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Glossary of Geology Terms
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Laccolith A large, dome-shaped space formed when an intrusive igneous body has pushed in between strata with force enough to arch up the overlying beds. It has a flat bottom and a domed top and is parallel to the layers above and below it. A laccolith is typically composed of granite. The country rock above it often erodes away.

Landform Any conspicuous, surface feature of the Earth's surface of natural origin. Examples include a hill, plain, valley, and alluvial fan.

Land Gate A way or route over land.

Lapilli Small round to angular unconsolidated volcanic rock fragments with diameters of 4 to 32 mm that were ejected into the air in either a solid or molten state.

Lateral Moraine A low ridge of glacial debris formed along the side of a glacier by erosion and an avalanche from the valley wall. It is deposited as a ridge as the glacier recedes. See also Moraine

Lava Magma (molten rock) that has reached the Earth's surface through a volcanic eruption. The term is used most commonly in reference to the streams of liquid rock that flow from a crater or fissure. It is also used for the cooled and solidified rock. When cooled, the lava forms igneous rock.

Leaching A geological term, which denotes the loss of soluble substances, including plant nutrients, from the upper layer of soil by percolating precipitation. These materials are carried downward through the soil and generally are redeposited in a lower layer. The rate of leaching is greater in areas of high temperature and rainfall. It is also influenced by the amount of protective vegetation. Leaching also denotes the removal of liquid waste products of sewage by percolation through the soil.

Lead (Pb) A heavy, soft, gray, malleable, ductile, inelastic, corrosion resistant, metallic element. It has an atomic number 82. There are four stable isotopes of lead. They have atomic weights of 204, 206, 207, and 208 and appear in nature in the following relative concentrations: 1.4%, 24.1%, 22.1%, and 52.4%, respectively. Most lead is obtained from ores: galena, anglesite, cerrusite, and minum.

Levee An embankment, particularly along the edge of a river, stream, or channel, which has been built to protect against flooding. Also, any natural embankment, such as those formed by mud or lava flows or by sediment tracing the edge of a stream, or by sediment left behind when floodwater, which had overflowed the channel, recedes. Further, levee describes an embankment or rise used to control the direction of water flow in irrigation. See also Dike, Flood Plain

Lignite A soft, brownish to blackish, imperfectly formed coal that has a distinct woody texture. It is formed by the further compression of peat. Lignite is a product that is intermediate between peat and bituminous coal. Dry lignite contains about 60 to 75 percent carbon. Raw lignite has a water content as high as 60 percent, some of which is lost through weathering. Many lignite beds lie close to the surface and can be easily worked. Although 45 percent of the world's total coal reserves are lignite, it is inferior to bituminous coal in heating value and other properties. See also Anthracite, Bituminous Coal.

Limestone A sedimentary rock that consists mainly of calcium carbonate, primarily in the form of calcite. Limestone is normally formed from shells of dead marine life. It is estimated that limestone accounts for 10% to 15% of all sedimentary rocks.

Limonite An important iron ore. Limonite is a hydrated ferric oxide, that ranges in color from yellow to dark brown. Weathering imparts this color. Rust.

Limy Consisting, containing, or like lime. An adjective that describes sediments and rocks that contain a large amount of lime, calcium oxide (CaO).

Lithic Of, or pertaining to, stone.

Lithification The transformation of loose sediments into sedimentary rock as a result of processes such as compaction of grains, cementation, and crystallization of minerals.

Lithology The field of study that is concerned with the mineral composition, structure, and description of rocks, particularly with such characteristics as color, hardness, structure, grain size, etc.

Lithosphere The Earth's rigid, outer shell that contains plates and continents. It consists of the Earth's crust and the uppermost part of the Earth's mantle. It has a thickness of about 60 miles.

Littoral The noun littoral denotes a shore and its adjacent areas. Littoral rights are the rights of the owners of waterfront property along an ocean or sea. Although laws may vary from state to state, the property owner generally owns the shore facing the property out to the high tide mark. Further, the submerged land beyond that point usually cannot be sold (by the state) without the approval of the owner of the waterfront property. Littoral rights correspond to Riparian Rights, which concern property along such watercourses as rivers and streams.

Locating The marking of boundaries and staking of a claim to the mineral rights of the designated area.

Lode An unusually large vein-like deposit, usually metalliferous. Any body of ore set off from adjacent rock formations.

Lode Mining See Hardrock Mining.

Loess An unconsolidated deposit transported and deposited by wind, usually of silt, yellowish, and calcareous. Most loess was formed during the Pleistocene Epoch.

London Metal Exchange (LME) Established in 1877, this important London-based commodity exchange dominates trading in non-ferrous metals for industrial use, particularly primary high grade aluminum, grade A copper, special high grade zinc, primary nickel, standard lead, tin, aluminum alloy, and silver. Hedging is the prime function of this international market with industrial clients accounting for 70-80% of its volume. The London Clearing House (LCH) clears the transactions and guarantees performance of its members.

Lopolith A bowl-shaped mass of intrusive igneous rock that is similar to a laccolith, but concave downward rather than upward. See Laccolith. A saucer-shaped intrusive body of igneous rock.

Low-Grade An ore that is low in the concentration of the metal that is being mined for.

Luster The appearance of the surface of a mineral in regard to how it reflects light. Luster may be described as metallic, sub metallic, glassy, dull, resinous, etc.

Mafic A term used in reference to rocks that are rich in iron, magnesium, or other dark minerals. An igneous rock that has a high proportion of dark-colored minerals. Such minerals include biotite, hornblende, pyroxene, basalt, and gabbro.

Magma Molten rock that has not yet reached the earth's surface. Once magma reaches, the surface of the earth, it is called lava. Magma may be a mixture of rock, melted rock, and gases, or entirely liquid. It solidifies upon cooling, after it has reached the surface of the earth, to form igneous rocks. Magma is formed deep within the earth in a region of very high pressure and temperature. See Lava.

Magnetic Anomaly A magnetic reading that differs from that of the surrounding materials. A higher than normal reading indicates the presence of a metal object.

Magnetic Survey A geophysical survey that a geophysicist uses in a search for mineral-bearing ore bodies, or even oil-bearing sedimentary structures. The survey measures the intensity of the Earth's magnetic field and sometimes the magnetic inclination, and departure from the geographic north at several points. The data enables the geophysicist to prepare a magnetic map showing lines of equal intensity that may indicate the size and extent of an anomalous body to a trained eye.

Magnetite A common, iron oxide mineral (Fe3O4) that is an important iron ore. It is 72% iron. Magnetite is blackish-red to dark red color in color and is slightly darker than hematite. It has a metallic luster. Magnetite is strongly magnetic. That is, it is attracted by a magnet. It occasionally is itself a magnet. It is the chief member of the spinel group of minerals. Magnetite is also called lodestone or magnetic iron ore. Magnetite is found in association with igneous or metamorphic rocks

Magnetometer An instrument used to measure the intensity of a magnetic field at various points, especially the Earth's magnetic field. Device for measuring magnetism.

Malachite Basic copper carbonate is a copper ore that is widely used for making ornamental articles. Because of its distinctive bright-green color and the fact that it a drop of acid causes it to effervesce, it is easily identified. Malachite generally occurs in irregular masses. The crystals are extremely small and needle-like.

Mantle The middle layer of the Earth that lies between the crust and the core at depths ranging from depths of about 40 to 3,480 kilometers. The mantle consists of relatively dense rocks. Its uppermost part is rigid and, along with the crust, forms the plates of plate tectonics. The lower mantle is denser due to the greater pressure caused by the depth. See also Core, Crust.

Marble A coarse-grained, nonfoliated metamorphic rock that is derived from limestone or dolomite. It is calcite and/or dolomite in a crystalline state. Marble can take a high polish, is found in a range of colors and variegations, and is used in sculpture and architecture.

Marcasite A common mineral, iron disulfide is chemically similar to pyrite, but crystallizes in the orthorhombic system. This mineral is similar to chalcopyrite in its characteristics. It tarnishes easily, going from brassy yellow to yellowish or grayish brown, has a metallic luster, a greenish-gray streak, and a hardness of 6. Marcasite is most often are found as irregularly shaped, fibrous masses.

Marsh A tract of low-lying ground, which is flooded in winter and usually wet throughout the remainder of the year. In contrast to swamps, marshes are dominated by grasses. See also Bog, Sink, Swamp

Massive Homogeneous. A term that is applicable to rock that is relatively unbroken by cracks, joints, foliation, or bedding.

Massive Sulfide Rock An unusually large body of rock made up mainly or wholly of sulfide minerals, such as pyrite, pyrrhotite, or chalcopyrite.

Matrix The fine-grained material that surrounds larger grains in sedimentary rock. The solid matter in which crystals or fossils are embedded.

Mcf Thousand cubic feet. A unit of volume measurement for natural gas.

Meander A pronounced U-bend in a stream, typically one of a series of such bends. Meanders are most often formed in alluvial materials (streams-deposited sediments) within an alluvial valley. The cause of meanders is not completely understood. However, it is recognized that water follows a meandering path even on ice and within larger bodies, as illustrated by the path of the Gulf Stream within the Atlantic Ocean. See also Meander Line, Stream

Measured Mineral Resource An indicated mineral resource. See Indicated Mineral Resource.

Mechanical Weathering The total of all processes by which a rock or mineral disintegrates into smaller fragments and particles without altering its chemical composition. See also Chemical Weathering.

Mesa A high, broad, and flat tableland bordered on at least one side by a steep, rocky cliff. A flat-topped mountain. As the area of the summit decreases in relation to the mesa's height, the mesa becomes what is better described as a butte. Mesa is Spanish for table.

Mesh Size: The number of openings per one-inch square of screen used to sift materials. The commonly used sizes of screens for concentrates in mining are: #20, #30, #40, #60, #80, and #100 mesh size.

Metaconglomerate A metamorphosed conglomerate. See Conglomerate.

Metamorphic Rock A rock that has undergone chemical or structural changes due to heat and/or pressure to which it has been subjected. Any of a class of rocks that have been altered because of a change in factors, such as temperature, pressure, mechanical stress, or the addition or subtraction of chemical components. The rock may have been igneous, sedimentary, or another metamorphic rock previously. The word, metamorphic, comes from the Greek "meta" (change) and "morph" (form).

Metamorphism The process by which conditions within the Earth, alter the mineral content, chemical composition, and structure of solid rock over time without melting it. The factors responsible are heat and pressure. All classes of rock, igneous, sedimentary, and metamorphic, can undergo metamorphism.

Methane A colorless, odorless hydrocarbon that is a gas at normal temperatures and the main component of natural gas. Methane is also a component of firedamp in coal mines, and is produced by anaerobic bacterial decomposition of vegetable matter under water. In addition, it is produced industrially by the destructive distillation of bituminous coal. Methane is the simplest member of the paraffin series of hydrocarbons. Its molecule (CH4) contains one carbon atom and four hydrogen atoms. Methane is lighter than air and only slightly soluble in water. It burns readily to form carbon dioxide and water. In addition to its use as a heating fuel, methane is used in the manufacture of carbon black, methanol, formaldehyde, chloroform, carbon tetrachloride, and nitromethane.

Mica Any member of a group of hydrous potassium, magnesium, lithium, and iron, silicate minerals of aluminum that separate readily into thin, tough, often transparent, and usually elastic, flat, plate-like crystals. Micas are common in all three major rock varieties- igneous, sedimentary, and metamorphic. They are widespread minerals that are easily recognized by their cleavage. The most abundant variety of mica is Muscovite, common in acid igneous rocks, such as granites. Two other principal types of mica are biotite and phlogopite.

Migmatite A rock consisting of a metamorphic host that is streaked or veined with granite rock. Migmatite rocks are usually banded (gneissic) and acidic, rather than basic. Migmatites can form when metamorphic rock is subjected to multiple injections of igneous rock that solidify to form a network of bands. Others represent the partial fusion of the metamorphic host during extreme metamorphism. The components of the rock that has the lower melting temperatures fused to produce the streaks of granite. Migmatite means mixed rock.

Mill A machine for crushing or pulverizing. Also, a processing plant for crushing and pulverizing ore in order to produce a concentrate by flotation or gravity separation for subsequent smelting and refining.

Mine A pit or excavation in the earth from which minerals are taken. If the ore is extracted underground, a system of excavations in the rock (shaft, tunnels) are required in order to access the ore. If the ore is mined from the surface, it is extracted from one or several pits.

Mine Gas Any of various harmful gases produced during mining operations. Firedamp occurs naturally in coal seams and is highly inflammable and explosive when present in the air in a concentration of 5 to 14 percent. It is nearly always methane. White damp, carbon monoxide, is formed in coal mines by the oxidation of coal, particularly in mines where spontaneous combustion occurs. It can cause death within one hour in a concentration of only 0.35 percent. Stinkdamp is the name that miners have given to hydrogen sulfide. After damp refers to the mixture of gases present in a mine following an explosion or fire. Blackdamp contains an excess of carbon dioxide and is an atmosphere in which a flame lamp will not burn.

Miner A laborer who works in a mine. One whose occupation is the extraction of ore, coal, or other valuable materials from the earth's crust.

Mineral A naturally occurring, usually inorganic, crystalline solid that consists of a single element or compound, or a limited mixture of chemical compounds, with a distinctive set of chemical and physical properties, which can be used to identify it. It has a regular internal lattice structure.

Mineral Deposit Any naturally occurring body of minerals that have economic value. The value lies in the ore minerals, rather than the body of minerals as a whole.

Mineralization The concentration of metals and their chemical compounds within a body of rock. Also, the process by which minerals are laid down into rock. Finally, the process of replacing an organism's original material with a mineral.

Mineral Lease A lease agreement, which gives a lessee the right to explore a designated tract of land for minerals and to remove them, if discovered. The lease permits the removal of minerals upon payment of a royalty for a specified term or as long as they may be removed economically. See also Mineral Rights, Oil and Gas Lease, Royalty

Mineralogy The field of geology that is concerned with the study of minerals, including their appearance, physical properties, chemical composition, internal crystal structure, stability, occurrence, and their origins.

Mineraloid A mineral substance that does not have a definite chemical formula or crystal form. Examples include opal and obsidian.

Mineral Processing The process of extracting the minerals contained in ore. This usually involves crushing and separating the valuable substance from waste by any of a variety of techniques that rely on flotation, gravimetric or magnetic properties, etc.

Mineral Rights The right to extract natural resources, such as oil, gas, etc., from below the surface of the land. The right to take the minerals from below the ground is restricted in some areas. In some parts of the country, the rights to the minerals were alienated years ago. In others, the purchaser of land receives all rights, including mineral rights, when title to the land is conveyed. See also Mineral Lease, Oil and Gas Lease, Royalty, Subsurface Rights, Surface Mining

Mining The excavation of minerals and ores from the earth for subsequent processing. It includes development of the mineral deposits, construction of the mine and associated facilities, and the initial processing of the ore. >P> Once the location and extent of the mineral deposit has been determined, mining engineers determine how to best remove it. This may involve the construction of an adit, a straight vertical shaft, stopes, or inclined shafts; a long strip excavation; or the drilling of a well. >P> An adit or crosscut tunnel is a horizontal opening into the base of a hill or mountain. A stope is a cavernous opening below ground where minerals are excavated are called stopes and accessed from the vertical shaft or a drift. An inclined shaft is constructed parallel to the vein and connects to the vein at intervals through horizontal tunnels. A long strip excavation or excavated pit is used to access the mineral in the process that is called strip mining.

MMBtu A unit of thermal energy equal to one million British Thermal Units. This is the energy equivalent to one thousand cubic feet of natural gas, which has a heating value of 1,000 Btus per cubic foot.

Monadock: A residual hill or mountain of resistant rock rising conspicuously above the surface of a surrounding peneplain.

Monticle A small mountain or hill. Also, a small mound created by volcanic eruption. Also called monticule. See Mountain

Monzonite Any of a group of coarse-grained, igneous rocks that contain approximately equal amounts of orthoclase ad plagioclase feldspar. Monzonite is between syenite and diorite in composition.

Moraine An accumulation of debris carried down from a mountain and deposited at its lower edge by a glacier. This material ranges from blocks or boulders to sand and clay. It is not sorted or stratified. See also Lateral Moraine, Terminal Moraine.

Mother Lode The most important vein or source.

Mound A small natural elevation, such as a hillock. See Hillock

Mountain A landform which rises conspicuously above its surroundings, possesses steep slopes, has a relatively limited summit area, and, entails substantial local relief. Although it is generally agreed that mountains are larger than hills, there is no standardized geological definition of either. What residents of one area might term a mountain, particularly if it is prominent and dominates the nearby landscape, others may describe as a hill. Mountains rarely appear alone. Instead, they usually appear in ranges or groupings. Mountains are sometimes classified by their origin and structure. Dome, fault block, fold, and volcanic are major categories. See also Peak, Range, Summit

Muck The earth, broken rock, or other useless matter to be taken away in the process of excavating or removing the ore, mineral or other substances sought.

Mucker In mining, one who removes muck. A shoveler.

Mudflow A flow of water, which contains a great quantity of particles and silt in suspension. It develops on steep slopes, which have insufficient vegetation to prevent erosion, but can occur on gentle slopes during periods of heavy precipitation if easily erodible materials are available. Mudflows occur most frequently in arid and semi-arid areas. They leave behind unsorted mixtures of silt, boulders, organic materials, and debris. This can be extensive on alluvial fans and about the bases of volcanoes. See also Earthflow

Mudstone A fine-grained sedimentary rock consisting of clay-sized particles. Mudstone is a clayey rock of almost uniform texture throughout, with little or no lamination.

Muscovite The commonest mica, Muscovite is always present in mica schist and often in granite and gneiss. The best crystals occur in granites. It is clear or sometimes pale-green or brown with well-formed crystals, usually hexagonal.

Muskeg A bog or deep marsh, which fills a depression in land and is formed by successive deposits of leaves, mosses, muck, etc. See also Bog, Marsh

Native Metal: A deposit of a metallic element that is found in nature in pure metallic form. A native metal has not been oxidized or combined with sulfur or other elements. Native silver occurs in Canada and Norway.

Natural Gas Natural gas is extracted, sometimes with oil, from the Earth's crust where pockets of it have been trapped for hundreds of thousands of years. Once it has been brought to the surface, the gas is refined in order to remove water, sand, and other impurities. Then it is distributed through pipelines. Natural gas is comprised primarily of methane, a colorless and odorless hydrocarbon gas, which burns cleanly, leaving only carbon dioxide and water vapor. An odor is added to natural gas by local distributors as a safety measure. Natural gas is traded on the New York Mercantile Exchange and Kansas City Board of Trade. The minimum futures contract on either exchange is 10,000 million BTUs (about 10 million cubic feet).

New York Mercantile Exchange (NYMEX) North America's most important trading forum for futures and options of energy and precious metals. The exchange operates two divisions. The NYMEX division handles futures and/or options for crude oil, heating oil, unleaded gasoline, natural gas, propane, electricity, platinum, and palladium. The COMEX Division handles futures and/or options on gold, silver, copper, and the Eurotop 100, an index of European equities. The exchange is located at One North End Avenue, near the former World Trade Center.

Non-Associated Gas A term to describe natural gas that is found in a field, unaccompanied by crude oil.

Nonconformity A type of unconformity in which younger sedimentary rock covers an erosion surface of older metamorphic or igneous rock.

Norite An igneous rock composed of calcium plagioclase feldspars and orthorhombic pyroxenes. It is very similar in appearance to gabbro.

Normal Fault A fault that causes rock on one side of the fault to drop relative to the rock on the other side. A dip-slip fault.

Nugget A small lump or piece of a precious metal found in nature, often rounded by stream or water action. Also, any piece of gold larger than a flake.

Obsidian A black or dark-colored non-crystalline rock, usually of felsic composition, that was formed when lava cooled too rapidly to permit crystal growth. Obsidian normally has the same chemical composition as the extrusive igneous rock, rhyolite.

Oil and Gas Lease A lease of a parcel of land which provides the lessee with the exclusive right to explore for oil and gas and to extract it from beneath from the surface of the land. The lease agreement covers the term, lease extension terms, payment of royalties, damage to the surface of the land, assignment of the lease, and the date by which drilling is to begin. The term of an oil or gas lease is usually several years in length. A lessor's arrangement with an exploration company would include a royalty or negotiated share of profits or revenue resulting from the production and/or sale of any oil or gas from the land. Oil and gas leases are frequently awarded by state or federal lottery.

Oil Shale A brown or black shale from which a heavy oil can be extracted by crushing, heated, distillation, and condensation.

Olivine A common mineral, magnesium iron silicate. It occurs in olive-green to gray-green, irregular masses as an important constituent of basic igneous rocks. It has a vitreous luster, a hardness of 6.5 to 7, and rarely occurs in crystals.

OPT Troy ounces of gold per short ton of ore.

Ore A naturally occurrence of a metal-bearing mineral or rock, or a mineral that serves as a source of a nonmetallic substance, that is present in a concentration sufficient to make mining economically feasible. A natural source from which valuable matter is extracted.

Ore Reserves That portion of a known mineral resource that has been actually discovered and measured, but has not yet been exploited, although it would be profitable to do so. In the case of ore, the reserve is normally described as the number of tons that are available of a specified average grade.

Outcrop The edge or surface of a mass of rock or mineral deposit that lies exposed at the Earth's surface. A section of bedrock that is exposed to the atmosphere.

Overburden The worthless or low-grade material that covers a mineral deposit. An example is the soil or rock that covers a seam of coal. Overburden is removed before surface mining and replaced after the coal or other mineral has been removed.

Paleontology The field of science that studies the forms of life existing in former geological periods as represented by fossil animals and plants. It is also spelled palaeontology,

Palisade An extended rocky cliff or precipice, which rises steeply above the edge of a stream or lake. Also, a fence formed by poles or stakes imbedded in the ground to create an enclosure.

Panning The process which heavier minerals, such as gold or sulfides, are separated lighter metals in a stream sediment, crushed rock, or loose soil, using a pan-shaped container.

Parent Rock The pre-existing rock from which a metamorphic rock was formed. Also, the source, usually bedrock or a sediment, from which a certain soil derives.

Pass A narrow and difficult or dangerous passage between peaks in a mountain range or through a mountainous region. Also, a way or opening through which one may pass in any region otherwise obstructed or impassable. In addition, the word, pass, is used as a verb to denote the transfer of ownership or title from one person to another. See also Gap, Gorge

Peak The pointed or cone-shaped top of a mountain or hill, a conspicuous mountain, or a mountain having a single conspicuous summit. Also, a point of land extending into the sea. A promontory or headland.

Peat Partially carbonized vegetable matter, which has been formed over time by the partial decomposition of vegetable matter in water. It is formed primarily in temperate, humid environments in conditions of deficient drainage and represents the first stage in the formation of coal. Peat is often found in bogs. It is a light, spongy material. It is brown in color and has a carbon content of 50%. Peat is cut into blocks, pressed, dried and used as a fuel for domestic heating. It is sometimes called Turf. See also Bog, Peat Bog

Peat Bog A marsh which contains an accumulation of peat. See also Bog, Marsh

Pegmatite Any very coarsely crystalline igneous rock, typically a granite. The crystals are extremely large grains and more than a centimeter in diameter. Pegmatites differ little from common igneous rocks in their chemical composition. Granite and syenitic pegamatite deposits are the main sources of commercial feldspar, sheet mica, beryllium, tantalum-niobium, and lithium minerals.

Pelite Any clay rock. A fine-grained sedimentary rock consisting mostly of clay or silt.

Peridotite Any of a group of coarse-grained igneous rocks, composed chiefly of olivine, with accessory amounts of other minerals, but with little or no feldspar.

Permeability The capacity of a rock to permit the passage of water. A rock's permeability depends on the size of its pores and the extent to which they are interconnected. A good aquifer requires highly permeable bedrock. See also Porosity.

Petrified Wood A fossil formed when minerals, usually silica (silicon dioxide), replace the natural tissue in wood over a very long period. Both the external shape and internal structure of the wood are reproduced accurately.

Petroleum A word derived from two Latin words, petra and oleum, which mean rock and oil respectively. Its name reflects the fact that petroleum was first noticed as a liquid seeping up from the earth. Today, the word is used interchangeably with oil or crude oil. See Barrel, Crude Oil, Depletion, Hydrocarbons.

Petrology The scientific study of rocks and minerals that is concerned with their composition, texture, structure; distribution, and origin in relation to physiochemical conditions and geologic processes.

Phaneritic A word to describe texture. It applies to any igneous rock or mineral that has mineral grains large enough to be seen with the unaided eye.

Phenocryst Any of the large and conspicuous crystals embedded in a porphyritic rock.

Phlogopite A yellowish-brown, but sometimes reddish-brown, mica. Phlogopite is a basic aluminosilicate of potassium, magnesium, and iron. It forms flat, plate-like crystals that cleave into smooth flakes. It typically occurs as a metamorphic product and in ultrabasic igneous rocks.

Phosphorite Any member of a series of phosphate minerals. Also, a complex mineral consisting of calcium fluoride phosphate or calcium chloride phosphate and appearing in glassy crystals, nodules, or compact masses. Phosphorite is also called phosphate rock.

Phyllite A foliated metamorphic rock that is generally derived from slate. Phyllite has a silky sheen and very fine grain size. It is usually black or dark gray in color.

Pipe The vertical conduit in a volcano through which magma rose to the Earth's surface. In a dormant or inactive volcano, a pipe is typically filled with volcanic breccia and fragments of older rock.

Pitch The inclination of a seam or bedrock.

Pitchblende A dark, lustrous heavy mineral that is a source of uranium and radium. It is largely uranium dioxide or trioxide, but does contain lead and trace amounts of thorium, polonium and radium. It is greenish, brownish, or black in color with a pitchy to sub metallic luster. It is massive in form and frequently has a grape-cluster appearance. It has a 50%-80% uranium content and a specific gravity of 6-9. Pitchblende is also called uraninite.

Placer A deposit, or the site of a deposit, of a mass of gravel, sand, or similar material resulting from the crumbling and erosion of solid rocks, which contain particles of valuable minerals that are heavy and durable, such as gold or silver, etc. The valuable minerals are found in the form of dust, flakes, grains, and nuggets. Placers are typically found where the flow of water abruptly slows.

Plateau Tableland. A mesa. See Tableland

Plate Tectonics The theory that the earth's crust consists of about a dozen plates (fragments), which move in relation to each another, shifting continents, forming new ocean crust, stimulating volcanic eruptions, and creating mountains and oceans.

Platinum A rare chemical element, platinum is a precious, very heavy, silver-white metal. It is soft and ductile and has a high melting point (1,769 degrees C or 3,216 degrees F) and good resistance to corrosion and chemical attack. Platinum is the best known and most widely used of the six metals comprising the platinum group of metals. Platinum is even heavier than gold. A six-inch cube of the metal weighs about as much as an average man.

Pluton Any body of igneous rock that solidified far below the Earth's surface. Also, an intrusive rock, rather than the preexisting country rock that surrounds it. Plutons include batholiths, stocks, laccoliths, sills, dikes, and other forms of intrusions. Plutonic rocks typically have large crystals due to the slow cooling.

Polymorph Any of the forms assumed by a polymorphous substance. A mineral that is identical chemically to another mineral, but differs from it in crystal structure.

Pool A natural subterranean reservoir of oil or gas that is held in porous sedimentary rock.

Porosity The ratio of the volume of the open spaces within a rock or strata to the total volume of its mass. In oil fields, the oil and gas are contained in the interconnected pores in the rock.

Porphyroblast A large mineral crystal of metamorphic origin that developed in the finer crystalline groundmass of a metamorphic rock.

Porphyry A word that describes the structure, which is formed with larger crystals set in a fine-grained or glassy ground-mass. Porphyritic structure may appear in any rock, but the true porphyries are those in which there is a marked contrast between the large crystals and the ground-mass.

Potash Any of several potassium salts, such as potassium carbonate or potassium chloride, mined and processed for use in industry and as a fertilizer in agriculture.

Potassium-Argon Dating An isotope dating technique that is based on the rate of decay of radioactive potassium-40 to argon-40. The extremely long half-life of potassium-40 enables the ages of rocks containing argon that are between 100,000 and 4 billion years old to be estimated. See also Carbon-14 Dating.

ppb Parts per billion

Precious Metals Gold, silver, platinum, and palladium;

Primary Crushing The initial process of reducing ore into smaller fragments for transportation to the processing plant. The primary crusher is often located underground in the case of underground mines, or at the entrance to the processing plant.

Primary Mineral Any mineral in igneous rock that was formed during the original solidification of the rock, rather than by subsequent alteration. Primary minerals include the dominant minerals that may appear in the name of the rock and the accessory minerals that are present in smaller quantities.

Primary Silver Unworked silver. Primary silver is usually in bullion or grain form and newly recovered from mining operations.

Principle of Original Horizontality The proposition that all sedimentary bedding was deposited in layers that parallel to the Earth's surface.

Principle of Superposition The hypothesis that, except in extremely deformed strata, each stratum in a sequence is younger than the one beneath it and older than the one above it.

Principle of Uniformitarianism The hypothesis that current geological processes, which are observable today, operated similarly in the past and therefore can be used to explain past geologic events.

Probable Mineral Reserve The estimated size and grade of an indicated fossil fuel or mineral resource that is believed to be economically feasible. Deposits of a size that has been confirmed by samplings sufficient to prepare draft preliminary mining or drilling plans. An estimate of reserves, but needing further information to be classified as a proven reserve.

Production Well A well that is producing petroleum or natural gas.

Prospect To explore, especially for mineral deposits. It usually means to search for new deposits.

Proven Mineral Reserves Deposits of minerals, ore, or fossil fuels whose location and quantity are known from known fields and that are economically recoverable under existing economic and operating conditions. The tonnage or volume and grade are known in detail. They have been determined on the basis of drilling results, production, and historical trends.

Pumice A light-colored form of very light, frothy, volcanic rock. It is so filled with small cavities formed by the entrapment of gases that it resembles a sponge. Its texture was formed by the rapidly expanding gas in erupting lava. Pumice is similar in composition to rhyolite.

P wave The primary or fastest wave traveling away from a seismic event through the solid rock. It consisting of a series of compressions and dilations of the material through which it passes. P wave is an abbreviation for primary wave. See also S wave.

Pyrite A naturally occurring iron disulfide mineral (FeS). Pyrite is a hard, heavy, shiny, silvery to brassy colored, metallic mineral that generally forms cubic crystals or masses. It occurs as an accessory mineral in igneous rocks, in vein deposits with quartz and sulfide minerals, and in sedimentary rocks, such as shale, coal, and limestone. It is the most common sulfide found in coal mines. Pure pyrite contains 46.67 percent iron and 53.33 percent sulfur. Pyrite can be produced by magmatic segregation, hydrothermal solutions, or the process of sublimation.

Pyrite is also called iron pyrites, fool's gold, and sulfur balls. It is the mineral that is most commonly mistaken for gold. However, it has a greater hardness, lower specific gravity; lighter weight; striations on its crystal faces, and makes a dark streak on a scratch plate. Because there are better sources of iron, pyrite is not generally used as an iron ore.

Pyroclastic Pertaining to rock fragments formed in a volcanic eruption. The word also applies to ash and other material ejected during a volcanic eruption.

Pyroclastic Flow An extremely hot stream of gases and unsorted pyroclastic materials (fragments, pumice, glass shards, ash) that travel down the flanks of a volcano or along the ground at speeds of 50 to 100 miles per hour. The temperature of the flow may be as hot as 800C (1472 F) or more.

Pyroclastics Chunks and particles of igneous rock that have been ejected from the vent of a volcano during eruption. The term, pyroclastic rocks, is also used.

Pyroxene A common group of minerals of many varieties, silicates of magnesium, iron, calcium, and other elements, occurring as important constituents of many kinds of rocks, especially basic igneous rocks. They are similar in composition to the amphibole group, but have short and stout crystals, instead of blade- or needle-like crystals. Further, pyroxene crystals are eight-sided, whereas amphibole crystals are six-sided. Three common members of the group are enstatite, hypersthene, and augite.

Pyroxenite Any rock composed largely of pyroxene of any kind.

Pyrrhotite: An iron sulfide mineral that varies in iron content and is slightly magnetic. It is bronze-yellow in color and is sometimes mistaken for fool's gold. Pyrrhotite rarely occurs in crystalline form. It is usually found in veins in igneous rocks and less frequently in limestone.

Quad A unit of measurement for a quantity of heat energy equal to one quadrillion British Thermal Units. This is approximately equal to the energy contained in one trillion cubic feet of natural gas. Although these figures may appear astronomical, it may be noted that the minimum natural gas futures contract at both the New York Mercantile Exchange and the Kansas City Board of Trade is 10,000 million British Thermal Units (i.e., 10 billion BTUs). See also British Thermal Unit, Cubic Foot, Natural Gas.

Quarry An open excavation in the Earth's crust from which stone is mined. Quarried stone may be crushed and broken or used as dimensional stone, blocks, or slabs cut to size. Quarrying was undertaken in the past primarily to supply dimensional stone for use in building. Consequently, quarries were limited to areas of availability of rock possessing uniform color and texture, such as granite and limestone. Crushed stone is used today for concrete aggregate, road building, and other applications. The production of crushed stone involves deep drilling, blasting to fragment the rock, crushing to provide smaller pieces, and screening to provide uniform size classes. See Stone

Quartz The second most common mineral in the Earth's crust after feldspar. Quartz appears in many forms, differing in color, luster, etc., but is composed primarily of silicon dioxide (SiO2). When pure, it is a six-sided crystal and clear in color. It is usually found in all-white color, or marbled white with brown or red.

Quartz is a component in nearly all acid igneous, metamorphic, and sedimentary rocks and is an essential mineral in granites, granodiorites, rhyolites, and sandstones. It is resistant to weathering. Quartz is also called silica.

Quartzite A hard, clean, white, granular metamorphic rock consisting essentially of quartz in interlocking grains. Sandstone that has been converted into solid quartz rock. Unlike sandstone, quartzite is free from pores and, when struck, fractures smoothly.

Raise In mining, to excavate a vertical or inclined underground working upward from below.

Ravine A long, narrow, steep-sided cleft, valley, or pass through the mountains, which has been created by erosion due to a stream or other rapidly running water. See also Barranca, Gulch

Raw Natural Gas Natural gas that contains impurities or unwanted substances that must be removed. See also Sour Gas.

Recoverable Reserves The volume of hydrocarbons, ore, or minerals that can be recovered from the ground by the use of existing techniques.

Recrystallization The growth of new mineral crystals in a rock, often of the same composition as the old ones, which supply the material.

Reef A ridge or line of rocks, sand, coral, small stones, etc., at or just above or below the surface of the water. Also, a lode or vein. See also Skerry

Refining The process of reducing to a pure state. Removing the precious metals from the alloying metals to improve the purity of the former.

Regolith Mantle rock. Any solid material, such as rock fragments, soil, and alluvium, that lies on top of bedrock.

Relative Dating The fixing of rocks and geological structures in correct chronological sequence. Relative dating does not involve the determination of age in years. See also Radiometric Dating.

Relief The differences in elevation among various points in a region, as between, the summit of a mountain and an adjacent valley.

Replacement The chemical process in which hydrothermal liquids, which are absorbed by permeable rocks, dissolve original minerals and replace them with other minerals.

Reservoir Continuous rock strata that contain a network of pores and cracks through which water, liquid hydrocarbons, and gases can travel.

Resource An occurrence of a mineral or fuel that has not been sufficiently defined to be classified as ore or a reserve. However, a resource is, an occurrence of the material in such a form, concentration, quantity, and quality that its prospects for economic extraction are favorable.

Rhyolite A fine-grained or glassy looking, usually light-colored, volcanic rock that has the same composition as granite. It has a silica content of 69% or more and is rich in potassium and sodium. Rhyolite often contains tiny visible crystals of quartz and/or feldspar in a glassy white, green, or pink groundmass. Any of a group of felsic igneous rocks that are the extrusive equivalents of granite.

Rich Gas Natural gas that is largely methane, but which contains a relatively high proportion of other hydrocarbons.

Richter Scale A 10-point scale developed in 1935 by Charles Richter of the California Institute of Technology to express and compare the magnitude of earthquakes. Its values are based on the measurement of seismic waves by the use of a seismograph. The Richter scale is logarithmic. The incidence of earthquakes declines by a factor of ten for each successive higher value of the Richter scale. Earthquakes typically have a magnitude of three to four on the Richter scale. An earthquake of magnitude less than five usually leaves no discernable property. However, an earthquake of magnitude 6.5 typically is catastrophic due to the great extent of shaking. Earthquakes of greatest known magnitude have been in the range of eight to nine. The Richter scale was the first of several scales used to describe earthquake intensity. Many seismologists today prefer to use the moment-magnitude scale to more precisely determine the energy released by large earthquakes. See also Earthquake, Seismograph, Seismology

Ridge The crest of a roof formed by the intersection of its sloped surfaces. Also, a long, narrow elevation of land or a string of hills or mountains.

Ring of Fire The zone of volcanic, earthquake, and mountain-building activity that encircles the Pacific Ocean.

River Terrace A bench or step, which extends along the side of a valley and marks a previous floor of the valley. A river terrace is created by a hydrological or climatic change, which renewed erosion of the valley floor. A river terrace generally has a horizontal surface. Frequently, this is formed of sedimentary deposits. The terrace is usually the vestige of an earlier floodplain, which the river has cut through and left standing above the level of the present floodplain.

Rock Hard, naturally occurring, consolidated or unconsolidated mineral matter of various composition assembled in masses. There are three basic types of rocks. These are igneous, sedimentary, and metamorphic. The origins and characteristics of these three classes of rock differ. Also, stone of any size.

Royalty Payment for the right to use another's property. This may involve the right to use a patented method or copyrighted material or, instead, the right to extract a valuable resource from an owner's land. Land owners, who lease their mineral rights to oil or mining companies, receive royalty payments, which are negotiated in advance and calculated on the quantity of minerals extracted from their land. See also Mineral Lease, Oil and Gas Lease

Rubidium-Strontium Dating A radiometric dating method that is used to determine the age of rocks that are at least 10 million years old. It is based on the 47-billion-year half-life of radioactive rubidium-87, which decays into isotopes of strontium, to determine the age of rocks in which strontium is present. See also Carbon-14 Dating, Potassium-Argon Dating.

Runnel A brook, rill, or rivulet. See also Rivulet

Runoff The flow of precipitation, which exceeds what the soil can absorb and which drains naturally from the land in rills or streams. Alternatively, the excess precipitation so drained.

Saddle A depression in a hill or line of hills. A shallow gap in a ridge. A col. Also, a similar formation of ice or snow. See Col

Salina A salt flat or alkaline flat. See Alkaline Flat

Salt See Halite.

Saltation The Intermittent, short leaps of particles of sand or gravel above the ground or a stream, as a result of the wind or force of running water. Rocks that bounce along during floods are said to move by saltation.

Salt Dome An immense domelike rock structure below the earth's surface that has been formed by the upward movement of a mass of salt pushing its way through the overlying formations. It forms an anticline. The mass is generally circular in shape, may reach a height of thousands of feet and is associated with pools of oil and gas.

Salt Flat A salina or alkaline flat.

Salting The surreptitious introduction of metals, minerals, or rich ore into a mine, the ground, or a sample, in order to make it appear to be rich in valuable mineral(s) and create a false impression of value. Salting is done with the intention to defraud.

Salt Lick A naturally or artificially occurring deposit of exposed salt that animals can visit to lick when they crave salt.

Salt Marsh An area of low-lying, flat, poorly drained ground, which is subject to frequent or occasional flooding by salt or brackish water. The typical vegetation of a salt marsh is grass and grass-like plants, such as rushes. Such marshes are often found along sea coasts, inside barrier bars and beaches, in estuaries, and on deltas or other districts occasionally subject to overflow by saltwater. See also Marsh, Swamp

Salt Mine A mine from which rock salt is excavated.

Sample A small quantity of rock, ore, gravel, or dirt, taken systematically from a deposit in order to judge its quality or to analyze.

Sampling The process or act of selecting a small, but representative, portion of an ore body or mineral deposit, for analysis in order to determine the value of the latter.

Sand Naturally occurring, finely-divided loose particles of rock that result from the disintegration of rock. Sand grains often consist largely of quartz, interspersed with grains of mica, magnetite, ilmenite, garnet, etc. The sand grains may be rounded or angular. Particle sizes range from 0.063 mm to 2 mm.

Sandbar A wholly or partially submerged rise of sand or sediment deposited offshore near a beach. Waves approaching the beach excavate a trough in the sandy bottom as they break, depositing some sand on the beach and carrying some sand offshore to deposit in the backwash to form a bar. The plunge of waves breaking over the sandbar keeps its top below still-water level. Bars and troughs are highest during the seasons of heaviest surf. They migrate shoreward in gentle seas and seaward in high seas. A sandbar is also called an Offshore Bar. See also Bar, Sand Dune

Sand Dune An elongated mound, ridge, or hill of sand formed by the action of the wind, particularly on a shore of a lake or seacoast or on the border of a river. Also, a similarly shaped ridge or mound of clay created by the action of wind. As dunes become larger, they move slowly downwind and adopt more asymmetrical shapes. Sand dunes are usually associated with desert areas covered extensively by windblown sand. See also Ridge, Sandbar

Sandstone Sedimentary rock that consists of sand, usually quartz, that has been cemented together by silica, calcium carbonate, iron oxide, and clay. It is the second most common sedimentary rock after shale. In general, there is a considerable amount of space between grains of sand. As a result, a sandstone will absorb up to 25% of its bulk in water.

Savanna An open, fertile, relatively treeless plain, which is covered with long grass and other low vegetation. Any large expanse of tropical or subtropical grassland covered with long grass and scattered drought-resistant trees and shrubs.

Scarp See Escarpment.

Schist Any of a class of crystalline rocks in which metamorphic changes have created a structure of thin layers or sheets. It can be easily split or cleaved because of this foliated structure. Most schists are composed largely of platy minerals such as muscovite, chlorite, talc, sericite, biotite, and graphite. Schists are named for their mineral constituents. A mica schist, for example, is rich in mica. There is much less feldspar and quartz in schist than in gneiss.

Scoria A bomb-sized piece of cinderlike, cellular lava. Like pumices, scoria develops its porous texture when the escaping gas is trapped as the lava solidifies. However, scoria is darker and denser than pumice.

Seam A comparatively thin stratum of ore of coal that is, nevertheless, thick enough to be mined at a profit.

Secondary Enrichment The process by which a metal in an ore becomes concentrated as minerals are eliminated from it, whether by weathering dissolution, precipitation, etc.

Section One of the 36 equal parcels of land forming a township. The section provides an important reference in legal descriptions of land. Each section is a one-mile square having an area of 640 acres. In all townships, section number 1 is located in the extreme northeast corner of the township. The numbers, which identify sections, rise consecutively from east to west across the northernmost row of sections to number six. The second horizontal row begins directly under number six and proceeds from west to east from 7 through 12. Sections in the third row are numbered from east to west, etc. Sections may be subdivided into half sections and quarter sections. Quarter Sections are identified as corners, such as the Northwest Corner of Section 11 (NW1/4 of Sec.11). See Acre.

Sediment Any mineral or organic matter that has been deposited by water, air, or ice. It normally accumulates in loose layers, as of mud or sand. With the passage of time the sediments become sedimentary rock.

Sedimentary Formed by the deposition of sediment, as sedimentary rocks.

Sedimentary Basin Terrain that consists of layers of rock that were formed during long periods of time by the depositing of sediments and their subsequent lithification as buried sediments were subjected to increasing pressure.

Sedimentary Facies Characteristics that differentiate one section of sedimentary rock from other nearby sections. These characteristics may include mineral content, grain size, shape, and density. Also, any section of sedimentary rock so described.

Sedimentary Rock Rock formed by the accumulation of sediments, later consolidated by pressure and natural cementing. The sediments may consist of rock fragments worn or ground to a powder, shells of animals or plants, or minerals that had settled to the bottom of water.

Sedimentary Structure The structure of sedimentary rock that was formed when the material was deposited. This includes such physical characteristics as bedding, cross-bedding, ripples, scour marks, and mud-cracks. It applies only to weakly metamorphosed rock.

Sedimentation The process of sediment formation. See Sediment.

Seismic Pertaining to, or caused by, an earthquake.

Seismic Discontinuity A surface within the Earth where a change in velocity of the P-wave or S-wave occurs, usually by more than 0.2 kilometer/second. See also Seismic wave.

Seismic Survey A method by which the rock structure below the Earth's surface may be examined. Seismic surveys are used particularly in the exploration for oil, gas, and ore deposits. They are based on the differences in time between the initiation of a sound wave and the arrival of its reflected or refracted impulses at seismic detectors. The sound waves are usually created below the surface of the ground, typically by the explosion of dynamite. The amplitude and timing of the returning sound waves are recorded by the detectors. The resulting seismogram is studied in order to gain an understanding of the depths and densities of subsurface strata.

Seismic Waves The waves of energy caused within the earth by an explosion or the sudden breaking of rock. They travel through the earth and are recorded by seismographs.

Body waves and surface waves are the two main types of waves. Body waves can travel through the earth, but surface waves move only along the earth's surface. The energy of earthquakes is radiated as seismic energy in both body and surface waves.

The first type of body wave is the P wave. This is the fastest of seismic waves and the first to arrive. It is also called compression wave, primary wave, pressure wave, and longitudinal wave. The vibration takes place longitudinally in the direction of the wave. P-waves can pass through rock and liquids.

The second type of body wave is the S wave. The S-wave is also called secondary wave, shear wave, or transverse wave. It always arrives after the P-wave. It moves from side to side in a shear motion that is perpendicular to the direction of propagation. It cannot pass through liquids. It can move only through rock. It is the S-wave that one feels in an earthquake.

The first kind of surface wave is known as a Love wave, after the mathematician who developed the mathematical model for it. It is the fastest surface wave and moves the ground from side-to-side.

The Rayleigh wave is the second type of surface wave. It was named after the mathematician, who predicted its existence. A Rayleigh wave rolls along the ground as a wave rolls across a body of water. It moves the ground up and down, and side-to-side in the direction that the wave is moving. The Raleigh wave is responsible for most of the shaking felt from an earthquake.

Seismogram A visual record created by a seismograph It shows the times of arrival and magnitudes of seismic waves.

Seismograph Any of various instruments that can measure and record ground oscillations, whether the result of an earthquake, an explosion, or another ground-shaking event. See Richter Scale, Seismic Survey

Seismologist A geophysicist who studies earthquake waves and our planet's mechanical characteristics.

Seismology The study of earthquakes and the creation of seismic waves. Seismology includes the study of island arcs, and oceanic trenches and ridges and covers efforts to locate subsurface faults and other petroleum or mineral bearing structures. Earthquake studies involve the recording of the arrival times and intensities of seismic waves on seismographs at three or more locations in order to locate the earthquake's point of origin. See also Earthquake, Richter Scale, Seismic Survey, Seismograph

Seismometer A seismograph that measures the direction, intensity, and duration of earthquakes by measuring the resulting motion of the ground.

Separation The first stage in refining petroleum. This makes use of the different boiling temperature ranges of the various hydrocarbons present in the crude oil in order to separate them in a distillation column.

Separator Machinery that makes uses of the relative densities of the oil, gases, and water that are contained in the effluent at the exit of a producing well, in order to separate them.

Serpentine A common mineral, composed of hydrous magnesium silicate, it is usually green in color and sometimes spotted. It has a smooth or slippery feel and a greasy or waxy luster. It is a secondary mineral formed by the alteration of such igneous rocks as peridotite or dunite, or of metamorphic rocks. Composed largely of pyroxene or hornblende.

Shaft A vertical access to a mine. Shafts are used for the transportation of men and materials, including ore and non-mineralized rock. The term, shaft, can also refer to an inclined opening of a limited and uniform cross section excavated for mining ore, ventilating underground areas, or removing water.

Shale A sedimentary rock formed by the consolidation under pressure of clay, mud, or silt, and, to a lesser extent, cementing. Shale has thin layers or laminas, which are easily separated. It varies in composition and sometimes is mostly clay. Shales are typically red, brown, black, or gray in color. They usually begin in still waters.

Shoot A narrow vein of ore. Also, a small tunnel branching off from a larger tunnel in a mine.

Shoreland Land which, although bordering a sea, lake, or river, is not exposed to tidal action. That is, the tide does not ebb and flow over shoreland. Land over which the tide ebbs and flows is known as tideland. See also Tideland

Siderite A common mineral of iron carbonate that usually occurs in yellowish to deep-brown cleavable masses. A minor ore that is also known as ironstone and chalybite.

Silica Silicon dioxide (SiO2), one of the most common compounds in the Earth's crust, occurring as quartz, sand, flint, and agate.

Silicate Any salt derived from the silicic acids or from silica. A mineral that contains silicon, oxygen, and normally one or more other common elements.

Siliceous Containing, consisting of, or resembling silica. The word is normally used to describe a rock that is rich in quartz.

Silicification The process in which silica, especially quartz, replaces an original mineral of a substance.

Sill A sheet-like igneous intrusion between beds of sedimentary rocks or layers of volcanic ejecta.

Sillimanite A mineral, aluminum silicate that is also known as fibrolite. Occurs in long, slender, and often fibrous crystals. Sillimanite is found only in metamorphic rocks.

Silt Loose earthy material, fine sand, mud, soil, etc. carried by moving or running water (especially a river) and deposited as a sediment in still water. Silt is finer than sand, but coarser than clay. A soil that contains a high proportion of silt has a greater water-holding capacity than do other soils.

Silver A white, lustrous metallic element that conducts heat and electricity better than any other metal. Silver resists atmospheric oxidation and, with the exception of gold, is the most malleable and ductile of all metals. It is one of the so-called precious metals, along with gold, iridium, palladium, and platinum. Silver has a specific gravity of 10.5 at 20 degrees C and a melting point of 960.8 degrees C (1,861.4 degrees F).

Silver occurs in the pure state to a limited extent. Electrum is a naturally occurring alloy of silver and pure gold. Silver is also found in many naturally occurring minerals. The majority of silver-containing ores also contain lead, copper, or zinc, or a combination of the three. Most silver comes from Mexico, Peru, Canada, the United States and Australia in that order of production.

Alloys of silver and copper are harder, tougher, and more fusible than pure silver. The proportion of silver in such alloys is expressed in terms of fineness. Sterling silver containing 92.5 percent of silver and 7.5 percent of another metal, usually copper, has a fineness of 0.925. Jewelry silver is an alloy containing 80 percent silver and 20 percent copper. It has a fineness of 0.800.

Sink A natural pool, marsh, basin or other depression in the land surface in which water collects to form a bog, marsh, or pool and from which it disappears by evaporation or percolation. See also Marsh

Sinkhole A natural depression in the surface of the Earth. Sinkholes are often formed by the collapse of the roof of a cavern or subterranean passage in regions of limestone or some other soluble rock.

Skerry An island, which consists of a single rock or series of rocks. Also, a reef. See Reef

Skip A container in which ore is lifted up a vertical mine shaft to the surface.

Slag The waste product of the smelting process.

Slate A dense, fine-grained , metamorphic rock formed by the compression of clay, shale, etc., that tends to split along parallel cleavage planes to form thin plates, flat sheets. Slate is fairly soft and it can be cut and punched. Its color ranges from gray through red, green and purple to black. Slate is composed mostly of quartz and mica, with a wide range of accessory minerals, like chlorite, feldspar, magnetite, hematite, pyrite, calcite, graphite, etc.

Slide The exposed surface left on a hillside by an avalanche or landslide.

Slip The distance between the broken ends of the ore body, measured along the fault.. Also, a small fault.

Smelting The process by which a metal is extracted from its ore by heating beyond the melting point, normally in the presence of an oxidizing agent, such as air, or a reducing agent, such as coke. A flux, which is added, and the impurities combine to form a slag, which can be separated from the metal by taking advantage of the slag's lighter weight.

Soapstone See Talc.

Soil The ground or upper layer of the Earth in which plants grow and which consists of disintegrated rock, usually with vegetable or animal matter and mold. Soil is a source of plant nutrients, including water and oxygen, for growth and reproduction. A soil's most important chemical characteristic for plant growth is its degree of acidity or alkalinity. This affects its suitability as a growing medium. Texture, structure and porosity of a soil also affect plant growth.

Texture refers to the feel of a soil. Sandy soils are coarse and gritty, whereas clay soils are smooth and sticky. Soil texture is defined by the relative proportions of sand, silt and clay particles. Soils having a high content of sand or gravel are porous and have a low water-holding capacity, but afford good drainage. Conversely, soils having a high proportion of silt possess greater water-holding capacity. In addition, they contain a variety of minerals. Particularly important is the clay content of a soil. Clay particles influence the water retention capacity of soil, in addition to providing nutrients. The proportion of clay particles also affect the capacity of a soil to swell and shrink and the stickiness of the soil. Sand particles are approximately 100 times larger than clay particles. Silt particles are intermediate in size.

Soil structure refers to the physical arrangements of mineral and organic particles and determines the shapes, sizes and porosities of different aggregates. A good soil structure permits movement of air and water and increases the soil's resistance to erosion.

Soil porosity refers to the total space of pores filled with either air or water. It determines soil drainage, the volume of water or air, which the soil can hold, and the depth to which roots may penetrate. Compaction of soil during construction may remove good soil porosity.

Soil names are based on the sizes of particles, which comprise them. Examples include silty clays and sandy clays. Loam describes a soil, which has approximately equal parts of clay, silt, and sand. Organic matter comprises only a small percent of most soils. The micro-organisms in soil contain millions of individual microbes, primarily bacteria, fungi, algae, and protozoa, which serve to decompose plant residues leaving nutrients behind for roots to absorb.

Soil Creep The slow drifting or sliding movement of surface soil down a slope or grade.

Soil Mantle A term referring to the soil as a blanket or a cover of the rock which lies below it. See Bedrock, Excavate

Sounding Well A well hole drilled for the purpose of obtaining information about the field's characteristics.

Source Rock A term used in reference to the sedimentary rock in which hydrocarbons originate.

Sour Crude Crude oil that contains a high degree of sulfur or sulfur compounds. A crude oil having a sulfur content of 2.5% by weight or more is termed a sour crude. The term sweet applies to crude oils having a sulfur content of less than 0.5%. See Crude Oil.

Sour Gas Natural gas in its natural state and containing such a high level of sulfur or sulfur compounds that it requires purifying before shipment or use because of its corrosive effect on equipment and pipes. See also Natural Gas, Sweet Gas.

Specific Gravity The weight of a substance in comparison to the weight of an equal volume of pure water at a temperature of 4 degrees Celsius. It indicates how tightly packed the atoms of a substance are. The specific gravity varies by substance. The specific gravity of a mineral is a factor in determining whether simple gravity concentrators, such as sluice boxes, can be used in recovering it. placer gold has a specific gravity of 18.5, whereas the specific gravity of quartz is far lower.

Sphalerite A common mineral, zinc sulfide (ZnS), the chief ore mineral of zinc. It is found associated with galena in most important lead-zinc deposits. It usually contains some iron and a little cadmium. Sphalerite occurs in yellow, brown, or black crystals or cleavable masses with resinous luster. Sphalerite is also called blende, or zincblende.

Speleothem A mineral deposit that was formed when calcium carbonate (CaCO3) or another mineral precipitated from solution within a cave. Stalactites and stalagmites are examples of speleothems.

Spodumene A mineral, a lithium aluminum silicate mineral in the pyroxene family that is an important ore of lithium and a source of ceramic materials. It is ordinarily found in lithium-bearing granite pegmatites. When brilliant and glassy, clear spodulene is valued as a semiprecious gem. Spodumene is also called triphane. Spring An opening at or near the surface of the Earth and serving as an exit point for water from an underground source. A spring is the natural point at which subterranean water emerges from the surface of the ground or from the bed of a stream, lake or other body of water. A spring from which water emerges without a noticeable current is termed a seep. Water in springs and seeps normally enters the soil as rainfall. It percolates downward into aquifers. These are underlying rocks, such as limestone, sandstone, and basalt, through which water can migrate. The aquifers, which receive and discharge the greatest quantity of groundwater, are comprised of unconsolidated material such as sand and gravel.

Spur A sharp part, such as a crag or mountain peak, which is suggestive of a spur, and which projects for some distance from the side of a range, ridge, mountain, or hill. See also Peak, Mountain

Squatter One who takes possession of land (settles on it) without having title or a claim to it and without the agreement of its owner or having paid for its use. The land may be public or private. Historically, the term, squatter, has referred to a person, who settles on government land, subject to regulations governing it, with the objective of obtaining title to it.

Stalactite An icicle-like mineral formation s that hangs downward from the ceiling or walls of a cavern. It normally consists of travertine, which precipitates when water that contains dissolved limestone drips from the cave's ceiling. See also stalagmite.

Stalagmite A cone-shaped mineral deposit that has formed on the floor of a cavern and built upward as water that contains dissolved limestone dripped down from the cave's ceiling. See also stalactite.

Steep A steep slope or declivity, especially of a hill. See also Acclivity, Declivity

Stone The hard substance formed from minerals and which comprises rock. Also, a piece of rock smaller than a boulder. A material of adequate integrity and quality that can be sawed, cut, split, or finished for specific purposes. Granite, limestone, marble, sandstone, and slate are used in construction. The uses of stone include lintels, window sills, flooring, and thresholds. A disadvantage is stone's low insulating value.

Stope Any excavation made inside a mine to remove the ore. It often takes the form of a step-like excavation as the ore is mined in successive layers.

Strand A shore or beach of a sea, lake, or river and, in particular, that portion of the shore, which lies between high and low tides. See also Shore, Tide

Strata See Stratum.

Stratified Arranged in strata. Formed or deposited in layers. A reference to geologic deposits.

Stratiform Arranged in strata. Having the form of a stratum. Occurring as a bed or beds. A term used in reference to a mineral deposit that occurs as beds or layers.

Stratigraphic Sequence The sequence in which various strata were deposited, an indication of a region's geologic history.

Stratigraphy A branch of geology that is concerned with the study of rock layers, including the description, correlation, classification, nomenclature, and correlation of strata in sedimentary rocks, particularly when and where they were deposited. It is based on the principle that in any undisturbed deposit, the oldest layers are normally located at the lower level.

Stratum A single layer of sedimentary rock, usually consisting of one type of matter and representing continuous deposition. Strata is the plural form of stratum.

Streak The color of a mineral in its powdered form. This is normally evident in the line of powder left on a streak plate when the mineral is scratched or rubbed hard upon a piece of unglazed porcelain. This streak of color often differs significantly from the color of the mineral's surface. It is of great importance as an identifying characteristic for some minerals, especially metallic ores.

Streak Plate A small plate of unglazed porcelain on which a mineral is rubbed in order to determine its color when powdered.

Stream A watercourse through which water flows at least periodically and which has a bed, banks, a source, and a destination, usually another stream, river, lake, or ocean. A stream flows in one direction. Also, a current of water or anything liquid, which flows in a course or in one direction. A river, brook, or rivulet.

Stria Any of a number of scratches or parallel grooves engraved in bedrock by the action of moving ice, as of a glacier. Also, any of a series of parallel lines or grooves on the surface of a crystal, or on a cleavage face of a crystal, due to its molecular organization.

Striation One of many parallel striae. Also, a striated condition or appearance.

Strike The direction of the break or fracture of the rock. Also, the angle between this line and the true North.

Strike Fault A fault that has a strike parallel to that of the ore-body.

Stringer A narrow vein. A thin, discontinuous mineral vein.

Stripping The act of removing the overburden from a mineral or placer deposit.

Structural Geology The branch of geology that is concerned with the deformation of rock bodies and the interpretation of the natural forces that caused them.

Subsidence The gradual downward sinking, or sometimes abrupt collapse, of a broad area of the Earth's surface with little or no appreciable deformation. Subsidence may be caused by a decrease in groundwater or the removal of oil. Also, the collapse of rock and soil layers into an underground mine.

Subsoil The layer of earth immediately below the surface soil. The character of the subsoil can influence the ease and expense of excavation and the time it requires. Further, the subsoil has an effect on how a building settles in years following construction. Consequently, it is common practice before constructing any building structure to undertake test borings in order to check subsurface conditions. Subsoil is known also as undersoil. See also Excavation, Soil, Topsoil

Subsurface The land and all material below the surface of the Earth (minerals, oil, gas, etc.) within a space forming the shape of a V and extending downward toward the center of the Earth.

Subsurface Rights Mineral rights and a right or easement permitting access to the land for purposes of extracting the minerals. See also Mineral Rights.

Subterranean Stream A stream which is situated below the surface of the land. Water below the surface of the land and restricted to a specific channel or bed is called a subterranean stream or lake. Subterranean water not so restricted is known as percolating water.

Sulfate Any of several minerals that contain positive sulfur ions bound to negative oxygen ions.

Sulfide Any of several minerals that contain negative sulfur ions bound to one or more positive metallic ions. Most sulfides are simple structurally, are highly symmetry in crystalline forms, and possess many properties of metals, such as electrical conductivity and metallic luster. Sulfides have a high specific gravity, a low hardness and are often strikingly colored.

Sulfur A non-metallic element that occurs in a natural state, as well as in compounds, such as sulfides or sulfates.

Summit The highest point of a ridge, mountain, hill, or any undulating land. See also Mountain, Ridge

Surface Mining The removal of the soil and rock from the surface of the area of land, which covers a seam of coal or other mineral located near the surface, followed by the recovery of the exposed mineral. This mining method is called strip-mining if it involves coal or open-pit mining or opencast mining if it involves mining of iron, copper, phosphate, or other ores.

Surface mining is best used where the coal or other mineral is not deeply buried. It is most economical for flat terrain and horizontal seams. If deposits occur in rolling, a contour method is used. This creates a shelf with a slope on one side and a nearly vertical wall on the other.

Suspended Load The fine sediment that remains suspended in a stream and travels with it because the upward velocity of eddies is greater than the settling velocity.

Swaliet An underground stream, such as might burst upon miners at work below ground. See also Subterranean Stream

Swaliet Hole An opening in the ground into which a stream disappears underground. See also Swaliet

Swamp An area or expanse where the water table reaches the surface of the land. As a result, the ground is saturated with water and is wet and spongy. Swamps are characterized by heavy vegetation, often under water. The swamp's abundance of trees differentiates it from a marsh. Swamps are often found in regions of low relief with rivers providing the source of water. Swamps and some marshes produce peat, but in smaller quantities than do bogs. See also Marsh

Sweet Gas Natural gas in its natural state and containing so little hydrogen sulfide and/or other sulfur compounds that it can be used without purifying without fear of its effect on piping and equipment. See also Natural Gas, Sour Crude, Sour Gas.

Syenite A coarse-grained, intrusive igneous rock consisting chiefly of feldspar with minor amounts of quartz and mica.

Syncline A concave fold in layered rock. A fold in rock in which the strata slope downward from both sides toward the center, as in the form of a bowl. The youngest strata are in the center. A syncline is the opposite of an anticline. See anticline.

Table A tableland. A plateau. See Tableland

Table-Land An extensive, elevated region, which has a level surface and is usually treeless. A table-land is normally bounded by an escarpment on all sides, but sometimes is bounded by mountains. A plateau. See also Mesa, Plateau

Talc A light-colored, magnesium silicate mineral that has a soft, soapy feel. It has a fibrous or flaky texture. It is distinguished by almost all other minerals by its extreme softness and can be easily carved with a knife. Talc's soapy or greasy feel accounts for its other name, soapstone. It generally occurs in very fine grained masses. Talc is found as a mineral in veins, in foliated masses, and in certain rocks. It is always associated with carbonates, such as calcite, dolomite, or magnesite, and occasionally with serpentine, tremolite, and forsterite.

Talus The sloping mass of fallen rock fragments and disintegrated material, which has fallen from the face of a cliff and lies at its base. Also, the inclination of the surface of this sloping mass.

Tar Sands An unconsolidated combination of sand, clay, bitumen, and water. Tar sands are mined using strip mining techniques. Tar sands are also known as oil sands and bituminous sands.

Tectonically Active A term used in reference to regions that are affected by movement of the Earth's tectonic plates. Volcanoes and earthquakes are features of these regions.

Tetrahedite A steel-gray or blackish, metallic mineral, consisting of copper and antimony sulfide with a high silica content, but often containing other elements, as silver, iron, and zinc. It occurs in gray to black metallic crystals or masses with a brilliant luster. An important ore of copper and sometimes of silver.

Tephra Pyroclastic materials of all types and sizes that are ejected into the air by an erupting volcano. These include fine dust particles to objects as large as massive blocks.

Terminal Moraine A low ridge of alluvial debris pushed forward by the leading edge of a glacier and left at its terminus as it melts and recedes. A terminal moraine forms a convex curve pointing down a valley and may extend up its sides to form lateral moraines. See also Moraine

Terrane Any rock formation or series of formations. A region in which the geology differs from that of adjoining regions.

Terrain A term used in reference to land, ground, or a piece or plot of ground. More often, the word denotes a larger area or tract of land, especially in reference to its natural features, configuration, military advantages, etc.

Thermal Spring A spring, which has a water temperature higher than the mean air temperature of the surrounding area. A thermal spring also is called a hot spring.

Tidal Flat A level muddy surface, which borders an estuary and is alternately submerged and exposed by the changing level of the tide. Because of the alternating submergence and exposure of the tidal flat and the influences of fresh river water and saline marine waters, the temperature, salinity, and acidity of a tidal flat varies widely. The mud usually is rich in nutrients and supports an abundance of crustaceans and other small creatures, although vegetation is sparse.

Tide The periodic rise and fall of the oceans' waters and connected waters due to the gravitational pull of the moon and sun. There are two high tides and two low tides in each lunar day, at equal intervals of flood and ebb. A lunar day is 24 hours and 51 minutes in length. Tides are most easily observed along seacoasts, where their amplitudes are magnified.

Tideland Land at the edge of a body of water and over which the tide regularly flows and ebbs. Tide land belongs to the state. Land, which is not subject to tidal action, is called shoreland and may be individually owned. See also Shoreland, Tidal Flat, Tide

Tidemark A mark left or reached by the tide at high water and which shows the highest elevation reached by the tide. For example, a line of dirt left on a surface may reveal the maximum advance of the waters. Also, a post or other object positioned as a reference mark to observe the rise and ebb of the tide or highest point reached by the tide. See also Low Water Mark, Tide

Tidewater Water which inundates land at high tides. Also, any water affected by the tide, such as water at the seacoast or an adjacent river. See also Tide

Till Unsorted, unstratified glacial drift that consists of a heterogeneous mixture of clay, sand, gravel, stones, and boulders carried and deposited by a glacier. It has not been worked subsequently by meltwater.

Tipple See Dump.

Ton A unit of measurement of weight. A short ton or net ton is equal to 2,000 pounds. A British ton or long ton is 2,240 pounds. A metric ton is equal in weight to 1,000 kilograms, or 2,204.6 pounds. The term, tonne, refers to as metric ton.

Topographic Map A map on which the topographical features, of the area portrayed on the map, are recorded to scale using conventional symbols. A topographic map is a contour map, which shows elevation, high ground, and all natural barriers to development. The information contained on such maps can be classified as relief, hydrographic features, vegetation, or man-made works. Topographic maps available from the Geologic Survey are useful in forecasting the directions of future growth. See Topography, United States Geological Survey

Topography The physical features of land, whether natural or man-made, and the conformation of its surface, in particular. On a map, this is represented by contour lines of equal height, which indicate the elevations, and unevenness of land surface. Poor topography can cause an improvement to be extremely expensive. For example, rocky land above grade might be expensive to level. Land below grade might require a great deal of fill. General topographic conditions could present drainage problems. The analysis of topography is a matter usually requiring professional advice from an architect or engineer. Developers consider not only the contours of the land, but also its soils and vegetation. U.S. Geological Survey topographical maps are available for many areas. These maps are drawn with contours representing increments in elevation of 25 feet. See also Landform, United States Geological Survey

Topsoil The surface soil of land. That upper portion of the soil, which is rich in organic matter and particularly, suited to plant growth. It has been estimated that more than three billion tons of topsoil are eroded from America's croplands each year. This is much greater than the replacement of topsoil by natural means. See also Soil, Subsoil, Topsoil Stripping

Trachybasalt A fine-grained igneous rock that is intermediate in composition between trachyte and basalt. See Basalt, Trachyte.

Trachyte A fine-grained, light-colored, extrusive, igneous rock consisting mainly of alkali feldspar and one or more subordinate minerals, such as hornblende or mica. Most trachytes exhibit porphyritic texture in which large phenocrysts are embedded in a fine-grained matrix. Trachyte is the extrusive equivalent of syenite.

Trap: A natural configuration of layers of sedimentary or metamorphic rock in which non-porous layers serve as a barrier to impede the upward movement of oil and gas to the Earth's surface and cause them to collect beneath the barrier.

Travertine A type of limestone deposit formed around hot springs by precipitation from carbonate-saturated groundwater that cools when exposed to the air.

Trend The general direction in which a vein, fault, or rock outcrop extends.

Troy Weight A system of weights used for precious metals and gems. The weight of gold and silver is usually measured in troy ounces. Only 12 troy ounces constitute one troy pound. Each troy ounce is equal to 1.0971 avoirdupois ounces. Other equivalents are: 24 grains = 1 pennyweight 20 pennyweights = 1 troy ounce 480 grains = 1 troy ounce = 31.10 34 grams 1,000 grams = 1 kilogram = 32.15 troy ounces 1,000 kilograms = 1 metric ton = 32,150 troy ounces 1 troy ounce = 31.1035 grams When applied to bullion, the word ounce always refers to troy ounces. See also Avoirdupois, Grain, Precious Metals.

Tufa A porous limestone formed from calcium carbonate that was created by evaporation around springs or the like.

Tuff A consolidated rock composed of pyroclastic fragments and fine ash. If particles are melted slightly together from their own heat, the rock is called a welded tuff. It is relatively soft and porous.

Unconformity A discontinuity in sedimentary rock sequence indicating interruption of sedimentation or uplift followed by erosion of rocks before the break. A boundary between two or more strata of significantly different ages. It represents an interval of time during which there was deposition or erosion removed some sediments and rock. An unconformity is a gap in the geological record.

Unconsolidated Loose sediment that lacks consistency or cement binding its grains. An adjective that describes any sediment that has not been lithified.

Underground Mine A mine located below the earth's surface, usually several hundred feet or more. The coal or ore is removed mechanically and transported to the surface by conveyor or shuttle car. An underground mine is also called a deep mine.

Undersoil Subsoil. See Subsoil

Understratum The layer of soil or rock underlying the surface of the land. Substratum. Also known as subsoil. See Subsoil

United States Geological Survey (USGS) The U.S. Geological Survey (USGS) was established by an Act in 1879. It was mandated to classify the public lands and examine the geological structure, mineral resources, and products of the national domain. Topographic mapping and chemical and physical research were assigned in 1888. The measurement of the country's streams and water supply was added in 1894. Its mandate was broadened subsequently to include examinations beyond the national domain.

Besides evaluating the Nation's land, water, energy, and mineral resources, the Geological Survey conducts research on global change; investigates natural hazards such as earthquakes, volcanoes, landslides, floods, and droughts; and conducts the National Mapping Program. In fulfilling its objectives, USGS prepares maps and digital and cartographic data; collects and interprets data on energy and mineral resources; conducts nationwide assessments of the quality, quantity, and use of the nation's water resources; and undertakes fundamental and applied research in the sciences and the techniques involved. In addition, the USGS is authorized to establish a National Cooperative Geologic Mapping Program, to facilitate the production of a geologic map database, to establish the Federal Geographic Data Committee, and to serve as the lead agency for the Federal Water Information Coordination Program.

The USGS publishes the results of its investigations in thousands of new maps and reports each year, including aerial photographs, geodetic control data, cartographic data, and spacecraft and aircraft-remote sensor data. An aerial photo or topographical map of one's property or town may be obtained from the USGS Earth Science Information Center at 12201 Sunrise Valley Dr., MS-507, Reston, VA 22092.

Uplands Elevated areas or districts of a country. A term used to describe a region or district of elevation higher than that adjacent to it. Unlike a meadow or marsh, it is a stretch of high ground. In addition, the word is used to denote interior districts and high or mountainous areas remote from the sea. See also Highland

Uraninite See Pitchblende.

Uranium-Thorium-Lead Dating An isotope dating technique that relies on the extremely long half-life of radioactive isotopes of uranium, which decay into isotopes of lead, to determine the age of rocks in which uranium and lead are present. It facilitates the estimation of the ages of rocks containing lead that are between 100,000 years old and 1,200,000 years old. It helps to bridge the gap between the carbon-14 dating method and the potassium-argon dating method. See also Potassium-Argon Dating.

Value A term used in reference to the particular mineral or substance sought and specifically a precious metal contained in rock, gravel, etc. In the case of gold, the term is synonymous with color.

Varve In certain geological formations, an annual deposit consisting of two layers, one of fine materials and the other of course. The two layers typically are a thick, coarse, light-colored bed deposited during the spring runoff or flooding and a thin, fine-grained, dark-colored bed deposited during the winter.

Vein A mineral-filled fracture, crack, fissure, or fault in rock. Any body of minerals, igneous rock, or the like, that is clearly separated or defined, such as a vein of coal.

Vent An opening at the Earth's surface through which lava, gases, and other volcanic materials are ejected. Also called volcano or volcanic vent.

Ventifact A rock or stone that exhibits the effects of sandblasting. It has been flattened and sharpened. Ventifacts are often found loose on the floors of deserts.

Vesicle A cavity or small air pocket that was formed by escaping gas in igneous rock while it cooled and solidified.

Virgin Unworked, untouched. A term to describe that which has never been mined before, especially in coal mining. It is used as an adjective in conjunction with a noun (e.g., virgin bed, virgin bedrock, virgin placer, virgin ground, virgin materials).

Viscosity A measure of a liquid's resistance to flow. Water has a low viscosity, whereas molasses has a high viscosity. Viscosity increases as temperatures decrease.

Vitreous Glassy. Having the nature of glass, resembling glass, as in transparency, brittleness, hardness, glossiness, etc.

Volcanic Pipe A massive pillar of rock that filled the vertical conduit of a volcano and is more resistant to erosion than the materials that made up the surrounding cone. It may remain standing after the rest of the original structure has eroded away.

Volcanic Rock Igneous rock that has cooled and solidifies at, or very near, the surface of t he Earth.

Volcano Any opening in the Earth's surface through which magma reaches the surface and deposits surrounding it. The solid structure, usually conical, that erupts to release lava, hot fragments, ash, and gases to the Earth's surface.

A volcano is active if it is erupting or has erupted recently. It is dormant if it has not erupted recently, but is believed probable to erupt in the future. A volcano is extinct if it is not expected to erupt in the future.

Vug An unfilled part of a vein, often lined with crystals.

Wash A coulee. See Arroyo, Coulee

Watershed The ridge, summit, or high land, which separates two drainage systems. The line of demarcation between two contiguous drainage valleys or river basins. Also, the area or region drained by a stream or river. The territory from which a river draws its supply of water. A watershed is sometimes called a water parting or divide.

Water Table The groundwater's upper surface. See Groundwater.

Weathering The act and process by which exposure to atmospheric agents, such as air or moisture, causes rocks and minerals at or near the Earth's surface to break down. Weathering includes two processes that work in concert. Chemical weathering involves a chemical change in minerals within a rock, whereas mechanical weathering involves the freezing and expansion of water in cracks and changes in temperature that cause individual minerals to expand and shrink sufficiently to cause them to break apart. Little or no movement of the rock is involved in weathering. See also erosion.

Well A hole or shaft or hole drilled in the ground for the exploration of oil and gas.

Wellhead See Christmas Tree.

Well Log The record of information concerning a well. The well log includes the location of the well, date and depth excavated or drilled, capacity, and pumping rate, etc. It is of particular use in the gas and oil industry.

West Texas Intermediate A grade of light, sweet crude oil which is traded on the New York Mercantile Exchange and used as the benchmark crude for the United States oil industry. See also Crude Oil, New York Mercantile Exchange.

Wet Gas Natural gas containing condensable hydrocarbons (i.e., compounds that are heavier than ethane) that are condensable when brought to the surface. For a gas to be classified as wet, it must contain more than 0.3 gallons of condensables per 1,000 cubic feet of gas.

Wetlands Land areas having a water table so close to the surface of the land during such a long period of the year that their terrain is characterized by marshes and swamps. See also Marsh, Swamp

Wildcat An exploratory well that is drilled in an effort to discover deposits of oil or gas. A prospective well.

Xenocryst A rock or crystal surrounded by magma and retained as an inclusion in the resulting igneous rock. It is similar to a phenocryst in igneous rock, but is foreign to the rock in which it occurs.

Xenolith A rock fragment that is foreign to the igneous rock in which it is embedded. A pre-existing rock that has been embedded in a newer igneous rock. .

Zeolite Any of a group of hydrated silicates of aluminum with alkali metals. Zeolites commonly occur as secondary minerals in cavities in basic igneous volcanic rocks.

Zircon A common mineral, zirconium silicate, and the principal source of zirconium. It occurs in small, four-sided crystals or grains of various colors, usually opaque. Zircon is widespread as an accessory mineral in acid igneous rocks, but also occurs in metamorphic rocks and detrital deposits. It is a common heavy mineral in sedimentary rocks. Its high refractive index and dispersion enable zircon to approach diamond in fire and brilliance.

Zone A horizon. See Horizon.

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