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Geology Dictionary
Geological Terms Beginning With "P"
For Terms Beginning With Other Letters Please Click Below
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.
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.
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.
Peak Flow:
The maximum instantaneous discharge of a stream at a specific
location. Corresponds to the highest stage of a flood.
Peat:
An accumulation of unconsolidated plant debris that if buried and preserved could become coal. Special conditions are required to accumulate and preserve plant materials. These conditions are most often found in a marsh or swamp where water cover prevents oxidation and attack by most organisms.
Pediment:
A broad, gently sloping erosional surface of low local relief adjacent to an eroding cliff or mountain range. The area is likely covered with sediments.
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. Picture of Pegmatite.
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.
A dark-colored, coarse-grained igneous rock that is made up mainly of olivine and pyroxene, with very little quartz or feldspar. Picture of Peridotite.
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.
A foliate metamorphic rock that is made up mainly of very fine-grained mica. The surface of phyllite is typically lustrous and sometimes wrinkled. It is intermediate in grade between slate and schist. Picture of Phyllite.
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.
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.
PPM:
An abbreviation for parts per million.
Precipitation:
Movement of water from the atmosphere to the land or to a
surface water body. Rain, hail, snow, dew, and sleet are all examples
of precipitation.
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.
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. Picture of Pumice.
P-wave:
Primary seismic waves. The fastest set of earthquake vibrations. 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.
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.
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