Category: Geology Dictionary

Hot spring : A natural spring resulting from the emergence of geothermally heated groundwater from beneath the Earth’s crust.

Horst : A raised fault block bounded by normal faults.

Hornfels : A hard, compact, fine-textured contact-altered argillaceous rock that breaks into splintery fragments.

Hornblende : An important rock-forming mineral of complex composition, essentially a silicate of calcium, magnesium and iron, with smaller amounts of potash, soda and hydroxyl. Hornblende crystallises in the monoclinic system and occurs as black crystals or grains in many different types of igneous and metamorphic rocks, including hornblende-granite, syenite, diorite, andesite, hornblende-schist, and amphibole.

Holocrystalline : Having the properties of those igneous rocks in which all of the components are crystalline; glass is absent. Contrast hemicrystalline.

Hinge line : A line joining the points of maximum curvature along the hinge of a fold.

Hinge :The zone of maximum curvature of a fold.

Hemicrystalline : Having the properties of certain rocks of igneous origin which contain some interstitial glass in addition to crystalline minerals. Contrast holocrystalline.

Half-life : The time it takes for a given amount of a radioactive isotope to be reduced by one-half.

Habit : See crystal habit.

Grus : Freshly eroded, angular grains of quartz and feldspar derived from a granitoid.

Greenstone : An omnibus term lacking precision and applied indiscriminately to basic and intermediate igneous rocks of Lower Paleozoic age in which much chlorite has been produced at the expense of the original coloured minerals, staining the rocks green.

Graywacke : A variety of sandstone generally characterized by its hardness, dark colour, and poorly sorted, angular grains of quartz, feldspar, and small rock fragments (lithic fragments) set in a compact, clay-fine matrix. Also spelled greywacke.

Granodiorite : An intrusive, felsic, igneous rock similar to granite but containing more plagioclase than potassium feldspar. Dacite is the volcanic equivalent.

Granoblastic : An arrangement of mineral grains in a rock of metamorphic origin similar to that of a normal granite, but produced by recrystallisation in the solid and not by crystallisation from a molten condition.

Granitoid : Any granite-like rock, including granodiorite, diorite, monzonite, and granite itself, among others.

Granite : A coarse-grained, often porphyritic, intrusive, felsic, igneous rock containing megascopic quartz, averaging 25%, much feldspar (orthoclase, microcline, sodic plagioclase) and mica or other coloured minerals. Rhyolite is the volcanic equivalent.

Graben : A depressed block of the Earth’s crust bordered by parallel faults.

Gondwanaland : The southern part of the supercontinent of Pangaea which eventually separated to form present-day South America, Africa, India, Australia and Antarctica.

Gneiss : A coarse-grained, pale-coloured gneissose rock, containing abundant feldspar with quartz, mica, hornblende, and garnet.

Glauconite : A green-coloured, hydrated silicate mineral of potassium and iron that forms on submerged banks. Its occurrence in sands and sandstones is considered an indication of accumulation under marine conditions.

Glass : A hard, brittle, transparent solid, such as used for windows, many bottles, or eyewear, including soda-lime glass, acrylic glass, sugar glass, isinglass (Muscovy-glass), or aluminium oxynitride.

Geosyncline : A mobile down-warping of the Earth’s crust, either elongate or basin-like, measured in scores of kilometres, which is subsiding as sedimentary and volcanic rocks accumulate to thicknesses of thousands of metres.

Geological time scale : Also geologic time scale.

Geologic map : A special-purpose map made to show geological features.

Gemology :Also spelled gemmology.

Gastrolith : A rock which is or was once held inside the digestive tract of a living animal.

Gabbro : A dark, coarse-grained, intrusive igneous rock chemically equivalent to basalt.

Fracture : Any crack or discontinuity. In its geological definition, it is only used when no displacement can be distinguished.

Fossiliferous : Bearing or being composed of fossils in rocks or strata.

Fossil : Any mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, or other once-living organisms.

Foliation : The parallel alignment of textural and structural features of a rock.

Fold : A stack of originally flat and planar surfaces, such as sedimentary strata, which have become bent or curved as a result of plastic (i.e. permanent) deformation.

Fission track dating : A method that uses tracks that are visible under the microscope to date minerals.

Ferromagnesian mineral : Any iron/magnesium-bearing mineral, such as augite, hornblende, olivine, or biotite.

Ferricrete : A conglomerate consisting of surficial sand and gravel cemented into a hard mass by iron oxide derived from oxidation of percolating solution of iron salts. A ferruginous duricrust.

Felsic : Silicate minerals, magmas, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Light minerals (quartz and feldspar) greater than 60% – acid. [Granite (Rhyolite), Adamellite (Rhyo-dacite), Granodiorite (Dacite)]. The term is a mnemonic adjective for igneous rocks having light-coloured minerals in their mode, from “feldspar” and “silica”. Contrast mafic (Geology Glossary).

Fels : A massive metamorphic rock lacking schistosity or foliation.

Feldspar : Any of a set of the most common minerals in the Earth’s crust. All feldspars contain silicon, aluminium, and oxygen and may contain potassium, calcium and sodium.

Fault zone : The zone where exist different discrete fault planes.

Fault : A discrete planar rock fracture which shows evidence of a displacement (the throw of the fault). A fault is a discrete surface.

Fanning : Rock deformation related to shear stress.

Facies : The sum of the lithological and faunal characters of a sediment is its facies. Lithological facies involves composition, grain size, texture, colour, as well as such mass characteristics as current bedding, nature of stratification, ripple marks, etc. Similarly, metamorphic facies involves the degree of crystallisation and the mineral assemblage in a group of metamorphic rocks.

Extrusive : A mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff.

Extension : Strain involving an increase in length. Extension can cause thinning and faulting.

Exfoliation : The stripping of concentric rock slabs from the outer surface of a rock mass.

Evaporite : Any of a diverse set of water-soluble mineral sediments that result from the evaporation of bodies of surface water.

Eustatic movements : Changes of sea level, constant over wide areas, due to alterations in the volume of the oceans resulting from the formation or melting of ice caps.

Euhedral : Bounded by the crystal faces peculiar to the species, used of minerals. Synonymous with idiomorphic.

Eugeosyncline : A geosyncline in which volcanism is associated with clastic sedimentation; the volcanic part of an orthosyncline located away from the craton.

Estuary : A semi-enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea.

Esker : A long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North America. Eskers are frequently several miles in length and, because of their peculiar uniform shape, somewhat resemble railroad embankments.

Escarpment : A transition zone between different physiogeographic provinces that involves an elevation differential, characterized by a cliff or steep slope.

Erratic : A piece of rock that deviates from the size and type of rock native to the area in which it rests. Erratics usually occur as stones ranging in size from pebbles to large boulders which were transported by glacial ice, which upon melting left them stranded far from their original source. The name “erratic” is based on the errant location of these boulders.

Erosion : The displacement of solids (sediment, soil, rock and other particles) usually by the agents of currents such as wind, water, or ice by downward or down-slope movement in response to gravity or by living organisms (in the case of bioerosion).

Epoch : A division of the standard geologic time scale subordinate to periods. An example is the Pleistocene Epoch of the Quaternary Period.

Epirogenetic : The simultaneous rising and falling movements of continents, maintaining isostasy.

Epidiorite : Any of a set of altered gabbroic and doleritic rocks in which the original pyroxene has been replaced by fibrous amphibole. The rock may be regarded as a first step in the conversion by dynamothermal metamorphism of a basic igneous rock into a green schist.

Epicenter : The point on the Earth’s surface that is directly above the hypocenter or focus at which an earthquake or underground explosion originates.

Eon :The largest unit of geologic time.

Eemian transgression : The portion of the Late Pleistocene spanning the period between 120 Ka and 8m before present.

Eclogite : A generally coarse- to medium-grained pyroxene in which are set red garnets. The colour is pistachio green when fresh, but mottled with red when weathered.

Duricrust : A general term for hard crust existing as a layer in or on the surface of the upper horizons of a soil in semi-arid climates. Duricrust is formed by the accumulation of solid minerals deposited by water moving upwards by capillary action and evaporating in the dry season. Compare hardpan.

Dunite : An ultrabasic plutonic rock in which the mafic material is almost entirely olivine, with accessory chromite almost always present. Feldspar mainly plagioclase. See also peridotite.

Drumlin : An elongated, whale-shaped hill formed by glacial action.

Drill core : A drill specifically designed to remove a cylinder of material, much like a hole saw.

Dolomite : A sedimentary carbonate rock and mineral, both composed of crystalline calcium magnesium carbonate CaMg(CO₃)₂.

Divergent boundary : The boundary separating two tectonic plates that are moving away from each other. Contrast convergent boundary.

Disconformity : A surface that represents missing rock strata but beds above and below that surface are parallel to one another.

Dip slope : A geological formation often created by erosion of tilted strata.

Dike: Also spelled dyke.

A type of sheet intrusion referring to any geologic body that cuts discordantly across. A form of minor intrusion injected into the crust during its subjection to tension, the dyke being thin with parallel sides, and maintaining a constant direction in some cases for long distances. Some are more resistant to weathering than the surrounding rock and stand up like walls, while others weather faster and form long narrow depressions.

Detachment fault : A major fault in a mountain belt above which rocks have been intensely folded or faulted.

Deposition : The geological process by which material is added to a landform or landmass.

Dendrite : A crystal that develops with a typical multi-branching tree-like form.

Degradation : The lowering of a fluvial surface, such as a stream bed or floodplain, through erosional processes.

Delta : A landform where the mouth of a river flows into an ocean, sea, desert, estuary, lake or another river.

Daughter product : Any distinct isotope produced by the radioactive decay of an atomic nucleus.

Dacite : An igneous, volcanic rock with a high iron content. It is an extrusive rock of the same general composition as andesite, but a less calcic feldspar. Synonymous with quartz andesite.

Crust : The outermost solid layer of a planet or moon, referring especially to the Earth’s crust.

Crude oil : A liquid mixture of naturally occurring hydrocarbons.

Cross-bedding : An inclined sedimentary structure in a horizontal unit of rock. Such tilted structures indicate the type of depositional environment, not post-depositional deformation.

Craton : An old and stable part of the continental crust that has survived the merging and splitting of continents and supercontinents for at least 500 million years.

Country rock : The rock native to an area, as opposed to rock that formed elsewhere and was later transported to the area.

Corestone : An ellipsoidal or broadly rectangular joint block of granite formed by subsurface weathering in the same manner as a tor but entirely separated from bedrock.

Core : The innermost layer(s) of a planet, referring especially to the Earth’s core.

Cordierite : A silicate of aluminium, iron and magnesium with water, which crystallises in the orthorhombic system and occurs mainly in metamorphic rocks.

Coprolite : A fossilized specimen of human or animal dung.

Copal : A type of resin produced by plant or tree secretions, particularly identified with the forms of aromatic tree resins used by the cultures of pre-Columbian Mesoamerica as a ceremonially burned incense, as well as for a number of other purposes.

Convergent boundary : The boundary between two tectonic plates that are moving toward each other. Contrast divergent boundary.

Continental shelf : Extended perimeter of a continent and its associated coastal plain, which is covered, during interglacial periods (such as the current epoch), by gulfs, and relatively shallow seas known as shelf seas.

Continental margin : Zone of the ocean floor, separating the thin oceanic crust from thicker continental crust.

Continental crust : The layer of granitic, sedimentary, and metamorphic rocks which form the parts of the Earth’s crust that comprise the continents, and the areas of shallow seabed close to their shores (known as continental shelves).

Contact metamorphism : Metamorphism due to the local heating of rocks by the intrusion of magma nearby.

Conglomerate : Any type of rock consisting of individual stones that have become cemented together.

Concretion : A volume of sedimentary rock in which a mineral cement fills the porosity (i.e. the spaces between the sediment grains).

Conchoidal : A type of fracture that results in smoothly curved surface faces.

Compression : The process by which rocks shorten or decrease in volume when exposed to certain forces.

Compaction : The process by which a newly deposited sediment progressively loses its original water content due to the effects of loading. This forms part of the process of lithification.

Coccolithophore : Also called a coccolithophorid.

A type of microfossil of single-celled algae, protists and phytoplankton belonging to the division of haptophytes. These fossils are distinguished by special calcium carbonate plates called coccoliths.

Coccolith : An individual plate of calcium carbonate formed by coccolithophores which are arranged around them in a so-called coccosphere.