Historical Geology/Glossary and index
100,000 year problem
editThe 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
editA 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
editThe 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
editErosion of rocks caused by the sediments carried by wind or water. Article: Mechanical weathering and erosion.
Absolute dating
editDating 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
editThe flat terrain found at the bottom of the ocean beyond the continental margin. Article: Marine sediments.
Accretionary prism
editAn accumulation of sediment which forms in a trench. Article: Subduction.
Accretionary wedge
editA synonym for accretionary prism. Article: Subduction.
Accumulation zone
editThe beginning of a glacier; the zone in which snowfall exceeds the loss of snow by melting or evaporation. Article: Glaciers.
Acidic rock
editAn outdated and inaccurate term for felsic rock. Article: Igneous rocks.
ACM
editAbbreviation for atmospheric circulation model. Article: Climate models.
Actualism
editThe 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
editHaving to do with the wind. Article: Deserts.
Aeolian sandstone
editSandstone formed from sand deposited by the wind, i.e. desert sand. Article: Deserts.
Al
editChemical symbol for the element aluminum. Article: Chemistry for geologists.
Alkenones
editOrganic molecules produced by certain planktonic organisms, used in the temperature proxy known as Uk'37. Article: Uk'37.
Alluvial fan
editA fan-shaped deposit of sediment left where a mountain stream reaches a plain. Articles: Deserts, Rivers.
Alpha decay
editRadioactive decay involving the emission of an alpha particle. Article: Radioactive decay.
Alpha particle
editA particle consisting of two protons and two neutrons. Article: Radioactive decay.
Aluminosilicate
editAny of an important class of silicate minerals in which the structure involves aluminum-based as well as silicon-based tetrahedra. Article: Minerals.
Alpine glacier
editSynonym for valley glacier. Article: Glaciers.
Amber
editA mineraloid formed from the solidified resin of trees. Article: Fossils.
Amino acid dating
editA rather unreliable method of absolute dating based on measuring the racemization of organic remains. Article: Amino acid dating.
Amorphous
editLacking a crystal structure. Article: Minerals.
Ammonite
editAn extinct marine mollusc. Articles: Geological column, Index fossils.
Amphibole
editA group of silicate minerals in which the SiO44- units are bonded to form a double chain. Article: Silicate minerals, Igneous rocks.
Angle of repose
editThe maximum angle from the horizontal that a heap of a given type of sediment can assume without collapsing. Article: Steno's principles.
Angular unconformity
editAn 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
editIn 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
editA river which is present before the uplift of the hills through which it flows. Article: Rivers.
Anthracite
editA very black, hard, and shiny form of coal produced by metamorphism. Article: Peat and coal.
Anticline
editStructure formed when rocks are folded upwards. Article: Folds.
Antidune
editA 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
editAn upward fold in rocks. Article: Folds.
Aphanitic
editAn 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
editApparent secular variation recorded in the paleomagnetic record which is actually caused by the motion of plates relative to the poles. Article: Continental drift.
Ar
editChemical symbol for the element argon. Articles: Chemistry for geologists, K-Ar dating, Ar-Ar dating.
Ar-Ar dating
editArgon-argon dating, a form of radiometric dating. Article: Ar-Ar dating.
Aseismic ridge
editA long linear trail of volcanic islands and seamounts caused by a plate passing over a hotspot. Article: Hotspots.
Archaecyathids
editEarly 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
editHaving 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
editHaving 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
editAlternative term for sandstone. Article: Sedimentary rocks.
Arkose
editSandstone 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
editThe portion of the mantle just below the lithosphere. Article: Structure of the Earth.
Atom
editA nucleus of protons and neutrons orbited by electrons arranged in electron shells. Articles: Chemistry for geologists, Radioactive decay.
Atmospheric circulation model
editA climate model which only takes into account the circulation of the atmosphere and not the oceanic circulation. Article: Climate models.
Atomic number
editThe number of protons in the nucleus of an atom. Article: Chemistry for geologists.
Atomic weight
editThe sum of the number of protons and neutrons in the nucleus of an atom. Article: Chemistry for geologists.
Attrition
editErosional processes whereby the clasts transported by wind or water are broken or worn down. Article: Mechanical weathering and erosion.
Aureole
editA ring of metamorphic rock formed around an igneous intrusion by contact metamorphism. Article: Metamorphic rocks.
B
editChemical symbol for the element boron. Article: Chemistry for geologists.
Ba
editChemical symbol for the element barium. Articles: Chemistry for geologists, Other isochron methods.
Backshore
editThat part of a beach which is above the high-water line. Article: Nearshore sediments.
Bajada
editThe merging of two or more alluvial fans. Article: Deserts.
Banded iron formation
editSedimentary rock consisting of alternating bands of iron oxide and other sedimentary rock, typically chert. Article: Banded iron formations.
Bar
editA 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
editAn island formed at the mouth of a river running at right angles to the direction of the distributary streams. Article: Deltas.
Basalt
editA mafic intrusive igneous rock, black in color and aphanitic. Article: Igneous rocks.
Basic rock
editAn obsolete and inaccurate term for mafic rock. Article: Igneous rocks.
Be
editChemical symbol for the element berylium. Article: Chemistry for geologists.
Bed
editA layer in a sedimentary rock. Article: Sedimentary rocks.
Bedding
editA structure found in sedimentary rocks in which the rock is visibly composed of numerous layers (beds). Article: Sedimentary rocks.
Bedding planes
editThe planes dividing the beds in a bedded rock. Article: Sedimentary rocks.
Bedrock
editThe solid rock underlying unlithified sediment. Articles: Glaciers, Rivers.
Beta decay
editA 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
editA 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
editA form of radioactive decay in which a proton is converted into a neutron by the emission of a positron. Article: Radioactive decay.
BIF
editAbbreviation for banded iron formation. Article: Banded iron formations.
Biogeography
editThe study of the geographical distribution of living or extinct organisms. Articles: Continental drift, Biogeography and climate.
Bioturbation
editChanges in the structure of sediment caused by the activity of living things. Article: Nearshore sediments.
Bituminous coal
editThe 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
editA 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
editSeismic 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
editAn evaporite mineral having the chemical formula Na2B4O7·10H2O. Article: Deserts.
Bottom-set beds
editHorizontal beds of sediment deposited on the sea or lake floor in front of a delta. Article: Deltas.
Bouma sequence
editThe characteristic pattern of sediment deposited by a turbidity current. Article: Turbidites.
Br
editChemical symbol for the element bromine. Articles: Chemistry for geologists, Other isochron methods.
Braided stream
editA 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
editA rock consisting of large unrounded fragments cemented together. Articles: Sedimentary rocks, Faults.
Brittle
editA 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
editTerm occasionally used for pelagic clay. Articles: Pelagic clay, Marine sediments.
C
editChemical symbol for the element carbon. Articles: Chemistry for geologists, Radiocarbon dating.
Ca
editChemical symbol for the element calcium. Article: Chemistry for geologists.
Calcareous
editHaving to do with calcium carbonate. Articles: Calcareous ooze, Marine sediments.
Calcareous ooze
editA calcareous sediment found over large areas of the ocean floor, consisting of the shells of small organisms. Articles: Calcareous ooze, Marine sediments.
Calcite
editA mineral consisting of calcium carbonate in a trigonal crystal system.
Calcium carbonate
editThe chemical CaCO3. Most shells are formed of this, as are the rocks limestone and marble. Articles: Calcareous ooze, Marine sediments.
Carbonate
editA molecule with the negative ion CO32-; also a rock consisting of carbonates, particularly limestone. Articles: Calcareous ooze, Marine sediments.
Carbonate compensation depth
editThe 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
editAlternative term for radiocarbon dating. Article: Radiocarbon dating.
C-C dating
editAlternative term for radiocarbon dating. Article: Radiocarbon dating.
Carbon dioxide
editThe 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
editThe 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
editA fossil produced when a mold is filled with minerals. Article: Fossils.
Cave formation
editAlternative term for a speleothem. Articles: U-Th, U-Pa, and Ra-Pb dating, U-Pb, Pb-Pb, and fission track dating.
CCD
editAbbreviation for carbonate compensation depth. Article: Calcareous ooze.
14C dating
editAlternative term for radiocarbon dating. Article: Radiocarbon dating.
Ce
editChemical symbol for the element cerium. Articles: Chemistry for geologists, Other isochron methods.
Cementation
editThe 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
editVolcanic ash which has lithified by the process of cementation, as opposed to welded tuff. Article: Volcanic ash.
Chain silicate
editA 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
editRock which, under a microscope, is clearly composed of the tests of calcium carbonate-secreting micro-organisms. Article: Calcareous ooze.
Chemical sediment
editA 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
editWeathering 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
editA sedimentary rock composed of silica, having an amorphous or very fine-grained structure. Article: Siliceous ooze.
Chirality
editThe handedness of an organic molecule. Article: Amino acid dating.
Cirque
editA 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
editChemical symbol for the element chlorine. Article: Chemistry for geologists.
Clast
editA piece of rock detached by erosion or weathering from a larger rock. Article: Sedimentary rocks.
Clastic
editComposed of clasts. Article: Sedimentary rocks.
Clay
editThe 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
editSedimentary rock composed of clay. Article: Sedimentary rocks.
Climate
editBroad trends in the weather; i.e. the tendency of a location to be hot and humid, or dry and cold. Article: Paleoclimatology: introduction.
Closure
editThe 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
editThe time between snowfall and closure, varying from location to location. Article: Ice cores.
Coal
editCoal is peat which has been lithified by compaction, heat, or both. Article: Peat and coal.
Coalification
editThe chemical processes by which peat is turned into coal. Article: Peat and coal.
Coarse-grained
editComposed of crystals of large size; the opposite of fine-grained. Article: Igneous rocks.
Cobble
editA clast between 64 and 256mm in diameter, especially one that exhibits rounding. Articles: Sedimentary rocks, Rivers.
Coccolith
editA calcareous plate forming part of the shell of a coccolithophore; a common constituent of calcareous ooze. Article: Calcareous ooze.
Coccolithophores
editA group of micro-organisms clad in coccoliths. Article: Calcareous ooze.
Compaction
editDecrease in volume of sediment, caused by the pressure induced by being buried under yet more sediment. Articles: Sedimentary rocks, Ice cores.
Complacent
editA tree is said to be complacent if the thickness of its growth rings is unaffected by annual variations in temperature. Article: Dendrochronology.
Compression
editStress that produces shortening of a solid along the direction in which force is applied. Article: Physical properties of rocks.
Concordant
editOf dates, in agreement with one another. Articles: U-Pb, Pb-Pb, and fission track dating, Absolute dating: an overview.
Conglomerate
editA 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
editTerm used ambiguously to refer either to a conodont animal or a conodont structure; context usually makes it clear which. Article: Fossils.
Conodont animals
editA group of extinct primitive chordates having no hard parts except for conodont structures. Article: Fossils.
Conodont structures
editThe hard parts of a conodont animal. Article: Fossils.
Contact metamorphism
editMetamorphism 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
editThe theory that continents have shifted their positions over time; now subsumed into the theory of plate tectonics. Article: Continental drift.
Continental glacier
editA 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
editThe continental shelf, slope, and rise. Article: Marine sediments.
Continental rise
editThe shallowly sloping (approximately 1 degree from horizontal) terrain between the continental slope and the abyssal plain. Article: Marine sediments.
Continental shelf
editThat part of a continent which is underwater, lying between the unsubmerged portion of a continent and the continental slope. Article: Marine sediments.
Continental slope
editA shallow slope, typically between 4 and 10 degrees from horizontal, found between the continental shelf and the continental rise. Article: Marine sediments.
Corals
editA group of marine organisms. Hard corals secrete skeletons of calcium carbonate and so act as reef-forming organisms. Article: Reefs.
Core
editThe 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
editDust 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
editStreams of high-energy particles which bombard the Earth from outer space. Article: Cosmogenic surface dating.
Cosmogenic
editOf isotopes, produced by cosmic rays. Articles: Cosmogenic surface dating, Radiocarbon dating.
Cosmogenic surface dating
editA method of absolute dating which gives the time since a rock became exposed on the surface. Article: Cosmogenic surface dating.
Country rock
editThe rock into which an igneous rock intrudes. Article: Igneous rocks.
Covalent bond
editA bond between atoms in which they share electrons. Article: Chemistry for geologists.
Creep
editTransport 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
editBedding 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
editAn 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
editThe correlation of dates from different sources. Article: Dendrochronology.
Crust
editThe 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
editA large molecule composed of smaller chemical units chemically bonded together in a regular repetitive arrangement. Article: Minerals.
Crystal habit
editThe shape or shapes in which a mineral will typically grow. Article: Minerals.
Crystal system
editOne 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
editVery roughly speaking, the temperature above which a material cannot be magnetized and below which it can. Article: Geomagnetic reversals.
Cyclosilicate
editA synonym for ring silicate. Article: Silicate minerals.
δ18O
editA proxy for temperature based on oxygen isotope ratios. Articles: Scleroclimatology, Ice cores.
Decay
editThe destruction of organic remains by organic processes. Articles: Peat and coal, Soils and paleosols, Fossils.
- or
Radioactive decay. Articles: Radioactive decay.
Decay chain
editA sequence of events in which one isotope decays to another via an intermediate sequence of unstable isotopes. Article: Radioactive decay.
Deflation
editThe erosion of fine particles from dry soil by the wind. Article: Deserts.
Deflation lake
editA lake caused when deflation has caused a hollow the bottom of which lies below the water table. Article: Deserts.
Delta
editThe body of sediment deposited when a river flows into a lake or the sea. Article: Deltas.
Dendrochronology
editA method of dating wood by studying the annual growth rings produced by the tree. Article: Dendrochronology.
Deposition
editAll those processes which add sediment to a surface; the opposite of erosion. Article: Sedimentary rocks.
Desert
editAn 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
editDessication crack
editAlternative term for a mud crack. Article: Way-up structures.
Detrital
editComposed of clasts; synonymous with clastic. Article: Sedimentary rocks.
Dextral
editA 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
editA device used in experimental petrology to subject small samples of rock to large amounts of stress. Article: Physical properties of rocks.
Diapir
editA sedimentary structure formed by one type of sediment flowing upwards through another as a result of pressure. Article: Way-up structures.
Diatomite
editA very light and porous rock formed from diatom tests that have undergone little in the way of compaction and recrystallization. Article: Siliceous ooze.
Diatoms
editA group of single-celled algae which produce siliceous tests; a major source of siliceous ooze. Article: Siliceous ooze.
Differentiation
editThe mechanism by which an originally homogeneous Earth separated into crust, mantle, and core. Article: Structure of the Earth.
Dike
editA 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
editA 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
editAn unconformity in which the underlying strata are parallel with the overlying strata. Article: Unconformities.
Dissolved
editA 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
editA smaller stream flowing out of a larger river, as opposed to a tributary, which flows in. Article: Rivers.
Diurnal
editOccurring once daily. Article: Tidal rhythmites and dating.
Docking
editThe union of a terrane with the landmass to which it becomes attached. Article: Terranes.
Drag fold
editA structure formed in a rock as friction drags the material in it backwards relative to its motion along a fault. Article: Folds.
Drift
editDrifters
editTerm for the early supporters of continental drift; the opposite of "fixists". Article: Continental drift.
Dropstone
editA 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
editA smallish hill shaped somewhat like the back of a spoon, deposited by glaciers in a manner not fully understood. Article: Glaciers.
Ductile
editA 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
editA mound of sand formed by the action of wind or water. Articles: Rivers, Deserts.
Dunnite
editAn ultramafic rock consisting entirely of olivine. Article: Igneous rocks.
Elastic
editA 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
editA particle with negative charge and negligible mass found orbiting the nucleus of an atom. Articles: Chemistry for geologists, Radioactive decay.
Electron capture
editA 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
editAn orbit followed by electrons about an atomic nucleus. Articles: Chemistry for geologists, K-Ar dating, Ar-Ar dating.
Element
editAtoms 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
editMolecules which are mirror images of one another. Article: Amino acid dating.
Entire margins
editLeaf margins which are smooth rather than serrated, characteristic of warm humid climates. Article: Leaf shape and temperature.
Epicenter
editThe point on the Earth's surface directly above the focus of an earthquake. Article: Seismic waves.
Erg
editErosion
editAny process capable of breaking up rocks or soils and transporting the resulting clasts. Article: Mechanical weathering and erosion.
Erratic boulder
editA boulder which does not fit in with the geology of its surroundings, transported from its place of origin by a glacier. Article: Glaciers.
Evaporite
editAny 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
editAn 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
editIn 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
editArgon which is not radiogenic; a potential source of error in Ar-Ar dating. Articles: K-Ar dating, Ar-Ar dating.
Exponential decay
editA 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
editAny 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
editA facies is a body of sediment or sedimentary rock characteristic of a particular depositional environment. Article: Walther's principle.
Fault
editA planar fracture or discontinuity in a volume of rock. Articles: Faults, Terranes.
Fault breccia
editBreccia produced by the crushing action of motion along a fault. Article: Faults.
Fault gouge
editMaterial similar to fault breccia but finer in texture. Article: Faults.
Fault mirror
editAlternative term for slickenside. Article: Faults.
Fauna
editAnimals (in the broadest possible sense, including birds, fish, crustaceans, molluscs, etc). Article: Principle of faunal succession.
Fe
editChemical symbol for the element iron. Article: Chemistry for geologists.
Feldspar
editA group of aluminosilicate minerals with a lattice structure. Article: Igneous rocks.
Felsic rocks
editRocks which are high in silica and feldspar and low in magnesium and iron. The opposite of mafic rocks. Article: Igneous rocks.
Fine-grained
editComposed of crystals or clasts of small size; the opposite of coarse-grained. Article: Igneous rocks.
Fining-up sequence
editA form of grading upwards from coarse to fine sediments associated with rivers. Article: Rivers.
Firn
editSnow which has compacted, but not so far as to become ice. Article: Ice cores.
Fissile
editOf a rock, having the property of splitting easily in a given direction (e.g. between bedding planes). Article: Sedimentary rocks.
Fission track dating
editA form of absolute dating which involves counting the fission tracks in a rock. Article: U-Pb, Pb-Pb, and fission track dating.
Fission tracks
editMicroscopic scars left in minerals by alpha particles. Article: U-Pb, Pb-Pb, and fission track dating.
Fixist
editTerm for the early opponents of continental drift; the opposite of "drifter". Article: Continental drift.
Flame structure
editA 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
editDeposits in which light and heavy sediments alternate, characteristic of nearshore environments. Article: Nearshore sediments.
Flood plain
editThe flattened, sediment-rich area formed by the action of rivers on a landscape. Article: Rivers.
Flume
editAn artificial channel used by geologists to study the transport of sediment by water in the laboratory. Articles: Rivers.
Fluvial
editHaving to do with rivers. Article: Rivers.
Focus
editThe point in the Earth at which an earthquake originates. Article: Seismic waves.
Foliation
editThe 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
editRocks lying above a dip-slip or oblique fault. Article: Faults.
Foram
editShort form of foraminiferan. Article: Calcareous ooze.
Foraminiferans
editA group of micro-organisms which secrete calcareous tests; one of the most common constituents of calcareous ooze. Article: Calcareous ooze.
Foreset beds
editBeds of sediment sloping down at the front of a delta into the sea or lake into which it discharges. Article: Deltas.
Foreshore
editThat part of the nearshore which is uncovered at high tide. Article: Nearshore sediments.
Fossil
editOrganic remains, or traces of organic activity such as footprints, preserved in the geological record. Article: Fossils.
Gabbro
editMafic intrusive igneous rock; the intrusive equivalent of basalt. Article: Igneous rocks.
GCM
editAbbreviation for general circulation model. Article: Climate models.
GDGTs
editGlycerol 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
editA climate model which takes into account both the atmospheric and the oceanic circulation. Article: Climate models.
Geological column
editA table showing the order of the faunal succession in the fossil record. Article: Geological column.
Geomagnetic reversal
editA change in state from normal polarity to reversed polarity, or vice versa. Articles: Geomagnetic reversals, Paleomagnetic dating.
Geopetal structure
editA 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
editHaving to do with glaciers. Articles: Glaciers, Ice ages, Glacial marine sediment, Ice cores.
Glacial outwash
editSediment carried out of a glacier by meltwater. Article: Glaciers.
Glacial polish
editThe smooth (but striated) surface produced on a rock by the polishing action of a glacier passing over it. Article: Glaciers.
Glaciation
editGlacier
editA moving mass of ice. Articles: Glaciers, Ice ages, Glacial marine sediment, Ice cores.
Glass
editAny 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
editA 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
editA metamorphic rock of high grade with a distinctive streaky appearance produced by the separation out of chain silicates into streaks. Article: Metamorphic rocks.
GOE
editAbbreviation for Great Oxygen Event. Article: Banded iron formations.
Goethite
editThe iron oxide mineral FeO(OH). Articles: Deserts, Banded iron formations.
GPS
editAbbreviation for Global Positioning System. Article: Continental drift.
Grade
editThe degree to which a rock has undergone metamorphism, depending on the amount of heat to which it has been exposed. Article: Metamorphic rocks.
Grading
editChange 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
editA lamina formed in sand dunes when sand at the crest of the dune avalanches down the lee face of the dune. Article: Deserts.
Granite
editA felsic intrusive igneous rock; the intrusive counterpart of rhyolite. Article: Igneous rocks.
Gravel
editSediment consisting of clasts 2mm in diameter and upwards. Article: Sedimentary rocks.
Greenhouse gas
editA gas such as carbon dioxide or methane which helps keep the Earth warm by trapping heat in the atmosphere. Article: Ice ages.
Greywacke
editSandstone consisting of quartz, feldspar, and small rock fragments embedded in a clay matrix. Article: Sedimentary rocks.
Ground mass
editFiner 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
editIn 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
editA layer of wood produced by a tree on an annual basis, used in dendrochronology. Articles: Dendrochronology, Dendroclimatology.
Gypsum
editA mineral consisting of hydrated calcium sulfate (CaSO4.H2O) with a monoclinic crystal system. Article: Saline giants.
H
editChemical symbol for the element hydrogen. Article: Chemistry for geologists.
Half-life
editThe 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
editRock salt (NaCl). Article: Saline giants.
Hallam curve
editA reconstruction of past variations of sea level based on sedimentary evidence of transgressions and regressions. Article: Sea level variations.
Hanging wall
editRocks lying above a dip-slip or oblique fault. Article: Faults.
Hematite
editThe iron oxide mineral Fe2O3. Articles: Deserts, Banded iron formations.
Herringbone crossbedding
editA form of cross-bedding in which the direction of slope alternates, as a result of oscillatory flow. Article: Nearshore sediments.
Hf
editChemical symbol for the element hafnium. Articles: Chemistry for geologists, Other isochron methods.
Hoodoo
editA pillar of rock produced by erosion. Article: Steno's principles.
Horizon
editA distinct layer in a soil, formed by pedogenetic processes. Article: Soils and paleosols.
Hornfels
editA large group of metamorphic rocks produced from sedimentary rocks by contact metamorphism. Article: Metamorphic rocks.
Hotspot
editA stationary point of high volcanic activity above which plates pass, creating an aseismic ridge. Article: Hotspots.
Humus
editDecaying organic matter in soil. Article: Soils and paleosols.
Humic coal
editCoal produced by the deposition of land plants in swamps, as opposed to sapropelic coal. Article: Peat and coal.
Hydrothermal
editHaving to do with hot water. Article: Metamorphic rocks.
Ice age
editA time at which sheet glaciers are present on some regions of the Earth's surface. Article: Ice ages.
Ice sheet
editSynonym for continental glacier. Articles: Glaciers, Ice ages.
Igneous rock
editRock 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
editA 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
editAny 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
editA species suitable for the production of index fossils. Article: Index fossils.
Inert
editOf an element, unable to participate in chemical reactions. Article: K-Ar dating.
Inosilicate
editA synonym for chain silicate. Article: Silicate minerals.
Insolation
editThe quantity per area of solar radiation reaching a given location. Articles: Milankovitch cycles, Climate models.
Insoluble
editIncapable of becoming dissolved. Article: Chemistry for geologists.
Interference ripples
editRipples caused by two currents flowing (one after the other) at or near right-angles to one another. Article: Nearshore sediments.
Interglacial
editA time of glacial retreat during an ice age. Article: Ice ages.
Interfingering
editA 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
editA drainage pattern typical of deserts, in which rivers flow into the desert and evaporate. Article: Deserts.
Intrusive rock
editRock 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
editGrading where the size of clasts varies from small clasts at the bottom of a layer to large clasts at the top. Article: Turbidites.
Ion
editAn atom which has gained or lost electrons, giving it a negative or positive charge respectively. Article: Chemistry for geologists.
Ionic bond
editA bond between atoms in which one atom donates electrons to another. Article: Chemistry for geologists.
Iron oxides
editMinerals containing both iron and oxygen, as the name suggests: examples include hematite and goethite. Articles: Deserts, Banded iron formations.
Isochron dating
editA form of radiometric dating involving the construction of an isochron diagram. Articles: Rb-Sr dating, Other isochron methods.
Issochron diagram
editA 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
editThe 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
editAtoms are classified into isotopes according to their atomic number and their atomic weight. Articles: Chemistry for geologists, Radioactive decay.
K
editChemical symbol for the element potassium. Articles: Chemistry for geologists, K-Ar dating.
K-Ar dating
editPotassium-argon dating, a form of radiometric dating. Article: K-Ar dating.
Karst topography
editThe distinctive landscape produced by the chemical weathering of limestone. Article: Chemical weathering.
K-Ca dating
editPotassium-argon dating, a form of radiometric dating. Article: Other isochron methods.
Kettle
editA 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
editChemical symbol for the element lanthanum. Articles: Chemistry for geologists, Other isochron methods.
La-Ba dating
editLanthanum-barium dating, a form of radiometric dating. Article: Other isochron methods.
La-Ce dating
editLanthanum-cerium dating, a form of radiometric dating. Article: Other isochron methods.
Lacolith
editAn intrusion between two strata, similar to a sill but thicker and lens-shaped. Article: Igneous rocks.
Lacustrine
editLake
editAn inland body of water fed by rivers, streams, or sometimes by seepage of groundwater. Article: Lakes.
Lamina
editA very thin bed, no more than a few millimeters thick. Article: Deserts.
Lateral moraine
editSediment which accumulates along the sides of a valley glacier, having fallen or been scraped off the walls of the valley. Article: Glaciers.
Laterite
editA soil type characteristic of a tropical climate alternating between a monsoon season and a dry season. Article: Sediments and climate.
Lattice silicates
editSilicate minerals in which the silicate tetrahedra are bonded together to form a three-dimensional lattice. Article: Silicate minerals.
Lava
editMolten rock which has reached the surface, as opposed to magma, which is sill trapped beneath it. Article: Igneous rocks.
Lava flow
editIgneous rock formed by lava flowing on the surface.
Leached ions
editIons dissolved in water as a result of chemical weathering; as opposed to residual minerals. Article: Chemical weathering.
Lee
editThe 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
editAbbreviation for last glacial maximum. Article: Climate models.
Lignite
editThe softest form of coal; the next stage in the formation of coal from peat after peat itself. Article: Peat and coal.
Limestone
editRock formed from calcium carbonate, usually in the form of calcite. Article: Calcareous ooze.
Limiting stand
editTrees 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
editThe 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
editThe conversion of sediment into a sedimentary rock by such processes as compaction and cementation. Article: Sedimentary rocks.
Lithosphere
editThe 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
editHaving to do with the coast. Article: Index fossils.
Loess
editFine wind-borne sediment produced by the action of glaciers. Article: Ice cores.
Longshore bars
editBars of sediment running parallel to a beach. Article: Nearshore sediments.
Longshore current
editThe component of a nearshore current that flows parallel to the shoreline. Article: Nearshore sediments.
Longshore drift
editThe 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
editA type of seismic wave which travels along the surface of the Earth rather than through it. Article: Seismic waves.
Lu
editChemical symbol for the element lutetium. Articles: Chemistry for geologists, Other isochron methods.
Lu-Hf dating
editLutetium-hafnium dating, a form of radiometric dating. Article: Other isochron methods.
Mafic rock
editA rock rich in magnesium and iron, and poor in quartz and feldspar. Article: Igneous rocks.
Magma
editMolten 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
editA zone of ultramafic rock lying below the Earth's crust and above its core. Article: Structure of the Earth.
Mantle plume
editA column of hot rock rising in the mantle below a hotspot. Article: Hotspots.
Marble
editA metamorphic rock formed by the metamorphism of limestone. Article: Metamorphic rocks.
Massive rock
editRock 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
editErosion caused by gravity. Article: Mechanical weathering and erosion.
Matrix
editFiner 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
editA broad loop in a stream or river. Article: Rivers.
Meandering stream
editA stream which flows in a series of meanders. Article: Rivers.
Mechanical weathering
editWeathering caused by mechanical processes that break up a rock, as opposed to chemical weathering. Article: Mechanical weathering and erosion.
Medial moraine
editA moraine formed by the union of two lateral moraines when two valley glaciers flow together to forms a single glacier. Article: Glaciers.
Metamorphic grade
editThe degree of metamorphism undergone by a metamorphic rock. Article: Metamorphic rocks.
Metamorphic rock
editA rock which has had its texture or composition changed by heat and/or pressure. Article: Metamorphic rocks.
Metamorphism
editChanges in the texture or composition of a rock brought about by heat and/or pressure. Article: Metamorphic rocks.
Metasomatism
editChanges 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
editMg
editChemical symbol for the element magnesium. Article: Chemistry for geologists.
Mica
editA group of sheet aluminosilicate minerals. Articles: Igneous rocks, Metamorphic rocks.
Mid-ocean ridge
editElevated sea-floor on either side of a mid-ocean rift. Articles: Sea floor spreading, Sea level variations.
Mid-ocean rift
editThe rift between two plates at which sea floor spreading occurs. Article: Sea floor spreading.
Milankovitch cycles
editPeriodic changes in the inclination of the Earth's axis and the shape of its orbit. Article: Milankovitch cycles.
Mineral
editA solid with a particular chemical composition and structure. Articles: Minerals, Silicate minerals.
Mineraloid
editAnything which is like a mineral in some respects but doesn't quite fit the definition. Article: Minerals.
Mixing plot
editA 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
editA 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
editA collection of atoms bonded together. Article: Chemistry for geologists.
Moraine
editMud crack
editA small-scale geological structures produced in mud as it dries. Article: Way-up structures.
Mudstone
editRock formed from clay or silt which is not bedded: lithified mud which is bedded is known as shale. Article: Sedimentary rocks.
Mylonite
editA 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
editChemical symbol for the element sodium. Article: Chemistry for geologists.
Nd
editChemical symbol for the element neodymium. Articles: Chemistry for geologists, Other isochron methods.
Ne
editChemical symbol for the element neon. Articles: Chemistry for geologists, Radioactive decay.
Nearshore
editThe zone in which the sea bed is affected by waves. Article: Nearshore sediments.
Neosilicate
editA silicate mineral in which the silicate tetrahedra are isolated from one another. Article: Silicate minerals.
Neutron
editA 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
editAn unconformity in which the older rocks are igneous or metamorphic. Article: Unconformities.
Normal fault
editA dip-slip fault in which the hanging wall moves downwards relative to the foot wall. Article: Faults.
Normal polarity
editThe 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
editA 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
editChemical symbol for the element oxygen. Articles: Chemistry for geologists, Banded iron formations.
Oasis
editObduction
editA process in which one platecolliding with another is thrust over it instead of beneath it; the opposite of subduction. Article: Ophiolites.
Oblique fault
editA fault which combines elements of a dip-slip fault and a strike-slip fault. Article: Faults.
Obsidian
editFelsic volcanic glass. Article: Igneous rocks.
Olivine
editA silicate mineral with the chemical formula (Mg,Fe)2SiO4 and an orthorhombic crystal system. Articles: Minerals, Igneous rocks, Structure of the Earth.
Ooid
editA 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
editLimestone composed of ooids cemented together. Article: Ooids and oolite.
Oolith
editEither a rock formed from ooids, or a single ooid, depending on context. Article: Ooids and oolite.
Oolitic limestone
editSynonym for oolite. Article: Ooids and oolite.
Opal
editAmorphous hydrated silica, of which the precious stone known as opal is only one particularly pretty example. Article: Siliceous ooze.
Opal compensation depth
editThe 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
editA section of oceanic crust which has been thrust up above sea-level. Article: Ophiolites.
Orthosilicate
editA synonym for neosilicate. Article: Silicate minerals.
Orogeny
editThe 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
editChemical symbol for the element osmium. Articles: Chemistry for geologists, Other isochron methods.
Oscillatory flow
editThe washing back and forth of water on the foreshore as a result of the action of the tide. Article: Nearshore sediments.
Outwash
editLight sediment carried by meltwater from the ablation zone of a glacier. Article: Glaciers.
Outwash plain
editA flat area of outwash sediment in front of a glacier. Article: Glaciers.
Oxbow lake
editA 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
editSynonym for the Great Oxygenation Event Article: Banded iron formations.
Oxygen crisis
editSynonym for the Great Oxygenation Event Article: Banded iron formations.
Oyster
editPa
editAbbreviation for pascals.
- or
The chemical symbol for the element protactinium. Articles: Chemistry for geologists, U-Th, U-Pa, and Ra-Pb dating.
Pahoehoe
editA 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
editThe study of ancient climates. Article: Paleoclimatology: introduction.
Paleocurrents
editAncient currents of wind and water the direction of which can be deduced from the analysis of sedimentary rocks. Article: Paleocurrents.
Paleomagnetic dating
editA form of absolute dating based on analysis of the paleomagnetic data in the rocks. Article: Paleomagnetic dating.
Paleomagnetism
editThe geological record of the past history of the Earth's magnetic field. Articles: Geomagnetic reversals, Sea floor spreading, Paleomagnetic dating.
Paleosol
editFossilized soil. Article: Soils and paleosols.
Pangaea
editThe last supercontinent to exist, prior to its rifting and the formation of the Atlantic Ocean. Article: Continental drift.
Pannotia
editA supercontinent that existed before Pangea. Article: Continental drift.
Paraconformity
editAn unconformity without an erosional surface. Article: Unconformities.
Parent isotope
editA radioactive isotope which undergoes radioactive decay to produce a daughter isotope. Article: Radioactive decay.
Parent rock
editThe original rock from which a metamorphic rock is formed by metamorphism. Article: Metamorphic rocks.
Pascal
editUnit of stress: 1 pascal = 1 newton/square meter. Article: Physical properties of rocks.
Pb
editChemical 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
editLead-lead dating, a form of radiometric dating. Article: U-Pb, Pb-Pb, and fission track dating.
Peat
editWaterlogged 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
editThe partial decomposition of waterlogged vegetable matter, turning it into peat. Article: Peat and coal.
Pedogenesis
editThe 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
editHaving to do with the open sea. Articles: Marine sediments, Index fossils.
Pelagic clay
editFine-textured sediment deposited on the abyssal plain. Articles: Pelagic clay, Marine sediments.
Peridotite
editAn ultramafic igneous rock consisting mainly of olivine with a little pyroxene and amphibole. Article: Igneous rocks.
Periodic table
editA tabular arrangement of the elements which gives insight into their chemical properties. Article: Chemistry for geologists.
Permineralization
editA process forming mineralized fossils in which the voids in the original material are filled by minerals. Article: Fossils.
Petrification
editA process forming mineralized fossils in which they undergo both replacement and permineralization. Article: Fossils.
Phaneritic
editAn 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
editWhether a substance is solid, liquid, or gas; if solid, its crystal structure or lack thereof. Article: Chemistry for geologists.
Phenocryst
editA large crystal embedded in the more finely-grained ground mass of a porphyritic rock. Article: Igneous rocks.
Philosophical naturalism
editThe rejection a priori of the existence of the supernatural; a position completely unnecessary to the practice of geology. Article: Actualism.
Phylosilicate
editA synonym for sheet silicate. Article: Silicate minerals.
Pillow basalt
editBasalt 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
editA very thin lamina of very fine clasts, formed in and characteristic of aeolian sand dunes. Article: Deserts.
Plastic
editA 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
editA flat-bottomed basin in a desert which periodically fills with water to form a shallow temporary lake. Article: Deserts.
Plutonic rock
editAlternative term for intrusive rock. Article: Igneous rocks.
Plate
editA piece of the lithosphere bounded by faults. Articles: Plate tectonics: overview, Sea floor spreading, Subduction, Hotspots, Terranes.
Plate tectonics
editThe study of the motion of the Earth's plates. Articles: Plate tectonics: overview, Sea floor spreading, Subduction, Hotspots, Terranes, Ophiolites, Orogeny.
PMIP
editThe Paleoclimate Modeling Intercomparison Project, a project comparing results from climate models with the evidence from paleoclimatic proxies. Article: Climate models.
Point bar
editA bar of sediment formed on the inner bank of a meander. Article: Rivers.
Polymorph
editTwo minerals having the same chemical formula but a different crystal structure are said to be polymorphs. Article: Minerals.
Pond
editPorphyritic
editOf a rock, containing some large crystals embedded in a more finely-grained ground mass. Article: Igneous rocks.
Precipitation
editWhen a chemical formerly dissolved in water settles out of it as a solid sediment, this is called precipitation. Article: Saline giants.
Prehistoric
editPreceding written human history; the fairly arbitrary line before which organic remains are considered to be fossils. Article: Fossils.
Primary rock
editTerm sometimes used for igneous rock. Article: Igneous rocks.
Principle of cross-cutting relationships
editThe 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
editRoughly 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
editThe 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
editThe 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
editThe principle that when sediment is laid down, it is usually laid down more or less flat. Article: Steno's principles.
Principle of superposition
editThe principle that when sediment is laid down, the sediment most recently deposited will be on the top. Article: Steno's principles.
Proglacial lake
editProgradation
editThe building out of a delta into the sea by deposition of sediment. Article: Deltas.
Proton
editA 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
editA 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
editA 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
editA form of volcanic glass filled with air bubbles. Article: Igneous rocks.
Pyroclastic flow
editA current of air laden with volcanic ash, which resists dispersion into the surrounding air because of its greater density. Article: Volcanic ash.
Pyroxene
editAn important group of rock-forming chain silicates. Article: Igneous rocks.
P-waves
editBody waves consisting of moving zones of compression and tension. Article: Seismic waves.
Quartz
editA 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
editA metamorphic rock formed by metamorphism of quartz sandstone. Article: Metamorphic rocks.
Quartz sandstone
editSandstone of which the sand grains are almost entirely quartz. Article: Sedimentary rocks.
Ra
editChemical symbol for the element radium. Articles: Chemistry for geologists, U-Th, U-Pa, and Ra-Pb dating.
Racemic
editComposed of equal amounts of both forms of enantiomers. Article: Amino acid dating.
Racemization
editThe process by which a collection of chiral molecules become racemic. Article: Amino acid dating.
Racemization dating
editAn alternative term for amino acid dating. Article: Amino acid dating.
Radioactive
editHaving a tendency to radioactive decay. Article: Radioactive decay.
Radioactive decay
editAny 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
editRadiometric dating of organic material by analysis of the isotopes of carbon it contains. Article: Radiocarbon dating.
Radiogenic
editAn atom is said to be radiogenic if it is the product of radioactive decay. Article: Radioactive decay.
Radiolaria
editA 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
editA 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
editA 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
editA type of seismic wave which travels on the surface of the Earth rather than through it. Article: Seismic waves.
Rank
editThe degree to which coal has undergone metamorphism. Article: Peat and coal.
Ra-Pb dating
editRadium-lead dating, a form of radiometric dating. Article: U-Th, U-Pa, and Ra-Pb dating.
Rb
editChemical symbol for the element rubidium. Articles: Chemistry for geologists, Rb-Sr dating.
Rb-Sr dating
editRubidium-strontium dating, a form of radiometric dating. Article: Rb-Sr dating.
Re
editChemical symbol for the element rhenium. Articles: Chemistry for geologists, Other isochron methods.
Reaction
editIn chemistry, a process in which molecules form, break apart, or recombine. Article: Chemistry for geologists.
Recrystalization
editChange in the texture of a rock. Article: Metamorphic rocks.
Recumbent fold
editA fold in a rock which has been pushed so far over as to lie on its side. Articles: Orogeny, Folds.
Redbeds
editSedimentary rocks cemented together chiefly by iron oxide, characteristic of dry climates. Article: Sediments and climate.
Red clay
editA synonym sometimes used for pelagic clay. Articles: Pelagic clay, Marine sediments
Reef
editAn 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
editLimestone resulting from the intact preservation of hard parts of coral or other organisms. Article: Reefs.
Reflection seismology
editA method of examining the structure of buried rocks by studying the reflections of Seismic waves produced by artificial explosions. Article: Sea level variations.
Refraction
editThe 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
editMetamorphism over a wide region, caused by deep burial or wide-acting tectonic forces; as opposed to contact metamorphism. Articles: Metamorphism, Orogeny.
Regression
editAn event in which the shoreline moves in a seaward direction; the opposite of a transgression. Articles: Sea level variations, Walther's principle.
Relative dating
editDating 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
editRhenium-osmium dating, a form of radiometric dating. Article: Other isochron methods.
Replacement
editA process forming fossils in which the original organic material is replaced by minerals. Article: Fossils.
Residence time
editThe 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
editMinerals which are not dissolved by chemical weathering. Article: Chemical weathering.
Reversed polarity
editA 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
editA dip-slip fault in which the hanging wall moves upwards relative to the foot wall. Article: Faults.
Rhyolite
editA felsic extrusive igneous rock; the extrusive counterpart of granite. Article: Igneous rocks.
Rhythmite
editA sedimentary rock which display a repetitive vertical succession of types of sediment. Articles: Tidal rhythmites and dating, Varves, Milankovitch cycles.
Ring silicates
editSilicate minerals in which the silicate tetrahedra are bonded together to form rings. Article: Silicate minerals.
Ripple
editRoche moutonnée
editA 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
editAn aggregate of one or more minerals or mineraloids. Article: Rocks.
Rock cycle
editThe set of processes by which rocks are formed, altered, destroyed, and reformed. Article: Rocks.
Rock flour
editExtremely fine sediment formed by the grinding action of a glacier. Article: Glaciers.
Rock glacier
editA glacier consisting mainly of rocks held together by ice. Article: Glaciers.
Rock salt
editCommon salt (NaCl) when it occurs naturally as a rock; a synonym for halite. Articles: Minerals, Saline giants.
Rounding
editA 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
editTerm for conglomerates and breccias. Article: Sedimentary rocks.
Rudists
editA group of reef-building molluscs that went extinct at the end of the Cretaceous period. Article: Reefs.
S
editChemical symbol for the element sulfur. Article: Chemistry for geologists
Sabkha
editA salt flat found between the sea and a desert above the high-water line. Article: Deserts.
Saltation
editThe 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
editAn 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
editParticles of sediment between 1/16mm and 2mm in diameter. Article: Sedimentary rocks.
Sandstone
editRock formed by the cementation of sand. Article: Sedimentary rocks.
Saprolite
editCompletely weathered rock. Article: Chemical weathering.
Sapropelic coal
editCoal 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
editA 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
editA high-grade metamorphic rock exhibiting pronounced foliation. Article: Metamorphic rocks.
Schistosity
editThe kind of foliation found in schist. Article: Metamorphic rocks.
Sclerochronology
editAn absolute dating method based on the study of growth patterns in shells and corals. Article: Sclerochronology.
Scleroclimatology
editThe study of past climates by the analysis of the composition of shells. Article: Scleroclimatology.
Sea floor spreading
editThe 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
editThe sediments carried by a wave. Article: Mechanical weathering and erosion.
Seamount
editA marine mountain which is entirely underwater. Article: Hotspots.
Seat earth
editThe paleosol underlying coal beds. Article: Peat and coal.
Secular equilibrium
editA 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
editThe wandering of the magnetic poles over time. Article: Geomagnetic reversals.
Sediment
editParticles transported and/or deposited by wind, water, glaciers, precipitation, etc; the constituents of sedimentary rocks. Article: Sedimentary rocks.
Sedimentary rock
editSediment lithified by cementation and/or compaction, or as a result of simple crystal growth in the case of evaporites. Article: Sedimentary rocks.
Sediment trap
editA device that collects sediment as it settles. Article: Deposition rates.
Seismic tomography
editThe 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
editWaves in the body or surface of the Earth generated by earthquakes. Articles: Seismic waves, Structure of the Earth.
Seismometer
editA device for detecting earthquakes and measuring their properties. Article: Seismic waves.
Semidiurnal
editOccurring twice daily. Article: Tidal rhythmites and dating.
Series
editA stratigraphic unit smaller than a system but larger than a stage. Article: Geological column.
Serpentinite
editA metamorphic rock produced from peridotite in the presence of heat and water. Article: Ophiolites.
Serrated margins
editEdges of leaves which are not smooth, characteristic of a temperate climate. Article: Leaf shape and temperature.
Shale
editA sedimentary rock formed from silt or clay which exhibits bedding. Article: Sedimentary rocks.
Shear
editStress 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
editDikes which stand side by side with one another like books on a shelf rather than intruding into some other rock. Article: Ophiolites.
Sheet glacier
editSynonym for continental glacier. Articles: Glaciers, Historical Geology/Ice ages.
Sheet silicate
editAny silicate in which the silicate tetrahedra bond together to form a sheet. Article: Silicate minerals.
Shoestring
editA geological feature which is long and thin, e.g. a river or a shoreline. Articles: Rivers, Nearshore sediments.
Si
editChemical symbol for the element silicon. Articles: Chemistry for geologists, Silicate minerals.
Siderite
editThe mineral FeCO3. Article: Banded iron formations.
Silica
editSilicon dioxide, (SiO2). This occurs in many forms, such as quartz, opal, and chert Article: Minerals.
Silicate
editA shorter way of saying silicate mineral. Article: Silicate minerals.
Silicate mineral
editAny of a large and important class of minerals the chemistry of which is based on the silicate tetrahedron. Article: Silicate minerals.
Silicate tetrahedron
editThe 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
editComposed of silica. Article: Siliceous ooze.
Siliceous ooze
editOoze on the sea floor, composed of the siliceous test of radiolaria and diatoms. Articles: Siliceous ooze, Marine sediments.
Sill
editA sheet of intrusive rock forced between strata. Articles: Igneous rocks, Igneous rocks and stratigraphy.
Silt
editClasts between 1/16mm and 1/256mm in diameter. Article: Sedimentary rocks.
Siltstone
editSedimentary rock composed of silt. Article: Sedimentary rocks.
Sinstral
editA 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
editA rock produced by metasomatism. Article: Metamorphic rocks.
Sky island
editA mountain habitat which is home to species which are isolated by their inability to cross the drier hotter surrounding plain. Article: Ice ages.
Slab
editThe portion of a plate being thrust into the athenosphere during subduction. Article: Subduction.
Slate
editA metamorphic rock formed by metamorphism of shale, exhibiting pronounced foliation. Article: Metamorphic rocks.
Slatey foliation
editThe sort of foliation found in slate. Article: Metamorphic rocks.
Slickenside
editA smoothed and striated surface produced by the friction between the two sides of a fault. Article: Faults.
SLR
editAbbreviation for Satelite Laser Ranging. Article: Continental drift.
Sm
editChemical symbol for the element samarium. Articles: Chemistry for geologists, Other isochron methods.
Sm-Nd dating
editSamarium-neodymium dating, a form of radiometric dating. Article: Other isochron methods.
Snell's Law
editA 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
editSediment 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
editA mark made in sediment when it is scoured by a current. Article: Turbidites.
Solid solution
editA mineral in which some positions in the crystal lattice may be filled by different elements. Article: Minerals.
Soluble
editCapable of becoming dissolved. Article: Chemistry for geologists.
Solute
editA substance dissolved in a solvent. Article: Chemistry for geologists.
Solvent
editThe medium in which a substance is dissolved. Article: Chemistry for geologists.
Sorosilicates
editSilicates in which the silica tetrahedra are bonded together in pairs. Article: Minerals.
Sorting
editSediment is said to be well-sorted if it consists of particles of about the same grain-size. Article: Sedimentary rocks.
Speleothem
editA 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
editA 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
editChemical symbol for the element strontium. Articles: Chemistry for geologists, Rb-Sr dating.
Stage
editA stratigraphic unit smaller than a series but larger than a zone. Article: Geological column.
Stalactite
editA speleothem hanging like an icicle from the roof of a cave. Article: U-Pb, Pb-Pb, and fission track dating.
Stalagmite
editA 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
editA rock of known age used in Ar-Ar dating. Article: Ar-Ar dating.
Steno's principles
editThe principles of superposition, of original horizontality, and of original continuity. Article: Steno's principles.
Step heating
editA process used in Ar-Ar dating in which a rock sample is heated in steps of progressively higher temperatures. Article: Ar-Ar dating.
Strain
editThe deformation of a solid body as a result of stress. Article: Physical properties of rocks.
Stratigraphy
editThe 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
editA 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
editDeltas 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
editThe 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
editSynonym for striations. Article: Glaciers.
Striation
editGrooves left by the movement of a glacier over a rock, parallel to the direction of motion. Article: Glaciers.
Strike-slip fault
editA 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
editSponges 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
editCoal of a higher rank than lignite and a lower rank than bituminous coal. Article: Peat and coal.
Subduction
editThe motion of one plate beneath another and into the mantle. Article: Subduction.
Supercontinent
editA landmass consisting of most or all of the continental crust joined together to form a single continent. Article: Continental drift.
Supercontinent cycle
editA process in which supercontinents repeatedly form and then rift again into separate continents. Article: Continental drift.
Superposed river
editA river which exists before the creation by erosion of the hills through which it flows. Article: Rivers.
Superimposed river
editSynonym for superposed river. Article: Rivers.
Suspension
editA 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
editThe line along which a continent becomes joined to another continent, microcontinent, or island arc. Article: Terranes.
Swamp
editAn 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
editBody 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
editStructure formed when rocks are folded downwards. Article: Folds.
System
editThe largest stratigraphic unit. Article: Geological column.
Tarn
editA lake that forms in the former cirque of a glacier after the glacier has melted. Article: Glaciers.
Tectonic window
editA 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
editA synonym for lattice silicate. Article: Silicate minerals.
Tension
editStress that produces elongation of a solid along the direction in which force is applied. Article: Physical properties of rocks.
Terminal lake
editA lake which water flows into but not out of, usually salty as a result of the accumulation of dissolved minerals. Article: Lakes.
Terminal moraine
editA moraine deposited at the ablation zone of a glacier. Articles: Glaciers, Ice ages.
Terragenic
editHaving an origin on land. Article: Marine sediments.
Terrane
editPart 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
editHaving to do with the land. Articles: Sedimentary rocks, Lakes, Radiocarbon dating.
Test
editThe shell of a micro-organism such as a diatom or a foraminiferan. Articles: Calcareous ooze, Siliceous ooze.
TEX86
editA temperature proxy based on measurement of the different varieties of GDGTs in sediments. Article: TEX86.
Texture
editPhysical 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
editThe 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
editDeep-water circulation driven by density differences in the temperature and salinity of sea water. Article: Climate models.
Thrust fault
editA reverse fault in which the angle of the fault is more than 45° from the vertical. Article: Faults.
Tide-dominated delta
editA 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
editUnsorted and usually unbedded sediment deposited by a glacier. Article: Glaciers.
Tidal braking
editSlowing 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
editTopset beds
editThe flat beds of sediment deposited on the top surface of a delta. Article: Deltas.
Trace fossil
editA fossil such as a footprint which is not of an animal but which was produced by one. Article: Fossils.
Transgression
editAn event in which the shoreline moves inland; the opposite of a regression. Articles: Walther's principle, Sea level variations.
Transitive
editA 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
editA depression in the sea floor formed along the line where one plate subducts beneath another. Article: Subduction.
Trona
editAn evaporite mineral having the chemical formula Na3(CO3)(HCO3)·2H2O.
Tsunami
editA 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
editTuff
editLithified Volcanic ash Article: Volcanic ash.
Turbid
editLoaded with sediment. Article: Turbidites.
Turbidite
editRock formed from sediment deposited by a turbidity current. Article: Turbidites.
Turbidity current
editA 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
editSediment deposited by a turbidity current. Article: Turbidites.
U
editChemical 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
editA proxy for temperature based on measurements of the different kinds of alkenones preserved in sediment. Article: Uk'37.
Ultrabasic rock
editAn obsolete and inaccurate term for ultramafic rock. Article: Igneous rocks.
Ultramafic rock
editA rock which is extremely mafic; that is, particularly low in silicate tetrahedra and high in magnesium and iron. Article: Igneous rocks.
Unconformity
editA surface between successive strata representing a period of erosion or of no deposition. Article: Unconformities.
Underclay
editSynonym for seat-earth. Article: Peat and coal.
Uniformitarianism
editAn alternative term for actualism, not used in this textbook because of ambiguities and inconsistencies in its usage. Article: Actualism.
Unstable
editProne to radioactive decay. Article: Radioactive decay.
U-Pa dating
editUranium-protactinium dating, a form of radiometric dating. Article: U-Th, U-Pa, and Ra-Pb dating.
U-Pb
editUranium-lead dating, a form of radiometric dating. Article: U-Pb, Pb-Pb, and fission track dating.
Upper plane bed
editA 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
editThe mineral UO2 Article: Banded iron formations.
U-Th dating
editUranium-thorium dating, a form of radiometric dating. Article: U-Th, U-Pa, and Ra-Pb dating.
Vail curve
editA reconstruction of past variations of sea level based on the study of unconformities in the geological record. Article: Sea level variations.
Valley glacier
editA 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
editOne of the two shells of a bivalve. Article: Way-up structures.
Varve
editA 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
editA 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
editInformally speaking, the reluctance of a liquid to flow; so for example maple syrup is more viscous than water. Article: Igneous rocks.
Vis plastica
editThe name of an imaginary force once thought to cause fossils to grow in rocks. Article: Fossils.
VLBI
editAbbreviation for Very Long Baseline Inferometry. Article: Continental drift.
Volcanic ash
editFine 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
editThe 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
editThe greatest depth at which the action of a wave has any effect. Article: Nearshore sediments.
Wave ripples
editRipples in sand or other sediment caused by the action of the tide. Article: Nearshore sediments.
Wave-dominated deltas
editdeltas in which longshore drift forms barrier islands in front of the delta. Article: Deltas.
Way-up structure
editA geological feature which enable us to discover which way up a rock was when it was originally formed. Article: Way-up structures.
Weathering
editProcesses which break up rock but do not themselves transport it, as distinct from erosion. Article: Mechanical weathering and erosion, Chemical weathering.
Weathering rind
editThe 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
editTuff which forms when a fall of Volcanic ash is still hot enough to weld itself together. Article: Volcanic ash.
Xenotime
editThe 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
editChemical symbol for the element yttrium. Article: Chemistry for geologists.
Zircon
editThe 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
editThe 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
editChemical symbol for the element zirconium. Article: Chemistry for geologists.