H

HALF-LIFE: The time required for half of a sample of a given radioactive isotope to decay.

HANGING VALLEY: The valley left by a melted glacial tributary that enters a larger glacial valley above its base, high up on the valley wall.

HARDNESS (MINERAL): A measure of the ease with which the surface of a mineral can be scratched.

HEAT CONDUCTION: The transfer of the vibrational energy of atoms and molecules, which constitutes heat energy, by the mechanism of atomic or molecular impact. (Compare Convection.)

HEAT RESERVOIR: A subsurface region containing enough heat to be used for geothermal energy.

HOGBACK: A formation similar to a cuesta in that it is a ridge formed by slower erosion of hard strata, but having two steep, equally inclined slopes.

HORNFELS: A high-temperature metamorphic rock of uniform grain size showing no foliation. Usually formed by contact metamorphism.

HOT SPOT: The volcanic surface expression of a mantle plume.

HUMUS: The decayed part of the organic matter in a soil.

HYDRATION: The absorption of water by a mineral, usually in weathering.

HYDROLOGIC CYCLE: The cyclical movement of water from the ocean to the atmosphere, through rain to the surface, through runoff and groundwater to streams, and back to the sea.

HYDROLOGY: The science of that part of the hydrologic cycle between rain and return to the sea; the study of the movement and characteristics of water on and within the land.

HYDROTHERMAL ACTIVITY: Any process involving high-temperature groundwater, especially the alteration and emplacement of minerals and the formation of hot springs and geysers.

HYDROTHERMAL METAMORPHISM: A form of metamorphism, frequently associated with mid-ocean ridges, in which hot fluids percolate through the crust and metamorphose the invaded rocks.

HYDROTHERMAL VEIN: A cluster of minerals precipitated by hydrothermal activity in a rock cavity.