U

ultramafic rock

An intrusive igneous rock very rich in iron and magnesium and with much less silicon and aluminum than most crustal rocks. Most come from the Earth's mantle.

unconformity

The contact between older rocks and younger sedimentary rocks in which at least some erosion has removed some of the older rocks before deposition of the younger. An angular unconformity shows that the older rocks have been deformed and eroded before the younger sedimentary rocks were deposited; there is an angle between the beds of the older and the younger.

unconsolidated

Loose sediment; lacking cohesion or cement.

V

vein

A mineral-filled fracture or fault in a rock.

vein

Tabular rock or mineral filling of a generally small crack such as a quartz vein. A product of chemical precipitation from a watery solution, in contrast to a dike crystallized from magma, although gradations exist.

veneer

A thin, widespread layer of sediment covering an older surface.

vesicle

A small bubble formed in volcanic rock during solidification.

vitreous

Glassy.

vitrophyre

A volcanic rock with larger crystals (phenocrysts) embedded in a glassy groundmass.

volcanic rock

Igneous rock that cools and solidifies at or very near the Earth's surface. Volcanoes produce volcanic rock.

volcanic arc

Arcuate chain of volcanoes formed above a subducting plate. The arc forms where the downgoing descending plate becomes hot enough to release water and gases that rise into the overlying mantle and cause it to melt. Arc rocks are mostly volcanic rocks from the volcanoes and sedimentary rocks made up of eroded debris from the volcanoes. Melted rock in the deeper plumbing of the arc which may crystallizes at depth to become an arc root plutons.