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.

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.

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.