GeologyRocks

Phenotype

The observable physical or biochemical characteristics of an organism, as determined by both genetic makeup (genotype) and environmental influences.

Phylogenetic species concept

A species is the smallest monophyletic group of common ancestry. See also biological species concept and morphospecies concept.

Plate Tectonics

Plate tectonics is the underpinning theory used in geology. Developed in the late 20th centuary, plate tectonics explains the location of earthquakes and volcanos, the apparent changes in magnetic pole location and shifts in climate recorded in the geological record across the continents. The theory states that the lithosphere is composed of a series of plates which move around the surface. The plates are created at mid-ocean ridges and destroyed at subduction zones.

Polymorph

A chemical compound to exists as two or more forms whilst having the same chemical composition. For example, calcite and aragonite.

Polyploidy

A major mechanism of speciation in plants. Caused by a failure of reduction of chromosome number during meiosis so 2n gametes are produced. Polyploid organisms have >2 sets of chromosomes. If new species are formed by polyploidy the polyploids must have higher fitness than the parental forms, they can achieve this by occupying new niches. Two types: autopolyploidy and allopolyploidy.

Porphyritic

A type of texture exhibited by igneous rocks where some minerals occur as significantly larger crystals in an otherwise fine matrix. See Phenocryst.

Porphyroblast

Unusually large crystal contained in a finer crystalline matrix occurring in metamorphic rocks. The resultant texture is a porphyroblastic texture.

Pseudomorph

A mineral with the outward form/shape of another species of mineral that it has replaced. There are three types of replacement: infiltration pseudomorph, paramorph, and encrustation pseudomorph.

See http://www.gc.maricopa.edu/earthsci/imagearchive/pseudomorphs.htm for examples and pictures.

Pyroclastic Flow

Extremely hot (700°C) clouds of gas and ash particles occurring during some explosive volcanic eruptions. They can travel at speeds comparable to a hurricane (100-150 mph). Also known as Nuees Ardentes.

Pyroxene

Silicate mineral composed of chains of silica tetrahedra. Cations occupy the sites between oxygen atoms. The general formula is:
A_1-x(B,C)_1-xT₂O₆.
A is commonly sodium or calcium, B is magnesium or iron (²⁺), C is aluminium or iron (³⁺) and T is silicon or aluminium. It is common in igneous and metamorphic rocks.