Index fossil A fossil that is
associated with a particular span of
geologic time.
Index mineral A mineral that is a
good indicator of the metamorphic
environment in which it formed.
Used to distinguish different zones
of regional metamorphism.
Inertia Objects at rest tend to
remain at rest, and objects in motion
tend to stay in motion unless acted
upon by an outside.
Infiltration The movement of
surface water into rock or soil
through crack and pore spaces.
Infiltration capacity The
maximum rate at which soil can
absorb water.
Inner core The solid, innermost
layer of Earth, about 1216
kilometers (754 miles) in radius.
Inselberg An isolated mountain
remnant characteristic of the late
stage of erosion in a mountainous
region.
Intensity (earthquake) A
measure of the degree of earthquake
shaking at a given locale based on
the amount of damage.
Interface A common boundary
where different parts of a system
interact.
Interior drainage A
discontinuous pattern of
intermittent streams that do not
flow to the ocean.
Intermediate Compositional
category for igneous rocks found
near the middle of Bowen’ s reaction
series, mainly amphibole and the
intermediate plagioclase feldspars.
Intermediate focus An
earthquake focus at a depth of
between 60 and 300 kilometers.
Internal process A process such
as mountain building or volcanism
that derives its energy from Earth’ s
interior and elevates Earth’ s surface.
Intraplate volcanism Igneous
activity that occurs within a tectonic
plate away from plate boundaries.
Intrusive rock Igneous rock that
formed below Earth’ s surface.
Ion An atom or molecule that
possesses an electrical charge.
Ionic bond A chemical bond
between two oppositely charged
ions formed by the transfer of
valence electrons from one atom to
another.
Island arc See Volcanic island arc.
Isostasy The concept that Earth’ s
crust is floating in gravitational
balance upon the material of the
mantle.
Isotopes Varieties of the same
element that have different mass
numbers; their nuclei contain the
same number of protons but
different numbers of neutrons.
Joint A fracture in rock along
which there has been no movement.
Kame A steep-sided hill,
composed of sand and gravel,
originating when sediment collected
in openings in stagnant glacial ice.
Kame terrace A narrow,
terracelike mass of stratified drift
deposited between a glacier and an
adjacent valley wall.
Karst A topography consisting of
numerous depressions called
sinkholes.
Kettle holes Depressions created
when blocks of ice become lodged
in glacial deposits and subsequently
melt.
Laccolith A massive, concordant
igneous body intruded between
preexisting strata.
Lahar Mudflows on the slopes of
volcanoes that result when unstable
layers of ash and debris become
saturated and flow downslope,
usually following stream channels.
Laminar flow The movement of
water particles in straight-line paths
that are parallel to the channel. The
water particles move downstream
without mixing.
Lateral moraine A ridge of till
along the sides of a valley glacier
composed primarily of debris that
fell to the glacier from the valley
walls.
Laterite A red, highly leached soil
type found in the tropics and rich in
oxides of iron and aluminum.
Laurasia The northern portion of
Pangaea consisting of North
America and Eurasia.
Lava Magma that reaches Earth’ s
surface.
Lava dome A bulbous mass
associated with an old-age
volcano, produced when thick lava
is slowly squeezed from the vent.
Lava domes may act as plugs to
deflect subsequent gaseous
eruptions.
Lava tube Tunnel in hardened
lava that acts as a horizontal conduit
for lava flowing from a volcanic
vent. Lava tubes allow fluid lavas to
advance great distances.
Law of superposition In any
undeformed sequence of
sedimentary rocks or surfacedeposited igneous materials, each
layer is older than the one above it
and younger than the one below.
Leaching The depletion of soluble
materials from the upper soil by
downward-percolating water.
Light silicate Silicate minerals
that lack iron and/or magnesium.
They are generally lighter in color
and have lower specific gravities
than dark silicates.
Lithification The process,
generally by cementation and/or
compaction, of converting
sediments to solid rock.
Lithosphere The rigid outer layer
of Earth, including the crust and
upper mantle.
Lithospheric plate A coherent
unit of Earth’ s rigid outer layer that
includes the crust and upper
mantle.
Local (temporary) base level
See Temporary (local) base level.
Loess Deposits of windblown silt,
lacking visible layers, generally buff
colored, and capable of maintaining
a nearly vertical cliff.
Longitudinal dunes Long ridges
of sand oriented parallel to the
prevailing wind; these dunes form
where sand supplies are limited.
Longitudinal profile A cross
section of a stream channel along its
descending course from the head to
the mouth.
Longshore current A nearshore
current that flows parallel to the
shore.
Losing stream Streams that lose
water to the groundwater system by
outflow through the streambed.
Lower mantle The part of the
mantle that extends from a depth of
660 kilometers (410 miles) to the
top of the core, at a depth of 2900
kilometers (1800 miles).
Luster The appearance or quality
of light reflected from the surface of
a mineral.
Mafic Because basaltic rocks
contain a high percentage of
ferromagnesian minerals, they are
also called mafic (from magnesium
and ferrum, the Latin name for
iron).
Magma A body of molten rock
found at depth, including any
dissolved gases and crystals.
Magnetic reversal A change in
Earth’ s magnetic field from normal
to reverse or vice versa.
Magnetic time scale The detailed
history of Earth’ s magnetic reversals
developed by establishing the
magnetic polarity of lava flows of
known age.
Magnetometer A sensitive
instrument used to measure the
intensity of Earth’ s magnetic field at
various points.
Magnitude (earthquake) The
total amount of energy released
during an earthquake.
Manganese nodules A type of
hydrogenous sediment scattered on
the ocean floor, consisting mainly of
manganese and iron, and usually
containing small amounts of copper,
nickel, and cobalt.
Mantle The 2885-kilometer
(1789-mile) thick layer of Earth
located below the crust.
Mantle plume A mass of
hotter-than-normal mantle material
that ascends toward the surface,
where it may lead to igneous
activity. These plumes of solid yet
mobile material may originate as
deep as the core–mantle boundary.
Mass number The sum of the
number of neutrons and protons in
the nucleus of an atom.
Mass wasting The downslope
movement of rock, regolith, and
soil under the direct influence of
gravity.
Massive An igneous pluton that is
not tabular in shape.
Meander A looplike bend in the
course of a stream.
Meander scar A floodplain
feature created when an oxbow lake
becomes filled with sediment.
Mechanical weathering The
physical disintegration of rock,
resulting in smaller fragments.
Medial moraine A ridge of till
formed when lateral moraines from
two coalescing valley glaciers join.
Melt The liquid portion of magma
excluding the solid crystals.
GLOSSARY
538
associated with a particular span of
geologic time.
Index mineral A mineral that is a
good indicator of the metamorphic
environment in which it formed.
Used to distinguish different zones
of regional metamorphism.
Inertia Objects at rest tend to
remain at rest, and objects in motion
tend to stay in motion unless acted
upon by an outside.
Infiltration The movement of
surface water into rock or soil
through crack and pore spaces.
Infiltration capacity The
maximum rate at which soil can
absorb water.
Inner core The solid, innermost
layer of Earth, about 1216
kilometers (754 miles) in radius.
Inselberg An isolated mountain
remnant characteristic of the late
stage of erosion in a mountainous
region.
Intensity (earthquake) A
measure of the degree of earthquake
shaking at a given locale based on
the amount of damage.
Interface A common boundary
where different parts of a system
interact.
Interior drainage A
discontinuous pattern of
intermittent streams that do not
flow to the ocean.
Intermediate Compositional
category for igneous rocks found
near the middle of Bowen’ s reaction
series, mainly amphibole and the
intermediate plagioclase feldspars.
Intermediate focus An
earthquake focus at a depth of
between 60 and 300 kilometers.
Internal process A process such
as mountain building or volcanism
that derives its energy from Earth’ s
interior and elevates Earth’ s surface.
Intraplate volcanism Igneous
activity that occurs within a tectonic
plate away from plate boundaries.
Intrusive rock Igneous rock that
formed below Earth’ s surface.
Ion An atom or molecule that
possesses an electrical charge.
Ionic bond A chemical bond
between two oppositely charged
ions formed by the transfer of
valence electrons from one atom to
another.
Island arc See Volcanic island arc.
Isostasy The concept that Earth’ s
crust is floating in gravitational
balance upon the material of the
mantle.
Isotopes Varieties of the same
element that have different mass
numbers; their nuclei contain the
same number of protons but
different numbers of neutrons.
Joint A fracture in rock along
which there has been no movement.
Kame A steep-sided hill,
composed of sand and gravel,
originating when sediment collected
in openings in stagnant glacial ice.
Kame terrace A narrow,
terracelike mass of stratified drift
deposited between a glacier and an
adjacent valley wall.
Karst A topography consisting of
numerous depressions called
sinkholes.
Kettle holes Depressions created
when blocks of ice become lodged
in glacial deposits and subsequently
melt.
Laccolith A massive, concordant
igneous body intruded between
preexisting strata.
Lahar Mudflows on the slopes of
volcanoes that result when unstable
layers of ash and debris become
saturated and flow downslope,
usually following stream channels.
Laminar flow The movement of
water particles in straight-line paths
that are parallel to the channel. The
water particles move downstream
without mixing.
Lateral moraine A ridge of till
along the sides of a valley glacier
composed primarily of debris that
fell to the glacier from the valley
walls.
Laterite A red, highly leached soil
type found in the tropics and rich in
oxides of iron and aluminum.
Laurasia The northern portion of
Pangaea consisting of North
America and Eurasia.
Lava Magma that reaches Earth’ s
surface.
Lava dome A bulbous mass
associated with an old-age
volcano, produced when thick lava
is slowly squeezed from the vent.
Lava domes may act as plugs to
deflect subsequent gaseous
eruptions.
Lava tube Tunnel in hardened
lava that acts as a horizontal conduit
for lava flowing from a volcanic
vent. Lava tubes allow fluid lavas to
advance great distances.
Law of superposition In any
undeformed sequence of
sedimentary rocks or surfacedeposited igneous materials, each
layer is older than the one above it
and younger than the one below.
Leaching The depletion of soluble
materials from the upper soil by
downward-percolating water.
Light silicate Silicate minerals
that lack iron and/or magnesium.
They are generally lighter in color
and have lower specific gravities
than dark silicates.
Lithification The process,
generally by cementation and/or
compaction, of converting
sediments to solid rock.
Lithosphere The rigid outer layer
of Earth, including the crust and
upper mantle.
Lithospheric plate A coherent
unit of Earth’ s rigid outer layer that
includes the crust and upper
mantle.
Local (temporary) base level
See Temporary (local) base level.
Loess Deposits of windblown silt,
lacking visible layers, generally buff
colored, and capable of maintaining
a nearly vertical cliff.
Longitudinal dunes Long ridges
of sand oriented parallel to the
prevailing wind; these dunes form
where sand supplies are limited.
Longitudinal profile A cross
section of a stream channel along its
descending course from the head to
the mouth.
Longshore current A nearshore
current that flows parallel to the
shore.
Losing stream Streams that lose
water to the groundwater system by
outflow through the streambed.
Lower mantle The part of the
mantle that extends from a depth of
660 kilometers (410 miles) to the
top of the core, at a depth of 2900
kilometers (1800 miles).
Luster The appearance or quality
of light reflected from the surface of
a mineral.
Mafic Because basaltic rocks
contain a high percentage of
ferromagnesian minerals, they are
also called mafic (from magnesium
and ferrum, the Latin name for
iron).
Magma A body of molten rock
found at depth, including any
dissolved gases and crystals.
Magnetic reversal A change in
Earth’ s magnetic field from normal
to reverse or vice versa.
Magnetic time scale The detailed
history of Earth’ s magnetic reversals
developed by establishing the
magnetic polarity of lava flows of
known age.
Magnetometer A sensitive
instrument used to measure the
intensity of Earth’ s magnetic field at
various points.
Magnitude (earthquake) The
total amount of energy released
during an earthquake.
Manganese nodules A type of
hydrogenous sediment scattered on
the ocean floor, consisting mainly of
manganese and iron, and usually
containing small amounts of copper,
nickel, and cobalt.
Mantle The 2885-kilometer
(1789-mile) thick layer of Earth
located below the crust.
Mantle plume A mass of
hotter-than-normal mantle material
that ascends toward the surface,
where it may lead to igneous
activity. These plumes of solid yet
mobile material may originate as
deep as the core–mantle boundary.
Mass number The sum of the
number of neutrons and protons in
the nucleus of an atom.
Mass wasting The downslope
movement of rock, regolith, and
soil under the direct influence of
gravity.
Massive An igneous pluton that is
not tabular in shape.
Meander A looplike bend in the
course of a stream.
Meander scar A floodplain
feature created when an oxbow lake
becomes filled with sediment.
Mechanical weathering The
physical disintegration of rock,
resulting in smaller fragments.
Medial moraine A ridge of till
formed when lateral moraines from
two coalescing valley glaciers join.
Melt The liquid portion of magma
excluding the solid crystals.
GLOSSARY
538