U

ultramafic
A type of igneous rock that contains more than 90% mafic minerals.
ultraviolet (UV)
Electromagnetic radiation having wavelengths shorter than the violet.
ultraviolet (UV) radiation
High energy radiation from the Sun that can be dangerous to Earth’s life.
umbra
Inner cone shaped part of a shadow when all light is blocked during an eclipse.
unconformity
A gap between rocks of very different ages. Unconformities are often marked by an erosional surface.
uniformitarianism
Natural processes operated the same way throughout Earth’s history as they do today.
universe
Everything that exists; all matter and energy; also includes all of space and time.
uplift
The upward rise of rock material.
upwelling
Cold, nutrient-rich water that rises from oceanic depths.