Historical Geology/Glossary and index< Historical Geology
100,000 year problemEdit
The question of why over the last million years, climatic variation has been driven by the 100,000 year Milankovitch cycle rather than the 41,000 year Milankovitch cycle. Article: Milankovitch cycles.
Dating methods which tell us how old a rock or fossil is, as opposed to relative dating. Articles: Concepts in absolute dating, Erosion, deposition, and time, Dendrochronology, Varves, Amino acid dating, Radioactive decay, K-Ar dating, Ar-Ar dating, Rb-Sr dating, Other isochron methods, U-Pb, Pb-Pb, and fission track dating, Radiocarbon dating, Cosmogenic surface dating, U-Th, U-Pa, and Ra-Pb dating, Paleomagnetic dating, Sclerochronology, Tidal rhythmites and dating, Fossils and absolute dating, Absolute dating: an overview
Having to do with the wind. Article: Deserts.
Amino acid datingEdit
Lacking a crystal structure. Article: Minerals.
Angle of reposeEdit
In geology, the term "anomaly" means a measurement at some place of some quantity which is different from the average or background value for that quantity.This should not be confused with the usage of the term "anomaly" in the philosophy of science, where it means a measurement or observation which cannot be reconciled with current theory. In geology, the term has no such implication. Article: Sea floor spreading.
A river which is present before the uplift of the hills through which it flows. Article: Rivers.
Structure formed when rocks are folded upwards. Article: Folds.
A rounded dune-like structure found in rivers of the right velocity and having a sandy bottom. Because they erode by the transport of sand grains from the lee side of one antidune to the stoss side of the next, the net effect is that while the sand moves downstream, the antidunes move upstream. Article: Rivers.
An upward fold in rocks. Article: Folds.
An igneous rock is said to be aphanitic if the crystals in it are too small to be seen with the naked eye. In this textbook I have tended to use the more straightforward term "fine-grained". Article: Igneous rocks.
Apparent polar wanderEdit
Having to do with mud. May be used to qualify the nature of a rock, e.g. argillaceous sandstone would be sandstone with a significant amount of mud mixed in with the sand. Article: Sedimentary rocks.
Having to do with sand. May be used to qualify the nature of a rock, e.g. arenaceous mudrock would be mudrock with a significant amount of sand mixed in with the mud. Article: Sedimentary rocks.
Alternative term for sandstone. Article: Sedimentary rocks.
Atmospheric circulation modelEdit
That part of a beach which is above the high-water line. Article: Nearshore sediments.
Banded iron formationEdit
Beta minus decayEdit
Beta plus decayEdit
A stream or river in which the current repeatedly splits into smaller streams which merge back together and then split again, and so forth. Article: Rivers.
A mineral consisting of calcium carbonate in a trigonal crystal system.
Carbonate compensation depthEdit
The acid H2CO3. Although this is a very weak acid, it is extremely common, because it can be formed from the reaction between carbon dioxide and water. Because of this, it plays an important role in chemical weathering. Article: Chemical weathering.
A silicate mineral in which the silicate tetrahedra are bonded together in the form of a chain, i.e. each tetrahedron is attached to just two other tetrahedra (except, of course, at each end of the chain). Article: Silicate minerals.
A chemical sediment is one deposited by precipitation rather than by mechanical processes such as wind or water; or by biological processes such as the growth of coral. Note however that some authors will include biological processes as a subcategory of chemical processes; our articles do not follow this usage. Article: Sedimentary rocks.
Weathering caused by chemical processes (most commonly by some or all of the constituent minerals of a rock being dissolved by carbonic acid); as opposed to mechanical weathering. Article: Chemical weathering.
The handedness of an organic molecule. Article: Amino acid dating.
The term clay can either, depending on the context, refer to a class of sheet aluminosilicate minerals, or to clasts with a diameter of less than 1/256 mm. As clay in the second sense is usually also clay in the first sense, this causes less confusion than you might think. Article: Sedimentary rocks.
Broad trends in the weather; i.e. the tendency of a location to be hot and humid, or dry and cold. Article: Paleoclimatology: introduction.
The point at which snow has been so far compacted into ice that the air trapped in it is completely sealed off from the atmosphere. Article: Ice cores.
A conglomerate is a rock consisting of large clasts (pebble-sized or larger) cemented together; it is common usage (which we have followed in this text) to use the term to imply that the clasts are rounded, as distinct from a breccia. Article: Sedimentary rocks.
The innermost 3,400 km of the Earth, composed mainly of iron. Article: Structure of the Earth.
A sample of ice or rock recovered from the Earth's crust by drilling. Article: Ice cores.
Dust fallen from outer space, i.e. micrometorites. Although they can be found in pretty much all kinds of sediment, they are proportionally most abundant in pelagic clay due to its slow rate of deposition. Article: Pelagic clay.
Streams of high-energy particles which bombard the Earth from outer space. Article: Cosmogenic surface dating.
Cosmogenic surface datingEdit
Bedding in which the beds, instead of being deposited horizontally, are deposited at an angle, as a result of deposition by a current of wind or water; in the simplest case, where the current has a continuous direction, the beds will have a downward slope in the direction of the current. Articles: Sedimentary rocks, Deserts, Rivers, Deltas, Nearshore sediments, Way-up structures, Paleocurrents.
The correlation of dates from different sources. Article: Dendrochronology.
The upper layer of the Earth, varying from about 5 - 50 km thick, distinct from the mantle by having a different chemical composition, being composed of less dense and more felsic rocks. Article: Structure of the Earth.
Very roughly speaking, the temperature above which a material cannot be magnetized and below which it can. Article: Geomagnetic reversals.
The erosion of fine particles from dry soil by the wind. Article: Deserts.
An area of exceptionally low rainfall. Note that although the stereotypical desert is hot and sandy, in geological terms a desert is defined solely by a shortage of rain or snow. Article: Deserts.
Diamond anvil cellEdit
A sedimentary structure formed by one type of sediment flowing upwards through another as a result of pressure. Article: Way-up structures.
A substance (a solute) is said to be dissolved in another substance (a solvent) if it is mixed with it in such a way as to acquire the phase of the solvent. Articles: Chemistry for geologists, Chemical weathering.
Occurring once daily. Article: Tidal rhythmites and dating.
A material is said to be elastic if it recovers from stress: that is, if when the stress is removed it returns to its original conformation. The opposite of plastic. Article: Physical properties of rocks.
Atoms are classified into elements according to their atomic numbers, which determine their chemical properties; this is a broader classification then the division into isotopes, which also takes into account their atomic weights. Articles: Chemistry for geologists, Radioactive decay.
Molecules which are mirror images of one another. Article: Amino acid dating.
The point on the Earth's surface directly above the focus of an earthquake. Article: Seismic waves.
Any chemical sedimentary rock the precipitation of which was produced by the partial or complete evaporation of the water containing the dissolved minerals or which the rock is composed. Articles: Sedimentary rocks, Deserts, Saline giants.
In biology, heritable change in a line of descent. Outside of biology, the term may be used colloquially to refer to any sort of change or development, as in (for example) "the evolution of jazz from ragtime". Article: Principle of faunal succession.
A quantity is said to undergo exponential decay if its magnitude as a function of time t can be expressed in the form ab-ct. Article: Radioactive decay.
Any igneous rock formed by lava pouring out on the surface (where the "surface" includes on the sea floor, under a glacier, or anywhere except under rock) as opposed to intrusive rock, which remains trapped within the country rock into which it intrudes. Extrusive rock can be distinguished from intrusive rock by its larger crystal size. Article: Igneous rocks.
Animals (in the broadest possible sense, including birds, fish, crustaceans, molluscs, etc). Article: Principle of faunal succession.
Fission track datingEdit
A sedimentary structure formed when a denser sediment (typically sand) is deposited on top of a less dense sediment (typically mud) which then penetrates it by seeping upwards; hence, a kind of small diapir. Article: Way-up structures.
Having to do with rivers. Article: Rivers.
The point in the Earth at which an earthquake originates. Article: Seismic waves.
The arrangement of sheet silicates in parallel planes in some metamorphic rocks, due to pressure causing realignment of the sheets in planes at right angles to the direction of pressure. Article: Metamorphic rocks.
Organic remains, or traces of organic activity such as footprints, preserved in the geological record. Article: Fossils.
General circulation modelEdit
Global Positioning SystemEdit
In chemistry, elements which lie in the same column of the periodic table, with similar chemical properties as a result of having similar situations in their outer electron shells. Article: Chemistry for geologists.
Having to do with hot water. Article: Metamorphic rocks.
Rock formed by the cooling of lava (in which case the rock is said to be extrusive) or magma (in which case the rock is said to be intrusive). igneous rocks can also be classified by their mineral composition from felsic to ultramafic. Article: Igneous rocks.
A fossil of a species that was sufficiently widely distributed that its fossils can be used to correlate the deposition of fossils and sediments in widely separated locations. Articles: Index fossils, Fossils and absolute dating.
Any mineral which forms only at certain temperatures and pressures, and which can therefore be used as an index to the conditions under which certain metamorphic rocks were formed. Article: Metamorphic rocks.
A complex pattern of sediments in which different sedimentary types (e.g. sand and mud) interpenetrate in interlocking wedges broadly similar to the pattern made by the fingers of two hands laced together. Article: Deltas.
Rock formed by magma penetrating country rock but not reaching the surface as lava. As the magma will cool slowly, intrusive rock can be distinguished from extrusive rock by the relatively large size of the crystals of which the former is composed. Such rock is said to intrude into the country rock. Article: Igneous rocks.
A small lake formed by glacial outwash being deposited around a largish chunk of ice left behind by a retreating glacier; when the residual chunk of ice melts, this leaves a depression which will typically fill with water, producing a kettle. Article: Glaciers.
An inland body of water fed by rivers, streams, or sometimes by seepage of groundwater. Article: Lakes.
Trees the growth of which we would expect to be limited by a single factor (such as temperature) because they grow in an environment with an abundant supply of other factors necessary for growth (such as rainfall). Article: Dendroclimatology.
The arrangement of chain silicates in parallel lines in certain metamorphic rocks formed under pressure: the pressure forces these silicates to orientate themselves at right-angles to the direction of pressure. Article: Metamorphic rocks.
Having to do with the coast. Article: Index fossils.
Molten rock which has not reached the surface, as opposed to lava. Some authors will define magma as any molten rock, in which case it would be proper to say that "lava is magma on the surface". However, in this text I have preferred the usage which makes magma and lava two distinct non-overlapping categories of molten rock. Article: Igneous rocks.
Rock which does not display bedding (in the case of sedimentary rocks) or foliation (in the case of metamorphic rocks), giving the rock a uniform and homogeneous appearance. This term is not used in our articles, so as to avoid confusion with the common use of "massive" to mean "very big"; we have instead used more transparent terms such as "unbedded". Article: Sedimentary rocks.
A broad loop in a stream or river. Article: Rivers.
Periodic changes in the inclination of the Earth's axis and the shape of its orbit. Article: Milankovitch cycles.
The zone in which the sea bed is affected by waves. Article: Nearshore sediments.
The polarity of the Earth's magnetic field as it is at present. (Note that there is nothing particularly normal about this state of affairs.) The opposite of reversed polarity. Article: Geomagnetic reversals.
A small roughly spherical particle consisting of calcium carbonate layers formed around a nucleus of sand or shell. Required the action of waves for formation, and is therefore formed in shallow seas. Article: Ooids and oolite.
Opal compensation depthEdit
A section of oceanic crust which has been thrust up above sea-level. Article: Ophiolites.
Abbreviation for pascals.
Unit of stress: 1 pascal = 1 newton/square meter. Article: Physical properties of rocks.
Waterlogged and partially decomposed vegetable matter. Note that in geological usage peat does not just refer to gardeners' peat (formed from sphagnum moss) but to any vegetable matter that has undergone peatification. Peat is the sediment from which coal is formed. Article: Peat and coal.
The process of turning sediment into soil by chemical weathering and the activity of organisms (plants growing in it, burrowing animals such as worms, the addition of humus etc). Article: Soils and paleosols.
A tabular arrangement of the elements which gives insight into their chemical properties. Article: Chemistry for geologists.
The rejection a priori of the existence of the supernatural; a position completely unnecessary to the practice of geology. Article: Actualism.
A material is said to be plastic if it does not recover from stress: that is, having been squeezed by stress into a given form, it retains that form when the stress is removed. The opposite of elastic. Article: Physical properties of rocks.
Principle of cross-cutting relationshipsEdit
The principle that when one geological feature cuts through another, the former is the younger and the latter is the older of the two features. Article: Cross-cutting relationships.
Principle of faunal successionEdit
Roughly speaking, the principle that if the fauna and flora in one location are found in one stratigraphic order, the same species will not be found in a different order in another location. Article: Principle of faunal succession.
Principle of least timeEdit
The principle in physics that a wave traveling through a medium will take the quickest route between two points. Article: Seismic waves.
Principle of original continuityEdit
The principle that when sediment is laid down, it will extend continuously until either it meets an obstacle or tapers off with increasing distance from the source of the sediment. Article: Steno's principles.
Principle of original horizontalityEdit
Principle of superpositionEdit
A quantity which we can measure which bears a known relationship to a quantity that we can't measure but would like to; for example measuring past oxygen isotope ratios in shellfish as a substitute for measuring past temperatures. Articles: Paleoclimatology: introduction, Leaf shape and temperature, Scleroclimatology, Uk'37, TEX86.
A mineral consisting entirely of silicate tetrahedra in a lattice structure, so that each oxygen atom of each tetrahedron is shared with one other tetrahedron, giving quartz the chemical formula SiO2 Articles: Silicate minerals, Igneous rocks.
A collection of methods of absolute dating which depend on the constancy of radioactive decay rates. Articles: K-Ar dating, Ar-Ar dating, Rb-Sr dating, Other isochron methods, U-Pb, Pb-Pb, and fission track dating, Radiocarbon dating, Cosmogenic surface dating, U-Th, U-Pa, and Ra-Pb dating, Absolute dating: an overview.
An underwater ridge or mound formed from the calcareous shells of organisms (typically coral in the present day, but the term is not restricted to coral reefs). Note that the geological usage is more restricted than the nautical usage, in which a sandbar or rock sufficiently near the surface of the water to cause a hazard to shipping would also be considered a reef. Article: Reefs.
The change of direction undergone by a wave when it passes from a material which permits travel at one speed to a material which permits travel at another speed. A consequence of the principle of least time. Article: Seismic waves.
Dating methods which allow us to put fossils and/or rocks in order of age, but without telling us how old they are, as opposed to absolute dating, which does. Articles: Steno's principles, Principle of faunal succession, Index fossils, Geological column, Cross-cutting relationships, Igneous rocks and stratigraphy.
A clast is said to be rounded if its sharp edges and corners have been worn away by erosion. Note that the term does not imply that the clast in question is spherical or near-spherical, just that its shape is smooth. Article: Sedimentary rocks.
The motion of a wind-blown or water-borne clast along the ground, river bed, sea bed, etc, by a series of short hops, when the particle is too large and the current too weak for it to be transported in suspension. Articles: Mechanical weathering and erosion, Rivers.
An accumulation of minerals on dry land by the evaporation of water containing dissolved minerals. While the commonest mineral in salt flats is indeed rock salt, other minerals such a gypsum may be deposited. Article: Deserts.
Satelite Laser RangingEdit
A system in which ground-based observation stations measure the round-trip time of ultrashort pulses of light traveling to and from satelites. Used by geologists to measure plate motion and isostatic rebound. Article: Continental drift.
Sea floor spreadingEdit
A marine mountain which is entirely underwater. Article: Hotspots.
The wandering of the magnetic poles over time. Article: Geomagnetic reversals.
A device for detecting earthquakes and measuring their properties. Article: Seismic waves.
Occurring twice daily. Article: Tidal rhythmites and dating.
The ion SiO44-, consisting of four oxygen atoms arranged around a silicon atom in a tetrahedron. Such units can link together with each other by sharing oxygen atoms at their corners to form a variety of structures including sheet silicates, chain silicates and quartz. Article: Silicate minerals.
A mountain habitat which is home to species which are isolated by their inability to cross the drier hotter surrounding plain. Article: Ice ages.
Abbreviation for Satelite Laser Ranging. Article: Continental drift.
A law relating the density of two mediums to the angle of refraction undergone by a wave when it passes from one medium to the other. Article: Seismic wave.
Sediment which has been altered by the effects of chemical weathering and and the activity of organisms (plants growing in it, burrowing animals such as worms, the addition of humus etc). Article: Soils and paleosols.
A feature in a cave, such as a stalactite or stalagmite, formed by the precipitation of dissolved minerals, typically calcium carbonate. Articles: U-Th, U-Pa, and Ra-Pb dating, U-Pb, Pb-Pb, and fission track dating.
A body (in this textbook, invariably the Earth) is said to be spherically symmetric with respect to some property if the value of that property at any given point in it depends only on the distance of that point from the center, and not on the longitude and latitude of the point. Article: Structure of the Earth.
The study of strata, in particular their order of deposition. Articles: Actualism, Steno's principles, Way-up structures, Fossils, Principle of faunal succession, Index fossils, Unconformities, Faults, Folds, Geological column, Walther's principle, Cross-cutting relationships, Igneous rocks and stratigraphy
The force per unit area exerted on a surface of a deformable body; also by extension the external pressure which creates the internal force. Article: Physical properties of rocks.
Synonym for superposed river. Article: Rivers.
The line along which a continent becomes joined to another continent, microcontinent, or island arc. Article: Terranes.
Body waves consisting of waves of shear: that is, of displacement at right angles to the direction of travel of the wave, resembling the waves produced by shaking the end of a rope. Article: Seismic waves.
Structure formed when rocks are folded downwards. Article: Folds.
Having an origin on land. Article: Marine sediments.
Physical characteristics of a rock including crystal size (in igneous or metamorphic rocks), and clast size and the degree of sorting and rounding of clasts (in sedimentary rocks). Articles: Igneous rocks, Sedimentary rocks, Metamorphic rocks.
Theory of evolutionEdit
The explanation of the facts of evolution in terms of such mechanisms as mutatation, recombination, lateral gene transfer, genetic drift, and natural selection. The explanation for the principle of faunal succession. Article: Principle of faunal succession.
Deep-water circulation driven by density differences in the temperature and salinity of sea water. Article: Climate models.
A relation is said to be transitive if when A stands in that relation to B, and B stands in that relation to C, then A stands in that relation to C. For example, the relation "is smaller than" is a transitive relation: if A is smaller than B, and B is smaller than C, then A is smaller than C. Article: Principle of faunal succession.
A sea-wave caused by any high-intensity, short-duration submarine event, most usually an earthquake. Often colloquially and completely inaccurately known as a "tidal wave". Article: Actualism.
Upper plane bedEdit
A glacier which has its accumulation zone on a mountain (typically in a cirque) and which flows down through valleys under both gravity and its own pressure; as distinct from a continental glacier. Article: Glaciers.
Very Long Baseline InterferometryEdit
A technique in astronomy involving widely separated radio telescopes observing the same object, such as a quasar. Used by geologists to measure the motion of tectonic plates by inferring the motion of the radio telescopes necessary to account for the data. Article: Continental drift.
Informally speaking, the reluctance of a liquid to flow; so for example maple syrup is more viscous than water. Article: Igneous rocks.
The principle that if sediment A is succeeded vertically by sediment B without an unconformity between them, then sediment A will also be succeeded horizontally by sediment B in some direction. Article: Walther's principle.
The greatest depth at which the action of a wave has any effect. Article: Nearshore sediments.