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Geologic Glossary

This glossary contains simplified definitions for technical terms used within this site.
For more complete, technical definitions, click here.

A B C D E F G H I J K L M N O P Q R S T U V W X Z

O

obsidian
Dark-colored volcanic glass. Usually has the same chemical composition as the extrusive igneous rock, rhyolite.

oceanic crust
The relatively thin, dense crust that forms the ocean basins.

oceanic rocks
Rocks formed in the deep ocean. Includes sedimentary rocks deposited on the deep ocean floor as well as the basalt of the oceanic crust. Commonly include some slices of the underlying mantle (ultramafic rocks) as well.

olivine
Silicate mineral containing iron and magnesium. A green glassy mineral formed at high temperature. Common in basalt, especially ocean-floor basalt, and in ultramafic rocks. Gem-quality olivine is called peridote. Rock made up entirely of olivine is called dunite.

Ordovician
A Period in the Paleozoic Era that includes the time interval from about 505 to 438 million years ago.
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ore
A mineral deposit that can be mined at a profit.

orogeny
An episode of mountain building and/or intense rock deformation.

orthogneiss
Gneiss formed by squeezing (deformation and usually some recrystallization) of a granitic igneous plutonic rock.

outcrop
A mass of rock that appears at the Earth surface.

outer core
The liquid outer layer of the core that lies directly beneath the mantle.
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outwash
Glacial outwash is the deposit of sand, silt, and gravel formed below a glacier by meltwater streams and rivers. An outwash plain is an extensive, relatively flat area of such deposits.

overbank deposits
Silt and clay deposited on a flood plain by a flooding stream.

oxidation
Removal of electrons from an atom or ion. Usually by combining with oxygen ions. Minerals exposed to air may oxidize as a form of chemical weathering.

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P

pahoehoe
A lava flow with a smooth, ropy surface.

paleobiogeography
The science that studies the past distribution of plants and animals.

paleoclimatology
The study of how global climate has changed through time.

paleomagnetism
The magnetism of an iron-bearing rock imparted to it by the Earth’s magnetic field when the rock formed. Literally, early magnetism; meaning magnetism formed in a past geologic era.

paleoseismology
The study of ancient seismic (earthquake) events.

Paleozoic Era
Includes the time from about 570-245 million years ago.
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palynology
The study of pollen, living and fossil.

Pangaea
The supercontinent which formed at the end of the Paleozoic Era and began breaking up about 200 million years ago to form today’s continents.
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parent isotope
A radioactive isotope that changes to a different, daughter isotope when its nucleus decays.

parent rock
The preexisting rock from which a metamorphic rock forms.

passive margin
A tectonically inactive continental margin characterized by a lack of earthquakes and volcanic activity.

pavement
A bare rock surface that provides a protective rock cover over the material beneath it.

pediment
A sloping bedrock surface at the base of a mountain, formed when erosion removes much of the mountain’s mass.

pebble
Loose particles of rock or mineral (sediment) that range in size from 2 - 64 millimeters in diameter. Pebbles are the smallest type of gravel.
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pegmatite
A very coarse-grained igneous rock, commonly with a granitic composition. Usually forms from molten rock rich in water or other volatiles that facilitate the growth of large crystals. Forms sills and dikes.

pelite
A fine-grained sedimentary rock consisting mostly of clay and/or silt. Mudstone, shale, siltstone, and claystone are all pelitic.

perennial stream
A stream that runs continuously throughout the year.

permeability
The ability of a rock or other material to allow water to flow through its interconnected spaces. Permeable bedrock makes a good aquifer, a rock layer that yields water to wells. See porosity. (3 MB porosity animation available)

petrology
The study of rocks.

petrophile
Lover of rocks. May be a petrologist, a stone mason, a rock climber.

phaneritic
An igneous rock texture in which the mineral grains are large enough to be seen with the unaided eye and are of approximately equal size.

Phanerozoic Eon
The eon beginning about 570 million years ago and continuing to the present. The portion of Earth history with rocks containing abundant fossils.
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phenocryst
A term used to describe large crystals embedded in a mass of finer crystals (groundmass) in an igneous rock. See 'porphyritic'.

phlogopite
A magnesium-rich member of the mica mineral family. Phlogopite is a yellowish-brown to coppery-colored mica. Like all micas, phlogopite forms flat, plate-like crystals that cleave into smooth flakes.

phyllite
A very fine-grained, foliated metamorphic rock, generally derived from shale or fine-grained sandstone. Phyllites are usually black or dark gray; the foliation is commonly crinkled or wavy. Differs from less recrystallized slate by its sheen, which is produced by barely visible flakes of muscovite (mica).

plagioclase
A member of the feldspar mineral family. Plagioclase feldspars are silicates that contain considerable sodium and calcium. Feldspar crystals are stubby prisms, generally white to gray and a glassy luster.
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plankton
Generally tiny animals or plants that live floating in water.

plastic deformation
Permanent deformation (change in size or shape) of soft, but solid rock by folding or flowing without fracturing.

plate
A slab of rigid lithosphere (crust and uppermost mantle) that moves over the asthenosphere.
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plate tectonics
The theory that the Earth’s outer shell is made up of about a dozen lithospheric plates that move about and interact at their boundaries.
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playa
Playas are shallow, short-lived lakes that form where water drains into basins with no outlet to the sea and quickly evaporates. Playas are common features in arid (desert) regions and are among the flattest landforms in the world.
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Pleistocene Epoch
The earliest Epoch of the Quaternary Period, beginning about 1.6 million years ago and ending 10,000 years ago. Commonly known as the 'Ice Age', a time with episodes of widespread continental glaciation.
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Pliocene
The latest Epoch of the Tertiary Period, beginning about 5.3 million years ago and ending 1.6 million years ago.
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pluton
A large body of intrusive igneous rock that solidified within the crust. Batholiths and Stocks are types of plutons.

plutonic rock
Any igneous rock that cools beneath the surface. (=intrusive rock).

pluvial lake
A lake formed in a land-locked basin during a period of increased rainfall associated with glacial advance elsewhere.

porosity
The percentage of open spaces (pores) in rock or soil. When these spaces are interconnected, water, air, or other fluids can migrate from space to space. Interconnected spaces make the soil or bedrock permeable.

porphyritic
An igneous rock texture characterized by larger crystals (phenocrysts) in a matrix of distinctly finer crystals (groundmass).

porphyroblasts
Large mineral grains that grow during metamorphism.

porphyry
An igneous rock, usually a dike or sill, with larger, generally conspicuous, early-formed crystals contained within a matrix of much smaller crystals.

Precambrian
The 'unofficial' time period that encompasses all time from the Earth’s formation, 4.55 billion years ago to 570 million years ago, the beginning of the Paleozoic Era.
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precarious boulder
A large rock resting on another in an unstable position. Precarious boulders may remain in place for thousands of years until an earthquake or human-caused tremor dislodges them.

precipitate (verb)
The process that separates solids from a solution.

precipitate (noun)
Mineral precipitate. A mineral deposited from a water solution in pores or other openings in rocks. Chemical reaction with the surrounding rock, changes in pressure or temperature, or just drying up (evaporation) can cause a mineral to precipitate out of solution. Quartz veins are common products of mineral precipitation.

Proterozoic Eon
The 'Precambrian' time interval from 2.5 billion to 570 million years ago.
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pumice
A light-colored, frothy, glassy volcanic rock. The texture is formed by rapidly expanding gas in erupting lava.

pyrite
Iron sulfide mineral (FeS). Forms silvery to brassy metallic cubes or masses. Common in many rocks. Known as fool’s gold. Weathered pyrite produces limonite (iron oxide) that stains rock brown. or yellow.

pyroclastic
An igneous rock texture produced from consolidation of fragmented volcanic material ejected during a violent eruption. Also used to describe ash, bombs and other material forcefully ejected during a volcanic eruption (=tephra)
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pyroclastic eruption
A volcanic eruption that produces a large volume of solid volcanic fragments (pyroclastics) rather than fluid lava. This type of eruption is typical of volcanoes with high silica, viscous, gas-rich magma.

pyroclastic flow
An extremely hot mixture of gas, ash and pumice fragments, that travels down the flanks of a volcano or along the surface of the ground at speeds of 50 to 100 miles per hour.

pyroxene
Family of silicate minerals containing iron, magnesium, and calcium in varying amounts. Differ from amphibole family by lack of water in the crystals. The most common variety, augite, contains aluminum as well. Generally forms very dark green to black stubby prisms.

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Q

quartz
One of the most common minerals in the Earth’s crust (and in some new-age boutiques). Made up of silicon dioxide (SiO2),lso called silica. Commonly in white masses. Crystals are clear, glassy 6-sided prisms.
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quartzite
A nonfoliated metamorphic rock formed from pure, dominantly quartz sandstone.

quartzite
Hard, somewhat glassy-looking rock made up almost entirely of quartz. Metamorphosed quartz sandstone and chert are quartzites.

Quaternary
The most recent Period of the Cenozoic Era. Encompasses the time interval of 1.6 million years ago through today.
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R

radiocarbon dating
The age of organic material determined by the amounts of carbon isotopes 12, 13 and 14. The ratio of 12 to 14 is about the same in all living things but when a plant or animal dies, no more carbon is taken on. Carbon 12 and 13 are stable isotopes and the amounts remain the same even in dead material. Carbon 14 is an radioactive isotope that decays radioactively until none is left; . Thus, the ratio records the time elapsed since death. Since carbon 14 decays relatively rapidly, the method is only reliable for the last 40,000 years. See radiometric age.

radiolarian chert
A rock made up of the spherical siliceous shells of radiolarians which are single-celled planktonic animals (protozoans).
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radiometric age
The approximate age of a geologic event, feature, fossil, or rock in years. Radiometric ages, sometimes termed 'absolute' ages, are determined by using natural radioactive 'clocks'. See radiocarbon dating.
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radiometric dating
A dating method that uses measurements of certain radioactive isotopes to calculate the ages in years (absolute age) of rocks and minerals.
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regional metamorphism
Metamorphism affecting a large region that is associated with mountain building events.

relief
Refers to differences in elevation of different points in a region.

relative dating
The process of placing rocks and geologic structures in the correct chronological order. This process does not yield ages in number of years. See radiometric dating.

rhyolite
A volcanic rock chemically equivalent to granite. Usually light colored, very fine-grained or glassy-looking. May have tiny visible crystals of quartz and/or feldspar dispersed in a glassy white, green, or pink groundmass.

ribbon chert
Chert and shale in thin alternating beds. The beds resemble parallel ribbons stretched over an outcrop.

rift zone
A region of Earth’s crust along which divergence is taking place. A linear zone of volcanic activity and faulting usually associated with diverging plates or crustal stretching.

ring of fire
A zone of volcanoes, earthquakes, and mountain-building encircling the Pacific Ocean formed where plates collide.

rock
Rocks are made of different kinds of minerals, or broken pieces of crystals, or broken pieces of rocks. Some rocks are made of the shells of once-living animals, or of compressed pieces of plants. Rocks are divided into three basic types, igneous, sedimentary and metamorphic, depending upon how they were formed.
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rockfall
Falling, bouncing, and rolling of debris down slope.

root of a volcano
Plutonic igneous rock formed from magma that crystallized beneath the volcano it once fed.

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Continue to glossary entries S through Z



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This page was last updated on 10/11/00