Obsidian: Dark volcanic glass of felsic composition.
Octahedral coordination: The packing of six ions around an ion of opposite charge to form an octahedron.
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 hydrocarbons.
Oolite: A sedimentary carbonate particle composed of spherical grains precipitated from warm ocean water on carbonate platforms. Also a rock composed of such particles.
Opaque mineral: A mineral which transmits no light through a thin section under a microscope. Usually a native metal, sulfide, or metallic oxide mineral.
Ophiolite suite: An assemblage of mafic and ultra-mafic igneous rocks with deep-sea sediments supposedly associated with divergence zones and the sea-floor environment.
Orbit: The elliptical or hyperbolic path traced by a planet or meteorite or satellite in the presence of a more massive body.
Orogenic belt: A linear region, often a former geo-syncline, that has been subjected to folding, and other deformation in a mountain-building episode.
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 mountains.
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. Overturned fold: A fold in which a limb has tilted past vertical so that the older strata are uppermost. Oxbow lake: A long, broad, crescent-shaped lake formed when a stream abandons a meander and takes a new course.
Oxidation: A chemical reaction in which electrons 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 forms.