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A type of lava resulting from the rapid motion of highly fluid basalt. It cools into smooth glassy flows, or can form twisted, ropey shapes. Pahoehoe is formed from lava that has a low viscosity and strain rate, as well as a low rate of gas effusion.


The study of the geographic distribution of fossil organisms in the geologic past.
See also: fossil


A geologic time interval spanning from about 66 to 56 million years ago. It is an epoch of the Paleogene period.


The study of the relationships of fossil organisms to one another and their environment.


The geologic time interval extending from 66 to 23 million years ago. During the Paleogene, mammals and birds diversified into many of the niches that had previously been held by dinosaurs.
The Paleogene is the first part of the Cenozoic.

paleogeographic maps

Maps that portray the estimated ancient geography of the Earth. They often appear in sequences designed to show the geologic development of a region. Because an enormous amount of data is required to construct even a small paleogeographic map, reconstructions often show only general details and are frequently subject to considerable uncertainty.


A geologic time period that extends from 541 to 252 million years ago. Fossil evidence shows that during this time period, life evolved in the oceans and gradually colonized the land.
The Paleozoic includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.


A supercontinent, meaning “all Earth,” which formed over 300 million years ago and lasted for almost 150 million years, during which all of the Earth’s continents were joined in a giant supercontinent. Pangaea eventually rifted apart and separated into the continents in their current configuration.

parent material

The original geologic material from which soil formed. This can be bedrock, preexisting soils, or other transported sediment such as till or loess.

passive margin

A tectonically quiet continental edge, such as the eastern margin of North America, where crustal collision or rifting is not occurring.

patterned ground

Patterns and sorting in the soil caused by repeated freezing and thawing, which causes repeated heaving upwards and settling of the rocks and pebbles in the soil.


An accumulation of partially decayed plant matter. Under proper heat and pressure, it will turn into lignite coal over geologic periods of time.
As much as 9 meters (30 feet) of peat might need to accumulate to produce an economically profitable coal seam. By the time that a peat bed has been turned into a layer of anthracite, the layer is one-tenth its original thickness.


Clumps of soil, identified by their shape, which may take the form of balls, blocks, columns, and plates. These structures are easiest to see in recently plowed fields, where the soil is often granular and loose or lumpy.


A very coarse-grained igneous rock that formed below the surface, usually rich in quartz, feldspars, and micas. Pegmatite magmas are very rich in water, carbon dioxide, silicon, aluminum, and potassium, and form as the last fluids to crystallize from magma or the first minerals to melt at high temperatures during metamorphism.


Free-swimming; of or in a zone of open water that is neither close to the bottom nor near the shore.


A subperiod of the Carboniferous, spanning from 323 to 299 million years ago.


Continuous; year-round or occurring on a yearly basis.


A coarse-grained plutonic rock containing minerals, such as olivine, which make up the Earth’s mantle.

periglacial zone

A region directly next to an ice sheet, which, although it was never covered or scoured by ice, has its own distinctive landscape and features because it was next to the ice margin.
The average annual air temperature in a periglacial area is between -12o and 3oC (10° and 37°F). Though the surface of the ground may melt in the summer, it refreezes in the winter.


A layer of soil below the surface that remains frozen all year round. Its thickness can range from tens of centimeters (inches) to a few meters (yards). Permafrost is typically defined as any soil that has remained at a temperature below the freezing point of water for at least two years.

permeable, permeability

A capacity for fluids and gas (such as water, oil and natural gas ) to move through fractures within a rock, or the spaces between its grains.
Sandstone, limestone, and fractured rocks of any kind generally are permeable. Shale, on the other hand, is usually impermeable because the small, flat clay particles that make up the rock are tightly packed into a dense rock with very little space between particles. Poorly sorted sedimentary rocks can also be impermeable because smaller grains fill in the spaces between the bigger grains, restricting the movement of fluids.


The geologic time period lasting from 299 to 252 million years ago. During the Permian, the world’s landmass was combined into the supercontinent Pangaea.
The Permian is the last period of the Paleozoic. It ended with the largest mass extinction in Earth’s history, which wiped out 70% of terrestrial animal species and 90% of all marine animal species.


A fossilization method in which empty spaces (such as in a bone or shell) are filled by minerals.


A naturally occurring, flammable liquid found in geologic formations beneath the Earth’s surface and consisting primarily of hydrocarbons. Petroleum, also called oil, is a fossil fuel, formed when large masses of dead organisms (usually algae or plankton) are buried underneath sediments and subjected to intense heat and pressure. Today, petroleum is used to manufacture a wide variety of materials, and it is commonly refined into various types of fuels. It is estimated that 90 million barrels are consumed globally every day.


A generalized term used to describe the entirety of geological history after the Precambrian, from 541 million years ago to the present.


A large and generally conspicuous crystal which has been enclosed in a much finer-grained igneous rock. Phenocrysts may occur in all types of igneous rock, but are most common in felsic rocks.


An extrusive igneous rock of intermediate composition, which forms from magma with a relatively low silica content. The name phonolite comes from Greek meaning "sounding stone" due to the metallic sound it produces if struck.


An inorganic salt of phosphoric acid, and a nutrient vital to biological life.


A metamorphic rock that is intermediate in grade between slate and schist.


A subfield of geography that studies the Earth’s physical processes and patterns, including consideration of the shape (not just the height) of land forms, as well as the bedrock, soil, water, vegetation, and climate of an area, and how they interacted in the past to form the landscape we see today.


An extinct reptile from the late Triassic period. Phytosaurs were semi-aquatic relatives of the crocodile with heavily armored bodies. Their fossils have been found in North America, Europe, and India.

piercement dome

A dome or anticlinal fold in which the core has ruptured the more brittle overlying rock.Also called diapir.See also anticline.

pillow basalt

Basaltic lava that forms in a characteristic "pillow" shape due to its extrusion underwater.

placer deposit

A mineral deposit occurring in rivers and streams where less dense sediment has been carried downstream but denser minerals such as gold have been left behind.


An extinct class of heavily armored fishes. Placoderms lived from the Silurian to the Devonian.

plate tectonics

The process by which Earth’s tectonic plates move and interact with one another at their boundaries. The Earth is dynamic, consisting of constantly moving plates that are made of rigid continental and oceanic lithosphere overlying a churning, plastically flowing asthenosphere. These plates are slowly pulling apart, colliding, or sliding past one another with great force, creating strings of volcanic islands, new ocean floor, earthquakes, and mountains.


Large, rigid pieces of the Earth’s crust and upper mantle, which move and interact with one another at their boundaries.
See also: plate tectonics


See craton


playa lakes

Ephemeral or dry lakebeds that occasionally contain only a thin layer of quickly evaporating water. Soluble minerals such as halite, gypsum, and calcite precipitate from evaporating playa lakes, leaving behind rock salt, gypsum, and limestone.


A subset of the Quaternary, lasting from 2.5 million to about 11,700 years ago. During the Pleistocene, continental ice sheets advanced south and retreated north several dozen times.


A member of a group of extinct, long-necked, Mesozoic marine reptiles.


A geologic time interval extending from roughly 5 to 2.5 million years ago. The Pliocene epoch is a subdivision of the Neogene period, and is the time period directly preceding the onset of Pleistocene glaciations.


Process by which a glacier "plucks" sediments and larger chunks of rock from the bedrock. The flowing ice cracks and breaks rock as it passes over, pieces of which become incorporated into the sheet or bulldozed forward, in front of the glacier’s margin.

plunge pool

A stream pool, lake, or pond scoured at the foot of a waterfall by the erosional force of falling water. Also called plunge basins, these pools are small in diameter, but deep.

pluton, plutonic rock

A body of intrusive igneous rock that formed under the Earth’s surface through the slow crystallization of magma. The term comes from the name of Pluto, Roman god of the underworld.

pluvial lake

A landlocked basin that fills with rainwater or meltwater during times of glaciation.
See also: glacier

polar vortex

A regularly occurring area of low pressure that circulates in the highest levels of the upper atmosphere. Typically, the polar vortex hovers above Canada. However, a pocket of the counter-clockwise rotating low-pressure center can break off and shift southward at a lower altitude. The jet stream then shifts to a more southward flow than usual. A polar vortex can lock the jet stream in this new pattern for several days to more than a week


The percentage of openings in a body of rock such as pores, joints, channels, and other cavities, in which gases or liquids may be trapped or migrate through.

porphyry, porphyritic

An igneous rock consisting of large grained crystals, or phenocrysts, embedded in a fine-grained matrix.


A name used for a variety of salts containing potassium, with mined potash being primarily potassium chloride (KCl). The majority of potash is used as fertilizer, but an increasing amount is being used in a variety of other ways: water softening, snow melting, a variety of industrial processes, as a medicine, and to produce potassium carbonate (K2CO3).


A shallow, rounded depression eroded in bedrock by a glacier.

power (energy)

The rate at which energy is transferred, usually measured in watts or, less frequently, horsepower.

pre-Illinoian glaciation

A grouping of the Midwestern glacial periods that occurred before the Wisconsinian and Illinoian glaciations.


A geologic time interval that spans from the formation of Earth (4.6 billion years ago) to the beginning of the Cambrian (541 million years ago). Relatively little is known about this time period since very few fossils or unaltered rocks have survived. What few clues exist indicate that life first appeared on the planet as long as 3.9 billion years ago in the form of single-celled organisms.
The Precambrian contains the Hadean, Archean, and Proterozoic eons.


A pale green mineral (a hydrous silicate of alumina and lime) occurring in crystalline aggregates.


Prehnite is used as a gem mineral.


See also aggregate, gem, mineral.

primary energy source

A source of energy found in nature that has not been subject to any human-induced energy transfers or transformations (like conversion to electricity). Examples include fossil fuels, solar, wind, and hydropower.


Outward building of strata toward the sea in the form of a beach, fan, or delta, caused by continuous deposition of sediment by rivers, or by the progressive accumulation of material thrown up by waves or other shoreline processes.


Single-celled organisms, with simple cells containing no nucleus or organelles.


A geologic time interval that extends from 2.5 billion to 541 million years ago. It is part of the Precambrian.
During this eon, the Earth transitioned to an oxygenated atmosphere and eukaryotic cells, including fungi, plants, and animals, originated.


A diverse group of single-celled eukaryotes.


The original parent rock from which a metamorphosed rock is formed.


Extinct flying reptiles with wingspans of up to 15 meters (49 feet). They lived during the same time as the dinosaurs.


A pyroclastic rock that forms as frothing and sputtering magmatic foam cools and solidifies. It is so vesicular that it can float. Pumice is a common product of explosive eruptions. Today it is used in a variety of mediums, including construction materials and abrasives.


A group of metamorphic silicate minerals that produce translucent green crystals with a fibrous texture.
See also: metamorphic rock, mineral, silica


An iron sulfide mineral (FeS2). Pyrite’s superficial resemblance to gold has led to the common nickname "fool’s gold."

pyroclastic rocks

Rocks that form during explosive volcanic eruptions, and are composed from a variety of different volcanic ejecta. The term comes from Greek, and means “broken fire.” Pyroclastic debris of all types is known as tephra.


Dark-colored, rock-forming silicate minerals containing iron and magnesium, found in many igneous and metamorphic rocks. They are often present in volcanic rocks.