Geological Glossary - O

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Ordovician	A period in the geologic time scale that spans from 505 to 438 million years ago.
Original Horizontality, Principle of	The proposition of Steno, that all sedimentary bedding is horizontal at the time of deposition.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
oblique-slip fault	A fault that combines some strike slip motion with some dip-slip motion.
obsidian	A black or dark-colored volcanic glass usually of rhyolitic (felsic) composition.
ocean crust	The relatively thin, dense crust that form the ocean basin. The crust is mostly basalt with an average depth of 3.7 km below sea level.
ocean floor	The oceanic crust is mostly basalt, undeformed, and, due to plate tectonics, younger than 150 million years old.The major features of the ocean floor are oceanic ridge, abyssal floor, seamount, trenches and continental margin.
oceanic floor	"The bed or bottom of the ocean. A comparatively level valley bottom; any low-lying ground surface"
oceanic magnetic anomalies	Systematic fluctuations in magnetic field intensity that are a result of the juxtaposition of ocean floor formed (and magnetized) during intervals of normal and reversed geomagnetic polarity.
oceanic ridge	"A continuous, seismic, median mountain range extending through the North and South Atlantic Oceans, the Indian Ocean, and the South Pacific Ocean. It is a broad, fractured swell with a central rift valley and usually extremely rugged topography; it is 1 to 3 km in elevation, about 1,500 km in width, and over 84,000 km in length. According to the hypothesis of sea-floor spreading, the mid-ocean ridge is the source of new crustal material"
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.
octahedral coordination	The packing of six ion around an ion of opposite charge to form an octa-hedron.
oil field	An underground accumulation of oil and gas concentrated beneath an impermeable trap, preventing its escape upward.
oil shale	A dark-colored shale containing organic material that can be crushed and heated to liberate gaseous hydrocarbon.
old age	A stage in the geomorphic cycle, characterized by formation of a peneplain near sea level.
olivine	Silicate mineral containing iron and magnesium, (MgFe)2SiO4. It is the main component of the upper mantle. A green glassy mineral formed at high temperature. Common in basalt, especially ocean-floor basalt, and in ultramafic rock. Gem-quality olivine is called peridote. Rock made up entirely of olivine is called dunite.
oncolites	Small accumulations of algae that formed in warm shallow water and were rolled back and forth by currents to become shaped liked little balls. (Found in 350Myr Lower Paleozoic Mississippian Leadville Limestone at Horseshoe Cirque, Fairplay Co)
oolite	A sedimentary carbonate particle composed of spherical grain precipitated from warm ocean water on carbonate platform. Also a rock composed of such particles.
opaque	A mineral which transmits no light through a thin section under a microscope. Usually a native metal, sulfide, or metallic oxide mineral.
open hydrochemical system	Portion in which the mass of a chemical element is not conserved locally and migrates across the boundaries of the system in solution as a solute or as a suspended mineral particle.
ophiolite	"Assemblage of rock units including pillow basalt, diabase sheeted dike, gabbro and serpentized periodites lying uncomformably on continental lithosphere, which likely represent sections of oceanic lithosphere accreted to the continent during plate collision. Body of rock composed of some or all of the following principal rock units: basaltic lava; sheeted diabase dikes; cumulate gabbro, norite, pyroxenite, and periodite; metamorphic-textured herzolite, harzburgite and dunite. Layered suite of basic and ultrabasic silicate rocks consisting of pillow basalts, basaltic dikes, gabbros and periodites that are thought to represent segments of oceanic lithosphere that have been thrust onto a convergent plate margins."
ophiolite suite	An assemblage of mafic and ultra-mafic igneous rocks with deep-sea sediment supposedly associated with divergence zones and the sea-floor environment.
orbicular rocks	Ellipsoidal-shaped masses of rock consisting of successive shells of dark mineral (biotite) and light minerals (feldspar). 200 known localities.
orbit	The elliptical or hyperbolic path traced by a planet or meteorite or satellite in the presence of a more massive body.
order of magnitude	"Ten times; thus an increase of two orders of magnitude is 10 x 10 or 100 times."
ore	A natural deposit in which a valuable metallic element occurs in high enough concentration to make mining economically feasible.
ore mineral	The mineral of an ore that contains the useful element.
orogenic belt	A linear region, often a former geo-syncline, that has been subjected to folding, and other deformation in a mountain-building episode.
orogenic belt	Linear region of the crust, where the lithosphere is relatively mobile and deformed into high-standing mountain belts. Most commonly forms in association with subduction zone or continental collision.
orogeny	The tectonic process in which large areas are folded, thrust-faulted, metamorphosed, and subjected to plutonism. The cycle ends with uplift and the formation of mountain.
orthogneiss	Gneiss formed by squeezing (deformation and usually some recrystallization) of a granitic igneous plutonic rock.
orthoquartzite	A clastic sedimentary rock consisting almost entirely of silica cemented quartz grain. The rock has few, if any, heavy mineral or fossil.
oscillation ripple	A ripple with a symmetrical cross section and a sharp peak formed by wave.
outcrop	A segment of bedrock exposed to the atmosphere.
outer core	The liquid outer layer of the core that lies directly beneath the mantle.
outgassing	The release of juvenile gases to the atmosphere and oceans by volcanism.
outwash	A glaciofluvial sediment that is deposited by meltwater streams emanating from a glacier.
outwash plain	An extensive, relatively flat area of sand, silt, and gravel formed below a glacier by meltwater streams and rivers.
overbank deposits	Silt and clay deposited on a flood plain by a flooding stream.
oxbow lake	A lake formed when a meander of a stream is cut off from the main stream by erosion.
oxidation	A chemical reaction in which electron are lost from an atom and its charge becomes more positive.
oxidized element	An element occurring in the more positively charged of two common ionic form.