Science / Geology Glossary

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 gra . . . View Full Definition

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 s . . . View Full Definition

Plinian Eruption: An explosive eruption where large volumes of rock, ash and gas are blasted at high velocity from a vent. These eruptions produce huge clouds that can rise tens of kilometers into the atmosphere.

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. E . . . View Full Definition

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 agreeable to the taste and not dangerous to the health.

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.

Pratt Isostatic Compensation: The mechanism in which variations in crustal density act to counterbalance the varying weight of topographic features. The crust is here assumed to be of approximately uniform thickness, thu . . . View Full Definition

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.

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-u . . . View Full Definition

Primary Recovery: Any crude oil or natural gas that is recovered from a well as a result of the natural pressure within the reservoir.

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 be . . . View Full Definition

Prospecting: The activities associated with the search for an area of probable mineralization. It can include: topographical, geological, geochemical and geophysical studies. Prospecting is usually done . . . View Full Definition

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: Deposits of fossil fuels whose location and extent are known, as opposed to potential but unproved ('discovered') deposits.

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