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:
- Uniaxial crosses that do not break up or rotate as the stage is rotated.
- 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 <30o. 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 60o-80oC, 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.
