Geological Terms Beginning With "P"
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A Hawaiian term for a lava flow that has a surface flow structure that looks like coiled rope or cord. Lavas with a relatively low viscosity can form a "ropy" surface. The term is also used for the rock materials after the flow has cooled.
The climate of a given area at a specific time in the past. Paleoclimates of an area can be determined by examining the characteristics of sedimentary rocks such as their fossils, minerals, sedimentary structures. Paleoclimates can also be determined from ice cores. The photo shows pollen grains that can be used to determine climatic conditions in their source area.
A map that shows the directions of currents at the time of sediment deposition. These directions can be determined through the study of cross bedding, ripple marks, tool marks and other sedimentary structures. The image is a field-note representation of the rock types present at a given location and the paleocurrent directions inferred from sedimentary structures.
A map that shows the distribution of sedimentary environments at a specific time in the past. These maps are made by studying the rock record to correlate rock units that were deposited at the same time, then relating rock characteristics to specific sedimentary environments. They can also show the relative positions of landmasses on the face of the Earth at specific times in the past. The map at left shows the relative position of Earth's landmasses and seas approximately 210 million years ago.
The study of Earth's magnetic field over time. When rocks that contain magnetic minerals are deposited, the character (vertical and horizontal orientation along with polarity) of Earth's magnetic field is locked within the rocks. This information can be used to study changes in Earth's magnetic field as well as the movement of plates over time. The image shows the current configuration of Earth's magnetic field.
The study of the changes in Earth's life over space and time through the study of rock units and the fossils contained within them.
A large continental landmass that existed from about 300 million years ago through about 200 million years ago. It included most of the continental lithosphere present at that time. It has since broken up and the fragments have drifted to become the configuration of Earth's present day continents.
The ancient ocean that surrounded the Pangaea landmass.
Sand dunes with a U-shaped outline. They have a convex nose and two trailing horns that point upwind. Parabolic dunes form in areas with a single dominant wind direction that blows up the gentle slope on the trailing side of the dune and drops the sand on the steep side of the dune. The dunes move slowly down wind and are common in coastal areas.
A radioactive element that spontaneously decays into a new substance. The product of this decay is known as a "daughter" element.
The maximum instantaneous discharge of a stream at a specific location. Corresponds to the highest stage of a flood.
An accumulation of unconsolidated plant debris that if buried and preserved could become coal. Special conditions are required to accumulate and preserve plant materials. These conditions are most often found in a marsh or swamp where water cover prevents oxidation and attack by most organisms.
A broad, gently sloping erosional surface of low local relief adjacent to an eroding cliff or mountain range. The area is likely covered with sediments.
A very coarse grained igneous rock, normally of granitic composition. Typically forms during the final states of magma chamber crystallization when the high water content solutions allow rapid crystal growth.
A ocean sediment that accumulates far enough from land that detrital materials are a minor component. These sediments are largely composed of the tiny shell debris of radiolarians and foraminifera.
Perched Water Table
A water table that is isolated from and higher than the regional water table. This can occur when a hilltop is underlain by an impermeable rock unit. Infiltrating waters stack up on the impermeable unit, creating an isolated water table that is higher than the water table of the surrounding land.
A dark-colored, coarse-grained igneous rock that is made up mainly of olivine and pyroxene, with very little quartz or feldspar.
A unit of geologic time that is longer than an epoch but shorter than an era. From oldest to youngest, Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary are periods of the geologic time scale,
A measure of how well a material can transmit water. Materials such as gravel, that transmit water quickly, have high values of permeability. Materials such as shale, that transmit water poorly, have low values. Permeability is primarily determined by the size of the pore spaces and their degree of interconnection. Permeability measures are expressed in units of velocity, such as centimeters per second, and assume a gradient of one vertical foot of drop per linear foot.
Organic and inorganic compounds and mixtures that are derived from petroleum. These include: organic chemicals, cyclic interŽmediates, plastics, resins, synthetic fibers, elastomers, organic dyes, organic pigments, detergents, surface active agents, carbon black, and ammonia.
A group of liquid hydrocarbons that includes: crude oil, lease condensate, unfinished oils, refined products obtained from the processing of crude oil, and natural gas liquids.
A relative measure of the acidity or alkalinity of a water based upon a scale that ranges between 0 and 14 with 7 being neutral. Values of pH below 7 indicate acid solutions and values of pH above 7 indicate basic solutions.
A thin slice of rock prepared for microscopic examination. It is prepared by gluing a small cube of rock to a microscope slide and cutting/grinding it to such a thinness that light will pass through the non-opaque minerals. Then, when observed in reflected and transmitted light using various filters, the mineral and other components of the rock can be identified by a skilled observer. Quantifying the relative abundances of the minerals will allow the rock to be more specifically named and its history understood. The image here is a photomicrograph of basalt in polarized light with visible plagioclase (shades of gray) and olivine (orange-yellow-green) crystals visible.
An explosive volcanic eruption initiated by the interaction of magma and water (usually either meteoric or groundwater).
A foliate metamorphic rock that is made up mainly of very fine-grained mica. The surface of phyllite is typically lustrous and sometimes wrinkled. It is intermediate in grade between slate and schist.
A general term applied to a variety of weathering processes that result in the particle size reduction of rock materials with no change in composition. Frost action, salt crystal growth and pressure relief fracturing are examples. Also known as mechanical weathering.
A mass of stream sediment that contains an economically significant concentration of mineral particles. This accumulation of mineral particles is a result of their being of high specific gravity or resistant to abrasion. Gold, magnetite, and diamonds can be found in placer deposits.
A sequence of parallel to subparallel basalt flows that were formed during a geologically brief interval of time and which covered an extensive geographic area. Thought to have formed from simultaneous or successive fissure eruptions.
Play of Color
A display of spectral colors produced with light passing through a gemstone encounters a regular pattern of tiny spheres, layers or other structures that diffract the light into its component colors. Opal is especially well-known for its "play-of-color" in which an observer can move the stone under a source of light and see the colorful flashes as the angle of light in the stone is changed. The colors will also change if the observer's eye moves or if the source of the light is moved. It is this play-of-color that defines precious opal. Without that display of color the opal is "common".
A change of a gem's body color when it is viewed from different crystallographic directions. As an example, if you have a crystal of transparent iolite, hold it between your eye and a source of light and rotate the stone through 360 degrees, the color of the stone will change with every 90 degrees of rotation. The color of the specimen depends upon the angle of observation. When pleochroic stones are faceted or cut en cabochon the cutter must be knowledgeable of the pleochroic characteristics of the stone so it can be oriented in a way that shows optimal color to the observer.
An explosive eruption where large volumes of rock, ash and gas are blasted at high velocity from a vent. These eruptions produce huge clouds that can rise tens of kilometers into the atmosphere.
Water contamination that can be traced to a single point. A toxic material spill and a sewage discharge pipe are examples of point sources.
An interval of time between reversals of Earth's magnetic field.
A specific event in the history of Earth's magnetic field. Usually used in reference to a specific polarity reversal.
A change in the polarity of Earth's magnetic field in which the north magnetic pole becomes the south magnetic pole and vice versa. Also known as geomagnetic reversal or magnetic reversal. Earth's magnetic field has reversed many times in the past and the time intervals between these changes are known as polarity epochs.
The volume of pore space in a rock, sediment or soil. Usually expressed as a percentage. This pore space can include openings between grains, fracture openings and caverns.
Water that is of adequate quality for drinking.
A cylindrical or hemispherical hold in the bedrock of a stream that is formed from the continual swirling motion of sand and gravel by swirling currents.
An abbreviation for parts per million.
Movement of water from the atmosphere to the land or to a surface water body. Rain, hail, snow, dew, and sleet are all examples of precipitation.
Any crude oil or natural gas that is recovered from a well as a result of the natural pressure within the reservoir.
Primary Seismic Waves
The fastest set of earthquake vibrations - also known as P-waves. They move through the Earth in compression and expansion motions (much like sound waves move through air). Called primary because they are the first recorded at a seismograph. Primary waves are able to travel through both solids and liquids.
The activities associated with the search for an area of probable mineralization. It can include: topographical, geological, geochemical and geophysical studies. Prospecting is usually done prior to the acquisition of mineral rights.
A subatomic particle, contained in the nucleus of an atom. It has a positive electrical charge and a mass similar to that of a neutron.
An intermediate stage in the development of a star in which a large cloud of dust and gases gradually condenses through gravitational actions.
Mineral deposits that have been explored thoroughly enough to be quantified but which are still in the ground.
A vesicular volcanic glass of granitic composition. It has so many vesicles that it has a very low specific gravity - sometimes low enough to float on water.
A pump used on an oil well when there is not enough bottom hole pressure to push the oil to Earth's surface. The pumpjack is a mechanical device that converts the rotary action of a motor to a lifting action needed for a reciprocating piston pump. They can be powered by electricity but where that is not available, gasoline, diesel, propane and natural gas are used.
Primary seismic waves. The fastest set of earthquake vibrations. They move through the Earth in compression and expansion motions (much like sound waves move through air). Called primary because they are the first recorded at a seismograph. Primary waves are able to travel through both solids and liquids.
The word "pyroclastic" is an adjective used for volcanic clasts that are blown from a volcanic vent. Rocks and rock materials of a volcanic origin are also called "pyroclastics".
A hot, high-velocity mixture of ash, gas and fragmented rock that flows like a liquid down slopes and over terrain.
A coarse-grained contact metamorphic rock that is formed at high temperatures and low pressures and which is rich in pyroxene minerals.
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