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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 scientific study of the effects of music on the brain. Neuromusicology is a scientific discipline concerned with the function of the brain in music processing. Although neuromusicology is an area of study that utilizes present-day neurological…

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…

An explosion whose energy is produced by a nuclear transformation, either fission or fusion. Nuclear explosions can be either natural or artificial, for example in the form of a star undergoing a supernova or the deliberate detonation of an…

An anxiety disorder defined by the presence of recurrent, intrusive thoughts (known as obsessions) or repetitive behaviors that a person feels driven to perform (known as compulsions). Individuals with obsessive-compulsive disorder (OCD) experience…

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 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…

The subjective experiences of objects or events that ordinarily result from stimulation of the receptor organs of the body. Perception encompasses recognition in response to sensory stimuli. It involves the act or process by which the memory of…

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…

An optical instrument that permits viewing along a displaced or deflected axis, providing an observer with the view from a position which may be inaccessible or dangerous. Periscopes range in complexity from the simple unit-power tank periscope to…

A branch of psychology concerned with developing a scientifically defensible conception of human nature—in modern parlance, a general theory of behavior. A personality theory is a set of assumptions and expectations about oneself and other people…

An interactive technology intentionally designed for the purpose of changing users' attitudes or behaviors without coercion or deception. Persuasive technology melds the fields of psychology and information/communication systems. Researchers are…

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 scientific discipline examining human happiness and the psychological bases by which humans flourish as individuals and in communities. Positive psychology as a “movement" began in 1998 when Martin Seligman proposed, during his term as president…

An anxiety disorder in some individuals who have experienced a stressful and traumatic event that poses a direct threat to the individual's or another person's life. Posttraumatic stress disorder (PTSD) is a debilitating mental-health condition that…

The analysis and solution of problems by the use of mental processes. Cognitive psychology, which is a branch of psychology concerned with mental activities such as learning, memory, perception, and reasoning, seeks to understand the integral…

The adaptive capacity of an individual to cope with stress and other psychological challenges in life. Psychological resilience refers to positive adaptation among individuals at risk for maladjustment. It is characterized either by positive…

The study of human behavior and mental processes. Psychology is the scientific field that deals with the functions of the mind (brain) [see illustration] and the behavior of an individual in relation to its environment. Psychology is distinguished…

A discipline that merges psychology—the study of cognition, emotion, and behavior—and pharmacology, which characterizes the effects of drugs. Psychopharmacology is a field of psychology that investigates the use of drugs and medications to treat…

Methods for the quantitative study of the relations between physical stimulus magnitudes and the corresponding magnitudes of sensation. Psychophysical methods are psychological investigative techniques that study the relations of stimuli to the…

Physiological symptoms or illnesses that are caused or exacerbated by the mind. Psychosomatic disorders are physical diseases of the body that are causally related to or worsened by mental factors. Starting in the 1930s, the term psychosomatic…

Any treatment that applies psychological principles to produce changes that enhance the health and well-being of participants. Psychotherapy is concerned with the treatment of mental disorders by psychological means and techniques (Fig. 1). In…

A class of drugs, also called psychedelic or hallucinogenic drugs, that induce transient states of altered perception resembling or mimicking the symptoms of psychosis and are characterized by profound alterations in mood. Psychotomimetic drugs…

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…

Behavior related to the production of offspring, including such patterns as the establishment of mating systems, courtship, sexual behavior, parturition, and the care of young. Reproductive behavior encompasses all of the behavior patterns in…

A brain disorder that is characterized by unusual mental experiences, including hallucinations, delusions, changes in emotional experience, and severe decrements in social, cognitive, and occupational functioning. Schizophrenia is usually a…

A state of rest in which consciousness and activity are diminished, and in which an involuntary series of visual, auditory, or kinesthetic images, emotions, and thoughts occur in the mind, which take the form of a sequence of events or of a story,…

The neurological means by which sleep affects memory. Many of the functions of sleep remain largely unknown. However, sleep contributes importantly to the brain plasticity that underlies learning and memory. In particular, one function of sleep, in…

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 ship that can operate completely submerged in the water. The term "submarine" formerly applied to any ship capable of operating completely underwater, but now usually describes a ship built for military purposes (Fig. 1). The expected military…

A chemical sense by which flavors are perceived by receptors in the oral cavity. Taste, or gustation, is one of the senses used to detect (via taste buds; Fig. 1) the chemical makeup of ingested food, thereby establishing its palatability and…

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…

A group of psychiatric conditions, also known as mood disorders, that are characterized by disturbances of affect, emotion, thinking, and behavior. Affective or mood disorders are strong mental disturbances that interfere with daily life (Fig. 1).…

A psychiatric disorder in which a dramatic reduction in caloric intake consequent to excessive dieting leads to significant bodily, physiological, biochemical, emotional, psychological, and behavioral disturbances. Anorexia nervosa is one of three…