Geology The science that
examines Earth, its form and
composition, and the changes
it has undergone and is
undergoing.
Geosphere The solid Earth; one
of Earth’ s four basic spheres.
Geothermal energy Natural
steam used for power generation.
Geothermal gradient The
gradual increase in temperature
with depth in the crust. The average
is 30 °C per kilometer in the
upper crust.
Geyser A fountain of hot water
ejected periodically from the
ground.
Glacial budget The balance, or
lack of balance, between
accumulation at the upper end
of a glacier and loss at the
other end.
Glacial drift An all-embracing
term for sediments of glacial origin,
no matter how, where, or in what
shape they were deposited.
Glacial erratic An
ice-transported boulder that was not
derived from the bedrock near its
present site.
Glacial striations Scratches and
grooves on bedrock caused by
glacial abrasion.
Glacial trough A mountain valley
that has been widened, deepened,
and straightened by a glacier.
Glacier A thick mass of ice
originating on land from the
compaction and recrystallization of
snow. The ice shows evidence of
past or present flow.
Glass (volcanic) Natural glass
produced when molten lava cools
too rapidly to permit crystallization.
Volcanic glass is a solid composed of
unordered atoms.
Glassy texture A term used to
describe the texture of certain
igneous rocks, such as obsidian, that
contain no crystals.
Gneissic texture The texture
displayed by the metamorphic rock
gneiss in which dark and light
silicate minerals have separated,
giving the rock a banded
appearance.
Gondwanaland The southern
portion of Pangaea consisting of
South America, Africa, Australia,
India, and Antarctica.
Graben A valley formed by the
downward displacement of a
fault-bounded block.
Graded bed A sediment layer
characterized by a decrease in
sediment size from bottom to top.
Graded stream A stream that has
the correct channel characteristics to
maintain the exact velocity required
to transport the material supplied
to it.
Gradient The slope of a stream;
generally measured in feet per mile.
Granitic Igneous rocks composed
mainly of light-colored silicates
(quartz and feldspar) are said to
have this composition.
Gravitational collapse The
gradual subsidence of mountains
caused by lateral spreading of weak
material located deep within these
structures.
Greenhouse effect Carbon
dioxide and water vapor in a planet’ s
atmosphere absorb and reradiate
infrared wavelengths, effectively
trapping solar energy and raising the
temperature.
Groin A short wall built at a right
angle to the seashore to trap moving
sand.
Ground moraine An undulating
layer of till deposited as the ice front
retreats.
Groundmass The matrix of
smaller crystals within an igneous
rock that has porphyritic texture.
Groundwater Water in the zone
of saturation.
Guyot A submerged flat-topped
seamount.
Habit The common or
characteristic shape of a crystal or
aggregate of crystals.
Hadean eon An informal term
that refers to the earliest interval
(eon) of Earth history—before the
oldest known rocks.
Half-graben A tilted fault block in
which the higher side is associated
with mountainous topography and
the lower side is a basin that fills
with sediment.
Half-life The time required for
one half of the atoms of a radioactive
substance to decay.
Hanging valley A tributary valley
that enters a glacial trough at a
considerable height above the floor
of the trough.
Hardness A mineral’ s resistance to
scratching and abrasion.
Head The vertical distance
between the recharge and discharge
points of a water table. Also, the
source area or beginning of a valley.
Headward erosion The extension
upslope of the head of a valley due
to erosion.
Historical geology A major
division of geology that deals with
the origin of Earth and its
development through time. Usually
involves the study of fossils and
their sequence in rock beds.
Hogback A narrow, sharp-crested
ridge formed by the upturned edge
of a steeply dipping bed of resistant
rock.
Horn A pyramidlike peak formed
by glacial action in three or more
cirques surrounding a mountain
summit.
Horst An elongated, uplifted
block of crust bounded by faults.
Hot spot A proposed
concentration of heat in the mantle
capable of introducing magma that
in turn extrudes onto Earth’ s
surface. The intraplate volcanism
that produced the Hawaiian Islands
is one example.
Hot spot track A chain of
volcanic structures produced as a
lithospheric plate moves over a
mantle plume.
Hot spring A spring in which the
water is 6° to 9 °C (10 °C to 15 °F)
warmer than the mean annual air
temperature of its locality.
Humus Organic matter in soil
produced by the decomposition of
plants and animals.
Hydraulic conductivity A factor
relating to groundwater flow; it is a
coefficient that takes into account
the permeability of the aquifer and
the viscosity of the fluid.
Hydraulic gradient The slope of
the water table. It is determined by
finding the height difference
between the two points on the
water table and dividing by the
horizontal distance between the
two points.
Hydrogenous sediment Seafloor
sediments consisting of minerals
that crystallize from seawater. The
principal example is manganese
nodules.
Hydrologic cycle The unending
circulation of Earth’ s water supply.
The cycle is powered by energy
from the Sun and is characterized by
continuous exchanges of water
among the oceans, the atmosphere,
and the continents.
Hydrolysis A chemicalweathering process in which
minerals are altered by chemically
reacting with water and acids.
Hydrosphere The water portion
of our planet; one of the traditional
subdivisions of Earth’ s physical
environment.
Hydrothermal metamorphism
Chemical alterations that occur as
hot, ion-rich water circulates
through fractures in rock.
Hydrothermal solution The hot,
watery solution that escapes from
a mass of magma during the
latter stages of crystallization.
Such solutions may alter the
surrounding country rock and are
frequently the source of significant
ore deposits.
Hypothesis A tentative
explanation that is then tested to
determine if it is valid.
Ice cap A mass of glacial ice
covering a high upland or plateau
and spreading out radially.
Ice sheet A very large, thick mass
of glacial ice flowing outward in all
directions from one or more
accumulation centers.
Ice-contact deposit An
accumulation of stratified drift
deposited in contact with a
supporting mass of ice.
Igneous rock A rock formed by
the crystallization of molten magma.
Immature soil A soil lacking
horizons.
Impact metamorphism
Metamorphism that occurs when
meteorites strike Earth’ s surface.
Incised meander Meandering
channel that flows in a steep,
narrow valley. These meanders form
either when an area is uplifted or
when base level drops.
Inclusion A piece of one rock
unit contained within another;
inclusions are used in relative
dating. The rock mass adjacent to
the one containing the inclusion
must have been there first in order
to provide the fragment.
537
GLOSSARY
examines Earth, its form and
composition, and the changes
it has undergone and is
undergoing.
Geosphere The solid Earth; one
of Earth’ s four basic spheres.
Geothermal energy Natural
steam used for power generation.
Geothermal gradient The
gradual increase in temperature
with depth in the crust. The average
is 30 °C per kilometer in the
upper crust.
Geyser A fountain of hot water
ejected periodically from the
ground.
Glacial budget The balance, or
lack of balance, between
accumulation at the upper end
of a glacier and loss at the
other end.
Glacial drift An all-embracing
term for sediments of glacial origin,
no matter how, where, or in what
shape they were deposited.
Glacial erratic An
ice-transported boulder that was not
derived from the bedrock near its
present site.
Glacial striations Scratches and
grooves on bedrock caused by
glacial abrasion.
Glacial trough A mountain valley
that has been widened, deepened,
and straightened by a glacier.
Glacier A thick mass of ice
originating on land from the
compaction and recrystallization of
snow. The ice shows evidence of
past or present flow.
Glass (volcanic) Natural glass
produced when molten lava cools
too rapidly to permit crystallization.
Volcanic glass is a solid composed of
unordered atoms.
Glassy texture A term used to
describe the texture of certain
igneous rocks, such as obsidian, that
contain no crystals.
Gneissic texture The texture
displayed by the metamorphic rock
gneiss in which dark and light
silicate minerals have separated,
giving the rock a banded
appearance.
Gondwanaland The southern
portion of Pangaea consisting of
South America, Africa, Australia,
India, and Antarctica.
Graben A valley formed by the
downward displacement of a
fault-bounded block.
Graded bed A sediment layer
characterized by a decrease in
sediment size from bottom to top.
Graded stream A stream that has
the correct channel characteristics to
maintain the exact velocity required
to transport the material supplied
to it.
Gradient The slope of a stream;
generally measured in feet per mile.
Granitic Igneous rocks composed
mainly of light-colored silicates
(quartz and feldspar) are said to
have this composition.
Gravitational collapse The
gradual subsidence of mountains
caused by lateral spreading of weak
material located deep within these
structures.
Greenhouse effect Carbon
dioxide and water vapor in a planet’ s
atmosphere absorb and reradiate
infrared wavelengths, effectively
trapping solar energy and raising the
temperature.
Groin A short wall built at a right
angle to the seashore to trap moving
sand.
Ground moraine An undulating
layer of till deposited as the ice front
retreats.
Groundmass The matrix of
smaller crystals within an igneous
rock that has porphyritic texture.
Groundwater Water in the zone
of saturation.
Guyot A submerged flat-topped
seamount.
Habit The common or
characteristic shape of a crystal or
aggregate of crystals.
Hadean eon An informal term
that refers to the earliest interval
(eon) of Earth history—before the
oldest known rocks.
Half-graben A tilted fault block in
which the higher side is associated
with mountainous topography and
the lower side is a basin that fills
with sediment.
Half-life The time required for
one half of the atoms of a radioactive
substance to decay.
Hanging valley A tributary valley
that enters a glacial trough at a
considerable height above the floor
of the trough.
Hardness A mineral’ s resistance to
scratching and abrasion.
Head The vertical distance
between the recharge and discharge
points of a water table. Also, the
source area or beginning of a valley.
Headward erosion The extension
upslope of the head of a valley due
to erosion.
Historical geology A major
division of geology that deals with
the origin of Earth and its
development through time. Usually
involves the study of fossils and
their sequence in rock beds.
Hogback A narrow, sharp-crested
ridge formed by the upturned edge
of a steeply dipping bed of resistant
rock.
Horn A pyramidlike peak formed
by glacial action in three or more
cirques surrounding a mountain
summit.
Horst An elongated, uplifted
block of crust bounded by faults.
Hot spot A proposed
concentration of heat in the mantle
capable of introducing magma that
in turn extrudes onto Earth’ s
surface. The intraplate volcanism
that produced the Hawaiian Islands
is one example.
Hot spot track A chain of
volcanic structures produced as a
lithospheric plate moves over a
mantle plume.
Hot spring A spring in which the
water is 6° to 9 °C (10 °C to 15 °F)
warmer than the mean annual air
temperature of its locality.
Humus Organic matter in soil
produced by the decomposition of
plants and animals.
Hydraulic conductivity A factor
relating to groundwater flow; it is a
coefficient that takes into account
the permeability of the aquifer and
the viscosity of the fluid.
Hydraulic gradient The slope of
the water table. It is determined by
finding the height difference
between the two points on the
water table and dividing by the
horizontal distance between the
two points.
Hydrogenous sediment Seafloor
sediments consisting of minerals
that crystallize from seawater. The
principal example is manganese
nodules.
Hydrologic cycle The unending
circulation of Earth’ s water supply.
The cycle is powered by energy
from the Sun and is characterized by
continuous exchanges of water
among the oceans, the atmosphere,
and the continents.
Hydrolysis A chemicalweathering process in which
minerals are altered by chemically
reacting with water and acids.
Hydrosphere The water portion
of our planet; one of the traditional
subdivisions of Earth’ s physical
environment.
Hydrothermal metamorphism
Chemical alterations that occur as
hot, ion-rich water circulates
through fractures in rock.
Hydrothermal solution The hot,
watery solution that escapes from
a mass of magma during the
latter stages of crystallization.
Such solutions may alter the
surrounding country rock and are
frequently the source of significant
ore deposits.
Hypothesis A tentative
explanation that is then tested to
determine if it is valid.
Ice cap A mass of glacial ice
covering a high upland or plateau
and spreading out radially.
Ice sheet A very large, thick mass
of glacial ice flowing outward in all
directions from one or more
accumulation centers.
Ice-contact deposit An
accumulation of stratified drift
deposited in contact with a
supporting mass of ice.
Igneous rock A rock formed by
the crystallization of molten magma.
Immature soil A soil lacking
horizons.
Impact metamorphism
Metamorphism that occurs when
meteorites strike Earth’ s surface.
Incised meander Meandering
channel that flows in a steep,
narrow valley. These meanders form
either when an area is uplifted or
when base level drops.
Inclusion A piece of one rock
unit contained within another;
inclusions are used in relative
dating. The rock mass adjacent to
the one containing the inclusion
must have been there first in order
to provide the fragment.
537
GLOSSARY