Glossary
A definition will show up in this window whenever you select a key term from
an Overview page. You can also find a key term manually by scrolling through
the glossary or choosing a letter from the alphabetized menu below.
Choose a letter:
a b c d e f g h i j k l m n o p q r s t u v w x y z
Scroll down to see the "N" terms 
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 hazard
A natural feature of the environment that could cause damage to people or their property.
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 planetary nebula.
negative anomaly
An area where the magnetic field strength is less than expected.
negative feedback
Feedback that slows a process down or reverses it.
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 that come 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.
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.
nuée ardente
Pyroclastic flow.