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.