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P


paleomagnetism
1. The fixed orientation of a rock's crystals, based on the Earth's magnetic field at the time of the rock's formation, that remains constant even when the magnetic field changes. 2. The study of such phenomena as indicators of the Earth's magnetic history.
paleosol
An ancient, buried soil whose composition may reflect a climate significantly different from the climate now prevalent in the area where the soil is found.
Paleozoic Era
The earliest era of the Phanerozoic Eon, marked by the presence of marine invertebrates, fish, amphibians, insects, and land plants.
parabolic dune
A horseshoe-shaped dune having a concave windward slope and a convex leeward slope. Parabolic dunes tend to form along sandy ocean and lake shores. They may also develop from transverse dunes through deflation.
parent isotope
A radioactive isotope that changes into a different isotope when its nucleus decays. See also daughter isotope.
parent material
The source from which a given soil is chiefly derived, generally consisting of bedrock or sediment.
partial melting
The incomplete melting of a rock composed of minerals with differing melting points. When partial melting occurs, the minerals with higher melting points remain solid while the minerals whose melting points have been reached turn to magma.
passive continental margin
A border that lies between continental and oceanic lithosphere, but is not a plate margin. It is marked by lack of seismic and volcanic activity.
peat
A soft brown mass of compressed, partially decomposed vegetation that forms in a water-saturated environment and has a carbon content of 50%. Dried peat can be burned as fuel.
pediment
A broad surface at the base of a receding mountain. The pediment develops when running water erodes most of the mass of the mountain.
pegmatite
A coarse-grained igneous rock with exceptionally large crystals, formed from a magma that contains a high proportion of water.
perched water table
A saturated area that lies within a zone of aeration.
peridotite
An igneous rock composed primarily of the iron-magnesium silicate olivine and having a silica content of less than 40%.
permafrost
Permanently frozen regolith, ranging in thickness from 30 centimeters to over 1000 meters.
permeability
The capability of a given substance to allow the passage of a fluid. Permeability depends on the size of and the degree of connection among a substance's pores.
petroleum
Any of a group of naturally occurring substances made up of hydrocarbons. These substances may be gaseous, liquid, or semi-solid.
phosphorescence
Emission of visible light by a substance, such as a mineral, that is exposed to ultraviolet light and absorbs radiation from it. The light appears in the form of glowing, distinctive colors. The emission continues after the exposure to ultraviolet light ends.
phyllite
A foliated metamorphic rock that develops from slate and is marked by a silky sheen and medium grain size.
placer deposit
A deposit of heavy or durable minerals, such as gold or diamonds, typically found where the flow of water abruptly slows.
plastic deformation
A permanent strain that entails no rupture.
plate tectonics
The theory that the Earth's lithosphere consists of large, rigid plates that move horizontally in response to the flow of the asthenosphere beneath them, and that interactions among the plates at their borders cause most major geologic activity, including the creation of oceans, continents, mountains, volcanoes, and earthquakes.
playa
A dry lake basin found in a desert.
Pleistocene Epoch
The first epoch of the Quaternary Period, beginning 2 to 3 million years ago and ending approximately 10,000 years ago. See also Holocene Epoch.
pluton
An intrusive rock, as distinguished from the preexisting country rock that surrounds it.
plutonic rock
An intrusive rock formed inside the Earth.
pluvial lake
A lake that formed from rainwater falling into a landlocked basin during a glacial period marked by greater precipitation than is found in the region in prior or subsequent periods.
point bar
A low ridge of sediment that forms along the inner bank of a meandering stream.
polymorph
A mineral that is identical to another mineral in chemical composition but differs from it in crystal structure.
porosity
The percentage of a soil, rock, or sediment's volume that is made up of pores.
porphyry copper deposit
A crystallized rock, typically porphyritic, having hairline fractures that contain copper and other metals.
potassium-argon dating
A form of isotope dating that relies on the extremely long half-life of radioactive isotopes of potassium, which decay into isotopes of argon, to determine the age of rocks in which argon is present. Potassium-argon dating is used for rocks between 100,000 and 4 billion years old.
potentia
The combined influence of gravity and water pressure on groundwater flow at a given depth.
potentiometric surface
The level to which the water in an artesian aquifer would rise if unaffected by friction with the surrounding rocks and sediments.
primary coast
A coast shaped primarily by nonmarine processes, such as glacial erosion or biological processes.
principle of cross-cutting relationships
The scientific law stating that a pluton is always younger than the rock that surrounds it.
principle of faunal succession
The scientific law stating that specific groups of animals have followed, or succeeded, one another in a definite sequence through Earth history.
principle of original horizontality
The scientific law stating that sediments settling out from bodies of water are deposited horizontally or nearly horizontally in layers that lie parallel or nearly parallel to the Earth's surface.
principle of superposition
The scientific law stating that in any unaltered sequence of rock strata, each stratum is younger than the one beneath it and older than the one above it, so that the youngest stratum will be at the top of the sequence and the oldest at the bottom.
principle of uniformitarianism
The scientific law stating that the geological processes taking place in the present operated similarly in the past and can therefore be used to explain past geologic events.
property
A characteristic that distinguishes one substance from another.
proton
A positively charged particle that is found in the nucleus of an atom and has a mass approximately 1836 times that of an electron.
P wave(abbreviation for primary wave)
A body wave that causes the compression of rocks when its energy acts upon them. When the P wave moves past a rock, the rock expands beyond its original volume, only to be compressed again by the next P wave. P waves are the fastest of all seismic waves. See also S wave.
P-wave shadow zone
The region that extends from 103 to 143 from the epicenter of an earthquake and is marked by the absence of P waves. The P-wave shadow zone is due to the refraction of seismic waves in the liquid outer core. See also S-wave shadow zone.
pyroclastic
Being or pertaining to rock fragments formed in a volcanic eruption.
pyroclastic cone
A usually steep, conic volcano composed almost entirely of an accumulation of loose pyroclastic material. Pyro-clastic cones are usually less than 450 meters high. Because no lava binds the pyroclastics, pyroclastic cones erode easily.
pyroclastic eruption
A volcanic eruption of viscous, gas-rich magma. Pyroclastic eruptions tend to produce a great deal of solid volcanic fragments rather than fluid lava.
pyroclastic flow
A rapid, extremely hot, downward stream of pyroclastics, air, gases, and ash ejected from an erupting volcano. A pyroclastic flow may be as hot as 800C or more and may move at speeds exceeding 150 kilometers per hour.
pyroclastics
(used only in the plural) Particles and chunks of igneous rock ejected from a volcanic vent during an eruption.

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