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The group of sciences concerned with the study of the Earth, including its structure, long-term history, composition, and origin.

Physical geology

This deals with the structure and composition of the Earth and the forces of change altering them. The divisions that make up physical geology include mineralogy (the arrangement of minerals), petrology (the study of rocks and their combinations of minerals), geodesy, geomorphology, geophysics, and seismology (see earthquake). Much of modern physical geology is based on the theory of plate tectonics.

Historical geology

This deals with the Earth in past ages, and with the evolution of life upon it. It embraces such sciences as paleoclimatology, paleomagnetism, paleontology, and stratigraphy, ; and relies heavily on dating, events being related to the geological time scale, whose derivation is primarily stratigraphical, to a lesser extent paleontological.

Economic geology

This lies between physical and historical geology, and borrows from both. Concerned with the location and exploitation of the Earth's natural resources, it is generally taken to include the disciplines of crystallography, mineralogy, and petrology. Its practical manifestations are prospecting and mining.

branches of geology and allied sciences
The different branches of geology (red) and their links with allied sciences (yellow)

Development of geology

Most early geological knowledge came from the experience of mining engineers, some of the earliest geological treatises coming from the pen of Georgius Agricola. The interest of the 16th century in fossils was also reflected in the writings of K. von Genner. In the 17th century the biblical timescale of about 6,000 years from the Creation to the present largely constrained the many speculative "Theories of the Earth" that were issued. The centuries most notable geological observations were made by N. Steno. The late 18th century saw the celebrated controversy between Alfred Wegener's "Neptunism" and J. Hutton's "Plutonists" as to the origin of the rocks. The first decades of the 19th century, however, witnessed the decline of speculative geology as field observations became ever more detailed. William Smith (1769–1839), the "father of stratigraphy," show how the succession of fossils could be used to index the stratigraphic column, and he and others produced impressive geological maps. Charles Lyell's classic Principles of Geology (1830–33) restated the Huttonian principle of uniformitarianism and provided the groundwork for much of the later development of the science. Louis Agassiz pointed to the importance of glacial action in the recant history of the Earth (1840), while mining engineering continued to contribute to the pool of geologic data. Among the most significant 20th century developments in the earth sciences was the acceptance of plate tectonics foreshadowed by Wegener's 1912 theory of continental drift.

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