Pahoehoe: A basaltic lava flow with a glassy, smooth, and undulating, or ropy, surface.

Paleoclimate: The average state or typical conditions of climate during some past geologic period.

Paleomagnetism: The science of the reconstruction of the Earth's ancient magnetic field and the positions of the continents from the evidence of remnant magnetization in ancient rocks.

Paleontology: The science of fossils, of ancient life-forms, and their evolution.

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.

Pangaea: According to some theories, a great proto-continent from which all present continents have broken off by the mechanism of sea-floor spreading and continental drift..

Panthalassa: A hypothetical primeval ocean covering two-thirds of the world except for the continent of Pangaea.

Peat: A marsh or swamp deposit of water-soaked plant remains containing more than 50 percent carbon.

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.

Pediment: A planar, sloping rock surface forming a ramp up to the front of a mountain range in an arid region. It may be covered locally by thin alluvium.

Pedocal: A common soil type of arid regions, characterized by accumulation of calcium carbonate in the A-horizon.

Pegmatite: An igneous rock with extremely large grains, more than a centimeter in diameter. It may be of any composition but most frequently is granitic.

Perched groundwater: An isolated body of ground-water that is perched above and separated from the main water table by an aquiclude.

Peridotite: A coarse-grained mafic igneous rock composed of olivine with accessory amounts of pyroxene and amphibole but little or no feldspar.

Potable water: Water that is agreeable to the taste and not dangerous to the health.

Pothole: A semispherical hole in the bedrock of a stream bed, formed by abrasion of small pebbles and cobbles in a strong current.

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, thus a mountain range would be underlain by lighter rocks.

PREPOSITION:it es a word used before nouns, pronouns, or other substantives to form phrases functioning as modifiers of verbs, nouns, or adjectives, and that typically express a spatial, temporal, or other relationship, as in, on, by, to, since.

PROCESS (engineering): refers to engineering which is collaborative and concerned with completing a project as a whole.

PRONOUN:The part of speech that substitutes for nouns or noun phrases and designates persons or things asked for, previously specified, or understood from the context.

Proto-sun: A large cloud of dust and gas gradually coalescing into a star under the force of gravity.

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

pulley is a mechanism composed of a wheel with a groove between two flanges around the wheel's circumference. A rope cable or belt usually runs inside the groove. Pulleys are used to change the direction of an applied force, transmit rotational motion, or realize a mechanical advantage in either a linear or rotational system of motion.

Pumice: A form of volcanic glass, usually of silicic composition, so filled with vesicles that it resembles a sponge and is very light.

Pyroclastic rock: A rock formed by the accumulation of fragments of volcanic rock scattered by volcanic explosions.

Pyroxene granulite: A coarse-grained contact metamorphic rock containing pyroxene, formed at high temperatures and low pressures.