Geology

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The School of Geology

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.^[1] An older meaning of the word was first used by Richard de Bury to distinguish between earthly and theological jurisprudence.

Main article: History of geology

A mosquito and a fly in this Baltic amber necklace are between 40 and 60 million years old

The work Peri Lithon (On Stones) by the ancient Greek scholar Theophrastus (372-287 BC), a student of ancient Greek philosopher 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.^[2][3] 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.^[4] 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.^[5] 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".^[6] Avicenna's scientific methodology of field observation was also original in the Earth sciences, and remains an essential part of modern geological investigations.^[3]

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.

William Smith's geologic map of England, Wales, and southern Scotland. Completed in 1815, it was the first national-scale geologic map, and by far the most accurate of its time.

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.^{[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.^[7] 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).

The geologist, 19th century painting by Carl Spitzweg.

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.

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.

There are a number of important principles that were developed near the beginning of geology as a formal science. Many of these involve the ability to provide the relative ages of strata or the manner in which they were formed. These principles are still often used today as a means to provide information about geologic history and the timing of geologic events.

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.

Main articles: Radiometric dating and Age of the Earth

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 used radioactive decay to establish the age of the Earth at about 4.54 billion (4.5x10⁹) years and the age of the oldest planetary material (Carbonaceous Chondrite meteorites) at 4.567 billion years.^{[citation needed]}

Oceanic-continental convergence resulting in subduction and volcanic arcs illustrates one effect of plate tectonics.

Main article: plate tectonics

Geologists have determined that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into a number of tectonic plates. These tectonic plates move across the plastically-deforming, solid, upper mantle, which is called the asthenosphere. There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle: plate motions and mantle convection currents always move in the same direction. This coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics.

Main article: Structure of the Earth

Earth layered structure. (1) inner core; (2) outer core; (3) lower mantle; (4) upper mantle; (5) lithosphere; (6) crust

Advances in seismology, computer modeling, and mineralogy and crystallography at high temperatures and pressures give insights into the internal composition and structure of the Earth.

Earth layered structure. Typical wave paths from earthquakes like these gave early seismologists insights into the layered structure of the Earth

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 propagate) 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.

On this diagram, subducting slabs are in blue, and continental margins and a few plate boundaries are in red. The blue blob in the cutaway section is the seismically-imaged Farallon Plate, which is subducting beneath North America. The remnants of this plate on the Surface of the Earth are the Juan de Fuca Plate and Explorer plate in the Northwestern USA / Southwestern Canada, and the Cocos Plate on the west coast of Mexico.

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.

Surface of Mars as photographed by the Viking 2 lander December 9, 1977.

Main article: Planetary geology

Main article: Geology of solar terrestrial planets

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. Although the Greek-language-origin prefix geo refers to Earth, "geology" is often used in conjunction with the names of other planetary bodies when describing their composition and internal processes: examples are "the geology of Mars" and "Lunar geology". Specialised terms such as selenology (studies of the moon), areology (of Mars), etc., are also in use.

Main article: Economic Geology

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.

Main articles: soil mechanics and geotechnical engineering

In the field of civil engineering, geological principles and analyses are used in order to ascertain the mechanical principles of the material on which structures are built. This allows tunnels to be built without collapsing, bridges and skyscrapers to be built with sturdy foundations, and buildings to be built that will not settle in clay and mud.

Geology and geologic principles can be applied to various environmental problems, such as stream restoration, the restoration of brownfields, and the understanding of the interactions between natural habitat and the geologic environment. Groundwater hydrology, or hydrogeology, is used to provide water in arid regions and to monitor the spread of contaminants in groundwater wells.

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.

Main article: Natural hazard

Geologists and geophysicists study natural hazards in order to enact safe building codes and warning systems that are used to prevent loss of property and life. Examples of important natural hazards that are pertinent to geology (as opposed those that are mainly or only pertinent to meteorology) are:

• Tsunamis
• Earthquakes
• Sinkholes
• Liquefaction
• Landslides and Debris flows
• Avalanches
• Floods and river channel migration and avulsion
• Subsidence

An illustrated depiction of a syncline and anticline commonly studied in Structural geology and Geomorphology.

• Geology of the Alps
• Geology of the Appalachians
• Geology of the Himalaya

• Planetary Geology
  • Geology of Mercury
  • Geology of Venus
  • Geology of the Moon
  • Geology of Mars

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• James Hutton's Theory of the Earth
• James Hutton's Theory of the Earth & Abstract of the Theory of the Earth