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Glossary O-P



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.


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.


A Period in the Paleozoic Era that includes the time interval from about 505 to 438 million years ago.


A mineral deposit that can be mined at a profit.


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


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


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.


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.


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.




A lava flow with a smooth, ropy surface.


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


The study of how global climate has changed through time.


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.


The study of ancient seismic (earthquake) events.

Paleozoic Era

Includes the time from about 570-245 million years ago.


The study of pollen, living and fossil.


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.

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.


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


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


Loose particles of rock or mineral (sediment) that range in size from 2 - 64 millimeters in diameter. Pebbles are the smallest type of gravel.


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.


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.


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)


The study of rocks.


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


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.


A term used to describe large crystals embedded in a mass of finer crystals (groundmass) in an igneous rock. See 'porphyritic'.


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.


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).


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.


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.


A slab of rigid lithosphere (crust and uppermost mantle) that moves over the asthenosphere.

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.


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.

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.


The latest Epoch of the Tertiary Period, beginning about 5.3 million years ago and ending 1.6 million years ago.


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.


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.


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


Large mineral grains that grow during metamorphism.


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


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.

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.


A light-colored, frothy, glassy volcanic rock. The texture is formed by rapidly expanding gas in erupting lava.


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.


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)

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.


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.

Rock types:
Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks