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Glossary of Geological Terminology from Quartz 'N' All Category: Earth and Environment > Geology Date & country: 15/11/2007, UK Words: 308

aa
A solidified lava flow with a very rough or clinkery surface.

Aalenian
The lowest stage of the M. Jurrasic (dogger) 178.0 - 173.5 Ma

Ab
Method of indicating Plagioclase feldspar composition as a percentage of albite, ie Ab15 indicates a composition of 15% albite, 85% anorthite

Abandonment
The voluntary surrender of legal rights or title to a mining claim

Abbe refractometer
An instrument for determining the refractive index of minerals and liquids.

Ablation
The removal of detritus by wind action.

Abrasion
Mechanical erosion by debris-charged wind, water or ice, which also removes the eroded material.

Abrasion Platform
Any horizontal surface cut into a slope.

Abrasive
Any material suitable for grinding, polishing, scouring or cutting. e.g. diamond, corundum, sand, umpice.

Absarokite
A porphyritic basalt with a small amount of orthoclase in the groundmass.

Absolute age
The age of a rock or formation with respect to the present. Determined by radiometric dating methods.

Absolute Permeability
The ability of a rock at 100% saturation to transmit a particular fluid.

Absolute Plate Motion
The motion of a plate with respect to the interior of the Earth. Absolute motions can be determined by making use of the hotspot reference frame.

Absolute Temperature
Temperature measured on the Kelvin scale, with respect to absolute zero (-273.15oC)

Abukuma-type metamorphism
Morphism at a high geothermal gradient and low pressure, characterised by the presence of andalusite and sillimanite in pelitic rocks.

Abutment load
Weight transferred to adjacent solid rock in a deep excavation.

Abyss
A very deep oceanic depression

Acanthite
Ag2S An ore mineral of silver

Accessory lithic
A clast in a pyroclastic rock formed of material torn from a volcanic vent's walls during a volcanic eruption.

Accessory Mineral
A mineral comprising less than ~10% of a rock which is insignificant to nomenclature or classification.

Accidental lithic
A clast in a pyroclastic rock plucked from the ground during the transport of magma.

Accomodation structure
A small structure which allows a bed to fill all available space created during deformation.

Accoustic basement
The boundary between overlying sediments and underlying igneous or metamorphic rocks, characterised by strong seizmic reflections on a siesmogram.

Accretion Vein
A mineral vein in which more than one phase of fracture and infilling has occurred.

Achnelithe ( Pele's hair )
Hair-like volcanic glass formed by lava exuding through a small orifice and blown by the wind.

Achondrite
A class of stony meteorite with no chondrules

Achroite
A white potassium-rich variety of tourmaline.

Acicular
(see Habit) Fine, needle-shaped crystals as in Rutile

Acid
(Of igneous rocks) Containing at least 10% quartz, and chemically more than 65% silica

Acidic rock
An igneous rock with >10% free silica

Adamantine
A high degree of sparkle, as demonstrated by a diamond.

Adinole
An argillaceous rock which has undergone albitization during contact metamorphism.

Adobe
A clay similar to loess

Adularescence
Bluish-white schiller which appears on the surface in moonstone.

Adularia
(KALSi3O8) A colourless translucent variety of potash feldspar.

Advection
Transport of heat, or any other physical property, as a reslut of movement of a fluid, such as occurs in a convection current.

Aeolian
Pertaining to wind-driven processes.

Aeronian
A stage of the silurian 436.9 - 432.6

Agalmatolite
A compact variety of pyrophyllite.

Agate
Concentric layers of chalcedony with different colours and porosity.

Alabandite
(MnS) A rare ore mineral of manganese.

Alaskite
A bimineralic leucocratic granite rock composed of quartz and alkali feldspar.

Albertite
A pitch-black solid bitumen of the asphltite group

Albian
A stage of the cretaceous 112.0 - 97.0

Albite Twinning
Twinning which forms zebra-like stripes, seen in plagioclase feldspars in thin section.

Albitization
The metasomatic replacement of an existing mineral, usually another feldspar by albite as sodium ions are introduced into the rock.

Alexandrian
An ordovician / silurian succession in North America covering part of the U. ordovician, the rhuddanian and part of the aeronian.

Alkali Feldspar
A general term for feldspar of the K-Na solid solution, orthoclase/sandine - orthoclase.

Alkaline
(of igneous rock) with a high content of alkali metals, namely sodium and potassium

Aller
A permian succession in NW Europe covering the lower part of the capitanian.

Allochemical metamorphism
Metamorphism in which there is removal or addition of material and the chemical composition of the rock is altered.

Allochromatic
Describes minerals of variable colour. (See idiochromatic)

Alluvial Deposits
Carried by a river, minerals are separated out according to density, and chemically resistant materials, such as gold, cassiterite, ruby and garnet, may form important deposits.

Alteration
Once a mineral has formed there are many things that can impact upon it and change or alter it to other minerals. Examples are weathering and hydrothermal activity. The alteration products can give clues to the identity of the original mineral.

Aminikan
An era of the precambrian, 2,200 - 1650 Ma

Amorphous
A structureless mass that does not naturally form crystals.

Amygdales
Vesicles found in volcanic rocks, originally formed by gas bubbles in the magma and later filled with secondary minerals

An
Percentage of anorthite in plagioclase feldspar.

Anhedral
Descriptive of a mineral with no crystal form developed.

Anionic
Most minerals are compounds containing one, or more elements,. They have two charged parts called ions The negatively charged anions often contain oxygen

Anisian
The lower stage of the M. Triassic, 241.1 - 239.5 Ma

Aptian
A stage of the cretaceous, 124.5 - 112.0 Ma

Aquifer
A geological unit containing suffieient saturated permeable rock to yield significant amounts of water.

Aquitanian
The lowest stage of the micene23.3 - 21.5 Ma

Arenaceous
(of sedimentary rocks) medium grained rocks

Argillaceous
(of sedimentary rocks) Fine grained rocks

Arnsbergian
A stage of the carboniferous, 331.1 - 328.3 Ma

Artiniskian
A stage of the permian 268.8 - 259.7 Ma

Arundian
A stage of the carboniferous, 345.0 - 342.8 MA

Asbian
A stage of the carboniferous 339.4 - 336.0 Ma

Aseimic
Earthquake free.

Ashgill
The oldest epoch of the ordovician, 443.1 - 439.0 Ma

Asphaltite
A group name for the organic compounds abertite, anthraxolite, grahamite, impsonite, nigrate and uintaite.

Asselian
The lowest stage of the permian, 290.0 - 281.5 Ma

Assemblage
Refers to the total number of minerals included in a metamorphic rock

Asterism
Star-shaped rays of light which meet at a point. Formed like cat's eye, except that the reflecting fibres lie in different directions.

Asteroid
A small rocky or metallic interplanetary body (bolide). A concentration of these bodies is found in the Asteroid belt between Mars and Jupiter.

Asthenosphere
A mechanically weak layer of the mantle immediately beneath the lithosphere, corresponding to the depth range within the Earth where the melting temperature is most closely approached. The top is near the surface beneath oceanic ridges, 120 - 180 km deep under old ocean basins and at least 250 km deep, if present at all beneath the continents.

Astrobleme
A terrestrial crater over 10km in diameter formed by the impact of an extraterrestrial body.

Atdabanian
A stage of the cambrian, 560 - 554 Ma

Atoll
An irregular subcircular annular coral - algal reef surrounding a central lagoon in oceanic waters.

Atomic absorption spectroscopy
A method of chemical analysis in which a solution of a sample is passed into a flame which atomizes it. The amount of light absorbed from a source of the wave-length or a particular element focussed on the flame gives a measure of that element's concentration.

Atterberg Limit
The result of tests oon chohesive soil that characterize changes from solid to plastic to liquid states.

Augen gneiss
A gneiss with a planar or linear shape fabric consisting of eye-like lensoid shapes, often resluting from the deformation of porphyritic coarse-grained igneous rocks or by the growth of porphyroblasts.

Aureole
Country rock affected by contact metamorphism around igneous intrusion.

Autoclastic
descriptive of a rock that has fractured in situ

Autointrusion
The injection of magma into fissures of its earlier crystallized rock.

Autometamorphism
The process whereby metamorphic changes take place by the action of residual fluids as an igneous body cools.

Autometasomatism
The metasomatism of newly crystallized igneous rock by its residual fluids.

Aventurism
Varigated play of colours; glistening reflections of leaf-like inclusions on an almost opaque background.

Back - arc spreading
A process whereby new lithosphere is created by rifting and subsequent sea floor spreading in the over-riding slab of a subduction zone to form a back-arc basin

Backshore
The area of a beach between normal high tide level and the highest point reached by marine action such as storm waves.

Backthrust
A low-angle reverse fault with a vergence different from the majority of reverse faults in the area.

Backwash
The gravity-fed return flow of water after a wave breaks on a beach. An important factor in determining beach gradient.

Backwearing ( parallel retreat )
Erosional hillslope retreat without change in slope morphology. Occurs when the rate of erosion on different sections of the slope is proportional to the dip.

Baddeleyite
Zr)2 A zircon material. A by-product of working certain carbonatites.

Bafflestone
An autochthonous carbonate rock whose original components were bound organically during deposition, the organisms forming baffles to trap finer matrix material.

Bagenov
A jurassic succession in W. Siberia

Bajada
A low-lying area of confluent pediment slopes and alluvial fans at the base of mountains around a desert.

Bajocian
A stage of the jurassic, 173.5 - 166.1 Ma