Historical Geology/Glossary and index

100,000 year problem
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/.

Aa
A type of lava flow, or the cooled and solidified rock produced by it, characterized by the rough jagged surface of the resulting rock. Article: ../Way-up structures/.

Ablation zone
The end of a glacier; the point at which loss of ice by melting exceeds the supply of ice by the movement of the glacier. Articles: ../Glaciers/, ../Glacial marine sediment/, ../Ice ages/.

Abrasion
Erosion of rocks caused by the sediments carried by wind or water. Article: ../Mechanical weathering and erosion/.

Absolute dating
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/

Abyssal plain
The flat terrain found at the bottom of the ocean beyond the continental margin. Article: ../Marine sediments/.

Accretionary prism
An accumulation of sediment which forms in a trench. Article: ../Subduction/.

Accretionary wedge
A synonym for accretionary prism. Article: ../Subduction/.

Accumulation zone
The beginning of a glacier; the zone in which snowfall exceeds the loss of snow by melting or evaporation. Article: ../Glaciers/.

Acidic rock
An outdated and inaccurate term for felsic rock. Article: ../Igneous rocks/.

ACM
Abbreviation for atmospheric circulation model. Article: ../Climate models/.

Actualism
The observation that the geological record can be explained in terms of the sort of geological processes that actually happen. Articles: ../Actualism/, ../Steno's principles/.

Aeolian
Having to do with the wind. Article: ../Deserts/.

Aeolian sandstone
Sandstone formed from sand deposited by the wind, i.e. desert sand. Article: ../Deserts/.

Al
Chemical symbol for the element aluminum. Article: ../Chemistry for geologists/.

Alkenones
Organic molecules produced by certain planktonic organisms, used in the temperature proxy known as Uk'37. Article: Uk'37.

Alluvial fan
A fan-shaped deposit of sediment left where a mountain stream reaches a plain. Articles: ../Deserts/, ../Rivers/.

Alpha decay
Radioactive decay involving the emission of an alpha particle. Article: ../Radioactive decay/.

Alpha particle
A particle consisting of two protons and two neutrons. Article: ../Radioactive decay/.

Aluminosilicate
Any of an important class of silicate minerals in which the structure involves aluminum-based as well as silicon-based tetrahedra. Article: ../Minerals/.

Alpine glacier
Synonym for valley glacier. Article: ../Glaciers/.

Amber
A mineraloid formed from the solidified resin of trees. Article: ../Fossils/.

Amino acid dating
A rather unreliable method of absolute dating based on measuring the racemization of organic remains. Article: ../Amino acid dating/.

Amorphous
Lacking a crystal structure. Article: ../Minerals/.

Ammonite
An extinct marine mollusc. Articles: ../Geological column/, ../Index fossils/.

Amphibole
A group of silicate minerals in which the SiO44- units are bonded to form a double chain. Article: ../Silicate minerals/, ../Igneous rocks/.

Angle of repose
The maximum angle from the horizontal that a heap of a given type of sediment can assume without collapsing. Article: ../Steno's principles/.

Angular unconformity
An unconformity in which the older strata meet the younger strata at an angle, the older strata being truncated by the erosional surface. Article: ../Unconformities/.

Anomaly
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/.

Antecedent river
A river which is present before the uplift of the hills through which it flows. Article: ../Rivers/.

Anthracite
A very black, hard, and shiny form of coal produced by metamorphism. Article: ../Peat and coal/.

Anticline
Structure formed when rocks are folded upwards. Article: ../Folds/.

Antidune
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/.

Antisyncline
An upward fold in rocks. Article: ../Folds/.

Aphanitic
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 wander
Apparent secular variation recorded in the paleomagnetic record which is actually caused by the motion of plates relative to the poles. Article: ../Continental drift/.

Ar
Chemical symbol for the element argon. Articles: ../Chemistry for geologists/, ../K-Ar dating/, ../Ar-Ar dating/.

Ar-Ar dating
Argon-argon dating, a form of radiometric dating. Article: ../Ar-Ar dating/.

Aseismic ridge
A long linear trail of volcanic islands and seamounts caused by a plate passing over a hotspot. Article: ../Hotspots/.

Archaecyathids
Early reef-building organisms, shaped rather like goblets and secreting skeletons of calcium carbonate; they went extinct at the end of the Cambrian period. Article: ../Reefs/.

Argillaceous
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/.

Arenaceous
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/.

Arenite
Alternative term for sandstone. Article: ../Sedimentary rocks/.

Arkose
Sandstone which contains an appreciable quantity of feldspar as well as the more usual quartz. The grains are often poorly sorted and not well rounded. Article: ../Sedimentary rocks/.

Athenosphere
The portion of the mantle just below the lithosphere. Article: ../Structure of the Earth/.

Atom
A nucleus of protons and neutrons orbited by electrons arranged in electron shells. Articles: ../Chemistry for geologists/, ../Radioactive decay/.

Atmospheric circulation model
A climate model which only takes into account the circulation of the atmosphere and not the oceanic circulation. Article: ../Climate models/.

Atomic number
The number of protons in the nucleus of an atom. Article: ../Chemistry for geologists/.

Atomic weight
The sum of the number of protons and neutrons in the nucleus of an atom. Article: ../Chemistry for geologists/.

Attrition
Erosional processes whereby the clasts transported by wind or water are broken or worn down. Article: ../Mechanical weathering and erosion/.

Aureole
A ring of metamorphic rock formed around an igneous intrusion by contact metamorphism. Article: ../Metamorphic rocks/.

B
Chemical symbol for the element boron. Article: ../Chemistry for geologists/.

Ba
Chemical symbol for the element barium. Articles: ../Chemistry for geologists/, ../Other isochron methods/.

Backshore
That part of a beach which is above the high-water line. Article: ../Nearshore sediments/.

Bajada
The merging of two or more alluvial fans. Article: ../Deserts/.

Banded iron formation
Sedimentary rock consisting of alternating bands of iron oxide and other sedimentary rock, typically chert. Article: ../Banded iron formations/.

Bar
A local accumulation of sediment, usually sand, such as forms in between the channels of a braided stream or offshore from a beach. Article: ../Rivers/.

Barrier island
An island formed at the mouth of a river running at right angles to the direction of the distributary streams. Article: ../Deltas/.

Basalt
A mafic intrusive igneous rock, black in color and aphanitic. Article: ../Igneous rocks/.

Basic rock
An obsolete and inaccurate term for mafic rock. Article: ../Igneous rocks/.

Be
Chemical symbol for the element berylium. Article: ../Chemistry for geologists/.

Bed
A layer in a sedimentary rock. Article: ../Sedimentary rocks/.

Bedding
A structure found in sedimentary rocks in which the rock is visibly composed of numerous layers (beds). Article: ../Sedimentary rocks/.

Bedding planes
The planes dividing the beds in a bedded rock. Article: ../Sedimentary rocks/.

Bedrock
The solid rock underlying unlithified sediment. Articles: Glaciers, Rivers.

Beta decay
A type of radioactive decay including beta plus and beta minus decay; the term is sometimes used to include electron capture as well. Article: ../Radioactive decay/.

Beta minus decay
A form of radioactive decay in which one of the neutrons in the atom is converted to a proton by emitting an electron. Article: ../Radioactive decay/.

Beta plus decay
A form of radioactive decay in which a proton is converted into a neutron by the emission of a positron. Article: ../Radioactive decay/.

BIF
Abbreviation for banded iron formation. Article: ../Banded iron formations/.

Biogeography
The study of the geographical distribution of living or extinct organisms. Articles: ../Continental drift/, ../Biogeography and climate/.

Bioturbation
Changes in the structure of sediment caused by the activity of living things. Article: ../Nearshore sediments/.

Bituminous coal
The commonest form of coal: less peat-like than sub-bituminous coal, but not as hard, black, and pure as anthracite. Article: ../Peat and coal/.

Bivalve
A member of a group of molluscs characterized by being enclosed in two shells (valves). Common examples are mussels, clams, and oysters. Articles: ../Reefs/, ../Way-up structures/.

Body waves
Seismic waves which pass through the body of the Earth rather than traveling on its surface; a collective term for S-waves and P-waves. Articles: ../Seismic waves/, ../Structure of the Earth/.

Borax
An evaporite mineral having the chemical formula Na2B4O7·10H2O. Article: ../Deserts/.

Bottom-set beds
Horizontal beds of sediment deposited on the sea or lake floor in front of a delta. Article: ../Deltas/.

Bouma sequence
The characteristic pattern of sediment deposited by a turbidity current. Article: ../Turbidites/.

Br
Chemical symbol for the element bromine. Articles: ../Chemistry for geologists/, ../Other isochron methods/.

Braided stream
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/.

Breccia
A rock consisting of large unrounded fragments cemented together. Articles: ../Sedimentary rocks/, ../Faults/.

Brittle
A material is said to be brittle if with increasing stress it undergoes very little plastic deformation between elastic deformation and shattering. Article: ../Physical properties of rocks/.

Brown clay
Term occasionally used for pelagic clay. Articles: ../Pelagic clay/, ../Marine sediments/.

C
Chemical symbol for the element carbon. Articles: ../Chemistry for geologists/, ../Radiocarbon dating/.

Ca
Chemical symbol for the element calcium. Article: ../Chemistry for geologists/.

Calcareous
Having to do with calcium carbonate. Articles: ../Calcareous ooze/, ../Marine sediments/.

Calcareous ooze
A calcareous sediment found over large areas of the ocean floor, consisting of the shells of small organisms. Articles: ../Calcareous ooze/, ../Marine sediments/.

Calcite
A mineral consisting of calcium carbonate in a trigonal crystal system.

Calcium carbonate
The chemical CaCO3. Most shells are formed of this, as are the rocks limestone and marble. Articles: ../Calcareous ooze/, ../Marine sediments/.

Carbonate
A molecule with the negative ion CO32-; also a rock consisting of carbonates, particularly limestone. Articles: ../Calcareous ooze/, ../Marine sediments/.

Carbonate compensation depth
The depth at which calcium carbonate will dissolve faster than it is deposited; hence, the depth below which calcareous ooze will not accumulate. Article: ../Calcareous ooze/.

Carbon dating
Alternative term for radiocarbon dating. Article: ../Radiocarbon dating/.

C-C dating
Alternative term for radiocarbon dating. Article: ../Radiocarbon dating/.

Carbon dioxide
The molecule CO2. A gas at temperatures and pressures found on Earth, and forming 0.038% of the Earth's atmosphere. Article: ../Chemical weathering/.

Carbonic acid
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/.

Cast
A fossil produced when a mold is filled with minerals. Article: ../Fossils/.

Cave formation
Alternative term for a speleothem. Articles: ../U-Th, U-Pa, and Ra-Pb dating/, ../U-Pb, Pb-Pb, and fission track dating/.

CCD
Abbreviation for carbonate compensation depth. Article: ../Calcareous ooze/.

14C dating
Alternative term for radiocarbon dating. Article: ../Radiocarbon dating/.

Ce
Chemical symbol for the element cerium. Articles: ../Chemistry for geologists/, ../Other isochron methods/.

Cementation
The binding of clasts together by a finer material, typically silica, calcium carbonate, or iron oxide, to form a clastic rock. Article: ../Sedimentary rocks/.

Cemented tuff
Volcanic ash which has lithified by the process of cementation, as opposed to welded tuff. Article: ../Volcanic ash/.

Chain silicate
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/.

Chalk
Rock which, under a microscope, is clearly composed of the tests of calcium carbonate-secreting micro-organisms. Article: ../Calcareous ooze/.

Chemical sediment
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/.

Chemical weathering
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/.

Chert
A sedimentary rock composed of silica, having an amorphous or very fine-grained structure. Article: ../Siliceous ooze/.

Chirality
The handedness of an organic molecule. Article: ../Amino acid dating/.

Cirque
A large bowl-shaped depression formed at the accumulation point of a valley glacier, with the bowl lacking about a quarter of its rim to let the glacier flow out. Article: ../Glaciers/.

Cl
Chemical symbol for the element chlorine. Article: ../Chemistry for geologists/.

Clast
A piece of rock detached by erosion or weathering from a larger rock. Article: ../Sedimentary rocks/.

Clastic
Composed of clasts. Article: ../Sedimentary rocks/.

Clay
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/.

Claystone
Sedimentary rock composed of clay. Article: ../Sedimentary rocks/.

Climate
Broad trends in the weather; i.e. the tendency of a location to be hot and humid, or dry and cold. Article: ../Paleoclimatology: introduction/.

Closure
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/.

Closure time
The time between snowfall and closure, varying from location to location. Article: ../Ice cores/.

Coal
Coal is peat which has been lithified by compaction, heat, or both. Article: ../Peat and coal/.

Coalification
The chemical processes by which peat is turned into coal. Article: ../Peat and coal/.

Coarse-grained
Composed of crystals of large size; the opposite of fine-grained. Article: ../Igneous rocks/.

Cobble
A clast between 64 and 256mm in diameter, especially one that exhibits rounding. Articles: ../Sedimentary rocks/, ../Rivers/.

Coccolith
A calcareous plate forming part of the shell of a coccolithophore; a common constituent of calcareous ooze. Article: ../Calcareous ooze/.

Coccolithophores
A group of micro-organisms clad in coccoliths. Article: ../Calcareous ooze/.

Compaction
Decrease in volume of sediment, caused by the pressure induced by being buried under yet more sediment. Articles: ../Sedimentary rocks/, ../Ice cores/.

Complacent
A tree is said to be complacent if the thickness of its growth rings is unaffected by annual variations in temperature. Article: ../Dendrochronology/.

Compression
Stress that produces shortening of a solid along the direction in which force is applied. Article: ../Physical properties of rocks/.

Concordant
Of dates, in agreement with one another. Articles: ../U-Pb, Pb-Pb, and fission track dating/, ../Absolute dating: an overview/.

Conglomerate
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/.

Conodont
Term used ambiguously to refer either to a conodont animal or a conodont structure; context usually makes it clear which. Article: ../Fossils/.

Conodont animals
A group of extinct primitive chordates having no hard parts except for conodont structures. Article: ../Fossils/.

Conodont structures
The hard parts of a conodont animal. Article: ../Fossils/.

Contact metamorphism
Metamorphism caused by close proximity to a source of heat, such as an intrusion of magma; as opposed to regional metamorphism. Article: ../Metamorphic rocks/.

Continental drift
The theory that continents have shifted their positions over time; now subsumed into the theory of plate tectonics. Article: ../Continental drift/.

Continental glacier
A glacier covering a large area and flowing outwards from its accumulation zone under the pressure of its own weight, as distinct from a valley glacier. Articles: ../Glaciers/, ../Ice ages/.

Continental margin
The continental shelf, slope, and rise. Article: ../Marine sediments/.

Continental rise
The shallowly sloping (approximately 1 degree from horizontal) terrain between the continental slope and the abyssal plain. Article: ../Marine sediments/.

Continental shelf
That part of a continent which is underwater, lying between the unsubmerged portion of a continent and the continental slope. Article: ../Marine sediments/.

Continental slope
A shallow slope, typically between 4 and 10 degrees from horizontal, found between the continental shelf and the continental rise. Article: ../Marine sediments/.

Corals
A group of marine organisms. Hard corals secrete skeletons of calcium carbonate and so act as reef-forming organisms. Article: ../Reefs/.

Core
The innermost 3,400 km of the Earth, composed mainly of iron. Article: ../Structure of the Earth/.


 * or

A sample of ice or rock recovered from the Earth's crust by drilling. Article: ../Ice cores/.

Cosmic dust
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/.

Cosmic rays
Streams of high-energy particles which bombard the Earth from outer space. Article: ../Cosmogenic surface dating/.

Cosmogenic
Of isotopes, produced by cosmic rays. Articles: ../Cosmogenic surface dating/, ../Radiocarbon dating/.

Cosmogenic surface dating
A method of absolute dating which gives the time since a rock became exposed on the surface. Article: ../Cosmogenic surface dating/.

Country rock
The rock into which an igneous rock intrudes. Article: ../Igneous rocks/.

Covalent bond
A bond between atoms in which they share electrons. Article: ../Chemistry for geologists/.

Creep
Transport of clasts by wind or water by means of rolling them along the ground, river bed, sea bed, etc. Articles: ../Mechanical weathering and erosion/, ../Rivers/.

Cross-bedding
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/.

Cross-cutting
An igneous rock such as a dike which cuts through the beds of country rock is said to be cross-cutting. Article: ../Cross-cutting relationships/.

Crossdating
The correlation of dates from different sources. Article: ../Dendrochronology/.

Crust
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/.

Crystal
A large molecule composed of smaller chemical units chemically bonded together in a regular repetitive arrangement. Article: ../Minerals/.

Crystal habit
The shape or shapes in which a mineral will typically grow. Article: ../Minerals/.

Crystal system
One of the seven basic geometrical arrangements in which the atoms of a crystal can be arranged: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal or cubic. Article: ../Minerals/.

Curie temperature
Very roughly speaking, the temperature above which a material cannot be magnetized and below which it can. Article: ../Geomagnetic reversals/.

Cyclosilicate
A synonym for ring silicate. Article: ../Silicate minerals/.

&#948;18O
A proxy for temperature based on oxygen isotope ratios. Articles: ../Scleroclimatology/, ../Ice cores/.

Decay
The destruction of organic remains by organic processes. Articles: ../Peat and coal/, ../Soils and paleosols/, ../Fossils/.


 * or

Radioactive decay. Articles: ../Radioactive decay/.

Decay chain
A sequence of events in which one isotope decays to another via an intermediate sequence of unstable isotopes. Article: ../Radioactive decay/.

Deflation
The erosion of fine particles from dry soil by the wind. Article: ../Deserts/.

Deflation lake
A lake caused when deflation has caused a hollow the bottom of which lies below the water table. Article: ../Deserts/.

Delta
The body of sediment deposited when a river flows into a lake or the sea. Article: ../Deltas/.

Dendrochronology
A method of dating wood by studying the annual growth rings produced by the tree. Article: ../Dendrochronology/.

Deposition
All those processes which add sediment to a surface; the opposite of erosion. Article: ../Sedimentary rocks/.

Desert
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/.

Desert pavement
A stony surface often found in deserts. Article: ../Deserts/.

Dessication crack
Alternative term for a mud crack. Article: ../Way-up structures/.

Detrital
Composed of clasts; synonymous with clastic. Article: ../Sedimentary rocks/.

Dextral
A fault is said to be dextral if someone standing on one side of the fault and looking at the other when there is motion along the fault would see the other side moving to the right. Article: ../Faults/.

Diamond anvil cell
A device used in experimental petrology to subject small samples of rock to large amounts of stress. Article: ../Physical properties of rocks/.

Diapir
A sedimentary structure formed by one type of sediment flowing upwards through another as a result of pressure. Article: ../Way-up structures/.

Diatomite
A very light and porous rock formed from diatom tests that have undergone little in the way of compaction and recrystallization. Article: ../Siliceous ooze/.

Diatoms
A group of single-celled algae which produce siliceous tests; a major source of siliceous ooze. Article: ../Siliceous ooze/.

Differentiation
The mechanism by which an originally homogeneous Earth separated into crust, mantle, and core. Article: ../Structure of the Earth/.

Dike
A vertical or near-vertical sheet of igneous rock which intrudes into the country rock. Articles: ../Igneous rocks/, ../Cross-cutting relationships/, ../Igneous rocks and stratigraphy/, ../Ophiolites/.

Dip-slip fault
A fault in which much of the motion of the rocks on either side of the fault is vertical: hence either a normal fault or a reverse fault. Article: ../Faults/.

Disconformity
An unconformity in which the underlying strata are parallel with the overlying strata. Article: ../Unconformities/.

Dissolved
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/.

Distributary
A smaller stream flowing out of a larger river, as opposed to a tributary, which flows in. Article: ../Rivers/.

Diurnal
Occurring once daily. Article: ../Tidal rhythmites and dating/.

Docking
The union of a terrane with the landmass to which it becomes attached. Article: ../Terranes/.

Drag fold
A structure formed in a rock as friction drags the material in it backwards relative to its motion along a fault. Article: ../Folds/.

Drift
Any sediment deposited by a glacier. Article: ../Glaciers/.

Drifters
Term for the early supporters of continental drift; the opposite of "fixists". Article: ../Continental drift/.

Dropstone
A stone which has traveled out to sea on a "raft" of ablated glacial ice, and has been deposited when the ice melted. Articles: ../Glaciers/, ../Glacial marine sediment/.

Drumlin
A smallish hill shaped somewhat like the back of a spoon, deposited by glaciers in a manner not fully understood. Article: ../Glaciers/.

Ductile
A material is said to be ductile if, under stress, it will undergo a great deal of plastic deformation before it breaks. The opposite of brittle. Article: ../Physical properties of rocks/.

Dune
A mound of sand formed by the action of wind or water. Articles: ../Rivers/, ../Deserts/.

Dunnite
An ultramafic rock consisting entirely of olivine. Article: ../Igneous rocks/.

Elastic
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/.

Electron
A particle with negative charge and negligible mass found orbiting the nucleus of an atom. Articles: ../Chemistry for geologists/, ../Radioactive decay/.

Electron capture
A form of radioactive decay in which of the radioactive atom's own electrons combines with one of its protons, converting the proton into a neutron. Article: ../Radioactive decay/.

Electron shell
An orbit followed by electrons about an atomic nucleus. Articles: ../Chemistry for geologists/, ../K-Ar dating/, ../Ar-Ar dating/.

Element
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/.

Enantiomers
Molecules which are mirror images of one another. Article: ../Amino acid dating/.

Entire margins
Leaf margins which are smooth rather than serrated, characteristic of warm humid climates. Article: ../Leaf shape and temperature/.

Epicenter
The point on the Earth's surface directly above the focus of an earthquake. Article: ../Seismic waves/.

Erg
A sandy desert. Article: ../Deserts/.

Erosion
Any process capable of breaking up rocks or soils and transporting the resulting clasts. Article: ../Mechanical weathering and erosion/.

Erratic boulder
A boulder which does not fit in with the geology of its surroundings, transported from its place of origin by a glacier. Article: ../Glaciers/.

Evaporite
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/.

Event horizon
An extensive geological feature all of which was laid down at exactly the same time; e.g. volcanic ash from a single volcanic eruption. Article: ../Volcanic ash/.

Evolution
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/.

Excess argon
Argon which is not radiogenic; a potential source of error in Ar-Ar dating. Articles: ../K-Ar dating/, ../Ar-Ar dating/.

Exponential decay
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/.

Extrusive rock
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/.

Facies
A facies is a body of sediment or sedimentary rock characteristic of a particular depositional environment. Article: ../Walther's principle/.

Fault
A planar fracture or discontinuity in a volume of rock. Articles: ../Faults/, ../Terranes/.

Fault breccia
Breccia produced by the crushing action of motion along a fault. Article: ../Faults/.

Fault gouge
Material similar to fault breccia but finer in texture. Article: ../Faults/.

Fault mirror
Alternative term for slickenside. Article: ../Faults/.

Fauna
Animals (in the broadest possible sense, including birds, fish, crustaceans, molluscs, etc). Article: ../Principle of faunal succession/.

Fe
Chemical symbol for the element iron. Article: ../Chemistry for geologists/.

Feldspar
A group of aluminosilicate minerals with a lattice structure. Article: ../Igneous rocks/.

Felsic rocks
Rocks which are high in silica and feldspar and low in magnesium and iron. The opposite of mafic rocks. Article: ../Igneous rocks/.

Fine-grained
Composed of crystals or clasts of small size; the opposite of coarse-grained. Article: ../Igneous rocks/.

Fining-up sequence
A form of grading upwards from coarse to fine sediments associated with rivers. Article: ../Rivers/.

Firn
Snow which has compacted, but not so far as to become ice. Article: ../Ice cores/.

Fissile
Of a rock, having the property of splitting easily in a given direction (e.g. between bedding planes). Article: ../Sedimentary rocks/.

Fission track dating
A form of absolute dating which involves counting the fission tracks in a rock. Article: ../U-Pb, Pb-Pb, and fission track dating/.

Fission tracks
Microscopic scars left in minerals by alpha particles. Article: ../U-Pb, Pb-Pb, and fission track dating/.

Fixist
Term for the early opponents of continental drift; the opposite of "drifter". Article: ../Continental drift/.

Flame structure
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/.

Flaser deposits
Deposits in which light and heavy sediments alternate, characteristic of nearshore environments. Article: ../Nearshore sediments/.

Flood plain
The flattened, sediment-rich area formed by the action of rivers on a landscape. Article: ../Rivers/.

Flume
An artificial channel used by geologists to study the transport of sediment by water in the laboratory. Articles: ../Rivers/.

Fluvial
Having to do with rivers. Article: ../Rivers/.

Focus
The point in the Earth at which an earthquake originates. Article: ../Seismic waves/.

Foliation
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/.

Foot wall
Rocks lying above a dip-slip or oblique fault. Article: ../Faults/.

Foram
Short form of foraminiferan. Article: ../Calcareous ooze/.

Foraminiferans
A group of micro-organisms which secrete calcareous tests; one of the most common constituents of calcareous ooze. Article: ../Calcareous ooze/.

Foreset beds
Beds of sediment sloping down at the front of a delta into the sea or lake into which it discharges. Article: ../Deltas/.

Foreshore
That part of the nearshore which is uncovered at high tide. Article: ../Nearshore sediments/.

Fossil
Organic remains, or traces of organic activity such as footprints, preserved in the geological record. Article: ../Fossils/.

Gabbro
Mafic intrusive igneous rock; the intrusive equivalent of basalt. Article: ../Igneous rocks/.

GCM
Abbreviation for general circulation model. Article: ../Climate models/.

GDGTs
Glycerol dibiphytanyl glycerol tetraethers, organic molecules produced by the single-celled organisms known as Crenarchaeota, used in the TEX86 temperature proxy. Article: TEX86.

General circulation model
A climate model which takes into account both the atmospheric and the oceanic circulation. Article: ../Climate models/.

Geological column
A table showing the order of the faunal succession in the fossil record. Article: ../Geological column/.

Geomagnetic reversal
A change in state from normal polarity to reversed polarity, or vice versa. Articles: ../Geomagnetic reversals/, ../Paleomagnetic dating/.

Geopetal structure
A structure formed when a hollow object is partially filled with sediment, allowing us to use it as a way-up structure. Article: ../Way-up structures/.

Glacial
Having to do with glaciers. Articles: ../Glaciers/, ../Ice ages/, ../Glacial marine sediment/, ../Ice cores/.

Glacial outwash
Sediment carried out of a glacier by meltwater. Article: ../Glaciers/.

Glacial polish
The smooth (but striated) surface produced on a rock by the polishing action of a glacier passing over it. Article: ../Glaciers/.

Glaciation
Synonym for ice age. Article: ../Ice ages/.

Glacier
A moving mass of ice. Articles: ../Glaciers/, ../Ice ages/, ../Glacial marine sediment/, ../Ice cores/.

Glass
Any igneous rock with an amorphous structure, produced by lava cooling too fast to allow the formation of crystals. Articles: ../Rocks/, ../Volcanic ash/.

Global Positioning System
A method for finding one's location on the surface of the Earth; used in geology to measure the motion of plates. Article: ../Continental drift/.

Gneiss
A metamorphic rock of high grade with a distinctive streaky appearance produced by the separation out of chain silicates into streaks. Article: ../Metamorphic rocks/.

GOE
Abbreviation for Great Oxygen Event. Article: ../Banded iron formations/.

Goethite
The iron oxide mineral FeO(OH). Articles: ../Deserts/, ../Banded iron formations/.

GPS
Abbreviation for Global Positioning System. Article: ../Continental drift/.

Grade
The degree to which a rock has undergone metamorphism, depending on the amount of heat to which it has been exposed. Article: ../Metamorphic rocks/.

Grading
Change in size of clasts between two points; most typically from large clasts at the bottom of a layer to small clasts at the top. Articles: ../Turbidites/, ../Way-up structures/.

Grainflow lamina
A lamina formed in sand dunes when sand at the crest of the dune avalanches down the lee face of the dune. Article: ../Deserts/.

Granite
A felsic intrusive igneous rock; the intrusive counterpart of rhyolite. Article: ../Igneous rocks/.

Gravel
Sediment consisting of clasts 2mm in diameter and upwards. Article: ../Sedimentary rocks/.

Greenhouse gas
A gas such as carbon dioxide or methane which helps keep the Earth warm by trapping heat in the atmosphere. Article: ../Ice ages/.

Greywacke
Sandstone consisting of quartz, feldspar, and small rock fragments embedded in a clay matrix. Article: ../Sedimentary rocks/.

Ground mass
Finer material in which larger clasts (in the case of sedimentary rock) or crystals (in the case of igneous rock) are embedded. A synonym for matrix. Article: ../Igneous rocks/.

Group
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/.

Growth ring
A layer of wood produced by a tree on an annual basis, used in dendrochronology. Articles: ../Dendrochronology/, ../Dendroclimatology/.

Gypsum
A mineral consisting of hydrated calcium sulfate (CaSO4.H2O) with a monoclinic crystal system. Article: ../Saline giants/.

H
Chemical symbol for the element hydrogen. Article: ../Chemistry for geologists/.

Half-life
The half-life of an isotope is the length of time in which an atom of that isotope has a 50% chance of undergoing radioactive decay. Article: ../Radioactive decay/.

Halite
Rock salt (NaCl). Article: ../Saline giants/.

Hallam curve
A reconstruction of past variations of sea level based on sedimentary evidence of transgressions and regressions. Article: ../Sea level variations/.

Hanging wall
Rocks lying above a dip-slip or oblique fault. Article: ../Faults/.

Hematite
The iron oxide mineral Fe2O3. Articles: ../Deserts/, ../Banded iron formations/.

Herringbone crossbedding
A form of cross-bedding in which the direction of slope alternates, as a result of oscillatory flow. Article: ../Nearshore sediments/.

Hf
Chemical symbol for the element hafnium. Articles: ../Chemistry for geologists/, ../Other isochron methods/.

Hoodoo
A pillar of rock produced by erosion. Article: ../Steno's principles/.

Horizon
A distinct layer in a soil, formed by pedogenetic processes. Article: ../Soils and paleosols/.

Hornfels
A large group of metamorphic rocks produced from sedimentary rocks by contact metamorphism. Article: ../Metamorphic rocks/.

Hotspot
A stationary point of high volcanic activity above which plates pass, creating an aseismic ridge. Article: ../Hotspots/.

Humus
Decaying organic matter in soil. Article: ../Soils and paleosols/.

Humic coal
Coal produced by the deposition of land plants in swamps, as opposed to sapropelic coal. Article: ../Peat and coal/.

Hydrothermal
Having to do with hot water. Article: ../Metamorphic rocks/.

Ice age
A time at which sheet glaciers are present on some regions of the Earth's surface. Article: ../Ice ages/.

Ice sheet
Synonym for continental glacier. Articles: ../Glaciers/, ../Ice ages/.

Igneous rock
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/.

Index fossil
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/.

Index mineral
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/.

Index species
A species suitable for the production of index fossils. Article: ../Index fossils/.

Inert
Of an element, unable to participate in chemical reactions. Article: ../K-Ar dating/.

Inosilicate
A synonym for chain silicate. Article: ../Silicate minerals/.

Insolation
The quantity per area of solar radiation reaching a given location. Articles: ../Milankovitch cycles/, ../Climate models/.

Insoluble
Incapable of becoming dissolved. Article: ../Chemistry for geologists/.

Interference ripples
Ripples caused by two currents flowing (one after the other) at or near right-angles to one another. Article: ../Nearshore sediments/.

Interglacial
A time of glacial retreat during an ice age. Article: ../Ice ages/.

Interfingering
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/.

Internal drainage
A drainage pattern typical of deserts, in which rivers flow into the desert and evaporate. Article: ../Deserts/.

Intrusive rock
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/.

Inverse grading
Grading where the size of clasts varies from small clasts at the bottom of a layer to large clasts at the top. Article: ../Turbidites/.

Ion
An atom which has gained or lost electrons, giving it a negative or positive charge respectively. Article: ../Chemistry for geologists/.

Ionic bond
A bond between atoms in which one atom donates electrons to another. Article: ../Chemistry for geologists/.

Iron oxides
Minerals containing both iron and oxygen, as the name suggests: examples include hematite and goethite. Articles: ../Deserts/, ../Banded iron formations/.

Isochron dating
A form of radiometric dating involving the construction of an isochron diagram. Articles: ../Rb-Sr dating/, ../Other isochron methods/.

Issochron diagram
A graph showing the isotope ratios of various minerals found in the same rock, used in isochron dating. Articles: ../Rb-Sr dating/, ../Other isochron methods/.

Isostatic rebound
The process whereby land which has formerly been depressed by overlying weight (for example of an ice sheet) rises when the weight is removed. Articles: ../Glaciers/, ../Ice ages/.

Isotope
Atoms are classified into isotopes according to their atomic number and their atomic weight. Articles: ../Chemistry for geologists/, ../Radioactive decay/.

K
Chemical symbol for the element potassium. Articles: ../Chemistry for geologists/, ../K-Ar dating/.

K-Ar dating
Potassium-argon dating, a form of radiometric dating. Article: ../K-Ar dating/.

Karst topography
The distinctive landscape produced by the chemical weathering of limestone. Article: ../Chemical weathering/.

K-Ca dating
Potassium-argon dating, a form of radiometric dating. Article: ../Other isochron methods/.

Kettle
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/.

La
Chemical symbol for the element lanthanum. Articles: ../Chemistry for geologists/, ../Other isochron methods/.

La-Ba dating
Lanthanum-barium dating, a form of radiometric dating. Article: ../Other isochron methods/.

La-Ce dating
Lanthanum-cerium dating, a form of radiometric dating. Article: ../Other isochron methods/.

Lacolith
An intrusion between two strata, similar to a sill but thicker and lens-shaped. Article: ../Igneous rocks/.

Lacustrine
Having to do with lakes. Article: ../Lakes/.

Lake
An inland body of water fed by rivers, streams, or sometimes by seepage of groundwater. Article: ../Lakes/.

Lamina
A very thin bed, no more than a few millimeters thick. Article: ../Deserts/.

Lateral moraine
Sediment which accumulates along the sides of a valley glacier, having fallen or been scraped off the walls of the valley. Article: ../Glaciers/.

Laterite
A soil type characteristic of a tropical climate alternating between a monsoon season and a dry season. Article: ../Sediments and climate/.

Lattice silicates
Silicate minerals in which the silicate tetrahedra are bonded together to form a three-dimensional lattice. Article: ../Silicate minerals/.

Lava
Molten rock which has reached the surface, as opposed to magma, which is sill trapped beneath it. Article: ../Igneous rocks/.

Lava flow
Igneous rock formed by lava flowing on the surface.

Leached ions
Ions dissolved in water as a result of chemical weathering; as opposed to residual minerals. Article: ../Chemical weathering/.

Lee
The side of a mountain, dune, antidune, or generally any hill-shaped geological feature, which is on the down-stream side of a current of wind or water. The opposite of stoss. Article: ../Rivers/.

LGM
Abbreviation for last glacial maximum. Article: ../Climate models/.

Lignite
The softest form of coal; the next stage in the formation of coal from peat after peat itself. Article: ../Peat and coal/.

Limestone
Rock formed from calcium carbonate, usually in the form of calcite. Article: ../Calcareous ooze/.

Limiting stand
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/.

Lineation
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/.

Lithification
The conversion of sediment into a sedimentary rock by such processes as compaction and cementation. Article: ../Sedimentary rocks/.

Lithosphere
The Earth's crust together with that portion of the mantle which, like the crust, is brittle and elastic rather than plastic and ductile. Article: ../Structure of the Earth/.

Littoral
Having to do with the coast. Article: ../Index fossils/.

Loess
Fine wind-borne sediment produced by the action of glaciers. Article: ../Ice cores/.

Longshore bars
Bars of sediment running parallel to a beach. Article: ../Nearshore sediments/.

Longshore current
The component of a nearshore current that flows parallel to the shoreline. Article: ../Nearshore sediments/.

Longshore drift
The motion of sediment along the shore as a result of the fact that waves that approach the shore obliquely will recede from it at right-angles to the shoreline. Article: ../Nearshore sediments/.

Love waves
A type of seismic wave which travels along the surface of the Earth rather than through it. Article: ../Seismic waves/.

Lu
Chemical symbol for the element lutetium. Articles: ../Chemistry for geologists/, ../Other isochron methods/.

Lu-Hf dating
Lutetium-hafnium dating, a form of radiometric dating. Article: ../Other isochron methods/.

Mafic rock
A rock rich in magnesium and iron, and poor in quartz and feldspar. Article: ../Igneous rocks/.

Magma
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/.

Mantle
A zone of ultramafic rock lying below the Earth's crust and above its core. Article: ../Structure of the Earth/.

Mantle plume
A column of hot rock rising in the mantle below a hotspot. Article: ../Hotspots/.

Marble
A metamorphic rock formed by the metamorphism of limestone. Article: ../Metamorphic rocks/.

Massive rock
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/.

Mass wasting
Erosion caused by gravity. Article: ../Mechanical weathering and erosion/.

Matrix
Finer material in which larger clasts (in the case of sedimentary rock) or crystals (in the case of igneous rock) are embedded. A synonym for groundmass. Article: ../Igneous rocks/.

Meander
A broad loop in a stream or river. Article: ../Rivers/.

Meandering stream
A stream which flows in a series of meanders. Article: ../Rivers/.

Mechanical weathering
Weathering caused by mechanical processes that break up a rock, as opposed to chemical weathering. Article: ../Mechanical weathering and erosion/.

Medial moraine
A moraine formed by the union of two lateral moraines when two valley glaciers flow together to forms a single glacier. Article: ../Glaciers/.

Metamorphic grade
The degree of metamorphism undergone by a metamorphic rock. Article: ../Metamorphic rocks/.

Metamorphic rock
A rock which has had its texture or composition changed by heat and/or pressure. Article: ../Metamorphic rocks/.

Metamorphism
Changes in the texture or composition of a rock brought about by heat and/or pressure. Article: ../Metamorphic rocks/.

Metasomatism
Changes associated with contact metamorphism in which the parent rock mixes and/or reacts with the intrusive igneous rock and the hot fluids associated with its eruption. Article: ../Metamorphic rocks/.

Methane
The gas CH4, a potent greenhouse gas. Article: ../Ice ages/.

Mg
Chemical symbol for the element magnesium. Article: ../Chemistry for geologists/.

Mica
A group of sheet aluminosilicate minerals. Articles: ../Igneous rocks/, ../Metamorphic rocks/.

Mid-ocean ridge
Elevated sea-floor on either side of a mid-ocean rift. Articles: ../Sea floor spreading/, ../Sea level variations/.

Mid-ocean rift
The rift between two plates at which sea floor spreading occurs. Article: ../Sea floor spreading/.

Milankovitch cycles
Periodic changes in the inclination of the Earth's axis and the shape of its orbit. Article: ../Milankovitch cycles/.

Mineral
A solid with a particular chemical composition and structure. Articles: ../Minerals/, ../Silicate minerals/.

Mineraloid
Anything which is like a mineral in some respects but doesn't quite fit the definition. Article: ../Minerals/.

Mixing plot
A graph showing the composition of a rock on which the plotted points will fall in a straight line if the rock was produced by the mixing of different sources of magma. Article: ../Rb-Sr dating/.

Mold
A fossil formed when sediment is packed around organic remains, which are then destroyed, leaving a void in the sediment in the shape of the remains. Article: ../Fossils/.

Molecule
A collection of atoms bonded together. Article: ../Chemistry for geologists/.

Moraine
A deposit of till. Article: ../Glaciers/.

Mud crack
A small-scale geological structures produced in mud as it dries. Article: ../Way-up structures/.

Mudstone
Rock formed from clay or silt which is not bedded: lithified mud which is bedded is known as shale. Article: ../Sedimentary rocks/.

Mylonite
A rock produced at depth by the ation of two sides of a fault rubbing against one another, having a distinctive "grain" indicating the direction of motion of the fault. Article: ../Faults/.

Na
Chemical symbol for the element sodium. Article: ../Chemistry for geologists/.

Nd
Chemical symbol for the element neodymium. Articles: ../Chemistry for geologists/, ../Other isochron methods/.

Ne
Chemical symbol for the element neon. Articles: ../Chemistry for geologists/, ../Radioactive decay/.

Nearshore
The zone in which the sea bed is affected by waves. Article: ../Nearshore sediments/.

Neosilicate
A silicate mineral in which the silicate tetrahedra are isolated from one another. Article: ../Silicate minerals/.

Neutron
A particle with no charge and approximately the same mass as a proton; together with protons, neutrons make up the nucleus of an atom. Articles: ../Chemistry for geologists/, ../Radioactive decay/.

Nonconformity
An unconformity in which the older rocks are igneous or metamorphic. Article: ../Unconformities/.

Normal fault
A dip-slip fault in which the hanging wall moves downwards relative to the foot wall. Article: ../Faults/.

Normal polarity
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/.

Nucleus
A fragment of shell or stone around which an ooid forms. Article: ../Ooids and oolite/.


 * or

The core of an atom, consisting of protons and neutrons. Articles: ../Chemistry for geologists/, ../Radioactive decay/.

O
Chemical symbol for the element oxygen. Articles: ../Chemistry for geologists/, ../Banded iron formations/.

Oasis
A small lake found in a desert. Article: ../Deserts/.

Obduction
A process in which one platecolliding with another is thrust over it instead of beneath it; the opposite of subduction. Article: ../Ophiolites/.

Oblique fault
A fault which combines elements of a dip-slip fault and a strike-slip fault. Article: ../Faults/.

Obsidian
Felsic volcanic glass. Article: ../Igneous rocks/.

Olivine
A silicate mineral with the chemical formula (Mg,Fe)2SiO4 and an orthorhombic crystal system. Articles: ../Minerals/, ../Igneous rocks/, ../Structure of the Earth/.

Ooid
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/.

Oolite
Limestone composed of ooids cemented together. Article: ../Ooids and oolite/.

Oolith
Either a rock formed from ooids, or a single ooid, depending on context. Article: ../Ooids and oolite/.

Oolitic limestone
Synonym for oolite. Article: ../Ooids and oolite/.

Opal
Amorphous hydrated silica, of which the precious stone known as opal is only one particularly pretty example. Article: ../Siliceous ooze/.

Opal compensation depth
The depth at which siliceous material will dissolve faster than it is deposited; hence, the depth below which marine chert will not form. Article: ../Siliceous ooze/.

Ophiolite
A section of oceanic crust which has been thrust up above sea-level. Article: ../Ophiolites/.

Orthosilicate
A synonym for neosilicate. Article: ../Silicate minerals/.

Orogeny
The formation of mountains; or the faulting and folding of a large area by lateral pressure; or the formation of mountains by this process. Article: ../Orogeny/.

Os
Chemical symbol for the element osmium. Articles: ../Chemistry for geologists/, ../Other isochron methods/.

Oscillatory flow
The washing back and forth of water on the foreshore as a result of the action of the tide. Article: ../Nearshore sediments/.

Outwash
Light sediment carried by meltwater from the ablation zone of a glacier. Article: ../Glaciers/.

Outwash plain
A flat area of outwash sediment in front of a glacier. Article: ../Glaciers/.

Oxbow lake
A crescent-shaped lake formed when a meandering stream changes its course, leaving one of its meanders cut off from the stream. Article: ../Rivers/.

Oxygen catastrophe
Synonym for the Great Oxygenation Event Article: ../Banded iron formations/.

Oxygen crisis
Synonym for the Great Oxygenation Event Article: ../Banded iron formations/.

Oyster
A reef-forming bivalve. Article: ../Reefs/.

Pa
Abbreviation for pascals.


 * or

The chemical symbol for the element protactinium. Articles: ../Chemistry for geologists/, ../U-Th, U-Pa, and Ra-Pb dating/.

Pahoehoe
A type of lava flow, or the cooled and solidified rock produced by it, characterized by a ropey and billowy surface texture. Article: ../Way-up structures/.

Paleoclimatology
The study of ancient climates. Article: ../Paleoclimatology: introduction/.

Paleocurrents
Ancient currents of wind and water the direction of which can be deduced from the analysis of sedimentary rocks. Article: ../Paleocurrents/.

Paleomagnetic dating
A form of absolute dating based on analysis of the paleomagnetic data in the rocks. Article: ../Paleomagnetic dating/.

Paleomagnetism
The geological record of the past history of the Earth's magnetic field. Articles: ../Geomagnetic reversals/, ../Sea floor spreading/, ../Paleomagnetic dating/.

Paleosol
Fossilized soil. Article: ../Soils and paleosols/.

Pangaea
The last supercontinent to exist, prior to its rifting and the formation of the Atlantic Ocean. Article: ../Continental drift/.

Pannotia
A supercontinent that existed before Pangea. Article: ../Continental drift/.

Paraconformity
An unconformity without an erosional surface. Article: ../Unconformities/.

Parent isotope
A radioactive isotope which undergoes radioactive decay to produce a daughter isotope. Article: ../Radioactive decay/.

Parent rock
The original rock from which a metamorphic rock is formed by metamorphism. Article: ../Metamorphic rocks/.

Pascal
Unit of stress: 1 pascal = 1 newton/square meter. Article: ../Physical properties of rocks/.

Pb
Chemical symbol for the element lead. Articles: ../Chemistry for geologists/, ../U-Pb, Pb-Pb, and fission track dating/, ../U-Th, U-Pa, and Ra-Pb dating/.

Pb-Pb dating
Lead-lead dating, a form of radiometric dating. Article: ../U-Pb, Pb-Pb, and fission track dating/.

Peat
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/.

Peatification
The partial decomposition of waterlogged vegetable matter, turning it into peat. Article: ../Peat and coal/.

Pedogenesis
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/.

Pelagic
Having to do with the open sea. Articles: ../Marine sediments/, ../Index fossils/.

Pelagic clay
Fine-textured sediment deposited on the abyssal plain. Articles: ../Pelagic clay/, ../Marine sediments/.

Peridotite
An ultramafic igneous rock consisting mainly of olivine with a little pyroxene and amphibole. Article: ../Igneous rocks/.

Periodic table
A tabular arrangement of the elements which gives insight into their chemical properties. Article: ../Chemistry for geologists/.

Permineralization
A process forming mineralized fossils in which the voids in the original material are filled by minerals. Article: ../Fossils/.

Petrification
A process forming mineralized fossils in which they undergo both replacement and permineralization. Article: ../Fossils/.

Phaneritic
An igneous rock is said to be phaneritic if the crystals in it are large enough to be seen with the naked eye. The opposite of aphenatic. Article: ../Igneous rocks/.

Phase
Whether a substance is solid, liquid, or gas; if solid, its crystal structure or lack thereof. Article: ../Chemistry for geologists/.

Phenocryst
A large crystal embedded in the more finely-grained ground mass of a porphyritic rock. Article: ../Igneous rocks/.

Philosophical naturalism
The rejection a priori of the existence of the supernatural; a position completely unnecessary to the practice of geology. Article: ../Actualism/.

Phylosilicate
A synonym for sheet silicate. Article: ../Silicate minerals/.

Pillow basalt
Basalt with a distinctive shape consisting of a set of "pillows"; formed underwater as a result of the more rapid cooling of lava on contact with water. Articles: ../Igneous rocks/, ../Ophiolites/.

Pinstripe lamina
A very thin lamina of very fine clasts, formed in and characteristic of aeolian sand dunes. Article: ../Deserts/.

Plastic
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/.

Playa
A flat-bottomed basin in a desert which periodically fills with water to form a shallow temporary lake. Article: ../Deserts/.

Plutonic rock
Alternative term for intrusive rock. Article: ../Igneous rocks/.

Plate
A piece of the lithosphere bounded by faults. Articles: ../Plate tectonics: overview/, ../Sea floor spreading/, ../Subduction/, ../Hotspots/, ../Terranes/.

Plate tectonics
The study of the motion of the Earth's plates. Articles: ../Plate tectonics: overview/, ../Sea floor spreading/, ../Subduction/, ../Hotspots/, ../Terranes/, ../Ophiolites/, ../Orogeny/.

PMIP
The Paleoclimate Modeling Intercomparison Project, a project comparing results from climate models with the evidence from paleoclimatic proxies. Article: ../Climate models/.

Point bar
A bar of sediment formed on the inner bank of a meander. Article: ../Rivers/.

Polymorph
Two minerals having the same chemical formula but a different crystal structure are said to be polymorphs. Article: ../Minerals/.

Pond
A small lake. Article: ../Lakes/.

Porphyritic
Of a rock, containing some large crystals embedded in a more finely-grained ground mass. Article: ../Igneous rocks/.

Precipitation
When a chemical formerly dissolved in water settles out of it as a solid sediment, this is called precipitation. Article: ../Saline giants/.

Prehistoric
Preceding written human history; the fairly arbitrary line before which organic remains are considered to be fossils. Article: ../Fossils/.

Primary rock
Term sometimes used for igneous rock. Article: ../Igneous rocks/.

Principle of cross-cutting relationships
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 succession
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 time
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 continuity
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 horizontality
The principle that when sediment is laid down, it is usually laid down more or less flat. Article: ../Steno's principles/.

Principle of superposition
The principle that when sediment is laid down, the sediment most recently deposited will be on the top. Article: ../Steno's principles/.

Proglacial lake
A lake fed by meltwater from a glacier. Article: ../Glaciers/.

Progradation
The building out of a delta into the sea by deposition of sediment. Article: ../Deltas/.

Proton
A positively charged particle of about the same mass as the neutron; together with neutrons, protons form the nuclei of atoms. Articles: ../Chemistry for geologists/, ../Radioactive decay/.

Proxy
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.

Pseudostratigraphy
A term used to describe a situation where rock is layered, but the layers do not represent successive deposition; for example, the layers found in an ophiolite. Article: ../Ophiolites/.

Pumice
A form of volcanic glass filled with air bubbles. Article: ../Igneous rocks/.

Pyroclastic flow
A current of air laden with volcanic ash, which resists dispersion into the surrounding air because of its greater density. Article: ../Volcanic ash/.

Pyroxene
An important group of rock-forming chain silicates. Article: ../Igneous rocks/.

P-waves
Body waves consisting of moving zones of compression and tension. Article: ../Seismic waves/.

Quartz
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/.

Quartzite
A metamorphic rock formed by metamorphism of quartz sandstone. Article: ../Metamorphic rocks/.

Quartz sandstone
Sandstone of which the sand grains are almost entirely quartz. Article: ../Sedimentary rocks/.

Ra
Chemical symbol for the element radium. Articles: ../Chemistry for geologists/, ../U-Th, U-Pa, and Ra-Pb dating/.

Racemic
Composed of equal amounts of both forms of enantiomers. Article: ../Amino acid dating/.

Racemization
The process by which a collection of chiral molecules become racemic. Article: ../Amino acid dating/.

Racemization dating
An alternative term for amino acid dating. Article: ../Amino acid dating/.

Radioactive
Having a tendency to radioactive decay. Article: ../Radioactive decay/.

Radioactive decay
Any process by which the composition of the nucleus of an atom is changed, such as alpha decay, beta decay, and electron capture. Article: ../Radioactive decay/.

Radiocarbon dating
Radiometric dating of organic material by analysis of the isotopes of carbon it contains. Article: ../Radiocarbon dating/.

Radiogenic
An atom is said to be radiogenic if it is the product of radioactive decay. Article: ../Radioactive decay/.

Radiolaria
A group of single-celled organisms which produce tiny intricate tests, usually siliceous; these form a major component of siliceous ooze. Article: ../Siliceous ooze/.

Radiometric dating
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/.

Rain shadow
A dry area on the lee side of a mountain, caused by the tendency of clouds to burst on the stoss side. Article: ../Deserts/.

Raleigh wave
A type of seismic wave which travels on the surface of the Earth rather than through it. Article: ../Seismic waves/.

Rank
The degree to which coal has undergone metamorphism. Article: ../Peat and coal/.

Ra-Pb dating
Radium-lead dating, a form of radiometric dating. Article: ../U-Th, U-Pa, and Ra-Pb dating/.

Rb
Chemical symbol for the element rubidium. Articles: ../Chemistry for geologists/, ../Rb-Sr dating/.

Rb-Sr dating
Rubidium-strontium dating, a form of radiometric dating. Article: ../Rb-Sr dating/.

Re
Chemical symbol for the element rhenium. Articles: ../Chemistry for geologists/, ../Other isochron methods/.

Reaction
In chemistry, a process in which molecules form, break apart, or recombine. Article: ../Chemistry for geologists/.

Recrystalization
Change in the texture of a rock. Article: ../Metamorphic rocks/.

Recumbent fold
A fold in a rock which has been pushed so far over as to lie on its side. Articles: ../Orogeny/, ../Folds/.

Redbeds
Sedimentary rocks cemented together chiefly by iron oxide, characteristic of dry climates. Article: ../Sediments and climate/.

Red clay
A synonym sometimes used for pelagic clay. Articles: ../Pelagic clay/, ../Marine sediments/

Reef
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/.

Reef limestone
Limestone resulting from the intact preservation of hard parts of coral or other organisms. Article: ../Reefs/.

Reflection seismology
A method of examining the structure of buried rocks by studying the reflections of Seismic waves produced by artificial explosions. Article: ../Sea level variations/.

Refraction
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/.

Regional metamorphism
Metamorphism over a wide region, caused by deep burial or wide-acting tectonic forces; as opposed to contact metamorphism. Articles: ../Metamorphism/, ../Orogeny/.

Regression
An event in which the shoreline moves in a seaward direction; the opposite of a transgression. Articles: ../Sea level variations/, ../Walther's principle/.

Relative dating
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/.

Re-Os dating
Rhenium-osmium dating, a form of radiometric dating. Article: ../Other isochron methods/.

Replacement
A process forming fossils in which the original organic material is replaced by minerals. Article: ../Fossils/.

Residence time
The average amount of time a given type of atom or molecule will spend in the ocean or in the atmosphere. Article: ../Radiocarbon dating/.

Residual minerals
Minerals which are not dissolved by chemical weathering. Article: ../Chemical weathering/.

Reversed polarity
A condition in which the north and south magnetic poles of the Earth were opposite in orientation to their present position. The opposite of normal polarity. Article: ../Geomagnetic reversals/.

Reverse fault
A dip-slip fault in which the hanging wall moves upwards relative to the foot wall. Article: ../Faults/.

Rhyolite
A felsic extrusive igneous rock; the extrusive counterpart of granite. Article: ../Igneous rocks/.

Rhythmite
A sedimentary rock which display a repetitive vertical succession of types of sediment. Articles: ../Tidal rhythmites and dating/, ../Varves/, ../Milankovitch cycles/.

Ring silicates
Silicate minerals in which the silicate tetrahedra are bonded together to form rings. Article: ../Silicate minerals/.

Ripple
A very small dune. Article: ../Rivers/.

Roche moutonnée
A hump of rock with one side shallow, polished, and striated and the other side steep and ragged, caused by a glacier flowing over the rock. Article: ../Glaciers/.

Rock
An aggregate of one or more minerals or mineraloids. Article: ../Rocks/.

Rock cycle
The set of processes by which rocks are formed, altered, destroyed, and reformed. Article: ../Rocks/.

Rock flour
Extremely fine sediment formed by the grinding action of a glacier. Article: ../Glaciers/.

Rock glacier
A glacier consisting mainly of rocks held together by ice. Article: ../Glaciers/.

Rock salt
Common salt (NaCl) when it occurs naturally as a rock; a synonym for halite. Articles: ../Minerals/, ../Saline giants/.

Rounding
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/.

Rudaceous rocks
Term for conglomerates and breccias. Article: ../Sedimentary rocks/.

Rudists
A group of reef-building molluscs that went extinct at the end of the Cretaceous period. Article: ../Reefs/.

S
Chemical symbol for the element sulfur. Article: ../Chemistry for geologists/

Sabkha
A salt flat found between the sea and a desert above the high-water line. Article: ../Deserts/.

Saltation
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/.

Salt flat
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/.

Sand
Particles of sediment between 1/16mm and 2mm in diameter. Article: ../Sedimentary rocks/.

Sandstone
Rock formed by the cementation of sand. Article: ../Sedimentary rocks/.

Saprolite
Completely weathered rock. Article: ../Chemical weathering/.

Sapropelic coal
Coal where the original organic material comes from the deposition of algae in lakes; as opposed to humic coal. Article: ../Peat and coal/.

Satelite Laser Ranging
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/.

Schist
A high-grade metamorphic rock exhibiting pronounced foliation. Article: ../Metamorphic rocks/.

Schistosity
The kind of foliation found in schist. Article: ../Metamorphic rocks/.

Sclerochronology
An absolute dating method based on the study of growth patterns in shells and corals. Article: ../Sclerochronology/.

Scleroclimatology
The study of past climates by the analysis of the composition of shells. Article: ../Scleroclimatology/.

Sea floor spreading
The motion of two plates away from one another, producing a rift which is continuously filled by magma, producing fresh oceanic crust. Article: ../Sea floor spreading/.

Seaload
The sediments carried by a wave. Article: ../Mechanical weathering and erosion/.

Seamount
A marine mountain which is entirely underwater. Article: ../Hotspots/.

Seat earth
The paleosol underlying coal beds. Article: ../Peat and coal/.

Secular equilibrium
A condition in which the rate of production of a radioactive isotope in a rock is exactly balanced by the radioactive decay of the same isotope. Article: ../U-Th, U-Pa, and Ra-Pb dating/.

Secular variation
The wandering of the magnetic poles over time. Article: ../Geomagnetic reversals/.

Sediment
Particles transported and/or deposited by wind, water, glaciers, precipitation, etc; the constituents of sedimentary rocks. Article: ../Sedimentary rocks/.

Sedimentary rock
Sediment lithified by cementation and/or compaction, or as a result of simple crystal growth in the case of evaporites. Article: ../Sedimentary rocks/.

Sediment trap
A device that collects sediment as it settles. Article: ../Deposition rates/.

Seismic tomography
The science of discovering the internal structure of an object (typically, the Earth) by studying the passage of body waves through it. Articles: ../Seismic waves/, ../Structure of the Earth/.

Seismic waves
Waves in the body or surface of the Earth generated by earthquakes. Articles: ../Seismic waves/, ../Structure of the Earth/.

Seismometer
A device for detecting earthquakes and measuring their properties. Article: ../Seismic waves/.

Semidiurnal
Occurring twice daily. Article: ../Tidal rhythmites and dating/.

Series
A stratigraphic unit smaller than a system but larger than a stage. Article: ../Geological column/.

Serpentinite
A metamorphic rock produced from peridotite in the presence of heat and water. Article: ../Ophiolites/.

Serrated margins
Edges of leaves which are not smooth, characteristic of a temperate climate. Article: ../Leaf shape and temperature/.

Shale
A sedimentary rock formed from silt or clay which exhibits bedding. Article: ../Sedimentary rocks/.

Shear
Stress that causes an object to skew, e.g. the stress that would deform a rectangle into a parallelogram. Article: ../Physical properties of rocks/.

Sheeted dikes
Dikes which stand side by side with one another like books on a shelf rather than intruding into some other rock. Article: ../Ophiolites/.

Sheet glacier
Synonym for continental glacier. Articles: ../Glaciers/, ../Ice ages.

Sheet silicate
Any silicate in which the silicate tetrahedra bond together to form a sheet. Article: ../Silicate minerals/.

Shoestring
A geological feature which is long and thin, e.g. a river or a shoreline. Articles: ../Rivers/, ../Nearshore sediments/.

Si
Chemical symbol for the element silicon. Articles: ../Chemistry for geologists/, ../Silicate minerals/.

Siderite
The mineral FeCO3. Article: ../Banded iron formations/.

Silica
Silicon dioxide, (SiO2). This occurs in many forms, such as quartz, opal, and chert Article: ../Minerals/.

Silicate
A shorter way of saying silicate mineral. Article: ../Silicate minerals/.

Silicate mineral
Any of a large and important class of minerals the chemistry of which is based on the silicate tetrahedron. Article: ../Silicate minerals/.

Silicate tetrahedron
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/.

Siliceous
Composed of silica. Article: ../Siliceous ooze/.

Siliceous ooze
Ooze on the sea floor, composed of the siliceous test of radiolaria and diatoms. Articles: ../Siliceous ooze/, ../Marine sediments/.

Sill
A sheet of intrusive rock forced between strata. Articles: ../Igneous rocks/, ../Igneous rocks and stratigraphy/.

Silt
Clasts between 1/16mm and 1/256mm in diameter. Article: ../Sedimentary rocks/.

Siltstone
Sedimentary rock composed of silt. Article: ../Sedimentary rocks/.

Sinstral
A fault is said to be sinstral if someone standing on one side of the fault and looking at the other when there is motion along the fault would see the other side moving to the left. Article: ../Faults/.

Skarn
A rock produced by metasomatism. Article: ../Metamorphic rocks/.

Sky island
A mountain habitat which is home to species which are isolated by their inability to cross the drier hotter surrounding plain. Article: ../Ice ages/.

Slab
The portion of a plate being thrust into the athenosphere during subduction. Article: ../Subduction/.

Slate
A metamorphic rock formed by metamorphism of shale, exhibiting pronounced foliation. Article: ../Metamorphic rocks/.

Slatey foliation
The sort of foliation found in slate. Article: ../Metamorphic rocks/.

Slickenside
A smoothed and striated surface produced by the friction between the two sides of a fault. Article: ../Faults/.

SLR
Abbreviation for Satelite Laser Ranging. Article: ../Continental drift/.

Sm
Chemical symbol for the element samarium. Articles: ../Chemistry for geologists/, ../Other isochron methods/.

Sm-Nd dating
Samarium-neodymium dating, a form of radiometric dating. Article: ../Other isochron methods/.

Snell's Law
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/.

Soil
Sediment which has been altered by the effects of 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/.

Sole mark
A mark made in sediment when it is scoured by a current. Article: ../Turbidites/.

Solid solution
A mineral in which some positions in the crystal lattice may be filled by different elements. Article: ../Minerals/.

Soluble
Capable of becoming dissolved. Article: ../Chemistry for geologists/.

Solute
A substance dissolved in a solvent. Article: ../Chemistry for geologists/.

Solvent
The medium in which a substance is dissolved. Article: ../Chemistry for geologists/.

Sorosilicates
Silicates in which the silica tetrahedra are bonded together in pairs. Article: ../Minerals/.

Sorting
Sediment is said to be well-sorted if it consists of particles of about the same grain-size. Article: ../Sedimentary rocks/.

Speleothem
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/.

Spherical symmetry
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/.

Sr
Chemical symbol for the element strontium. Articles: ../Chemistry for geologists/, ../Rb-Sr dating/.

Stage
A stratigraphic unit smaller than a series but larger than a zone. Article: ../Geological column/.

Stalactite
A speleothem hanging like an icicle from the roof of a cave. Article: ../U-Pb, Pb-Pb, and fission track dating/.

Stalagmite
A speleothem in the form of a mound or column rising from the floor of a cave. Article: ../U-Pb, Pb-Pb, and fission track dating/.

Standard
A rock of known age used in Ar-Ar dating. Article: ../Ar-Ar dating/.

Steno's principles
The principles of superposition, of original horizontality, and of original continuity. Article: ../Steno's principles/.

Step heating
A process used in Ar-Ar dating in which a rock sample is heated in steps of progressively higher temperatures. Article: ../Ar-Ar dating/.

Strain
The deformation of a solid body as a result of stress. Article: ../Physical properties of rocks/.

Stratigraphy
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/

Stratum
A layer of sedimentary rock with distinctive mineralogical, structural, or fossil characteristics such that it can be distinguished from the strata above and below it. Article: ../Sedimentary rocks/.

Stream-dominated deltas
Deltas with long distributary channels reaching seaward; deltas in which the most important factor in their formation is the river discharging via the delta. Article: ../Deltas/.

Stress
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/.

Striae
Synonym for striations. Article: ../Glaciers/.

Striation
Grooves left by the movement of a glacier over a rock, parallel to the direction of motion. Article: ../Glaciers/.

Strike-slip fault
A fault in which the blocks on either side of the fault move laterally but not vertically with respect to one another in a direction parallel to the fault. Article: ../Faults/.

Stromatoporoids
Sponges which secrete a calcium carbonate skeleton and so were once important reef-forming organisms. While not actually extinct, they now live only in marginal habitats. Article: ../Reefs/.

Subbituminous coal
Coal of a higher rank than lignite and a lower rank than bituminous coal. Article: ../Peat and coal/.

Subduction
The motion of one plate beneath another and into the mantle. Article: ../Subduction/.

Supercontinent
A landmass consisting of most or all of the continental crust joined together to form a single continent. Article: ../Continental drift/.

Supercontinent cycle
A process in which supercontinents repeatedly form and then rift again into separate continents. Article: ../Continental drift/.

Superposed river
A river which exists before the creation by erosion of the hills through which it flows. Article: ../Rivers/.

Superimposed river
Synonym for superposed river. Article: ../Rivers/.

Suspension
A form of transport of clasts by wind or water where the particles are carried above the ground, sea bed, river bed, etc. Articles: ../Mechanical weathering and erosion/.

Suture zone
The line along which a continent becomes joined to another continent, microcontinent, or island arc. Article: ../Terranes/.

Swamp
An area of waterlogged ground in which the water is shallow enough for land plants to grow. Articles: ../Peat and coal/, ../Sediments and climate/.

S-waves
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/.

Syncline
Structure formed when rocks are folded downwards. Article: ../Folds/.

System
The largest stratigraphic unit. Article: ../Geological column/.

Tarn
A lake that forms in the former cirque of a glacier after the glacier has melted. Article: ../Glaciers/.

Tectonic window
A place at which a rift in the Earth's crust allows us to see deeper into the crust than is normally possible. Article: ../Ophiolites/.

Tectosilicate
A synonym for lattice silicate. Article: ../Silicate minerals/.

Tension
Stress that produces elongation of a solid along the direction in which force is applied. Article: ../Physical properties of rocks/.

Terminal lake
A lake which water flows into but not out of, usually salty as a result of the accumulation of dissolved minerals. Article: ../Lakes/.

Terminal moraine
A moraine deposited at the ablation zone of a glacier. Articles: ../Glaciers/, ../Ice ages/.

Terragenic
Having an origin on land. Article: ../Marine sediments/.

Terrane
Part of a landmass, bounded by tectonic faults, which is different in many ways from the main landmass to which it is attached. Article: ../Terranes/.

Terrestrial
Having to do with the land. Articles: ../Sedimentary rocks/, ../Lakes/, ../Radiocarbon dating/.

Test
The shell of a micro-organism such as a diatom or a foraminiferan. Articles: ../Calcareous ooze/, ../Siliceous ooze/.

TEX86
A temperature proxy based on measurement of the different varieties of GDGTs in sediments. Article: TEX86.

Texture
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 evolution
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/.

Thermohaline circulation
Deep-water circulation driven by density differences in the temperature and salinity of sea water. Article: ../Climate models/.

Thrust fault
A reverse fault in which the angle of the fault is more than 45° from the vertical. Article: ../Faults/.

Tide-dominated delta
A delta in which the most important factor in its dynamics is the tide; characterized by the formation of offshore bars running parallel to the direction of the tide. Article: ../Deltas/.

Till
Unsorted and usually unbedded sediment deposited by a glacier. Article: ../Glaciers/.

Tidal braking
Slowing of the Earth's rotation as a result of the tidal interaction between the Earth and the Moon. Articles: ../Sclerochronology/, ../Tidal rhythmites and dating/.

Tillite
The lithified equivalent of till. Article: ../Glaciers/.

Topset beds
The flat beds of sediment deposited on the top surface of a delta. Article: ../Deltas/.

Trace fossil
A fossil such as a footprint which is not of an animal but which was produced by one. Article: ../Fossils/.

Transgression
An event in which the shoreline moves inland; the opposite of a regression. Articles: ../Walther's principle/, ../Sea level variations/.

Transitive
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/.

Trench
A depression in the sea floor formed along the line where one plate subducts beneath another. Article: ../Subduction/.

Trona
An evaporite mineral having the chemical formula Na3(CO3)(HCO3)·2H2O.

Tsunami
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/.

Tsunamite
Sediment deposited by a tsunami. Article: ../Actualism/.

Tuff
Lithified Volcanic ash Article: ../Volcanic ash/.

Turbid
Loaded with sediment. Article: ../Turbidites/.

Turbidite
Rock formed from sediment deposited by a turbidity current. Article: ../Turbidites/.

Turbidity current
A current which manages to keep from mixing with the medium through which it flows because, being turbid, it is denser than the surrounding medium. Article: ../Turbidites/.

Turbidity sediment
Sediment deposited by a turbidity current. Article: ../Turbidites/.

U
Chemical symbol for the element uranium. Articles: ../Chemistry for geologists/, ../U-Pb, Pb-Pb, and fission track dating/, ../U-Th, U-Pa, and Ra-Pb dating/.

Uk'37
A proxy for temperature based on measurements of the different kinds of alkenones preserved in sediment. Article: Uk'37.

Ultrabasic rock
An obsolete and inaccurate term for ultramafic rock. Article: ../Igneous rocks/.

Ultramafic rock
A rock which is extremely mafic; that is, particularly low in silicate tetrahedra and high in magnesium and iron. Article: ../Igneous rocks/.

Unconformity
A surface between successive strata representing a period of erosion or of no deposition. Article: ../Unconformities/.

Underclay
Synonym for seat-earth. Article: ../Peat and coal/.

Uniformitarianism
An alternative term for actualism, not used in this textbook because of ambiguities and inconsistencies in its usage. Article: ../Actualism/.

Unstable
Prone to radioactive decay. Article: ../Radioactive decay/.

U-Pa dating
Uranium-protactinium dating, a form of radiometric dating. Article: ../U-Th, U-Pa, and Ra-Pb dating/.

U-Pb
Uranium-lead dating, a form of radiometric dating. Article: ../U-Pb, Pb-Pb, and fission track dating/.

Upper plane bed
A flat layered surface formed in a river bed when the river is travelling at too great a velocity to form ripples or dunes. Article: ../Rivers/.

Uraninite
The mineral UO2 Article: ../Banded iron formations/.

U-Th dating
Uranium-thorium dating, a form of radiometric dating. Article: ../U-Th, U-Pa, and Ra-Pb dating/.

Vail curve
A reconstruction of past variations of sea level based on the study of unconformities in the geological record. Article: ../Sea level variations/.

Valley glacier
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/.

Valve
One of the two shells of a bivalve. Article: ../Way-up structures/.

Varve
A lamina of coarse light sediment grading into fine dark sediment, often found in lakes fed by meltwater from a glacier and representing one year's deposition. Articles: ../Glaciers/, ../Lakes/, ../Varves/.

Very Long Baseline Interferometry
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/.

Viscosity
Informally speaking, the reluctance of a liquid to flow; so for example maple syrup is more viscous than water. Article: ../Igneous rocks/.

Vis plastica
The name of an imaginary force once thought to cause fossils to grow in rocks. Article: ../Fossils/.

VLBI
Abbreviation for Very Long Baseline Inferometry. Article: ../Continental drift/.

Volcanic ash
Fine debris formed when a volcano sprays out fine particles of lava. Note that the term "ash" is a misnomer, since volcanic "ash" is not a product of combustion. Article: ../Volcanic ash/.

Walther's principle
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/.

Wave base
The greatest depth at which the action of a wave has any effect. Article: ../Nearshore sediments/.

Wave ripples
Ripples in sand or other sediment caused by the action of the tide. Article: ../Nearshore sediments/.

Wave-dominated deltas
deltas in which longshore drift forms barrier islands in front of the delta. Article: ../Deltas/.

Way-up structure
A geological feature which enable us to discover which way up a rock was when it was originally formed. Article: ../Way-up structures/.

Weathering
Processes which break up rock but do not themselves transport it, as distinct from erosion. Article: ../Mechanical weathering and erosion/, ../Chemical weathering/.

Weathering rind
The outer, weathered volume of a rock in which the outside has undergone weathering but the weathering process has not yet penetrated all the way through the rock. Article: ../Chemical weathering/.

Welded tuff
Tuff which forms when a fall of Volcanic ash is still hot enough to weld itself together. Article: ../Volcanic ash/.

Xenotime
The mineral YPO4, useful because it can be used in the radiometric dating of sedimentary rocks. Article: ../U-Pb, Pb-Pb, and fission track dating/.

Y
Chemical symbol for the element yttrium. Article: ../Chemistry for geologists/.

Zircon
The mineral ZrSiO4, useful for radiometric dating because of its resistance to erosion, weathering, and metamorphosis. Article: ../U-Pb, Pb-Pb, and fission track dating/.

Zone
The smallest stratigraphic unit. Can also be used in the usual informal sense of a region or area, as in the term "ablation zone". Article: ../Geological column/.

Zr
Chemical symbol for the element zirconium. Article: ../Chemistry for geologists/.