Objective Function – When geophysicists wish to invert data to obtain a model, they must pose an optimization problem.

Oblique-Slip Fault – Fault that combines some strike slip motion with some dip-slip motion.

Obsidian – Dark volcanic glass of felsic composition.

Oceanic Crust – Earth’s crust which is formed at mid-oceanic ridges.

Oceanic Trench – Deep steep-sided depression in the ocean floor caused by the subduction of oceanic crust beneath another oceanic crust or a continental crust.

Octahedral Coordination – Packing of six ions around an ion of opposite charge to form an octahedron.

Ohm – Unit of electrical resistance through which 1 amp of current will flow when the potential difference is 1 V.

Ohm-Meter – Unit of electrical resistivity measurement.

Oil Field – Underground accumulation of oil and gas concentrated beneath an impermeable trap preventing its escape upward.

Oil Shale – Dark-colored shale containing organic material that can be crushed and heated to liberate gaseous hydrocarbons.

Old Age – Stage in the geomorphic cycle that is typically characterized by the formation of a peneplain near sea level.

Oligocene Epoch – Epoch from 33.9 to 23.03 million years ago with two Ages, the 1) Rupelian and the 2) Chattian ages.

Olivine – Green magnesium iron silicate with the formula (Mg,Fe)2SiO4 is a common mineral below the earth’s surface particularly in basalt, gabbro and ultramafic rock. It tends to weather quickly on the surface. Peridot is gem-quality olivine variety that approaches the formula of forsterite Mg2SiO4 and Fayalite (Fe2SiO4) is the iron-rich end-member of the olivine solid-solution variety.

Oolite – Sedimentary carbonate particle composed of spherical grains precipitated from warm ocean water on carbonate platforms, or a rock composed of such particles.

Opaque Mineral – Mineral which transmits no light through a thin section under a microscope. Most often it is a native metal, sulfide, or metallic oxide mineral.

Open Hole – Uncased intervals of a drill hole.

Open Pit – Large-scale hard-rock surface mine.

Ophiolite – Group of rocks originally formed as oceanic crust, consisting of ultramafic rock, gabbro, diabase, basalt and deep-sea sedimentary rocks.

Ophiolite Suite – Assemblage of mafic and ultramafic igneous rocks with deep-sea sediments supposedly associated with divergence zones and the sea-floor environment.

Optimization – Branch of mathematics related to determining the best or optimum choice from a large, possibly infinite, range of possibilities.

Optimum Offset – Seismic reflection technique employing optimum window.

Orbit – Elliptical or hyperbolic path traced by a planet or meteorite or satellite in the presence of a more massive body.

Ore – Naturally occurring deposit of valuable minerals or materials from which minerals of economic value can be profitably extracted. They must be in a high enough concentration to make the process of mining them economically feasible.

Original Horizontality, Principle of – Proposition that all sedimentary bedding is horizontal at the time of deposition.

Orocline – Bend about a vertical axis formed by buckling of an originally linear orogen.

Orogen – Mountain formation.

Orogenic Belt – Linear region that is often a former geo-syncline that has been subjected to folding, and other deformation in a mountain-building episode.

Orogenic Event – Period of mountain building usually accompanied by over-thrusting of rocks along continental margins.

Orogeny – Major tectonic processes of folding, faulting, and uplifting of the earth’s crust that result in the formation of mountains.

Orthoclase – Common constituent of most granites and other felsic igneous rocks.

Oscillation Ripple – Ripple with a symmetrical cross section and a sharp peak formed by waves.

Ounce – Troy ounce equal to 31.1035 grams).

Outcrop – Exposure of bedrock on a surface of the earth..

Outgassing – Release of gases to the atmosphere and oceans by volcanism.

Outwash – Glaciofluvial sediment that is deposited by meltwater streams coming from a glacier.

Overburden Drill Hole – Drillhole that does not penetrate to bedrock.

Overturned Fold – Fold in which a limb has tilted past vertical so that the older strata are uppermost.

Oxbow Lake – Long, wide, crescent-shaped lake formed when a stream abandons a meander and takes a new course.

Oxidation – Chemical reaction in which electrons are lost from an atom and its charge becomes more positive.

Oxide – Mineral that contains oxygen

Oxidized Element – Element occurring in the more positively charged of two common ionic forms.