Glossary Starting with the letter: U

Ultraviolet Radiation

(UV) A portion of the electromagnetic spectrum. Ultraviolet radiation has a shorter wavelength than visible light.


An erosional surface representating a gap in the geologic record between rock layers of different ages indicating that deposition was not continuous.


The principle that the same geological processes and natural laws that modify the Earth’s crust operating today have operated in the same manner or way throughout geologic time.

Upper Mantle

The part of the mantle which lies above the transition zone. It is presumed to be composed of peridotite.


In the solid earth, the act of warm mantle rising upwards relative to surrounding, cooler mantle. In the ocean, the rising of deeper waters to replace migrating surface waters. Upwelling may bring waters rich in nutrients to the surface, resulting in a region where ocean productivity is high.