Essentials of Geology, 2e
Glossary
ultimate base level
Sea level; the level below which a trunk stream cannot cut at its mouth.
ultramafic
A term used to describe igneous rocks or magmas that are rich in iron and magnesium and very poor in silica.
unconfined aquifer
An aquifer that intersects the surface of the Earth.
unconformity
A boundary between two different rock sequences representing an interval of time during which new strata were not deposited and/or were eroded.
unconsolidated
Consisting of unattached grains.
undercutting
Excavation at the base of a slope that results in the formation of an overhang.
undersaturated
A term used to describe a solution capable of holding more dissolved ions.
unsaturated zone
The region of the subsurface above the water table.
unstable air
Air that is significantly warmer than air above and has a tendency to rise quickly.
unstable ground
Land capable of moving downslope in a matter of seconds to weeks.
unstable slope
A slope on which sliding will likely happen.
updraft
Upward-moving air.
uplift
336.
upper mantle
The uppermost section of the mantle, reaching down to a depth of 400 km.
upwelling zone
A place where deep water rises in the ocean, or hot magma rises in the asthenosphere.
U-shaped valley
A steep-walled valley shaped by glacial erosion into the form of a U.
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