P-Wave - A seismic wave that moves material in a push-pull fashion in the direction of its travel. This type of seismic wave can travel through solids, liquids, and gases. Also called a primary wave.
Pacific High - A high pressure system that develops over the central Pacific Ocean near the Hawaiian Islands. Also called the Hawaiian High.
Pacific Ocean - The Pacific Ocean is the largest ocean, covering about 30% of Earth's surface area. It has a total surface area of about 155,557,000 square kilometers (60,045,000 square miles). The ocean floor of the Pacific is quite uniform in depth, with an average depth of 4,300 meters (14,100 feet) below sea level. This fact makes it the deepest ocean region on average. The Pacific Ocean is also home to the lowest elevation on our planet. The deepest point in the Mariana Trench lies some 10,971 meters (35,994 feet) below sea level. About 25,000 islands can be found in the Pacific Ocean region. This is more than the combined total for the other four ocean regions.
Pack Ice - A large area of floating sea ice that is held together as a dense, continuous mass by wind and ocean currents. Also called ice pack or drift ice.
Pahoehoe - The Hawaiian name for a type of fluid basaltic lava flow. Pahoehoe forms when the lava emerges from a volcanic vent at high temperatures (over 1100°C or 2000°F) and flows as thin, quick-moving streams.
Paleoarctic - One of eight biogeographic realms found on Earth, where many species have a unique evolutionary history because of geographic isolation. Geographically, this realm covers all of Eurasia and Northern Africa. This realm’s climate is mainly subtropical to polar.
Paleocene - Geologic epoch that occurred from 65.5 ± 0.3 to 55.8 ± 0.2 million years ago. The Paleocene followed the Cretaceous-Paleogene Extinction Event. Notable events during this time include: modern plant species increasing in species numbers and expanding their coverage on the planet, angiosperms continue to proliferate with their co-evolved insect pollinators, mammals increasing in body size, mammals residing in more habitat types, and birds become more diverse and also occupy more habitats. Climate cooler and dryer than the pervious Cretaceous. One of three epochs during the Paleogene Period. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale. Also spelled Palaeocene.
Paleochannel - A relic stream channel created in the past when hydrological conditions were different and there was enough water in form of stream discharge to fill it. Paleochannels often contain streams that do not have the discharge to fill it. Also spelled palaeochannel.
Paleoclimate - A description of climatic conditions sometime in the geological past as reconstructed from a direct or indirect data source. Also spelled palaeoclimate.
Paleoclimatology - The scientific study of the Earth's past climate. Also spelled Palaeoclimatology.
Paleoecology - A field of ecology that studies the abiotic and biotic factors that influence the distribution and abundance of species in the past. Paleoecology often uses microfossils (such as pollen, spores, and diatoms) and macrofossils to describe past conditions and infer ecological processes that may have operated at this time. Also spelled Palaeoecology.
Paleogene - Geologic period that starts 65.5 ± 0.3 million years ago and ends 23.03 ± 0.05 million years ago. One important event of the Paleogene is the evolution of mammals from tiny organisms into much larger ones. The continued speciation of birds occurs, and they begin to fill habitats once occupied by flying dinosaurs. Tropical plants became more restricted to regions around the equator because of a cooling global climate. Deciduous trees and their forests became more common. Grasses first evolved, and subsequent speciation of these plants led to the appearance of grassland and savanna-type ecosystems. This geologic period contains three epochs: the Paleocene, Eocene, and Oligocene. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale. Also spelled Palaeogene.
Paleolake - An ancient lake that existed in the past when hydrological conditions were different. Also spelled palaeolake.
Paleomagnetism - The study of the relic magnetism preserved in sediments, minerals, rocks, and some archeological materials. These substances can record the direction and intensity of Earth's magnetic field when they are formed. See magnetic reversal. Also spelled palaeomagnetism.
Paleosol - A soil exhibiting features that developed as the result of some past environmental conditions and associated processes. Also spelled palaeosol.
Paleozoic - Is the earliest of three geologic eras. It occurred between 542 and 251 million years ago during the Phanerozoic Eon. The Paleozoic is subdivided into six geologic periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale. Also spelled Palaeozoic.
Pallid Zone - A layer within a soil profile that has a bleached appearance. The pallid zone is characterized by a lack of iron minerals.
Palsa - A mound of peat that develops as the result of the formation of a number of ice lenses beneath the ground surface. Typical size is 1 to 7 meters (3 to 23 feet) high, 10 to 30 meters (30 to 100 feet) wide, and 15 to 150 meters (50 to 500 feet) long. This landform is found in the high latitudes. Similar to a pingo.
Paludal - With reference to marsh environments, processes, and things.
Paludal Sediments - Sediments found in swamps and marshes.
Paludification - Refers to the expansion of a bog, fen, or peatland caused by a rising water table. The rising water table is often facilitated by drainage impediments caused by peat accumulation.
Palynology - (1) The science of studying inorganic and organic microscopic particles. (2) The science that studies existing and fossilized pollen, spores, and other organic particulate matter. Scientists have used palynology to study paleoecology, climate change, stratigraphy, evolution, forensic science, and archeology.
Pampas - A fertile, extensive grassland area found in South America. It is primarily located in Argentina but also extends into parts of Uruguay and Brazil. Much of the natural vegetation of the Pampas has been replaced by crops or is used for cattle grazing.
Pan - (1) Compact soil horizon that has a high clay content. (2) A large natural basin or depression found in desert climates.
Pangaea - Was a supercontinent that existed on the Earth's surface between 335 and 175 million years ago. The break-up of Pangaea, beginning about 175 million years ago, created the current configuration of landmasses found on the Earth. Other supercontinents that existed in Earth's geologic past include Laurasia and Gondwana.
Panplain - A low relief landscape created by sideward erosion by streams.
Parabolic Dune - A crescent-shaped sand dune whose long axis is transverse to the dominant wind direction. These dunes form when scattered vegetation stabilizes surface sediments, and U-shaped blowouts form between clumps of plants. The points of this dune curve upwind. Parabolic dunes also have multiple slip-faces.
Parallel - A line placed on a globe or simulated representation of the Earth that is parallel to the equator and connecting all places of the same latitude.
Parametric Test - A statistical test that assumes the sample data is normally distributed.
Parasite - A consumer-type organism that feeds on a host for an extended time. Feeding causes the host to be less fit and may eventually cause premature death.
Parasitic Cone - A small volcanic cone found on the sides of a much larger volcano.
Parasitism - Biological interaction between species where a parasite species feeds on a host species.
Parent Material - The mineral and rock material from which a soil forms.
Paris Agreement - International climate change treaty negotiated at the Conference of the Parties in Paris, France, from November 30 to December 11, 2015. This treaty was signed by 177 parties (individual nation-states and the European Union) on April 22, 2016 (Earth Day). The main goal of this legally binding agreement is to restrict the increase in annual mean global temperature to well below 2.0°C (relative to the pre-industrial temperature level) and to vigorously pursue efforts to limit this temperature increase to just 1.5°C. This effort will primarily be accomplished by gradually reducing our emissions of various greenhouse gases into the atmosphere to zero by around 2070. For this agreement to become effective, at least 55 parties, accounting for at least 55% of global greenhouse gas emissions, must ratify it. The Paris Agreement is part of the framework of the United Nations Framework Convention on Climate Change (UNFCCC) and a logical extension of the Kyoto Protocol.
Park Savanna - A type of savanna where the trees are scattered and relatively few within the grass habitat.
Particle Shape - Refers to the three-dimensional form of individual particles found in a soil, sediment, or rock. Particle shape analysis aims to quantify attributes such as roundness, flatness, and angularity.
Particle Size - The diameter measurement of the size of mineral particles in a soil, sediment, or rock. Particle size analysis is typically done in soil science, geomorphology, and sedimentology. Using the Wentworth Scale, standard particle diameter sizes are grouped as clay - less than 0.0039 millimeters; silt - 0.0039 to 0.0625 millimeters; sand - 0.0625 to 2 millimeters; granule - 2 to 4 millimeters; pebble - 4 to 64 millimeters; cobble - 64 to 256 millimeters; and boulder - greater than 256 millimeters.
Particulate Matter - Tiny bits of dust, soot, salt, sulfate compounds, pollen, spores, or other particles suspended in the atmosphere.
Parts Per Billion (ppb) - Number of parts of a substance found in one billion parts of a particular gas, liquid, or solid.
Parts Per Million (ppm) - Number of parts of a substance found in one million parts of a particular gas, liquid, or solid.
Parts Per Thousand (ppt) - Number of parts of a substance found in one thousand parts of a particular gas, liquid, or solid.
Pascal (Pa) - An International System of Units quantity for measuring force. Used to measure atmospheric pressure. Equal to one newton over an area of one square meter.
Passive Glacier - Is a glacier that is not currently advancing or retreating. Instead, the glacier is maintaining a relatively stable position within its environment because of a balance between snow and ice accumulation and ablation. Also see active glacier and dead glacier.
Passive Margin - A type of continental plate edge that arises from the fragmentation of a once much larger continental mass. As a result, these passive margins are characterized by a much lower level of tectonic activity when compared to other continental margins. Passive margins are usually not influenced by convergent plate processes and/or the subduction of plates.
Passive Movement - A form of dispersal where the organism uses some external factor to initiate movement. Some commonly used external factors include wind and currents in water bodies. Compare with active movement.
Passive Remote Sensing - A form of remote sensing where the sensor passively captures electromagnetic radiation reflected or emitted by an object being imaged.
Paternoster Lakes - A linear series of mountain valley lakes that form from glacial erosion. Pasternoster lakes normally form behind glacial moraines or in glacially carved out rock basins. The name of this feature is related to the series of lakes looking like a string of beads.
Pathogen - A microscopic parasitic organism that causes disease in a host. Disease causes the host to be less fit and may eventually cause premature death.
Patterned Ground - Term used to describe a variety of unique surface features found in periglacial environments. These features can resemble circles, polygons, nets, steps, and stripes. The development of some of these shapes is thought to result from freeze-thaw action.
Pattern Recognition - An automated process in remote sensing that classifies patterns seen in images according to defined classes using mathematical algorithms and statistical methods with the help of a computer.
Peak - (1) A pointed mountain top. (2) The time of maximum output from a cyclical phenomenon.
Peak Annual Flow - The maximum discharge produced by a stream during a one-year period.
Peak Discharge - The most significant volume of stream discharge measured for a stream at a particular location over a specified span of time.
Peat - Is the partially decomposed remains of plants that once flourished in a waterlogged environment. Some of the plants making up peat are mosses, sedges, and wetland shrubs. Peat can be classified as being either fibric, hemic, or sapric. Fibric peats are the least decomposed and are primarily made of undamaged plant fibers. Hemic peats show more decomposition. Sapric peats are so well decomposed that it is often difficult to find recognizable plant fibers.
Peatland - A treeless wetland ecosystem dominated by mosses, sedges, and wetland shrubs that decay to produce peat. Peatlands make up between 50-70% of Earth's wetlands.
Pebble - A rounded piece of rock that is larger than gravel. Typically, a pebble is between 2.0 and 63 millimeters (0.079 and 2.48 inches) in diameter (Wentworth Scale 4.0 to 64 millimeters). Can be synonymous with the term gravel.
Pedalfer - A soil in a humid environment that is well-drained and has most of its soluble minerals leached from its upper horizons. Compare with pedocal.
Pediment - A gradually sloping bedrock surface located at the base of a fluvial-eroded mountain range. Found in arid locations and normally covered by fluvial deposits.
Pediplain - An arid landscape of little relief that is occasionally interrupted by the presence of scattered inselbergs. Formed by the coalescence of several pediments.
Pedocal - A soil in a humid environment that is poorly drained and has none of its soluble minerals leached from its upper horizons. As a result, there is a precipitation and accumulation of minerals into larger structures like nodules, seams, and/or layers. Compare with pedalfer.
Pedogenesis - The process of soil formation.
Pedogenic Regime - The primary soil-forming process that normally operates under a particular climate. Some of the main processes are laterization, salinization, podzolization, calcification, and gleization.
Pedology - The scientific study of soils.
Pedon - A basic soil sampling unit. Often viewed as a soil profile that goes from the surface to a depth where soil meets bedrock.
Peds - Larger aggregates of mineral and/or organic particles in a soil.
Pelagic Zone - Is a layer of water found in a lake, sea, or ocean that extends from the water surface down to the bottom of the water body. In this extensive column of water, environmental conditions vary significantly with depth. With increasing depth in the pelagic zone, pressure increases, temperature decreases, life becomes less common, dissolved oxygen and nutrients become less available, and sunlight intensity decreases significantly. In our oceans, we can find two major zones in the pelagic zone classified according to light penetration: the photic and aphotic zones. Below this layer are the demersal zone and the benthic zone. Some other layers found in the pelagic zone include the mesopelagic zone, bathyal zone, abyssal zone, and hadal zone.
Peneplain - A landscape or plain with little topographic relief. The final stage in the Davis' cycle of erosion.
Penitent Rocks - Beds of residual resistant rocks in an intensely weathered landscape.
Pennsylvanian - A sub-period during the Carboniferous geologic period that occurred roughly from 299 to 318 million years ago. During this period, the first reptiles and winged insects appear. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Perched Groundwater - A layer of unconfined groundwater, isolated and found above the main groundwater for an area. The separation is usually the result of the presence of a zone of impermeable rock that is impervious to horizontal water flow. This situation will, of course, create a perched water table.
Perched Water Table - A water table that is positioned above the main water table for an area because of the presence of an impermeable rock layer.
Percolation - The vertical movement or infiltration of water from the Earth's surface to its subsurface. This movement usually stops when the downward-flowing water reaches the water table.
Percolation Gauge - A scientific instrument used to measure the downward flow of water through a soil by percolation.
Percoline - The zone within a soil where water moves laterally by way of throughflow.
Perennial Plant - A species of plant that lives for more than two years.
Perennial Stream - A stream that has water flow for the whole year. Compare with an intermittent stream and an ephemeral stream.
Peridotite - Coarse-grained ultramafic igneous rock composed mainly of olivine and pyroxene. The mantle is thought to be composed primarily of this rock type.
Perigean Tide - The tide that is produced on the Earth’s surface when the Moon is in perigee. The Moon’s gravitational attraction causes the high tide to increase in height.
Perigee - (1) The location in the Moon's orbital path when it is nearest Earth. (2) The location in any of our Solar System’s planets' orbital path when it is nearest Earth.
Periglacial - A term used to describe landscapes and processes that are connected with recent deglaciation or found in areas in close proximity to glaciers. Often associated with very active freeze-thaw action. Some landforms that are considered periglacial include pingoes, palsas, patterned ground, block fields, and nivation hollows.
Periglacial Landform - Landforms created by processes associated with intense freeze-thaw action in a high latitude areas or near an alpine or continental glacier.
Perihelion - The point in the Earth's orbit when it is closest to the Sun. This distance is about 147.3 million kilometers (91.5 million miles). Perihelion occurs on the 3rd or 4th of January. Compare with aphelion.
Period - A geologic time unit that is shorter than an era but longer than an epoch. See the International Commission on Stratigraphy (stratigraphy.org) for more information on the geologic time scale.
Periodic Table - A table that describes some of the chemical properties of the known elements. See the following website for more information:
https://www.webelements.com
Permafrost - Zone of permanently frozen water found in high latitude soils and sediments. Five types of permafrost have been recognized: continuous permafrost, discontinuous permafrost, sporadic permafrost, alpine permafrost, and subsea permafrost.
Permeability - A measure of the ability of soil, sediment, and rock to transport water (and other liquids) horizontally and vertically. Permeability is primarily dependent on the porosity of the medium through which the water is flowing. Some rocks, like granite, have very low permeability, while others, like shale, are quite porous. As for soils, sand is the most porous, while clay has the lowest permeability. Silt usually is somewhere in the middle.
Permian - The last geologic period in the Paleozoic Era. The Permian occurred from 299.0 ± 0.8 to 251.0 ± 0.4 million years ago. This period saw the mass extinction of many corals, brachiopods, and trilobites. It also saw the diversification and growing dominance of reptiles. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Permian-Triassic Extinction Event - A mass extinction event that occurred 251 million years ago at the end of the Permian. Considered our planet's most severe mass extinction event, with the loss of 96% of all marine life, about 70% of land vertebrate species, and about 80% of insect genera. Numerous possible causes have been hypothesized, including asteroid impact, severe volcanism, climate change, gamma-ray exposure from a nearby supernova, and the sudden release of methane clathrates from the ocean floor.
Peroxyacetyl Nitrate (PAN) - A common toxic chemical found in photochemical smog. Formed from photochemical reactions involving nitric oxide (NO) and volatile organic compounds (VOCs). A very damaging substance to plants.
Perviousness - Term used to describe the ability of soil, sediment, and rock to move water (and other liquids) through them.
Pesticide - A chemical substance used to attract, deter, reduce, or kill a pest organism. Many pesticides are toxic. Also see biocide, herbicide, insecticide, and fungicide.
Petroleum - A yellow-to-black liquid composed of hydrocarbons and other organic substances and found primarily in sedimentary deposits below the ground surface. Petroleum is mainly recovered by ground drilling and pumping. We refine petroleum to create a variety of products, including gasoline and other fuels, lubricants, asphalt, tar, chemical substances used to make plastics, paraffin wax, and pharmaceuticals.
pH - Scale used to measure the alkalinity or acidity of a substance by determining the concentration of hydrogen ions in solution. A pH of 7.0 is considered neutral. Values below 7.0, to a minimum of 0.0, indicate increasing acidity. Values above 7.0, up to 14.0, indicate increasing alkalinity.
Phacolith - A subsurface mass of younger igneous rock found within older rock. Usually lens-shaped and associated with the top of an anticline or the bottom of a syncline. In the image shown, the phacoliths are colored red. Compare with batholith, dyke, sill, laccolith, and lopolith.
Phanerozoic - A geologic eon that occurs from 542 million years ago to today. During this time, life becomes more diversified and increasingly complex. The Phanerozoic is divided into three geologic eras: Paleozoic, Mesozoic, and Cenozoic. Also see Hadean, Archean, and Proterozoic Eons. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Phase Change - The reorganization of a substance at the atomic or molecular level resulting in a change of the physical state of matter. For example, a change from solid to liquid to a gas.
Phenology - Is the scientific study of how periodic life cycle events cause organisms to change their behavior, physiology, morphology, or metabolism. Some events that can trigger a phenological response include changes in seasons, temperature, moisture, and light availability. For example, the fall coloration and leaf drop of temperate deciduous trees are due to biochemical reactions triggered by changes in day length (photoperiod) and air temperature.
Phloem - Conducting tissue found in a vascular plant that is used to transport organic nutrients (like sucrose).
Phosphate - Forms of inorganic phosphorus are often found in the natural environment. Mined inorganic phosphates are commonly used as a fertilizer in agriculture. Inorganic phosphate is also important in industry. Phosphorus is an essential macronutrient for plants and animals, and has an important biogeochemical role in ecosystems. The typical chemical formula for the phosphate ion is PO43-.
Photic Zone - The topmost layer in a lake, sea, or ocean that receives enough sunlight to promote photosynthesis in aquatic plants. The depth of this zone may only be a few centimeters (inches) thick in a very murky lake, or more than 100 meters (330 feet) in extremely clear ocean waters. Also called the euphotic zone. Compare with the aphotic zone.
Photochemical - Something chemically related to or a byproduct of the absorption of light energy.
Photochemical Smog - Photochemical smog is a condition that develops when primary pollutants (oxides of nitrogen and volatile organic compounds produced by fossil fuel combustion) interact under sunlight to form a mixture of hundreds of hazardous secondary pollutants. Also see industrial smog.
Photodissociation - The splitting of a molecule by a photon from sunlight.
Photogrammetry - The science of using aerial photographs and other remote sensing imagery to obtain measurements of natural and human-made features on the Earth.
Photoperiod - The duration of the daylight period.
Photoperiodism - (1) Mechanism possessed by some organisms to use photoperiod to sense seasonal time. (2) Response by organisms to changes in the duration of day and night.
Photon - A discrete unit of electromagnetic radiation.
Photosphere - Visible surface of the Sun from which solar radiation is emitted.
Photosynthesis - Is the chemical process by which plants and some bacteria capture and organically fix the radiant energy of the Sun. This chemical reaction can be described by the following simple equation:
6CO2 + 6H2O + light energy ➜ C6H12O6 + 6O2
The main products of photosynthesis are carbohydrates, such as the sugar glucose, and oxygen, which is released into the atmosphere. All of the sugar produced in the photosynthetic cells of plants and other organisms is derived from the initial chemical reaction of carbon dioxide and water, driven by sunlight. This chemical reaction is catalyzed by chlorophyll, which acts in concert with other pigments, lipids, sugars, proteins, and nucleic acid molecules. Sugars created in photosynthesis can be later converted by the plant to starch for storage, or they can be combined with other sugar molecules to form specialized carbohydrates such as cellulose, or they can be combined with other nutrients such as nitrogen, phosphorus, and sulfur, to build complex molecules such as proteins and nucleic acids. Also see chemosynthesis.
Photosynthetic Autotroph - An organism that produces food molecules inorganically by using light and the chemical process of photosynthesis. Plants are the dominant photosynthetic autotrophs on the Earth. This organism does not require outside sources of organic food energy for survival.
Photosynthetically Active Radiation (PAR) - This term refers to the portion of electromagnetic radiation that can be used for photosynthesis. For most plants, this radiation falls in a wavelength band between 0.4 and 0.7 micrometers (µm). Of the sunlight received at the Earth's surface, less than half is photosynthetically active radiation.
Phreatic Divide - An imaginary line on the Earth's surface that marks the source of groundwater flow from an underground watershed for a particular stream.
Phreatophytes - A type of plant that produces an extremely deep root system. This evolutionary adaptation allows the plant to access water from areas deep within a soil or from the water table. Phreatophytes are common in arid and desert climates.
Phyllosilicate Mineral - A type of mineral that has a chemical structure that consists of silicate tetrahedra arranged in layers. The following minerals fall into this group: serpentine, muscovite, biotite, chlorite, and talc.
Phylogenic Classification - A classification system of organisms based on measured genetic connections between other species.
Phylum - A group or category used in the taxonomic and/or phylogenetic classification of organisms. A phylum is composed of one or more classes. In plant classification, the term 'category division' is often used synonymously.
Physical Geography - A scientific field of knowledge that studies natural features and phenomena on the Earth from a spatial perspective. Sub-discipline of Geography and Earth Science.
Physical Meteorology - A subfield of meteorology that studies the physical properties and processes of the atmosphere. This includes topics such as electricity, acoustics, radiation dynamics, chemistry, the physics of aerosols, clouds, precipitation, and physical climatology.
Physical Weathering - The breakdown of minerals and rocks into smaller-sized particles through mechanical stress. Also called mechanical weathering.
Phytogeography - A subfield of biogeography and ecology that studies the spatial distribution of plants on our planet.
Phytokarst - A type of karst landform composed of limestone bedrock and dominated by geomorphic features created by biotic processes of plants. These biotic processes can include enhanced solution chemical weathering due to the formation of chelates and erosion by root action.
Phytoplankton - Small photosynthetic organisms, mostly algae and bacteria, found inhabiting aquatic ecosystems. Also see plankton and zooplankton.
Picrite - A coarse-grained ultramafic igneous rock containing the minerals olivine and augite with small quantities of plagioclase feldspar.
Piedmont - A zone of foothills running parallel to a mountain range.
Piedmont Glacier - A large glacier formed from the merger of several alpine glaciers. Compare with cirque glacier, alpine glacier, and continental glacier.
Pigment - An organic substance found in plant and animal cells that creates coloring.
Pingo - A large conical mound that contains an ice core. Pingos can reach 60 to 70 meters (200 to 300 feet) in height. Form in regions of permafrost. Common in the Mackenzie Delta region of Canada. Also see the related palsa.
Pioneer Community - A plant community dominated by pioneer species.
Pioneer Species - The particular plant species that dominate a developing community in the early stages of plant succession.
Pitted Topography - Landscape characterized by numerous kettle holes on a glacial outwash plain.
Place - A term used in geography that describes the factors that make the location of natural and human-made phenomena unique.
Plagioclase Feldspar - A type of feldspar that is rich in sodium and calcium. Common rock-forming mineral.
Planar Map Projection - Is a two-dimensional map projection system that projects the Earth onto a flat surface or plane. The Earth's surface touches this plane at only one point or tangent. These maps are plagued by area and shape distortions that increase toward the tangent point. The ancient Greeks developed the first planar maps around 600 BCE. Modern versions of this projection are commonly used to map the polar regions of our planet. An alternative name for this system is Azimuthal Map Projection.
Plane of the Ecliptic - See ecliptic plane.
Planet - (1) According to the International Astronomical Union (IAU), this is a celestial body orbiting the Sun that is massive enough to have a spherical shape because of its own gravity. Further, this celestial body has cleared its orbital path through space of objects. Planets are not massive enough to create their own light. Our Sun has eight celestial bodies orbiting that are considered planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. (2) A similar body orbiting another star in the Universe.
Planetary Albedo - The albedo of an entire planet. This measurement includes all of the ground and atmospheric surfaces that intercept incoming solar radiation. Earth's planetary albedo is about 0.31.
Planimeter - A mechanical instrument used to measure the area of a two-dimensional surface. Also called a platometer.
Plankton - Minute plant (phytoplankton) and animal organisms (zooplankton) that are found in fresh and marine aquatic ecosystems.
Plant - Organisms belonging to the kingdom Plantae. These organisms have the following general characteristics: lack of locomotion, lack of a nervous system, and cellulose cell walls. Most plants can photosynthesize.
Plant Succession - The cumulative directional change in the types of plant species that occupy a given area through time, after a site was partially or completely removed of plant species by some agent of disturbance. Also called succession.
Plantae - Group, at the kingdom level, in the classification of life. Multicellular organisms that have a eukaryotic cell type, chloroplasts, mitochondria, and a cell wall composed of cellulose.
Plasma - A fourth state of matter, with the other three being gas, liquid, and solid. Plasma consists of an ionized gas composed of positive ions and electrons freed from the orbits around their atoms. Plasmas, like a gas, do not have a shape and have conductive properties similar to metals. A gas can be converted to a plasma by adding heat or other forms of energy. Plasma is believed to make up 99% of the matter found in the Universe. Most stars are in a plasma state. Plasma is also found in neon signs, lightning, and plasma televisions.
Plastic Deformation - Irreversible change in the shape of a material without fracture due to the force of compression or expansion.
Plastic Limit - A measurement made on fine-grained soil in Geology and Civil Engineering. Determines the amount of water in a soil at the point when this material changes from being a solid to having the ability to be plastic.
Plasticity - Is the tendency for some materials to undergo permanent deformation under a threshold level of stress or load. Many materials can undergo plastic deformation, including soil, rock, metals, concrete, foams, and bone.
Plate Tectonics - Theory suggesting that the Earth's surface is composed of a number of oceanic and continental plates. Driven by convection currents in the mantle, these plates can slowly move across the Earth's plastic asthenosphere. This theory is critical to Geology and Geomorphology because it helps explain the formation and occurrence of mountains, folds, faults, volcanoes, earthquakes, ocean trenches, and the mid-oceanic ridges.
Plateau - An extensive area with flat terrain that is elevated above the surrounding landscape.
Plateau Basalt - An accumulation of horizontal flows of basaltic lava. Also called flood basalt.
Platform - Horizontal sedimentary deposits found on top of continental shield deposits.
Playa - A dry lakebed found in a desert.
Pleistocene - Geologic epoch from about 2.588 million to 11,700 years ago. During this time, extensive areas of land in the Northern Hemisphere at high and middle latitudes were covered with glacial ice. One of two epochs during the Quaternary Period. Sometimes called the Ice Age. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Plinian Eruption - A type of volcanic eruption that is characterized by a massive explosion that can send gas and volcanic ash high into the stratosphere (up to 45 kilometers or 28 miles). This type of volcanic eruption is usually associated with very viscous magmas (composed of dacite and rhyolite). Some examples of powerful Plinian eruptions include Vesuvius (29 CE), Tambora (1815), Krakatoa (1883), Santa María (1902), Novarupta (1912), Agung (1963), El Chichón (1982), and Pinatubo (1991). Also called a Vesuvian eruption.
Pliocene - Geologic epoch that occurred from 5.332 to 2.588 million years ago. At the midpoint of this time interval, the average global temperature was approximately 2 to 3°C (3 to 5°F) warmer than today, while sea level was about 25 meters (80 feet) higher than current levels. By the end of the Pliocene, the global climate had become significantly cooler and drier. Deciduous and coniferous forests were common in the mid-latitudes, boreal forests and tundra biomes covered much of the high latitudes, and grasslands were found on all the continents, with the exception of Antarctica. Grasslands and savannas supported a large variety of grazing mammals. One of two epochs during the Neogene Period. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Plucking - Erosive process of particle detachment by moving glacial ice. In this process, basal ice freezes in rock surface cracks. As the main body of the glacial ice moves, material around the ice in the cracks is pulled and plucked out. Also called quarrying.
Pluton - A mass of intrusive igneous rock.
Plutonic - An intrusive igneous rock produced by the solidification of magma deep beneath the Earth's surface.
Pluvial - A period of relatively wetter climatic conditions sandwiched in between two drier climatic phases. Compare with interpluvial.
PM2.5 - Air pollution particulate matter that measures less than (<) 2.5 micrometers (µm) in size.
PM10 - Air pollution particulate matter that measures less than (<) 10 micrometers (µm) in size.
Poaching - Is a type of hunting where a hunter kills an animal illegally. Often, this form of hunting is done because the animal has great value on the black market. Compare with sport hunting, subsistence hunting, and commercial hunting.
Podzolization - A soil-forming process that produces a strongly leached soil with a distinctive iron hardpan layer in the B horizon. Common in cool, moist forest environments.
Podzol Soil - (1) Soil order (type) of the Canadian System of Soil Classification. This soil type is often found under coniferous forests. Its main identifying traits are a poorly decomposed organic layer, an eluviated A horizon, and a B horizon with illuviated organic matter, aluminum, and iron. For more information on this soil type, see the textbook Canadian System of Soil Classification, 3rd Edition, available online - https://sis.agr.gc.ca/cansis/publications/manuals/1998-cssc-ed3/index.html. (2) Soil commonly found under coniferous forests.
Point Bar - A stream bar deposit that is normally located on the inside of a stream channel bend.
Polar Axis - Is a line drawn through the Earth around which the planet rotates. The point where the polar axis intersects the Earth's surface in the Northern Hemisphere is called the North Pole. Likewise, the point where the polar axis intersects Earth's surface in the Southern Hemisphere is called the South Pole.
Polar Cell - Three-dimensional atmospheric circulation cell located at roughly 60 to 90° North and South of the equator. Vertical airflow in the polar cell consists of rising air at the polar font and descending air at the polar vortex (see image). On the Earth's surface, this global-scale circulation system generates the Polar Easterlies. Compare with Hadley Cell and Ferrel Cell.
Polar Climate Zone - A region on our planet found roughly between 60 to 90° North and South latitude. This region is dominated by D and E type climates according to the Köppen Climate Classification system.
Polar Easterlies - Surface winds that originate at the polar highs and blow to the subpolar lows in an east-to-west direction (see image above).
Polar Front - A weather front located typically in the mid-latitudes that separates arctic and polar air masses from tropical air masses (Image Source: Wikipedia Commons, NOAA). Along the polar front, we get the development of mid-latitude cyclones. Above the polar front exists the polar jet stream.
Polar High - Surface area of atmospheric high pressure located at about 90° North and South latitude. These high pressure systems are produced by vertically descending air currents from the polar vortex.
Polar Jet Stream - Relatively fast, uniform winds concentrated within the upper atmosphere in a narrow band. The polar jet stream exists in the mid-latitudes at an altitude of approximately 10 kilometers (6.2 miles) above the ground surface. Polar jet stream flows from west to east at an average speed of between 180 and 210 kilometers per hour (110 to 130 miles per hour). Also see the jet stream and the subtropical jet stream.
Polar Stratospheric Clouds - High altitude clouds found in the stratosphere where the temperature is less than -85°C (-121°F). Commonly found over Antarctica. These clouds play a role in creating the ozone hole over Antarctica.
Polar Vortex - High pressure system located in the upper atmosphere at the polar regions. In this system, air in the upper troposphere moves into the vortex center and then descends to the Earth's surface, creating the polar highs.
Polder - An area of land surface that has been artificially reclaimed from a lake, sea, or ocean by pumping out water.
Polje - A karst landform feature that consists of a depressed, broad plain covering an area from 5 to 1,000 square kilometers (2 to 400 square miles) with underlying limestone bedrock. These features can often be completely enclosed by steep valley sides. Water drainage on these plains can be surficial and/or subterranean, with streams disappearing into sinkholes. Smaller poljes can result from the coalescence of many sinkholes.
Pollen - A very fine grain of organic matter containing male gametes that is produced by seed plants. Used for sexual reproduction in cycadophyta, ginkophyta, pinophyta, gnetophyta, and angiosperm plants.
Pollen Analysis - See palynology.
Pollutant - A substance that has a harmful effect on the health, survival, or activities of humans or other living organisms.
Pollution - Physical, chemical, or biological change in the characteristics of some component of the atmosphere, hydrosphere, lithosphere, or biosphere that adversely influences the health, survival, or activities of humans or other living organisms.
Polyclimax - This idea suggests that the outcome of plant succession can be more than one type of climax community, as many factors act to control the succession process in an area. Compare with monoclimax.
Polycyclic Landform - A landform that shows the repeated influence of one or more major geomorphic processes over geological time. Major geomorphic processes are: weathering, erosion, deposition, and massive crustal movements caused by plate tectonics.
Polygenetic Landform - A landform that shows the influence of two or more major geomorphic processes. Major geomorphic processes are: weathering, erosion, deposition, and massive earth movements caused by plate tectonics.
Polygonal Karst - A karst landscape where the ground surface is marked by a network soil filled depressions.
Polymorphism - Something that exists in multiple forms. In biology, many species are polymorphic because they have two or more genetic phenotypes.
Polypedon - An identifiable soil with distinct characteristics found in a location or region. Composed of numerous pedons.
Pond - A small depression filled with water that is smaller than a lake.
Pool - A scoured depression found on the stream bed that develops because of erosional forces. Often associated with riffles. Also see bedforms.
Population - (1) Refers to all the individuals of a given species in a specific area or region at a certain time. Its significance is more than the sum of individuals, because not all individuals are identical. Populations contain genetic variation within themselves and between other populations. (2) A statistical population is the entire collection of people, animals, plants, or things from which we may collect data and analyze this data using qualitative or quantitative techniques.
Population Crash - Sudden decline in the number of individuals found in a population because of a scarcity of environmental resources required for survival, growth, and reproduction.
Population Density - The number of individuals of a particular species found in a specified unit area.
Population Dynamics - The numerical and statistical study of the changes in the size and age distribution of a population over time. One goal of this analysis is to determine the responsible causal factors. Traditionally, this body of knowledge is a subfield of Mathematical Biology or Ecology. When applied to humans, population dynamics is called demography and is studied by demographers.
Population Parameter - A value used to represent a certain quantifiable characteristic of a statistical population. For example, the population mean is a parameter that is often used to indicate the central value of a quantity.
Pore Ice - A form of periglacial ground ice that is found in the spaces that exist between particles of soil.
Pore Space - Refers to the voids found between mineral and organic particles within a mass of soil. These voids vary in size and can be filled with air and/or water.
Pore Water Pressure - Is the pressure that exists in the water held in the pore spaces of soil, sediment, or rock. Positive pressure exists when the pore spaces are fully saturated with water. Negative pressure (suction) occurs when the pore spaces are not filled with water.
Porosity - The void spaces found in rock, sediment, or soil. Commonly measured as the percentage of space found in a volume of substance.
Positive Feedback - Change in the state of a system that enhances the measured effect of the initial alteration. Compare with negative feedback.
Potassium-Argon Dating - A dating method that determines past dates from the radioactive decay of potassium-40 isotopes into stable argon-40. Potassium is found in mica-type minerals, clay minerals, tephra, and evaporites. This technique is used to date items believed to be over 100,000 years old. Also called P-K dating.
Potential Energy - Is the energy that a body possesses by virtue of its position, and that is potentially transformable into another form of energy.
Potential Evaporation - Is a measure of the atmosphere's ability to remove water from a surface through the processes of evaporation, assuming no limit on water supply.
Potential Evapotranspiration - Is a measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration, assuming no limitation on water supply.
Potential Resource - Is a substance or thing (resource) known to exist in the environment and may be consumed by humans in the future.
Pothole - A round and sometimes deep hole found on the rocky beds of streams. Formed by abrasive material twirling in a stream flow eddy that erodes the stream bed. Geomorphologists can use these features to identify the past presence of a stream.
Power - In physics, this term refers to the rate of work or the rate at which energy is transferred, consumed, or converted into other forms.
ppb - See parts per billion.
ppm - See parts per million.
ppt - See parts per thousand.
Precambrian - Span of geologic time that dates from 4.6 billion to 570 million years ago. The Precambrian is made up of three geologic eras: Hadean, Archean, and Proterozoic. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Precambrian Shield - Another term for shield.
Precession of the Equinox - A wobble in the Earth's polar axis that influences the timing of aphelion and perihelion over a 23,000-year cycle.
Precipitable Water - Amount of water potentially available in the atmosphere for precipitation. It is usually measured along a vertical column extending from Earth's surface to the upper edge of the troposphere.
Precipitate - The solidification of a previously dissolved substance from a solution.
Precipitation - (1) Is any aqueous deposit, in liquid or solid form, that develops in a saturated atmosphere (relative humidity equals 100%) and falls to the ground generally from clouds. Most clouds, however, do not produce precipitation. In many clouds, water droplets and ice crystals are too small to overcome natural updrafts found in the atmosphere. As a result, the tiny water droplets and ice crystals remain suspended in the atmosphere as clouds. Some forms of precipitation include rain, snow, drizzle, hail, ice pellets, and snow pellets. (2) The state of a substance being precipitated into a solid from a solution.
Predation - A biological interaction between species where a predator species consumes a prey species.
Predator - A consumer type organism that feeds on prey. The process of consumption involves the killing of the prey.
Predator-Prey Relationship - A cyclical numerical relationship that exists between the number of individuals of a prey species and the predator species that consumes it. This relationship will exhibit a temporal lag. If the number of individuals in the prey population increases because food is plentiful, there will be a corresponding increase in predator numbers, as more prey to consume means more predators being born. However, the number of prey a habitat can support is finite, and at some point, higher predator numbers will begin to suppress prey numbers significantly. This, in turn, will cause predator numbers to crash as the food supply dwindles. At this point, the cycle begins again, as an abundant food supply will cause prey numbers to increase again.
Prediction - A forecast or extrapolation of the future state of a system from current or past states.
Pressure - Is defined as the force acting on a surface from another mass per unit area.
Pressure Gradient Force - The force due to spatial differences in atmospheric pressure. Usually expressed in millibars or kilopascals per unit distance measured in meters or kilometers. This force is mainly responsible for the development of wind.
Pressure Melting Point - (1) The temperature at which minerals deep within the Earth melt because of high pressure levels. (2) The temperature at which ice below the surface of a glacier melts because of high pressure levels.
Pressure Release - A process where a large rock exfoliates sheets of rock from its surface. This occurs because pressures at Earth's surface are much lower than those deep underground, where the rock was formed. As a result, reduced pressure causes layers near the rock's surface to expand, leading to the formation of fractures. Sheets are created because the fractures are roughly parallel to the surface of the rock. Erosion is the primary process that moves these rocks from within the Earth to the ground surface.
Prevailing Wind - The primary direction that the wind blows from for a location or region over a specific period of time. Compare with the dominant wind.
Prey - An organism that is consumed by a predator.
Primary Carnivore - Carnivore-type organism that occupies the third trophic level in a grazing food chain. Also known as a secondary consumer.
Primary Consumer - Organisms that occupy the second trophic level in the grazing food chain. These organisms are herbivores.
Primary Minerals - Minerals that form from the solidification and crystallization of magma into rock. Compare with secondary minerals.
Primary Pollutant - Air pollutants that enter the atmosphere directly. Compare with secondary pollutants.
Primary Producer - Organisms that occupy the first trophic level in the grazing food chain. These organisms are photosynthetic autotrophs.
Primary Productivity - The organic chemical energy or biomass fixed mainly by plants in an ecosystem via the processes of photosynthesis. Also see gross primary productivity and net primary productivity.
Primary Succession - A plant succession on soil or sediments that does not contain an active seed bank.
Primary Wave - See P-Wave.
Prime Meridian - The location from which meridians of longitude are measured. Has the measure of 0° of longitude. The prime meridian was selected by international agreement to run through Greenwich, England.
Probability - The statistical chance that an event will occur. For example, the probability of a coin landing heads or tails is equal to 50 percent.
Process-Response System - This system integrates the characteristics of both morphological and cascading systems. In a process-response system, we can model the processes involved in the movement, storage, and transformation of energy and/or matter between system elements, and we fully understand the system's form in terms of the measured features.
Producer - Generally, an organism that can synthesize organic matter by way of photosynthesis.
Productivity - The rate of energy fixation or storage of biomass by plants. Usually expressed per unit area and time.
Proglacial - Refers to something next to or near the edge of a glacier or an ice sheet. Compare with englacial, subglacial, and supraglacial.
Progradation - The natural extension of a shoreline seaward because of an increase in sediment deposition at beaches. Compare with retrogradation.
Progressive Succession - A plant succession where the developing plant community becomes complex and contains more species and biomass over time.
Prokaryote - Organisms whose cells have their genetic material in the form of loose strands of DNA found in the cytoplasm. They also do not have a membrane-bound nucleus and have few specialized structures located within their cell boundary.
Prominence - A localized eruption of a filament of extremely hot ionized gas originating from the Sun's photosphere and extending hundreds of thousands of kilometers (miles) into space. Often, these solar features are suspended by magnetic field loops and bend back towards the Sun's surface. Prominences can exist for days to weeks.
Propagule - A structure that develops into a plant.
Proportional - A cause and effect relationship between two variables where a positive or negative change in the quantity of one causes a predictable, similar quantity change in the other.
Protein - Organic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements, which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds.
Proterozoic - A geologic eon that occurred from 542 to 2,500 million years ago. During this time, the first single-celled and multicelled eukaryotic organisms evolved and developed. Also see Hadean, Archean, and Phanerozoic Eons. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Protista - Group, at the kingdom level, in the classification of life. Organisms that are mainly unicellular and have a eukaryotic cell type. A few multicellular members exist. See the Encyclopedia of Life for more information on this group of organisms. https://eol.org/docs/discover/protists-or-protozoa
Protobiont - Non-living precursor to prokaryotic cells that consisted of organized proteins and nucleic acids bounded by an organic membrane. These complex molecular systems had a chemical environment quite different from their surroundings. Protobionts had complex organic molecules that resembled RNA and DNA but were unable to reproduce accurately.
Proton - A type of subatomic particle of an atom that contains a positive electromagnetic charge. These particles are found in an atom's nucleus. Compare with an electron and a neutron.
Protoplasm - Substances making up a cell, including its exterior membrane.
Protozoa - Heterotrophic eukaryotic unicellular organisms that belong to the kingdom Protista.
Provisioning Services - One of four benefits humans receive from ecosystems, as described in the United Nations’ Millennium Ecosystem Assessment. This service suggests that humans can benefit from ecosystems through the natural resources they provide, such as food, medicines, chemicals, and other commercial products. Compare with cultural services, regulating services, and supporting services.
Proxy Data - Data that measures the cause-and-effect relationship between two variables indirectly.
Pseudokarst - A landscape that shows significant effects from solution-type chemical weathering processes and resembles karst, but the bedrock is not limestone.
Psychrometer - Instrument used to measure atmospheric humidity (relative humidity) and dew point. It consists of two thermometers (wet-bulb and dry-bulb), one of which has its bulb covered by a moistened wick. Humidity is determined by the difference in readings between the two thermometers after air has passed over both of them for a specific time period.
Psychrometric Table - A table of values that allows for the determination of relative humidity and dew point from dry-bulb and wet-bulb temperatures recorded on a psychrometer.
Pumice - A fine-grained, low mass, porous igneous rock of volcanic origin. The numerous pores in this rock came from gas bubbles found in the lava before it solidified. This rock differs from scoria in that it will float in water.
Puna - A cold climate grassland found at high altitudes in the Andes.
Pyramid of Biomass - A graphic model describing biomass distribution in a community or an ecosystem at the trophic level. Also see the pyramid of numbers.
Pyramid of Numbers - A graphical model describing the number of organisms that exist at each trophic level in a community or an ecosystem. Also see the pyramid of biomass.
Pyroclastic Material - Pieces of volcanic rock thrown out in an explosive volcanic eruption.
Pyroxene - A group of single-chained inosilicate minerals whose basic chemical unit is the silicate tetrahedron (SiO4). Pyroxenes are common rock-forming minerals and are found in most igneous and metamorphic rocks. They form at high temperatures with very little water in the crystallization environment.
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