face oceanic crust, and behind have backarc basins, that may also be floored by oceanic crust. The greatest modern concentration of island arcs is the circum-Pacific (Ring of Fire). Cf. Continental arcs.

Isoclinal fold: A fold where the interlimb angle is <30^o. The beds on opposite fold limbs can sometimes appear parallel. Good younging indicators are essential for deciphering these kinds of structural complexity.

Isopach: A contour that delineates a sedimentary, volcanic or volcaniclastic unit thickness, either as a single bed or succession of beds. Unit thickness is measured directly in the field, from core or borehole logs (gamma and SP logs are commonly used to do this), or from seismic reflection traces. Isopachs are used to map thickness trends.

Isopachous cement: Cements that rim clast surfaces and are of approximately equal thickness throughout. Common types include aragonite and calcite as fibrous, bladed or drusy crystals, prismatic quartz, and clays like kaolinite and illite. They are commonly overlain by later pore-filling cements. Isopachous cementation implies fluid saturated pore spaces.

Isostasy:   Isostasy describes the tendency to equilibrium of a lighter lithosphere floating on a more dense and perhaps more ductile mantle asthenosphere; it describes the state of balance between the lithosphere and asthenosphere. The two foundational models are Pratt Isostasy that allows density to vary from one lithospheric block to another, and Airy Isostasy where density is the same across all blocks. A later modification of the theory recognises the elastic nature of the lithosphere, allowing it to bend under loads – this is flexural isostasy, also known as regional isostasy. At equilibrium the lithosphere and asthenosphere are isostatically compensated. See also compensation depth, glacio-isostatic rebound.

Isotropic minerals: Unlike anisotropic minerals, this group cannot reorient plain polarized light which means that no light will pass through the upper polarizer; they will appear black through all rotations of the stage. All isometric minerals (cubic system) are isotropic (e.g., garnet, fluorite, halite, spinels). Note that an anisotropic mineral oriented at right angles to its optic axis will appear isotropic (because there is no resolution of fast and slow rays along the optic axis, i.e., no birefringence).

Isotropy: (Hydrogeology) An aquifer or aquitard is considered isotropic if its permeability or hydraulic conductivity is the same in all directions, usually specified by three principal orthogonal axes. Isotropy is often assumed in groundwater modelling as a reasonable simplification. In reality, most porous media are anisotropic.

Jacob’s Staff: A measuring stick with an inclinometer at one end, that is used to measure directly true stratigraphic thickness in dipping beds. Use as a surveying instrument dates to Medieval times.

James Hutton: (1726-1797). The founder of modern geology, he proposed the principle of uniformity (later called uniformitarianism), recognised the unfathomable depths of geological time and the value of unconformities, and sorted the problem for the origin of granites – viz. from melts, rather than ocean precipitates.

Jet streams: Stream-like bands of fast-moving air in the upper levels of the troposphere, close to the boundary with the stratosphere, generally at 9-16 km altitude. They are the product of an interaction between cold (polar) and warm (tropical, sub-tropical) air masses. Wind speeds are as high as 400 km/hour. The jets move north and south with the seasons and changes in temperature, and meanders come and go, sometimes disappearing completely for brief periods.  Jet-streams have a direct effect on large-scale weather pattern. They also occur on other planets that have atmospheres.

Johannes Walther: (1860-1937). We remember Walther primarily for his ‘Law’, that is an essential part of any modern analysis of sedimentary facies and depositional systems: ‘‘. . . only those facies and facies-areas can be superimposed primarily which can be observed beside each other at the present time’’ (Walther 1894). It provides a rational means for interpretation of ancient environments by inviting us to examine modern analogues.

Joints: Open fractures in hard rock formed by extension. Joints lack displacement – cf faults. Joints commonly occur in three dimensional networks. Joints can form during faulting, folding, or by extension during unloading of the crust, for example during erosion, or melting of icesheets. Open fractures provide pathways for subsurface fluid flow.

Juvenile fragments: In volcaniclastic deposits, the granular material derived directly by fragmentation of new magma. Airfall deposits and pyroclastic density currents consist almost entirely of juvenile debris. C.f. accidental clasts plucked from existing rocks in a vent, or the substrate to ground-hugging flows. Also called Essential clasts. Cf. Accidental pyroclasts, Cognate pyroclasts.

Kaolinite: A triclinic clay mineral with the general formula Al₂Si₂O₅(OH)₄, presented as flakes a few microns wide, or aggregates of flakes into mica-like books. A common weathering product of feldspars, a common diagenetic product as a cement or replacement mineral.

Karst:  A landscape of gullies, canyons, and steep-sided pinnacles resulting from intense meteoric diagenesis (dissolution) of thick limestones. The relief on karst landforms ranges from 1-2 m to 100s of metres. The corresponding subterranean structures include sinkholes, caverns and underground streams.

Kelvin Helmholtz instabilities (or waves): At the top of a PDC near the flow head, intense shear at the contact between the rapidly moving flow and overlying air leads to instabilities manifested as billows, vortices and waves.

Kerogen: Kerogens are complex organic polymers that form during the breakdown of organic matter during the early stages of sediment burial. Three main types are identified depending on the O/C and H/C ratios of the polymer molecules: Type 1 is derived from algal organic matter, Type II from mainly marine micro-organisms, and Type III from plant material. Kerogen itself begins to break down at temperatures around 60^o-80^oC, as part of the organic diagenetic-maturation process. Identification of the kerogen types preserved in hydrocarbon deposits provides a good indication of the original organic matter.

Kinematics: The branch of classical mechanics that studies movement. In Earth sciences this centres on deformed rock, the kind that produces fault zones and landslides, thrust sheets and folds, or entire mountain belts and the evolving boundaries of tectonic plates. A kinematic analysis can probe single crystals or entire mountains.

Klippe: Plural = klippen. A remnant of a thrust panel or other allochthonous structure, isolated by erosion, that overlies and is surrounded by autochthonous rock. Cf. Window.

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.

Genetic sequences: An alternative to the standard Exxon sequence stratigraphic model, introduced by W. Galloway (1989). Genetic sequence boundaries are bound by maximum flooding surfaces, rather than subaerial unconformities. They are based in part on D. Frazier’s (1974) depositional episodes,   that begin with a period of progradation and end with transgression (maximum flooding).

Geochronology: The study of rocks and minerals to determine their age. Modern techniques use the decay profiles of radioactive isotopes, particularly uranium, lead, thorium, and potassium, to give us numerical ages (in comparison fossils provide relative ages).

Geofluids: Below the watertable (local or regional) all sediment and rock is saturated with fluid – aqueous, or non-aqueous. Geofluids include:

• Fresh and saline water (aqueous fluids in aquifers and aquitards) and hydrocarbons (oil and gas).
• Depth of flow ranges from near surface to the deepest parts of the crust.
• Rates of fluid flow rates range from cm/second near the surface, to cm/million years deep in the crust.
• Aqueous fluids are involved in all chemical reactions and distribute dissolved mass through the crust, including those that form rocks, hydrocarbons, and ore deposits.
• Fluids play an important role in how the earth deforms by reducing shear strength and elevating fluid pressures.
• Hydrous igneous melts have lower melting points.

Geographic poles: The points where lines of longitude converge, north and south. These poles are close to Earth’s pole of rotation; the coincidence is not exact or permanent because Earth wobbles about the rotation axis. Cf. Magnetic poles.

Geohistory: A numerical analysis that attempts to tease apart the different components of basin subsidence and basin accommodation space. The method developed by Van Hinte (1978) calculates the effects of compaction (decompaction), bathymetry, and sea level change. The contribution to total subsidence by tectonics is determined by backstripping the sedimentary column – this method was developed by Watts and Ryan, 1976.

Geoid: A hypothetical surface of equal gravitational potential, that coincides with sea level in the absence of tides, waves, currents, and changes in air pressure. Sea level in this context is an ideal surface. Because it depends on gravitational potential, the geoid, and therefore sea level will not be a smooth surface, but will have long wavelength hill and valley like relief. Satellite altimetric measurements if sea level are referenced to the geoid.

Geopetal: Textures and fabrics that allow the interpretation of stratigraphic top, or ‘way-up’. This definition would include normal grain size grading in a turbidite. However, there is a tendency these days to restrict the meaning to structures where cements or sediments partially fill a void, such that the top of the fill represents a depositional or precipitation surface. Examples include fossils that have preserved chambers, the interstices between pillow lavas, and cavernous porosity in reef frameworks or caves.

Geosyncline: The term coined in the 19th century for basins containing great thicknesses of sedimentary rock. Early hypotheses, popularized by folk like J. Dana, James Hall, and Marshall Kay, linked geosynclines to orogenic belts as cause and effect, rather than just being exposed in mountain uplifts. Eugeosynclines were deep-water oceanic basins, commonly bordered by an uplifted ridge or geanticline; Miogeosynclines, or miogeoclines were relatively shallow water sedimentary depocenters, likened to passive margin sedimentary prisms. Interpretations of thick sedimentary successions were frequently forced to comply with geosynclinal models. One of the major problems with these models was a general lack of modern analogues. The advent of plate tectonics and the recognition of basins associated with plate interactions, rendered the terms redundant.

Geostatic pressure: An alternative term for lithostatic pressure.

Geothermal gradient: Temperature generally increases with depth in the crust; the gradient for a particular location is stated as the temperature increase per unit depth. The global average is 3^o C/ 100 m although there can be large departures from these values in regions of geothermal and volcanic activity, or regions that have cooled significantly over geological time, such as old oceanic crust.

Ghyben-Herzberg equation: (hydrogeology)

z = h.ρf / ρs – ρf

where z is the depth to the interface from sea level, h the watertable elevation, ρs (1.025 gm/cc) and ρf (1 gm/cc) the densities of seawater and freshwater respectively, such that:

z = 40h

This relationship is an important approximation of the interface between freshwater and seawater in coastal aquifers. The equation states that for every unit decrease or increase in watertable depth (h) there will be a corresponding 40 unit rise or fall respectively in the interface between seawater and freshwater. In practice, it provides a reasonable approximation of potential seawater intrusion into coastal aquifers that have been over-produced.

Gilbert delta: Originally described by G. Gilbert for coarse-grained deltas that display a 3-fold architecture: horizontal to shallow dipping topset beds (analogous to a delta plain), foresets beds, and bottom set beds. They form where coarse bedload rivers empty into lakes and marine basins. They are included in the general category of fan deltas.

Glacial outwash: Deposits, usually coarse-grained, deposited downstream of glacier ice fronts by fluvial processes. Streams are commonly braided. Outwash streams may be linked to subglacial channels. Small outwash fans may also form where subglacial streams exit the ice. Outwash streams and fans may drain into or from lakes.

Glacial striae: Grooves and scratches produced by ice-drag of rocky clasts over a subglacial bedrock surface. Measurement of striae bearings provides useful information on ice flow directions.

Glaciofluvial: A broad term that includes a variety of fluvial environments and processes associated with glaciers, ice caps and ice sheets. This includes subglacial and other ice contact deposits (such as eskers), as well as outwash streams originating at ice fronts. Most are coarse-grained.

Glaciolacustrine: Lakes that form from glacier or ice cap meltwaters, and receive glacial outwash sediment. Lakes may be located in antecedent drainage lows, or damming of outwash streams by ice or landslides. Coarse-grained deposits will form as beaches  or small deltas (e.g. Gilbert-type deltas). Mud carried by outwash streams will settle in the lower energy parts of lakes. Lake varves are a characteristic product of seasonal freeze-thaw.

Glacio-isostatic rebound:  The isostatic response where a landmass elevation rises following the melting and removal of an ice sheet. Evidence for rebound is commonly observed as raised beaches. Well-known examples of post-glacial rebound where uplift continues today  are Scandinavia and Hudson Bay.

Glowing avalanche: A hot pyroclastic flow that reveals a glowing flow head as it careens down the volcano slope. Also called Nuées ardentes.

The staircase of raised beach ridges, over an altitude gain of about 100 m from present sea level, has formed in response to lithospheric rebound following melting of the Laurentide Ice sheet. The present rate of uplift is about 1 m/100 years. Tukarak Island, Hudson Bay.

Goldilocks zone: The zone around a star in which orbiting planets maintain conditions for life to evolve. Earth is the primary example, where conditions are neither too hot or too cold.  The zone depends on the planet’s orbit (distance) the size of the star, and atmospheric greenhouse condtions. Exoplanets can be categorized as having the potential for life forms, according to these conditions.

Graben: A fault depression or basin formed between paired, normal dip-slip faults. The downward displaced hanging wall block is common to both faults. The adjacent upthrown blocks are horsts. They form in regions of extensional tectonics, such as rift zones. Cf. Half graben

Graded bedding: A depositional unit in which there is an upward decrease in grain size (normal grading). This structure is indicative of deposition from turbulent sediment-water mixtures; it is one of the defining characteristics of turbidites.

Grain flow: Sediment gravity flows consisting mostly of sand, in which the primary mechanism of grain support are dispersive pressures generated by grain-to-grain collisions. Maintenance of grain flows requires relatively steep slopes compared with debris flows and turbidity currents.

Grain size:      There are several ways to measure the size of clasts: grain diameters (maximum, intermediate, minimum), surface area, or by using the assumption that grains approximate spheres. Sieve sizes use the minimum diameter where a grain will pass through a particular mesh. The most popular scale is the Wentworth scale, based on a geometric progression of sizes, centred on silt and sand populations. An important modification of the Wentworth model is the Phi scale (Φ), calculated as Φ = -Log₂ of the grain size in millimetres.

Grain size distribution: A measure of the size distribution of particles in granular sediments. Measurement of the range of grain sizes in a sample provides data for detailed descriptions, calculation of population statistics such as mean, mode, and sorting, and may even provide clues to deciphering depositional hydraulics.

Grainstone: The cousin to siliciclastic sandstones, where sand-sized carbonate particles (<2 mm) form a grain-supported framework, relatively free of or carbonate mud (micrite).

Dunham’s (1962) limestone classification scheme reviewed and modified by Lockier and Junaibi (2016).

Gravel: The unconsolidated equivalent of conglomerate, comprising clasts coarser than 2 mm. The general class is divided into granules (2-4 mm; -1 to -2 phi); pebbles (4-64 mm; -2 to -6 phi); cobbles (64 – 256 mm; -6 to -8 phi); and boulders (larger than 256 mm).

Gravitational friction: Also called tidal friction. The gravitational pull on Earth by the Moon (principally) and to a lesser extent the Sun and other planets, causes minor distortion in Earth’s shape. This pull-push effect is enough  to generate frictional heating of Earth’s interior. The effect is also known to be important on other celestial bodies, particularly Jupiter’s moon  Europa and Saturn’s Enceladus, where the frictional heat may be sufficient to maintain a liquid ocean beneath its frozen surface. It is one of three heat sources on Earth – the other two are radiogenic heat and primordial heat.

Greenhouse effect: The heating of an atmosphere when gas molecules absorb certain frequencies of solar infrared energy. On Earth this involves water vapour, carbon dioxide, methane, and to a lesser extent nitrous oxide. Molecular oxygen and nitrogen do not absorb infrared energy. Carbon dioxide and water vapour absorb energy at different frequencies. Note that the amount of water vapour in the atmosphere depends on temperature, unlike carbon dioxide.

Greywacke: A muddy sandstone having 15-75% muddy matrix. Typically they are poorly sorted, and contain grains having variable roundness-angularity. It can be difficult distinguishing between original matrix (detrital) and diagenetic cements, particularly with the recrystallization of clays, the transformation of illite to white mica, and the addition of minerals like chlorite. Likewise, distinguishing between lithic fragments and diagenetically altered matrix can be difficult. Greywackes are commonly highly indurated.

Gridiron twinning: A common diagnostic twin in potassium feldspars twins that belongs exclusively to microcline. It is presented as a cross-hatching of thin albite and perthite twin lamellae. Also called tartan twins.

Groove casts: Straight to slightly arcuate furrows formed when objects are dragged across a soft sediment surface. The grooves are subsequently filled with sediment and exposed as casts at the base of the overlying bed. Groove casts provide ambiguous paleocurrent azimuths (180^o apart). They are common at the base of turbidites, and frequently accompany other sole marks like flute casts, roll and skip marks.

Groundwater: Water that resides in porous and permeable sediment and rock beneath the surface. The term applies equally to fresh and saline waters, in aquifers and aquitards at any depth in the crust. The term does not apply to chemically bound water, although such water may be released to groundwater during diagenesis and metamorphism. See also aquifer, hydraulic gradient.

Groundwater discharge: Natural discharge of groundwater as springs, seeps, or baseflow, at the surface or in streams, lakes, or the sea floor. Discharge occurs where the watertable (unconfined aquifers) or potentiometric surface (confined aquifers) intersect the land or water body surface, and the hydraulic gradient is sufficient to drive flow. Cf. Groundwater recharge.

Groundwater recharge: The infiltration of water from precipitation into an aquifer. For unconfined aquifers this recharge occurs at the watertable. For confined aquifers recharge occurs by slow seepage from the confining aquitards.

Groundwater residence time: The time from recharge (usually at the surface) to discharge. Residence times are briefest in unconfined aquifers, ranging from days to years. In regional groundwater flow systems these times are measured in 10⁵ to 10⁶ years. Groundwater dating utilises trace compounds such as fluorocarbons, isotopes like ³H (tritium from atmospheric atomic device testing), and cosmogenic isotopes such as Carbon-14, and Beryllium-10.

Growth fault: Faults having a listric geometry, that are active during sedimentation.  Stratigraphically they are represented by preferential sediment thickening above the downward displaced hanging wall block, and angular stratigraphic discordances. They are common on shelf, platform, and delta margins.

Grykes:  Elevated blocks of limestone bound by fracture networks, or clints. They are common in karst landscapes.

Gutter casts: Straight scours several centimetres deep, that in some cases may represent erosion downflow of objects on the depositional surface, and in others the product of helical flow or eddies. Like other sole marks, they present as casts on the base of an overlying bed.

Gypsum divide: The stage during evaporation of brines where gypsum precipitation determines the succession of minerals in waters subsequently depleted in Ca²⁺ and SO₄^2-.  It determines whether the brines evolve as SO₄ rich – Ca poor, or SO₄ poor  https://www.geological-digressions.com/mineralogy-of-evaporites-saline-lake-brines/

Half graben: A depression or basin formed by extension, and (usually) rotation of a hanging wall block above a single, normal dip-slip fault (cf. paired faults in grabens). They commonly form above listric faults in continental rift zones, but analogous structures also form in rotational slumps.

Half life: The time taken to reduce a quantity of a radioactive isotope by half. This means that after one half life, it is probable that 50% of the isotope will remain; after the second half life 25%, and so on. Isotope decay is exponential.

Halides: Anions of the Chemical Periodic Table halogen group: Fluoride F‾, chloride Cl‾, bromide Br‾, Iodide I‾, and astatide At‾. Many inorganic halides are water-soluble; most organic halides are not.

Halokinesis: Halokinesis, or salt tectonics, studies the movement of (stratiform) salt during burial, the kinds of diapiric structures that form, the response of the surrounding bedrock (such as faulting), their impact on depositional processes, and their influence on stratigraphic architecture.

Hanging wall: Displaced fault blocks across a dipping fault plane are described as hanging wall and foot wall blocks, depending on whether they are located above or below the fault plane. Hanging wall blocks lie above – if you are standing on the fault plane, the hanging wall block will hang over your head. The term derives from old mining jargon.

Hanging wall cut-off: The intersection point of a marker layer or bed in the hanging wall block that is truncated by a fault. It is used in conjunction with the corresponding foot wall cut-off to determine fault slip.

Hardness (mineralogy): A measure of a mineral resistance to scratching. The common Moh Scale has hardness gradations from 1 – the softest (e.g. talc), to 10 (diamond). The scale does not include mineral propensity for breakage (along cleavage planes). Common indicator minerals are (increasing hardness):

1 Talc          2 Gypsum        3 Calcite          4 Fluorite          5 Apatite          6 Orthoclase

7 Quartz          8 Topaz          9 Corundum          10 Diamond.

Hawaiian eruption: Effusive eruptions of fluid basaltic lava in lava lakes and associated flank fissures. VEI = 0-1. Flows are mostly pahoehoe type.

Head loss: Fluid flow through pipes or an aquifer will only take place if there is an energy gradient from one location to another. In hydraulics, this is represented by the change (decrease) in hydraulic head. If the head loss can be measured, then the local hydraulic gradient can be calculated.

Heat flow: The transfer of heat from Earth’s core and deep mantle to the surface, primarily by conduction and convection. It is expressed as milli-Watts per square metre (mWm^-2).

Heavy minerals: A broad group of minerals having specific gravities 2.9 and greater (quart and feldspar are 2.6), that make up <1% of most arenites; exceptions are iron sands (high proportions of magnetite and ilmenite) and some volcanic sands with abundant pyroxenes and olivines. Common groups include tourmalines, zircon, sphene, amphiboles, pyroxenes, micas, iron oxides, and other ferromagnesian minerals like olivine. They are important for assessing sediment provenance.  Zircon is important because it is both mechanically and chemically stable, and it contains measurable Uranium and lead isotopes making it useful for dating and tracking potential source rocks.

Hemipelagic sediment: Very fine-grained siliciclastic sediment (clays, silt) that is deposited from suspension in the ocean water column; it may be mixed with pelagic sediment. Hemipelagite tends to accumulate in relatively deep-water slope, rise, and ocean basins remote from strong bottom currents.

Herringbone crossbedding: Two crossbed sets, one above the other, where the foresets in each dip in opposite directions. The opposing foresets can be interpreted as representing tidal current asymmetry – one set formed during flood tide, the other during ebb tide. This structure is best identified where the 3-dimensional attributes of bedforms can be observed to avoid the ambiguity of apparent foreset dips.

Heterotroph: An organism that requires the assistance of other organisms to generate energy and food. In other words, they eat other heterotrophs and autotrophs. The group includes omnivores, herbivores, carnivores, and critters or plants that use decomposition processes.

Hiatus: (plural hiatuses). A stratigraphic surface that records a break in sedimentation. A hiatus can be any duration. A depositional hiatus may be a few minutes or 1000s of years. The hiatus recorded by angular unconformities can be many millions of years. The term was introduced by A.W. Grabau to describe the absence of a rock record between pulsations, or cycles. See also Lacuna.

Highstand systems tract (HST): Formed during the late stage of relative sea level rise and the beginning of sea level fall. Depositional systems that make up the HST can include fluvial, delta, barrier island – lagoon, coastal, and shelf deposits. The lower boundary is a maximum flooding surface. The upper boundary is a sequence boundary that includes the subaerial unconformity, and offshore the basal surface of forced regression (FSST). The HST underlies the FSST and overlies the TST.

Hinge line: A line defined by points of maximum curvature (hinge points) connected along a fold surface; the hinge line may be straight or curved. It is used with an axial surface to define the geometry of a fold. Cf. Fold axis. See also axial plane.

Hinterland: In collisional orogens, the mountainous region in the upper plate underlain primarily by metamorphic core complexes and crystalline rocks. The hinterland is separated from the cratonward foreland by a collisional suture zone. Hinterland lithosphere may be arc or continent related.

Hogback: A sharp-crested and symmetric ridge produced by differential erosion of resistant layers underlain by less resistant rocks. One side of the Hogback is a dip slope (dips > 25°-30°) underlain by resistant rock; The other side of the ridge is underlain by softer rock.

Holomict: Lakes or seas in which there is mixing of surface and deeper waters. Bottom waters tend to be oxygenated Cf. Meromict.

Horse: A panel of rock bound on all sides by thrusts. Duplexes are a stack of horses.

Horsetail splay: A splay of curved faults at the end of a strike-slip fault, where each strand merges with the master fault.

Horst: See Graben.

Hummocky cross-stratification: (HCS). Low relief, laminated, mound-like bedforms and intervening troughs or swales. Mound amplitudes are only a few centimetres, and intermound spacings of 2-4 metres. They are mostly fine-grained sandstone, but may contain basal pebble layers. Successive generations of HCS truncate underlying bedforms. HCS forms during storms where unidirectional flowing bottom currents, possibly as sediment gravity flows generated during storm surges, are simultaneously moulded by the oscillatory motion of large storm waves. They are good indicators of storm wave-base. Their preservation potential above storm wave-base (i.e. over the shoreface) is low.

Hyaloclastite: Fragmental volcaniclastics that form when lava is quenched rapidly under water, beneath ice, or in saturated sediment, such that is shatters into angular fragments. Hyaloclastitic debris is commonly glassy and highly angular, with straight, arcuate, or bubble texture margins.

Hyalotuff: Fragmental deposits formed by explosive, phreatomagmatic eruptions when magma comes into contact with seawater or groundwater. Ash particles are generally angular and commonly have bubble or vesicle textures. Cf. Hyaloclastites.

Hydraulic conductivity (hydrogeology):  This is the proportionality constant in Darcy’s Law. It has dimensions of length/time. Hence it is also called the Darcian velocity. It is a measure of the ease with which a fluid will flow through a porous medium. Importantly, it is a function of the porous medium and the fluid, particularly the fluid viscosity. This means that oil flowing through an aquifer will have a lower hydraulic conductivity than water through the same medium. Hydraulic conductivity is used in all hydrogeological studies. In contrast, the oil and gas industry uses a different proportionality constant – the Darcy, that depends only on the porous medium.

Hydraulic gradient (hydrogeology):  The change in hydraulic head from one location to another can be stated as a gradient, which is the head difference divided by the distance between the two locations. Gradients can also be calculated from contoured potentiometric surface maps. Groundwater always flows towards locations at lower head.

Hydraulic head (hydrogeology):  Also called hydraulic potential, is a measure of the potential energy available to drive groundwater flow. From Bernoulli’s equation, the total head is:

H_Total = h (the elevation head) + P (pressure head)/ρg

For which the dimensions are in units of length, or height/depth measured to some datum. The total head is the same anywhere along a line of equal potential (equipotential); however, the elevation and pressure head components change.

Hydraulic head – elevation head (hydrogeology):  If the point of measurement is the bottom of a borehole, then the elevation head is the depth from this point to the datum. It is a component of the total head measured at that point; the other component is the pressure head. The point of measurement can be anywhere along the line of the borehole. In most cases, this line will represent an equipotential.  For example, if the point of measurement was the watertable, then total head would be made up entirely of the elevation head; the pressure head would be zero.

Hydraulic head – pressure head (hydrogeology): If the point of measurement is the bottom of a borehole, then the pressure head is the depth from this point to the watertable or other equipotential surface. It is a component of the total head measured at that point; the other component is the elevation head. The point of measurement can be anywhere along the line of the borehole. For example, if the point of measurement was the watertable, then total head would be made up entirely of the elevation head; the pressure head would be zero.

Hydraulic jump: A region of turbulence that develops in channels when Upper Flow Regime conditions slow to Lower Flow Regime.

Hydraulic potential (hydrogeology): The statement of hydraulic potential derived from Bernoulli’s equation is a statement about the potential energy that drives groundwater flow. Mathematically this simplifies to potential energy E = ρgz + fluid pressure P (ignoring the kinetic energy component), where ρ = fluid density; g = gravity constant; z = depth relative to a datum. The more common expression for this is hydraulic head.

Hydraulics: A general term for the conditions promoting flow in water, air, and sediment-water mixtures, and the processes of sediment movement and deposition. Involves consideration of flow velocity, turbulence, laminar flow, frictional drag, and shear stress.

Hydrogeology: The study of subsurface fluids, particularly groundwater and its utilization,, aquifers and aquitards, fluid chemistry, its influence on rock strength and slope stability, its role in tectonics, hydrocarbon migration and trapping, and mineralization.

Hydrolysis: Also called dissociation. The reversible reaction where H₂0 splits into a hydrogen ion and a hydroxyl ion, as in H₂0 = H⁺ + OH^–. The equilibrium constant is written as:

K_w = (H⁺).(OH^–)/( H₂0). The activity of H₂0 is usually taken as 1.0, so that K_w = (H⁺).(OH^–). At 25ºC K= 10^-14.0 . Where the concentration, or activity of (H⁺) > (OH^–) is acidic, and (H⁺) < (OH^–) is basic. This is the basis for the pH scale, calculated as the -log₁₀  of the activities.

Hydrostatic pressure:  At any depth, the pressure exerted by a (theoretical) overlying column of water having unit-area cross-section, is calculated from the expression P = ρgz where ρ = density of water, g = gravity constant, and z = depth from some datum, commonly sea level. Note that, assuming a cross-section of unit-area reduces volume to units of depth. It is analogous to lithostatic pressure.

Hyperconcentrated flow: Sediment laden flows that behave mechanically between two end-member flow types: normal stream flow with little or no suspended sediment load, and debris flows having high matrix content. Hyperconcentrated flows have no yield strength, like water, but do have a viscosity that depends on strain rate. Rheologically, they behave somewhere between a Newtonian fluid and a plastic (or hydroplastic). A typical example is a mud-laden river flood. https://www.geological-digressions.com/sedimentary-structures-alluvial-fans/

Hypersaline: Having salinity greater than seawater (>35 parts/1000). Modern hypersaline environments are most common between the tropics but are found in such diverse places as the Antarctic dry valleys. Plant and animal life require specialized adaptations to survive these conditions. Prolonged hypersalinity may result in evaporite deposits in lakes and seas.