Geoscience Reference
In-Depth Information
Hogback ridge
A ridge underlain by gently tipped rock
strata with a long, gradual slope on one side and a relatively
steep scarp or cliff on the other. 345
Hook echo
The diagnostic feature in Doppler radar
indicating that strong rotation is occurring within a
thunderstorm and that tornado development is thus
possible. 189
Horn
A mountain with three or more arêtes on its
flanks. 459
Horst
An upthrown block of rock that lies between two
steeply inclined fault blocks. 352
Hotspot
A stationary zone of magma upwelling that is
associated with volcanism within the interior of a crustal
plate. 365
Humidity
A measure of how much water vapor is in the
air. The ability of air to hold water vapor is dependent on
temperature. 142
Humus
Decomposed organic matter, typically dark, that is
contained within the soil. 266
Hurricane
A tropical circulatory system with maximum
sustained winds greater than 63 knots (73 mph). 190
Hydraulic fracturing
Fracturing of rock at depth by the
injection of fluids to release oil and natural gas contained
within shale beds. 562
Hydrologic cycle
A model that illustrates the way that
water is stored and moves on Earth from one reservoir to
another. 141
Hydrolysis
A chemical weathering process that results in the
decomposition of silicate molecules within rock through the
reaction of hydrogen and hydroxyl ions in water. 377
Hydrosphere
The part of Earth where water, in all its forms,
flows and is stored. 8
Hygroscopic water
Soil water held so tightly by sediment
grains that it is unavailable for plant use.
Inner core
The inner part of the Earth's core. This area is
about 1220 km (760 mi) thick and consists of solid iron and
nickel. 306
Insolation
Amount of solar radiation measured in watts per
square meter (
W
/m
2
) that strikes a surface perpendicular to the
Sun's incoming rays. 70
Interfluves
Topographic high points in a drainage basin that
separate one tributary from another. 412
International Date line
This line generally occurs at 180°
longitude, with some variations due to political boundar-
ies, and marks the transition from one day to another on
Earth. 48
Intertropical Convergence Zone (ITCZ)
Band of low
pressure, calm winds, and clouds in tropical latitudes
where air converges from the Southern and
Northern
Hemispheres. 116
Intrusive igneous rocks
Rocks that form when magma
cools within the Earth's crust. 311
Isobars
Isolines that connect points of equal atmospheric
pressure. 30
Isohyets
Isolines that connect points of equal
precipitation. 30
Isolines
Lines on a map that connect data points of equal
value. 29
Isopachs
Isolines that connect points of equal sediment or
rock thickness.
30
Isotherms
Isolines that connect points of equal
temperature.
30
J
Jetties
Long stone or concrete structures used to create a
permanent opening for a channel by reinforcing both sides of
the passageway. 532
Joint
A crack or fissure along horizontal or vertical planes in
a rock mass that divides the rock into large blocks.
392
373
I
Ice cap
A dome-shaped sheet of ice that covers an area less
than 50,000 km
2
(~19,000 mi
2
) in size. 452
Ice field
A topographically constrained sheet of ice in
mountainous areas that frequently has glaciers streaming away
from it. 452
Igneous rocks
Rocks that form when magma rises from
the mantle and cools, either within the Earth's crust or on the
surface. 311
Illuviation
The recrystallization of minerals that occurs
directly below the zone of eluviation. 270
Inceptisols
Soils that have one or more weakly devel-
oped horizons due to some alteration and removal of soluble
minerals. 298
Indirect radiation
Radiation that reaches Earth after it has
been scattered or reflected.
K
Kame
A large mound of sediment deposited along the front
of a slowly melting or stationary glacier. 464
Karst topography
Terrain that is underlain by soluble rocks,
such as limestone and dolomite, where the landscape evolves
largely through the dissolution of rock. 402
Katabatic winds
Downslope airflow that evolves when pools
of cold air develop over ice caps and subsequently descend
into valleys. 129
Kettle lake
A lake that forms when a block of ice falls off
the glacial front, is buried by glacial drift, and then melts,
forming a depression that fills with water. 464
Kinetic energy
The energy of motion in a body, measured
as temperature, that is derived from movement of molecules
within the body.
74
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