Mesozoic era A time span on the
geologic time scale between the
Paleozoic and Cenozoic eras—from
about 251 million to 65.5 million
years ago.
Metallic bond A chemical bond
present in all metals that may be
characterized as an extreme type of
electron sharing in which the
electrons move freely from atom to
atom.
Metamorphic rock Rock formed
by the alteration of preexisting rock
deep within Earth (but still in the
solid state) by heat, pressure, and/or
chemically active fluids.
Metamorphism The changes in
mineral composition and texture of
a rock subjected to high
temperature and pressure within
Earth.
Metasomatism A significant
change in the chemical composition
of a rock, usually by the addition or
removal of ions in solution.
Microcontinents Relatively small
fragments of continental crust that
may lie above sea level, such as the
island of Madagascar, or be
submerged, as exemplified by the
Campbell Plateau located on the
seafloor near New Zealand.
Mid-ocean ridge See Oceanic
ridge.
Migmatite A rock exhibiting both
igneous and metamorphic rock
characteristics. Such rocks may
form when light-colored silicate
minerals melt and then crystallize,
while the dark silicate minerals
remain solid.
Mineral A naturally occurring,
inorganic crystalline material with a
unique chemical structure.
Mineral resource All discovered
and undiscovered deposits of a
useful mineral that can be extracted
now or at some time in the future.
Modified Mercalli intensity scale
A 12-point scale developed to
evaluate earthquake intensity based
on the amount of damage to various
structures.
Mohorovi˘ cic ´ discontinuity
(Moho) The boundary separating
the crust and the mantle, discernible
by an increase in seismic velocity.
Mohs scale A series of 10
minerals used as a standard in
determining hardness.
Moment magnitude A more
precise measure of earthquake
magnitude than the Richter scale, it
is derived from the amount of
displacement that occurs along a
fault zone.
Monocline A one-limbed flexure
in strata. The strata are usually flat
lying or very gently dipping on both
sides of the monocline.
Mouth The point downstream
where a river empties into another
stream or water body.
Mud crack A feature in some
sedimentary rocks that forms when
wet mud dries out, shrinks, and
cracks.
Mudflow See Debris flow.
Natural levees The elevated
landforms composed of alluvium
that parallel some streams and act to
confine their waters, except during
floodstage.
Neap tide The lowest tidal range,
occurring near the times of the first
and third quarters of the Moon.
Nearshore zone The zone of a
beach that extends from the
low-tide shoreline seaward to where
waves break at low tide.
Nebular theory A model for the
origin of the solar system that
assumes a rotating nebula of dust
and gases that contracted to
produce the Sun and planets.
Negative feedback mechanism
As used in climate change, any effect
that is opposite of the initial change
and tends to offset it.
Neutron A subatomic particle
found in the nucleus of an atom.
The neutron is electrically neutral,
with a mass approximately equal to
that of a proton.
Nonclastic A term for the texture
of sedimentary rocks in which the
minerals form a pattern of
interlocking crystals.
Nonconformity An unconformity
in which older metamorphic or
intrusive igneous rocks are overlain
by younger sedimentary strata.
Nonferromagnesian silicate See
Light silicate.
Nonfoliated texture
Metamorphic rocks that do not
exhibit foliation.
Nonmetallic mineral resource
Mineral resource that is not a fuel or
processed for the metals it contains.
Nonrenewable resource
Resource that forms or accumulates
over such long time spans that it
must be considered as fixed in total
quantity.
Normal fault A fault in which the
rock above the fault plane has
moved down relative to the rock
below.
Normal polarity A magnetic field
the same as that which presently
exists.
Nucleus The small, heavy core of
an atom that contains all of its
positive charge and most of its mass.
Nuée ardente Incandescent
volcanic debris that is buoyed up by
hot gases and moves downslope in
an avalanche fashion.
Numerical date Date that
specifies the actual number of years
that have passed since an event
occurred.
Oceanic plateau An extensive
region on the ocean floor composed
of thick accumulations of pillow
basalts and other mafic rocks that in
some cases exceed 30 kilometers in
thickness.
Oceanic ridge A continuous
mountainous ridge on the floor of
all the major ocean basins and
varying in width from 500 to 5000
kilometers (300 to 3000 miles). The
rifts at the crests of these ridges
represent divergent plate
boundaries.
Octet rule Atoms combine in
order that each may have the
electron arrangement of a noble gas;
that is, the outer energy level
contains eight electrons.
Offshore zone The relatively flat
submerged zone that extends from
the breaker line to the edge of the
continental shelf.
Oil trap A geologic structure that
allows for significant amounts of oil
and gas to accumulate.
Ore Usually a useful metallic
mineral that can be mined at a
profit. The term is also applied to
certain nonmetallic minerals such as
fluorite and sulfur.
Organic sedimentary rock
Sedimentary rock composed of
organic carbon from the remains of
plants that died and accumulated on
the floor of a swamp. Coal is the
primary example.
Original horizontality Layers of
sediment are generally deposited in
a horizontal or nearly horizontal
position.
Orogenesis The processes that
collectively result in the formation
of mountains.
Outer core A layer beneath the
mantle about 2270 kilometers
(1410 miles) thick that has the
properties of a liquid.
Outgassing The release of gases
dissolved in molten rock.
Outlet glacier A tongue of ice
normally flowing rapidly outward
from an ice cap or ice sheet, usually
through mountainous terrain to the
sea.
Outwash plain A relatively flat,
gently sloping plain consisting of
materials deposited by meltwater
streams in front of the margin of an
ice sheet.
Oxbow lake A curved lake
produced when a stream cuts off a
meander.
Oxidation The removal of one
or more electrons from an atom
or ion. So named because
elements commonly combine with
oxygen.
Oxygen isotope analysis A
method of deciphering past
temperatures based on the precise
measurement of the ratio between
two isotopes of oxygen,
16
O and
18
O. Analysis is commonly made of
seafloor sediments and cores of
glacial ice.
P wave The fastest earthquake
wave; travels by compression and
expansion of the medium.
Pahoehoe A lava flow with a
smooth-to-ropy surface.
Paleoclimatology The study of
ancient climates; the study of
climate and climate change prior to
the period of instrumental records
using proxy data.
Paleomagnetism The natural
remnant magnetism in rock bodies.
The permanent magnetization
acquired by rock that can be used to
determine the location of the
magnetic poles and the latitude of
the rock at the time it became
magnetized.
Paleontology The systematic
study of fossils and the history of life
on Earth.
539
GLOSSARY
geologic time scale between the
Paleozoic and Cenozoic eras—from
about 251 million to 65.5 million
years ago.
Metallic bond A chemical bond
present in all metals that may be
characterized as an extreme type of
electron sharing in which the
electrons move freely from atom to
atom.
Metamorphic rock Rock formed
by the alteration of preexisting rock
deep within Earth (but still in the
solid state) by heat, pressure, and/or
chemically active fluids.
Metamorphism The changes in
mineral composition and texture of
a rock subjected to high
temperature and pressure within
Earth.
Metasomatism A significant
change in the chemical composition
of a rock, usually by the addition or
removal of ions in solution.
Microcontinents Relatively small
fragments of continental crust that
may lie above sea level, such as the
island of Madagascar, or be
submerged, as exemplified by the
Campbell Plateau located on the
seafloor near New Zealand.
Mid-ocean ridge See Oceanic
ridge.
Migmatite A rock exhibiting both
igneous and metamorphic rock
characteristics. Such rocks may
form when light-colored silicate
minerals melt and then crystallize,
while the dark silicate minerals
remain solid.
Mineral A naturally occurring,
inorganic crystalline material with a
unique chemical structure.
Mineral resource All discovered
and undiscovered deposits of a
useful mineral that can be extracted
now or at some time in the future.
Modified Mercalli intensity scale
A 12-point scale developed to
evaluate earthquake intensity based
on the amount of damage to various
structures.
Mohorovi˘ cic ´ discontinuity
(Moho) The boundary separating
the crust and the mantle, discernible
by an increase in seismic velocity.
Mohs scale A series of 10
minerals used as a standard in
determining hardness.
Moment magnitude A more
precise measure of earthquake
magnitude than the Richter scale, it
is derived from the amount of
displacement that occurs along a
fault zone.
Monocline A one-limbed flexure
in strata. The strata are usually flat
lying or very gently dipping on both
sides of the monocline.
Mouth The point downstream
where a river empties into another
stream or water body.
Mud crack A feature in some
sedimentary rocks that forms when
wet mud dries out, shrinks, and
cracks.
Mudflow See Debris flow.
Natural levees The elevated
landforms composed of alluvium
that parallel some streams and act to
confine their waters, except during
floodstage.
Neap tide The lowest tidal range,
occurring near the times of the first
and third quarters of the Moon.
Nearshore zone The zone of a
beach that extends from the
low-tide shoreline seaward to where
waves break at low tide.
Nebular theory A model for the
origin of the solar system that
assumes a rotating nebula of dust
and gases that contracted to
produce the Sun and planets.
Negative feedback mechanism
As used in climate change, any effect
that is opposite of the initial change
and tends to offset it.
Neutron A subatomic particle
found in the nucleus of an atom.
The neutron is electrically neutral,
with a mass approximately equal to
that of a proton.
Nonclastic A term for the texture
of sedimentary rocks in which the
minerals form a pattern of
interlocking crystals.
Nonconformity An unconformity
in which older metamorphic or
intrusive igneous rocks are overlain
by younger sedimentary strata.
Nonferromagnesian silicate See
Light silicate.
Nonfoliated texture
Metamorphic rocks that do not
exhibit foliation.
Nonmetallic mineral resource
Mineral resource that is not a fuel or
processed for the metals it contains.
Nonrenewable resource
Resource that forms or accumulates
over such long time spans that it
must be considered as fixed in total
quantity.
Normal fault A fault in which the
rock above the fault plane has
moved down relative to the rock
below.
Normal polarity A magnetic field
the same as that which presently
exists.
Nucleus The small, heavy core of
an atom that contains all of its
positive charge and most of its mass.
Nuée ardente Incandescent
volcanic debris that is buoyed up by
hot gases and moves downslope in
an avalanche fashion.
Numerical date Date that
specifies the actual number of years
that have passed since an event
occurred.
Oceanic plateau An extensive
region on the ocean floor composed
of thick accumulations of pillow
basalts and other mafic rocks that in
some cases exceed 30 kilometers in
thickness.
Oceanic ridge A continuous
mountainous ridge on the floor of
all the major ocean basins and
varying in width from 500 to 5000
kilometers (300 to 3000 miles). The
rifts at the crests of these ridges
represent divergent plate
boundaries.
Octet rule Atoms combine in
order that each may have the
electron arrangement of a noble gas;
that is, the outer energy level
contains eight electrons.
Offshore zone The relatively flat
submerged zone that extends from
the breaker line to the edge of the
continental shelf.
Oil trap A geologic structure that
allows for significant amounts of oil
and gas to accumulate.
Ore Usually a useful metallic
mineral that can be mined at a
profit. The term is also applied to
certain nonmetallic minerals such as
fluorite and sulfur.
Organic sedimentary rock
Sedimentary rock composed of
organic carbon from the remains of
plants that died and accumulated on
the floor of a swamp. Coal is the
primary example.
Original horizontality Layers of
sediment are generally deposited in
a horizontal or nearly horizontal
position.
Orogenesis The processes that
collectively result in the formation
of mountains.
Outer core A layer beneath the
mantle about 2270 kilometers
(1410 miles) thick that has the
properties of a liquid.
Outgassing The release of gases
dissolved in molten rock.
Outlet glacier A tongue of ice
normally flowing rapidly outward
from an ice cap or ice sheet, usually
through mountainous terrain to the
sea.
Outwash plain A relatively flat,
gently sloping plain consisting of
materials deposited by meltwater
streams in front of the margin of an
ice sheet.
Oxbow lake A curved lake
produced when a stream cuts off a
meander.
Oxidation The removal of one
or more electrons from an atom
or ion. So named because
elements commonly combine with
oxygen.
Oxygen isotope analysis A
method of deciphering past
temperatures based on the precise
measurement of the ratio between
two isotopes of oxygen,
16
O and
18
O. Analysis is commonly made of
seafloor sediments and cores of
glacial ice.
P wave The fastest earthquake
wave; travels by compression and
expansion of the medium.
Pahoehoe A lava flow with a
smooth-to-ropy surface.
Paleoclimatology The study of
ancient climates; the study of
climate and climate change prior to
the period of instrumental records
using proxy data.
Paleomagnetism The natural
remnant magnetism in rock bodies.
The permanent magnetization
acquired by rock that can be used to
determine the location of the
magnetic poles and the latitude of
the rock at the time it became
magnetized.
Paleontology The systematic
study of fossils and the history of life
on Earth.
539
GLOSSARY