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The phenomenon occurring when the Sun does not set, but only approaches the horizon at midnight. The effect occurs near the time of the summer solstice, on June 21, for latitudes north of the Arctic Circle. The same effect occurs near the time of…

A naturally occurring solid phase with a restricted chemical composition and a definite atomic arrangement. This is really an idealized definition insofar as some minerals (especially those that form at low temperature) have poorly defined atomic…

The first star recognized to have a periodic brightness variation. Mira (officially designated Omicron Ceti, in the constellation Cetus, the Whale) was discovered in 1596 by David Fabricius (or Fabricus), a clergyman and an amateur astronomer from…

A distorted scene of distant objects resulting from the passage of light through a nonuniform medium. In a uniform atmosphere light would travel in straight lines and distant scenes would appear without distortion. Since the density of the air…

The second-magnitude star that marks the bend in the handle of the Big Dipper, ζ Ursae Majoris. Mizar was the first telescopic visual binary to be discovered (1650), the first ever to be photographed (1857), and the first star discovered to be a…

The smallest of the four major deserts in the North American Desert region in the continental United States and is located in the southwestern quadrant of the Basin and Range physiographic province in southeastern California and portions of Arizona,…

A large and relatively dense cloud of cold gas and dust in interstellar space from which new stars are born. Molecular clouds consist primarily of molecular hydrogen (H2) gas, have temperatures in the range 10–100 K, and contain 1031–1036 kg of mass…

Any of several units of time based on the revolution of the Moon around Earth.The calendar month is one of the 12 arbitrary periods into which the calendar year is divided. CalendarThe synodic month, the period of the lunar phases, is the average…

A group name for all clay minerals with an expanding structure, except vermiculite, and also a specific mineral name for the high alumina end member of the group. VermiculiteMontmorillonite clays have wide commercial use. The high colloidal,…

A concentration of interstellar matter that is largely gaseous but can contain almost all states of matter, including solid particles, and that is a constituent of the Milky Way Galaxy or another galaxy. The term “nebula” was originally used to…

Ground-level, or tropospheric ozone (O3), is a strong oxidizing agent that reacts with living tissue. Consequently, tropospheric ozone is considered an air pollutant and a respiratory irritant at low concentrations. At higher concentrations,…

Using old data from the NASA spacecraft MESSENGER, researchers have demonstrated how to measure the average lifetime of a neutron from space. This finding is important because, despite decades of efforts, physicists still cannot definitively say how…

An isolated fertile area, usually limited in extent and surrounded by desert. The term was initially applied to small areas in Africa and Asia typically supporting trees and cultivated crops with a water supply from springs and from seepage of water…

The general circulation of the ocean. The term is usually understood to include large-scale, nearly steady features, such as the Gulf Stream (Fig. 1), as well as current systems that change seasonally but are persistent from one year to the next,…

Propagating oscillations in the ocean that carry energy and momentum from one region to another. Most ocean waves (Fig. 1) are caused directly or indirectly by wind blowing across the sea surface. Many waves can propagate through the ocean thousands…

The science of the sea, including physical oceanography, marine chemistry, marine geology, and marine biology. The need to know more about the impact of marine pollution and possible effects of the exploitation of marine resources, together with the…

Waters of varying mineral content which are found associated with petroleum and natural gas or have been encountered in the search for oil and gas. They are also called oil-field brines, or brines. They include a variety of underground waters,…

A name given to a group of magnesium-iron silicate minerals crystallizing in the orthorhombic system. Crystals are usually of simple habit, a combination of the dipyramid with prisms and pinacoids. The luster is vitreous and the color olive green,…

The study of the abundance and composition of naturally occurring organic substances, their origins and fate, and the processes that affect their distributions on Earth and in extraterrestrial materials.Organic geochemistry was born from a curiosity…

A mineral having composition As2S3 and crystallizing in the monoclinic system. Crystals are small, tabular, and rarely distinct (see illustration); the mineral occurs more commonly in foliated or columnar masses. There is one perfect cleavage…

A geographic designation that includes thousands of mainly small coral and volcanic islands scattered across the Pacific Ocean from Palau in the west to Easter Island in the east. Island archipelagos off the coast of the Asian mainland, such as…

The largest of the world's oceans, having an area of 1.65 × 108 km2 (6.3 × 107 mi2) and a mean depth of 4282 m (14,049 ft).The Pacific Ocean covers 32% of the Earth's surface and 46% of the surface of all oceans and seas and its area is greater than…

A soil of the past, that is, a fossil soil. Paleosols are most easily recognized when they are buried by sediments. They also include surface profiles that are thought to have formed under very different conditions from those now prevailing, such as…

A northern constellation named for a Greek hero (see illustration). Perseus and adjacent Cassiopeia are prominently located in the northern Milky Way. In Johan Bayer's (1603) star atlas, the bright star Mirfak marks Perseus' shoulder.…

A rare and very large area of severely depleted ozone (03) opened up in the stratosphere above the Arctic Circle in March 2020. Scientists attribute this ozone hole to the persistence of a very strong polar vortex (a jet stream that circles the…

Any of the oriented intergrowths of potassium- and sodium-rich feldspars, (K,Na)AlSi3O8, whose proportions are determined in part by the initial composition of the alkali feldspar from which they exsolved and whose physical properties are thus…

The systematic study of the fabrics of rocks, generally involving statistical study of the orientations and distributions of large numbers of fabric elements. The term fabric denotes collectively all the structural or spatial characteristics of a…

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 Fishes, a large zodiacal constellation in the northern sky. None of the stars in the constellation are very bright, but many are joined in the shape of a large V, making them easy to notice. The constellation also contains the Circlet of stars.…

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 ejected outer layers of stars born with mass between approximately 1 and 8 times the solar mass. Toward the end of their lives, such stars go through the asymptotic giant branch phase. At the end of this phase they suffer a dramatic increase in…

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…

A typical open cluster of stars (see illustration), and the best example in the sky of an open cluster for the casual observer. (Open clusters are one of the two basic morphological types of star clusters, distinguished from globular clusters by…

The star α Ursae Minoris, also known as the North Star or Pole Star. It is perhaps the best-known star in the northern sky. Its location only 1° of arc from the north celestial pole, the point where the Earth's axis of rotation intersects the…

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…

An instrument used in astronomical research to study naturally occurring radio emission from stars, galaxies, quasars, pulsars, interstellar clouds, and other astronomical bodies between wavelengths of about 1 mm (300 GHz) and 10 m (30 MHz). At 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…

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…

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 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…