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 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—Precambrian time and the 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.
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