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Hadean	The oldest of the Precambrian eons; the time between Earth’s origin and the formation of the first rocks that have been preserved.
Hadley cells	The name given to the low-latitude convection cells in the atmosphere.
hail	Falling ice balls from the sky, formed when ice crystallizes in turbulent storm clouds.
hail streak	An approximately 2-by-10-km stretch of ground, elongate in the direction of a storm, onto which hail has fallen.
half-graben	A wedge-shaped basin in cross section that develops as the hanging-wall block above a normal fault slides down and rotates; the basin develops between the fault surface and the top surface of the rotated block.
half-life	The time it takes for half of a group of a radioactive element’s isotopes to decay.
halocline	The boundary in the ocean between surface-water and deep-water salinities.
hamada	Barren rocky highlands in a desert.
hanging valley	A glacially carved tributary valley whose floor lies at a higher elevation than the floor of the trunk valley.
hanging wall	The rock or sediment above an inclined fault plane.
hard water	Groundwater that contains dissolved calcium and magnesium, usually after passing through limestone or dolomite.
head	(1) The elevation of the water table above a reference horizon; (2) the edge of ice at the origin of a glacier.
head scarp	The distinct step along the upslope edge of a slump where the regolith detached.
headland	A place where a hill or cliff protrudes into the sea.
headward erosion	The process by which a stream channel lengthens up its slope as the flow of water increases.
headwaters	The beginning point of a stream.
heat	Thermal energy resulting from the movement of molecules.
heat capacity	A measure of the amount of heat that must be added to a material to change its temperature.
heat flow	The rate at which heat rises from the Earth’s interior up to the surface.
heat-transfer melting	Melting that results from the transfer of heat from a hotter magma to a cooler rock.
heliocentric Universe concept	An idea proposed by Greek philosophers around 250 b.c.e. suggesting that all heavenly objects including the Earth orbited the Sun.
Hercynian orogen	The late Paleozoic orogen that affected parts of Europe; a continuation of the Alleghenian orogen.
heterosphere	A term for the upper portion of the atmosphere in which gases separate into distinct layers on the basis of composition.
hiatus	The interval of time between deposition of the youngest rock below an unconformity and deposition of the oldest rock above the unconformity.
high-altitude westerlies	Westerly winds at the top of the troposphere.
high-grade metamorphic rocks	Rocks that metamorphose under relatively high temperatures.
high-level waste	Nuclear waste containing greater than 1 million times the safe level of radioactivity.
hinge	The portion of a fold where curvature is greatest.
hogback	A steep-sided ridge of steeply dipping strata.
Holocene	The period of geologic time since the last glaciation.
Holocene climatic maximum	The period from 5,000 to 6,000 years ago, when Holocene temperatures reached a peak.
homosphere	The lower part of the atmosphere, in which the gases have stirred into a homogenous mixture.
hoodoo	The local name for the brightly colored shale and sandstone chimneys found in Bryce Canyon National Park in Utah.
horn	A pointed mountain peak surrounded by at least three cirques.
hornfels	Rock that undergoes metamorphism simply because of a change in temperature, without being subjected to differential stress.
horse latitudes	The region of the subtropical high in which winds are weak.
horst	The high block between two grabens.
hot spot	A location at the base of the lithosphere, at the top of a mantle plume, where temperatures can cause melting.
hot spring	A spring that emits water ranging in temperature from about 30° to 104°C.
hot-spot track	A chain of now-dead volcanoes transported off the hot spot by the movement of a lithosphere plate.
hot-spot volcano	An isolated volcano not caused by movement at a plate boundary, but rather by the melting of a mantle plume.
hummocky surface	An irregular and lumpy ground surface.
hurricane	A huge rotating storm, resembling a giant spiral in map view, in which sustained winds blow over 119 km per hour.
hurricane track	The path a hurricane follows.
hyaloclastite	A rubbly extrusive rock consisting of glassy debris formed in a submarine or sub-ice eruption.
hydration	The absorption of water into the crystal structure of minerals; a type of chemical weathering.
hydraulic conductivity	The coefficient K in Darcy’s law; hydraulic conductivity takes into account the permeability of the sediment or rock as well as the fluid’s viscosity.
hydraulic gradient	The slope of the water table.
hydrocarbon	A chain-like or ring-like molecule made of hydrogen and carbon atoms; petroleum and natural gas are hydrocarbons.
hydrocarbon system	The association of source rock, migration pathway, reservoir rock, seal, and trap geometry that leads to the occurrence of a hydrocarbon reserve.
hydrologic cycle	The continual passage of water from reservoir to reservoir in the Earth System.
hydrolysis	The process in which water chemically reacts with minerals and breaks them down.
hydrosphere	The Earth’s water, including surface water (lakes, rivers, and oceans), groundwater, and liquid water in the atmosphere.
hydrothermal deposit	An accumulation of ore minerals precipitated from hot-water solutions circulating through a magma or through the rocks surrounding an igneous intrusion.
hypsometric curve	A graph that plots surface elevation on the vertical axis and the percentage of the Earth’s surface on the horizontal axis.