P Index Glossary geology Science
potentia
The combined influence of gravity and water pressure on groundwater flow at a given depth.
Pahoehoe
A Hawaiian term for a lava flow that has a surface flow structure appearance that looks like coiled rope or cord. See aa for contrast.
Paleoclimate
The climate of a given area at a specific time in the past. Paleoclimates can be read from the rocks much as areas with different types of climates produce sediments with specific characteristics today.
Paleocurrent Map
A map that shows the directions of currents at the time of sediment deposition. These directions can be determined through the study of cross bedding, ripple marks, tool marks and other sedimentary structures.
Paleogeographic Map
A map that shows the distribution of sedimentary environments at a specific time in the past. These maps are made by studying the rock record to correlate rock units that were deposited at the same time, then relating rock characteristics to specific sedimentary environments.
Paleomagnetism
The study of Earth's magnetic field over time. When rocks that contain magnetic minerals are deposited, the character (vertical and horizontal orientation) of Earth's magnetic field is locked within the rocks. This information can be used to study changes in Earth's magnetic field as well as the movement of plates over time.
Paleontology
The study of ancient life through fossils.
Paleowind
A prevailing wind direction in an area, inferred from dune structure or the distribution of volcanic ash for one particular time in geologic history.
Pedalfer
A common soil type in humid regions, characterized by an abundance of iron oxides and clay minerals deposited in the B-horizon by leaching.
Pedocal
A common soil type of arid regions, characterized by accumulation of calcium carbonate in the A-horizon.
Peneplain
A hypothetical extensive area of low elevation and relief reduced to near sea level by a long period of erosion and representing the end product of the ideal geomorphic cycle.
Perched groundwater
An isolated body of ground-water that is perched above and separated from the main water table by an aquiclude.
Preferred orientation
Any deviation from randomness in the distribution of the crystallographic or grain shape axes of minerals of a rock (including flow cleavage and foliation), produced by deformation and non-uniform stress during crystallization in metamorphic rocks or by depositional currents in sediments.
Pangaea
A large continental landmass that existed from about 300 million years ago through about 200 million years ago. It included most of the continental lithosphere present at that time. It has since broken up and the fragments have drifted to become the configuration of Earth's present day continents.
Panhandle
A narrow projection of a larger territory (as a state).
Physiographic Region
A portion of the Earth's surface with a basically common topography and common morphology.
Lying or formed at the base of mountains; in the
Platted Land
Land that has been divided into surveyed lots.
Polynodal
Many-centered.
Post
industrial
An economy that gains its basic character from economic activities developed primarily after manufacturing grew to predominance. Most notable would be quaternary economic patterns.
Presidio
A military post (Spanish).
Primary Sector
That portion of a region's economy devoted to the extraction of basic materials (e.g., mining, lumbering, agriculture).
Panthalassa
The ancient ocean that surrounded the Pangaea landmass.
Parent Element
A radioactive element that spontaneously decays into a new substance. The product of this decay is known as a "daughter" element.
Pegmatite
A very coarse grained igneous rock, normally of granitic composition. Typically forms during the final states of magma chamber crystallization when the high water content solutions allow rapid crystal growth.
Pelagic Sediment
A ocean sediment that accumulates far enough from land that detrital materials are a minor component. These sediments are largely composed of the tiny shell debris of radiolarians and foraminifera.
Perched Water Table
A water table that is isolated from and higher than the regional water table. This can occur when a hilltop is underlain by an impermeable rock unit. Infiltrating waters stack up on the impermeable unit, creating an isolated water table that is higher than the water table of the surrounding land.
Permeability
A measure of how well a material can transmit water. Materials such as gravel, that transmit water quickly, have high values of permeability. Materials such as shale, that transmit water poorly, have low values. Permeability is primarily determined by the size of the pore spaces and their degree of interconnection. Permeability measures are expressed in units of velocity, such as centimeters per second, and assume a gradient of one vertical foot of drop per linear foot.
pH
A relative measure of the acidity or alkalinity of a water based upon a scale that ranges between 0 and 14 with 7 being neutral. Values of pH below 7 indicate acid solutions and values of pH above 7 indicate basic solutions.
Physical Weathering
A general term applied to a variety of weathering processes that result in the particle size reduction of rock materials with no change in composition. Frost action, salt crystal growth and pressure relief fracturing are examples. Also known as mechanical weathering.
Placer Deposit
A mass of stream sediment that contains an economically significant concentration of mineral particles. This accumulation of mineral particles is a result of their being of high specific gravity or resistant to abrasion. Gold, magnetite, and diamonds can be found in placer deposits.
Plateau Basalt
A sequence of parallel to subparallel basalt flows that were formed during a geologically brief interval of time and which covered an extensive geographic area. Thought to have formed from simultaneous or successive fissure eruptions.
Point-Source Pollution
Water contamination that can be traced to a single point. A toxic material spill and a sewage discharge pipe are examples of point sources.
Polarity Epoch
An interval of time between reversals of Earth's magnetic field.
Polarity Event
A specific event in the history of Earth's magnetic field. Usually used in reference to a specific polarity reversal.
Polarity Reversal
A change in the polarity of Earth's magnetic field in which the north magnetic pole becomes the south magnetic pole and vice versa. Also known as geomagnetic reversal or magnetic reversal. Earth's magnetic field has reversed many times in the past and the time intervals between these changes are known as polarity epochs.
Porosity
The volume of pore space in a rock, sediment or soil. Usually expressed as a percentage. This pore space can include openings between grains, fracture openings and caverns.
Potable Water
Water that is of adequate quality for drinking.
A line of bold cliffs.
Permafrost
A permanently frozen layer of soil.
- Permanently frozen regolith, ranging in thickness from 30 centimeters to over 1,000 meters.
Physiography
Physical geography.
Pleistocene
Period in geologic history (basically the last one million years) when ice sheets covered large sections of the Earth's land surface not now covered by glaciers.
Plural Society
A situation in which two or more culture groups occupy the same territory but maintain their separate cultural identities.
Post industrial
An economy that gains its basic character from economic activities developed primarily after manufacturing grew to predominance. Most notable would be quaternary economic patterns.
Precambrian Rock
The oldest rocks, generally more than 600 million years old.
Primary Product
A product that is important as a raw material in developed economies; a product consumed in its primary (i.e., unprocessed) state (see Staple Product).
prime meridian
An imaginary line running from north to south through
A type of Indian village constructed by some tribes in the southwestern
Pothole
A cylindrical or hemispherical hold in the bedrock of a stream that is formed from the continual swirling motion of sand and gravel by swirling currents.
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 1030 to 1430 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.
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 wind ward 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.
parent rock
The preexisting rock from which a metamorphic rock forms.
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.
peridotite
An igneous rock composed primarily of the iron-magnesium silicate olivine and having a silica content of less than 40%.
petroleum
Any of a group of naturally occurring substances made up of hydrocarbons. These substances may be gaseous, liquid, or semisolid.
phaneritic
Of or being an igneous rock containing components large enough to be seen with the unaided eye.
Phanerozoic Eon
The eon that started 570 million years ago, when numerous fossils of sea shells began to be formed, and that continues to the present time.
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.
plastic deformation
A permanent strain that entails no rupture.
plate
One of the large, thin, rigid units making up the Earth's lithosphere. Plates may be continental, oceanic, or both.
playa
A dry lake basin found in a desert.
particle size
the diameter, in millimeters, of suspended sediment or bed material. Particle-size classifications are:
[1] Clay—0.00024-0.004 millimeters (mm);
[2] Silt—0.004-0.062 mm;
[3] Sand—0.062-2.0 mm;
[4] Gravel—2.0-64.0 mm.
parts per million
the number of "parts" by weight of a substance per million parts of water. This unit is commonly used to represent pollutant concentrations.
pathogen
a disease-producing agent; usually applied to a living organism. Generally, any viruses, bacteria, or fungi that cause disease.
peak flow
the maximum instantaneous discharge of a stream or river at a given location. It usually occurs at or near the time of maximum stage.
per capita use
the average amount of water used per person during a standard time period, generally per day.
percolation
(1) The movement of water through the openings in rock or soil.
(2) the entrance of a portion of the streamflow into the channel materials to contribute to ground water replenishment.
polychlorinated biphenyls
(PCBs)-a group of synthetic, toxic industrial chemical compounds once used in making paint and electrical transformers, which are chemically inert and not biodegradable. PCBs were frequently found in industrial wastes, and subsequently found their way into surface and ground waters. As a result of their persistence, they tend to accumulate in the environment. In terms of streams and rivers, PCBs are drawn to sediment, to which they attach and can remain virtually indefinitely. Although virtually banned in 1979 with the passage of the Toxic Substances Control Act, they continue to appear in the flesh of fish and other animals.
primary wastewater treatment
the first stage of the wastewater-treatment process where mechanical methods, such as filters and scrapers, are used to remove pollutants. Solid material in sewage also settles out in this process.
prior appropriation doctrine
the system for allocating water to private individuals used in most Western states. The doctrine of Prior Appropriation was in common use throughout the arid West as early settlers and miners began to develop the land. The prior appropriation doctrine is based on the concept of "First in Time, First in Right." The first person to take a quantity of water and put it to beneficial use has a higher priority of right than a subsequent user. The rights can be lost through nonuse; they can also be sold or transferred apart from the land. Contrasts with riparian water rights.
public supply
water withdrawn by public governments and agencies, such as a county water department, and by private companies that is then delivered to users. Public suppliers provide water for domestic, commercial, thermoelectric power, industrial, and public water users. Most people's household water is delivered by a public water supplier. The systems have at least 15 service connections (such as households, businesses, or schools) or regularly serve at least 25 individuals daily for at least 60 days out of the year.
Pleistocene Epoch
The first epoch of the Quaternary Period, beginning two to three million years ago and ending approximately 10,000 years ago. See also Holocene Epoch.
plume
An upward flow of hot material from the Earth's mantle into the crust
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 land locked 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.
porphyritic
Of or being an igneous rock containing some large grains within a smaller-grained matrix.
porphyry copper deposit
A crystallized rock, typically porphyritic, having hairline fractures that contain copper and other metals.
potassium-argon dating
A form of radiometric 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.
potential
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.
precipitate
To separate from solution in solid form. Minerals may precipitate because of cooling, evaporation, or loss of acidity.
precipitation
1. The process by which a substance becomes precipitated. 2. Water that falls from the atmosphere to Earth's surface in the form of rain, snow, sleet, or hail.
primary coast
A coast shaped primarily by nonmarine processes, such as glacial erosion or biological processes.
principle of cross-cutting relationships
The scienhfic law stating that a pluton is always younger than the rock that surrounds it.
principle of faunal succession
The scientific law stating that an organism is always simpler than those that evolved later and more - complex than those that evolved earlier.
principle of inclusions
The scientific law stating that rock fragments contained within a larger body of rock are always older than the surrounding body of rock.
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.
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. Pyroclastic 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 8000C or more and may move at speeds higher than 150 kilometers per hour.
Pyroclastics
used only in the plural-Particles and chunks of igneous rock ejected from a volcanic vent during an eruption.
quake
See earthquake
quartzite
An extremely durable, nonfoliated metamorphic rock derived from pure sandstone and consisting primarily of quartz.
PPM
An abbreviation for parts per million.
Primary Seismic Waves
The fastest set of earthquake vibrations - also known as P-waves. They move through the Earth in compression and expansion motions (much like sound waves move through air). Called primary because they are the first recorded at a seismograph. Primary waves are able to travel through both solids and liquids.
Proto-Sun
An intermediate stage in the development of a star in which a large cloud of dust and gases gradually condenses through gravitational actions.
Proven Reserves
Mineral deposits that have been explored thoroughly enough to be quantified but which are still in the ground.
Pumice
A vesicular volcanic glass of granitic composition. It has so many vesicles that it has a very low specific gravity - sometimes low enough to float on water.
Pyroclastic Rock
A rock formed when small particles of magma are blown from the vent of a volcano by escaping gas.
Pyroxene Granulite
A coarse-grained contact metamorphic rock that is formed at high temperatures and low pressures and which is rich in pyroxene minerals.