Thursday, November 23rd, 2017

* P wave-See "primary (P) wave."
* Pahoehoe-A basaltic lava flow with a smooth, billowy, or "ropy" surface.
* Paleoclimatology-The study of ancient climates.
* Paleontology-The study of life that existed in the past.
* Paleozoic -A geological term denoting the time in Earth history between 540 and 245 million years ago.
* Paleozoic era-The part of geologic time 570 to 245 million years ago. During this era invertebrates, fishes, amphibians, reptiles, ferns, and cone-bearing trees were dominant.
* Parabolic dune-A crescent-shaped dune with tips pointing into the wind.
* Parallax-The apparent displacement of an object caused by the movement of the observer.
* Parallelism of the axis - the behavior of Earth's axis as it orbits the Sun. The axis always points to Polaris. The axis at any point on Earth's orbit is parallel to the axis at any other point.
* Parent isotope-A radioactive isotope that decays to produce a daughter isotope.
* Parent material- rock material from which soil is formed.
* Parent rock-Any original rock before it is changed by weathering, metamorphism, or other geological processes.
* Parsec-The distance to an object that would have a stellar parallax angle of 1 arc second. One parsec is about 3.2 light years or 3 x 1013 km.
* Partial melting-The process in which a silicate rock only partly melts as it is heated, to form magma that is more silica rich than the original rock.
* Particles-In air pollution terminology, any pollutant larger than a molecule.
* Passive continental margin-A margin characterized by a firm connection between continental and oceanic crust where little tectonic activity occurs.
* Paternoster lake-One of a series of lakes, strung out like beads and connected by short streams and waterfalls, created by glacial erosion.
* Peat-A loose, unconsolidated, brownish mass of partially decayed plant matter; a precursor to coal.
* Pebble-A sedimentary particle between 2 and 64 millimeters in diameter, larger than sand and smaller than a cobble.
* Pedalfer-A common soil type that forms in humid environments, characterized by abundant iron and aluminum oxides and a concentration of clay in the B horizon.
* Pediment-A gently sloping erosional surface, that forms along a mountain front uphill from a bajada, usually covered by a patchy veneer of gravel only a few meters thick.
* Pedocal-A soil formed in arid and semiarid climates characterized by an accumulation of calcium carbonate.
* Pegmatite-An exceptionally coarse-grained igneous rock, usually with the same mineral content as granite.
* Pelagic sediment-Muddy ocean sediment that consist of a mixture of clay and the skeletons of microscopic marine organisms.
* Penumbra - A) The outer filamentary region of a sunspot. B) The blurry edges of a shadow
* Penumbra-A wide band outside of the umbra, where only a portion of the Sun is eclipsed during a solar eclipse.
* Perched water table-The top of a localized lens of groundwater that lies above the main water table, formed by a layer of impermeable rock or clay.
* Peridotite-A coarse-grained plutonic rock composed mainly of olivine; it may also contain pyroxene, amphibole, or mica but little or no feldspar. The mantle is thought to be made of peridotite.
* Perigee -The point in the orbit closest to the Earth.
* Perihelion -The point in its orbit where a planet is closest to the Sun.
* Period-A geologic-time unit longer than an epoch and shorter than an era.
* Period-A) the amount of time (in Earth years) it takes a planet to orbit the sun once. B) a subdivision of a geologic era.
* Permafrost-A layer of permanently frozen soil or subsoil which lies from about a half meter to a few meters beneath the surface in arctic environments.
* Permeability- a measure of a rock's ability to transmit water or other liquids.
* Permeability-A measure of the speed at which fluid can travel through a porous material.
* Perturb -To cause a planet or satellite to deviate from a theoretically regular orbital motion.
* Petroleum-A natural occurring liquid composed of a complex mixture of hydrocarbons.
* Phanerozoic Eon-The most recent 570 million years of geologic time represented by rocks that contain evident and abundant fossils.
* Phases- A) daily changes in the moon's appearance. B) states of matter (solid, liquid, gas, etc.).
* Phenocryst-A large, early formed crystals in a finer matrix in igneous rock.
* Photon-The smallest particle or packet of electromagnetic energy, such as light, infrared radiation, etc.
* Photosphere-The surface of the Sun visible from Earth.
* Phyllite-A metamorphic rock with a silky appearance and commonly wrinkled surface, intermediate in grade between slate and schist.
* Physical weathering (mechanical weathering) - disintegration that takes place when rock is broken into smaller pieces without changing its chemical composition.
* Pillow basalt-Lava that solidified under water, forming spheroidal lumps like a stack of pillows.
* Pillow lava- Interconnected, sack-like bodies of lava formed underwater.
* Placer deposit-A surface mineral deposit formed by the mechanical concentration of mineral particles (usually by water) from weathered debris.
* Plains- Vast, flat areas with low elevation.
* Planetary nebula-A nebula created when a star the size of our Sun explodes.
* Plasma -A low-density gas in which the individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality.
* Plastic- Capable of being molded into any form, which is retained.
* Plastic deformation-A permanent change in the original shape of a solid that occurs without fracture.
* Plastic flow-See "plastic deformation."
* Plate boundary-A boundary between two lithospheric plates.
* Plate Tectonics- The theory that the earth's crust is broken into about 10 fragments (plates,) which move in relation to one another, shifting continents, forming new ocean crust, and stimulating volcanic eruptions.
* Plate tectonics theory-A theory of global tectonics in which the lithosphere is segmented into several plates that move about relative to one another by floating on and gliding over the plastic asthenosphere. Seismic and tectonic activity occur mainly at the plate boundaries.
* Plate-A relatively rigid independent segment of the lithosphere that can move independently of other plates.
* Plateau-A large elevated area of comparatively flat land.
* Platform-The part of a continent covered by a thin layer of nearly horizontal sedimentary rocks overlying older igneous and metamorphic rocks of the craton.
* Playa lake-An intermittent desert lake.
* Playa-A dry desert lake bed.
* Pleistocene- A epoch in Earth history from about 1.6 million years to 10,000 years ago. Also refers to the rocks and sediment deposited in that epoch.
* Pleistocene Ice Age-A span of time from roughly 2 or 3 million to 8,000 years ago characterized by several advances and retreats of glaciers.
* Plucking-A process in which glacial ice erodes rock by loosening particles and then lifting and carrying them downslope.
* Plume (of pollution)-The 3-dimensional zone of an aquifer or of surface water affected by a dispersing pollutant.
* Pluton- A large igneous intrusion formed at great depth in the crust (plutonic).
* Pluton-An igneous intrusion.
* Plutonic rock-An igneous rock that forms deep (a kilometer or more) beneath the Earth's surface.
* Pluvial lake-A lake formed during a time of abundant precipitation. Many pluvial lakes formed as continental ice sheets melted.
* Point bar-A stream deposit located on the inside of a growing meander.
* Point source pollution-Pollution which arises from a specific site such as a septic tank or a factory.
* Polar Front- The boundary separating air masses of tropical origin from those of polar origin. The front is usually only present in the far-northern polar regions in summer but dips down to the more southerly temperate latitudes during the winter. Many cyclonic storm systems form along the polar front.
* Polarity-The magnetically positive (north) or negative (south) character of a magnetic pole.
* Pollution-The reduction of the quality of a resource by the introduction of impurities.
* Population I star-A relatively young star, formed from material ejected by an older, dying star, composed mainly of hydrogen and helium, with 1 percent heavier elements. Our Sun is a population I star.
* Population II star-An old star with lower concentration of heavy elements than a population I star.
* Pore space-The open space between grains in rock, sediment, or soil.
* Porosity- percentage of a material's volume that is pore space.
* Porosity-The proportion of the volume of a material that consists of open spaces.
* Porphyry copper-A large body of porphyritic igneous rock that contains disseminated copper sulfide minerals, and is usually mined by open pit methods.
* Porphyry-Any igneous rock containing larger crystals (phenocrysts) in a relatively fine-grained matrix.
* 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.
* Precambrian -A geological term denoting the time in Earth history prior to 540 million years ago.
* Precambrian-All of geologic time before the Paleozoic era, encompassing approximately the first 4 billion years of Earth's history. Also, all rocks formed during that time.
* Precession-The circling or wobbling of the tilt of the Earth's orbit.
* Precipitation- A) Any condensed water falling from the atmosphere to the surface of the earth. Common types include rain, snow, sleet, and hail. B) Minerals that were once dissolved in the water settle to the bottom of a body of water.
* Precipitation-(1) A chemical reaction that produces a solid salt, or precipitate, from a solution. (2) Any form in which atmospheric moisture returns to the Earth's surface--rain, snow, hail, and sleet.
* Pressure gradient-The change in air pressure over distance.
* Pressure release melting-The melting of rock and the resulting formation of magma caused by a drop in pressure at constant temperature.
* Primary (P) wave-A seismic wave formed by alternate compression and expansion of rock. P waves travel faster than any other seismic waves.
* Primary air pollutant-A pollutant released directly into the atmosphere.
* Principle of cross-cutting relationships-The principle that a rock or feature must first exist before anything can happen to it, or another rock cuts across it.
* Principle of faunal succession-The principle that fossil organisms succeed one another in a definite and recognizable sequence, so that sedimentary rocks of different ages contain different fossils, and rocks of the same age contain identical fossils. Therefore, the relative ages of rocks can be identified from their fossils.
* Principle of original horizontality-The principle that most sediment is deposited as nearly horizontal beds, and therefore most sedimentary rocks started out with nearly horizontal layering.
* Principle of superposition-The principle that states that in any undisturbed sequence of sediment or sedimentary rocks, the age becomes progressively younger from bottom to top.
* Profile- a "side view" created from a topographic map. (Topographic maps show a "bird's eye view".)
* Prominence-A flame-like jet of gas rising from the Sun's corona.
* Proterozoic Eon-The portion of geological time from 2.5 billion to 570 million years ago.
* Proton-A dense, massive, positively charged particle found in the nucleus of an atom.
* Protoplanets- whirling gaseous masses within a giant cloud of gas and dust rotating around a star; thought to have given rise to the planets and moons of our solar system.
* Protoplanets-The planets in their earliest, incipient stage of formation.
* Protosun-The Sun in its earliest, incipient stage of formation. The protosun was a condensing agglomeration of dust and gas.
* Psychrometer- An instrument used to measure water vapor content of the atmosphere. A common type consists of two liquid-in-glass thermometers, one a dry bulb and another, with a cloth sock over the tip which is saturated with water, called the wet bulb. The thermometers are ventilated to cause evaporation off the wet bulb thermometer. The atmospheric humidity is determined from the readings of these two thermometers
* Pulsar-A neutron star that emits a pulsating radio signal.
* Pumice -A light vesicular form of volcanic glass with a high silica content; it is usually light in color and will float on water.
* Pumice-Frothy, usually rhyolitic magma solidified into a rock so full of gas bubbles that it can float on water.
* P-waves - primary (compressional) earthquake waves, that can travel through any material.
* Pyroclastic Flow- Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (50 to 100 miles an hour.) The term also can refer to the deposit so formed.
* Pyroclastic -Pertaining to clastic (broken and fragmented) rock material formed by volcanic explosion or aerial expulsion from a volcanic vent.
* Pyroclastic rock-Any rock made up to material ejected explosively from a volcanic vent.
* Pyroxene-A rock-forming silicate mineral group that consists of many similar minerals. Members of the pyroxene group are major constituents of basalt and gabbro.

* Quartz-A rock-forming silicate mineral, sio2. Quartz is a widespread and abundant component of continental rocks but is rare in the oceanic crust and mantle.
* Quartzite-A metamorphic rock composed mostly quartz formed by of recrystallization of sandstone.
* Quasar-An object, less than one light year in diameter and very distant from Earth, that emits an extremely large quantity of energy.
* Quasars - very distant radio sources that resemble stars but are far larger, more luminous, and more massive; also called quasi-stellar radio sources.
* Quaternary- The period of Earth's history from about 1.6 million years ago to the present; also, the rocks and deposits of that age.

* Radiation -Energy radiated in the form of waves or particles; photons.
* Radiative zone-The zone of a star surrounding the core where energy is transmitted by absorption and radiation.
* Radio waves- A type of electromagnetic radiation which has the lowest frequency, the longest wavelength, and is produced by charged particles moving back and forth.
* Radioactive decay- process by which unstable atoms break down into new, lighter atoms.
* Radioactivity-The natural spontaneous decay of unstable nuclei.
* Radiometric dating-The process of measuring the absolute age of geologic material by measuring the concentrations of radioactive isotopes and their decay products.
* Radiosonde- An instrument package connected to a weather balloon that collects, and transmits by radio, meteorological data as it ascends through the atmosphere.
* Rain Shadow- The region on the lee side of a mountain or mountain range, where precipitation is decreased compared to the windward side. The rain shadow results when moisture-laden air is forced to ascend the encircling mountains and thus drops most of that moisture on the windward side of the range summits. As the air descends from the ridge, it is drier and warmer. Not only does the compression of descent warm the air, it also evaporates much of the liquid water, causing clouds to disappear. The downward motion of the air masse further inhibits cloud formation on the lee of these ridges.
* Rain-shadow desert-A desert formed on the lee side of a mountain range.
* Recessional moraine-A moraine that forms at the terminus of a glacier as the glacier stabilizes temporarily during retreat.
* Recharge-The replenishment of an aquifer by the addition of water.
* Red giant -A star that has low surface temperature and a diameter that is large relative to the Sun.
* Red giant-A stage in the life of a star when its core is composed of helium that is not undergoing fusion. A hot shell of hydrogen around this core is fusing at a rapid rate, producing enough energy to cause the star to expand.
* Red shift- When light coming from a distant star is seen through a spectroscope (an instrument that separates light into its different colors), the light we receive continues to shift toward the red area of the spectrum, which is the least powerful. This means that, since the light is becoming weaker and weaker, the stars must be traveling away from us. This makes scientists believe that our universe is expanding.
* Red shift-The frequency shift of light waves observed in the spectrum of an object traveling away from an observer. This shift is caused by the Doppler effect.
* Red spot-A swirling cloud on the planet Jupiter is a raging storm of gases, mainly red phosphorus.
* Red stars- The coolest stars are red. Their surface temperature is less than 5,500°F.
* Reef-A wave-resistant ridge or mound built by corals or other marine organisms.
* Reflecting telescope-A type of telescope that uses a mirror or mirrors to form the primary image.
* Reflection-The return of a wave that strikes a surface.
* Refracting telescope-A type of telescope where the primary image is formed by a lens or lenses.
* Refraction-The bending of a wave that occurs when the wave changes velocity as it passes from one medium to another.
* Regional metamorphism - process of rock-forming that results from large areas of rocks being under the intense heat and pressure of mountain-building movements.
* Regional metamorphism-Metamorphism that is broadly regional in extent, involving very large areas and volumes of rock. Includes both regional dynamothermal and regional burial metamorphism.
* Regolith-The loose, unconsolidated, weathered material that overlies bedrock.
* Relative age-Time expressed as the order in which rocks formed and geological events occurred, but not measured in years.
* Relative dating - A technique geologists use to assign a sequential order to the age of rocks and geologic events. The oldest comes first, and subsequent events follow on a relative dating line.
* Relative Humidity- The ratio of the actual water vapor content of a given mass of air to that it would hold at saturation, usually expressed as a percentage. Also, the ratio of the water vapor pressure to the its saturation vapor pressure.
* Relative humidity-See "humidity."
* Relief- The vertical difference between the summit of a mountain and the adjacent valley or plain.
* Remediation-The treatment of a contaminated substance to remove or decompose a pollutant.
* Remote sensing-The collection of information about an object by instruments that are not in direct contact with it.
* Renewable resources - a resource that can be replaced in nature at a rate close to its rate of use.(oxygen, trees, food, solar energy)
* Reserves-Known geological deposits that can be extracted profitably under current conditions.
* Reservoir-Porous and permeable rock in which liquid petroleum or gas accumulate.
* Residual deposit-A mineral deposit formed when water dissolves and removes most of the soluble ions from soil and rock near the Earth's surface, leaving only relatively insoluble ions in the soil. Bauxite, the principal source of aluminum, forms as a residual deposit, and in some instances iron also concentrates by this process to become ore.
* Residual soil- soil that has the local bedrock as its parent material. It was never eroded to another location. It stayed where it formed.
* Resolution-The ability of an optical telescope to distinguish details
* Retrograde motion-Backwards (westwards) motion of a planet with respect to the stars.
* Retrograde motion-The apparent motion of the planets where they appear to move backwards (westward) with respect to the stars.
* Reverse fault-A fault in which the hanging wall has moved up relative to the footwall.
* Reversed polarity-Magnetic orientations in rock which are opposite to the present orientation of the Earth's field. Also, the condition in which the Earth's magnetic field is opposite to its present orientation.
* Revolution- the movement of an object in its orbit around another object.
* Revolutional period-The length of time is takes a planet or moon to cycle around the object that it orbits. Earth's revolutional period is 365 days.
* Revolve-To orbit a central point. A satellite revolves around the Earth, (see "rotate").
* Rhyolite -Fine-grained extrusive igneous rock, commonly composed of quartz and feldspar
* Rhyolite-A fine-grained extrusive igneous rock compositionally equivalent to granite.
* Richter scale - a numerical description of an earthquake's magnitude.
* Richter scale-A numerical scale of earthquake magnitude measured by the amplitude of the largest wave on a standardized seismograph.
* Ridge, Oceanic- A major submarine mountain range.
* Rift -A fracture or crack in a planet's surface caused by extension. On some volcanoes, subsurface intrusions are concentrated in certain directions; this causes tension at the surface and also means that there will be more eruptions in these "rift zones."
* Rift System- The oceanic ridges formed where tectonic plates are separating and a new crust is being created; also, their on-land counterparts such as the East African Rift.
* Rift valley-An elongate depression that develops at a divergent plate boundary. Examples include continental rift valleys and the rift valley along the center of the mid-oceanic ridge system.
* Rift zone-A zone of separation of tectonic plates at a divergent plate boundary.
* Right-hand rule-A rule to help you determine which way is north on a planet. Point your fingers in the direction the planet rotates. Your fingers should curl around the planet, like you are grabbing a ball. Then stick out your thumb. That way is north.
* Ring of Fire- The regions of mountain-building earthquakes and volcanoes which surround the Pacific Ocean.
* Ring of fire-The belt of subduction zones and major tectonic activity including extensive volcanism that borders the Pacific Ocean along the continental margins of Asia and the Americas.
* Rip current - a strong surface current that flows away from the beach.
* Rip current-A current created when water flows back toward the sea after a wave breaks against the shore. (syn; undertow)
* River Valley- a V-shaped valley formed by erosion from running water.
* Rock avalanche-A type of mass wasting in which a segment of bedrock slides over a tilted bedding plane or fracture. The moving mass usually breaks into fragments. (syn; rockslide)
* Rock cycle-The sequence of events in which rocks are formed, destroyed, altered, and reformed by geological processes.
* Rock dredge-An open-mouthed steel net dragged along the sea floor behind a research ship for the purpose of sampling rocks from submarine outcrops.
* Rock flour- Finely ground rock material, usually associated with glaciers (or faults). Can be mixed with water and formed into loaves which, when baked for 45 minutes at 350 deg., are totally inedible. Don't try it.
* Rock flour-Finely ground, silt-sized rock fragments formed by glacial abrasion.
* Rock-A naturally formed solid that is an aggregate of one or more different minerals.
* Rock-forming minerals-The most abundant minerals in the Earth's crust; the nine minerals or mineral "groups" that make up most rocks of the Earth's crust. They are olivine, pyroxene, amphibole, mica, the clay minerals, quartz, feldspar, calcite, and dolomite.
* Rocks - mixtures of minerals
* Rockslide-A type of slide in which a segment of bedrock slides along a tilted bedding plane or fracture. The moving mass usually breaks into fragments. (syn; rock avalanche)
* Rotate-To turn or spin on an axis. Tops and planets rotate on their axes, (see "revolve").
* Rotation- The spinning of an object on its axis.
* Rotational period-The length of time it takes a planet or moon to rotate 360 degrees on its axis. Earth's rotational period is 24 hours.
* Rubble-Angular particles with a diameter greater than 2 millimeters.
* Runoff- The movement of water across the earth's surface or just beneath the surface coming from liquid precipitation or the melting of ice/snow. Runoff ultimately reaches stream channels, rivers, lakes, or seas unless it is evaporated or taken up by the biosphere.
* Runoff-Water that flows back to the oceans in surface streams.