Paleozoic era A time span on the
geologic time scale between the
Proterozoic and Mesozoic eras–from
about 542 million to 251 million
years ago.
Pangaea The proposed
supercontinent that 200 million
years ago began to break apart and
form the present landmasses.
Parabolic dune A sand dune
similar in shape to a barchan dune
except that its tips point into the
wind. These dunes often form along
coasts that have strong onshore
winds, abundant sand, and
vegetation that partly covers the
sand.
Paradigm Theory that is held
with a very high degree of
confidence and is comprehensive in
scope.
Parasitic cone A volcanic cone
that forms on the flank of a larger
volcano.
Parent material The material
upon which a soil develops.
Parent rock The rock from which
a metamorphic rock formed.
Partial melting The process by
which most igneous rocks melt.
Because individual minerals have
different melting points, most
igneous rocks melt over a
temperature range of a few hundred
degrees. If the liquid is squeezed out
after some melting has occurred, a
melt with a higher silica content
results.
Passive continental margin A
margin that consists of a continental
shelf, continental slope, and
continental rise. They are not
associated with plate boundaries
and therefore experience little
volcanism and few earthquakes.
Pater noster lakes A chain of
small lakes in a glacial trough that
occupy basins created by glacial
erosion.
Pegmatite A very coarse-grained
igneous rock (typically granite)
commonly found as a dike
associated with a large mass of
plutonic rock that has smaller
crystals. Crystallization in a waterrich environment is believed to be
responsible for the very large
crystals.
Pegmatitic texture A texture of
igneous rocks in which the
interlocking crystals are all larger
than one centimeter in diameter.
Perched water table A localized
zone of saturation above the main
water table created by an
impermeable layer (aquitard).
Peridotite An igneous rock of
ultramafic composition thought to
be abundant in the upper mantle.
Period A basic unit of the geologic
calendar that is a subdivision of an
era. Periods may be divided into
smaller units called epochs.
Periodic table The tabular
arrangement of the elements
according to atomic number.
Permafrost Any permanently
frozen subsoil. Usually found in the
subarctic and arctic regions.
Permeability A measure of a
material’ s ability to transmit water.
Phaneritic texture An igneous
rock texture in which the crystals
are roughly equal in size and large
enough so that individual minerals
can be identified with the unaided
eye.
Phanerozoic eon That part of
geologic time represented by rocks
containing abundant fossil evidence.
The eon extending from the end of
the Proterozoic eon (about 540
million years ago) to the present.
Phenocryst Conspicuously large
crystals in a porphyry that are
imbedded in a matrix of finergrained crystals (the groundmass).
Physical geology A major
division of geology that examines
the materials of Earth and seeks to
understand the processes and forces
acting upon Earth’ s surface from
below.
Piedmont glacier A glacier that
forms when one or more valley
glaciers emerge from the confining
walls of mountain valleys and
spread out to create a broad sheet in
the lowlands at the base of the
mountains.
Pillow lava Basaltic lava that
solidifies in an underwater
environment and develops a
structure that resembles a pile of
pillows.
Pipe A vertical conduit through
which magmatic materials have
passed.
Placer Deposit formed when
heavy minerals are mechanically
concentrated by currents, most
commonly streams and waves.
Placers are sources of gold, tin,
platinum, diamonds, and other
valuable minerals.
Planetesimal A solid celestial
body that accumulated during the
first stages of planetary formation.
Planetesimals aggregated into
increasingly larger bodies,
ultimately forming the planets.
Plastic deformation Permanent
deformation that results in a change
in size and shape through folding or
flowing.
Plastic flow A type of glacial
movement that occurs within the
glacier, below a depth of
approximately 50 meters, in which
the ice is not fractured.
Plate See Lithospheric plate.
Plate tectonics The theory that
proposes Earth’ s outer shell consists
of individual plates, which interact
in various ways and thereby
produce earthquakes, volcanoes,
mountains, and the crust itself.
Playa The flat central area of an
undrained desert basin.
Playa lake A temporary lake in a
playa.
Pleistocene epoch An epoch of
the Quaternary period beginning
about 1.8 million years ago and
ending about 10,000 years ago. Best
known as a time of extensive
continental ice sheets.
Plucking The process by which
pieces of bedrock are lifted out of
place by a glacier.
Pluton A structure that results
from the emplacement and
crystallization of magma beneath
Earth’ s surface.
Pluvial lake A lake formed
during a period of increased rainfall.
For example, this occurred in many
nonglaciated areas during periods of
ice advance elsewhere.
Point bar A crescent-shaped
accumulation of sand and gravel
deposited on the inside of a
meander.
Polar wandering hypothesis As
the result of paleomagnetic studies
in the 1950s, researchers proposed
that either the magnetic poles
migrated greatly through time or the
continents gradually shifted their
positions.
Polymerization The process of
linking the same molecules together
to form a chain or
three-dimensional structure.
Polymorphs Two or more
minerals having the same chemical
composition but different crystalline
structures, exemplified by the
diamond and graphite forms of
carbon.
Porosity The volume of open
spaces in rock or soil.
Porphyritic texture An igneous
rock texture characterized by two
distinctively different crystal sizes.
The larger crystals are called
phenocrysts, and the matrix of
smaller crystals is termed the
groundmass.
Porphyroblastic texture A
texture of metamorphic rocks in
which particularly large grains
(porphyroblasts) are surrounded by
a fine-grained matrix of other
minerals.
Porphyry An igneous rock with a
porphyritic texture.
Positive-feedback mechanism
As used in climate change, any effect
that acts to reinforce the initial
change.
Pothole A depression formed in a
stream channel by the abrasive
action of the water’ s sediment load.
Precambrian All geologic time
prior to the Paleozoic era.
Principal shells See Energy levels.
Principle of fossil succession
Fossil organisms succeed one
another in a definite and
determinable order, and any time
period can be recognized by its
fossil content.
Principle of original horizontality
Layers of sediment are generally
deposited in a horizontal or nearly
horizontal position.
Proterozoic eon The eon
following the Archean and
preceding the Phanerozoic. It
extends between 2.5 billion and 542
million years ago.
Proton A positively charged
subatomic particle found in the
nucleus of an atom.
Protoplanet A developing
planetary body that grows by the
accumulation of planetesimals.
Proxy data Data gathered from
natural recorders of climate
GLOSSARY
540
geologic time scale between the
Proterozoic and Mesozoic eras–from
about 542 million to 251 million
years ago.
Pangaea The proposed
supercontinent that 200 million
years ago began to break apart and
form the present landmasses.
Parabolic dune A sand dune
similar in shape to a barchan dune
except that its tips point into the
wind. These dunes often form along
coasts that have strong onshore
winds, abundant sand, and
vegetation that partly covers the
sand.
Paradigm Theory that is held
with a very high degree of
confidence and is comprehensive in
scope.
Parasitic cone A volcanic cone
that forms on the flank of a larger
volcano.
Parent material The material
upon which a soil develops.
Parent rock The rock from which
a metamorphic rock formed.
Partial melting The process by
which most igneous rocks melt.
Because individual minerals have
different melting points, most
igneous rocks melt over a
temperature range of a few hundred
degrees. If the liquid is squeezed out
after some melting has occurred, a
melt with a higher silica content
results.
Passive continental margin A
margin that consists of a continental
shelf, continental slope, and
continental rise. They are not
associated with plate boundaries
and therefore experience little
volcanism and few earthquakes.
Pater noster lakes A chain of
small lakes in a glacial trough that
occupy basins created by glacial
erosion.
Pegmatite A very coarse-grained
igneous rock (typically granite)
commonly found as a dike
associated with a large mass of
plutonic rock that has smaller
crystals. Crystallization in a waterrich environment is believed to be
responsible for the very large
crystals.
Pegmatitic texture A texture of
igneous rocks in which the
interlocking crystals are all larger
than one centimeter in diameter.
Perched water table A localized
zone of saturation above the main
water table created by an
impermeable layer (aquitard).
Peridotite An igneous rock of
ultramafic composition thought to
be abundant in the upper mantle.
Period A basic unit of the geologic
calendar that is a subdivision of an
era. Periods may be divided into
smaller units called epochs.
Periodic table The tabular
arrangement of the elements
according to atomic number.
Permafrost Any permanently
frozen subsoil. Usually found in the
subarctic and arctic regions.
Permeability A measure of a
material’ s ability to transmit water.
Phaneritic texture An igneous
rock texture in which the crystals
are roughly equal in size and large
enough so that individual minerals
can be identified with the unaided
eye.
Phanerozoic eon That part of
geologic time represented by rocks
containing abundant fossil evidence.
The eon extending from the end of
the Proterozoic eon (about 540
million years ago) to the present.
Phenocryst Conspicuously large
crystals in a porphyry that are
imbedded in a matrix of finergrained crystals (the groundmass).
Physical geology A major
division of geology that examines
the materials of Earth and seeks to
understand the processes and forces
acting upon Earth’ s surface from
below.
Piedmont glacier A glacier that
forms when one or more valley
glaciers emerge from the confining
walls of mountain valleys and
spread out to create a broad sheet in
the lowlands at the base of the
mountains.
Pillow lava Basaltic lava that
solidifies in an underwater
environment and develops a
structure that resembles a pile of
pillows.
Pipe A vertical conduit through
which magmatic materials have
passed.
Placer Deposit formed when
heavy minerals are mechanically
concentrated by currents, most
commonly streams and waves.
Placers are sources of gold, tin,
platinum, diamonds, and other
valuable minerals.
Planetesimal A solid celestial
body that accumulated during the
first stages of planetary formation.
Planetesimals aggregated into
increasingly larger bodies,
ultimately forming the planets.
Plastic deformation Permanent
deformation that results in a change
in size and shape through folding or
flowing.
Plastic flow A type of glacial
movement that occurs within the
glacier, below a depth of
approximately 50 meters, in which
the ice is not fractured.
Plate See Lithospheric plate.
Plate tectonics The theory that
proposes Earth’ s outer shell consists
of individual plates, which interact
in various ways and thereby
produce earthquakes, volcanoes,
mountains, and the crust itself.
Playa The flat central area of an
undrained desert basin.
Playa lake A temporary lake in a
playa.
Pleistocene epoch An epoch of
the Quaternary period beginning
about 1.8 million years ago and
ending about 10,000 years ago. Best
known as a time of extensive
continental ice sheets.
Plucking The process by which
pieces of bedrock are lifted out of
place by a glacier.
Pluton A structure that results
from the emplacement and
crystallization of magma beneath
Earth’ s surface.
Pluvial lake A lake formed
during a period of increased rainfall.
For example, this occurred in many
nonglaciated areas during periods of
ice advance elsewhere.
Point bar A crescent-shaped
accumulation of sand and gravel
deposited on the inside of a
meander.
Polar wandering hypothesis As
the result of paleomagnetic studies
in the 1950s, researchers proposed
that either the magnetic poles
migrated greatly through time or the
continents gradually shifted their
positions.
Polymerization The process of
linking the same molecules together
to form a chain or
three-dimensional structure.
Polymorphs Two or more
minerals having the same chemical
composition but different crystalline
structures, exemplified by the
diamond and graphite forms of
carbon.
Porosity The volume of open
spaces in rock or soil.
Porphyritic texture An igneous
rock texture characterized by two
distinctively different crystal sizes.
The larger crystals are called
phenocrysts, and the matrix of
smaller crystals is termed the
groundmass.
Porphyroblastic texture A
texture of metamorphic rocks in
which particularly large grains
(porphyroblasts) are surrounded by
a fine-grained matrix of other
minerals.
Porphyry An igneous rock with a
porphyritic texture.
Positive-feedback mechanism
As used in climate change, any effect
that acts to reinforce the initial
change.
Pothole A depression formed in a
stream channel by the abrasive
action of the water’ s sediment load.
Precambrian All geologic time
prior to the Paleozoic era.
Principal shells See Energy levels.
Principle of fossil succession
Fossil organisms succeed one
another in a definite and
determinable order, and any time
period can be recognized by its
fossil content.
Principle of original horizontality
Layers of sediment are generally
deposited in a horizontal or nearly
horizontal position.
Proterozoic eon The eon
following the Archean and
preceding the Phanerozoic. It
extends between 2.5 billion and 542
million years ago.
Proton A positively charged
subatomic particle found in the
nucleus of an atom.
Protoplanet A developing
planetary body that grows by the
accumulation of planetesimals.
Proxy data Data gathered from
natural recorders of climate
GLOSSARY
540