variability such as tree rings, ice
cores, pollen, and ocean-floor
sediments.
Pyroclastic flow A highly heated
mixture, largely of ash and pumice
fragments, traveling down the flanks
of a volcano or along the surface of
the ground.
Pyroclastic material The
volcanic rock ejected during an
eruption. Pyroclastics include ash,
bombs, and blocks.
Pyroclastic texture An igneous
rock texture resulting from the
consolidation of individual rock
fragments that are ejected during a
violent eruption.
Quarrying The removal of
loosened blocks from the bed of a
channel during times of high flow
rates.
Radial drainage pattern A
system of streams running in all
directions away from a central
elevated structure, such as a
volcano.
Radioactivity The spontaneous
decay of certain unstable atomic
nuclei.
Radiocarbon (carbon-14) The
radioactive isotope of carbon
produced continuously in the
atmosphere and used in dating
events as far back as 75,000 years.
Radiometric dating The
procedure of calculating the
absolute ages of rocks and minerals
containing certain radioactive
isotopes.
Rainshadow desert A dry area
on the lee side of a mountain range;
many middle-latitude deserts are of
this type.
Rapids A part of a stream channel
in which the water suddenly begins
flowing more swiftly and
turbulently because of an abrupt
steepening of the gradient.
Recessional moraine An end
moraine formed as the ice front
stagnated during glacial retreat.
Rectangular pattern A drainage
pattern that develops on jointed or
fractured bedrock and is
characterized by numerous
right-angle bends.
Refraction A change in direction
of waves as they enter shallow water.
The portion of the wave in shallow
water is slowed, which causes the
wave to bend and align with the
underwater contours.
Regional metamorphism
Metamorphism associated with
large-scale mountain building.
Regolith The layer of rock and
mineral fragments that nearly
everywhere covers Earth’ s land
surface.
Relative dating Rocks are placed
in their proper sequence or order;
only the chronological order of
events is determined.
Renewable resource A resource
that is virtually inexhaustible or that
can be replenished over relatively
short time spans.
Reserve Already identified
deposits from which minerals can
be extracted profitably.
Reservoir rock The porous,
permeable portion of an oil trap that
yields oil and gas.
Residual soil Soil developed
directly from the weathering of the
bedrock below.
Reverse fault A fault in which the
material above the fault plane moves
up in relation to the material below.
Reverse polarity A magnetic field
opposite to that which presently
exists.
Richter scale A scale of
earthquake magnitude based on the
motion of a seismograph.
Ridge push A mechanism that
may contribute to plate motion. It
involves the oceanic lithosphere
sliding down the oceanic ridge
under the pull of gravity.
Rift A region of Earth’ s crust along
which divergence (separation) is
taking place.
Rip current A strong, narrow
surface or near-surface current of
short duration and high speed that
moves seaward through the breaker
zone at nearly a right angle to the
shore.
Ripple marks Small waves of
sand that develop on the surface of a
sediment layer by the action of
moving water or air.
River A general term for a stream
that carries a substantial amount of
water and has numerous tributaries.
Roche moutonnée An
asymmetrical knob of bedrock
formed when glacial abrasion
smoothes the gentle slope facing the
advancing ice sheet and plucking
steepens the opposite side as the ice
overrides the knob.
Rock A consolidated mixture of
minerals.
Rock avalanche The very rapid
downslope movement of rock and
debris. These rapid movements
may be aided by a layer of air
trapped beneath the debris, and
they have been known to reach
speeds in excess of 200 kilometers
per hour.
Rock cleavage The tendency of
rock to split along parallel, closely
spaced surfaces. These surfaces are
often highly inclined to the bedding
planes in the rock.
Rock cycle A model that
illustrates the origin of the three
basic rock types and the
interrelatedness of Earth’ s materials
and processes.
Rock flour Ground-up rock
produced by the grinding effect of a
glacier.
Rockslide The rapid slide of a
mass of rock downslope along
planes of weakness.
Runoff Water that flows over the
land rather than infiltrating into the
ground.
S wave An earthquake wave,
slower than a P wave, that travels
only in solids.
Salt flat A white crust on the
ground produced when water
evaporates and leaves its dissolved
materials behind.
Saltation Transportation of
sediment through a series of leaps or
bounces.
Schistosity A type of foliation
characteristic of coarser-grained
metamorphic rocks. Such rocks
have a parallel arrangement of platy
minerals such as the micas.
Scoria Hardened lava that has
retained the vesicles produced by
escaping gases.
Scoria cone See Cinder cone.
Sea arch An arch formed by wave
erosion when caves on opposite
sides of a headland unite.
Sea stack An isolated mass of
rock standing just offshore,
produced by wave erosion of a
headland.
Seafloor spreading The
hypothesis first proposed in the
1960s by Harry Hess, suggesting
that new oceanic crust is produced
at the crests of mid-ocean ridges,
which are the sites of divergence.
Seamount An isolated volcanic
peak that rises at least 1000 meters
(3300 feet) above the deep-ocean
floor.
Seawall A barrier constructed to
prevent waves from reaching the
area behind the wall. Its purpose is
to defend property from the force of
breaking waves.
Secondary enrichment The
concentration of minor amounts of
metals that are scattered through
unweathered rocks into
economically valuable
concentrations by weathering
processes.
Secondary (S) wave A seismic
wave that involves oscillation
perpendicular to the direction of
propagation.
Sediment Unconsolidated
particles created by the
weathering and erosion of rock,
by chemical precipitations from
solution in water, or from the
secretions of organisms, and
transported by water, wind, or
glaciers.
Sedimentary rock Rock formed
from the weathered products of
preexisting rocks that have been
transported, deposited, and
lithified.
Seiche The rhythmic sloshing of
water in lakes, reservoirs, and other
smaller enclosed basins. Some
seiches are initiated by earthquake
activity.
Seismic gap A segment of an
active fault zone that has not
experienced a major earthquake
over a span when most other
segments have. Such segments are
probable sites for future major
earthquakes.
Seismic sea wave A rapidly
moving ocean wave generated by
earthquake activity and capable of
inflicting heavy damage in coastal
regions.
Seismogram The record made by
a seismograph.
Seismograph An instrument that
records earthquake waves.
Seismology The study of
earthquakes and seismic waves.
541
GLOSSARY
cores, pollen, and ocean-floor
sediments.
Pyroclastic flow A highly heated
mixture, largely of ash and pumice
fragments, traveling down the flanks
of a volcano or along the surface of
the ground.
Pyroclastic material The
volcanic rock ejected during an
eruption. Pyroclastics include ash,
bombs, and blocks.
Pyroclastic texture An igneous
rock texture resulting from the
consolidation of individual rock
fragments that are ejected during a
violent eruption.
Quarrying The removal of
loosened blocks from the bed of a
channel during times of high flow
rates.
Radial drainage pattern A
system of streams running in all
directions away from a central
elevated structure, such as a
volcano.
Radioactivity The spontaneous
decay of certain unstable atomic
nuclei.
Radiocarbon (carbon-14) The
radioactive isotope of carbon
produced continuously in the
atmosphere and used in dating
events as far back as 75,000 years.
Radiometric dating The
procedure of calculating the
absolute ages of rocks and minerals
containing certain radioactive
isotopes.
Rainshadow desert A dry area
on the lee side of a mountain range;
many middle-latitude deserts are of
this type.
Rapids A part of a stream channel
in which the water suddenly begins
flowing more swiftly and
turbulently because of an abrupt
steepening of the gradient.
Recessional moraine An end
moraine formed as the ice front
stagnated during glacial retreat.
Rectangular pattern A drainage
pattern that develops on jointed or
fractured bedrock and is
characterized by numerous
right-angle bends.
Refraction A change in direction
of waves as they enter shallow water.
The portion of the wave in shallow
water is slowed, which causes the
wave to bend and align with the
underwater contours.
Regional metamorphism
Metamorphism associated with
large-scale mountain building.
Regolith The layer of rock and
mineral fragments that nearly
everywhere covers Earth’ s land
surface.
Relative dating Rocks are placed
in their proper sequence or order;
only the chronological order of
events is determined.
Renewable resource A resource
that is virtually inexhaustible or that
can be replenished over relatively
short time spans.
Reserve Already identified
deposits from which minerals can
be extracted profitably.
Reservoir rock The porous,
permeable portion of an oil trap that
yields oil and gas.
Residual soil Soil developed
directly from the weathering of the
bedrock below.
Reverse fault A fault in which the
material above the fault plane moves
up in relation to the material below.
Reverse polarity A magnetic field
opposite to that which presently
exists.
Richter scale A scale of
earthquake magnitude based on the
motion of a seismograph.
Ridge push A mechanism that
may contribute to plate motion. It
involves the oceanic lithosphere
sliding down the oceanic ridge
under the pull of gravity.
Rift A region of Earth’ s crust along
which divergence (separation) is
taking place.
Rip current A strong, narrow
surface or near-surface current of
short duration and high speed that
moves seaward through the breaker
zone at nearly a right angle to the
shore.
Ripple marks Small waves of
sand that develop on the surface of a
sediment layer by the action of
moving water or air.
River A general term for a stream
that carries a substantial amount of
water and has numerous tributaries.
Roche moutonnée An
asymmetrical knob of bedrock
formed when glacial abrasion
smoothes the gentle slope facing the
advancing ice sheet and plucking
steepens the opposite side as the ice
overrides the knob.
Rock A consolidated mixture of
minerals.
Rock avalanche The very rapid
downslope movement of rock and
debris. These rapid movements
may be aided by a layer of air
trapped beneath the debris, and
they have been known to reach
speeds in excess of 200 kilometers
per hour.
Rock cleavage The tendency of
rock to split along parallel, closely
spaced surfaces. These surfaces are
often highly inclined to the bedding
planes in the rock.
Rock cycle A model that
illustrates the origin of the three
basic rock types and the
interrelatedness of Earth’ s materials
and processes.
Rock flour Ground-up rock
produced by the grinding effect of a
glacier.
Rockslide The rapid slide of a
mass of rock downslope along
planes of weakness.
Runoff Water that flows over the
land rather than infiltrating into the
ground.
S wave An earthquake wave,
slower than a P wave, that travels
only in solids.
Salt flat A white crust on the
ground produced when water
evaporates and leaves its dissolved
materials behind.
Saltation Transportation of
sediment through a series of leaps or
bounces.
Schistosity A type of foliation
characteristic of coarser-grained
metamorphic rocks. Such rocks
have a parallel arrangement of platy
minerals such as the micas.
Scoria Hardened lava that has
retained the vesicles produced by
escaping gases.
Scoria cone See Cinder cone.
Sea arch An arch formed by wave
erosion when caves on opposite
sides of a headland unite.
Sea stack An isolated mass of
rock standing just offshore,
produced by wave erosion of a
headland.
Seafloor spreading The
hypothesis first proposed in the
1960s by Harry Hess, suggesting
that new oceanic crust is produced
at the crests of mid-ocean ridges,
which are the sites of divergence.
Seamount An isolated volcanic
peak that rises at least 1000 meters
(3300 feet) above the deep-ocean
floor.
Seawall A barrier constructed to
prevent waves from reaching the
area behind the wall. Its purpose is
to defend property from the force of
breaking waves.
Secondary enrichment The
concentration of minor amounts of
metals that are scattered through
unweathered rocks into
economically valuable
concentrations by weathering
processes.
Secondary (S) wave A seismic
wave that involves oscillation
perpendicular to the direction of
propagation.
Sediment Unconsolidated
particles created by the
weathering and erosion of rock,
by chemical precipitations from
solution in water, or from the
secretions of organisms, and
transported by water, wind, or
glaciers.
Sedimentary rock Rock formed
from the weathered products of
preexisting rocks that have been
transported, deposited, and
lithified.
Seiche The rhythmic sloshing of
water in lakes, reservoirs, and other
smaller enclosed basins. Some
seiches are initiated by earthquake
activity.
Seismic gap A segment of an
active fault zone that has not
experienced a major earthquake
over a span when most other
segments have. Such segments are
probable sites for future major
earthquakes.
Seismic sea wave A rapidly
moving ocean wave generated by
earthquake activity and capable of
inflicting heavy damage in coastal
regions.
Seismogram The record made by
a seismograph.
Seismograph An instrument that
records earthquake waves.
Seismology The study of
earthquakes and seismic waves.
541
GLOSSARY