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Glossary of Rock and Mineral Terms - U
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- ultrabasic
- Describing an igneous rock containing magnesium and iron with a very low
percentage of silica, less than 45%.
- ultraviolet light
- Type of electromagnetic radiation which cannot be seen by humans.
Ultraviolet, often abbreviated U.V. is term used when referring to
ultraviolet light. Ultraviolet Lamp (U.V. lamp) is one that gives off
ultraviolet light while blocking out white light, which causes fluorescent
minerals to react.
- unconformity
- A boundary separating two or more rocks of markedly different ages,
marking a gap in the geologic record.
- uneven fracture
- Fracture that leaves a rough or irregular surface.
- uniformitarianism
- The hypothesis that current geologic processes, such as the slow erosion
of a coast under the impact of waves, have been occurring in a similar
manner throughout the Earth's history and that these processes can account
for past geologic events. See also catastrophism.
- unstable
- Mineral with a crystal structure that is unable to exist at the current
temperature. When certain minerals become unstable, they transform into
their paramorphs. Also refers to radioactive minerals which cannot
withstand their composition and begin to decay.
- upwarped mountain
- A mountain consisting of a broad area of the Earth's crust that
has moved gently upward without much apparent deformation, and usually
containing sedimentary, igneous, and metamorphic
rocks.
- uranium-thorium-lead dating
- A form of isotope dating that relies on the extremely long half-life
of radioactive isotopes of uranium, which decay into isotopes of lead, to
determine the age of rocks in which uranium and lead are present.
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