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Geological Terms Beginning With "U"



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Ultrabasic Rock

Ubiquitous

Found everywhere, constantly encountered. Some geologists say that the minerals quartz, calcite and pyrite are ubiquitous.

Ultrabasic Rock

Ultrabasic Rock

An igneous rock with a very low silica content and rich in minerals such as hypersthene, augite and olivine. These rocks are also known as ultramafic rocks. Examples include: peridotite, kimberlite, lamprophyre, lamproite, dunite and komatiite. Shown in the photo is a specimen of peridotite.

Ultramafic Rock

Ultramafic Rock

An igneous rock with a very low silica content and rich in minerals such as hypersthene, augite and olivine. These rocks are also known as ultrabasic rocks. Examples include: peridotite, kimberlite, lamprophyre, lamproite, dunite and komatiite. Shown in the photo is a specimen of peridotite.

unakite

Unakite

A granite that contains significant amounts of epidote. It typically has a green and pink appearance revealing the epidote and orthoclase feldspar in its composition. Named after the Unaka Mountains of eastern Tennessee.

confined aquifer

Unconfined Aquifer

An aquifer that is not overlain by an impermeable confining unit and has a porous connection to the atmosphere through which it can receive recharge. In the image at left, the well on the right penetrates an unconfined aquifer. The water level in the well is the same as the water level in the surrounding aquifer.

Angular Unconformity

Unconformity

A contact between two rock units of significantly different ages. An unconformity is a gap in the time record for that location. Shown in the photo is a section of "The Great Unconformity" of the Grand Canyon.

unconsolidated

Unconsolidated

A term used when referring to sediment that has not been compacted, cemented and lithified into a rock. The photo shows layers of sand and gravel in an unconsolidated alluvial deposit in North Dakota.

underclay

Underclay

A name used for a layer of shale or claystone that underlies a coal seam. "Seat rock" and "seat earth" are also used. In some instance an underclay with a high kalonite content will be a refractory material, or a "fire clay", that can be used in the manufacture of ceramic products.

Uniformitarianism

Uniformitarianism

A basic geologic principle. Processes that act upon the Earth today are the same processes that have acted upon it in the past. "The present is the key to the past." Shown in the photo is James Hutton's unconformity at Jedburgh, which he used as an example of the principle of uniformitarianism.

Unit Cell

Unit Cell

The smallest sample of a substance that has a complete representation of its atomic structure. A crystal structure is formed by repetition of the unit cell in three dimensions. The image shows a unit cell of the mineral halite (NaCl).

Uplift

Uplift

A structurally high area of Earth's crust. Formed by movements that bend the crust into a structure such as a dome or an arch. A structurally high area can also be caused by subsidence of adjacent lands. Shown in the image is the Black Hills Uplift which is bounded on one side by the Powder River Basin and on the other by the Williston Basin.

Upwelling

Upwelling

Movement of cold water from the floor of a lake or ocean up into a shallow area.

U-shaped Valley

U-Shaped Valley

A deep valley with a flat floor and very steep walls. Shaped in cross-section like the letter "U". Valleys with this geometry are frequently cut by a glacier. In the photo is McDonald Valley, cut by a glacier and located in Glacier National Park, Montana.

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