GEOLOGY - Geology Home

Geology (from Greek: γη, ge, "earth"; and λόγος, logos, "knowledge") is the science and study of the solid matter that constitute the Earth. Encompassing such things as rocks, soil, and gemstones, geology studies the composition, structure, physical properties, history, and the processes that shape Earth's components. It is one of the Earth sciences. Geologists have helped establish the age of the Earth at about 4.6 billion (4.6x10⁹) years, and have determined that the Earth's lithosphere, which includes the crust, is fragmented into tectonic plates that move over a rheic upper mantle (asthenosphere) via processes that are collectively referred to as plate tectonics. Geologists help locate and manage the Earth's natural resources, such as petroleum and coal, as well as metals such as iron, copper, and uranium. Additional economic interests include gemstones and many minerals such as asbestos, perlite, mica, phosphates, zeolites, clay, pumice, quartz, and silica, as well as elements such as sulfur, chlorine, and helium.

Planetary geology (sometimes known as Astrogeology) refers to the application of geologic principles to other bodies of the solar system. However, specialised terms such as selenology (studies of the Moon), areology (of Mars), etc., are also in use.

The word "geology" was first used by Jean-André Deluc in the year 1778 and introduced as a fixed term by Horace-Bénédict de Saussure in the year 1779. The science was not included in Encyclopedia Britannica's third edition completed in 1797, but had a lengthy entry in the fourth edition completed by 1809.^[1] An older meaning of the word was first used by Richard de Bury to distinguish between earthly and theological jurisprudence.

The work Peri lithon (On Stones) by Theophrastus (372-287 BC), a student of Aristotle, remained authoritative for millennia. Its interpretation of fossils was not overturned until after the Scientific Revolution. It was translated into Latin and the other languages of Europe such as French.

Some modern scholars, such as Fielding H. Garrison, are of the opinion that modern geology began in the Muslim world.^[2] Abu al-Rayhan al-Biruni (973-1048 AD) was one of the earliest Muslim geologists, whose works included the earliest writings on the geology of India.^[3]

In China, the polymath Shen Kua (1031-1095) formulated a hypothesis for the process of land formation: based on his observation of fossil shells in a geological stratum in a mountain hundreds of miles from the ocean, he inferred that the land was formed by erosion of the mountains and by deposition of silt.

Georg Agricola (1494-1555)), a physician, wrote the first systematic treatise about mining and smelting works, De re metallica libri XII, with an appendix Buch von den Lebewesen unter Tage (Book of the Creatures Beneath the Earth). He covered subjects like wind energy, hydrodynamic power, melting cookers, transport of ores, extraction of soda, sulfur and alum, and administrative issues. The book was published in 1556. Nicolas Steno (1638-1686) is credited with the law of superposition, the principle of original horizontality, and the principle of lateral continuity: three defining principles of stratigraphy. Previous attempts at such statements meet accusations of heresy from the Church.^{[citation needed]}

By the 1700s Jean-Étienne Guettard and Nicolas Desmarest hiked central France and recorded their observations on geological maps; Guettard recorded the first observation of the volcanic origins of this part of France.

William Smith (1769-1839) drew some of the first geological maps and began the process of ordering rock strata (layers) by examining the fossils contained in them.

James Hutton is often viewed as the first modern geologist. In 1785 he presented a paper entitled Theory of the Earth to the Royal Society of Edinburgh. In his paper, he explained his theory that the Earth must be much older than had previously been supposed in order to allow enough time for mountains to be eroded and for sediments to form new rocks at the bottom of the sea, which in turn were raised up to become dry land. Hutton published a two-volume version of his ideas in 1795 (Vol. 1, Vol. 2).

Followers of Hutton were known as Plutonists because they believed that some rocks were formed by vulcanism which is the deposition of lava from volcanoes, as opposed to the Neptunists, who believed that all rocks had settled out of a large ocean whose level gradually dropped over time.

In 1811 Georges Cuvier and Alexandre Brongniart published their explanation of the antiquity of the Earth, inspired by Cuvier's discovery of fossil elephant bones in Paris. To prove this, they formulated the principle of stratigraphic succession of the layers of the earth. They were independently anticipated by William Smith's stratigraphic studies on England and Scotland.

By 1827 Charles Lyell's Principles of Geology reiterated Hutton's uniformitarianism, which influenced the thought of Charles Darwin.

Sir Charles Lyell first published his famous book, Principles of Geology, in 1830 and continued to publish new revisions until he died in 1875. He successfully promoted the doctrine of uniformitarianism. This theory states that slow geological processes have occurred throughout the Earth's history and are still occurring today. In contrast, catastrophism is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter. Though Hutton believed in uniformitarianism, the idea was not widely accepted at the time.

Plate tectonics - seafloor spreading and continental drift illustrated on relief globe of the Field Museum

19th Century geology revolved around the question of the Earth's exact age. Estimates varied from a few 100,000 to billions of years. The most significant advance in 20th century geology has been the development of the theory of plate tectonics in the 1960s. Plate tectonic theory arose out of two separate geological observations: seafloor spreading and continental drift. The theory revolutionized the Earth sciences.

The theory of continental drift was proposed by Frank Bursley Taylor in 1908, expanded by Alfred Wegener in 1912 and by Arthur Holmes, but wasn't broadly accepted until the late 1960s when the theory of plate tectonics was developed.