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oasis	A verdant region surrounded by desert, occurring at a place where natural springs provide water at the surface.
oblique-slip fault	A fault in which sliding occurs diagonally along the fault plane.
obsidian	An igneous rock consisting of a solid mass of volcanic glass.
occluded front	A front that no longer intersects the ground surface.
oceanic crust	The crust beneath the oceans; composed of gabbro and basalt, overlain by sediment.
oceanic lithosphere	Lithosphere topped by oceanic crust; it reaches a thickness of 100 km.
Oil Age	The period of human history, including our own, so named because the economy depends on oil.
oil field	A region containing a significant amount of accessible oil underground.
oil reserve	The known supply of oil held underground.
oil shale	Shale containing kerogen.
oil trap	A geologic configuration that keeps oil underground in the reservoir rock and prevents it from rising to the surface.
oil window	The narrow range of temperatures under which oil can form in a source rock.
olistotrome	A large, submarine slump block, buried and preserved.
ophiolite	A slice of oceanic crust that has been thrust onto continental crust.
ordinary well	A well whose base penetrates below the water table, and can thus provide water.
ore	Rock containing native metals or a concentrated accumulation of ore minerals.
ore deposit	An economically significant accumulation of ore.
ore minerals	Minerals that have metal in high concentrations and in a form that can be easily extracted.
organic carbon	Carbon that has been incorporated in an organism.
organic chemical	occurs in living organisms, or that resembles such compounds; it consists of carbon atoms bonded to hydrogen atoms along with varying amounts of oxygen, nitrogen, and other chemicals.
organic coast	A coast along which living organisms control landforms along the shore.
organic sedimentary rock	Sedimentary rock (such as coal) formed from carbon-rich relicts of organisms.
organic shale	Lithified, muddy, organic-rich ooze that contains the raw materials from which hydrocarbons eventually form.
orogen (or orogenic belt)	A linear range of mountains.
orogenic collapse	The process in which mountains begin to collapse under their own weight and spread out laterally.
orogeny	A mountain-building event.
orographic barrier	A landform that diverts air flow upward or laterally.
outcrop	An exposure of bedrock.
outer core	The section of the core, between 2,900 and 5,150 km deep, that consists of liquid iron alloy.
outwash plain	A broad area of gravel and sandbars deposited by a braided stream network, fed by the meltwater of a glacier.
overburden	The weight of overlying rock on rock buried deeper in the Earth’s crust.
overriding plate (or slab)	The plate at a subduction zone that overrides the downgoing plate.
oversaturated solution	A solution that contains so much solute (dissolved ions) that precipitation begins.
oversized stream valley	A large valley with a small stream running through it; the valley formed earlier when the flow was greater.
oxbow lake	A meander that has been cut off yet remains filled with water.
oxidation reaction	A reaction in which an element loses electrons; an example is the reaction of iron with air to form rust.
ozone	O<sub>3</sub>, an atmospheric gas that absorbs harmful ultraviolet radiation from the Sun.
ozone hole	An area of the atmosphere, over polar regions, from which ozone has been depleted.