Glossary
This is a place-keeping post. I shall insert glossary items in here every few posts, as and when it seems necessary.
In books the glossary goes at the end, but I can't arrange for any post to always be the
last post on this thread, whereas I can arrange for this one to go near the start.
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The glossary is currently complete up to the end of the articles on paleoclimatology.
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100,000 year problem
The question of why over the last million years, climatic variation has been driven by the 100,000 year Milankovitch cycle rather than the 41,000 year Milankovitch cycle.
Aa
A type of lava flow, or the cooled and solidified rock produced by it, characterized by the rough jagged surface of the resulting rock.
Ablation zone
The end of a glacier; the point at which loss of ice by melting exceeds the supply of ice by the movement of the glacier.
Abrasion
Erosion of rocks caused by the sediments carried by wind or water.
Absolute dating
Dating methods which tell us how old a rock or fossil is, as opposed to relative dating.
Abyssal plain
The flat terrain found at the bottom of the ocean beyond the continental margin.
Accretionary prism
An accumulation of sediment which forms in a trench.
Accretionary wedge
A synonym for accretionary prism.
Accumulation zone
The beginning of a glacier; the zone in which snowfall exceeds the loss of snow by melting or evaporation.
Acidic rock
An outdated and inaccurate term for felsic rock.
ACM
Abbreviation for atmospheric circulation model.
Actualism
The observation that the geological record can be explained in terms of the sort of geological processes that actually happen.
Aeolian
Having to do with the wind.
Aeolian sandstone
Sandstone formed from sand deposited by the wind, i.e. desert sand.
Alkenones
Organic molecules produced by certain planktonic organisms, used in the temperature proxy known as U
k'37.
Alluvial
Having to do with flowing water, usually rivers or streams.
Alluvial fan
A fan-shaped deposit of sediment left where a mountain stream reaches a plain.
Alpha decay
Radioactive decay involving the emission of an alpha particle.
Alpha particle
A particle consisting of two protons and two neutrons.
Aluminosilicate
Any of an important class of silicate minerals in which the structure involves aluminum-based as well as silicon-based tetrahedra.
Alpine glacier
Synonym for valley glacier.
Amber
A mineraloid formed from the solidified resin of trees.
Amino acid dating
A rather unreliable method of absolute dating based on measuring the racemization of organic remains.
Amorphous
Lacking a crystal structure.
Ammonite
An extinct marine mollusc.
Amphibole
A group of silicate minerals in which the SiO
44- units are bonded to form a double chain.
Angle of repose
The maximum angle from the horizontal that a heap of a given type of sediment can assume without collapsing.
Angular unconformity
An unconformity in which the older strata meet the younger strata at an angle, the older strata being truncated by the erosional surface.
Anomaly
In geology, the term "anomaly" means a measurement at some place of some quantity which is different from the average or background value for that quantity.
This should not be confused with the usage of the term "anomaly" in the philosophy of science, where it means a measurement or observation which cannot be reconciled with current theory. In geology, the term has no such implication.
Antecedent river
A river which is present before the uplift of the hills through which it flows.
Anthracite
A very black, hard, and shiny form of coal produced by metamorphism.
Antidune
A rounded dune-like structure found in rivers of the right velocity and having a sandy bottom. Because they erode by the transport of sand grains from the lee side of one antidune to the stoss side of the next, the net effect is that while the sand moves downstream, the antidunes move upstream.
Antisyncline
An upward fold in rocks.
Aphanitic
An igneous rock is said to be aphanatic if the crystals in it are too small to be seen with the naked eye. In our articles we have tended to use the more straightforward term "fine-grained".
Apparent polar wander
Apparent secular variation recorded in the paleomagnetic record which is actually caused by the motion of plates relative to the poles.
Ar-Ar dating
Argon-argon dating, a form of radiometric dating.
Aseismic ridge
A long linear trail of volcanic islands and seamounts caused by a plate passing over a hotspot.
Archaecyathids
Early reef-building organisms, shaped rather like goblets and secreting skeletons of calcium carbonate; they went extinct at the end of the Cambrian period.
Argillaceous
Having to do with mud. May be used to qualify the nature of a rock, e.g. argillaceous sandstone would be sandstone with a significant amount of mud mixed in with the sand.
Arenaceous
Having to do with sand. May be used to qualify the nature of a rock, e.g. arenaceous mudrock would be mudrock with a significant amount of sand mixed in with the mud.
Arenite
Alternative term for sandstone.
Arkose
Sandstone which contains an appreciable quantity of feldspar as well as the more usual quartz. The grains are often poorly sorted and not well rounded.
Athenosphere
The portion of the mantle just below the lithosphere.
Atmospheric circulation model
A climate model which only takes into account the ciculation of the atmosphere and not the oceanic circulation.
Atomic number
The number of protons in the nucleus of an atom.
Atomic weight
The sum of the number of protons and neutrons in the nucleus of an atom.
Attrition
Erosional processes whereby the clasts transported by wind or water are broken or worn down.
Aureole
A ring of metamorphic rock formed around an igneous intrusion by contact metamorphism.
Backshore
That part of a beach which is above the high-water line.
Bajada
The merging of two or more alluvial fans.
Banded iron formation
Sedimentary rock consisting of alternating bands of iron oxide and other sedimentary rock, typically chert.
Bar
A local accumulation of sediment, usually sand, such as forms in between the channels of a braided stream or offshore from a beach.
Barrier island
An island formed a the mouth of a river running at right angles to the direction of the distributary streams.
Basalt
A mafic intrusive igneous rock, black in color and aphanatic.
Basic rock
An obsolete and inaccurate term for mafic rock.
Bed
A layer in a sedimentary rock.
Bedding
A structure found in sedimentary rocks in which the rock is visibly composed of numerous layers (beds).
Bedding planes
The planes dividing the beds in a bedded rock.
Beta decay
A type of radioactive decay including beta plus and beta minus decay; the term is sometimes used to include electron capture as well.
Beta minus decay
A form of radioactive decay in which one of the neutrons in the atom is converted to a proton by emitting an electron.
Beta plus decay
A form of radioactive decay in which a proton is converted into a neutron by the emission of a positron.
BIF
Abbreviation for banded iron formation.
Biogeography
The study of the geographical distribution of living or extinct organisms.
Bioturbation
Changes in the structure of sediment cause by the activity of living things.
Bituminous coal
The commonest form of coal: less peat-like than sub-bituminous coal, but not as hard, black, and pure as anthracite.
Bivalve
A member of a group of molluscs characterized by being enclosed in two shells (valves). Common examples are mussels, clams, and oysters.
Body waves
Seismic waves which pass through the body of the Earth rather than traveling on its surface; a collective term for S-waves and P-waves.
Bottom-set beds
Horizontal beds of sediment deposited on the sea or lake floor in front of a delta.
Bouma sequence
The characteristic pattern of sediment deposited by a turbidity current.
Braided river
A river in which the current repeatedly splits into smaller streams which merge back together and then split again, and so forth.
Breccia
A rock consisting of large unrounded fragments cemented together.
Brittle
A material is said to be brittle if with increasing stress it undergoes very little plastic deformation between elastic deformation and shattering.
Brown clay
Term occasionally used for pelagic clay.
Calcareous
Having to do with calcium carbonate.
Calcareous ooze
A calcareous sediment found over large areas of the ocean floor, consisting of the shells of small organisms.
Calcite
A mineral consisting of calcium carbonate in a trigonal crystal system.
Calcium carbonate
The chemical CaCO
3. Most shells are formed of this, as are the rocks limestone and marble.
Carbonate
A molecule with the anion CO
32-; also a rock consisting of carbonates, particularly limestone and dolostone.
Carbonate compensation depth
The depth at which calcium carbonate will dissolve faster than it is deposited; hence, the depth below which calcareous ooze will not accumulate.
Carbon dating
Alternative term for radiocarbon dating.
C-C dating
Alternative term for radiocarbon dating.
Carbon dioxide
The molecule CO
2. A gas at temperatures and pressures found on Earth, and forming 0.038% of the Earth's atmosphere.
Carbonic acid
The acid H
2CO
3. 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.
Cast
A fossil produced when a mold is filled with minerals.
Cave formation
Alternative term for a speleothem.
CCD
Abbreviation for carbonate compensation depth.
14C dating
Alternative term for radiocarbon dating.
Cementation
The binding of clasts together by a finer material, typically silica, calcium carbonate, or iron oxide, to form a clastic rock.
Cemented tuff
Volcanic ash which has lithified by the process of cementation, as opposed to welded tuff.
Chain silicate
A silicate mineral in which the SiO
44- units are bonded together in the form of a chain, i.e. each unit is attached to just two other units (except, of course, at each end of the chain).
Chalk
Rock which, under a microscope, is clearly composed of the tests of calcium carbonate-secreting micro-organisms.
Chemical sediment
A chemical sediment is one deposited by precipitation rather than by mechanical processes such as wind or water; or by biological processes such as the growth of coral. Note however that some authors will include biological processes as a subcategory of chemical processes; our articles do not follow this usage.
Chemical weathering
Weathering caused by chemical processes (most commonly by some or all of the constituent minerals of a rock being dissolved by carbonic acid); as opposed to mechanical weathering.
Chert
A sedimentary rock composed of silica, having an amorphous or very fine-grained structure.
Chirality
The handedness of an organic molecule.
Cirque
A large bowl-shaped depression formed at the accumulation point of a valley glacier, with the bowl lacking about a quarter of its rim to let the glacier flow out.
Clast
A piece of rock detached by erosion or weathering from a larger rock.
Clastic
Composed of clasts.
Clay
The term clay can either, depending on the context, refer to a class of sheet aluminosilicate minerals, or to clasts with a diameter of less than 1/256 mm. As clay in the second sense is usually also clay in the first sense, this causes less confusion than you might think.
Claystone
Sedimentary rock composed of clay.
Cleavage
The tendency of minerals or of certain rocks to break along particular preferred planes. As the angles between these planes varies from mineral to mineral, cleavage can be used as a diagnostic tool to distinguish between minerals.
Climate
Broad trends in the weather; i.e. the tendency of a location to be hot and humid, or dry and cold.
Closure
The point at which snow has been so far compacted into ice that the air trapped in it is completely sealed off from the atmosphere.
Closure time
The time between snowfall and closure, varying from location to location.
Coal
Coal is peat which has been lithified by compaction, heat, or both.
Coalification
The chemical processes by which peat is turned into coal.
Coarse-grained
Composed of crystals of large size; the opposite of fine-grained.
Coccolith
A calcareous plate forming part of the shell of a coccolithophore; a common constituent of calcareous ooze.
Coccolithophores
A group of micro-organisms clad in coccoliths.
Compaction
Decrease in volume of sediment, caused by the pressure induced by being buried under yet more sediment.
Complacent
A tree is said to be complacent if the thickness of its growth rings is unaffected by annual variations in temperature.
Compression
Stress that produces shortening of a solid along the direction in which force is applied.
Concordant
Of dates, in agreement with one another.
Conglomerate
A conglomerate is a rock consisting of large clasts (pebble-sized or larger) cemented together; it is common usage (which we have followed in this text) to use the term to imply that the clasts are rounded, as distinct from a breccia.
Conodont
Term used ambiguously to refer either to a conodont animal or a conodont structure; context usually makes it clear which.
Conodont animals
A group of extinct primitive chordates having no hard parts except for conodont structures.
Conodont structures
The hard parts of a conodont animal.
Contact metamorphism
Metamorphism caused by close proximity to a source of heat, such as an intrusion of magma; as opposed to regional metamorphism.
Continental drift
The theory that continents have shifted their positions over time; now subsumed into the theory of plate tectonics.
Continental glacier
A glacier covering a large area and flowing outwards from its accumulation zone under the pressure of its own weight, as distinct from a valley glacier.
Continental margin
The continental shelf, slope, and rise.
Continental rise
The shallow (approximately 1 degree from horizontal) terrain between the continental slope and the abyssal plain.
Continental shelf
That part of a continent which is underwater, lying between the unsubmerged portion of a continent and the continental slope.
Continental slope
A shallow slope, typically between 4 and 10 degrees from horizontal, found between the continental shelf and the continental rise.
Corals
A group of marine organisms. Hard corals secrete skeletons of calcium carbonate and so act as reef-forming organisms.
Core
The innermost 3,400 km of the Earth, composed mainly of iron. The term also refers to a sample of rock recovered from the Earth's crust by drilling; context ensure that the two meanings are never in practice confused.
Cosmic dust
Dust fallen from outer space, i.e. micrometorites. Although they can be found in pretty much all kinds of sediment, they are proportionally most abundant in pelagic clay due to its slow rate of deposition.
Cosmic rays
Streams of high-energy particles which bombard the Earth from outer space.
Cosmogenic
Of isotopes, produced by cosmic rays.
Cosmogenic surface dating
A method of absolute dating which gives the time since a rock became exposed on the surface.
Country rock
The rock into which an igneous rock intrudes.
Creep
Transport of clasts by wind or water by means of rolling them along the ground, river bed, sea bed, etc.
Cross-bedding
Bedding in which the beds, instead of being deposited horizontally, are deposited at an angle, as a result of deposition by a current of wind or water; in the simplest case, where the current has a continuous direction, the beds will have a downward slope in the direction of the current.
Cross-cutting
An igneous rock such as a dike which cuts through the beds of country rock is said to be cross-cutting.
Crossdating
The correlation of dates from different sources.
Crust
The upper layer of the Earth, varying from about 5 - 50 km thick, distinct from the mantle by having a different chemical composition, being composed of less dense and more felsic rocks.
Crystal
A large molecule composed of small molecules chemically bonded together in a regular repetitive arrangement.
Crystal habit
The shape or shapes in which a mineral will typically grow.
Crystal system
A crystal, as defined above, is a large molecule composed of small molecules chemically bonded together in a regular repetitive arrangement. The possible arrangements can be classified into seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal and cubic.
Curie temperature
Very roughly speaking, the temperature above which a material cannot be magnetized and below which it can.
Cyclosilicate
A synonym for ring silicate.
δ18O
A proxy for temperature based on oxygen isotope ratios.
Decay chain
A sequence of events in which one isotope decays to another via an intermediate sequence of unstable isotopes.
Deflation
The erosion of fine particles from dry soil by the wind.
Deflation lake
A lake caused when deflation has caused a hollow the bottom of which lies below the water table.
Delta
The body of sediment deposited when a river flows into a lake or the sea.
Dendrochronology
A method of dating wood by studying the annual growth rings produced by the tree.
Desert
An area of exceptionally low rainfall. Note that although the stereotypical desert is hot and sandy, in geological terms a desert is defined solely by a shortage of rain or snow.
Desert pavement
A stony surface often found in deserts.
Dessication crack
Alternative term for a mud crack.
Detrital
Composed of clasts; synonymous with clastic.
Diamond anvil cell
A device used in experimental petrology to subject small samples of rock to large amounts of stress.
Diapir
A sedimentary stucture formed by one type of sediment flowing upwards through another as a result of pressure.
Diatomite
A very light and porous rock formed from diatom tests that have undergone little in the way of compaction and recrystallization.
Diatoms
A group of single-celled algae which produce siliceous shells; a major source of siliceous ooze.
Differentiation
The mechanism by which an originally homogeneous Earth separated into crust, mantle, and core.
Dike
A vertical or near-vertical sheet of igneous rock which intrudes into the country rock.
Disconformity
An unconformity in which the underlying strata are parallel with the overlying strata.
Distributary
A smaller stream flowing out of a larger river, as opposed to a tributary, which flows in.
Diurnal
Occurring once daily.
Docking
The union of a terrane with the landmass to which it becomes attached.
Drift
Any sediment deposited by a glacier.
Drifters
Term for the early supporters of continental drift; the opposite of "fixists".
Dropstone
A stone which has traveled out to sea on a "raft" of ablated glacial ice, and has been deposited when the ice melted.
Drumlin
A smallish hill shaped somewhat like the back of a spoon, deposited by glaciers in a manner not fully understood.
Ductile
A material is said to be ductile if, under stress, it will undergo a great deal of plastic deformation before it breaks. The opposite of brittle.
Dune
A mound of sand formed by the action of wind or water.
Dunite
An ultramafic rock consisting entirely of olivine.
Elastic
A material is said to be elastic if it recovers from stress: that is, if when the stress is removed it returns to its original conformation. The opposite of plastic.
Electron
A particle with negative charge and negligible mass found orbiting the nucleus of an atom.
Electron capture
A form of radioactive decay in which of the radioactive atom's own electrons combines with one of its protons, converting the proton into a neutron.
Element
Atoms are classified into elements according to their atomic numbers, which determine their chemical properties; this is a broader classification then the division into isotopes, which also takes into account their atomic weights.
Enantiomers
Molecules which are mirror images of one another.
Entire margins
Leaf margins which are smooth rather than serrated, characteristic of warm humid climates.
Epicenter
The point on the Earth's surface directly above the focus of an earthquake.
Erg
A sandy desert.
Erosion
Any process capable of breaking up rocks or soils and transporting the resulting clasts.
Erratic boulder
A boulder which does not fit in with the geology of its surroundings, transported from its place of origin by a glacier.
Evaporite
Any chemical sedimentary rock the precipitation of which was produced by the partial or complete evaporation of the water containing the dissolved minerals or which the rock is composed.
Event horizon
An extensive geological feature all of which was laid down at exactly the same time; e.g. volcanic ash from a single volcanic eruption.
Evolution
In biology, heritable change in a line of descent. Outside of biology, the term may be used colloquially to refer to any sort of change or development, as in (for example) "the evolution of jazz from ragtime".
Excess argon
Argon which is not radiogenic; a potential source of error in Ar-Ar dating.
Exponential decay
A quantity is said to undergo exponential decay if its magnitude as a function of time (
t) can be expressed in the form
ab-ct.
Extrusive rock
Any igneous rock formed by lava pouring out on the surface (where the "surface" includes on the sea floor, under a glacier, or anywhere except under rock) as opposed to intrusive rock which remains trapped within the country rock into which it intrudes. Extrusive rock can be distinguished from intrusive rock by its larger crystal size.
Facies
A facies is a body of sediment or sedimentary rock characteristic of a particular depositional environment.
Fault
A planar fracture or discontinuity in a volume of rock.
Fauna
Animals (in the broadest possible sense, including birds, fish, crustaceans, molluscs, etc).
Feldspar
A group of aluminosilicate minerals with a lattice structure.
Felsic rocks
Rocks which are high in silica and feldspar and low in magnesium and iron. The opposite of mafic rocks.
Fine-grained
Composed of crystals or clasts of small size; the opposite of fine-grained.
Fining-up sequence
A form of grading upwards from coarse to fine sediments associated with rivers.
Firn
Snow which has compacted, but not so far as to become ice.
Fissile
Of a rock, having the property of splitting easily in a given direction (e.g. between bedding planes).
Fission track dating
A form of absolute dating which involves counting the fission tracks in a rock.
Fission tracks
Microscopic scars left in minerals by alpha particles.
Fixist
Term for the early opponents of continental drift; the opposite of "drifter".
Flame structure
A sedimentary stucture 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.
Flaser deposits
Deposits in which light and heavy sediments alternate, characteristic of nearshore environments.
Flood plain
The flattened, sediment-rich area formed by the action of rivers on a landscape.
Fluvial
Having to do with rivers.
Focus
The point in the Earth at which an earthquake originates.
Foliation
The arrangement of sheet silicates in parallel planes in some metamorphic rocks, due to pressure causing realignment of the sheets in planes at right angles to the direction of pressure.
Foram
Short form of foramineferan.
Foraminiferans
A group of micro-organisms which secrete calcareous tests; one of the most common constituents of calcareous ooze.
Foreset beds
Beds of sediment sloping down at the front of a delta into the sea or lake into which it discharges.
Foreshore
That part of the nearshore which is uncovered at high tide.
Gabbro
Mafic intrusive igneous rock; the intrusive equivalent of basalt.
GCM
Abbreviation for general circulation model.
GDGTs
Glycerol dibiphytanyl glycerol tetraethers, organic molecules produced by the single-celled organisms known as Crenarchaeota, used in the TEX
86 temperature proxy.
General circulation model
A climate model which takes into account both the atmospheric and the oceanic circulation.
Geological column
A table showing the order of the faunal succession in the fossil record.
Geomagnetic reversal
A change in state from normal polarity to reversed polarity, or vice versa.
Geopetal structure
A structure formed when a hollow object is partially filled with sediment, allowing us to use it as a way-up structure.
Glacial outwash
Sediment carried out of a glacier by meltwater.
Glacial polish
The smooth (but striated) surface produced on a rock by the polishing action of a glacier passing over it.
Glaciation
Synonym for ice age.
Glacier
A moving mass of ice.
Glass
Any igneous rock with an amorphous structure, produced by lava cooling too fast to allow the formation of crystals.
Global Positioning System
A method for finding one's location on the surface of the Earth; used in geology to measure the motion of plates.
Gneiss
A metamorphic rock of high grade with a distinctive streaky appearance produced by the separation out of chain silicates into streaks.
GPS
Abbreviation for Global Positioning System.
Grade
The degree to which a rock has undergone metamorphism, depending on the amount of heat to which it has been exposed.
Grading
Change in size of clasts between two points; most typically from large clasts at the bottom of a layer to small clasts at the top.
Grainflow lamina
A lamina formed in sand dunes when sand at the crest of the dune avalanches down the lee face of the dune.
Granite
A felsic intrusive igneous rock; the intrusive counterpart of rhyolite.
GOE
Abbreviation for Great Oxygen Event.
Gravel
Sediment consisting of clasts 2mm in diameter and upwards.
Greenhouse gas
A gas such as carbon dioxide or methane which helps keep the Earth warm by trapping heat in the atmosphere.
Greywacke
Sandstone consisting of quartz, feldspar, and small rock fragments embedded in a clay matrix.
Ground mass
Finer material in which larger clasts (in the case of sedimentary rock) or crystals (in the case of igneous rock) are embedded. A synonym for matrix.
Growth ring
A layer of wood produced by a tree on an annual basis, used in dendrochronology.
Gypsum
A mineral consisting of hydrated calcium sulfate (CaSO
4.H
2O) with a monoclinic crystal system.
Half-life
The half-life of an isotope is the length of time in which an atom of that isotope has a 50% chance of undergoing radioactive decay.
Halite
Rock salt (NaCl).
Hallam curve
A reconstruction of past variations of sea level based on sedimentary evidence of transgressions and regressions.
Herringbone crossbedding
A form of cross-bedding in which the direction of slope alternates, as a result of oscillatory flow.
Hoodoo
A pillar of rock produced by erosion.
Hornfels
A large group of metamorphic rocks produced from sedimentary rocks by contact metamorphism.
Hotspot
A stationary point of high volcanic activity above which plates pass, creating an aseismic ridge.
Humus
Decaying organic matter in soil.
Humic coal
Coal produced by the deposition of land plants in swamps, as opposed to sapropelic coal.
Ice age
A time at which sheet glaciers are present on some regions of the Earth's surface.
Ice sheet
Synonym for continental glacier.
Igneous rock
Rock formed by the cooling of molten rock, i.e 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.
Index fossil
A fossil of a species that was sufficiently widely distributed that its fossils can be used to correlate the deposition of fossils and sediments in widely separated locations.
Index mineral
Any mineral which forms only at certain temperatures and pressures, and which can therefore be used as an index to the conditions under which certain metamorphic rocks were formed.
Index species
A species suitable for the production of index fossils.
Inert
Of an element, unable to participate in chemical reactions.
Inosilicate
A synonym for chain silicate.
Insolation
The quantity per area of solar radiation reaching a given location.
Interference ripples
Ripples caused by two currents flowing (one after the other) at or near right-angles to one another.
Interglacial
A time of glacial retreat during an ice age.
Interfingering
A complex pattern of sediments in which different sedimentary types (e.g. sand and mud) interpenetrate in interlocking wedges broadly similar to the pattern made by the fingers of two hands laced together.
Internal drainage
A drainage pattern typical of deserts, in which rivers flow into the desert and evaporate.
Intrusive rock
Rock formed by magma penetrating country rock but not reaching the surface as lava. As the magma will cool slowly, intrusive rock can be distinguished from extrusive rock by the relatively large size of the crystals of which the former is composed. Such rock is said to intrude into the country rock.
Inverse grading
Grading where the size of clasts varies from small clasts at the bottom of a layer to large clasts at the top.
Isostatic rebound
The process whereby land which has formerly been depressed by overlying weight (for example of an ice sheet) rises when the weight is removed.
Isotope
Atoms are classified into isotopes according to their atomic number and their atomic weight.
K-Ar dating
Potassium-argon dating, a form of radiometric dating.
Karst topography
The distinctive landscape produced by the chemical weathering of limestone.
K-Ca dating
Potassium-argon dating, a form of radiometric dating.
Kettle
A small lake formed by glacial outwash being deposited around a largish chunk of ice left behind by a retreating glacier; when the residual chunk of ice melts, this leaves a depression which will typically fill with water, producing a kettle.
La-Ba dating
Lanthanum-barium dating, a form of radiometric dating.
La-Ce dating
Lanthanum-cerium dating, a form of radiometric dating.
Lacolith
An intrusion between two strata, similar to a sill but thicker and lens-shaped.
Lamina
A very thin bed, no more than a few millimeters thick.
Lateral moraine
Sediment which accumulates along the sides of a valley glacier, having fallen or been scraped off the walls of the valley.
Laterite
A soil type characteristic of a tropical climate alternating between a monsoon season and a dry season.
Lattice silicates
Silicate minerals in which the silica tetrahedra are bonded together to form a three-dimensional lattice.
Lava
Molten rock which has reached the surface, as opposed to magma, which is sill trapped beneath it.
Leached ions
Ions dissolved in water as a result of chemical weathering; as opposed to residual minerals.
Lee
The side of a mountain, dune, antidune, or generally any hill-shaped geological feature, which is on the down-stream side of a current of wind or water. The opposite of stoss.
LGM
Abbreviation for last glacial maximum.
Lignite
The softest form of coal; the next stage in the formation of coal from peat after peat itself.
Limestone
Rock formed from calcium carbonate, usually in the form of calcite.
Limiting stand
Trees the growth of which we would expect to be limited by a single factor (such as temperature) because they grow in an environment with an abundant supply of other factors necessary for growth (such as rainfall).
Lineation
The arrangement of chain silicates in parallel lines in certain metamorphic rocks formed under pressure: the pressure forces these silicates to orientate themselves at right-angles to the direction of pressure.
Lithification
The conversion of sediment into a sedimentary rock by such processes as compaction and sedimentation.
Lithosphere
The Earth's crust together with that portion of the mantle which, like the crust, is brittle and elastic rather than plastic and ductile.
Littoral
Having to do with the coast.
Loess
Fine wind-borne sediment produced by the action of glaciers.
Longshore bars
Bars of sediment running parallel to a beach.
Longshore current
The component of a nearshore current that flows parallel to the shoreline.
Longshore drift
The motion of sediment along the shore as a result of the fact that waves that approach the shore obliquely will recede from it at right-angles to the shoreline.
Love waves
A type of seismic wave which travels along the surface of the Earth rather than through it.
Lu-Hf dating
Lutetium-hafnium dating, a form of radiometric dating.
Luster
The gloss on a mineral: pearly, greasy, metallic, dull, etc. While these are somewhat subjective qualities, they are often used in field-guides as criteria for identifying minerals.
Mafic rock
A rock rich in magnesium and iron, and poor in silica and feldspar.
Magma
Molten rock which has not reached the surface, as opposed to lava. Some authors will define magma as any molten rock, in which case it would be proper to say that "lava is magma on the surface". However, in this text we have preferred the usage which makes magma and lava two distinct non-overlapping categories of molten rock.
Mantle
A zone of ultramafic rock lying below the Earth's crust and above its core.
Mantle plume
A column of hot rock rising in the mantle below a hotspot.
Marble
A metamorphic rock formed by the metamorphism of limestone.
Massive rock
Rock which does not display bedding (in the case of sedimentary rocks) or foliation (in the case of metamorphic rocks), giving the rock a uniform and homogeneous appearance. This term is not used in our articles, so as to avoid confusion with the common use of "massive" to mean "very big"; we have instead used more transparent terms such as "unbedded".
Mass wasting
Erosion caused by gravity.
Matrix
Finer material in which larger clasts (in the case of sedimentary rock) or crystals (in the case of igneous rock) are embedded. A synonym for groundmass.
Meander
A broad loop in a stream.
Meandering stream
A stream which flows in a series of meanders.
Mechanical weathering
Weathering caused by mechanical processes that break up a rock, as opposed to chemical weathering.
Medial moraine
A moraine formed by the union of two lateral moraines when two valley glaciers flow together to forms a single glacier.
Metamorphic grade
The degree of metamorphism undergone by a metamorphic rock.
Metamorphic rock
A rock which has had its texture or composition changed by heat and/or pressure.
Metamorphism
Changes in the texture or composition of a rock brought about by heat and/or pressure.
Metasomatism
Changes associated with contact metamorphism in which the parent rock mixes and/or reacts with the intrusive igneous rock and the hot fluids associated with its eruption.
Methane
The gas CH
4, a potent greenhouse gas.
Mica
A group of sheet aluminosilicate minerals.
Mid-ocean ridge
Elevated sea-floor on either side of a mid-ocean rift.
Mid-ocean rift
The rift between two plates at which sea-floor spreading occurs.
Milankocitch cycles
Periodic changes in the inclination of the Earth's axis and the shape of its orbit.
Mineral
A solid with a particular chemical composition and crystal structure.
Mineraloid
Anything which is like a mineral in some respects but doesn't quite fit the definition.
Mixing plot
A graph showing the composition of a rock on which the plotted points will fall in a straight line if the rock was produced by the mixing of different sources of magma.
Mohs scale
A comparative scale of the hardness of minerals.
Mold
A void formed when sediment is packed around organic remains, which are then destroyed, leaving a void in the sediment in the shape of the remains.
Moraine
A deposit of till.
Mud crack
A small-scale geological structures produced in mud as it dries.
Mudstone
Rock formed from clay or silt which is not bedded: lithified mud which is bedded is known as shale.
Native metal
Metal occurring in pure form: for example, a nugget of gold.
Nearshore
The zone in which the sea bed is affected by waves.
Neutron
A particle with no charge and approximately the same mass as a proton; together with protons, neutrons make up the nucleus of an atom.
Nonconformity
An unconformity in which the older rocks are igneous or metamorphic.
Neosilicate
A silicate mineral in which the silica tetrahedra are isolated from one another.
Normal polarity
The polarity of the Earth's magnetic field as it is at present. (Note that there is nothing particularly normal about this state of affairs.) The opposite of reversed polarity.
Not fully understood
How geologists say "we don't know".
Nucleus
A fragment of shell or stone around which an ooid forms.
[indent]
or
The core of an atom, consisting of protons and neutrons.
Oasis
A small lake found in a desert.
Obduction
A process in which one plate colliding with another is thrust over it instead of beneath it; the opposite of subduction.
Obsidian
Felsic volcanic glass.
Olivine
A silicate mineral with the chemical formula (Mg,Fe)
2SiO4 and an orthorhombic crystal system.
Ooid
A small roughly spherical particle consisting of calcium carbonate layers formed around a nucleus of sand or shell. Required the action of waves for formation, and is therefore formed in shallow seas.
Oolite
Limestone composed of ooids cemented together.
Oolith
Either a rock formed from ooids, or a single ooid, depending on context.
Oolitic limestone
Synonym for oolite.
Opal
Amorphous hydrated silica, of which the precious stone known as opal is only one particularly pretty example.
Opal compensation depth
The depth at which siliceous material will dissolve faster than it is deposited; hence, the depth below which marine chert will not form.
Ophiolite
A section of oceanic crust which has been thrust up above sea-level.
Orthosilicate
A synonym for neosilicate.
Orogeny
The formation of mountains; or the faulting and folding of a large area by lateral pressure; or the formation of mountains by this process.
Oscillatory flow
The washing back and forth of water on the foreshore as a result of the action of the tide.
Outwash
Light sediment carried by meltwater from the ablation zone of a glacier.
Outwash plain
A flat area of outwash sediment in front of a glacier.
Oxbow lake
A crescent-shaped lake formed when a meandering stream changes its course, leaving one of its meanders cut off from the stream.
Oxygen catastrophe
Synonym for the Great Oxygenation Event
Oxygen crisis
Synonym for the Great Oxygenation Event
Oyster
A reef-forming bivalve.
Pa
Abbreviation for pascals.
Pahoehoe
A type of lava flow, or the cooled and solidified rock produced by it, characterized by a ropey and billowy surface texture.
Paleoclimatology
The study of ancient climates.
Paleocurrents
Ancient currents of wind and water the direction of which can be deduced from the analysis of sedimentary rocks.
Paleomagnetic dating
A form of absolute dating based on analysis of the paleomagnetic data in the rocks.
Paleomagnetism
The geological record of the past history of the Earth's magnetic field.
Paleosol
Fossilized soil.
Pangaea
The last supercontinent to exist, prior to its rifting and the formation of the Atlantic Ocean.
Pannotia
A supercontinent that existed before Pangea.
Paraconformity
An unconformity without an erosional surface.
Parent isotope
A radioactive isotope which undergoes radioactive decay to produce a daughter isotope.
Parent rock
The original rock from which a metamorphic rock is formed by metamorphosis.
Pascal
Unit of stress: 1 pascal = 1 newton/square meter.
Pb-Pb dating
Lead-lead dating, a form of radiometric dating.
Peat
Waterlogged and partially decomposed vegetable matter. Note that in geological usage peat does not just refer to gardeners' peat (formed from sphagnum moss) but to any vegetable matter that has undergone peatification. Peat is the sediment from which coal is formed.
Peatification
The partial decomposition of waterlogged vegetable matter, turning it into peat.
Pedogenesis
The process of turning sediment into soil by chemical weathering and the activity of organisms (plants growing in it, burrowing animals such as worms, the addition of humus etc).
Pelagic
Having to do with the open sea.
Pelagic clay
Fine-textured sediment deposited on the abyssal plain.
Peridotite
An ultramafic igneous rock consisting mainly of olivine with a little pyroxene and amphibole.
Permineralization
A process forming mineralized fossils in which the voids in the original material are filled by minerals.
Petrification
A process forming mineralized fossils in which they undergo both replacement and permineralization.
Phaneritic
An igneous rock is said to be phaneritic if the crystals in it are large enough to be seen with the naked eye. The opposite of aphenatic.
Phenocryst
A large crystal embedded in the more finely-grained ground mass of a porphyritic rock.
Philosophical naturalism
The rejection a priori of the existence of the supernatural; a position completely unnecessary to the practice of geology.
Phylosilicate
A synonym for sheet silicate.
Pillow basalt
Basalt with a distinctive shape consisting of a set of "pillows"; formed underwater by as a result of the more rapid cooling of lava on contact with water.
Pinstripe lamina
A very thin lamina of very fine clasts, formed in and characteristic of aeolian sand dunes.
Plastic
A material is said to be plastic if it does not recover from stress: that is, having been squeezed by stress into a given form, it retains that form when the stress is removed. The opposite of elastic.
Playa
A flat-bottomed basin in a desert which periodically fills with water to form a shallow temporary lake.
Plutonic rock
Alternative term for intrusive rock.
Plate
A piece of the lithosphere bounded by faults.
Plate tectonics
The study of the motion of the Earth's plates.
PMIP
The Paleoclimate Modeling Intercomparison Project, a project comparing results from climate models with the evidence from paleoclimatic proxies.
Point bar
A bar of sediment formed on the inner bank of a meander.
Polymorph
Two minerals having the same chemical formula but a different crystal structure are said to be polymorphs.
Porphyritic
Of a rock, containing some large crystals embedded in a more finely-grained ground mass.
Precipitation
When a chemical formerly dissolved in water settles out of it as a solid sediment, this is called precipitation. (The term has a wider application in chemistry, but for geological purposes this is all you need to know.)
Prehistoric
Preceding written human history; the fairly arbitrary line before which organic remains are considered to be fossils.
Primary rock
Term sometimes used for igneous rock.
Principle of cross-cutting relationships
The principle that when one geological feature cuts through another, the former is the younger and the latter is the older of the two features.
Principle of faunal succession
Roughly speaking, the principle that if the fauna and flora in one location are found in one stratigraphic order, the same species will not be found in a different order in another location.
Principle of least time
The principle in physics that a wave traveling through a medium will take the quickest route between two points.
Principle of original continuity
The principle that when sediment is laid down, it will extend continuously until either it meets an obstacle or tapers off with increasing distance from the source of the sediment.
Principle of original horizontality
The principle that when sediment is laid down, it is usually laid down more or less flat.
Principle of superposition
The principle that when sediment is laid down, the sediment most recently deposited will be on the top.
Proglacial lake
A lake fed by meltwater from a glacier.
Progradation
The building out of a glacier into the sea by deposition of sediment.
Proton
A positively charged particle of about the same mass as the neutron; together with neutrons, protons form the nuclei of atoms.
Proxy
A quantity which we can measure which bears a known relationship to a quantity that we can't measure but would like to; for example measuring past oxygen isotope ratios in shellfish as a substitute for measuring past temperatures.
Pseudostratigraphy
A term used to describe a situation where rock is layered, but the layers do not represent successive deposition; for example, the layers found in an ophiolite.
Pumice
A form of volcanic glass filled with air bubbles.
Pyroclastic flow
A current of air laden with volcanic ash, which resists dispersion into the surrounding air because of its greater density.
Pyroxine
An important group of rock-forming chain silicates.
P-waves
Body waves consisting of moving zones of compression and tension.
Quartz
A mineral consisting entirely of SiO44- units in a lattice structure, so that each oxygen atom of each unit is shared with one other unit, giving quartz the chemical formula SiO2
Quartzite
A metamorphic rock formed by metamorphism of quartz sandstone.
Quartz sandstone
Sandstone of which the sand grains are almost entirely quartz.
Racemic
Composed of equal amounts of both forms of enantiomers.
Racemization
The process by which a collection of chiral molecules become racemic.
Racemization dating
An alternative term for amino acid dating.
Radioactive
Having a tendency to radioactive decay.
Radioactive decay
Any process by which the composition of the nucleus of an atom is changed, such as alpha decay, beta decay, and electron capture.
Radiocarbon dating
Radiometric dating of organic material by analysis of the isotopes of carbon it contains.
Radiogenic
An atom is said to be radiogenic if it is the product of radioactive decay.
Radiolaria
A group of single-celled organisms which produce tiny intricate shells, usually siliceous; these form a major component of siliceous ooze.
Radiometric dating
A collection of methods of absolute dating which depend on the constancy of radioactive decay rates.
Rain shadow
A dry area on the lee side of a mountain, caused by the tendency of clouds to burst on the stoss side.
Raleigh wave
A type of seismic wave which travels on the surface of the Earth rather than through it.
Rank
The degree to which coal has undergone metamorphism.
Ra-Pb dating
Radium-lead dating, a form of radiometric dating.
Rb-Sr dating
Rubidium-strontium dating, a form of radiometric dating.
Recrystalization
Change in the texture of a rock.
Recumbent fold
A fold in a rock which has been pushed so far over as to lie on its side.
Redbeds
Sedimentary rocks cemented together chiefly by iron oxide, characteristic of dry climates.
Red clay
A synonym sometimes used for pelagic clay.
Reef
An underwater ridge or mound formed from the calcareous shells of organisms (typically coral in the present day, but the term is not restricted to coral reefs). Note that the geological usage is more restricted than the nautical usage, in which a sandbar or rock sufficiently near the surface of the water to cause a hazard to shipping would also be considered a reef.
Reef limestone
Limestone resulting from the intact preservation of hard parts of coral or other organisms.
Reflection seismology
A method of examining the structure of buried rocks by studying the reflections of seismic waves produced by artificial explosions.
Refraction
The change of direction undergone by a wave when it passes from a material which permits travel at one speed to a material which permits travel at another speed. A consequence of the principle of least time.
Regional metamorphism
Metamorphism over a wide region, caused by deep burial or wide-acting tectonic forces; as opposed to contact metamorphism.
Regression
An event in which the shoreline moves in a seaward direction; the opposite of a transgression.
Relative dating
Dating methods which allow us to put fossils and/or rocks in order of age, but without telling us how old they are, as opposed to absolute dating, which does.
Re-Os dating
Rhenium-osmium dating, a form of radiometric dating.
Replacement
A process forming mineralized fossils in which the orginal organic material is replaced by minerals.
Residence time
The average amount of time a given type of atom or molecule will spend in the ocean or in the atmosphere.
Residual minerals
Minerals which are not dissolved by chemical weathering.
Reversed polarity
A condition in which the north and south magnetic poles of the Earth were opposite in orientation to their present position. The opposite of normal polarity.
Rhyolite
A felsic extrusive igneous rock; the extrusive counterpart of granite.
Rhythmite
A sedimentary rock which display a repetitive vertical succession of types of sediment.
Ring silicates
Silicate minerals in which the silica tetrahedra are bonded together to form rings.
Ripple
A very small dune.
Roche moutonne
A hump of rock with one side shallow, polished, and striated and the other side steep and ragged, caused by a glacier flowing over the rock.
Rock
An aggregate of one or more minerals or mineraloids.
Rock cycle
The set of processes by which rocks are formed, altered, destroyed, and reformed.
Rock flour
Extremely fine sediment formed by the grinding action of a glacier.
Rock glacier
A glacier consisting mainly of rocks held together by ice.
Rock salt
Common salt (NaCl) when it occurs naturally as a rock; a synonym for halite.
Rounding
A clast is said to be rounded if its sharp edges and corners have been worn away by erosion. Note that the term does not imply that the clast in question is spherical or near-spherical, just that its shape is smooth.
Rudaceous rocks
Term for conglomerates and breccias.
Rudists
A group of reef-building molluscs that went extinct at the end of the Cretaceous period.
Sabkha
A salt flat found between the sea and a desert above the high-water line.
Saltation
The motion of a wind-blown or water-borne particle 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.
Salt flat
An accumulation of minerals on dry land by the evaporation of water containing dissolved minerals. While the commonest mineral in salt flats is indeed rock salt, other minerals such a gypsum may be deposited.
Sand
Particles of sediment between 1/16mm and 2mm in diameter.
Sandstone
Rock formed by the cementation of sand.
Saprolite
Completely weathered rock.
Sapropelic coal
Coal where the original organic material comes from the deposition of algae in lakes; as opposed to humic coal.
Satelite Laser Ranging
A system in which ground-based observation stations measure the round-trip time of ultrashort pulses of light traveling to and from satelites. Used by geologists to measure plate motion and isostatic rebound.
Schist
A high-grade metamorphic rock exhibiting pronounced foliation.
Schistosity
The kind of foliation found in schist.
Sclerochronology
An absolute dating method based on the study of growth patterns in shells and corals.
Scleroclimatology
The study of past climates by the analysis of the composition of shells.
Sea-floor spreading
The motion of two plates away from one another, producing a rift which is continuously filled by magma, producing fresh oceanic crust.
Seaload
The sediments carried by a wave.
Seamount
A marine mountain which is entirely underwater.
Seat earth
The paleosol underlying coalbeds.
Secular equilibrium
A condition in which the rate of production of a radioactive isotope in a rock is exactly balanced by the radioactive decay of the same isotope.
Secular variation
The wandering of the magnetic poles over time.
Sediment
Particles transported and/or deposited by wind, water, glaciers, precipitation, etc; the constituents of sedimentary rocks.
Sedimentary rock
Sediment lithified by cementation and/or compaction, or as a result of simple crystal growth in the case of evaporites.
Sediment trap
A device that collects sediment as it settles.
Seismic tomography
The science of discovering the internal structure of an object (typically, the Earth) by studying the passage of body waves through it.
Seismic waves
Waves in the body or surface of the Earth generated by earthquakes.
Seismometer
A device for detecting earthquakes and measuring their properties.
Semidiurnal
Occurring twice daily.
Series
A stratigraphic unit smaller than a system but larger than a stage.
Serpentinite
A metamorphic rock produced from peridotite in the presence of heat and water.
Serrated margins
Edges of leaves which are not smooth, characteristic of a temperate climate.
Shale
A sedimentary rock formed from silt or clay which exhibits bedding.
Shear
Stress that causes an object to skew, e.g. the stress that would deform a rectangle into a parallelogram.
Sheeted dikes
Dikes which stand side by side with one another like books on a shelf rather than intruding into some other rock.
Sheet glacier
Synonym for continental glacier.
Sheet silicate
Any silicate in which the SiO44- tetrahedra bond together to form a sheet.
Shoestring
A geological feature which is long and thin, e.g. a river or a shoreline.
Siderite
The mineral FeCO3.
Silica
Silicon dioxide, (SiO2). This occurs in many forms, such as quartz, opal, and chert
Silica tetrahedron
The ion SiO44-, consisting of four oxygen atoms arranged around a silicon atom in a tetrahedron. Such units can link together with each other by sharing oxygen atoms at their corners to form a variety of structures including sheet silicates, chain silicates and quartz.
Silicate
Any of a large and important class of minerals the chemistry of which is based on the silica tetrahedron.
Siliceous
Composed of silica.
Siliceous ooze
Ooze on the sea floor, composed of the siliceous shells of radiolaria and diatoms.
Sill
A sheet of intrusive rock forced between strata.
Silt
Clasts between 1/16mm and 1/256mm in diameter.
Siltstone
Sedimentary rock composed of silt.
Skarn
A rock produced from limestone and igneous rock by metasomatism.
Sky island
A mountain habitat which is home to species which are isolated by their inability to cross the drier hotter surrounding plain.
Slab
The portion of a plate being thrust into the athenosphere during subduction.
Slate
A metamorphic rock formed by metamorphism of shale, exhibiting pronounced foliation.
Slatey foliation
The sort of foliation found in slate.
SLR
Abbreviation for Satelite Laser Ranging.
Sm-Nd dating
Samarium-neodymium dating, a form of radiometric dating.
Snell's Law
A law relating the density of two mediums to the angle of refraction undergone by a wave when it passes from one medium to the other.
Soil
Sediment which has been altered by the effects of chemical weathering and and the activity of organisms (plants growing in it, burrowing animals such as worms, the addition of humus etc).
Sole mark
A mark made in sediment when it is scoured by a current.
Solid solution
A mineral in which some positions in the crystal lattice may be filled by different elements.
Sorosilicates
Silicates in which the silica tetrahedra are bonded together in pairs.
Sorting
Sediment is said to be well-sorted if it consists of particles of about the same grain-size.
Speleothem
A feature in a cave, such as a stalactite or stalagmite, formed by the precipitation of evaporated minerals, typically calcium carbonate.
Spherical symmetry
A body (in this textbook, invariably the Earth) is said to be spherically symmetric with respect to some property if the value of that property at any given point in it depends only on the distance of that point from the center, and not on the longitude and latitude of the point.
Stage
A stratigraphic unit smaller than a series but larger than a zone.
Stalactite
A speleothem hanging like an icicle from the roof of a cave.
Stalagmite
A speleothem in the form of a mound or column rising from the floor of a cave.
Standard
A rock of known age used in Ar-Ar dating.
Steno's principles
The principles of superposition, of original horizontality, and of original continuity.
Step heating
A process used in Ar-Ar dating in which a rock sample is heated in steps of progressively higher temperatures.
Strain
The deformation of a solid body as a result of stress.
Stratum
A layer of sedimentary rock with distinctive mineralogical, structural, or fossil characteristics such that it can be distinguished from the strata above and below it. Not to be confused with a bed.
Streak
The characteristic deposit left behind when a mineral is scraped across a surface; a diagnostic tool for distinguishing minerals.
Stream-dominated deltas
Deltas with long distributary channels reaching seaward; deltas in which the most important factor in their formation is the river discharging via the delta.
Stress
The force per unit area exerted on a surface of a deformable body; also by extension the external pressure which creates the internal force.
Striation
Grooves left by the movement of a glacier over a rock, parallel to the direction of motion.
Striae
Synonym for striations.
Stromatoporoids
Sponges which secrete a calcium carbonate skeleton and so were once important reef-forming organisms. While not actually extinct, they now live only in marginal habitats.
Subbituminous coal
Coal of a higher rank than lignite and a lower rank than bituminous coal.
Subduction
The motion of one plate beneath another and into the mantle.
Supercontinent
A landmass consisting of most or all of the continental crust joined together to form a single continent.
Supercontinent cycle
A process in which supercontinents repeatedly form and then rift again into separate continents.
Superposed river
A river which exists before the creation by erosion of the hills through which it flows.
Superimposed river
Synonym for superposed river.
Suspension
A form of transport of clasts by wind or water where the particles are carried above the ground, sea bed, river bed, etc.
Suture zone
The line along which a continent becomes joined to another continent, microcontinent, or island arc.
Swamp
An area of waterlogged ground in which the water is shallow enough for land plants to grow.
S-waves
Body waves consisting of waves of shear: that is, of displacement at right angles to the direction of travel of the wave, resembling the waves produced by shaking the end of a rope.
System
The largest stratigraphic unit.
Tarn
A lake that forms in the former cirque of a glacier after the glacier has melted.
Tectonic window
A place at which a rift in the Earth's crust allows us to see deeper into the crust than is normally possible.
Tectosilicate
A synonym for lattice silicate.
Tension
Stress that produces elongation of a solid along the direction in which force is applied.
Terminal moraine
A moraine deposited in the ablation zone of a glacier.
Terragenic
Having an origin on land.
Terrane
Part of a landmass, bounded by tectonic faults, which is different in many ways from the main landmass to which it is attached.
Test
The shell of a micro-organism such as a diatom or a foraminiferan.
TEX86
A temperature proxy based on measurement of the different varieties of GDGTs in sediments.
Texture
Physical characteristics of a rock including crystal size (in igneous or metamorphic rocks), and particle size and the degree of sorting and rounding of clasts (in sedimentary rocks).
Theory of evolution
The explanation of the facts of evolution in terms of such mechanisms as mutatation, recombination, lateral gene transfer, genetic drift, and natural selection. The explanation for the law of faunal succession.
Thermohaline circulation
Deep-water circulation driven by density differences in the temperature and salinity of sea water.
Tide-dominated deltas
Deltas in which the most important factor in their dynamics is the tide; characterized by the formation of offshore bars running parallel to the direction of the tide.
Till
Unsorted and usually unbedded sediment deposited by a glacier.
Tidal braking
Slowing of the Earth's rotation as a result of the tidal interaction between the Earth and the Moon.
Tillite
The lithified equivalent of till.
Topset beds
The flat beds of sediment deposited on the top surface of a delta.
Trace fossil
A fossil such as a footprint which is not of an animal but which was produced by one.
Transgression
An event in which the shoreline moves inland; the opposite of a regression.
Transitive
A relation is said to be transitive if when A stands in that relation to B, and B stands in that relation to C, then A stands in that relation to C. For example, the relation "is smaller than" is a transitive relation: if A is smaller than B, and B is smaller than C, then A is smaller than C.
Trench
A depression in the sea floor formed along the line where one plate subducts beneath another.
Tsunami
A sea-wave caused by any high-intensity, short-duration submarine event, most usually an earthquake. Often colloquially and completely inacurately known as a "tidal wave".
Tsunamite
Sediment deposited by a tsunami.
Tuff
Lithified volcanic ash
Turbid
Loaded with sediment.
Turbidite
Rock formed from sediment deposited by a turbidity current.
Turbidity current
A current which manages to keep from mixing with the medium through which it flows because, being turbid, it is denser than the surrounding medium.
Turbidity sediment
Sediment deposited by a turbidity current.
Uk'37
A proxy for temperature based on measurements of the different kinds of alkenones preserved in sediment.
Ultrabasic rock
An obsolete and inaccurate term for ultramafic rock.
Ultramafic rock
A rock which is extremely mafic; that is, particularly low in SiO44- units and high in magnesium and iron.
Unconformity
A surface between successive strata representing a period of erosion or of no deposition.
Underclay
Synonym for seat-earth.
Uniformitarianism
An alternative term for actualism, not used in this textbook because of ambiguities and inconsistencies in its meaning.
Unstable
Prone to radioactive decay.
U-Pa dating
Uranium-protactinium dating, a form of radiometric dating.
Upper plane bed
A flat stratified surface formed in a river bed when the river is travelling at too great a velocity to form ripples or dunes.
Uraninite
The mineral UO2
U-Th dating
Uranium-thorium dating, a form of radiometric dating.
Vail curve
A reconstruction of past variations of sea level based on the study of unconformities in the geological record.
Valley glacier
A glacier which has its accumulation zone on a mountain (typically in a cirque) and which flows down through valleys under both gravity and its own pressure; as distinct from a continental glacier.
Valve
One of the two shells of a bivalve.
Varve
A lamina of coarse light sediment grading into fine dark sediment, found in lakes fed by meltwater from a glacier and representing one year's deposition.
Very Long Baseline Interferometry
A technique in astronomy involving widely separated radio telescopes observing the same object, such as a quasar. Used by geologists to measure the motion of tectonic plates by inferring the motion of the radio telescopes necessary to account for the data.
Viscosity
Informally speaking, the reluctance of a liquid to flow; so for example maple syrup is more viscous than water.
Vis plastica
The name of an imaginary force once thought to cause fossils to grow in rocks.
VLBI
Abbreviation for Very Long Baseline Inferometry.
Volcanic ash
Fine debris formed when a volcano sprays out fine particles of lava. Note that the term "ash" is a misnomer, since volcanic "ash" is not a product of combustion.
Walther's principle
The principle that if sediment A is succeeded vertically by sediment B without an unconformity between them, then sediment A will also be succeeded horizontally by sediment B in some direction.
Wave base
The greatest depth at which the action of a wave has any effect.
Wave ripples
Ripples in sand or other sediment caused by the action of the tide.
Wave-dominated deltas
Deltas in which longshore drift forms barrier islands in front of the delta.
Way-up structure
A geological feature which enable us to discover which way up a rock was when it was originally formed.
Weathering
Processes which break up rock but do not themselves transport it, as distinct from erosion.
Weathering rind
The outer, weathered volume of a rock in which the outside has undergone weathering but the weathering process has not yet penetrated all the way through the rock.
Welded tuff
Tuff which forms when a fall of volcanic ash is still hot enough to weld itself together.
Xenotime
The mineral YPO4, useful because it can be used in the radiometric dating of sedimentary rocks.
Zircon
The mineral ZrSiO4, useful for radiometric dating because of its resistance to erosion, weathering, and metamorphosis.
Zone
The smallest stratigraphic unit. Can also be used in the usual informal sense of a region or area, as in the term "ablation zone".
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