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


Palaeozoic
Era comprising the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian, spanning 545-245Ma. The Paleozoic started with an apparent evolutionary burst (the Cambrian Explosion) and ended with the largest mass extinction seen on this planet, the P-T event.

Parallel Evolution
The evolution of similar or identical characters independently in related lineages.

Parapatric
Populations that have contiguous but non-overlapping geographic distibutions. See also sympatric and allopatric.

Period
A second order geological time unit.

Peripatric Speciation
A type of allopatric speciation in which a colony diverges from a parent population. Also known as founder effect speciation.

Permafrost
Soil, rock and sediment where the temperature stays below 0°C for at least two consecutive winters and the summer in between. Permafrost usually occurs in a layer some distance beneath the topsoil.

Permian
The final period of the Palaeozoic from 290Ma-245Ma. The end of the Permian saw a mass extinction that wiped out around 90% of all marine life and nearly 70% of terrestrial life. Many theories have been presented for the cause of the extinction, including plate tectonics, an impact event, a supernova, extreme volcanism, the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect, or some combination of factors.
Epochs:
Zechstein 260-245Ma
Rotliegendes 290-260Ma

Phenocrysts
Unusually large, often euhedral, crystals in an otherwise fine-grained igneous rock. The resultant texture is a porphyritic texture. Phenocrysts are the result of slower crystallisation of the magma than the fin-grained matrix. See Porphyritic texture.

Rare Earth Elements
Elements with atomic numbers 57 (Lanthanum) to 71 (Lutetium) inclusive plus scandium and yttrium.

Rayleigh Wave
A seismic surface wave which has a elliptical particle motion in the direction of travel. These are the waves that are responsible for the surface movement during an earthquake.

Regional Metamorphism
Metamorphism on a large scale involving heat and pressure.

Regolith
Loose material, such as dust and rock fragments, covering bedrock.

Regression
A lowering of sealevel observed in the geological record. See trangression.

Relative Age
The age of a rock or formation given relative to other rocks or formation, usually defined as a zone fossil name. See also Absolute age.

Relative sea level change
A local sea level change caused by a combination of global (eustatic) and local (isostatic) changes. This is the sea level variation 'recorded' by the rock record.

Reproductive character displacement
Where a character used in species recognition differs more greatly between sympatric than between allopatric populations/species.

Reproductive Isolation
The situation where two populations are prevented or restricted from interbreeding by isolating mechanisms.

Reproductive Reinforcement
Evolution of enhanced reproductive isolation between populations due to natural selection.

S Wave
A seismic body wave that has motion perpendicular to the direction of travel (i.e. like light waves). They cannot travel through liquids and are slower than P Waves.

Sandstone
A clastic rock composed of sand-sized grains, i.e. between 0.625 and 2mm. The term usually implies a quartz rich composition, although this is not strictly necessary.

Seismology
The study of earthquakes and the structure of the earth via seismic waves. This covers whole earth structure (global seismology) and crustal structure (exploration seismology). Exploration seismology is used in commercial operations, such as oil exploration. There are two types of seismic waves that travel through the interior of the Earth; P-waves and S-waves.

Sequence stratigraphy
Sequence stratigraphy is a branch of geology that attempts to link relative sea-level changes to sedimentary deposits. The essence of the method is mapping of strata based on identification of time lines, such as unconformities, maximum flooding surfaces and transgression surfaces. In doing so it places stratigraphy in chronostratigraphic framework.

Sexual Selection
Variation in reproductive success due to variation in the ability to acquire mates.

Silica Tetrahedra
All silicate minerals have a structure based on the silica tetrahedra, an arrangement of 1 silica atom and 4 oxygen atoms. The chemical formula for this is SiO4.

Silicate
A mineral formed from silica and oxygen. The majority of rock forming minerals are silicates. Examples are quartz and pyroxene.

Silliminite
A polymorph whose other two minerals are kyanite and andalusite.

Silurian
A geological period; the third of the Paleozoic Era from 443 to 416Ma. The fossil record from this period includes a huge variety of marine fossils, land plants and early invertebrate land animals.

Slickenside
A polished or smooth fault surface. The direction of the last fault movement can be determined by feeling which direction is smoothest with your hand.

Taxonomy
The science of classification of organisms according to resemblances and differences.

Transgression
A raising of sea level observed in the geological record. See regression.

Triassic
The first period of the Mesozoic. Spans 251-199Ma.

Turbidite
A deposit formed by a turbidity current. Turbidity currents are turbid, sub-aqueous, density flows driven by gravity. They can form on slopes of less than 1 degree and can flow uphill as they can travel at 10km/s. They form when a unstable build-up of sediment is triggered to collapse due to a storm, earthquake or other triggering event.

Ultrabasic
A rock which contains mainly ferro-magnesium minerals and feldspars with the virtual exclusion of quartz.

Unconformity
A break in the stratigraphic record which represents a period of no sediment deposition. The interval of geologic time not represented is called a hiatus. There are three types of unconformities: disconformity, nonconformity, and angular unconformity.

Unloading
Unloading usually results in pressure release formations. A good example is the tilting of the UK due to the removal of ice sheets in the north. Scotland is moving upwards, southern Britain is moving downwards. The evidence can be seen in raised beaches in Scotland, which gives the past sea level.

Vicariant Speciation
Two widespread populations are divided by the emergence of an extrinsic barrier, e.g. the formation of a mountain belt.

Volatile
Gases and impurities contained in magma which make volcanic explosions more violent. They readily vapourise at normal temperatures and pressures. Examples are carbon dioxide and water.

Volcano
A point on the Earth's surface where magma erupts and accumulates.

Weathering
The process of disintegrating rocks in the near surface by chemical or physical means.

Xenolith
A foreign inclusion in an igneous rock.

Yazoo
A tributary that runs parallel to the main river for some distance.

Zoned Crystal
A crystal with concentric layers differing in chemical composition representing a complex crystallisation history.