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PAHOEHOE: A basaltic lava flow with a glassy, smooth, and ropy surface.

PALEOMAGNETIC STRATIGRAPHY: A branch of stratigraphy in which the remanent magnetization recorded in a rock is used to place the rock on the "magnetic" time scale constructed from known temporal variations in the Earth's magnetic field.

PALEOMAGNETISM: The remanent magnetization recorded in ancient rocks; allows the reconstruction of Earth's ancient magnetic field and the positions of the continents.

PALEONTOLOGY: The science of fossils of ancient life forms and their evolution.

PANGAEA: Supercontinent that coalesced in the latest Paleozoic era and comprised all present continents. The breakup of Pangaea began in Mesozoic time, as inferred from paleomagnetic and other data.

PARTIAL MELTING: A process in which heating melts some of the minerals in a mass of rock while the rest remain solid. Partial melting occurs because the minerals that compose a rock melt at different temperatures.

PASSIVE MARGIN: A continental margin characterized by thick, flat-lying, shallow-water sediments with only limited tectonism related to divergent plate motion.

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

PEDALFER: A common soil type in temperate 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 retreating mountain range in an arid region. It may be covered locally by thin alluvial deposits.

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

PEGMATITE: A vein of extremely coarse grained granite, often containing economic amounts of rare elements.

PELAGIC SEDIMENT: A deep-sea sediment composed of fine-grained detritus that slowly settles from surface waters. Common constituents are clay, foraminiferal ooze, and silica ooze.

PERCHED WATER TABLE: The upper surface of an isolated body of groundwater that is perched above and separated from the main body of groundwater by an aquiclude.

PERIDOTITE: A coarse-grained ultramafic igneous rock composed of olivine with small amounts of pyroxene and amphibole.

PERIOD (GEOLOGIC): The most commonly used unit of geologic time, representing one subdivision of an era.

PERIOD (WAVE): The time interval between the arrival of successive crests in a homogeneous wave train.

PERMAFROST: A permanently frozen aggregate of ice and soil occurring in very cold regions.

PERMEABILITY: The ability of a formation to transmit groundwater or other fluids through pores and cracks.

PHENOCRYST: A large crystal surrounded by a finer matrix in an igneous rock. An igneous rock that contains abundant phenocrysts is called a porphyry.

PHOSPHORITE: A sedimentary rock composed largely of calcium phosphate, usually as a variety of the mineral apatite and largely in the form of concretions and nodules. Primary ore of phosphate minerals and elemental phosphorus.

PHOTOSYNTHESIS: The process by which green organisms use chlorophyll and the energy from sunlight to make organic matter (carbohydrates) out of carbon dioxide and water. (See also Respiration.)

PHREATIC EXPLOSION: A volcanic eruption of steam, mud, and debris caused by the expansion of steam formed when magma comes in contact with groundwater or seawater.

PHYLLITE: A metamorphic rock that is intermediate in grade between slate and mica schist. Small crystals of micas give a silky sheen to the cleavage surfaces.

PHYSICAL WEATHERING: The set of all physical processes by which an outcrop is broken up into smaller particles.

PILLOW LAVA: A basaltic lava that forms under water when many small tongues of lava break through the chilled ocean floor and quickly solidify into a rock formation resembling a pile of sandbags.

PLACER: A clastic sedimentary deposit of a valuable mineral or native metal in unusually high concentration, usually segregated because of its greater density.

PLANETARY DIFFERENTIATION: The process by which heating, cooling, and gravitation sorted the materials of our planet so that it evolved into concentric layers that differ chemically and physically.

PLASTIC FLOW: Deformation of the shape or volume of a substance without fracturing.

PLATE: One of the dozen or more segments of the lithosphere that ride as distinct units over the asthenosphere.

PLATEAU: An extensive upland region at high elevation with respect to its surroundings.

PLATE TECTONICS: The theory and study of plate formation, movement, interactions, and destruction; the attempt to explain seismicity, volcanism, mountain building, and evidence of paleomagnetism in terms of plate motions.

PLATFORM: A sediment-covered, tectonically stable, almost level region of a continent.

PLAYA, PLAYA LAKE: The flat floor of a closed basin in an arid region, usually rich in evaporate minerals. It may be occupied by an intermittent lake.

PLUNGING FOLD: A fold whose axis is not horizontal but dips.

PLUTON: A large igneous intrusion, at least 1 km³, formed at depth in the crust.

PLUTONISM: Igneous activity at depth in the crust.

POINT BAR: A deposit of sediment on the inner bank of a meander that forms because the stream velocity is lower against the inner bank.

POLYMORPH: One of two or more alternative possible structures for a single chemical compound; for example, the minerals calcite and aragonite are polymorphs of calcium carbonate (CaCO₃).

POROSITY: The percentage of the total volume of a rock that is pore space (not occupied by mineral grains).

PORPHYROBLAST: A large crystal in a finer-grained matrix in a metamorphic rock: analogous to a phenocryst in an igneous rock.

PORPHYRY: An igneous rock containing abundant phenocrysts suspended in a finely crystalline matrix.

POTABLE WATER: Water that is agreeable to the taste and not dangerous to the health.

POTHOLE: A hemispherical hole in the bedrock of a streambed, formed by abrasion of small pebbles and cobbles in a strong current.

PRECIPITATE: To drop out of a saturated solution as crystals; the crystals that drop out of a saturated solution.

PRESSURE: A force distributed over a surface divided by the area of the surface. Confining pressure is uniform in all directions, while directed pressure is exerted in a particular direction.

PROTON: With neutrons, one of the two types of particles in the atomic nucleus that account for most of the atom's mass. The mass of a proton is valued at 1 unit and is equivalent to the mass of 18 3 6 electrons. Each proton has a positive electrical charge of 1.6 3 10²¹⁹ coulombs, expressed as 11.

PUMICE: A form of volcanic glass, usually of felsic composition, so filled with holes from the escape of gas during quenching that it resembles a sponge and has very low density. (Compare Obsidian.)

P WAVE: The primary or fastest wave traveling away from a seismic event through the solid rock and consisting of a train of compressions and dilations of the material.

PYROCLAST: Fragment of volcanic material ejected during an eruption.

PYROCLASTIC FLOW: A glowing cloud of volcanic ash, fragments of volcanic rock, and gases that moves rapidly downhill away from the eruptive center during a volcanic eruption.

PYROXENE GRANULITE: A coarse-grained regional metamorphic rock containing pyroxene; formed at high temperatures and pressures deep in the crust.