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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.
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


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