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Geology P-S
| Question | Answer |
|---|---|
| Pahoehoe | A basaltic lava flow with a glassy, smooth, and undulating, or ropy, surface. |
| Paleoclimate | The average state or typical conditions of climate during some past geologic period. |
| Paleocurrent map | A map of depositional currents that have been inferred from cross-bedding, ripples, or other sedimentary structures. |
| Paleogeographic map | A map showing the surface landforms and coastline of an area at some time in the geologic past. |
| Paleomagnetism | The science of the reconstruction of the Earth's ancient magnetic field and the positions of the continents from the evidence of remnant magnetization in ancient rocks. |
| Paleontology | The science of fossils, of ancient life-forms, and their evolution. |
| 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. |
| Pangaea | According to some theories, a great proto-continent from which all present continents have broken off by the mechanism of sea-floor spreading and continental drift. |
| Panthalassa | A hypothetical primeval ocean covering two-thirds of the world except for the continent of Pangaea. |
| Parent element | An element that is transformed by radioactive decay to a different (daughter) element. |
| Peat | A marsh or swamp deposit of water-soaked plant remains containing more than 50 percent carbon. |
| Pedalfer | A common soil type in humid regions, characterized by an abundance of iron oxides and clay minerals deposited in the B-horizon by leaching. |
| Pediment | A planar, sloping rock surface 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 | An igneous rock with extremely large grains, more than a centimeter in diameter. It may be of any composition but most frequently is granitic. |
| Pelagic sediment | Deep-sea sediments composed of fine-grained detritus that slowly settles from surface waters. Common constituents are clay, radiolarian ooze, and foraminiferal ooze. |
| 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. |
| Peridotite | A coarse-grained mafic igneous rock composed of olivine with accessory amounts of pyroxene and amphibole but little or no feldspar. |
| Potable water | Water that is agreeable to the taste and not dangerous to the health. |
| Pothole | A semispherical hole in the bedrock of a stream bed, formed by abrasion of small pebbles and cobbles 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. |
| Preferred orientation | Any deviation from randomness in the distribution of the crystallographic or grain shape axes of minerals of a rock (including flow cleavage and foliation), produced by deformation and non-uniform stress during crystallization in metamorphic rocks or by |
| 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. |
| Pumice | A form of volcanic glass, usually of silicic composition, so filled with vesicles that it resembles a sponge and is very light. |
| P-wave | The primary or fastest wave traveling away from a seismic event through the solid rock, and consisting of a train of compressions and dilations of the 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 granulite | A coarse-grained contact metamorphic rock containing pyroxene, formed at high temperatures and low pressures. |
| Quartz arenite | A sandstone containing very little except pure quartz grains and cement. |
| Quartzite | (1) A very hard, clean, white metamorphic rock formed from a quartz arenite sandstone. (2) A quartz arenite containing so much cement that it resembles ( 1 ). |
| Quartzose sandstone | (1) A quartz arenite. (2) A clean quartz sandstone, less pure than a quartz arenite, that may contain a moderate amount of other detrital minerals and/or calcite cement. |
| Radial drainage | A system of streams running in a radial pattern away from the center of a circular elevation, such as a volcano or dome. |
| Radiative transfer | One mechanism for the movement of heat, in which it takes the form of long-wavelength infrared radiation. |
| Radiolarian | A class of one-celled marine animals with siliceous skeletons that have existed in the ocean throughout the Phanerozoic Eon. |
| Radiolarian ooze | A siliceous deep-sea sediment composed largely of the skeletons of radiolaria. Radiolarite |
| Ray | A linear landform of the lunar surface emanating from a large crater and extending as much as 100 kilometers outward, probably consisting of fine ejecta thrown out by the impact of a meteorite. |
| Reaction series | A series of chemical reactions occurring in a cooling magma by which a mineral formed at high temperature becomes unstable in the melt and reacts to form another mineral (see also Discontinuous reaction series). |
| Recharge | In hydrology, the replenishment of ground-water by infiltration of meteoric water through the soil. |
| Recrystallization | The growth of new mineral grains in a rock at the expense of old grains, which supply the material. |
| Rectangular drainage | A system of streams in which each straight segment of each stream takes one of two characteristic perpendicular directions, with right-angle bends between. The streams are usually following two perpendicular sets of joints. |
| Recumbent fold | An overturned fold with both limbs nearly horizontal. |
| Refraction (wave) | The departure of a wave from its original direction of travel at the interface with a material of different index of refraction (light) or seismic wave velocity (see also Seismic refraction). |
| Regional metamorphism | Metamorphism occurring over a wide area and caused by deep burial and high internal temperatures of the Earth. |
| Regolith | Any solid material lying on top of bedrock. Includes soil, alluvium, and rock fragments weathered from the bedrock. |
| Regression | A drop in sea level that causes an area of the Earth to be uncovered by seawater, ending marine deposition. |
| Relief | The maximum regional difference in elevation. |
| Remote sensing | The study of Earth surface conditions and materials from airplanes and satellites by means of photography, spectroscopy, or radar. |
| Replacement deposit | A deposit of ore minerals by hydrothermal solutions that have first dissolved the original mineral to form a small cavity. |
| Respiration | The chemical reaction by which carbohydrates are oxidized and by which all animals and plants convert their food into energy. Carbon dioxide is released and oxygen used up. |
| Reversible reaction | A chemical reaction which can proceed in either direction, depending on the concentration of reacting materials. |
| Rheidity | (1) The ability of a substance to yield to viscous flow under large strains. (2) One thousand times the time required for a substance to stop changing shape when stress is no longer applied. |
| Rhyolite | The fine-grained volcanic or extrusive equivalent of granite, light brown to gray and compact. Richter magnitude scale |
| Ridge (mid-ocean) | A major linear elevated landform of the ocean floor, from 200 to 20,000 kilometers in extent. It is not a single ridge, but resembles a mountain range and may have a central rift valley. |
| Rift valley | A fault trough formed in a divergence zone or other area of tension. |
| Right-lateral fault | A strike-slip fault on which the displacement of the far block is to the right when viewed from either side. |
| Ring dike | A dike in the form of a segment of a cone or cylinder, having an arcuare outcrop. |
| Rip current | A current that flows strongly away from the sea shore through gaps in the surf zone at intervals along the shoreline. |
| Ripple | A very small dune of sand or silt whose long dimension is formed at right angles to the current. River order |
| Rock cycle | The geologic cycle, with emphasis on the rocks produced; sedimentary rocks are metamorphosed to metamorphic rocks, or melted to create igneous rocks, and all rocks may be uplifted and eroded to make sediments, which lithify to sedimentary rocks. |
| Rock flour | A glacial sediment of extremely fine (silt-and clay-size) ground rock formed by abrasion of rocks at the base of the glacier. |
| Rock glacier | A glacier-like mass of rock fragments or talus with interstitial ice that moves downhill under the force of gravity. |
| Rockslide | A landslide involving mainly large blocks of detached bedrock with little or no soil or sand. Rounding |
| Runoff | The amount of rain water directly leaving an area in surface drainage, as opposed to the amount that seeps out as groundwater. |
| Rupture strength | The greatest stress that a material can sustain without fracturing at one atmosphere pressure. |
| Saltation | The movement of sand or fine sediment by short jumps above the ground or stream bed under the influence of a current too weak to keep it permanently suspended. |
| Sandblasting | A physical weathering process in which rock is eroded by the impact of sand grains carried by the wind, frequently leading to ventifact formation of pebbles and cobbles. |
| Sandstone | A detrital sedimentary rock composed of grains from 1/16 to 2 millimeters in diameter, dominated in most sandstones by quartz, feldspar, and rock fragments, bound together by a cement of silica, carbonate, or other minerals or a matrix of clay minerals. |
| Schist | A metamorphic rock characterized by strong foliation or schistosity. |
| Schistosity | The parallel arrangement of shaly or prismatic minerals like micas and amphiboles resulting from nonhydrostatic stress in metamorphism. |
| Scoria | Congealed lava, usually of mafic composition, with a large number of vesicles formed by gases coming out of solution. |
| Sea-floor spreading | The mechanism by which new sea floor crust is created at ridges in divergence zones and adjacent plates are moved apart to make room. This process may continue at 0.5 to 10 centimeters/year through many geologic periods. |
| Seamount | An isolated tall mountain on the sea floor that may extend more than 1 kilometer from base to peak (see also Guyot). |
| Secular variation | Slow changes in the orientation of the Earth's magnetic field that appear to be long lasting and internal in origin as opposed to rapid fluctuations, which are external in origin. |
| Sedimentary rock | A rock formed by the accumulation and cementation of mineral grains transported by wind, water, or ice to the site of deposition or chemically precipitated at the depositional site. |
| Sedimentary structure | Any structure of a sedimentary or weakly metamorphosed rock that was formed at the time of deposition; includes bedding, cross-bedding, graded bedding, ripples, scour marks, mud-cracks. |
| Sedimentation | The process of deposition of mineral grains or precipitates in beds or other accumulations. Seif dune |
| Seismic discontinuity | A surface within the Earth across which P-wave or S-wave velocities change rapidly, usually by more than +~0.2 kilometer/second. |
| Seismicity | The world-wide or local distribution of earthquakes in space and time; a general term for the number of earthquakes in a unit of time. |
| Seismic profile | The data collected from a set of seismographs arranged in a straight line with an artificial seismic source, especially the times of P-wave arrivals. |
| Seismic reflection | A mode of seismic prospecting in which the seismic profile is examined for waves that have reflected from near-horizontal strata below the surface. |
| Seismic refraction | A mode of seismic prospecting in which the seismic profile is examined for waves that have been refracted upward from seismic discontinuities below the profile. Greater depths may be reached than through seismic reflection. |
| Seismic surface wave | A seismic wave that follows the earth's surface only, with a speed less than that of S-waves. There are Raleigh waves (forward and vertical vibrations) and Love waves (transverse vibrations). |
| Seismic transition zone | A seismic discontinuity, found in all parts of the Earth, at which the velocity increases rapidly with depth; especially the one at 300 to 600 kilometers. |
| Stratification | A structure of sedimentary rocks, which have recognizable parallel beds of considerable lateral extent. |
| Stratigraphic sequence | A set of beds deposited that reflects the geologic history of a region. |
| Stratigraphy | The science of the description, correlation, and classification of strata in sedimentary rocks, including the interpretation of the depositional environments of those strata. |
| Stratovolcano | A volcanic cone consisting of both lava and pyroclastic rocks, often conical. |
| Streak | The fine deposit of mineral dust left on an abrasive surface when a mineral is scraped across it; especially the characteristic color of the dust. |
| Streak plate | A ceramic abrasive surface for streak tests. |
| Streaming flow | A tranquil flow slower than shooting flow. |
| Streamline | A curved line representing the successive positions of a particle in a flow as time passes. |
| Stream order | The hierarchical number of a stream segment in dendritic drainage |
| Stress | A quantity describing the forces acting on each part of a body in units of force per unit area. Striation |
| Strike | The angle between true North and the horizontal line contained in any planar feature (inclined bed, dike, fault plane, etc.); also the geographic direction of this horizontal line. |
| Strike-slip fault | A fault whose relative displacement is purely horizontal. |
| Stromatolite | A fossil form representing the growth habit of an algal mat |
| Subduction zone | A dipping planar zone descending away from a trench and defined by high seismicity, interpreted as the shear zone between a sinking oceanic plate and an overriding plate. |
| Sublimation | A phase change from the solid to the gaseous state, without passing through the liquid state. |
| Submarine canyon | An underwater canyon in the continental shelf. |
| Subsidence | A gentle epeirogenic movement where a broad area of the crust sinks without appreciable deformation. |
| Superposed stream | A stream that flows through resistant formations because its course was established at a higher level on uniform rocks before down-cutting began. |
| Superposition, Principle of | The principle stated by Steno that, except in extremely deformed strata, a bed that overlies another bed is always the younger. |
| Supersaturation | The unstable state of a solution that contains more solute than its solubility allows. |
| Surf | The breaking or tumbling forward of water waves as they approach the shore. |
| Surf zone | An offshore belt along which the waves collapse into breakers as they approach the shore. |
| Suspended load | The fine sediment kept suspended in a stream because the settling velocity is lower than the upward velocity of eddies. |
| Swash | The landward rush of water from a breaking wave up the slope of the beach. |
| S-wave | The secondary seismic wave, traveling slower than the P-wave, and consisting of elastic vibrations transverse to the direction of travel. It cannot penetrate a liquid. |
| Swell | An oceanic water wave with a wavelength on the order of 30 meters or more and a height of perhaps 2 meters or less that may travel great distances from its source. |
| Symbiosis | The interaction of two mutually supporting species that do not compete with or prey upon each other. |
| Syncline | A large fold whose limbs are higher than its center; a fold with the youngest strata in the center. |
| System (stratigraphy) | A stratigraphic unit larger than a series, consisting of all the rocks deposited in one period of an era. |
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