Geologic Glossary
This glossary contains simplified definitions for technical terms used within this site.
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OobsidianDark-colored volcanic glass. Usually has the same chemical composition as the extrusive igneous rock, rhyolite. oceanic crustThe relatively thin, dense crust that forms the ocean basins. oceanic rocksRocks 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. olivineSilicate 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.
oreA mineral deposit that can be mined at a profit. orogenyAn episode of mountain building and/or intense rock deformation. orthogneissGneiss formed by squeezing (deformation and usually some recrystallization) of a granitic igneous plutonic rock. outcropA mass of rock that appears at the Earth surface. outer coreThe liquid outer layer of the core that lies directly beneath the mantle.
outwashGlacial 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 depositsSilt and clay deposited on a flood plain by a flooding stream. oxidationRemoval 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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