Glossary
A definition will show up in this window whenever you select a key term from
an Overview page. You can also find a key term manually by scrolling through
the glossary or choosing a letter from the alphabetized menu below.
Choose a letter:
a b c d e f g h i j k l m n o p q r s t u v w x y z
Scroll down to see the "H" terms 
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 cell
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 falls.
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.
hardness
a measure of the relative ability of a mineral to resist scratching. It represents the resistance of bonds in the lattice to being broken.
head
(1) The elevation of the water table above a reference horizon; (2) the edge of ice at the origin of a glacier.
headland
A place where a hill or cliff protrudes into the sea.
head scarp
The distinct step along the upslope edge of a slump where the regolith detached.
headward erosion
The process in 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 based on 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
Cuesta.
Holocene
The period of geologic time since the last glaciation.
Holocene climatic optimum
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-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.
hot spring
A spring that emits water ranging in temperature from about 30° to 104°C.
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.
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.