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O

oblique slip

Fault motion that involves both dip-slip and strike-slip movement of fault blocks.

ocean trench

A deep, linear, relatively narrow depression in the sea floor, formed by the subduction of oceanic plates.

octet

rule A scientific law stating that all atoms, except those of hydrogen and helium, require eight electrons in the outermost energy level to maintain chemical stability.

oil sand

A mixture of unconsolidated sand and clay that contains a semi-solid bitumen.

oil shale

A brown or black clastic source rock containing kerogen.

ophiolite suite

The group of sediments, sedimentary rocks, and mafic and ultramafic igneous rocks that make up the oceanic lithosphere.

ore

A mineral deposit that can be mined for a profit.

orogenesis

Mountain formation, as caused by volcanism, subduction, plate divergence, folding, or the movement of fault blocks. Also called orogeny.

oscillatory motion

The circular movement of water up and down, with little or no change in position, as a wave passes.

outwash

A load of sediment, consisting of sand and gravel, that is deposited by meltwater in front of a glacier.

oxbow lake

A crescent-shaped body of standing water formed from a single loop that was cut off from a meandering stream, typically by a flood that allowed the stream to flow through its floodplain and bypass the loop.

oxidation

The process of combining with oxygen ions. A mineral that is exposed to air may undergo oxidation as a form of chemical weathering.

oxide

One of several minerals containing negative oxygen ions bonded to one or more positive metallic ions.

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