natural bridge
An arch-shaped stretch of bedrock remaining in a karst region when the surrounding bedrock has dissolved.
natural spring
A place where groundwater flows to the surface and issues freely from the ground.
neutron
A particle that is found in the nucleus of an atom, has a mass approximately equal to that of a proton, and has no electric charge.
normal fault
A dip-slip fault marked by a generally steep dip along which the hanging wall has moved downward relative to the footwall.
nuclear fission
The division of the nuclei of isotopes of certain heavy elements, such as uranium and plutonium, effected by bombardment with neutrons. Nuclear fission causes the release of energy, additional neutrons, and an enormous quantity of heat. Nuclear fission is used in nuclear power plants and nuclear weapons. A by product of nuclear fission is toxic radioactive waste. See also nuclear fusion.
nuclear fusion
The combination of the nuclei of certain extremely light elements, especially hydrogen, effected by the application of high temperature and pressure. Nuclear fusion causes the release of an enormous amount of heat energy, comparable to that released by nuclear fission. The principal by product of nuclear fusion is helium.
nucleus (plural nuclei)
The central part of an atom, containing most of the atom's mass and having a positive charge due to the presence of protons.
nuée ardente
A sometimes glowing cloud of gas and pyroclastics erupted from a volcano and moving swiftly down its slopes. Also called a pyroclastic flow.
numerical dating
The fixing of a geological structure or event in time, as by counting tree rings
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.
