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geology

Geology, derived from the Greek geo, "Earth," plus logos, "study," deals with the study of the planet Earth - the materials of which it is made, the processes that affect Earth materials, the products formed in the Earth, and the history of the planet and its inhabitants since its origin.

Geologists study the composition of Earth materials and the various geological processes in order to locate and exploit the Earth's mineral resources. They also investigate earthquakes, volcanoes, and other geologic hazards in order to predict and minimize the damaging effects of these natural phenomena. Geologists study geologic history to determine the former positions of the continents and oceans, to ascertain the nature of ancient climates, and to trace the evolution of life as revealed in the fossil record.

Geology draws heavily from other sciences; Earth science and the basic sciences overlap in many areas of investigation. For example, chemistry is used to analyze the rocks and minerals of the Earth's crust. Biology aids in understanding the nature of prehistoric organisms. Thus, botany provides information about ancient plants, and a knowledge of zoology is essential to the understanding of prehistoric animals. Physics helps to explain the various physical forces that affect the Earth and how Earth materials respond to these forces. Findings from astronomy reveal where the Earth fits into the universe, and astronomers have also attempted to explain the origin of the Earth.


Substudy of geology

Because of its broad scope, geology has been divided into two major divisions: physical geology and historical geology. Physical geology deals with the composition of the Earth, its structural arrangement, the movements within and on the Earth's crust, and the myriad geologic processes by which the Earth is, or has been, changed. The principal subfields of physical geology include mineralogy, the study and classification of minerals (Mineralogy); Petrology, dealing with the origin, structure, occurrence, and history of rocks; and structural geology, which has to do with the deformation of rocks and their structural attitude or arrangements. Plate tectonics is closely related to structural geology but generally treats the deformation and historical evolution of the Earth's larger structural features. Geomorphology is concerned with the general configuration of the Earth's surface and the origin, development, and classification of landforms. Economic geology has to do with geologic processes and materials that can be utilized by humans, and Geophysics and Geochemistry rely on data derived from the study of the physics and chemistry of the Earth. More specialized subfields of physical geology include seismology (the study of Earthquakes and the Earth's interior), volcanology (the study of volcanoes and volcanic phenomena), glaciology (see Glaciers and glaciation) and environmental geology (geological studies related to human environmental concerns), engineering geology (the application of the geological sciences to engineering practice), and marine geology, or geological oceanography (that aspect of the study of the ocean which deals specifically with the ocean floor and the ocean-continent margins).

Historical geology is concerned with the evolution of the Earth and its inhabitants from their origin to the present day. Subfields include Stratigraphy, dealing with the study, interpretation, and correlation of rock strata, and paleontology, the study of prehistoric plants and animals as revealed by their fossils and related to the chronology of the Earth's history. Geochronology is the study of time in relationship to the history of the Earth, and Paleogeography deals with the physical geography of all or part of the Earth's surface at some time in the geologic past. More specialized are Paleoclimatology (the study of climates of the geologic past), Paleoecology (the study of the relationship between ancient organisms and their environment), Paleomagnetism (the study of the Earth's magnetic field over geologic time), and Micropaleontology (the study of microscopically small fossils).

The major branches of historical geology--like those of physical geology--overlap in a number of areas and are interdependent. Thus the unification of physical and historical geology leads ultimately to a better understanding of the Earth.


History of geology

Although geology is a relatively young science, humans have long been interested in the Earth. Prehistoric people utilized stones as tools and weapons, formed clay into pottery, and sought shelter in rocky caves. But their knowledge of the Earth was restricted to the ground beneath their feet or the limited areas that they could explore on foot.

Ancient Myths

Curiosity probably prompted early humans to pick up stones that later became tools. Fear undoubtedly spurred speculation about such cataclysmic events as earthquakes and volcanic eruptions. In early interpretations such geologic phenomena were explained by means of unworldly or supernatural forces. According to an early Hindu legend, the Earth was supported by eight elephants that stood on the back of a giant turtle. The turtle, believed to be an incarnation of the god Vishnu, rested on the back of a coiled cobra, the symbol of water. When any of these creatures moved, the Earth would vibrate, producing an earthquake. In Japanese folklore, earthquakes are caused by a giant catfish that lives in mud beneath the Earth. The fish can only be controlled while pinned beneath a huge "keystone" possessing magical powers. When the catfish frees itself and thrashes about, earthquakes are produced.


Ancient Greek and Roman Beliefs

As early as the 4th century BC, Aristotle taught that the Earth was a sphere. He also believed that streams originated from springs and that minerals were formed from "exhalations" of the Earth. He said that earthquakes were caused when "pent-up" winds burst to the surface after being trapped in subterranean channels.

Titus Lucretius Carus, an early Roman, wrote that quakes were caused by the collapse of the roofs of great caverns. Other ancient Romans speculated about volcanic eruptions. Indeed, the term volcano probably derives from the island of Vulcano, one of the Lipari (Aeolian) Islands in the Tyrrhenian Sea. This island's active volcano was assumed to be the home of Vulcan, Roman god of fire. Vulcan was the blacksmith of the gods, and when his forge was heated, smoke issued from Vulcano's crater. The volcanic explosions were thought to be caused by Vulcan pounding on his anvil.

Other ancients pondered the origin of fossils. In the 5th century BC, Anaximander of Miletus noted fossil fish well above sea level and concluded that fish had been the ancestors of all living things, including humans. Xenophanes of Colophon (5th century BC) found fossils of marine organisms far inland and correctly inferred that they represented the remains of sea-dwelling animals. Approximately 100 years later Herodotus of Halicarnassus found nummulites (large foraminifera) in Egyptian limestones. He said that these small, disk-shaped fossils were lentils left over from the slaves who built the pyramids. Herodotus also stated that lower Egypt had been formed from sediment deposited by the Nile. Because its roughly triangular shape resembles the Greek letter delta, he named that region the delta. Today this term is used to describe similar river-laid deposits in all parts of the world.

In the 4th century BC, Empedoced of Agrigentum recognized fossils in Sicily. He proposed that plants, animals, and humans had appeared in this order because each depends on its predecessor for survival--an "evolutionary" approach far ahead of its time. Aristotle and Theophrastus of Lesbos also collected fossil fish, and the latter made one of the earliest attempts to classify minerals.



Medieval Thought

There were few attempts to solve the Earth's mysteries during the Middle Ages. No distinction was made between rocks, minerals, and fossils as the terms are now understood. Early in the 11th century, however, Avicenna, a Persian physician and Islamic scholar, perceived basic geologic processes that were not to be understood for centuries. He recognized water and wind as erosional agents, flooding of the land by prehistoric seas, the development of solid rock from soft, water-deposited sediment, the formation of soils, and fossils as the remains of ancient animals. Avicenna also grasped the importance of time in geological processes. A less important contribution was made by Albertus Magnus, a 13th-century German scholar who wrote about rocks, minerals, and mountains.



The Renaissance: An Awakening

Interest in the Earth was rekindled early in the Renaissance by Leonardo Da Vinci. He recognized the true nature of fossils and refuted the then-popular notions that fossils were freaks of nature or devices that Satan had put in the rocks to lead people astray. His approach to the study of the Earth was modern in that he explained features in the rocks in the light of natural processes that could still be observed.

In the mid-16th century Georg Bauer, a German writing under the name of Georgius Agricola, wrote on the origin of mountains, minerals, and underground water. Two of his books, De natura fossilium (1546) and De re metallica (1556), laid the foundations for mineralogy and mining geology.



The 17th Century

In 1667, Nicolaus Steno, a Danish physician and theologian, published the first true treatise on geology. He wrote on processes of sedimentation, the origin of rocks, the formation of crystals and fossils, and the interpretation of rock strata. His Prodromus (1669) established criteria for differentiating between freshwater and marine sediments. He was the first to recognize the principle of superposition--that the lower layers in a sequence of rock strata must be older than those deposited above them. He also stated that rock layers were formed from beds of sediment that were originally laid down in a nearly horizontal position--the principle of original horizontality. Steno was also the first to note that the crystal faces of a given mineral always have the same angle with respect to one another. This is now known as the law of constancy of interfacial angles.

Despite advances in the 17th century, however, the concept of geologic time was not understood. The creation of the Earth and the geologic record were still explained in terms of biblical chronology. Accordingly, the creation was believed to have taken place in 4004 BC, a date based on the calculations of James USSHER, a mid-17th-century Irish bishop.



18th- and 19th-Century Advances

Modern geologic thought began to develop in the 18th century and expanded steadily in the 19th. Benoit de Maillet, a French consul to Egypt, was probably the first to note that older rocks contain fewer fossil species than do younger rocks. He also concluded that layered rocks were deposited over long spans of time. In his Telliamed (de Maillet spelled backward) he wrote of the formation of the Earth and of the origin of animals.

Giovanni Arduino, an Italian mineralogist, wrote on metamorphism and attempted to arrange rocks chronologically from youngest to oldest. He classified them as "primitive" or "primary," "secondary," and "tertiary." The last term is still used today.

Two major geological controversies emerged during the late 18th century, both of which were largely resolved by Scottish geologist James Hutton. The first centered on the neptunism/plutonism debate. Proponents of neptunism, led by German mineralogist Abraham Gottlob Werner, held that all bedrock had been precipitated in an ancient universal sea. Accordingly, fossils were seen as victims of Noah's Flood, and granite and basalt were believed to be the oldest deposits of an ancient sea. Proponents of plutonism, led by Hutton, acknowledged that sedimentary buildup occurred but held that most rock had solidified from an original molten mass. Neptunism was eventually discounted when granite and basalt were proved to be igneous rocks.

The second controversy involved Catastrophism, a then widely held doctrine that the Earth's major physical features were caused by periodic, worldwide catastrophes. Hutton shared the belief that the Earth had undergone great changes, but he saw no concrete evidence for catastrophism. He proposed that all past geologic events were somehow connected, that the most important events occurred over immense time periods, and that they continue to occur in the present. Hutton's concepts, which became known as Uniformitarianism, established the significance of geologic time and provided the cornerstone of future geologic thought.

Field studies and mapmaking burgeoned in the 19th century. William Smith, an English civil engineer, observed that each layer of rock contained fossils characteristic of that particular stratum and that such strata might be identified by their fossils and thus correlated over wide areas. He laid the foundation for stratigraphical paleontology and published the first geologic map of England. William Maclure, a Scottish geologist who settled in Virginia, published the first maps of what was then the United States in 1809.

Near the end of the 19th century, geologists began to abandon a 6,000-year age for the Earth, and many thought that the Earth was at least tens of millions of years old. In 1896 radioactivity was discovered by Henri Becquerel, a French physicist. This paved the way for the 20th-century development of Radiometric Age-dating, which has found some rocks to be 3.8 billion years old.



Geology in the 20th Century

With an understanding of geologic time and with Hutton's concept of uniformitarianism to guide them, 20th century geologists began to forge geology as it is known today. Increasing knowledge of radioactive minerals, rates of decay, and decay products led to the development of the long-awaited "geologic clock," which suggests that the Earth is at least 4.6 billion years old. New sophisticated instruments and research techniques have been developed; the Earth is now studied from ships, aircraft, and orbiting satellites.

In 1912, Alfred Wagoner, a German meteorologist, put forth his theory of Continental drift. He proposed that a single protocontinent -Pangaea- had broken apart and the fragments had drifted away to form the continents as they are known today. Wegener's revolutionary proposal was rejected for decades, but in the early 1960s, geologist Harry H. Hess and Robert S. Dietz, a geological oceanographer, developed the theory of Seafloor Spreading. This theory, along with a better understanding of paleomagnetism, led to the theory of plate tectonics, or global tectonics. This important and now widely accepted theory assumes that the several moving plates of the Earth's crust are formed by volcanic activity at the oceanic ridges and destroyed in great seafloor trenches at the margins of the continents. One of the great geologic theories of all time, plate tectonics has revolutionized geologic thought and stimulated global research.

The concept of plate tectonics has advanced research in marine geology, and special attention is being directed to the ocean basins. Deep-sea cores recovered by research vessels have confirmed seafloor spreading and yielded much information about the composition and geologic history of the ocean floor. Studies of the continental shelves have provided clues to valuable deposits of oil and natural gas. Radar and infrared imagery from high-altitude aircraft and satellites are being used to study volcanoes and potential earthquake faults and to locate accumulations of valuable mineral deposits.

In the laboratory, computer models are being developed to assist in solving geological problems. The mass spectrometer, scanning electron microscope, and computerized axial tomography are probing rocks, minerals, and fossils as geologists use evidence from the past to understand the present and predict the future.



References