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Iowa State University - Geology terms Category: Education > University Date & country: 18/11/2013, USA Words: 782

phyllite
A metamorphosed mudstone with a silky sheen, more coarse-grained than a slate and less coarse-grained than a schist.

piedmont glacier
A glacier that spreads out at the foot of mountains, formed by the coalescence of two or more valley glaciers.

pillow
A structure observed in certain igneous rocks extruded into water, characterized by discontinuous, close-fitting, pillow-shaped masses, commonly 30 to 60 cm across.

pipe
A vertical conduit through the Earth

pirate stream
A stream that captures the headwaters of another stream.

placer
A surficial mineral deposit formed by mechanical concentration of valuable minerals from weathered debris, usually through the action of stream currents or of waves.

plate
A rigid segment of the Earth

plate boundaries
The zones of seismic activity long which plates are in contact. These may coincide with continental margins , but usually do not. Movement between plates is predominately horizontal, and may be divergent, or convergent, or side-by-side.

plate tectonics
A theory of global tectonics according to which the lithosphere is divided into mobile plates. The entire lithosphere is in motion, not simply those segments composed of continental material. compare continental drift

plate triple junction
A point from which three rifts emanate at roughly 120 degree angles. Example: the Afar triangle in East Africa.

playa
A broad flat desert basin, often containing an ephemeral playa lake.

plucking
(quarrying) A process of erosion in which the glacier pulls loose pieces of bed rock.

plume
The movement of water along flow lines from a point source of ground water pollution toward its eventual emergence at the surface.

plunging fold
A fold in which the axis is inclined at an angle from the horizontal.

pluton
An igneous intrusion.

pluvial lake
A lake formed during a pluvial period.

pluvial period
Time when a dryland area had greater effective moisture than at present.

pocket beach
Small, narrow beach, usually crescentic, at head of a bay or small inlet.

point bar
Accumulations of sand and gravel deposited in slack water on inside of a winding or meandering river.

polar deserts
Deserts in which most moisture is locked up in ground ice and unavailable as liquid water.

polar glacier
A glacier whose temperature throughout is always below freezing.

polish
A smooth, polished surface imparted to some rock types by glacier abrasion.

porosity
The percentage of material occupied by pore space.

porphyritic
A texture of an igneous rock in which large crystals (phenocrysts) are set in a matrix of relatively finer-grained crystals or of glass.

porphyroblast
A large crystal of a mineral such as garnet or staurolite set in a matrix of much finer-grained minerals in a metamorphic rock. compare phenocryst.

potentiometric surface
The level to which water will rise in an artesian system when its confining aquitard is pierced.

pothole
A hole or basin cut into bedrock of a stream by the abrasive action of pebbles and sand swirled by turbulent stream flow.

Precambrian
An informal term to include all geologic time from the beginning of the Earth to the beginning of the Cambrian period 570 million years ago.

precession of the equinox
The wobble of the Earth as it spins changes the direction in which its axis of rotation points. One wobble takes about 23,000 years.

pressure melting
The phenomenon causing increased melting of ice by increase of pressure.

prograde
A succession of metamorphic conditions, each of which is at a higher temperature and/or pressure than the preceding one.

Proterozoic
The geologic eon lying between the Archean and Phanerozoic eons, beginning about 2.5 billion years ago and ending about 0.57 billion years ago.

proton
A fundamental particle of matter. Provides a positive charge in the nucleus of an atom.

pyroclastic
Pertaining to clastic material formed by volcanic explosion or aerial expulsion from a volcanic vent.

quarrying
1. The process by which building stone, usually in blocks or sheets, is extracted from the Earth.. 2. see plucking

quartz arenite
A sandstone in which the sand grains are predominantly quartz.

quartzite
A metamorphic rock consisting largely of interlocking quartz grains; the metamorphic equivalent of a sandstone or chert.

radial drainage
A pattern in which streams radiate outward from a high central zone.

radiation
Heat transport without the intervention of matter, as in the transport of heat from the Sun to the Earth. compare conduction , convection .

radioactivity
The spontaneous decay of the nucleus of an element. It involves the change in the number of protons in the nucleus and therefore creates an atom of a new element.

radiocarbon
14C derived from 14N as cosmic ray bombardment adds a neutron to its nucleus and the nucleus emits a proton. Radiocarbon decays back to 14N by beta decay . Half life is 5730

rain shadow deserts
Deserts formed by blocking moisture-bearing winds with mountain barriers.

ramp
The planar surface sloping seaward from the foot of the shore face.

rapids
Turbulent stream water flow down a steep gradient, but not as steep as in a waterfall.

Rayleigh wave
A type of seismic surface wave in which particles follow a backward elliptical orbit in a vertical plane.

reaction rim
A peripheral zone around a mineral grain, composed of another mineral.

recessional moraine
Ridges of glacial till marking halt and slight readvance of glacier during its general retreat.

rectangular drainage
A pattern in which a stream and its tributaries follow courses marked by nearly right angle bends.

recumbent fold
A fold in which the axial plane is horizontal.

refraction
1. Bending of waves or rays of energy, e.g. seismic waves. 2. As applies to the near shore environment, the bending of wave crests as they approach the shore.

regional metamorphism
Metamorphism affecting an extensive region, associated with orogeny .

regolith
A layer of unconsolidated fragmental rock material.

rejuvenation
Renewed stream erosion, generally as the result of uplift. Generates features of youthful topography on a landscape that was previously worn down to a base level.

relative time
Dating of rocks and geologic events by their positions in chronological order without reference to number of years before the present.

remanent magnetism
Magnetism acquired by a rock as some time in the past.

reserves
That portion of the resources for a valuable mineral commodity that can be extracted from the Earth at a profit today.

reservoir rock
Any porous and permeable rock that yields oil or natural gas.

residual
(resistant) A mineral that persists in soil after weathering, either because it was resistant to weathering or because it was formed during the weathering process.

residual soil
A soil presumed to have developed in place as the product of decomposition and disintegration of bedrock .

resources
The reserves of a valuable mineral commodity plus all other mineral deposits that may eventually become available, even those that are presumed to exist but have not yet been discovered and those that are not economically or technologically exploitable at the moment. The total mineral endowment ultimately available for extraction.

retrograde
A succession of metamorphic conditions, each one of which is at a lower temperature and/or pressure than the preceding one.

reverse fault
A dip-slip fault on which the hanging wall block is offset upward relative to the foot wall block . compare normal fault .

reversed polarity
Time when a magnetic needle points to the south pole.

rhyolite
A fine-grained silica-rich igneous rock, the extrusive equivalent of granite.

Richter scale
A commonly used measure of earthquake magnitude , based on a logarithmic scale. Each integral step on the scale represents a tenfold increase in the extent of ground shaking, as recorded on a seismograph.

rift
(graben) A valley caused by extension of the Earth

rip current
Carries excess water in the longshore current out through the surf zone where it dissipates.

ripple marks
Small waves produced by wind or water moving across deposits of sand or silt.

ripple marks of oscillation
Symmetrical ripple marks formed by oscillating movement of water such as may be found along the coast just outside the surf zone.

rock
An aggregate of one or more minerals in varying proportions.

rock avalanche
see rockslide.

rock cleavage
see cleavage

rock cycle
The concept of a sequence of events involving the formation, alteration, destruction and reformation of rocks as a result of geologic processes and which is recurrent, returning to a starting point. It represents a closed system. compare rock system.

rock flour
Finely divided rock material ground by glacial action and fed by streams fed by melting glaciers.

rock glacier
A mass of ice-cemented rock rubble found on slopes of some high mountains. Movement is slow, averaging 30 to 40 cm/yr.

rock system
The concept of a sequence of events involving the formation, alteration, destruction and reformation of rocks as a result of geologic processes. Unlike the rock cycle it is an open system and does not return to a starting point. compare rock cycle

rock varnish
A thin, shiny veneer of clay minerals and iron and manganese oxides deposited on some rocks in a desert environment.

rock waste
Angular fragments of rock. Forms a talus if abundant enough.

rockfall
The sudden fall of one or more large pieces of a rock from a cliff.

rockslide
(rock avalanche) A slide involving a downward and usually sudden movement of newly detached segments of bedrock sliding or slipping over an inclined surface of weakness such as a bedding plane, fault plane, or joint surface.

roundness
The degree to which a sedimentary particle

runoff
The precipitation that runs directly off the surface to stream or body of standing water.

salinization
. A process by which salts accumulate in soil

saltation
A process of sediment transport in which a particle jumps from one point to another.

salt-water invasion
Displacement of fresh surface or ground water by the advance of salt water.

sand dune
An accumulation of wind driven sand into a distinctive shape.

sand sea
A large area completely, or nearly completely, covered with sand dunes.

sandstone
A clastic sedimentary rock in which the particles are dominantly of sand size, from 0.062 mm to 2 mm in diameter.

sandstorm
A blanket of wind-driven sand with an upper surface about a meter above ground level.

sanitary land fill
An artificial hill formed by the refuse of present-day civilization.

schist
A strongly foliated, coarsely crystalline metamorphic rock, produced during regional metamorphism, that can readily be split into slabs or flakes because more than 50% of its mineral grains are parallel to each other.

schistosity
The foliation in a schist, due largely to the parallel orientation of micas.

seafloor spreading
Process by which ocean floors spread laterally from crests of main ocean ridges. As material moves laterally from the ridge, new material replaces it along the ridge crest by welling upward from the mantle. compare continental drift , plate tectonics

seamount
(guyot) A volcanic mountain on the seafloor. If flat-topped, it is a guyot.

seawall
A wall at the shore and parallel to it for protection against wave erosion

sedimentary facies
An accumulation of deposits that exhibits specific characteristics and grades laterally into other sedimentary accumulations that were formed at the same time but exhibit different characteristics.

sedimentary rock
Rock formed from the accumulation of sediment, which may consist of fragments and mineral grains of varying sizes from pre-existing rocks, remains or products of animals and plants, the products of chemical action, or mixtures of these.

seismic gap
A segment of an active fault zone that has not experienced a major earthquake during a time period when most other segments of the zone have. Generally regarded as having a higher potential for future earthquakes.

seismic sea wave
(tsunami) A sea wave produced by any large-scale, short duration disturbance on the seafloor, commonly a shallow submarine earthquake but possibly also a submarine slide or volcanic eruption.

seismic tomography
A technique for three-dimensional imaging of the Earth