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pahoehoe	A lava flow with a surface texture of smooth, glassy, rope-like ridges.
paleoclimate	The past climate of the Earth.
paleomagnetism	The record of ancient magnetism preserved in rock.
paleopole	The supposed position of the Earth’s magnetic pole in the past, with respect to a particular continent.
paleosol	Ancient soil preserved in the stratigraphic record.
Paleozoic	The oldest era of the Phanerozoic Eon.
Pangaea	A supercontinent that assembled at the end of the Paleozoic Era.
Pannotia	A supercontinent that may have existed sometime between 800 Ma and 600 Ma.
parabolic dunes	Dunes formed when strong winds break through transverse dunes to make new dunes whose ends point upwind.
parallax	The apparent movement of an object seen from two different points not on a straight line from the object (e.g., from your two different eyes).
parallax method	A trigonometric method used to determine the distance from the Earth to a nearby star.
parent isotope	A radioactive isotope that undergoes decay.
partial melting	The melting in a rock of the minerals with the lowest melting temperatures, while other minerals remain solid.
passive margin	A continental margin that is not a plate boundary.
passive-margin basin	A thick accumulation of sediment along a tectonically inactive coast, formed over crust that stretched and thinned when the margin first began.
patterned ground	A polar landscape in which the ground splits into pentagonal or hexagonal shapes.
pause	An elevation in the atmosphere where temperature stops decreasing and starts increasing, or vice versa.
peat	Compacted and partially decayed vegetation accumulating beneath a swamp.
pedalfer soil	A temperate-climate soil characterized by well- defined soil horizons and an organic A-horizon.
pediment	The broad, nearly horizontal bedrock surface at the base of a retreating desert cliff.
pedocal soil	Thin soil, formed in arid climates. It contains very little organic matter, but significant precipitated calcite.
pegmatite	A coarse-grained igneous rock containing crystals of up to tens of centimeters across and occurring in dike-shaped intrusions.
Pelé’s tears	Droplets of basaltic lava that mold into tear-shaped glassy beads as they fall.
Pelé’s hair	Droplets of basaltic lava that mold into long glassy strands as they fall.
pelagic sediment	Microscopic plankton shells and fine flakes of clay that settle out and accumulate on the deep-ocean floor.
peneplain	A nearly flat surface that lies at an elevation close to sea level; thought to be the product of long-term erosion.
perched water table	A quantity of groundwater that lies above the regional water table because an underlying lens of impermeable rock or sediment prevents the water from sinking down to the regional water table.
percolation	The process by which groundwater meanders through tiny, crooked channels in the surrounding material.
peridotite	A coarse-grained ultramafic rock.
periglacial environment	A region with widespread permafrost but without a blanket of snow or ice.
period	An interval of geologic time representing a subdivision of a geologic era.
permafrost	Permanently frozen ground.
permanent magnet	A special material that behaves magnetically for a long time all by itself.
permanent stream	A stream that flows year-round because its bed lies below the water table, or because more water is supplied from upstream than can infiltrate the ground.
permeability	The degree to which a material allows fluids to pass through it via an interconnected network of pores and cracks.
permineralization	The fossilization process in which plant material becomes transformed into rock by the precipitation of silica from groundwater.
petrified	A term used by geologists to describe plant material that has transformed into rock by permineralization.
petroglyph	Drawings formed by chipping into the desert varnish of rocks to reveal the lighter rock beneath.
phaneritic	A textural term used to describe coarse-grained igneous rock.
Phanerozoic Eon	The most recent eon, an interval of time from 542 Ma to the present.
phenocryst	A large crystal surrounded by a finer-grained matrix in an igneous rock.
photochemical smog	Brown haze that blankets a city when exhaust from cars and trucks reacts in the presence of sunlight.
photosynthesis	The process during which chlorophyll- containing plants remove carbon dioxide from the atmosphere, form tissues, and expel oxygen back to the atmosphere.
phreatomagmatic eruption	An explosive eruption that occurs when water enters the magma chamber and turns into steam.
phyllite	A fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine-grained mica.
phyllitic luster	A silk-like sheen characteristic of phyllite, a result of the rock’s fine-grained mica.
phylogenetic tree	A chart representing the ideas of paleontologists showing which groups of organisms radiated from which ancestors.
physical (or mechanical) weathering	The process in which intact rock breaks into smaller grains or chunks.
piedmont glacier	A fan or lobe of ice that forms where a valley glacier emerges from a valley and spreads out into the adjacent plain.
pillow basalt	Glass-encrusted basalt blobs that form when magma extrudes on the sea floor and cools very quickly.
placer deposit	Concentrations of metal grains in stream sediment that develop when rocks containing native metals erode and create a mixture of sand grains and metal fragments; the moving water of the stream carries away lighter mineral grains.
planetesimal	Tiny, solid pieces of rock and metal that collect in a planetary nebula and eventually accumulate to form a planet.
plankton	Tiny plants and animals that float in sea or lake water.
plastic deformation	The deformational process in which mineral grains behave like plastic and, when compressed or sheared, become flattened or elongate without cracking or breaking.
plate	One of about twenty distinct pieces of the relatively rigid lithosphere.
plate boundary	The border between two adjacent lithosphere plates.
plate interior	A region away from the plate boundaries that consequently experiences few earthquakes.
plate-boundary earthquakes	The earthquakes that occur along and define plate boundaries.
plate-boundary volcano	A volcanic arc or mid-ocean ridge volcano, formed as a consequence of movement along a plate boundary.
playa	The flat, typically salty lake bed that remains when all the water evaporates in drier times; forms in desert regions.
Pleistocene ice age(s)	The period of time from about 2 Ma to 14,000 years ago, during which the Earth experienced an ice age.
plunge pool	A depression at the base of a waterfall scoured by the energy of the falling water.
plunging fold	A fold with a tilted hinge.
pluton	An irregular or blob-shaped intrusion; can range in size from tens of m across to tens of km across.
pluvial lake	A lake formed to the south of a continental glacier as a result of enhanced rainfall during an ice age.
point bar	A wedge-shaped deposit of sediment on the inside bank of a meander.
polar cell	A high-latitude convection cell in the atmosphere.
polar easterlies	Prevailing winds that come from the east and flow from the polar high to the subpolar low.
polar front	The convergence zone in the atmosphere at latitude 60°.
polar glacier	Dry-bottom glacier.
polar high	The zone of high pressure in polar regions created by the sinking of air in the polar cells.
polar wander	The phenomenon of the progressive changing through time of the position of the Earth’s magnetic poles relative to a location on a continent; significant polar wander probably doesn’t occur—in fact, poles seem to remain fairly fixed, while continents move.
polarity	The orientation of a magnetic dipole.
polarity chron	The time interval between polarity reversals of Earth’s magnetic field.
polarity subchron	The time interval between magnetic reversals if the interval is of short duration (less than 200,000 years long).
polarized light	A beam of filtered light waves that all vibrate in the same plane.
polar-wander path	The curving line representing the apparent progressive change in the position of the Earth’s magnetic pole, relative to a locality X, assuming that the position of X on Earth has been fixed through time (in fact, poles stay fixed while continents move).
pollen	Tiny grains involved in plant reproduction.
polymorphs	Two minerals that have the same chemical composition but a different crystal lattice structure.
pore	A small open space within sediment or rock.
pore collapse	The closer packing of grains that occurs when groundwater is extracted from pores, thus eliminating the support holding the grains apart.
porosity	The total volume of empty space (pore space) in a material, usually expressed as a percentage.
porphyritic	A textural term for igneous rock that has phenocrysts distributed throughout a finer matrix.
positive anomaly	An area where the magnetic field strength is stronger than expected.
positive-feedback mechanism	A mechanism that enhances the process that causes the mechanism in the first place.
potentiometric surface	The elevation to which water in an artesian system would rise if unimpeded; where there are flowing artesian wells, the potentiometric surface lies above ground.
pothole	A bowl-shaped depression carved into the floor of a stream by a long-lived whirlpool carrying sand or gravel.
Precambrian	The interval of geologic time between Earth’s formation about 4.57 Ga and the beginning of the Phanerozoic Eon 542 Ma.
precession	The gradual conical path traced out by Earth’s spinning axis; simply put, it is the “wobble” of the axis.
precious metals	Metals (like gold, silver, and platinum) that have high value.
precipitation	(1) The process by which atoms dissolved in a solution come together and form a solid; (2) rainfall or snow.
preferred mineral orientation	The metamorphic texture that exists where platy grains lie parallel to one another and/or elongate grains align in the same direction.
pressure	Force per unit area, or the “push” acting on a material in cases where the push is the same in all directions.
pressure gradient	The rate of pressure change over a given horizontal distance.
pressure solution	The process of dissolution at points of contact, between grains, where compression is greatest, producing ions that then precipitate elsewhere, where compression is less.
prevailing winds	Surface winds that generally flow in the same direction for long time periods.
primary porosity	The space that remains between solid grains or crystals immediately after sediment accumulates or rock forms.
principal aquifer	The geologic unit that serves as the primary source of groundwater in a region.
principle of baked contacts	When an igneous intrusion “bakes” (metamorphoses) surrounding rock, the rock that has been baked must be older than the intrusion.
principle of cross-cutting relations	If one geologic feature cuts across another, the feature that has been cut is older.
principle of fossil succession	In a stratigraphic sequence, different species of fossil organisms appear in a definite order; once a fossil species disappears in a sequence of strata, it never reappears higher in the sequence.
principle of inclusions	If a rock contains fragments of another rock, the fragments must be older than the rock containing them.
principle of original continuity	Sedimentary layers, before erosion, formed fairly continuous sheets over a region.
principle of original horizontality	Layers of sediment, when originally deposited, are fairly horizontal.
principle of superposition	In a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect.
principle of uniformitariansim	The physical processes we observe today also operated in the past in the same way, and at comparable rates.
prograde metamorphism	Metamorphism that occurs as temperatures and pressures are increasing.
Proterozoic	The most recent of the Precambrian eons.
protocontinent	A block of crust composed of volcanic arcs and hot-spot volcanoes sutured together.
protolith	The original rock from which a metamorphic rock formed.
protoplanet	A body that grows by the accumulation of planetesimals but has not yet become big enough to be called a planet.
protoplanetary nebula	A ring of gas and dust that surrounded the newborn Sun, from which the planets were formed.
protostar	A dense body of gas that is collapsing inward because of gravitational forces and that may eventually become a star.
pumice	A glassy igneous rock that forms from felsic frothy lava and contains abundant (over 50%) pore space.
punctuated equilibrium	The hypothesis that evolution takes place in fits and starts; evolution occurs very slowly for quite a while and then, during a relatively short period, takes place very rapidly.
P-wave shadow zone	A band between 103° and 143° from an earthquake epicenter, as measured along the circumference of the Earth, inside which P-waves do not arrive at seismograph stations.
P-waves	Compressional seismic waves that move through the body of the Earth.
pycnocline	The boundary between layers of water of different densities.
pyroclastic debris	Fragmented material that sprayed out of a volcano and landed on the ground or sea floor in solid form.
pyroclastic flow	A fast-moving avalanche that occurs when hot volcanic ash and debris mix with air and flow down the side of a volcano.
pyroclastic rock	Rock made from fragments blown out of a volcano during an explosion that were then packed or welded together.