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—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.
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.
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).
Geology (from Greek: γη, gê, "earth"; and λόγος, logos, "speech" lit. to talk about the earth) is the science and study of the solid and liquid matter that constitute the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed. The field is important in academics, industry (due to mineral and hydrocarbon extraction), and for social issues such as geotechnical engineering, the mitigation of natural hazards, and knowledge about past climate and climate change.
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 Encyclopædia Britannica's third edition completed in 1797, but had a lengthy entry in the fourth edition completed by 1809. 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. Peri Lithon was translated into Latin and some other foreign languages. Its interpretation of fossils was the most dominant theory in classical Antiquity and the early Middle Ages, until it was replaced by Avicenna's theory of petrifying fluids (succus lapidificatus) in the late Middle Ages. In the Roman period, Pliny the Elder produced a very extensive discussion of many more minerals and metals then widely used for practical ends. He is among the first to correctly identify the origin of amber as a fossilized resin from pine trees by the observation of insects trapped within some pieces. He also laid the basis of crystallography by recognising the octahedral habit of diamond.
Some modern scholars, such as Fielding H. Garrison, are of the opinion that modern geology began in the medieval Islamic world. 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, hypothesizing that the Indian subcontinent was once a sea. Ibn Sina (Avicenna, 981-1037), in particular, made significant contributions to geology and the natural sciences (which he called Attabieyat) along with other natural philosophers such as Ikhwan AI-Safa and many others. He wrote an encyclopaedic work entitled “Kitab al-Shifa” (the Book of Cure, Healing or Remedy from ignorance), in which Part 2, Section 5, contains his essay on Mineralogy and Meteorology, in six chapters: Formation of mountains, The advantages of mountains in the formation of clouds; Sources of water; Origin of earthquakes; Formation of minerals; The diversity of earth’s terrain. These principles were later known in the Renaissance of Europe as the law of superposition of strata, the concept of catastrophism, and the doctrine of uniformitarianism. These concepts were also embodied in the Theory of the Earth by James Hutton in the Eighteenth century C.E. Academics such as Toulmin and Goodfield (1965), commented on Avicenna's contribution: "Around A.D. 1000, Avicenna was already suggesting a hypothesis about the origin of mountain ranges, which in the Christian world, would still have been considered quite radical eight hundred years later". Avicenna's scientific methodology of field observation was also original in the Earth sciences, and remains an essential part of modern geological investigations.
In China, the polymath Shen Kua (1031-1095) formulated a hypothesis for the process of land formation: based on his observation of fossil animal 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 met accusations of heresy from the Church.
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.
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.
Sir Charles Lyell first published his famous book, Principles of Geology, in 1830. Lyell continued to publish new revisions until he died in 1875. The book, which influenced the thought of Charles Darwin, 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.
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.
The principle of intrusive relationships concerns crosscutting intrusions. In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.
The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.
The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them.
The principle of uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now."
The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).
The principle of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.
The principle of faunal succession is based on the appearance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin's theory of evolution, the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils may be found globally at the same time.
A large advance in geology in the advent of the 20th century was the ability to use ratios of radioactive isotopes to find the amount of time that has passed since a rock passed through a particular temperature.
Geologists have established the age of the Earth at about 4.54 billion (4.6x109) years, and the age of the oldest planetary material (Carbonaceous Chondrite meteorites) at 4.567 billion years through the use of Uranium-lead dating.
Seismologists can use the arrival times of seismic waves in reverse to image the interior of the Earth. Early advances in this field showed the existence of a liquid outer core (where shear waves were not able to propigate) and a dense solid inner core. These advances led to the development of a layered model of the Earth, with a crust and lithosphere on top, the mantle below (separated within itself by seismic discontinuities at 410 and 660 kilometers), and the outer core and inner core below that. More recently, seismologists have been able to create detailed images of wave speeds inside the earth in the same way a doctor images a body in a CT scan. These images have led to a much more detailed view of the interior of the Earth, and have replaced the simplified layered model with a much more dynamic model.
Mineralogists have been able to use the pressure and temperature data from the seismic and modelling studies alongside knowledge of the elemental composition of the Earth at depth to reproduce these conditions in experimental settings and measure changes in crystal structure. These studies explain the chemical changes associated with the major seismic discontinuities in the mantle, and show the crystallographic structures expected in the inner core of the Earth.
With the advent of space exploration in the twentieth century, geologists have begun to look at other planetary bodies in the same way as the Earth. This has led to the oxymoron term, commonly used in the professional literature, of planetary geology.
Planetary geology (sometimes known as Astrogeology) refers to the application of geologic principles to other bodies of the solar system. Specialised terms such as selenology (studies of the moon), areology (of Mars), etc., are also in use. Colloquially, geology is most often used with another noun when indicating extra-Earth bodies (e.g. "the geology of Mars").
Geologists also obtain data through stratigraphy, boreholes, and core samples, including ice cores, which tell geologists about past and present climate and ecosystems. These data are our primary source of information on global climate change outside of instrumental data.