Documents for "Geology and Oceanography":

• aa see lava.

• abyssal plain see ocean.

• altitude vertical distance of an object above some datum plane, such as mean sea level or a reference point on the earth's surface. It is usually measured by the reduction in atmospheric pressure with height,...

• anticline see fold.

• aphotic zone see ocean.

• aquifer see artesian well.

• Archeozoic era see Precambrian era.

• artesian well deep drilled well through which water is forced upward under pressure. The water in an artesian well flows from an aquifer, which is a layer of very porous rock or sediment, usually sandstone, capable of holding and...

• asthenosphere region in the upper mantle of the earth's interior, characterized by low-density, semiplastic (or partially molten) rock material chemically similar to the overlying lithosphere. The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the earth's crust move about (see plate tectonics ). The asthenosphere is generally located between 45-155 miles (72-250 km) beneath the earth's surface, though under the oceans it is usually much nearer the surface and at mid-ocean ridges rises to...

• astrobleme large, circular structure ranging from c. 1/2 mi to 40 mi (.8-64 km) in diameter. Astroblemes are found at numerous places on the earth's surface, e.g., Meteor, or Barringer, Crater in Arizona, Brent Crater in Ontario, and Vredefort Ring in...

• atoll see coral reefs.

• avalanche rapidly descending large mass of snow, ice, soil, rock, or mixtures of these materials, sliding or falling in response to the force of gravity. Avalanches, which are natural forms of erosion and...

• backshore see beach.

• badlands area of severe erosion, usually found in semiarid climates and characterized by countless gullies, steep ridges, and sparse vegetation. Badland topography is formed on poorly cemented sediments...

• bar offshore: see beach.

• basalt fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color. Basalt is an igneous rock, i.e., one that has congealed from a molten state. Basaltic magma is derived by partial...

• base line see geodesy.

• batholith enormous mass of intrusive igneous rock, that is, rock made of once-molten material that has solidified below the earth's surface (see rock ). Batholiths usually are granitic (see granite ) in composition, have steeply inclined walls, have no visible floors, and commonly extend over areas of thousands of square miles. Batholiths are formed either as one large mass or many smaller...

• bayou [Louisiana Fr.; from Choctaw bayuk =small stream], term used mainly in U.S. Gulf states, especially Louisiana and Mississippi, to describe a stationary or sluggishly moving body of water that was once part of a lake, river, or gulf...

• beach a gently sloping zone where deposits of unconsolidated sediments are subject to wave action at the shore of an ocean or lake. Most of the sediment making up a beach is supplied by rivers or by the erosion of highlands adjacent to the coast. Beaches extend from a low waterline landward to a definite change in material or physiographic form, such as the presence of a cliff or dune complex marking a...

• bentonite see clay.

• berm see beach.

• bight broad bend or curve in a coastline, forming a large open bay. The New York bight, for example, is the curve in the coast described by the southern shore of Long Island and the eastern shore of New...

• bog very old lake without inlet or outlet that becomes acid and is gradually overgrown with a characteristic vegetation (see swamp ). Peat moss, or sphagnum , grows around the edge of the open water of a bog ( peat is obtained from old bogs) and out on the surface. With its continued growth, the moss forms a mat on the water in which other bog plants find a foothold, and humus and soil are slowly built up on...

• bore inrush of water that advances upstream with a wavelike front, caused by the progress of incoming tide from a wide-mouthed bay into its narrower portion. The tidal movement tends to be retarded by...

• boulder large rock fragment formed by detachment from its parent consolidated rock by weathering and erosion. In engineering and geology, especially in the United States, the term is applied to loose rocks...

• boulder clay see drift.

• breaker see wave , in oceanography.

• breakwater offshore structure to protect a harbor from wave energy or deflect currents. When it also serves as a pier, it is called a quay; when covered by a roadway it is called a mole. In the United States...

• breccia see conglomerate.

• brownstone red to brown variety of sandstone. Its unusual color is caused in some instances by the presence of red iron oxide which acts as a cement, binding the sand grains together. Vast thicknesses (up to 20,000 ft/6,096 m) of brownstone...

• butte an isolated hill with steep sides and a flat top, resulting from the more rapid erosion of the surrounding areas. Buttes are characteristic of the plains of the W United States. See mesa.

• calcareous rock see limestone ; marble.

• caldera see crater.

• Cambrian period [Lat. Cambria =Wales], first period of the Paleozoic geologic era (see Geologic Timescale , table) extending from approximately 570 to 505 million years ago. It was named by the 19th-century English geologist Adam Sedgwick, who first studied the great sequence of rocks characteristic of...

• Carboniferous period fifth period of the Paleozoic era of geologic time (see Geologic Timescale , table), from 350 to 290 million years ago.

• catastrophism in geology, the doctrine that at intervals in the earth's history all living things have been destroyed by cataclysms (e.g., floods or earthquakes) and replaced by an entirely different...

• catchment area or drainage basin, area drained by a stream or other body of water. The limits of a given catchment area are the heights of land—often called drainage divides, or watersheds—separating it from neighboring drainage...

• cave a cavity in the earth's surface usually large enough for a person to enter. Caves may be formed by the chemical and mechanical action of a stream upon soluble or soft rock, of rainwater seeping...

• Cenozoic era last major division of geologic time (see Geologic Timescale , table) lasting from 65 million years ago to the present. The Cenozoic is divided into the Tertiary (from 65 million years ago until 2 million years ago) and Quaternary (2 million years ago to the...

• Census of Marine Life an international program to assess and explain the diversity, distribution, and abundance of living organisms in the oceans. A 10-year project involving scientists in more than 70 nations, the...

• chalk mineral of calcium carbonate , similar in composition to limestone, but softer. It is characteristically a marine formation and sometimes occurs in great thickness; the chief constituents of these chalk deposits are the shells...

• Challenger expedition British oceanographic expedition under the direction of the Scottish professor Charles Wyville Thompson and the British naturalist Sir John Murray. Taking place from 1872 to 1876, it opened the era...

• chert see flint.

• china clay one of the purest of the clays, composed chiefly of the mineral kaolinite usually formed when granite is changed by hydrothermal metamorphism. Usage of the terms china clay and kaolin is not well defined; sometimes they are used synonymously for a group of similar clays, and sometimes kaolin refers to those obtained in the United States and china clay to those that are imported...

• clay common name for a number of fine-grained, earthy materials that become plastic when wet. Chemically, clays are hydrous aluminum silicates, ordinarily containing impurities, e.g., potassium, sodium,...

• clay pan see hardpan.

• coast land bordering an ocean or other large body of water. The line of contact between the land and water surfaces is called the shoreline. It fluctuates with the waves and tides. Sometimes the terms coast and shore are used synonymously, but often shore is interpreted to mean only the zone between the shorelines at high tide and low tide, and coast indicates a strip of land of indefinite width landward of the shore. Classically, coasts have been designated as submergent if they resulted from a rise in the relative sea level and emergent if...

• coast protection methods used to protect coastal lands from erosion. Beaches can exist only where a delicate dynamic equilibrium exists between the amount of sand supplied to the beach and the inevitable losses...

• concretion mass or nodule of mineral matter, usually oval or nearly spherical in shape, and occurring in sedimentary rock. It is formed by the accumulation of mineral matter in the pore spaces of the...

• conglomerate in geology, sedimentary rock composed largely of pebbles or other rounded particles whose diameter is larger than 2 mm (.08 in.). Essentially a cemented gravel, conglomerates are formed along beaches, as glacial drift, and in...

• continent largest unit of landmasses on the earth. The continents include Eurasia (conventionally regarded as two continents, Europe and Asia ), Africa , North America , South America , Australia , and...

• continental drift geological theory that the relative positions of the continents on the earth's surface have changed considerably through geologic time. Though first proposed by American geologist Frank Bursley...

• continental shelf see ocean.

• coral reefs limestone formations produced by living organisms, found in shallow, tropical marine waters. In most reefs, the predominant organisms are stony corals , colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone). The accumulation of skeletal material, broken and piled up by wave action, produces a massive calcareous formation...

• Coutchiching see Precambrian era.

• crater circular, bowl-shaped depression on the earth's surface. (For a discussion of lunar craters, see moon.) Simple craters are bowl-shaped with a raised outer rim. Complex craters have a raised central...

• craton see continent.

• Cretaceous period third and last period of the Mesozoic era of geologic time (see Geologic Timescale , table), lasting from approximately 144 to 65 million years ago. The Cretaceous was marked, in both North America and Europe, by extensive submergences of the continents. Changes both in the...

• crevasse large crack in the upper surface of a glacier , formed by tension acting upon the brittle ice. Transverse crevasses occur where the grade of the glacier bed becomes suddenly steeper; longitudinal crevasses, where the glacier spreads over a...

• cuesta asymmetric ridge characterized by a short, steep escarpment on one side, and a long, gentle slope on the other. The steep side exposes the edge of erosion-resistant rock layers that form the...

• Deep Sea Drilling Project U.S. program designed to investigate the evolution of ocean basins by core drilling of ocean sediments and underlying oceanic crust. Funded by the National Science Foundation, the project was...

• deep-sea exploration see oceanography.

• delta [from triangular shape of the Nile delta, like the Greek letter delta ], a deposit of clay, silt, and sand formed at the mouth of a river where the stream loses velocity and drops part of its sediment load. No delta is formed if the coast is sinking or if there is an...

• desert arid region, usually partly covered by sand, having scanty vegetation or sometimes almost none, and capable of supporting only a limited and specially adapted animal population. The so-called cold...

• Devonian period fourth period of the Paleozoic era of geologic time between 408 and 360 million years ago (see Geologic Timescale , table). It was named (1838) by the geologists Sir Roderick Impey Murchison and Adam Sedgwick for Devonshire, England, where they first investigated rocks formed during the period. The Devonian...

• diabase see basalt.

• diatomaceous earth see diatom.

• drainage basin see catchment area.

• drift deposit of mixed clay, gravel, sand, and boulders transported and laid down by glaciers. Stratified, or glaciofluvial, drift is carried by waters flowing from the melting ice of a glacier. The...

• dripstone see stalactite and stalagmite.

• drumlin smooth oval hill of glacial drift , elongated in the direction of the movement of the ice that deposited it. Drumlins, which may be more than 150 ft (45 m) high and more than 1/2 mi (.8 km) long,...

• dune mound or ridge of wind-blown sand formed in arid regions and along coasts. Dunes are common in most of the great deserts of the world. Often a dune begins to form because material is deposited by...

• earth in geology and astronomy, fifth largest planet of the solar system and the only planet definitely known to support life. Gravitational forces have molded the earth, like all celestial bodies, into...

• earthflow see landslide.

• earthquake trembling or shaking movement of the earth's surface. Most earthquakes are minor tremors. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent...

• echo sounder an older instrumentation system for indirectly determining ocean floor depth. Echo sounding is based on the principle that water is an excellent medium for the transmission of sound waves and that...

• elevation vertical distance from a datum plane, usually mean sea level to a point above the earth. Often used synonymously with altitude , elevation is the height on the earth's surface and altitude, the height in space above the surface. The elevation of a feature is calculated through such surveying techniques as trigonometric...

• Eocene epoch second epoch of the Tertiary period in the Cenozoic era of geologic time, from approximately 54.9 to 38 million years ago. The Eocene in North America was marked by the submergence of the Great Valley of California and a portion of the Atlantic and Gulf...

• epoch unit of geologic time that is a subdivision of a period. The Pleistocene and Holocene epochs, for example, are divisions of the Quaternary period. Epoch is also used to describe a short length of...

• erosion general term for the processes by which the surface of the earth is constantly being worn away. The principal agents are gravity, running water, near-shore waves, ice (mostly glaciers), and wind...

• escarpment or scarp, long cliff, bluff, or steep slope, caused usually by geologic faulting (see fault ) or by erosion of tilted rock layers. An example of a fault scarp is the north face of the San Jacinto Mts. in California. Examples of erosional escarpments include the Palisades along the Hudson...

• esker long, narrow, winding ridge of stratified sand-and-gravel drift. Eskers, many miles long and resembling abandoned railway embankments, occur in Scandinavia, Ireland, Scotland, and New England; they arose from deposition of sediment in the beds of streams...

• estuary partially enclosed coastal body of water, having an open connection with the ocean, where freshwater from inland is mixed with saltwater from the sea. One type of estuary, called a drowned river...

• euphotic zone see ocean.

• fall line boundary between an upland region and a coastal plain across which rivers from the upland region drop to the plain as falls or rapids. A fall line is formed in an area where the rivers have eroded...

• fault in geology, fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on other planets and satellites of...

• fetch see wave , in oceanography.

• fiord see fjord.

• fire clay clay that has a high degree of resistance to heat. By the best standards it should have a fusion point higher than 1,600°C. The term "fire clay" is commonly held to exclude kaolin and other refractory potter's clays. Fire clay should contain high percentages of silica and alumina, with as little as possible of such impurities as lime,...

• fjord or fiord , steep-sided inlet of the sea characteristic of glaciated regions. Fjords probably resulted from the scouring by glaciers of valleys formed by any of several processes, including faulting and...

• flagstone see silt.

• flint variety of quartz that commonly occurs in rounded nodules and whose crystal structure is not visible to the naked eye. Flint is dark gray, smoky brown, or black in color; pale gray flint is called chert. When found...

• flint clay see fire clay.

• flood inundation of land by the rise and overflow of a body of water. Floods occur most commonly when water from heavy rainfall, from melting ice and snow, or from a combination of these exceeds the...

• floodplain level land along the course of a river formed by the deposition of sediment during periodic floods. Floodplains contain such features as levees, backswamps, delta plains, and oxbow lakes. Floodplains may be extensive, such as below the conflux of the Ohio and the Mississippi, where they have a width up to 80 mi (130 km). Rivers with extensive floodplains are...

• fold in geology, bent or deformed arrangement of stratified rocks. These rocks may be of sedimentary or volcanic origin. Although stratified rocks are normally deposited on the earth's surface in...

• ford shallow place in a body of water, especially a river, that may be crossed by wading. Around the crossings habitually forded, cities sprang up; hence fords came to be the sites of numerous river...

• foreshore see beach.

• fuller's earth mineral substance characterized by the property of absorbing basic colors and removing them from oils. It is composed mainly of alumina, silica, iron oxides, lime, magnesia, and water, in extremely...

• gabbro see basalt.

• geochemistry study of the chemical changes on the earth. More specifically, it is the study of the absolute and relative abundances of chemical elements in the minerals, soils, ores, rocks, water, and...

• geode hollow, globular rock nodule ranging in diameter from 1 to 12 in. (2.54-30.5 cm) or more. Most geodes are partly filled with mineral matter; they have a thin layer of chalcedony ( "wavy" quartz) covering an inner lining of inward-projecting crystals. These spectacular crystals, generally quartz or, less often, calcite, make geodes highly prized by collectors. Geodes are formed in a...

• geodesy or geodetic surveying, theory and practice of determining the position of points on the earth's surface and the dimensions of areas so large that the curvature of the earth must be taken into account. It is distinguished...

• geologic dating see dating.

• geologic timescale a chronological scale of earth's history used to measure the relative or absolute age of any part of geologic time. Of the numerous timescales, the most common is based on geologic time units,...

• Geologic Timescale Geologic Timescale Era Period Epoch Approximate duration (millions of years) Approximate number of years ago (millions of years) Cenozoic Quaternary Holocene 10,000 years ago to the present...

• geology science of the earth's history, composition, and structure, and the associated processes. It draws upon chemistry, biology, physics, astronomy, and mathematics (notably statistics) for support of...

• geomorphology study of the origin and evolution of the earth's landforms, both on the continents and within the ocean basins. It is concerned with the internal geologic processes of the earth's crust, such as...

• Geophysical Year, International see International Geophysical Year.

• geophysics study of the structure, composition, and dynamic changes of the earth , its atmosphere , hydrosphere and magnetosphere, based on the principles of physics. The term was probably first used in Germany, where it appeared in scientific writings of the mid-19th cent. Geophysics, which embraces the concepts, data, and methods of various other sciences, is...

• geyser [Icel.], hot spring from which water and steam are ejected periodically to heights ranging from a few to several hundred feet. Notable geysers are found in Iceland, New Zealand, and W United...

• glacial periods times during which large portions of the earth's surface were covered with thick glacial ice sheets. In the Pleistocene epoch , in the Carboniferous and Permian periods of the Paleozoic era era, and in Huronian time of the Precambrian era , the earth experienced an overall cooling of the climate, resulting in great ice sheets covering great portions of the oceans and continents. More or less extensive continental glaciations, or glacial advances, may have occurred at other times. The study of glacial periods owed its first impetus to the Swiss-American naturalist Louis...

• glacier moving mass of ice that survives year to year, formed by the compacting of snow into névé and then into granular ice and set in motion outward and downward by the force of gravity and the stress of...

• gneiss coarse-grained, imperfectly foliated, or layered, metamorphic rock. Gneiss is characterized by alternating light and dark bands differing in mineral composition and having coarser grains than those of schist. The light bands of gneiss are generally composed of quartz and feldspar. Hornblende, biotite mica, garnet, or graphite commonly form the dark bands. Gneisses result from the metamorphism of many igneous or sedimentary rocks, and are the most common types of rocks found...

• Gondwanaland see continental drift.

• granite coarse-grained igneous rock of even texture and light color, composed chiefly of quartz and feldspars. It usually contains small quantities of mica or hornblende, and minor accessory minerals may be present. Depending on the...

• gravel particles of rock, i.e., stones and pebbles, usually round in form and intermediate in size between sand grains and boulders. Gravel is composed of various kinds of rock, the most common...

• groin in oceanography: see coast protection.

• groundmass see porphyry.

• gyre see ocean.

• hardpan condition of the soil or subsoil in which the soil grains become cemented together by such bonding agents as iron oxide and calcium carbonate, forming a hard, impervious mass. It is disadvantageous...

• hogback sharp-crested ridge with steep slopes on both sides, formed by the erosion of steeply tilted rock layers. Hogbacks are commonly formed along the eroded flanks of large, tightly folded anticlines...

• Holocene epoch or Recent epoch, most recent of all subdivisions of geologic time, ranging from the present back to the time (c.11,000 years ago) of almost complete withdrawal of the glaciers of the preceding Pleistocene epoch. During the Holocene epoch, the sculpturing of the earth's surface to its present form was completed. Withdrawal of the glacial ice resulted in the development of the present-day drainage basins of...

• hot spring natural discharge of groundwater having an elevated temperature. Most hot springs result from the emergence of groundwater that has passed through or near recently formed, hot, igneous rocks...

• hydrothermal vent crack along a rift or ridge in the deep ocean floor that spews out water heated to high temperatures by the magma under the earth's crust. Some vents are in areas of seafloor spreading , and in some locations water temperatures above 350°C (660°F) have been recorded. The vents' hot springs leach out valuable subsurface minerals and deposit them on the ocean floor. The disolved...

• Ice Age see Pleistocene epoch.

• iceberg mass of ice that has become detached, or calved, from the edge of an ice sheet or glacier and is floating on the ocean. Because ice is slightly less dense than water about one ninth of the total...

• igneous rock see rock.

• International Geophysical Year (IGY), 18-month period from July, 1957, through Dec., 1958, during a period of maximum sunspot activity, designated for cooperative study of the solar-terrestrial environment by the scientists of...

• island relatively small body of land surrounded entirely by water. (As the oceans form a continuous mass of water on the earth's surface, all continents are islands in the strict sense of the word.) The...

• isopleth line drawn on a map through all points of equal value of some measurable quantity. In many meteorologic, oceanographic, or geologic studies some physical or chemical property is examined that...

• isostasy see continent.

• isthmus narrow neck of land connecting two larger land areas. Since it commands the only land route between two large areas and is on two seas, an isthmus has great strategical and commercial importance...

• jetty see coast protection.

• joint in geology, fracture in rocks along which no appreciable movement has occurred (see fault ). Nearly vertical, or sheet, joints that result from shrinkage during cooling are commonly found in igneous rocks. Similar joints occur in thick beds of sandstone and gneiss, with the sheets...

• Jurassic period [from the Jura Mts.], second period of the Mesozoic era of geologic time, lasting from 213 to 144 million years ago. At the start of the Jurassic most of the continents were joined together until the Atlantic began to form and the Americas split off...

• kame low, steep, rounded hill or ridge of layered sand and gravel drift, developed from glacial deposits. Kames were probably formed by streams of melting glacial ice that deposited mud and sand along...

• kaolin see china clay.

• Keewatin see Precambrian era.

• kettle oval depression found in glacial moraines , which are landforms made up of rock debris. When a glacier melts and draws away from an area, a block of ice may break off and be covered by earth and rock. As the block melts, the ground above it...

• kimberlite see diamond.

• lake inland body of standing water occupying a hollow in the earth's surface. The study of lakes and other freshwater basins is known as limnology. Lakes are of particular importance since they act as...

• landslide rapid slipping of a mass of earth or rock from a higher elevation to a lower level under the influence of gravity and water lubrication. More specifically, rockslides are the rapid downhill movement of large masses of rock with little or...

• Laurasia see continental drift.

• lava molten rock that erupts on the earth's surface, either on land or under the ocean, by a volcano or through a fissure. It solidifies into igneous rock that is also called lava. Before reaching the earth's surface, the mixture of solid and liquid rock, and gases, is known as magma. Lavas are...

• lithosphere brittle uppermost shell of the earth, broken into a number of tectonic plates. The lithosphere consists of the heavy oceanic and lighter continental crusts, and the uppermost portion of the mantle. The crust and mantle are separated by the Moho or Mohorovicic discontinuity (see earth and seismology ). The thickness of the lithosphere varies from to around 1 mi (1.6 km) at the mid-ocean ridges to approximately 80 mi (130 km) beneath older oceanic crust. The thickness of the continental...

• littoral zone see ocean.

• magma see lava.

• magnetic pole the two nearly opposite ends of the planet where the earth's magnetic intensity is the greatest, as the north and south magnetic poles. For the magnetic north, it is the direction from any point on...

• mantle portion of the earth 's interior lying beneath the crust and above the core. No direct observation of the mantle, or its upper boundary, has been made; its boundaries have been determined solely by abrupt changes in the...

• marble metamorphic rock composed wholly or in large part of calcite or dolomite crystals, the crystalline texture being the result of metamorphism of limestone by heat and pressure. The term marble is loosely applied to any limestone or dolomite that takes a good polish and is otherwise suitable as a building stone or ornamental stone. Marbles range in color from snow-white to gray and black,...

• Mercalli scale see Richter scale.

• mesa [Span.,=table], name given in the SW United States to a small, isolated tableland or a flat-topped hill. Two or more of the sides are steep and usually perpendicular and some have all four sides...

• Mesozoic era [Gr.,=middle life], major division of geologic time (see Geologic Timescale , table) from 65 to 225 million years ago. Great crustal disturbances that marked the close of the Paleozoic and the beginning of the Mesozoic eras brought about drastic changes in the topography of...

• metamorphic rocks see rock.

• metamorphism in geology, process of change in the structure, texture, or composition of rocks caused by agents of heat, deforming pressure, shearing stress, hot, chemically active fluids, or a combination of these, acting while the rock being changed remains essentially in the solid state...

• mid-ocean ridge see plate tectonics.

• Miocene epoch fourth epoch of the Tertiary period in the Cenozoic era of geologic time (see Geologic Timescale , table), lasting from around 24.6 to 5.1 million years ago.

• Mississippian period see Carboniferous period.

• Moho or Mohorovičić discontinuity : see earth.

• Mohole, Project program proposed in 1957 to drill a hole down to the boundary between the crust and the mantle , known as the Mohorovičić discontinuity at about 4 to 43 mi (7 to 70 km) below the earth's surface. Initiated by the American Miscellaneous Society, a loose organization of scientists, the main...

• moraine a formation composed of unsorted and unbedded rock and soil debris called till, which was deposited by a glacier. The till that falls on the sides of a valley glacier from the bounding cliffs makes up lateral moraines, running parallel to the valley sides. When two or more valley glaciers unite, their lateral...

• moulin see pothole.

• mountain high land mass projecting conspicuously above its surroundings and usually of limited width at its summit. Although isolated mountains are not unusual, mountains commonly form ranges, comprising...

• mudflow see landslide.

• New Red Sandstone name for the thick red layer of the Triassic formation in Great Britain (see Triassic period ). It is many thousands of feet thick and is composed chiefly of red sandstones, clays, and conglomerate; the red color and the occurrence of workable quantities of salt and gypsum suggest markedly...

• nodule see concretion.

• ocean interconnected mass of saltwater covering 70.78% of the surface of the earth, often called the world ocean. It is subdivided into four (or five) major units that are separated from each other in...

• oceanography study of the seas and oceans. The major divisions of oceanography include the geological study of the ocean floor (see plate tectonics ) and features; physical oceanography, which is concerned with the physical attributes of the ocean water, such as currents and temperature; chemical oceanography, which focuses on the chemistry of...

• Old Red Sandstone series of red and brown sandstones, conglomerates, and shales deposited in Wales and Scotland and in England near the Welsh and Scottish borders in the Devonian period of geologic time. The Old Red Sandstone, in contrast to the typical formations of the Devonian, is largely a continental formation, laid down in freshwater and on land as a result of the erosion of...

• Oligocene epoch third epoch of the Tertiary period in the Cenozoic era of geologic time, lasting from 38 to 24 million years ago. More of North America was dry land during the Oligocene than in the preceding Eocene epoch. The Gulf Coast was flooded, but the Atlantic coast N of South Carolina became emergent; the principal formation of the Gulf district was the Vicksburg limestone. The Pacific coast, like the more...

• Ordovician period [from the Ordovices, ancient tribe of N Wales], second period of the Paleozoic era of geologic time (see Geologic Timescale , table) from 505 to 438 million years ago. It was similar to the preceding Cambrian period , with shallow seas spread for most of the time over the British Isles, Scandinavia, the Baltic region, the Mediterranean region, a large part of Siberia, and much of North America. The Ordovician...

• pahoehoe see lava.

• Paleocene epoch first epoch of the Tertiary period in the Cenozoic era of geologic time (see geologic timescale ) between 60 to 66 million years ago. In W North America, the uplift of the Rocky Mts. that marked the end of the Mesozoic era continued throughout the Paleocene, and the Cretaceous inland seas gradually withdrew from the Great Plains area and central and SW California. In Montana and Wyoming the Fort Union shales and...

• paleomagnetism study of the intensity and orientation of the earth's magnetic field as preserved in the magnetic orientation of certain minerals found in rocks formed throughout geologic time. Paleomagnetic...

• paleontology [Gr.,= study of early beings], science of the life of past geologic periods based on fossil remains. Knowledge of the existence of fossils dates back at least to the ancient Greeks, who appear to have regarded them as the remains of various mythological creatures. Because few fossils are found in rock older than the late Precambrian era paleontology is generally concerned with only the past 600 million years. Although paleontology deals with early forms of life, it is usually treated as a part of geology rather than of biology , as the environment of the animals and plants cannot be properly understood and reconstructed without knowledge of the age, structure, and composition of the rocks in which their remains are found...

• Paleozoic era a major division (era) of geologic time (see Geologic Timescale , table) occurring between 570 to 240 million years ago. It is subdivided into six periods, the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian (see each listed individually)...

• Pangaea see continental drift.

• pass opening or way by which a natural or artificial barrier can be crossed. The term pass is usually applied to a relatively narrow passage through a mountainous region. A pass, like an isthmus, may have great strategic and economic importance; the history of a nation has often been...

• pegmatite see granite.

• pelagic zone see ocean.

• Pennsylvanian period see Carboniferous period.

• period unit of time on the geologic timescale. Periods are shorter than an era and longer than an epoch. Periods are of variable length, generally lasting tens of millions of years, with characteristic fossils found preserved in the sediments...

• permafrost permanently frozen soil, subsoil, or other deposit, characteristic of arctic and some subarctic regions; similar conditions are also found at very high altitudes in mountain ranges. In 1962...

• Permian period [from Perm, Russia], sixth and last period of the Paleozoic era (see Geologic Timescale , table) from 250 to 290 million years ago.

• petrifaction see fossil.

• petrology branch of geology specifically concerned with the origin, composition, structure, and properties of rocks , primarily igneous and metamorphic, and secondarily sedimentary. It includes petrography, the systematic description and classification of rocks using microscopic examination of rock in thin...

• piedmont any area near the foot of a mountain, particularly the plateau (the Piedmont ) extending from New York to Alabama E of the Appalachian Mts. and W of the Atlantic coastal plain. In Maryland, Virginia, and North Carolina it is E of the Blue Ridge Mts. The plateau is cut by...