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A phylum of solitary, exclusively marine, coelomate, bivalved animals, with both valves symmetrical about a median longitudinal plane. Brachiopods are typically attached to the substrate by a posteriorly located cuticle-covered stalk called a…

A clastic rock composed of angular gravel-size fragments; the consolidated equivalent of rubble. The designation gravel-size refers to a mean particle diameter greater than 0.08 in. (2 mm), which means that 50% or more of the particles (by volume)…

In a narrow sense, the electromagnetic radiation emitted by electrons when they pass through matter. Charged particles radiate when accelerated, and in this case the electric fields of the atomic nuclei provide the force which accelerates the…

The irregular motion of a body arising from the thermal motion of the molecules of the material in which the body is immersed. Such a body will of course suffer many collisions with the molecules, which will impart energy and momentum to it.…

A phylum of sessile aquatic invertebrates (also called Polyzoa) that form colonies of zooids. Each zooid, in its basic form, has a lophophore of ciliated tentacles situated distally on an introvert, a looped gut with the mouth inside the lophophore…

Part of a clay and silt sequence that accumulated at the foot of a colossal “reef” during the Cambrian explosion, a dramatic evolutionary radiation of animals beginning about 545 million years ago. Although this explosion is most obvious from the…

A member of the plagioclase feldspar solid-solution series with a composition ranging from Ab30An70 to Ab10An90 (Ab = NaAlSi3O8 and An = CaAl2Si2O8) [see illustration]. In the high-temperature form, bytownite has the high-albite structure type in…

A term that may refer to either a zinc mineral, Zn4Si2O7(OH)2·H2O, which is also known as hemimorphite, or to zinc oxide, ZnO, which is used in medicinal or pharmaceutical products and in cosmetics. Hemimorphite ZincE. Eugene Weaver S. T. Irish,…

A class of the phylum Porifera, including exclusively marine sponges with a skeleton composed of spicules of calcium carbonate to which can be added a solid basal calcitic skeleton. The sponges that constitute the phylum Porifera are assigned to…

Primitive fossil members of the phylum Chordata with a calcite skeleton of echinoderm type. They occur in marine rocks of Cambrian to Pennsylvanian age (530–300 million years old) and, because of their skeletons, have traditionally been placed in…

A soil that is mineralogically an impure limestone. Such soils are also known as duricrust, kunkar, nari, kafkalla, Omdurman lime, croute, and race. Many soil profiles in semiarid climates (that is, those characterized by a rainfall of 4–20 in. or…

An extinct subclass of stalked Crinoidea; a class of the phylum Echinodermata. The Camerata include more than 235 genera confined to the Paleozoic Era, ranging from the Early Ordovician to the Late Permian. Camerate crinoids were one of the four…

An order of crepuscular (active during predawn or twilight hours) or mainly nocturnal birds collectively known as the nightjars or goatsuckers. Members of the order Caprimulgiformes are apparently most closely related to the owls (Strigiformes).…

A moderately coherent group of primitive reptiles constituting an order of the subclass Anapsida. Most members are characterized by a closed cheek (temporal) region. The order is divided into four suborders: Captorhinomorpha, Millerosauria,…

The unit of weight now used for all gemstones except pearls. It is also called the metric carat (m.c.). By international agreement, the carat weight is set at 200 milligrams. Pearls are weighed in grains, a unit of weight equal to 50 mg, or ¼…

Cameras in mobile phones or smartphones that can capture images and videos.The cellphone, the world’s favorite must-have gadget, has been with us for nearly a half-century, while experiencing continuous and often phenomenal improvements. Much of the…

A class in the phylum Mollusca that is exclusively marine and contains the extant members—squids, cuttlefishes, octopuses, and the chambered nautiluses—and the fossil taxa—ammonoids, nautiloids, and belemnoids. The earliest known cephalopod fossils…

An aquatic mammalian order comprising approximately 90 living species of whales, dolphins, and porpoises and their fossil relatives. Like all other mammals, members of the order Cetacea (cetaceans) nurse their young with milk produced by the mother,…

A thermodynamic law, also known as Gay-Lussac's law, which states that at constant pressure the volume of a fixed mass or quantity of gas varies directly with the absolute temperature. As such, Charles' law, V = kT, describes how gases expand when…

An experiment at the Large Hadron Collider (LHC), the world's largest particle accelerator, has revealed the first evidence for a phenomenon known as charge-parity (CP) violation in the decays of subatomic particles called charm quarks. The results…

An anapsid order of the class Reptilia, including the turtles, terrapins, and tortoises; also termed Chelonii, Testudinata, and Testudines. The order Chelonia (often referred to as Chelonii, Testudinata, and Testudines) comprises one of the oldest…

The force that holds atoms together in molecules and solids. Chemical bonds are very strong. To break one bond in each molecule in a mole of material typically requires an energy of many tens of kilocalories.It is convenient to classify chemical…

A subdiscipline of stratigraphy and geochemistry that involves correlation and dating of marine sediments and sedimentary rocks through the use of trace-element concentrations, molecular fossils, and certain isotopic ratios that can be measured on…

The name for several species of small ruminant mammals, also called mouse deer, which constitute the family Tragulidae in the order Artiodactyla. Chevrotains, often known as mouse deer (also spelled mouse-deer), belong to the family Tragulidae in…

The only extant order of the chondrichthyan subclass Holocephali, comprising the ratfishes. Members of the order Chimaeriformes (chimaeriforms or ratfishes; Fig. 1) are the closest living relatives of members of the subclass Elasmobranchii (sharks…

A mild, dry, extremely turbulent westerly wind on the eastern slopes of the Rocky Mountains and closely adjoining plains. The term is an Indian word which means “snow-eater,” appropriately applied because of the great effectiveness with which this…

A hydrous iron aluminum silicate mineral commonly found in low-grade metamorphic rocks such as slate and phyllite. Chloritoid is found as platy, black or dark green crystals, rarely more than a few millimeters in size. Its density ranges from 3.46…

A class of vertebrates comprising the cartilaginous, jawed fishes. Members of the Chondrichthyes (chondrichthyans, or the cartilaginous fishes) are characterized by well-developed jaws and bony teeth; pectoral and pelvic girdles, with each…

Chrysotile is a fibrous mineral with a tubular morphology for each fibril (Fig. 1). It is a member of the serpentine mineral group, as are antigorite and lizardite. Chrysotile aggregates make up serpentine asbestos, which is the most important type…

An order of sea urchins (phylum Echinodermata, class Echinoidea) in which the 20 columns of plates in the essentially spherical test consist of five pairs of narrow and numerous ambulacral plates and five pairs of broad and few interambulacral…

The process of producing the illusion of a moving picture. Cinematography includes two phases: taking the picture with a camera and showing the picture with a projector. The camera captures the action by taking a series of still pictures at regular…

Well-defined addition compounds formed by inclusion of molecules in cavities existing in crystal lattices or present in large molecules. The constituents are bound in definite ratios, but these are not necessarily integral. The components are not…

It has been a long-held belief that, as a result of hunting habits, early humans were partly or even primarily responsible for the decline and eventual extinction of most of the largest mammalian species that inhabited ancient Africa. These large…

Researchers from Rice University in Houston, Texas, United States, have found that climate change could potentially increase the size of stalled high-pressure weather systems (ridges) known as “blocking events,” according to global climate modelling…

Construction of a mathematical model of the climate system of the Earth capable of simulating its behavior under present and altered conditions. The Earth's climate is continually changing over time scales ranging from hundreds of years to a few…

The scientific study of climate. Climate is the expected mean and variability of the weather conditions for a particular location, season, and time of day. The climate is often described in terms of the mean values of meteorological variables such…

A suspension of minute droplets or ice crystals produced by the condensation of water vapor. This article presents an outline of cloud formation upon which to base an understanding of cloud classifications. For a more technical consideration of the…

The study of the physical and dynamical processes governing the structure and development of clouds and the release from them of snow, rain, and hail (collectively known as precipitation). Precipitation (meteorology)The factors of prime importance…

A diverse phylum of aquatic invertebrate animals, including jellyfish, hydroids, sea anemones, and corals. Members of the phylum Cnidaria (Fig. 1), termed cnidarians, are characterized by the possession of (1) intrinsic nematocysts (stinging…

A brown to black combustible carbonaceous sedimentary rock that is mined and used primarily as a fuel. Coal originated by accumulation and subsequent physical and chemical alteration of plant material over about 300 million years, and contains at…

Any interaction between particles, aggregates of particles, or rigid bodies in which they come near enough to exert a mutual influence, generally with exchange of energy. Broadly speaking, interactions can occur between particles that are simply…

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…

The principle stating that energy cannot be created or destroyed, although it can be changed from one form to another. The law of conservation of energy has been established by a multitude of meticulous measurements of gains and losses of all known…

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…

Charged, energetic particles, mostly hydrogen and helium nuclei, that travel at nearly the speed of light through space and bombard Earth from all directions. Cosmic rays are generated in astrophysical environments, ranging from the Sun locally to…

Trees provide substantial benefits to the environment. In addition to improving air quality, reducing stormwater runoff, serving as a wildlife habitat, and sequestering carbon by absorbing carbon dioxide, trees also cool the atmosphere through shade…

The science and technology of phenomena and processes at low temperatures, defined generally and arbitrarily as below 150°C (123 K or −238°F). Numerous novel and interesting phenomena occur at cryogenic temperatures. These include liquefaction (Fig.…

Sand is mined for many applications, but the largest use by far is for fine aggregate in concrete. Smaller quantities of sand are used for fracking, water treatment (filtration), metal casting, and making glass, silicon chips, and ceramics. Concrete…

Superposition is a confounding yet foundational principle of quantum physics in which a quantum system—for example, a group of subatomic particles or atoms—can exist in multiple quantum states simultaneously. For example, consider a coin that is…

A class of the phylum Porifera, including sponges with a skeleton of one- to four-rayed siliceous spicules or of spongin fibers, or both. Four taxonomic classes comprise the phylum Porifera—Calcarea, Demospongiae, Hexactinellida, and…

The mass of an object divided by its volume. Density is conventionally represented by the Greek letter rho: ρ. The most common units for density are g/cm3 or specific gravity, which is dimensionless. The modern SI units of kg/m3 are increasingly…