Glossary
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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.
paleogeography
Past geography of 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 (for example, 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 pentagon or hexagon 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 formed on granite and 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 desert climates and containing very little organic matter.
pegmatite
A coarse-grained igneous rock containing crystals of up to tens of centimeters across and occurring in dike-shaped intrusions.
peidmont glacier
A fan or lobe of ice that forms where a valley glacier emerges from a valley and spreads out into the adjacent plain.
pelagic sediment
Microscopic plankton shells and fine flakes of clay that settle out and accumulate on the deep-ocean floor.
Pelé’s hair
Droplets of basaltic lava that mold into long glassy strands as they fall.
Pelé’s tears
Droplets of basaltic lava that mold into tear-shaped glassy beads as they fall.
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 in 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 into 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 545 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 by which chlorophyll containing plants remove carbon dioxide from the atmosphere, form tissues, and expel oxygen back to the atmosphere.
photosynthetic_organisms
organisms that make their own food, and in the process remove carbon dioxide, water, methane, and ammonia from the atmosphere and return oxygen to it.
phreatomagmatic eruption
An explosive eruption that occurs when water is introduced into the magma chamber.
phyllite
A fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine-grained muscovite or chlorite.
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 weathering
The process in which intact rock breaks into smaller grains or chunks.
pillow basalt
Glass-encrusted basalt blobs formed when magma is extruded on the sea floor and cools very quickly.
pillow lava
submarine basaltic lava that forms a glass-encrusted blob (pillow) because of rapid cooling.
pitchblende
the mineral name for uranium oxide (UO2).
placer deposit
Concentrations of metal grains in stream sediment developed 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.
planetary nebula
A ring of gas and dust that surrounded the newborn Sun, from which the planets were formed.
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-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.
plate interior
A region away from the plate boundaries that consequently experiences few earthquakes.
plate tectonics
Theory of plate tectonics.
platy
Theory of plate tectonics.
playa
The flat, typically salty lake bed that remains when all the water evaporates in drier times; forms in desert regions.
Pleistocene ice age
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.
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
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.
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.
porphyry copper deposit
A hydrothermal deposit in which copper is precipitated out in igneous intrusions that are composed of large clasts of copper within a finer matrix material.
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 4.6 billion years ago and the beginning of the Phanerozoic eon 545 million years ago.
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.
precious stone
a gem that particularly rare and expensive (e.g. diamond, ruby, sapphire, emerald).
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 in which 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 ridge
An upward bulge alont the toe of a slump
pressure solution
The process of dissolution at points of contact where pressure is greatest, producing ions that then precipitate elsewhere, where compression is less.
prevailing winds
Belts in which surface winds generally flow in a consistent direction.
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.
prokaryotic cells
simple cells that lack complex internal structure.
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.
protostar
A dense body of gas that is collapsing inward because of gravitational forces and may eventually become a star.
pumice
A glassy igneous rock that forms from 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-waves
Compressional seismic waves that move through the body of the Earth.
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.
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 formed when hot volcanic ash and debris mix with air and flows 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.