GeologyRocks

P-Wave

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.

Pahoehoe

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

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.