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GeologyRocks -Glossary of geology
Category: Earth and Environment > Geology
Date & country: 26/09/2007, UK
Words: 142

A solidified lava flow with a blocky surface, formed from a viscous lava. The name is of Hawaiian origin.

Absolute Age
An age of a rock or formation given in years determined by radiometric means. See also Relative age.

Alkaline rocks having the alkalis over Al ratio in excess of 1, and containing a suite of complex alkali titano-, niobo- and zirconosilicates (e.g. eudialyte).

When applied to igneous rocks, this word implies the presence of Sodium (Na) and/or Potassium (K).

Where a population or species occupies a different geographic region to another population or species. See also sympatric and parapatric.

Allopatric Speciation
Where different populations are formed, without intermediates, if physical barriers restrict gene flow. There are two types: vicariant speciation and peripatric speciation.

Chromosomes donated from >1 parental species. See also polyploidy and autopolyploidy.

Variant of quartz, purple in colour.

The process by which evolutionary change along a single lineage creates a new species without any splitting of the phylogenetic tree (see cladogenesis).

A polymorph whose other two minerals are kyanite and sillimanite. A particularly notable variety of andalusite is chiastolite, which has crosses of carbon in it.

A centimetre to decimetre layer in a stratigraphy. Beds can be massive or contain structures, such as cross bedding, ripple marks or laminations. Beds are grouped into formations.

Biological species concept
Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. See also morphospecies concept and phylogenetic species concept.

A low temperature, high pressure regionally metamorphic rock, containing abundant glaucophane.

Bouma Sequence
The five part deposit produced by a waning turbidite flow. The deposit is split into:
Structureless massive sand with gravel-rich erosional base.
Parallel bedded sands produced by the upper flow regime.
Cross laminated sands with ripple marks.
Deposited in the lower flow regime.
Laminated silts.Fine pelagic muds.
A typical Bouma sequence is illustrated below, although all five units may not be developed in each flow:

A clastic rock composed of a matrix and angular clasts. A fine example of a breccia are the Devonian deposits at Siccar Point.

Calcite is a mineral composed of Calcium Carbonate (CaCO3). It exists as two forms, low and high Magnesium (Mg). Only low Mg is stable at the Earth's surface. Calcium Carbonate can also be found as aragonite, which is a polymorph of calcite. Calcite crystals may also show double refraction.

The earliest defined geological period spanning 542-483Ma. Time before the Cambrian is referred to as the Pre-Cambrian, making the Cambrian the first period of the Palaeozoic Era. All fossils from this period are marine and the beginning of this period saw the Cambrian Explosion; an apparent burst of evolution. The Cambrian is split into four Epochs, Lower Series, Series 2, Series 3 and Furongian.

The fifth period of the Palaeozoic. Spans 359-299Ma, and is famous for the extensive coal beds found throughout the UK.

The infill of a fossil mould.

A fine grained, white calcium carbonate composed of coccoliths. Common in the Cretaceous of western Europe. The White Cliffs of Dover are a classic example of chalk cliffs.

Microcrystalline silica (SiO2). Flint is the best known example.

Variety of quartz. Yellow-orange in colour.

The evolution of two or more daughter species from a single parent species by the splitting of a lineage.

The distinct planes of weakness that a mineral breaks along. The picture below shows clear cleavage lines.

A gradual change in an allele frequency or in the mean of a character over a geographic boundary.

The fourth period of the Palaeozoic. Spans 409-353Ma. The rocks of this period are referred to as Old Red Sandstone (ORS) as they are often red coloured sandstone. The name originates from the county of Devon in South West England. The colonisation of land was also well under way as shown by the famous Rynie Chert deposit.
Late Devonian 371-353Ma
Middle Devonian 380-371Ma
Early Devonian 409-380Ma

Directional selection
Fitness increases/decreases with trait magnitude. E.g. selection on bill size in Darwin`s medium ground finch – Geospiza fortis. Population crashes in 1977, 1980 and 1982, due to droughts, selected for increased bill depth due to abundance of hard seeds of Tribulus cistoides. Deep beaks are needed to crack these seeds and therefore large bills were selected for.

An unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

Disruptive selection
Individuals with extreme trait values have greatest fitness. E.g. selection on bill size in the black-bellied seedcracker – Pyrenestes o. ostrinus. This species of African finch exhibits non-sex related polymorphism in bill size. Members of the species have either one of two bill morphs; bills are relatively small or relatively large. The morphs feed on different sized seeds and this is directly related to natural selection on bill size.

A glacial deposit; an elongate shaped hill. The longest dimension is parallel to the movement of the ice flow (see arrow).

Dunham Classification
A classification of limestone based on depositional textures. The classification consists of mudstone (<10% grains, >90% mud), wackestone (>10% grains, mud supported), packstone (grain supported, but contains mud), grainstone (grain supported, no mud) and boundstone (grains bound in situ, i.e. reef).

A metamorphic rock of mantle origin. It is usually composed of garnet and omphacite (a pyroxene).

A measure of internal energy of a system. For a gas: H=E+pV Where H is the enthalpy, E is the internal energy, p is the pressure and V is the volume.

In thermodynamics entropy is the amount of energy not available to do work.

Eustatic Sea Level Change
Sea level change on a global scale. Causes include the switching between ice age and interglacial periods and changes in the global ocean volume due to plate tectonic movement. Combined with the isostatic changes make a relative sea level change.

A discontinuity over which there has been displacement of rock strata. See Reverse Fault, Normal Fault and Strike-Slip Fault. There are many terms associated with faults.

Fissure Eruption
An eruption occurring through long, open crack in the ground. They commonly form flood basalts which are repeated flows on low viscosity lava and can cover large areas.

The ability of an individual to survive and reproduce relative to conspecifics.

See clint.

Microcrystalline quartz (SiO2) found in chalk. Chert is the equivalent in any other rock, i.e. flint is chert.

Pertaining to a river or stream. A fluvial sandstone is a sandstone that was deposited in a river.

Folk's Classification
Folks classification of limestones based on the matrix/cement and the main lithological components. For example, a limestone with a micrite matrix containing mainly ooids would be Oomicrite. If the cement were sparite instead, then it would be an Oosparite. Folk used four main clast types: ooids, bioclast, peeloid and intraclast.

A formation is a group of beds used in stratigraphy. This is the smallest unit on a geological map. Formations are grouped together into a group. See also Member.

Any remains, trace, or imprint of an organism that has been preserved in the Earth's crust.

Founder Effect
The principle that the founders of a new colony carry only a fraction of the total genetic variation in the source population.

The unwanted material around ore minerals

Gardar Province
Geological area of south-west Greenland famous for its alkali intrusions especially Ilimaussaq which has over 200 minerals associated.

A cubic mineral of the general form of: A32+B23+Si3O12 A can be magnesium, iron, manganese or calcium, B is iron or aluminium (occasionally chromium). Commonly found in metamorphic and occasionally igneous rocks.

Gene Flow
The incorporation of genes into the gene pool of one population from one or more other populations.

Genetic Bottleneck
A severe, temporary reduction in variation in a population.

Genetic Drift
Random changes in the frequency of two or more alleles or genotypes within a population.

The genetic makeup, as distinguished from the physical appearance (phenotype), of an organism or a group of organisms.

A branch of geology where non-invasive techniques such as magnetics, gravity surveys and seismic surveys are used to map unseen geological structures, such as faults.

Gibbs Free Energy
Chemical reactions can be made to do work (e.g. produce heat). Energy which is, or which can be, available to do useful work is called Gibbs free energy (G). G = H - TS Where H is the enthalpy, S is the entropy, and T is the absolute temperature.

Metamorphic rock, showing distinct banding of light and dark minerals. Usually made of feldspars, micas and hornblende.

A channel or pass connecting an atoll lagoon with the open ocean, e.g. at Mururoa Atoll (French Polynesia)

An individual resulting from mating between genetically distinct populations/species. See also hybrid zone.

Hybrid Zone
A region in which genetically distinct populations come into contact and produce offspring off mixed ancestry. This process is known as hybridisation.

See hybrid zone and hybrid. If new species are formed by hybridisation the hybrids must have higher fitness than the parental forms, they can achieve this by occupying new niches.

A solid, liquid or gas composed of hydrogen and carbon. Examples include coal, oil and natural gas.

Ice Age
An extended period of time where there is extensive glaciation covering the Earth.

Inbreeding Coefficient (F)
The probability that an individual taken at random from the population will be autozygous.

Inbreeding Depression
Reduction, in inbred individuals, of the mean value of a character (usually one correlated with fitness).

Incompatible Elements
Incompatible relative to mantle minerals; those elements with large ionic radii such as Nb, Zr, U, Th, K, Na, Li, Be, REE, that preferentially go into the melt rather than remain in residual minerals.

Intrusion Magma
Magma that has crystallised beneath the surface.

Isolating Mechanism
A genetic difference between populations that prevents or restricts gene flow between them. See reproductive isolation.

The study of the responses of the Earth to loads (mountain belts, glaciers) placed on it. Isostatic theory states that at the depth of compensation all pressures exerted by rocks above are equal. Northern Europe is currently re-bounding from glaciers that were there during the Last Glacial Maximum, such that Scotland is raising and Kent is sinking.

Isostatic Sea Level Change
Sea level change on a local scale. Can be caused by glaciers making continents sink or melting glaciers making the continents rise again (isostatic rebound) or local tectonic activity. Combined with the eustatic changes make a relative sea level change.

An isotropic mineral appears opaque in thin section in polarised light.
Other meanings:
Materials: In the study of mechanical properties of materials, 'isotropic' means having identical values of a property in all crystallographic directions.
Optics: Optical isotropy means having the same optical properties in all directions.

The middle period of the Mesozoic from 199Ma to 145Ma.

Karst Topography
Karst topography arises from the dissolution of limestones or evaporites. Typical features include springs, limestone paving, caves and potholes.

See gryke.

A polymorph whose other two minerals are sillimanite and andalusite. Usually blue.

A mudflow associated with volcanic eruptions when volcanic ash mixes with water.

A rock composed mainly of calcium carbonate CaCO3. Limestones comprise a large variety of rocks including carbonate muds and oozes, chalks, ooliths and bioclastic limestone. They form in a variety of environments from deep sea (but above the carbonate compensation depth), temperate seas, tropical sea, lakes and rivers. Most caves are found in limestones due to erosion by acidic water. Limestones erode into karst topography.

Limestone Pavement
A formation found in areas of karstic topography where limestone has dissolved into pavement-like blocks. The dissolution occurs along joints forming grykes. This leaves blocks called clints.

The process of turning loose sediment into a sedimentary rock. This is done by heat, pressure and time. This process can take from years to millions of years. Hardgrounds are formed in inter-tidal carbonates and can form in years. Usually this process is a slow one. If this process continues, metamorphism occurs.

The outer, rigid shell of the earth. It is composed of the entire crust and uppermost part of the mantle. The thickness of the lithosphere varies from around 5km to up to 100km where there is thick continental crust. The lithosphere is above the more ductile asthenosphere. The boundary between these two layer is the Mohorovicic discontinuity. The lithosphere is fragmented into tectonic plates which move relative to one another. This movement of lithospheric plates is described as plate tectonics.

A melt, generally containing suspended crystals and other volatiles, formed by total or partial melting of solid mantle or crustal material.

The layer beneath the crust, but above the core in the interior of the Earth. It's composition is broadly that of ultrabasic rocks.

Area of dark basaltic lava on the Moon (pl. Maria).

A division of a formation, generally of distinct lithologic character and of only local extent.

Era comprising the Triassic, Jurassic and Cretaceous, spanning 245-65Ma.

Metamorphosis require heat, pressure and time and is an extension of lithification, but it can occur on any rocks type (including metamorphic rocks). There is a continuum of type from heat dominated (marble) to pressure dominated (blueschist). Metamorphism is ranked in terms of a grade. A high grade metamorphic rock is a gneiss, which has undergone intense heat and pressure. It is important to note metamorphism occurs in the solid state - there is no melting. If the rock starts to melt it is called a migmatite (Mixed IGneous and metamorphic rock).

Micro-crystalline carbonate mud.

A naturally occurring homogeneous solid with a highly ordered lattice and of a defined chemical composition.

The Mohorovicic discontinuity, usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. The Moho serves to separate both oceanic crust and continental crust from underlying mantle. The MohoroviÄ?ić discontinuity was first identified in 1909 by Andrija MohoroviÄ?ić, a Croatian seismologist, when he observed the abrupt increase in the velocity of earthquake waves (specifically P-waves) at this point.

Natural Selection
Differential reproductive success. Produces descent with modification, i.e. evolution.

A diagenetic transformation of a mineral and itself or a polymorph, including differences in size and shape.

A geometric element of a composite rock or mineral deposit, appearing to be younger than the main rock mass, for example the leucosomes in a migmatite are a neosome.

A silica-undersaturated volcanic igneous rock containing nepheline and pyroxene.

The method of naming species of animals and plants scientifically.

An unconformity that exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock was deposited on the pre-existing, eroded metamorphic or igneous rock.

Normal Fault
A fault caused by extension.

Nuees Ardentes
See Pyroclastic Flows.

A super-chilled igneous rock, black in colour and glassy in appearance. Fractures conchoidally. Often formed due to contact with cold seawater as the silicate minerals cool almost immediately, restricting mineral growth. Can also be called volcanic glass.

A spherical grain of calcium carbonate, either aragonite or calcite. See the Ooid Formation tutorial for more details of their formation and origin.

The second period of the Palaeozoic. Spans 510-443. The Ordovician is famous for its graptolite and trilobite-rich deposits, particularly in the UK. The Ordovician ended in a mass extinction, probably caused by an Ice Age.

A seismic body wave which propagate like sound waves, i.e. by compression and extension in the direction of travel. They can travel through solids and liquids and are the fastest of the seismic waves.

A solidified lava flow with a smooth, ropey surface, formed from less viscous lava than aa. The name is of Hawaiian origin.