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Air temperatures adjusted to represent human comfort or discomfort under prevailing conditions of temperature, humidity, radiation, and wind. Theoretical formulas attempt to compare the rate of heat loss to surroundings with rate of heat production…

Clays used as raw material in manufacturing. Commercial clays are among the most important nonmetallic mineral resources. The value of clays is related to their mineralogical and chemical composition, particularly the clay mineral constituents…

A loosely defined term used for a sedimentary mineral segregation that may range in size from inches to many feet. Concretions are usually distinguished from the sedimentary matrix enclosing them by a difference in mineralogy, color, hardness, and…

A protuberance of the Earth's crustal shell with an area of several million square miles and with sufficient elevation above neighboring depressions (the ocean basins) so that much of it is above sea level.The term was originally applied to the most…

The concept that the world's continents once formed part of a single mass and have since drifted into their present positions.The fundamental concept is that not only have the continents drifted, but the continents are merely parts of thicker…

The response of corals to environmental stress in which they eject the single-cell algae that live within the transparent coral tissue, making the white coral skeleton visible. Corals (Fig. 1) are animals that live in a partnership (symbiosis) with…

A crystalline form of carbon and the hardest known mineral. Diamond has been found on all continents except Antarctica, although rocks associated with it have been found there. It occurs in nature as single crystals of gem (Fig. 1) or industrial…

The third planet from the Sun and the largest of the four inner, or terrestrial, planets. The Sun is an average-sized, middle-aged star situated in the Milky Way Galaxy. So far as is known, Earth (Fig. 1) is unique in the solar system in having…

The sciences that attempt to understand the origins, evolution, and behavior of the Earth in a broad context, including its place in the solar system and the universe. Many of the advances in the earth sciences have resulted from the greatly…

A sudden movement of the Earth caused by the abrupt release of accumulated strain along a fault in the interior. The released energy passes through the Earth as seismic waves (low-frequency sound waves), which cause the shaking. Seismic waves…

The branch of geology that deals with the ways in which geology affects people. Examples of the effect of geology on human civilizations include (1) how fertile soils develop from rocks and how these soils can become polluted by human activities;…

The branch of ecology that studies the environmental changes that occur in ecosystems in relation to fire. Disturbance is a major environmental influence in many of Earth's ecosystems. Vital to the healthy functioning of forests, savannas,…

The discipline of Earth science that investigates the chemical composition of the Earth, other planets, and the universe as a whole, as well as the chemical processes that occur within them. Geochemistry is important because basic knowledge about…

Computer-based technologies for storing, manipulating, and analyzing geographically referenced information. Attribute information and spatial information are integrated in geographic information systems (GIS) through the notion of a data layer,…

The science of the Earth. The study of the Earth's materials and of the processes that shape them is known as physical geology. Historical geology is the record of past events. Earth Earth sciencesEnergy from two sources continually produces changes…

The study of landforms, including the description, classification, origin, development, and history of planetary surface features. Emphasis is placed on the interpretation of the erosional and depositional features of the Earth's surface. However,…

Thermal or electrical power produced from the thermal energy contained in the Earth (geothermal energy). Use of geothermal energy is based thermodynamically on the temperature difference between a mass of subsurface rock and water and a mass of…

A broad field encompassing all aspects of the study of ice. Many glaciologists focus their attention on glaciers, the largest ice masses on Earth, but glaciology also includes the study of ice that forms on rivers, lakes, and the sea; ice in the…

Significant and persistent changes in the state of Earth’s climate. Earth's past climates, or paleoclimates, have alternated between ice ages and periods warmer than today over relatively long time scales (millions to tens of millions of years)…

The rise in the surface temperature of the Earth. Global warming is one of the names given to the systematic increase of globally averaged surface temperature that has been measured since the mid-nineteenth century. There is abundant evidence that…

A mineral with the chemical composition MnWO4. Hübnerite (sometimes spelled huebnerite) is the manganese member of the wolframite solid-solution series. It commonly contains small amounts of iron. It occurs in monoclinic, short, prismatic crystals.…

A tropical cyclone with maximum sustained winds that reach or exceed a threshold of 119 km/h (74 mi/h). In the western North Pacific Ocean such a storm is known as a typhoon and in other parts as a severe tropical cyclone. Many tropical cyclones do…

Accelerating fluxes and stores of water in the atmosphere, oceans, and land regions because of global warming. Figure 1 shows the primary hydrologic cycle compartments and fluxes. The total volume of water in the Earth system is approximately 1.36 ×…

A large mass of glacial ice broken off and drifted from parent glaciers or ice shelves along polar seas. Icebergs are distinguished from polar pack ice, which is sea ice, and from frozen seawater, whose rafted or hummocked fragments may resemble…

The harvest of animals and plants from the ocean to provide food for people and animals. Important products in marine capture fisheries include fish (Fig. 1); mollusks such as oysters, clams, and squid; and crustaceans such as crabs and shrimps.…

A combustible gas that occurs beneath the surface of the earth, often found in conjunction with petroleum deposits. Its main use is for fuel, but it is also used to make carbon black, certain chemicals, and liquefied petroleum gas. Liquefied…

Solid, icelike combinations of water and natural gas. Gas hydrates occur wherever appropriately-sized gases and water are brought together in sufficient quantities under specific conditions of low temperature and moderately high pressure. Prior to…

The natural biological and chemical processes through which inorganic and organic nitrogen are interconverted. It includes the processes of ammonification, ammonia assimilation, nitrification, nitrate assimilation, nitrogen fixation, and…

One of the major subdivisions of the interconnected body of salt water that occupies almost three-quarters of the Earth’s surface. Earth is the only planet in the solar system whose surface is covered with significant quantities of water (Fig. 1).…

The decrease in the pH of the ocean over time, as carbon dioxide (CO2) from the atmosphere is absorbed by and dissolves in seawater. Since the Industrial Revolution, rising carbon dioxide levels in the atmosphere and increased absorption of CO2 by…

A member of the class of curves that are intersections of a plane with a cone of revolution. It is obtained (see illustration) when the cutting plane is parallel to an element of the cone. Conic section Parabola as (a) conic section and (b) locus…

An auxiliary variable, functions of which give the coordinates of a curve or surface. The coordinates of a curve are functions of one parameter. A curve in 3-space has parametric equations (1). 1 The coordinates of a surface are functions of two…

A type of mathematical equation used, typically, to represent curves in a plane or in space of three dimensions. In principle, however, there is no limitation to any particular number of dimensions. A parameter is actually an independent variable.…

A mathematical operation performed on functions of more than one variable. In this article only two or three variables are considered; however, the principles apply to functions of n variables, for any positive integer n > 1. If z = f(x,y), the…

A technique of selective chemical etching to reveal tracks of heavy nuclear particles in a wide variety of solid substances. Developed in order to see fossil particle tracks in extraterrestrial materials, the technique finds application in many…

Any mollusk-formed calcareous concretion that displays an orient and is lustrous. There are two major groups of bivalved mollusks in which gem pearls (see illustration) may form: the saltwater pearl oyster, of which there are several species…

A dark-brown or black residuum produced by the partial decomposition and disintegration of mosses, sedges, trees, and other plants that grow in marshes and other wet places. Forest-type peat, when buried and subjected to geological influences of…

A mineral inosilicate with composition Ca2NaSi3­O8 (OH), crystallizing in the triclinic system. Crystals are usually acicular in radiating aggregates (see illustration). There is perfect cleavage parallel to the front and basal pinacoids yielding…

Exceptionally coarse-grained and relatively light-colored crystalline rock composed chiefly of minerals found in ordinary igneous rocks. Extreme variations in grain size also are characteristic (Fig. 1), and close associations with dominantly…

The younger subperiod of the Carboniferous. The Pennsylvanian derives its name from a thick succession of mostly nonmarine, coal-bearing strata in Pennsylvania, United States. Radiometric ages place the beginning of the Pennsylvanian at…

A mineral having composition (Fe,Ni)9S8. Pentlandite is the major ore of nickel. It crystallizes in the isometric system, but crystals are rare. It is usually massive, showing a well-defined octahedral parting. The hardness is 3.5–4 (Mohs scale) and…

A rock consisting of more than 90% of millimeter-to-centimeter-sized crystals of olivine, pyroxene, and hornblende, with more than 40% olivine. Other minerals are mainly plagioclase, chromite, and garnet. Much of the volume of the Earth's mantle…

A scenario for the transition of a natural process from regular motion to chaos. Various natural processes develop in time in a way that depends upon prevailing environmental details. A quantity that specifies the particular state of the environment…

A naturally occurring complex liquid hydrocarbon found in source rocks beneath the Earth’s surface which, after distillation, yields combustible fuels, petrochemicals, and lubricants; also known as crude oil. Strictly speaking, petroleum consists…

The extent to which a plant, or plant ecosystem, is susceptible to, or unable to cope with, the adverse effects of climate change. Plants are constantly changing their ecophysiology and distributions over different timescales as they respond to…

A method of obtaining age estimates on organic materials which has been used to date samples as old as 75,000 years. The method was developed in the late 1940s by Willard F. Libby and coworkers, and has provided age determinations in archeology,…

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 recording of information about objects by the use of a device not in physical contact with the object. The word remote is derived from the Latin word remotus and describes an object situated at some distance away. Remote-sensing (RS) devices are…

The noxious mixture of gases and particles commonly associated with air pollution in urban areas.The term smog is a contraction of smoke and fog. Air pollutionThe only characteristic of smog that is readily apparent to the unaided observer is the…

The electromagnetic radiation and particles (electrons, protons, alpha particles, and rarer heavy atomic nuclei) emitted by the Sun. The electromagnetic radiation (light) that shines from the Sun covers a wavelength range from x-rays to radio waves,…

A series of long waves generated by underwater earthquakes, landslides, slumps, volcanic eruptions, meteorological events, and asteroid impacts that violently flood adjacent and distant coastlines and coastal communities, with devastating impact.…

The second largest body in the asteroid belt. Vesta constitutes about 9 percent of the mass in the asteroid belt, with the largest body, Ceres, constituting 25 percent. Because Vesta is not quite spherical, it does not qualify as a dwarf planet,…

A mountain or hill, generally steep-sided, formed by accumulation of magma (molten rock with associated gas and crystals) erupted through openings or volcanic vents in the Earth's crust; the term volcano also refers to the vent itself. During the…

The scientific study of volcanic phenomena, especially the processes, products, and hazards associated with active or potentially active volcanoes. Volcanology focuses on eruptive activity that has occurred within the past 10,000 years of the…

Human influence on the weather and, ultimately, climate. Human impacts on weather can be either intentional, as with cloud seeding (Fig. 1) to clear fog from airports or to increase precipitation, or unintentional, as with air pollution, which…

The coldest, windiest, and driest continent. The lowest temperature ever measured on Earth, −89.2°C (−128.5°F), was recorded at the Russian Antarctic station of Vostok in July 1983. Katabatic (cold, gravitational) winds with velocities up to 50 km/h…

The techniques concerned with constructing maps from geographic information. Maps are spatial representations of the environment. Typically, maps take graphic form, appearing on computer screens or printed on paper (Fig. 1), but they may also take…

The long-term records of precipitation, temperature, wind, and all other aspects of the Earth's climate. Climate, like the Earth itself, has a history extending over several billion years. Climatic changes have occurred at time scales ranging from…

Pathological conditions affecting the health of coral and causing its physiological dysfunction. Broadly defined, coral disease (Fig. 1) is any impairment to coral health resulting in physiological disturbance and breakdown. It usually involves…

A measure of the three-dimensional structure of live coral communities on reefs. Coral reef structural complexity is an intrinsic element of coral reef ecosystems and quantitates the surface area of coral in relation to its linear area. The…

A period of abnormally dry weather sufficiently prolonged so that the lack of water causes a serious hydrologic imbalance, such as crop damage or water-supply shortage, in the affected area. Droughts (Fig. 1) often have devastating economic, social,…

The dense kernel of Earth comprised primarily of iron and nickel that contains nearly one-third of the mass of the planet. Three distinct compositional spheres make up the Earth’s interior: the core, mantle, and crust (Fig. 1). A model of the…

The process by which water bodies, such as estuaries, coastal waters, and lakes, become enriched in dissolved nutrients, such as phosphorus and nitrogen, resulting in increased growth of algae and aquatic plants as well as the deterioration of the…

Any weather event that is unusual for a particular location or severe in its effects. The most commonly considered examples of extreme weather events include heat waves, cold snaps, heavy rainfall or snowfall, ice or hail storms, droughts,…

A troughlike glaciated valley recognized as a narrow, deep inlet of the sea between high cliffs or steep slopes. It differs from other glaciated valleys only in the fact of submergence (Fig. 1). The floors of many fjords are elongate basins…

A physiologically interactive system within which growth, adaptation, species composition, nutrient cycling, biological productivity, and energy flows among inland aquatic microbial, plant, and animal communities are integrated with the environment.…

An ordered, internally consistent, internationally recognized sequence of time intervals, each distinct in its own history and record of life on Earth, including the assignment of absolute time in years to each geologic interval. The geologic time…

The study of the Earth’s physical properties using the principles and practices of physics. Geophysics is distinguished from geology by its use of instruments to make direct and indirect measurements of the phenomena being studied in contrast to the…

The occurrence, circulation, distribution, and properties of any liquid water residing beneath the surface of the Earth. This article excludes any water molecules that constitute mineral species such as clays. Generally, groundwater is that fraction…

The period extending for several hundred million years from the formation of the Earth, probably around 4500 million years ago (MYA) to the first known occurrence of a preserved rock record around 4000 MYA. The length of the Hadean is comparable to…

A well stimulation technique, commonly referred to as “fracking,” that is part of a larger process of the development of unconventional oil and gas wells. During hydraulic fracturing, oil and gas operators inject a treatment fluid into an oil or gas…

The management of marine species and ecosystems to prevent their decline and extinction. As in terrestrial conservation, the goal of marine conservation is to protect biodiversity and ecosystem function through the preservation of species,…

The study of the atmosphere and its phenomena. Meteorology and climatology are rooted in different parent disciplines, the former in physics and the latter in physical geography. They have, in effect, become interwoven to form a single discipline…

The third largest continent, extending from the narrow isthmus of Central America to the Arctic Archipelago. The physical environments of North America, like the rest of the world, are a reflection of specific combinations of the natural factors…

The effect of the ocean absorbing excess heat gained by Earth's climate system. The ocean covers 71% of the Earth’s surface and, together with the atmosphere, terrestrial waters (lakes, rivers, and groundwater), and different forms of ice (snow, sea…

A rise in global mean sea level (GMSL) caused by an increase in ocean mass and temperature. Global mean sea level, the average height of the ocean surface relative to Earth’s surface, is a fundamental indicator of climate change and has considerable…

A class of economical spacecraft smaller in size and lower in mass than conventional satellites and used for diverse applications in academia, research, commerce, and national defense. Small satellites, or SmallSats, are defined here as spacecraft…

The field of knowledge concerned with collecting, analyzing, and presenting data. Statistical methods are regularly used by researchers in the physical, biological, and social sciences, as well as engineers, business managers, government officials…

The atmospheric layer that is immediately above the troposphere and contains most of the Earth's ozone. Here temperature increases upward when ozone absorbs solar ultraviolet light, protecting life from harmful electromagnetic radiation (Fig. 1).…

The ozone (O3) layer located above the troposphere, containing about 90 percent of the atmosphere's ozone and acting as an ultraviolet-radiation screen. While ozone is found in trace quantities throughout the atmosphere, the largest concentrations…

A violently rotating, tall, narrow column of air (that is, a vortex), typically about 300 ft (100 m) in diameter, that extends to the ground from a cumulonimbus cloud. The vast majority of tornadoes rotate cyclonically (counterclockwise in the…

An ecosystem that forms a transitional area between terrestrial and aquatic components of a landscape. Wetlands are areas characterized by a high content of soil moisture. They typically are shallow-water to intermittently flooded ecosystems,…

Precipitation that incorporates anthropogenic acids and acidic materials. Acid precipitation, strictly defined, contains a greater concentration of hydrogen (H+) ions than of hydroxyl (OH−) ions, resulting in a solution pH less than 7. Under this…

Natural and artificial substances in the atmosphere that affect human health or well-being, or the well-being of any other organisms in the environment. Air pollution also applies to situations in which contaminants affect structures and artifacts…

The now-buried remains of a large asteroid impact that occurred approximately 66 million years ago off the Yucatán Peninsula in Mexico, causing mass extinction on a global scale. Measuring approximately 200 km (124 mi) in diameter, the Chicxulub…

Coral reefs proliferate in ocean waters that are oligotrophic, that is, extremely poor in nutrients. This paradox, first noted by Charles Darwin when he passed through Tahiti in 1842, was difficult to comprehend until symbiotic relationships between…

The ability of a planetary atmosphere to inhibit heat loss from the planet's surface, thereby enhancing the surface warming that is produced by the absorption of solar radiation. The greenhouse effect acts like a thermal blanket, limiting heat loss…

The science dealing with all aspects of the waters of the Earth: their occurrence, circulation, and distribution; their chemical and physical properties; and their reaction with the environment, including their relation to living things. Water…

The Earth's water supply and its natural distribution. Freshwater is a renewable resource, and it is continually being replaced by precipitation, but it is not evenly distributed and is scarce in many areas. The uses of water worldwide are for…

Global climate change can act as a stressor on wild animal populations in several ways. Direct effects can occur when a change in climate exceeds physiological optima for species, such as when waterfowl or amphibians confront dry breeding ponds, or…

How will rapid urbanization affect how cities develop sustainably in the future? What are the best interventions for reducing crime rates in an area? Can we balance human resource requirements with environmental preservation? These are not trivial…

An interval of time in Earth history (Cambrian Period) and its rock record (Cambrian System). The Cambrian Period spanned about 60 million years and began with the first appearance of marine animals with mineralized (calcium carbonate, calcium…

The interval of geological time, and rocks deposited during that time, from the end of the Devonian Period to the beginning of the Permian Period. The Carboniferous Period spanned from about 359 to 299 million years ago during the Paleozoic Era…

The youngest and shortest of the three Phanerozoic geological eras. It represents the geological time (and rocks deposited during that time) extending from the end of the Mesozoic Era to the present day (see illustration). Cenozoic, the third and…

In geological time, the last period of the Mesozoic Era, preceded by the Jurassic Period and followed by the Tertiary Period. The rocks formed during Cretaceous time constitute the Cretaceous System. Omalius d'Halloys first recognized the…

The geologic period and system of the Paleozoic Era from the end of the Silurian Period to the beginning of the Carboniferous Period. The Devonian Peroid encompasses an interval of geologic time between 419 and 359 million years ago (MYA) based on…

The second oldest of the five major worldwide divisions (epochs) of the Tertiary Period; also referred to as the middle epoch of the Paleogene. The Eocene (Fig. 1) extends from 56.0 to 33.9 million years ago (MYA).Tertiary 1 Eocene, the second…

A hard, dense silicate mineral which occurs as crystals of cubic symmetry in a wide range of geologic environments. The general chemical formula of the silicate garnet group is A3B2(SiO4)3 where, in natural occurrences, the A cations are dominantly…

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…