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H

half-life

The time necessary for half of the atoms of a parent isotope to decay into the daughter isotope.

hardness

The degree of resistance of a given mineral to scratching, indicating the strength of the bonds that hold the mineral's atoms together. The hardness of a mineral is measured by rubbing it with substances of known hardness.

headland

A cliff that projects out from a coast into deep water.

Holocene Epoch

The second epoch of the Quaternary Period, beginning approximately 10,000 years ago and continuing to the pres-ent time. See also Pleistocene Epoch.

hook

A spit that curves sharply at its coastal end.

horn

A high mountain peak that forms when the walls of three or more cirques intersect.

hornfels

A hard, dark-colored, dense metamorphic rock that forms from the intrusion of magma into shale or basalt.

horst

A block of rock that lies between two faults and has moved upward relative to the two adjacent fault blocks. See also graben.

hot spot

An area in the upper mantle, ranging from 100 to 200 kilometers in width, from which magma rises in a plume to form volcanoes. A hot spot may endure for 10 million years or more.

hydraulic conductivity

The extent to which a given substance allows water to flow through it, determined by such factors as sorting and grain size and shape.

hydraulic gradient

The difference in potential between two points, divided by the lateral distance between the points.

hydraulic lifting

The erosion of a stream bed by water pressure.

hydrocarbon

A molecule that is entirely made up of hydrogen and carbon.

hydrogen bond

An intermolecular bond formed with hydrogen.

hydrologic cycle

The perpetual movement of water among the mantle, oceans, land, and atmosphere of the Earth.

hydrolysis

A form of chemical weathering in which ions from water replace equivalently charged ions from a mineral, especially a silicate.

hydrothermal deposit

A mineral deposit formed by the precipitation of metallic ions from water ranging in temperature from 50º to 700ºC.

hypothesis

A tentative explanation of a given set of data that is expected to remain valid after future observation and experimentation. See also theory.

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