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Pelean-type eruption: a violent explosion dominated by pyroclastic flows.
period: see geological period.
permeable rock: a rock that will allow a fluid
to pass through it.
Permian, Permian Period: the last period of the Palaeozoic Era, lasting from about 280 to 225 million years ago.
petrified: when the tissues of a dead plant or animal have been replaced by minerals, such as silica, they are said to be petrified (e.g. petrified wood).
petrified forest: a large number of fossil trees. Most petrified trees are replaced by silica.
petroleum: the carbon-rich, and mostly liquid, mixture produced by the burial and partial alteration of animal and plant remains. Petroleum is found in many sedimentary rocks. The liquid part of petroleum is called oil, the gaseous part is known as natural gas. Petroleum is an important fossil fuel.
petroleum field: a region from which petroleum can be recovered.
Phanerozoic Eon: the most recent eon, beginning at the Cambrian Period, some 570 million years ago, and extending up to the present.
phenocryst: an especially large crystal (in a porphyritic rock), embedded in smaller mineral grains.
phylum: (pl. phyla) biological classification for one of the major divisions of animal life and second in complexity to kingdom. The plant kingdom is not divided into phyla but into divisions.
placer deposit: a sediment containing heavy metal grains (e.g. gold) that have weathered out of the bedrock and are concentrated on a stream bed or along a coast.
plagioclase: the form of feldspar that is often white or grey and which contains sodium and calcium as important ions.
planetismals: small embryo planets. plate: see tectonic plate.
plateau: an extensive area of raised flat land. The cliff-like edges of a plateau may, when eroded, leave isolated features such as mesas and buttes. See also tableland.
plate tectonics: the theory that the earth’s crust and upper mantle (the lithosphere) are broken into a number of more or less rigid, but constantly moving, slabs or plates.
Plinian-type eruption: an explosive eruption that sends a column of ash high into the air.
plug: see volcanic plug
plunging fold: a fold whose axis dips, or
plunges, into the ground.
plutonic rock: an igneous rock that has solidified at great depth and contains large crystals due to the slowness of cooling (e.g. granite, gabbro).
porphyry, porphyritic rock: an igneous rock in which larger crystals (phenocrysts) are enclosed in a fine-grained matrix.
Precambrian, Precambrian time: the whole of earth history before the Cambrian Period. Also called Precambrian Era and Precambrian Eon.
precipitate: a substance that has settled out of a liquid as a result of a chemical reaction between two chemicals in the liquid.
Primary Era: an older name for the Palaeozoic Era.
prismatic: a word used to describe a mineral that has formed with one axis very much longer than the others.
Proterozoic Eon: see eon.
P wave, primary wave, primary seismic wave: P waves are the fastest body waves. The waves carry energy in the same line
as the direction of the wave. P waves can travel through all layers of the earth and are generally felt as a thump. See also S wave.
pyrite: iron sulphide. It is common in sedimentary rocks that were poor in oxygen, and sometimes forms fossil casts.
pyroclastic flow: solid material ejected from a volcano, combined with searingly hot gases, which together behave as a high-density fluid. Pyroclastic flows can do immense damage, as was the case with Mount Saint Helens.
pyroclastic material: any solid material ejected from a volcano.
Quaternary, Quaternary Period: the second period in the Cenozoic Era, beginning about 1.6 million years ago and continuing to the present day.
radiation: the transfer of energy between objects that are not in contact.
radioactive dating: the dating of a material by the use of its radioactive elements. The rate of decay of any element changes in a predictable way, allowing a precise date to be given of when the material was formed.
rank: a name used to describe the grade of coal in terms of its possible heat output. The higher the rank, the more the heat output.
Rayleigh wave: a type of surface wave having an elliptical motion similar to the waves caused when a stone is dropped into a pond. It is the slowest, but often the largest and most destructive, of the wave types caused by an earthquake. It is usually felt as a rolling or rocking motion and, in the case of major earthquakes, can be seen as they approach. Named after Lord Rayleigh, the English physicist who predicted its existence.
regional metamorphism: metamorphism resulting from both heat and pressure. It is usually connected with mountain building and occurs over a large area. See also contact metamorphism.
reniform: a kidney-shaped mineral habit (e.g. hematite).
reservoir rock: a permeable rock in which petroleum accumulates.
reversed fault: a fault where one slab of the earth’s crust rides up over another. Reversed faults are only common during plate collision.
rhyolite: acid, igneous, volcanic rock, typically light in colour; volcanic equivalent of granite.
ria: the name for a partly flooded coastal river valley in an area where the landscape is hilly.
Richter Scale: the system used to measure the strength of an earthquake. Developed by Charles Richter, an American, in 1935.
rift, rift valley: long troughs on continents and mid-ocean ridges that are bounded by normal faults.
rifting: the process of crustal stretching that causes blocks of crust to subside, creating rift valleys.
rock: a naturally occurring solid material containing one or more minerals.
rock cycle: the continuous sequence of events that cause mountains to be formed, then eroded, before being formed again.
rupture: the place over which an earthquake causes rocks to move against one another.
salt dome: a balloon-shaped mass of salt produced by salt being forced upwards under pressure.
sandstone: a sedimentary rock composed of cemented sand-sized grains 0.06–2 mm in diameter.
scarp slope: the steep slope of a cuesta. schist: a metamorphic rock characterised by
a shiny surface of mica crystals all orientated in the same direction.
scoria: the rough, often foam-like rock that forms on the surface of some lavas.
seamount: a volcano that rises from the sea bed.
Secondary Era: an older term for a geological era. Now replaced by Mesozoic Era.
sediment: any solid material that has settled out of suspension in a liquid.
sedimentary rock: a layered clastic rock formed through the deposition of pieces of mineral, rock, animal or vegetable matter.
segregation: the separation of minerals.
seismic gap: a part of an active fault where there have been no earthquakes in recent times.
seismic wave: a wave generated by an earthquake.