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native metal	A naturally occurring pure mass of a single metal in an ore deposit.
natural arch	An arch that forms when erosion along joints leaves narrow walls of rock; when the lower part of the wall erodes while the upper part remains, an arch results.
natural levees	A pair of low ridges that appear on either side of a stream and develop as a result of the accumulation of sediment deposited naturally during flooding.
natural selection	The process by which the fittest organisms survive to pass on their characteristics to the next generation.
neap tide	An especially low tide that occurs when the angle between the direction of the Moon and the direction of the Sun is 90°.
nebula	A cloud of gas or dust in space.
nebula theory of planet formation	The concept that planets grow out of rings of gas, dust, and ice surrounding a new-born star.
negative anomaly	An area where the magnetic field strength is less than expected.
negative feedback	Feedback that slows a process down or reverses it.
Neocrystallization	The growth of new crystals, not in the protolith, during metamorphism.
Nevadan orogeny	A convergent-margin mountain-building event that took place in western North America during the Late Jurassic Period.
nonconformity	A type of unconformity at which sedimentary rocks overlie basement (older intrusive igneous rocks and/or metamorphic rocks).
nonflowing artesian well	An artesian well in which water rises on its own up to a level that lies below the ground surface.
nonfoliated metamorphic rock	Rock containing minerals that recrystallized during metamorphism, but which has no foliation.
nonmetallic mineral resources	Mineral resources that do not contain metals; examples include building stone, gravel, sand, gypsum, phosphate, and salt.
nonplunging fold	A fold with a horizontal hinge.
nonrenewable resource	A resource that nature will take a long time (hundreds to millions of years) to replenish or may never replenish.
nonsystematic joints	Short cracks in rocks that occur in a range of orientations and are randomly placed and oriented.
nor’easter	A large, mid-latitude North American cyclone; when it reaches the East Coast, it produces strong winds that come out of the northeast.
normal fault	A fault in which the hanging-wall block moves down the slope of the fault.
normal force	The component of the gravitational force acting perpendicular to a slope.
normal polarity	Polarity in which the paleomagnetic dipole has the same orientation as it does today.
normal stress	The push or pull that is perpendicular to a surface.
North Atlantic deep-water mass	The mass of cold, dense water that sinks in the north polar regions.
northeast tradewinds	Surface winds that come out of the northeast and occur in the region between the equator and 30°N.
nuêe ardente	Pyroclastic flow.
nuclear fuel	Pellets of concentrated uranium oxide or a comparable radioactive material that can provide energy in a nuclear reactor.
nuclear fusion	The process by which the nuclei of atoms fuse together, thereby creating new, larger atoms.
nuclear reactor	The part of a nuclear power plant where the fission reactions occur.