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The practice of utilizing geological principles and applying geological concepts to the discovery and recovery of petroleum. Related fields in petroleum discovery include geochemistry and geophysics. The related areas in petroleum recovery are…

The relatively smooth sections of the continental surfaces, occupied largely by gentle rather than steep slopes and exhibiting only small local differences in elevation. Because of their smoothness, plains lands, if other conditions are favorable,…

The theory explaining the movement across the Earth's surface of the rigid plates that make up the Earth's lithosphere. Numerous other geological phenomena such as lateral variations in surface heat flow, the physiography and geology of ocean…

Any elevated area of relatively smooth land. Usually the term is used more specifically to denote an upland of subdued relief that on at least one side drops off abruptly to adjacent lower lands. In most instances the upland is cut by deep but…

The alteration of rocks or crystallization of minerals by gases or supercritical fluids (generically termed magmatic fluids) derived from solidifying magma. At surface conditions, magmatic fluids sampled from active volcanoes contain steam with…

The Pleistocene or Glacial Epoch is less remarkable for radical evolutionary changes in plant life than for repeated shifts of position because of climatic changes and the resulting ice movements. The influence of these changes is known to have…

A mineral having composition Ag3AsS3 and crystallizing in the hexagonal system. It occurs in prismatic crystals terminated by steep ditrigonal pyramids, but is more commonly massive or in disseminated grains. There is good rhombohedral cleavage.…

In sedimentary geology, all characteristics of the source area from which clastic sediments and sedimentary rocks are derived, including relief, weathering, and source rocks. Weathering processesThe goal of most provenance studies of sedimentary…

An instrument consisting of two thermometers which is used in the measurement of the moisture content of air or other gases. The bulb or sensing area of one of the thermometers either is covered by a thin piece of clean muslin cloth wetted uniformly…

A study of the physical and thermodynamic properties of the atmosphere. The properties of primary concern in air conditioning are (1) dry-bulb temperature, (2) wet-bulb temperature, (3) dew-point temperature, (4) absolute humidity, (5) percent…

A rock froth, formed by the extreme puffing up (vesiculation) of liquid lava by expanding gases liberated from solution in the lava prior to and during solidification. Some varieties will float in water for many weeks before becoming waterlogged.…

A large, geologically significant group of dark, rock-forming silicate minerals. Pyroxene is found in abundance in a wide variety of igneous and metamorphic rocks. Because of their structural complexity and their diversity of chemical composition…

The application of radar to the study of the atmosphere and to the observation and forecasting of weather. Meteorological radars transmit electromagnetic waves at microwave and radio-wave frequencies. Water and ice particles, inhomogeneities in the…

A branch of environmental geochemistry and isotope geology concerned with the occurrence of radioactive nuclides in sediment, water, air, biological tissues, and rocks. The nuclides have relatively short half-lives ranging from a few days to about…

A rigid and wave-resistant marine structure that stands above its surroundings. To navigators, a reef is any rocky structure that poses a threat to navigation. Marine biologists and geologists specify that reefs are constructed by organisms that…

The mineral form of manganese carbonate. Calcium, iron, magnesium, and zinc have all been reported to replace some of the manganese. The equilibrium replacement of manganese by calcium has been determined and found to increase with the temperature…

One of the geomorphological expressions between two tectonic plates that are opening relative to each other or sliding past each other. The term originally was used to describe the central graben structures of such classic continental rift zones as…

A relatively common aggregate of mineral grains. Some rocks consist essentially of but one mineral species (monomineralic, such as quartzite, composed of quartz); others consist of two or more minerals (polymineralic, such as granite, composed of…

Finding the age of rocks based on the presence of naturally occurring long-lived radioactive isotopes of several elements in certain minerals and rocks. Measurements of rock ages have enabled geologists to reconstruct the geologic history of the…

A secondary, planar structure of deformed rocks. A cleavage is penetrative and systematic, as opposed to fractures and shear zones which may occur alone or in widely spaced sets. It is generally better developed in fine grained rocks than in coarse…

The permanent and induced magnetism of rocks and minerals on scales ranging from the atomic to the global, including applications to magnetic field anomalies and paleomagnetism. Natural compasses, concentrations of magnetite (Fe3O4) called…

A dark to dark-brown coating on rock surfaces exposed to the atmosphere (Fig. 1). Rock varnish is probably the slowest-accumulating sedimentary deposit, growing at only a few micrometers to tens of micrometers per thousand years. Its thickness…

The red to red-violet variety of the gem mineral tourmaline. Perhaps the most sought-for of the many colors in which tourmaline occurs, it was named for its resemblance to ruby (see Illustration). The color is thought to be caused by the presence of…

The red variety of the mineral corundum, in its finest quality the most valuable of gemstones. Only medium to dark tones of red to slightly violet-red or very slightly orange-red are called ruby; light reds, purples, and other colors are properly…

Deposits of bedded sedimentary rocks composed of salts precipitated during solar evaporation of surface or near-surface brines derived from seawater or continental waters. Dominant minerals in ancient evaporite beds are anhydrite (along with varying…

An upwelling of crystalline rock salt and its aureole of deformed sediments. A salt pillow is an immature salt dome comprising a broad salt swell draped by concordant strata. A salt stock is a more mature, pluglike diapir of salt that has pierced,…

Unconsolidated granular material consisting of mineral, rock, or biological fragments between 63 micrometers and 2 millimeters in diameter. Finer material is referred to as silt and clay; coarser material is known as gravel. Sand is usually produced…

A clastic sedimentary rock comprising an aggregate of sand-sized (0.06–2.0-mm) fragments of minerals, rocks, or fossils held together by a mineral cement. Sandstone forms when sand is buried under successive layers of sediment. During burial the…

Any gem variety of the mineral corundum (Al2O3), except those called ruby because of their medium- to dark-red color. Sapphire has a hardness of 9 (Mohs scale), a specific gravity near 4.00, and refractive indices of 1.76–1.77.Although blue…

A term possessing genetic implications, originally defined as an aquatic sediment rich in organic matter that formed under reducing conditions (lack of dissolved oxygen in the water column) in a stagnant water body. This contrasts with the term…

The branch of meteorological science that uses meteorological sensing elements on satellites to define the past and present state of the atmosphere. Meteorological satellites can measure a wide spectrum of electromagnetic radiation in real time,…

An aluminosilicate mineral containing sodium (Na), calcium (Ca), chlorine (Cl), and carbonate (CO3). It is commonly found as light-colored, translucent tetragonal prisms (Fig. 1). Scapolite is normally white, but many other colors are known,…

A layer of organisms in the sea which causes sound to scatter and returns echoes. Recordings by sonic devices of echoes from sound scatterers indicate that the scattering organisms are arranged in approximately horizontal layers in the water,…

A mineral consisting of calcium tungstate, CaWO4. Scheelite occurs in colorless to white, tetragonal crystals (see illustration); it may also be massive and granular. Its fracture is uneven, and its luster is vitreous to adamantine. Scheelite has a…

Medium- to coarse-grained, mica-bearing metamorphic rock with well-developed foliation termed schistosity. Schist is derived primarily from fine-grained, mica-bearing rocks such as shales and slates. The schistosity is formed by rotation,…

The process whereby mapping technologies are used to produce highly detailed images of the seafloor. Ever since large ships began sailing the oceans, there has been a need to know what the seafloor looks like. At first this need was simply to avoid…

A seamount is a mountain that rises from the ocean floor; a submerged flat-topped seamount is termed a guyot. By arbitrary definition, seamounts must be at least 3000 ft (about 900 m) high, but in fact there is a continuum of smaller undersea…

Rocks formed of any of the materials that accumulate at Earth's solid surface at ambient temperatures, regardless of their consistency. Together with lava flows, sedimentary rocks form a cover of stratified material, known as the stratisphere, that,…

The study of natural sediments, both lithified (sedimentary rocks) and unlithified, and of the processes by which they are formed. Branches of this discipline have been known as sedimentation or sedimentary petrology, or they have been included in…

The probability that social or economic consequences of earthquakes will equal or exceed specified values at a site, at several sites, or in an area, during a specified exposure time. Historically the term seismic risk has been used to describe an…

Determination of the nature of sedimentary rocks and the fluids they contain from analysis of seismic data. Prior to the 1970s, seismic data were used only to map the structure of the Earth. Since then, improvements in seismic data quality have…

Various devices or systems of devices for measuring movement in the Earth. Ground motion is generally the result of passing seismic waves, gravitational tides, atmospheric processes, and tectonic processes. Seismographic instrumentation typically…

The study of the shaking of the Earth's interior caused by natural or artificial sources. The revolution in understanding the Earth brought about by the paradigm of plate tectonics in the early 1960s provided a rationale that has since guided an…

The study of stratigraphic sequences, defined as stratigraphic units bounded by unconformities. L. L. Sloss was the first to formally establish, in 1963, that Phanerozoic cratonic successions in North America could be subdivided into six broad…

A common rock composed of serpentine minerals, serpentinite is usually formed through the hydration of ultramafic rocks, dunites, and peridotites in a process known as serpentinization. The result is the formation of hydrated magnesium-rich…

A class of fine-grained clastic sedimentary rocks with a mean grain size of less than 0.0625 mm (0.0025 in.), including siltstone, mudstone, and claystone. One-half to two-thirds of all sedimentary rocks are shales (Fig. 1). Sedimentary…

Silica (SiO2) occurs naturally in at least nine different varieties (polymorphs), which include tridymite (high-, middle-, and low-temperature forms), cristobalite (high- and low-temperature forms), coesite, and stishovite, in addition to high (β)…

All silicates are built of a fundamental structural unit, the so-called SiO4 tetrahedron. The crystal structure may be based on isolated SiO4 groups or, since each of the four oxygen ions can bond to either one or two silicon (Si) ions, on SiO4…

Assemblages of silicate minerals. Studies of the nature of these minerals provide information on their origin and their role in rock formation. This article discusses kinetic processes in silicate systems, oxidation-reduction equilibria in…

Fine-grained sediment and sedimentary rock dominantly composed of the microscopic remains of the unicellular, silica-secreting plankton diatoms and radiolarians. Minor constituents include extremely small shards of sponge spicules and other…