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Land surfaces characterized by roughness and strong relief. The distinction between hills and mountains is usually one of relative size or height, but the terms are loosely and inconsistently used. Because of the prevalence of steep slopes, hill…

That portion of geologic time that postdates the latest episode of continental glaciation. The Holocene Epoch is synonymous with the Recent or Postglacial interval of Earth's geologic history and extends from 10,000 years ago to the present day…

Hyperaridity refers to extreme dryness. Locations on the Earth's surface that experience hyperaridity have high rates of evaporation and plant transpiration and extremely low annual precipitation. On a global scale, hyperarid landscapes cover about…

The 180° meridian, where each day officially begins and ends. As a person travels eastward, against the apparent movement of the Sun, 1 h is gained for every 15° of longitude; traveling westward, time is lost at the same rate. Two people starting…

The period and system of rocks deposited during the middle part of the Mesozoic Era from the end of the Triassic to the beginning of the Cretaceous. The Jurassic Period encompasses an interval of time between about 201 and 145 million years ago,…

A plagioclase feldspar with composition Ab50An50 to Ab30An70 (Ab = NaAlSi3O8; An = CaAl2Si2O8), occurring in basic igneous rocks, that is, basalts, gabbros and anorthosites. Its hardness is 6–6.5; mean refractive index, 1.56; and specific gravity,…

An inland body of standing water occupying a depression on the Earth's surface. The surface of a lake is exposed to the atmosphere; in addition, subterranean lakes occur. The term lake includes expanded parts of rivers, reservoirs behind dams, and…

The middle era of the three major divisions of the Phanerozoic Eon (Paleozoic, Mesozoic, and Cenozoic eras) of geologic time. The Mesozoic encompasses an interval from 252 to 66 million years ago (MYA) based on various isotopic-age dates (Fig. 1).…

A mineral deposit formed by the interactions between microbes and their environment and thought to be one of the oldest ecosystems on Earth. Through the metabolic activities of microbialites, these ancient ecosystems have profoundly affected the…

The fourth oldest of the five major worldwide divisions (epochs) of the Tertiary Period; also referred to as the first epoch of the Neogene. Named by by Charles Lyell in 1833, the Miocene represents the interval of time from the end of the…

The system (referring to rocks) or subperiod (referring to time during which these rocks were deposited) denoting the lower (or older) subdivision of the Carboniferous. The name Mississippian is derived from rock exposures on the banks of the…

The effects of mountains on the atmosphere, ranging over all scales of motion, including very small (such as turbulence), local (for instance, cloud formations over individual peaks or ridges), and global (such as the monsoons of Asia and North…

That end of the Earth's axis which points toward the North Star, Polaris (Alpha Ursae Minoris). It is the geographical pole located at 90° N latitude where all meridians converge, and should not be confused with the north magnetic pole, which is in…

The third oldest of the seven geological epochs of the Cenozoic Era. It corresponds to an interval of geological time (and rocks deposited during that time) from the close of the Eocene Epoch to the beginning of the Miocene Epoch (Fig. 1). The most…

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 second-oldest period in the Paleozoic Era. The Ordovician is remarkable because not only did one of the most significant Phanerozoic radiations of marine life take place (early Middle Ordovician), but also one of the two or three most severe…

The oldest of the seven geological epochs of the Cenozoic Era, and the oldest of the five epochs that make up the Tertiary Period. The Paleocene Epoch represents an interval of geological time (and rocks deposited during that time) from the end of…

The earliest of the three Phanerozoic geological eras. The Paleozoic extends from about 541 to 252 million years ago (MYA) [Fig. 1]. It is the earliest era in which significant numbers of shelly fossils are found, and Paleozoic strata were among…

The name applied to the last period of geologic time in the Paleozoic Era and to the corresponding system of rock formations that originated during that period. The Permian Period commenced approximately 299 million years ago (MYA) and ceased about…

The uppermost eonothem (a time-stratigraphic unit composed of rocks formed during an eon of geologic time) of the Standard Global Chronostratigraphic Scale. The Phanerozoic Eonothem is the physical stratigraphic representation of the Phanerozoic…

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…

The older of the two epochs of the Quaternary Period (the younger being the Holocene). The Pleistocene Epoch/Series represents the interval of geological time (and rocks accumulated during that time) extending from the end of the Pliocene…

The youngest of the five geological epochs of the Tertiary Period. The Pliocene represents the interval of geological time (and rocks deposited during that time) that extends from the end of the Miocene Epoch to the beginning of the Pleistocene…

A major interval of geologic time between about 4600 and 541 million years ago (MYA), encompassing most of Earth history. The Precambrian is divided into the three eons: Hadean, Archean, and Proterozoic (Fig. 1). The Earth probably formed around…

A major division of geologic time spanning from 2500 to 541 million years ago (MYA). The beginning of Proterozoic time is an arbitrary boundary that roughly coincides with the transition from a tectonic style dominated by extensive recycling of the…

A period that encompasses the last 2.6 million years of the Cenozoic Era, and is concerned with major worldwide glaciations and their effect on land and sea, on worldwide climate, and on the plants and animals that lived then. The Quaternary is…

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…

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 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…

A mineral (FeCO3) with the same space group and hexagonal crystal system as calcite (CaCO3). Siderite has a gray, tan, brown, dark brown, or red color, has rhombohedral cleavages, and occasionally may show rhombohedral crystal terminations (see…

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)…

The third oldest period of the Paleozoic Era, spanning an interval from about 444 to 419 million years ago (MYA). The Silurian system includes all sedimentary rocks deposited and all igneous and metamorphic rocks formed in the Silurian Period (Fig.…

The older major subdivision (period) of the Cenozoic Era, extending from the Cretaceous (top of the Mesozoic Era) to the beginning of the Quaternary (younger Cenozoic Period). The term Tertiary corresponds to all the rocks and fossils formed during…

A cyclosilicate mineral family with (BO3) triangular groups and a complex chemical composition. The general formula can be written XY3Al6(OH)4(BO3)3(Si6O18), in which X = Na, Ca, and Y = Al, Fe3+, Li, Mg, Mn2+. The more common tourmalines are…

The oldest period of the Mesozoic Era. The Triassic encompasses an interval from about 252 to 201 million years ago (MYA) [Fig. 1]. It was named in 1848 by F. A. von Alberti for the threefold division of rocks at its type locality in central…

In this project, you will build a small, vertical-axis wind turbine, or VAWT for short. VAWTs are not as efficient as their horizontal-axis cousins, but they are better suited to urban environments where wind can come from different…

A continent that straddles the Equator, extending between 37°N and 35°S. It is the second largest continent, exceeded only by the Eurasia landmass (see illustration). The political geography of the continent continues to evolve as the traditional…

Although reefs occupy only around 0.2% of the ocean's area, they are a fundamental part of oceanic ecosystems. Geologically, reefs strongly influence local sedimentation and geomorphology. Biologically, they provide a habitat for a huge variety of…

Although long called a continent, in many physical ways Europe is but a great western peninsula of the Eurasian landmass. Its eastern limits are arbitrary and are conventionally drawn along the water divide of the Ural Mountains, the Ural River,…

Thurston's geometrization conjecture (now, a theorem of Grigory Perelman), which aims to answer the question: How could one describe the possible shapes of our universe? Here we will be assuming (although, some scientists think otherwise) that our…

Several lines of evidence indicate that Earth in the Cryogenian Period [approximately 720–635 million years ago (MYA)] experienced at least two global glaciations during which glaciers grew at sea level at the Equator. The term Snowball Earth is…

An abrupt, high-current electric discharge that occurs in the atmospheres of the Earth and other planets and that has a path length ranging from hundreds of feet to tens of miles. Lightning occurs in thunderstorms because vertical air motions and…

The common name for a number of perciform fishes in the family Scombridae, characterized by a long slender body, pointed head, and large mouth. The family Scombridae comprises mackerels and tunas. Mackerels have a fusiform and moderately compressed…

Plastic polymers have transformed the way we live since they were invented almost a century ago. They are used in many consumer products because they are lightweight, durable, inexpensive, and good insulating materials. Plastics are difficult to…

An optical effect of the sky formed by sunlight falling on the spherical droplets of water associated with a rain shower. The circular arc of colors in the rainbow is seen on the side of the sky away from the Sun. The bright, primary rainbow shows…

The southernmost of the New World or Western Hemisphere continents, with three-fourths of it lying within the tropics. South America is approximately 4500 mi (7200 km) long and at its greatest width 3000 mi (4800 km). Its area is estimated to be…

An order of bioluminescent, deep-sea, neoteleost fishes, including dragonfishes, marine hatchetfishes, and bristlemouths. The order Stomiiformes is probably the sister group to all other neoteleosts (that is, Ateleopodomorpha, Cyclosquamata,…

A laminated, microbial structure in carbonate rocks (limestone and dolomite rocks). Stromatolites are the oldest macroscopic evidence of life on Earth, at least 3.5 billion years old, and they are still forming in the seas. During the approximately…

The study of systems consisting of arbitrary sets of elements of unspecified type, together with certain operations satisfying prescribed lists of axioms. Abstract algebra has been developed since the mid-1920s and has become a basic idiom of…

The branch of meteorology that deals with atmospheric effects on the operation of vehicles in the atmosphere, including winged aircraft, lighter-than-air devices such as dirigibles, rockets, missiles, and projectiles. The air which supports flight…

A branch of meteorology that examines the effects and impacts of weather and climate on crops, rangeland, livestock, and various agricultural operations. The branch of agricultural meteorology dealing with atmospheric-biospheric processes occurring…

A predominantly mechanical mixture of a variety of individual gases enveloping the terrestrial globe to form the Earth's atmosphere. In this sense air is one of the three basic components, air, water, and land (atmosphere, hydrosphere, and…

In meteorology, an extensive body of the atmosphere which is relatively homogeneous horizontally. An air mass may be followed on the weather map as an entity in its day-to-day movement in the general circulation of the atmosphere. The expressions…

The force per unit area that the air exerts on any surface in contact with it, arising from the collisions of the air molecules with the surface. It is equal and opposite to the pressure of the surface against the air, which for atmospheric air in…

The temperature of the atmosphere represents the average kinetic energy of the molecular motion in a small region, defined in terms of a standard or calibrated thermometer in thermal equilibrium with the air.TemperatureMeasurement Many different…

Visible, infrared, and ultraviolet emissions from the atoms and molecules in the atmosphere above 30 km (20 mi), generally in layers, and mostly between 70 and 300 km (45 and 200 mi).The airglow, together with the ionosphere, is found in the…

A lithium aluminum phosphate mineral of basic formula LiAl(PO4)(F). The structure of amblygonite consists of phosphate (PO4) groups of tetrahedra and AlO6 groups of octahedra. Each PO4 tetrahedron is connected to an AlO6 octahedron. Corner-sharing…

A supergroup of silicate minerals that have similar chemical compositions and atomic arrangements and occur as rock-forming or accessory constituents in a wide variety of rocks. The crystal structure of the amphiboles is very chemically compliant…

A class of metamorphic rocks with one of the amphibole minerals as the dominant constituent. Most of the amphibolites are dark green to black crystalline rocks that occur as extensive layers widely distributed in mountain belts and deeply eroded…

Earth's current climate is relatively “cold” in comparison to most intervals of geological time, with permanent ice caps at the poles. However, for most of its history, our planet was ice-free and is considered to have been in a greenhouse state.…

At first, it may appear paradoxical that we must study life in the context of modern Earth in order to understand the earliest life in the fossil record. The oldest part of Earth's history is preserved in rocks of the Archean time period, from about…

Arenite (rock composed of sand-size fragments) that contains a high proportion of feldspar in addition to quartz and other detrital minerals. Arkose is also known as feldspathic sandstone. Although there is no universal agreement, many geologists…

The science of sound in the atmosphere. Atmospheric acoustics is concerned with sound outdoors rather than indoors. The atmosphere has a structure that varies in both space and time, and these variations have significant effects on a propagating…

Electrical processes that take place in the lower atmosphere. This activity is ubiquitous and is of two kinds: the intense local electrification accompanying storms, and the much weaker fair-weather electrical activity occurring over the entire…

The statistical description of atmospheric motions over the Earth, their role in transporting energy, and the transformations among different forms of energy. Through their influence on the pressure distributions that drive the winds, spatial…

Those oscillations in any or all atmospheric fields whose periods are integral fractions of either lunar or solar days. Oscillations with a period of a day are called diurnal, with a period of a half day semidiurnal, and with a period of a third of…

A distribution of atmospheric pressure and mass such that the specific volume, or density, of air is a function of both pressure and temperature, but not either alone. When the field is baroclinic, solenoids are present, there is a gradient of air…

An absolute pressure gage which is specifically designed to measure atmospheric pressure. This instrument is a type of manometer with one leg at zero pressure absolute.The common meteorological barometer (see illustration) is a liquid-column gage…

A distribution of atmospheric pressure and mass such that the specific volume, or density, of air is a function solely of pressure. When the field is barotropic, there are no solenoids, air temperature is constant on a surface of constant pressure,…

One of the most important algebraic identities, with many applications in a variety of fields. The binomial theorem, discovered by Isaac Newton, expresses the result of multiplying a binomial by itself any number of times, as given in Eq. (1), 1…

A branch of mathematics that was first developed systematically, because of its applications to logic, by the English mathematician George Boole, around 1850. Closely related are its applications to sets and probability.Boolean algebra also…

A mineral composed of calcium carbonate (CaCO3); one of the most common and widespread minerals in the Earth's crust. Calcite may be found in a great variety of sedimentary, metamorphic, and igneous rocks. It is also an important rock-forming…

The branch of mathematics dealing with two fundamental operations, differentiation and integration, which are carried out on functions. The subject, as traditionally developed in college textbooks, is partly an elementary development of the purely…

An extension of the part of differential calculus which deals with maxima and minima of functions of a single variable. The functions of the calculus of variations depend in an essential way upon infinitely many independent variables. Classically…

In its simplest form, a vector is a directed line segment. Physical quantities, such as velocity, acceleration, force, and displacement, are vector quantities, or simply vectors, because they can be represented by directed line segments. The algebra…

A large volcanic collapse depression, typically circular to slightly elongate in shape, the dimensions of which are many times greater than any included vent (Fig. 1). Calderas range from a few miles to 45 mi (75 km) in diameter. A caldera may…

A type of carved gemstone in which the background is cut away to leave the subject in relief. Often cameos are cut from stones in which the coloring is layered, resulting in a figure of one color and a background of another. The term cameo, when…

A family of minerals, related to the scapolite family, characteristically occurring in basic rocks such as nepheline syenites and sodalite syenites. A typical specimen is shown in the illustration. Cancrinite is hexagonal, a = 1.28, c = 0.52…

The mineral species containing the CO32− unit as an essential structural and chemical component. Members are distinguished by chemical composition (type of metal cation and in some cases other anion present) and crystal structure. More than 200…

An igneous rock in which carbonate minerals make up at least half the volume. Individual occurrences of carbonatite are not numerous (527 have been claimed) and generally are small, but they are widely distributed. Carbonatites are important…

A heart-shaped curve generated by a point of a circle that rolls (without slipping) on a fixed circle of the same diameter. In point-wise construction of the curve, let O be a fixed point of a circle C of diameter a, and Q a variable point of C. Lay…

An unusual group of sponges, belonging to the class Demospongiae in the phylum Porifera, with a lie-and-wait mode of predation has been shown to be unique among multicellular taxa and has a body plan that defies the general description for this…

A mineral having the composition SnO2. Cassiterite is the principal ore of tin. Its crystals are of the tetragonal rutile structure type, usually in prisms terminated by dipyramids. Twins with a characteristic notch called visor tin are common.…

A theory of mathematical structure in which smooth continuous inputs lead to discontinuous responses. Water suddenly boils, ice melts, a building crashes to the ground, or the earth unexpectedly buckles and quakes. The French mathematician René Thom…

A natural cavity located underground or in the side of a hill or cliff, generally of a size to admit a human. Caves occur in all types of rocks and topographic situations. They may be formed by many different erosion processes. The most important…

A mineral with the chemical composition SrSO4. Celestite occurs commonly in colorless to sky-blue, orthorhombic, tabular crystals. Fracture is uneven and luster is vitreous. Hardness is 3–3.5 on Mohs scale and specific gravity is 3.97. It fuses…

A common mineral in the upper oxidized zone of ore deposits containing galena (PbS). It is usually associated with anglesite, smithsonite, malachite, and other secondary minerals. Normally cerussite is close in composition to PbCO3 with only minor…

A mineral belonging to the zeolite family of silicates. It commonly occurs in well-crystallized groupings of crystals with external rhombohedral symmetry (point group 32/m) with nearly cubic angles. This well-developed external form suggests an…

A mineral with the chemical composition CuSO4· 5H2O. Chalcanthite commonly occurs in blue to greenish-blue triclinic crystals or in massive fibrous veins or stalactites (see illustration). Fracture is conchoidal and luster is vitreous. Hardness is…

A fine-grained fibrous variety of quartz, silicon dioxide. The individual fibers that compose the mineral aggregate usually are visible only under the microscope. Subvarieties of chalcedony recognized on the basis of color differences (induced by…

The term “chalk” is sometimes used in a broad sense for any soft, friable, or weathered fine-grained limestone; however the term is mostly restricted to pelagic (biogenic) limestones. Chalk is a uniformly fine-grained, typically light-colored marine…

System behavior that depends so sensitively on the system's precise initial conditions that it is, in effect, unpredictable and cannot be distinguished from a random process, even though it is deterministic in a mathematical sense. This article…

A hard, dense sedimentary rock composed of fine-grained silica (SiO2), most commonly quartz. Chert is characterized by a semivitreous to dull luster and a splintery to conchoidal fracture, and is most commonly gray, black, reddish brown, or green.…

One of a group of layer silicate minerals, usually green in color, characterized by a perfect cleavage parallel to (001). The cleavage flakes are flexible but inelastic, with a luster varying from pearly or vitreous to dull and earthy. The hardness…

A mineral having composition BeAl2O4 and crystallizing in the orthorhombic system. Chrysoberyl crystals are usually tabular parallel to the front pinacoid and frequently in pseudohexagonal twins (see illustration). There is good prismatic cleavage.…

A silicate mineral with composition essentially (Cu,Al)2H2Si2O5(OH)4·nH2O; iron, and more rarely manganese, may substitute for copper. Although acicular monoclinic crystals have been reported, the wide variation in the composition of analyzed…

Fine-grained, hydrous, layer silicates that belong to the larger class of sheet silicates known as phyllosilicates. Their structure is composed of two basic units. (1) The tetrahedral sheet is composed of silicon-oxygen tetrahedra linked to…

Turbulence above the boundary layer but not associated with cumulus convection. The atmosphere is a fluid in turbulent motion. That turbulence of a scale sensed by humans in aircraft is primarily associated with the boundary layer within a kilometer…

Variously shaped nodules consisting of fossilized peat in which the individual cells and tissue systems of the plant parts are infiltrated by minerals, principally calcium carbonate, along with pyrite, dolomite, and occasionally silica. This type of…

A special branch of the paleobotanical sciences concerned with the origin, composition, mode of occurrence, and significance of the fossil plant materials that occur in, or are associated with, coal seams. Information developed in this field of…