science of the earth's history, composition, and structure, and the associated processes. It draws upon chemistry, biology, physics, astronomy, and mathematics (notably statistics) for support of its formulations.

Branches of Geology

Geology is divided into several fields, which can be grouped under the major headings of physical and historical geology.

Physical Geology

Physical geology includes mineralogy, the study of the chemical composition and structure of minerals; petrology, the study of the composition and origin of rocks; geomorphology, the study of the origin of landforms and their modification by dynamic processes; geochemistry, the study of the chemical composition of earth materials and the chemical changes that occur within the earth and on its surface; geophysics, the study of the behavior of rock materials in response to stresses and according to the principles of physics; sedimentology, the science of the erosion and deposition of rock particles by wind, water, or ice; structural geology, the study of the forces that deform the earth's rocks and the description and mapping of deformed rock bodies; economic geology, the study of the exploration and recovery of natural resources, such as ores and petroleum; and engineering geology, the study of the interactions of the earth's crust with human-made structures such as tunnels, mines, dams, bridges, and building foundations.

Historical Geology

Historical geology deals with the historical development of the earth from the study of its rocks. They are analyzed to determine their structure, composition, and interrelationships and are examined for remains of past life. Historical geology includes paleontology, the systematic study of past life forms; stratigraphy, of layered rocks and their interrelationships; paleogeography, of the locations of ancient land masses and their boundaries; and geologic mapping, the superimposing of geologic information upon existing topographic maps.

Historical geologists divide all time since the formation of the earliest known rocks (c.4 billion years ago) into four major divisions—the Precambrian, Paleozoic, Mesozoic, and Cenozoic eras. Each, except the Cenozoic, ended with profound changes in the disposition of the earth's continents and mountains and was characterized by the emergence of new forms of life (see geologic timescale). Broad cyclical patterns, which run through all historical geology, include a period of mountain and continent building followed by one of erosion and, in turn, by a new period of elevation.

Evolution of Geology

Early Geologic Studies

Observations on earth structure and processes were made by a number of the ancients, including Herodotus, Aristotle, Lucretius, Strabo, and Seneca. Their individual efforts in the natural history of the earth, however, provided no sustained progress. Their major contribution is that they attributed the phenomena they observed to natural and not supernatural causes. Many of the ideas expressed by these men were not to resurface until the Renaissance. Later Leonardo da Vinci correctly speculated on the nature of fossils as remains of ancient organisms and on the role that rivers play in the erosion of land. Agricola made a systematic study of ore deposits in the early 16th cent. Robert Hooke and Nicolaus Steno both made penetrating observations on the nature of fossils and sediments.

Evolution of Modern Geology

Modern geology began in the 18th cent. when field studies by the French mineralogist J. E. Guettard and others proved more fruitful than speculation. The German geologist Abraham Gottlob Werner, in spite of the many errors of his specific doctrines and the diversion of much of his energy into a fruitless controversy (in which he maintained that the origin of all rocks was aqueous), performed a great service for the science by demonstrating the chronological succession of rocks.

In 1795 the Scottish geologist James Hutton laid the theoretical foundation for much of the modern science with his doctrine of uniformitarianism, first popularized by the British geologist John Playfair. Largely through the work of Sir Charles Lyell, this doctrine replaced the opposing one of catastrophism. Geology in the 19th cent. was influenced also by the work of Charles Darwin and enriched by the researches of the Swiss-American Louis Agassiz.

In the 20th cent. geology has advanced at an ever-increasing pace. The unraveling of the mystery of atomic structure and the discovery of radioactivity allowed profound advances in many phases of geologic research. Important discoveries were made during the International Geophysical Year (1957–58), when scientists from 67 nations joined forces in investigating problems in all branches of geology. The systematic survey of the floors of the earth's oceans brought radical changes in concepts of crustal evolution (see seafloor spreading; plate tectonics).

As a result of numerous flyby spacecraft, geological studies have been extended to include remote sensing of other planets and satellites in the solar system and the moon. Laboratory analysis of rock samples brought back from the moon have provided insight into the early history of near-earth space. On-site analyses of Martian soil samples and photographic mapping of its surface have given clues about its composition and geologic history, including the possibility that Mars once had enough water to form oceans. Photographs of the many active volcanoes on Jupiter's moon Io have provided clues about earth's early volcanic activity. Geological studies also have been furthered by orbiting laboratories, such as the six launched between 1964 and 1969 in the Orbiting Geophysical Observatory (OGO) series and the Polar Orbiting Geomagnetic Survey (POGS) satellite launched in 1990; remote-imaging spacecraft, such as the U.S. Landsat program (Landsat 7, launched in 1999, was the most recent) and French SPOT series (SPOT 5, launched in 2002, was the most recent in the program); and geological studies on space shuttle missions.

Bibliography

See N. Coch and A. Ludman, Physical Geology (3d ed. 1991); L. S. Fichter et al., Earth Materials and Earth Processes (3d ed. 1991); L. Margulis and L. Olendenski, Environmental Evolution: Effects of the Origin and Evolution of Life on Planet Earth (1992); R. H. Dott, Jr., and D. R. Prothero, Evolution of the Earth (5th ed. 1994); E. A. Keller, Environmental Geology (7th ed. 1996); S. Chernicoff and C. Fox, Essentials of Geology (1998); E. J. Tarbuck and F. K. Lutgens, The Earth: An Introduction to Physical Geology (6th ed. 1998).

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