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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…

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…

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…

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…

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 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…

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 process by which human activities contribute to reductions in the biological productivity of semiarid lands. The term was made popular by André Aubreville’s 1949 book, Climats, forêts et desertification de l’Afrique tropicale (Climates, Forests…

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,…