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Geological Glossary - P

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
P wave Primary, longitudinal, irrotational, push, pressure, dilatational, compressional or push-pull wave. P wave are the fastest body waves and arrive at stations before the S wave or secondary waves. The waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave. P waves can travel through all layer of the Earth. P waves are generally felt by humans as a bang or thump.
Paleogene The Paleogene period is the interval of geological time from 65 to 24 million years ago. Paleogene is derived from the Greek word for ancient.
Paleozoic An era in geologic time that spans from 570 to 245 million years before the present.
Pangaea The supercontinent which formed at the end of the Paleozoic Era and began breaking up about 200 million years ago to form today’s continents.
Panthalassa A hypothetical primaeval ocean covering two-thirds of the world except for the continent of Pangaea.
Pennsylvanian A period in the geologic time scale that spans from 320 to 286 million years ago.
Permian A period in the geologic time scale that spans from 286 to 245 million years ago.
Phanerozoic The eon beginning about 570 million years ago and continuing to the present. The portion of Earth history with rocks containing abundant fossil.
Pleistocene The earlier of the two periods of the Quaternary period, starting 1.6 million years before the present and ending about 10,000 years ago. It was a time of glacial activity. A vast amount of evidence has been collected that indicates dramatic oscillations in climatic conditions of global proportion throughout the Pleistocene. It has been well documented that global ice mass has undergone huge fluctuations on the order of 10E19 kg (equivalent to more than 100 meters of sea level) during the last million years. In addition, it has been demonstrated that the concentrations of atmospheric CO2 vary by at least 30% through a glacial cycle, with glacial periods characterized by low pCO2 (~190 ppm) and interglacials by high pCO2 (~280 ppm). Ocean temperature and circulation have also undergone glacial-interglacial change.
Pliocene The latest Epoch of the Tertiary Period, beginning about 5.3 million years ago and ending 1.6 million years ago.
Precambrian The 'unofficial' time period that encompasses all time from the Earth’s formation, 4.55 billion years ago to 570 million years ago, the beginning of the Paleozoic Era.
Proterozoic The 'Precambrian' epoch from 2.5 billion to 570 million years ago
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
pahoehoe Hawaiian term for a fluid volcanic eruption resulting in broad basaltic shield volcanoes. The highly fluid magma flows readily and hardens into ropey form as it cools.
paleobiogeography The science that studies the past distribution of plants and animals.
paleoclimate The average state or typical conditions of climate during some past geologic period.
paleoclimatology The study of how global climate has changed through time.
paleocompetence Competence is a measure of a stream's abiility to entrain sediment particles, measured by the maximum grain size that can be transported. Empirical relationships between grain size and flow velocity or stream power are applied to paleoflood deposits to derive estimates of these same paleoflow parameters.
paleocurrent map A map of depositional currents that have been inferred from cross-bedding, ripple, or other sedimentary structure.
paleogeographic map A map showing the surface landforms and coastline of an area at some time in the geologic past.
paleogeography Working out by means of geologic criteria (chiefly chemically sensitive sedimentary rocks and environmentally sensitive rocks and structure, such as nonmarine shoreline or deep-water types) and structural geologic evidence for the displacement of rock bodies that originally were continuous geophysical criteria (chiefly the evidence of remanent magnetism, which provides data regarding the original latitudinal position of rock bodies, although now whether that latitude is north or south) and paleontologic criteria (chiefly the evidence of historical biogeography that helps to establish which rock bodies contain fossil suggesting original reproductive communication between same) of the ever-changing geographies of the past.
paleomagnetism "The magnetism of an iron-bearing rock imparted to it by the Earth’s magnetic field when the rock formed. Literally, early magnetism; meaning magnetism formed in a past geologic era. The study of paleomagnetism is important for determing the position of plate at various geological times."
paleontology The science of fossil, of ancient life-forms, and their evolution.
paleorift Dead or dormant rift structure with no remaining transient or thermal phenomena (recent or contemporary magmatism or enhanced heat flow).
paleoseismology Study of prehistoric earthquake, often by excavation across an active fault to examine evidence of past surface-rupturing events.
paleowind A prevailing wind direction in an area, inferred from dune structure or the distribution of volcanic ash for one particular time in geologic history.
palynology The study of pollen, living and fossil.
parabolic dune A dune in the shape of a parabola. The ends of the dune point in the direction from which the wind was blowing. They are found where thick grass or plants cover the sand. The sand is blown away from an area without plants forming the parabolic dune.
parent element An element that is transformed by radioactive decay to a different (daughter) element.
parent element A radioactive element that changes to a different, daughter element when its nucleus decays.
parent rock The preexisting rock from which a metamorphic rock form.
partitioning Process by which a given element distributes itself to different extents in two ohases in equilibrium with each other. The ratio of the concentration of the element in one phase limits and is known as the Nernst partition coefficient.
passive margin A tectonically inactive continental margin characterized by a lack of earthquake and volcanic activity.
pavement A bare rock urface that provides a protective rock cover over the material beneath it.
peat A marsh or swamp deposit of water-soaked plant remains containing more than 50 percent carbon.
pebble Loose particles of rock or mineral (sediment) that range in size from 2 - 64 millimeters in diameter. Pebbles are the smallest type of gravel.
pedalfer A common soil type in humid regions, characterized by an abundance of iron oxides and clay mineral deposited in the B-horizon by leaching.
pediment A planar, sloping rock urface forming a ramp up to the front of a mountain range in an arid region. It may be covered locally by thin alluvium.
pedocal A common soil type of arid regions, characterized by accumulation of calcium carbonate in the A-horizon.
pegmatite A very coarse-grained igneous rock, commonly with a granitic composition. Usually form from molten rock rich in water or other volatile that facilitate the growth of large crystal. Forms sill and dike.
pegmatold Name given to a coarse-grained rock in which mineral have a crystalline outline and range from one to several centimeters in size, as in pegmatite, but which lacks the large proportions of quartz and orthoclase that characterize pegmatite.
pelagic sediment Deep-sea sediment composed of fine-grained detritus that slowly settles from surface waters. Common constituents are clay, radiolarian ooze, and foraminiferal ooze.
pelagic sediment "Deposit found in deep water far from shore and may be predominantly either organic or inorganic in origin. Such deposits are light colored, reddish or brown, fine grained, and generally contain some skeletal remains of plankton organisms. Those that contain less than about 30% of organic remains are called red clay; those that contain more than about 30% of organic remains are known as oozes"
pelite A fine-grained sedimentary rock consisting mostly of clay and/or silt. Mudstone, shale, siltstone, and claystone are all pelitic.
peneplain A hypothetical extensive area of low elevation and relief reduced to near sea level by a long period of erosion and representing the end product of the ideal geomorphic cycle.
perched groundwater An isolated body of ground-water that is perched above and separated from the main water table by an aquiclude.
percolation The movement of water downward and radially through the subsurface soil layer, usually continuing downward to the groundwater.
perennial stream A stream that runs continuously throughout the year.
peridotite A coarse-grained mafic igneous rock composed of olivine with accessory amounts of pyroxene and amphibole but little or no feldspar.
period, wave The time between two successive wave crests.
permafrost "Perenially frozen ground that occurs whereever the ground temperatures remain continuously below 0ø C for two or more years. Discontinuous or patchy permafrost occurs north of the 0ø C mean annual air temperature isotherm; continuous permafrost up to several hundred meters in thickness is widespread in Siberia and northern Alaska and Canada."
permeability The ability of a rock or other material to allow water to flow through its interconnected spaces. Permeable bedrock makes a good aquifer, a rock layer that yields water to well.
perovskite Structure of the main mineral phase of the lower mantle. (MgFe)SiO3 perovskite represents approximately 40% by volume of the entire earth.
petrology The study of rocks.
petrophile Lover of rocks. May be a petrologist, a stone mason, a rock climber.
phaneritic An igneous rock texture in which the mineral grain are large enough to be seen with the unaided eye and are of approximately equal size.
phase The onset of a displacement or oscillation on a seismogram indicating the arrival of a different type of seismic wave.
phenocryst A term used to describe large crystal embedded in a mass of finer crystals (groundmass) in an igneous rock.
phlogopite A magnesium-rich member of the mica mineral family. Phlogopite is a yellowish-brown to coppery-colored mica. Like all micas, phlogopite form flat, plate-like crystal that cleave into smooth flakes.
phreatic eruption (explosion) An explosive volcanic eruption caused when water and heated volcanic rock interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.
phyllite "A very fine-grained, foliated metamorphic rock, generally derived from shale or fine-grained sandstone. Phyllites are usually black or dark gray; the foliation is commonly crinkled or wavy. Differs from less recrystallized slate by its sheen, which is produced by barely visible flakes of muscovite (mica)."
pillow lava When basalt erupt underwater, they commonly form pillow lava, which are mounds of elongate lava pillows formed by repeated oozing and quenching of the hot basalt. First, a flexible glassy crust forms around the newly extruded lava, forming an expanded pillow. Next, pressure builds until the crust breaks and new basalt extrudes like toothpaste, forming another pillow. This sequence continues until a thick sequence may be deposited.
pisolites Concretionary spherical nodules formed by transformation of duricrust by accretionary growth and dilation in the uppermost laterite zone.
plagioclase feldspar A member of the feldspar mineral family. Plagioclase feldspars are silicate that contain considerable sodium and calcium. Feldspar crystal are stubby prisms, generally white to gray and a glassy luster.
planetesimal Bodies in the solar nebul, formed from accumulation of dust grain. They vary greatly in size from a few microns up to the size of small planets.
plankton Generally tiny animals or plants that live floating in water.
planktonic foraminifera Planktonic protozoans which may form carbonate tests. The tests are preserved in sediment and form an important basis for paleoceanographic reconstructions.
plastic deformation Permanent deformation (change in size or shape) of soft, but solid rock by folding or flowing without fracturing
plate The Earth is made up of a dozen or so major plate and several minor plates. Tectonic plates are constantly on the move. The fastest plate races along at 15 centimeters (6 inches) per year while the slowest plates crawl at less than 2.5 centimeters (1 inch) per year.
plate boundary The place where two or more plate in the Earth's crust meet.
plate tectonics The theory that the Earth's crust and upper mantle (the lithosphere) is broken into a number of more or less rigid, but constantly moving, segments or plate which move in relation to one another, shifting continent, forming new ocean crust, and causing volcanic eruption
plateau A flat or nearly flat area of high ground with steep sides standing above the surrounding country.
plates Pacific plate, North American plate, Juan de Fuca plate, Cocos plate, Nazca plate, South American plate, Scotia Plate, Caribbean plate, African plate, Eurasian plate, Arabian plate, Australian-Indian plate, China subplate, Philippine plate, Somalian subplate, Antartic plate
playa Playas are shallow, short-lived lakes that form where water drains into basin with no outlet to the sea and quickly evaporates. Playas are common features in arid (desert) regions and are among the flattest landforms in the world.
plug Solidified lava that fills the conduit of a volcano. Plugs (also called volcanic neck) are usually more resistant to erosion than the material making up the surrounding cone and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.
plug dome The steep-sided, rounded mound formed when viscous lava well up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged.
pluton A large body of intrusive igneous rock that solidified within the crust. Batholiths and Stocks are types of pluton.
plutonic rock Any igneous rock that cools beneath the surface. (=intrusive rock).
pluvial lake A lake formed in a land-locked basin during a period of increased rainfall associated with glacial advance elsewhere.
podzolization Soil-forming process typified by formation of humic acids and leaching of acids and sesquioxides. Continuum of soil developed range from peat, podzol soils (upper mineral soil bleached nearly white by organic acids) to podzolic soils where leaching of sesquioxides is insufficient to bleach mineral soil.
polar motion Path of the earth's spin axis relative to an earth fixed-coordinate system. Movement of the rotation axis relative to geographic (earth-fixed) coordinates, consisting mainly of a slow drift, a 14-month Chandler wobble and a 12-month annual wobble.
polarity chron Time interval of constant geomagnetic polarity.
polarity zone Stratigraphic interval in which the rocks or sediment carry a magnetization indicating formation in a field of constant polarity.
porosity The percentage of open spaces (pores) in rock or soil. When these spaces are interconnected, water, air, or other fluids can migrate from space to space. Interconnected spaces make the soil or bedrock permeable.
porphyrictic A rock texture having large crystal or phenocryst in a finer groundmass.
porphyritic An igneous rock texture characterized by larger crystal (phenocrysts) in a matrix of distinctly finer crystals (groundmass).
porphyroblasts Large mineral grain that grow during metamorphism.
porphyry An igneous rock, usually a dike or sill, with larger, generally conspicuous, early-formed crystal contained within a matrix of much smaller crystals.
potable water Water that is agreeable to the taste and not dangerous to the health.
potential energy (gravitational) The stored energy of a substance. Water has a lot of this if there is an elevation difference. Potential energy can be converted to kinetic energy if the water (or other substance) is allowed to move.
potentiometric surface Imaginary surface defined by the levels to which water will rise in well that are open at the same elevation. The slope of the potentiometric surface determines the horizontal direction of groundwater flow.
pothole A semispherical hole in the bedrock of a stream bed, formed by abrasion of small pebble and cobble in a strong current.
ppm Abbreviation for parts per million.
pratt isostatic compensation The mechanism in which variations in crustal density act to counterbalance the varying weight of topographic features. The crust is here assumed to be of approximately uniform thickness, thus a mountain range would be underlain by lighter rocks.
precarious boulder A large rock resting on another in an unstable position. Precarious boulder may remain in place for thousands of years until an earthquake or human-caused tremor dislodges them.
precipitate (verb) The process that separates solid from a solution.
(noun) Mineral precipitate. A mineral deposited from a water solution in pores or other openings in rocks. Chemical reaction with the surrounding rock, changes in pressure or temperature, or just drying up (evaporation) can cause a mineral to precipitate out of solution. Quartz vein are common products of mineral precipitation.
precipitation Any condensed water falling from the atmosphere to the surface of the earth. Common types include rain, snow, sleet and hail.
preferred orientation Any deviation from random-hess in the distribution of the crystallographic or grain shape axes of mineral of a rock (including flow cleavage and foliation), produced by deformation and nonuniform stress during crystallization in metamorphic rocks or by depositional currents in sediment.
primary coast A young coast formed by erosion of a recent terrestrial activity such as erosion, deposition, volcanism, or fault movements.
primary magma Magma produced by melting a solid precursor. Magma only slightly differentiated in composition from primary magma is called primitive.
primary phase field Area or volume on a phase diagram depicting crystal-melt equilibria in which one crystalline phase precipitates.
principle of opposition Graphical technique used to model fractional crystallization of a magma that determines the composition of the extract assemblage.
proto-sun A large cloud of dust and gas gradually coalescing into a star under the force of gravity.
proven reserves Deposits of fossil fuels whose location and extent are known, as opposed to potential but unproved ('*discovered) deposits.
proxy climate indicators Dateable evidence of a biological or geological phenomenon whose condition, at least in part is attibutable to climatic conditions at the time of its formation. Proxy data are any material that provides an indirect measure of climate and include document
pumice Light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava and so filled with vesicle that it resembles a sponge and is very light. . Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles.
pyrite Iron sulfide mineral (FeS). Forms silvery to brassy metallic cubes or masses. Common in many rocks. Known as fool’s gold. Weathered pyrite produces limonite (iron oxide) that stains rock brown. or yellow.
pyroclastic Igneous material erupted explosively as finely pulverized lava (bombs, blocks, pumice, ash and hot gases).
pyroclastic eruption A volcanic eruption that produces a large volume of solid volcanic fragments (pyroclastics) rather than fluid lava. This type of eruption is typical of volcanoes with high silica, viscous, gas-rich magma.
pyroclastic flow A ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour or more. The temperature within a pyroclastic flow may be greater than 500° C, sufficient to burn and carbonize wood. Once deposited, the ash, pumice, and rock fragments may deform (flatten) and weld together because of the intense heat and the weight of the overlying material.
pyroclastic rock A rock formed by the accumulation of fragments of volcanic rock scattered by volcanic explosions.
pyroclastic texture The unsorted, angular, and un-rounded texture of the fragments in a pyroclastic rock.
pyroxene Family of silicate mineral containing iron, magnesium, and calcium in varying amounts. Differ from amphibole family by lack of water in the crystal. The most common variety, augite, contains aluminum as well. Generally form very dark green to black stubby prisms.
pyroxene granulite A coarse-grained contact metamorphic rock containing pyroxene, formed at high temperatures and low pressures.
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