half-life

The time required for one-half of the atoms of a radioactive substance to decay.
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hardness

A measure of a mineral’s resistance to scratching. The hardness of a mineral is measured by scratching it against another substance of known hardness.

headland

Headlands are projections of land that stick out into a sea or lake.
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Holocene

An epoch of the Quaternary Period beginning 10,000 years ago and continuing today.
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horst

An elongate block of rock uplifted along roughly parallel faults.

hornblende

See amphibole.

hornblende schist

A schist rich in hornblende. Generally with abundant plagioclase feldspar as well. Grades into amphibolite.

hornfels

A dark, very fine-grained metamorphic rock produced by the recrystallization of a fine-grained rock by heat from a nearby igneous intrusion. From the German, meaning horn rock.

hot spot

An area of concentrated heat in the mantle that produces magma that rises to the Earth’s surface to form volcanic islands. The volcanic activity of the Hawaiian Islands is one example. Hot spots generally persist for millions of years.

hydrology

The science that deals with water on and beneath the Earth surface.

hydrous

Literally, "with water". Refers to minerals or other materials which have water as a primary constituent.