Geology Vocabulary:
14C method: A method for determining the age in years of organic matter by calculating the amount ofradioactive carbon still remaining, as compared to the stable isotope, 12C.
40K/40Ar method: A method used for the dating of potassium-bearing rocks by using the ratio ofradioactive 40K to its daughter, 40Ar.
ablation: As applied to glacier ice, the process by which ice below the snow line is wasted by evaporationand melting.
absolute time: Geologic time expressed in years before the present.
abyssal plain: Large area of extremely flat ocean floor lying near a continent and generally over 4 km in depth.
accretion: The process by which the terrestrial planets grew, increasing their mass by graduallyaccumulating smaller bodies, called planetesimals.
active layer: The seasonally thawed zone above permafrost .
alluvial fan: Land counterpart of a delta . An assemblage of sediments marking place where a stream moves from a steep gradient to a flatter gradient and suddenly loses transporting power. Typical of arid and semiarid climates but not confined to them.
andesite line: The geographic boundary between rocks of the Pacific Basin, which are basaltic, and those around the rim of the basin, which are in part andesitic.
andesite: A fine-grained volcanic rock of intermediate composition, consisting largely of plagioclase and one or more mafic minerals.
aphanitic: A textural term meaning "fine-grained" that applies to igneous rocks.
aquifer: A permeable region of rock or soil through which ground water can move.
aquitard: A material of low permeability that greatly slows the movement of ground water.
Archean: An eon of geologic time extending from about 3.9 billion years to 2.5 billion years ago.
arête: A narrow, saw-toothed mountain ridge developed by glacier erosion in adjacent cirques .
arkose: A sedimentary rock formed by the cementation of sand-sized grains of feldspar and quartz.
asthenosphere: The weak or "soft" zone in the upper mantle just below the lithosphere , involved in plate movement and isostatic adjustments. It lies 70 to 100 km below the surface and may extend to a depth of 400 km. Corresponds to the seismic low-velocity zone .
atoll: A roughly circular reef with an occasional small, low, coral sand island surrounding a shallow lagoon.
banded iron formation (BIF): A sedimentary mineral deposit dominated by iron oxides, carbonates, or silicates that were deposited chemically from seawater. Most BIFs were formed between 2.5 and 3.5 billion years ago. Their formation is related to the rise of oxygen in the atmosphere.
barchan: A crescent-shaped sand dune with horns pointing downwind.
basalt: A dark colored extrusive igneous rock composed chiefly of calcium plagioclase and pyroxene. Extrusive equivalent of gabbro, underlies the ocean basins and comprises oceanic crust.
base flow: Ground water that enters a stream channel, maintaining stream flow at times when it is not raining.
batholith: A large, discordant, intrusive body of igneous rock.
bedrock: Any solid rock exposed at the Earth's surface or overlain by unconsolidated material.
beta decay: The process of radioactive decay in which a neutron loses a beta particle, which is physically identical to an electron. This increases the atomic number of the atom by one by turning the neutron into a proton. The atom's atomic mass number stays the same because the total number of protons and neutrons remain the same. The most common form of radioactive decay.
biogenic sediment: Sediments produced directly by the life processes of plants or animals.
bioturbation: The turning and mixing of sediments by organisms.
breccia: A clastic rock in which the gravel-sized particles are angular in shape and make up an appreciable volume of the rock.
caldera: A large, basin-shaped volcanic depression, more or less circular in form. Typically steep-sided, found at the summit of a shield volcano .
carbonate conservation depth: The water depth below which the calcium carbonate produced in the ocean is completely dissolved. There is no calcium carbonate deposition below this level.
carbonate rock: A rock consisting primarily of a carbonate mineral such as calcite or dolomite, the chief minerals in limestone and dolostone, respectively.
Cenozoic: The current geologic era, which began 66.4 million years ago and continues to the present.
chalk: A variety of limestone made up in part of biochemically derived calcite, in form of skeletons or skeletal fragments of microscopic oceanic plants and animals mixed with fine-grained calcite deposits of biochemical or inorganic-chemical origin.
clastic: Refers to rock or sediments made up primarily of broken fragments of pre-existing rocks orminerals.
col: Mountain pass formed by enlargement of two opposing cirques until their head walls meet and are broken down.
competence: The maximum size of particle that a stream can carry.
conglomerate: A clastic sedimentary rock composed of lithified beds of rounded gravel mixed with sand.
continental crust: The part of the crust that directly underlies the continents and continental shelves.Averages about 35 km in thickness, but may be over 70 km thick under largest mountain ranges.
continental ice glacier: An ice sheet that obscures all but the highest peaks of a large part of a continent.
continental rise: The portion of the continental margin that lies between the abyssal plain and thecontinental slope. The continental rise is underlain by crustal rocks of the ocean basin.
continental shelf: The portion of the continental margin that extends as a gently sloping surface from theshoreline seaward to a marked change in slope at the top of the continental slope . Seaward depth averages about 130 m.
continental slope: That part of the continental margin that lies between the continental shelf and the continental rise. Slope relatively steep, 3o - 6o. The continental slope is underlain by crustal rocks of the continent.
core: Innermost zone of Earth. Consists of two parts, an outer liquid section and an inner solidsection, both chiefly of iron and nickel with about 10 percent lighter elements. It is surrounded by the mantle
craton: The stable portions of the continents that have escaped orogenic activity for the last 2 billion years. Made predominantly of granite and metamorphic rocks. (compare to orogen).
creep: 1. The very slow, generally continuous downslope movement of soil and debris under the influence of gravity. 2. The movement of sand grains along the land surface.
Cretaceous: The geologic period from about 144-65 million years ago (Mya). The continents were approaching their current positions as Gondwana breaks up, the climate was much warmer, and no glaciers were in contact with the sea. Black shale deposits suggest oceans were anoxic at the bottom. Dinosaurs top predator.
crust: The upper part of the lithosphere , divided into oceanic crust and continental crust .
Curie point: The temperature above which a mineral loses its magnetism.
Darcy's law: A formula describing the flow of water through an aquifer.
decomposition (chemical weathering): Weathering processes that are the result of chemical reactions. Example: the transformation of orthoclase to kaolinite.
deflation: A process of erosion in which wind carries off particles of dust and sand.
dehydration: Any process by which water bound within a solid material is released. Example: Gypsum (CaSO4·2H2O) becomes anhydrite (CaSO4) by dehydration.
delta: An assemblage of sediments accumulated where a stream flows into a body of standing water and its velocity and transporting power are suddenly reduced. . A "delta plain" is the upper surface of a delta.
dendritic drainage: A stream pattern that, when viewed on a map or from the air, resembles the branching pattern of a deciduous tree such as a maple or oak.
denudation: The sum of the processes that result in the wearing away or the progressive lowering of theEarth's surface by weathering, erosion, mass wasting, and transportation.
depositional environment: The nature of the environment in which sediments are laid down. They are immensely varied and may range from the deep ocean to the coral reef and the glacial lake of the high mountains. The nature of the depositional environment may be deduced from the nature of the sediments and rock deposited there.
depositional remanent magnetism: Develops as magnetic minerals settle through water and align themselves in the Earth's magnetic field.
desert pavement: A large accumulation of pebbles or boulders that cuts off further deflation .
desertification: A process of land degradation initiated by human activity, particularly in the zones along the margins of deserts.
detrital sediments: Sediments made of fragments or mineral grains weathered from pre-existing rocks.
diagenesis: All the physical, chemical, and biologic changes undergone by sediments from the time of their initial deposition, through their conversion to solid rock, and subsequently to the brink of metamorphism.
diamict(on): A general term for any unsorted, unstratified sediment regardless of its genesis. Diamicts may be formed in various situations: glaciation, mudflow, landslide, avalanche, and turbidity current. Till is a special kind of diamicton that was formed directly from glacier ice. The terms diamictite and tillite are used for the ancient, consolidated equivalents of diamicton and till sediments.
differential weathering: Weathering that occurs at different rates, as the result of variations in composition and mechanical resistance of rocks, or differences in the intensity of weathering processes.
differentiation: The process of developing more than one rock type, in situ, from a common magma.
disintegration (mechanical weathering): The processes of weathering by which physical actions such as frost wedging break down a rock into fragments, involving no chemical change.
dissolution: A chemical reaction in which a solid material is dispersed as ions in a liquid. Example: Halite (NaCl) undergoes dissolution when placed in water.
Dole effect: The observed difference between the
dolostone: A carbonate rock made up predominately of the mineral dolomite, CaMg(C03)2.
drumlin: Streamlined hill, largely of till, with blunt end pointing into direction from which ice moved. Occur in clusters called drumlin fields.
ductile: Structural behavior in which a material deforms permanently without fracturing.
electron capture: Nuclear decay in which a proton in the nucleus acquires an electron from the outer cloud of the atom's electrons. This converts the proton to a neutron, reduces the number of protons in the nucleus by one and atomic number of the original element by one. Atomic mass number remains constant because the total number of protons and neutrons is unchanged.
eon: The primary division of geologic time which are, from oldest to youngest, the Hadean, Archean, Proterozoic, and Phanerozoic eons.
epoch: A division of geologic time next shorter than a period. Example: the Pleistocene epoch is in the Quaternary period.
equilibrium line: On a glacier the line separating the zone of accumulation from the zone of ablation .
era: A division of geologic time next smaller than the eon and larger than a period. Example: The Paleozoic era is in the Phanerozoic eon and includes, among others, the Devonian period.
erratic: A stone or boulder, glacially transported from place of origin and left in an area of different bedrock composition.
eustatic change in sea level: A worldwide change in sea level, such as caused by melting glaciers.
eutrophication: The process of aging of lakes by the addition of nutrients.
evaporite: A mineral or rock deposited directly from a solution (commonly seawater) during evaporation. For example, gypsum and halite are evaporite minerals.
ferromagnesian: Containing iron and magnesium, applied to the mafic minerals. Example: olivine.
fetch: Distance over which wave-forming winds blow.
firn (névé): Granular ice formed by the recrystallization of snow. Intermediate between snow and glacier ice.
fjord: Glaciated valleys now flooded by the sea.
fragipan: A dense layer of soil, containing silt and sand but no organic matter and little clay, whoseextreme hardness and impermeability are due primarily to compaction. compare caliche , claypan, hardpan.
fringing reef: A coral reef attached directly to the mainland.
gneiss: A coarse, foliated metamorphic rock in which bands of granular minerals (commonly quartz and feldspars) alternate with bands of flaky or elongate minerals (e.g., micas, pyroxenes). Generally less than 50% of the minerals are aligned in a parallel orientation.
Gondwana: The southern portion of the late Paleozoic supercontinent known as Pangea. It means, literally "Land of the Gonds" (a people of the Indian subcontinent). The variant Gondwanaland found in some books, therefore, is a tautology.
granite: Light colored, coarse grained, intrusive igneous rock characterized by the minerals orthoclaseand quartz with lesser amounts of plagioclase feldspar and iron-magnesium minerals. Underlies largesections of the continents.
Hadean: The oldest eon in Earth history, extending from the origin of the Earth to about 3.9 billion years ago.
humus: The generally dark, more or less stable part of the organic matter in a soil, so well decomposed that the original sources cannot be identified.
hydrograph: Graph of variation of stream flow over time.
hydrolysis: A decomposition reaction involving water, in which hydrogen ions (H+) or hydroxyl ions (OH-) replace other ions. The result is a new residual mineral. Example: the addition of water to orthoclase produces kaolinite and releases K+ and silica into solution.
ice sheet: A broad, mound-like mass of glacier ice that usually spreads radially outward from a central zone.
ice shelf: A floating ice sheet extending across water from a land-based glacier.
icecap: A small ice sheet.
igneous rock: A rock that has crystallized from a molten state.
inner core: The solid innermost part of the core with a diameter of a little over 1,200 km.
isostasy: The condition of equilibrium, comparable to floating, of units of the lithosphere above the asthenosphere .
isostatic change in sea level: A sea level change due to change in load on Earth's crust.
isotope: Atoms that differ in atomic mass number , but not in atomic number , are called isotopes. For example, oxygen (atomic number 8) may have an atomic mass number of 16, 17, or 18, depending on whether it has 8, 9, or 10 neutrons. It therefore has three isotopes.
karst: A landscape that develops from the action of ground water in areas of easily soluble rocks. Characterized by caves, underground drainage and sinkholes.
lahar: A mudflow composed chiefly of pyroclastic material on the flanks of a volcano.
lateral moraine: Moraine formed by valley glaciers along valley sides.
laterite: A highly weathered red soil rich in iron and aluminum oxides. Typically formed in a tropical to temperate climate where intense chemical weathering is common.
Laurasia: The northern portion of the late Paleozoic supercontinent called Pangea.
lithosphere: The rigid outer shell of the Earth. It includes the crust and uppermost mantle and is on theorder of 100 km in thickness.
loess: Deposits of wind-borne dust.
longshore current (littoral current): A current that flows parallel to the shore just inside the surf zone.Also called the littoral current.
longshore drift: The general movement of sediment parallel to the shoreline. Waves generally carrysediment up the shore face at an angle to the shoreline, but carry it straight out again, resulting in a net longshore displacement.
Love wave: A seismic surface wave that has a horizontal (side-to-side) component but no vertical component.
mafic: Referring to a generally dark-colored igneous rock with significant amounts of one or more ferromagnesian minerals, or to a magma with significant amounts of iron and magnesium.
magma ocean: A global-scale ocean of magma, according to some calculations several hundred kilometers deep, thought to have existed during the final stages of accretion as the Earth was forming.
magma: Molten rock, containing dissolved gases and suspended solid particles. At the Earth's surface, magma is known as lava .
magnetic anomaly: The amount by which a measurement of the local magnetic field intensity exceeds or falls below the intensity of the global magnetic field.
magnetic chron: Time during which magnetic polarity is dominantly normal or dominantly reversed.
magnetic polarity time scale: A chronology based on the shifting polarity of the Earth's magnetic field.
magnetic polarity: The direction, north (normal) or south (reversed), that a magnetic compass needle points.
mantle plume: A hypothetical column of hot, partially molten material that rises from an indeterminate depth in the mantle and is thought by some geologists to provide a driving force for plate movement. compare hot spot .
mantle: That portion of the Earth below the crust and reaching to about 2,780 km, where a transition zone of about 100 km thickness separates it from the core.
marble: A metamorphic rock composed largely of calcite. The metamorphic equivalent of limestone.
margin: The tectonic region that lies at the edge of a continent, whether it coincides with a plate boundary or not.
medial moraine: Formed by the merging of lateral moraines as two valley glaciers join.
metamorphic rock: A rock changed from its original form and/or composition by heat, pressure, or chemically active fluids, or some combination of them.
metamorphism: The processes of recrystallization, textural and mineralogical change that take place in the solid state under conditions beyond those normally encountered during diagenesis.
metasomatism: The metamorphic processes that occur as a result of the passage of chemically active fluids through a rock, adding to or removing constituents during metamorphism.
Mohorovicic discontinuity (Moho): The sharp seismic velocity discontinuity that separates the crust and the mantle.
moraine: Landform made largely of till.
névé: see firn.
oceanic crust: That part of the crust underlying the ocean basins. Composed of basalt and having a thickness of about 5 km.
orogen: Linear to arcuate in plan, intensely deformed crustal belt associated with mountain building. (compare to craton)
paleosol: A buried soil horizon of the geologic past. .
Paleozoic: An era of geologic time lasting from 570 to 245 million years ago.
Pangea: A supercontinent that existed from about 300 to 200 million years ago, and included most of thecontinental crust of the Earth.
pelagic ooze: A deep ocean sediment consisting of at least 30% skeletal remains of calcareous or siliceousmicroorganisms, the rest being clay minerals.
periglacial: Refers to conditions in a near glacial climate.
phaneritic: A textural term meaning "coarse-grained" that applies to igneous rocks.
Phanerozoic: the most recent eon of geologic time beginning 570 million years ago and continuing to thepresent.
piedmont glacier: A glacier that spreads out at the foot of mountains, formed by the coalescence of two or more valley glaciers.
placer: A surficial mineral deposit formed by mechanical concentration of valuable minerals from weathered debris, usually through the action of stream currents or of waves.
playa: A broad flat desert basin, often containing an ephemeral playa lake.
pluton: An igneous intrusion.
pluvial lake: A lake formed during a pluvial period.
pluvial period: Time when a dryland area had greater effective moisture than at present.
pocket beach: Small, narrow beach, usually crescentic, at head of a bay or small inlet.
polymetamorphism: A series of events in which two or more metamorphic episodes have left their imprint on the same rocks.
polymorphism: The circumstance in which two minerals with different crystalline structures have identical chemical compositions. Example: Diamond and graphite.
Precambrian: An informal term to include all geologic time from the beginning of the Earth to the beginning of the Cambrian period 570 million years ago.
Proterozoic: The geologic eon lying between the Archean and Phanerozoic eons, beginning about 2.5 billion years ago and ending about 0.57 billion years ago.
regolith: A layer of unconsolidated fragmental rock material.
rhyolite: A fine-grained silica-rich igneous rock, the extrusive equivalent of granite.
rock varnish: A thin, shiny veneer of clay minerals and iron and manganese oxides deposited on some rocks in a desert environment.
Rodinia: A giant supercontinent believed to have formed about 1.1 billion years ago in the Precambrian. North America formed the core and its eastern coast was adjacent to western South American, and the west coast of North America was next to Australia and Antarctica. Rodinia broke into two halves about 750 million years ago. Between the two halves was the Congo Craton. The earth is believed to have been heavily glaciated at this time, since each of the continents shows evidence of glaciation at this time. This is a time when ‘snowball Earth’ may have occurred.
solifluction: Turbulent movement of saturated soil or surficial debris.
striations: Scratches, or small channels, gouged by glacier action. Occur on boulders, pebbles, and bedrock. Striations along bedrock indicate direction of ice movement.
tailings: Washed or milled ore that is too poor to be further treated.
talus: A slope built up by the accumulation of rock waste at the foot of a cliff or ridge.
tephra: A general term for all pyroclastic material.
terminal moraine: (end moraine) Ridge of till marking farthest extent of glacier.
thermoremanent magnetism: The magnetism of a mineral that it is acquired as it cools below its Curie point.
till (unstratified drift): Glacial drift composed of rock fragments that range from clay to boulder size and randomly arranged without bedding.
travertine (tufa): Variety of limestone which forms stalactites and stalagmites and other deposits in limestone caves (dripstone) and the mouths of hot and cold calcareous springs.
tuff: A general term for all consolidated pyroclastic rock. Not to be confused with tufa.
unconformity: A buried erosion surface separating two rock masses.
uniformitarianism: The principle that applies to geology our assumption that the laws of nature are constant. As originally used it meant that the processes operating to change the Earth in the present also operated in the past and at the same rate and intensity and produced changes similar to those we see today. The meaning has evolved and today the principle of uniformitarianism acknowledges that past processes, even if the same as today, may have operated at different rates and with different intensities than those of the present. The term "actualism" is sometimes used to designate this later meaning.
U-shaped valley: A valley carved by glacier erosion and whose cross-valley profile has steep sidesand a nearly flat floor, suggestive of a large letter "U".
varve: A pair sedimentary units, one coarse-grained, the other fine-grained, interpreted as representing one year of sedimentation.
weathering: The process by which Earth materials change when exposed to conditions at or near the Earth's surface and different from the ones under which they formed. compare decomposition , disintegration .
zone of ablation: The area of wastage in a glacier.