History of geology
The history of geology encompasses the development of the science of geology as well as its precursors from prehistory to the present. It overlaps with the history of other geosciences , especially paleontology , mineralogy and petrography . It was Thales of Miletuswho was the first to try to replace the old mythological ideas about the earth with rational explanations. In the Christian late antiquity, however, many old ideas about the nature of the earth were lost again, in the Arab-Muslim cultural area, however, the ancient ideas about the origin of ores and rocks were further developed. Ibn Sina (Latinized: Avicenna , around 980-1037) made particular use of Aristotle. In addition, it provided a modern-looking classification of the mineral kingdom into salts, sulfur, metals and stones. In Europe, the reception of ancient ideas began in the Renaissance. In addition, the contemporary techniques of mining and mineralogical knowledge of the miners were by scholars such asGeorgius Agricola (1494–1555) systematized. The first steps in the direction of geological history were taken by the Danish doctor and naturalist Nicolaus Steno (1638–1686), who in 1669 designed the first geological profile that was really intended to be historical.
The development of geology into a modern natural science from the 18th century. occurred in the context of several - sometimes extremely violent - controversies about the prevailing paradigm : first Neptunism against plutonism , then catastrophism against actualism (or gradualism ). The first mobilistic ideas about the possibility that mainland masses can move sideways can be found in Alfred Wegener's (1880–1930) continental drift hypothesis from 1915, but it was only in the second half of the 20th century - in the paradigm of Plate tectonics - enforce against the prevailing fixism .
Antiquity
The origins of geology come from two very different sources: on the one hand, from the practical knowledge of the ore prospectors, miners and metallurgists who supplied the ancient civilizations with the raw materials they needed , and, on the other hand, from the very first seeds of Western philosophy.
Minerals, fossils and rocks in Ionic natural philosophy
It was Thales of Miletus (around 624 to 546 BC), the founder of Ionic natural philosophy , who was the first to try to replace the old mythological ideas about the earth with rational explanations. He no longer blamed the rumbling "earth shaker" Poseidon for the origin of the earthquake , but the movements of the earth disk floating on the primeval water . Likewise, by observing the sedimentation in sandbanks at the mouth of large rivers or the precipitation of minerals at the edge of hot springs , Thales seems to have come to his thesis that all things originated from water.
Anaximandros (around 610 to 546 BC) not only drew the first map of the inhabited world, but also extended Thales' ideas to the animate world. He taught that living beings emerged from moisture that evaporates under the action of the sun. As a result, humans evolved from fish-like creatures. Of course, it is pure coincidence that the discussion today is again about whether the first building blocks of life (“ primordial soup ”) were formed in the sea, or whether they were not concentrated in hot, mineral-saturated water holes. Yet Anaximandros' astonishing thesis anticipates modern evolution by more than 2,400 years. After all, he was the first thinker to consider a natural process of development in living beings. In any case, it shows that he was aware of the phenomenon of the precipitation of sea salt through exposure to sunlight ( evaporation ).
Xenophanes of Colophon (around 570 to around 470 v. Chr.) Indicated for the first time the imprints of shells and other marine animals in sea distant land lines as the remains of fossilized creatures ( fossils ). He explained their location by saying that the mountains once rose out of the sea. He also recognized the ongoing erosion on the coasts. From these two processes he concluded that there were large cycles in which mountain formation and erosion alternated. Every time the mainland is destroyed, the respective human race is annihilated.
Metaphysical Speculations in Greek Philosophy
All these approaches towards nature were already considered obsolete in the 4th century BC. The Greek philosophy devoted himself instead increasingly formal logical and transcendental problems. While the Pythagoreans in southern Italy transformed mathematics into a secret mystery religion, the sophists limited themselves to exercises in grammar, dialectics and rhetoric. The ideas about the formation of rocks and metals soon moved only in the realm of pure speculation, which largely dispensed with empirical observations. As z. B. Anaxagoras of Klazomenai (around 500-428 BC) claimed that the stony nature of the heavenly bodies had been proven by the fall of the meteorite from Aigospotamoi, which already earned him a conviction for blasphemy .
Plato (427–348 BC) combined the theory of the four elements of Empedocles with the mathematical speculations of the Pythagoreans about the geometric shape of atoms. Metals and minerals do not, like stones and earth, consist of mixed elements, but of specially compressed 'meltable water', in other words: particularly hard frozen ice.
Aristotle (384–322 BC) represented the momentous doctrine of the transformation ( transmutation ) of the elements in his work Meteorologia . He attributed the change to the deep penetration of the sun's rays into the earth's body. From the resulting dry evaporation the rocks would arise, and from the moist evaporation the metals. His idea of the formation of fossils inside rocks by an indeterminate creative force (Latin: vis plastica ) was also to remain valid well into modern times. He was familiar with the uplifts and subsidence of the earth's surface that cause alluvial deposits and erosion. In his opinion, they were based on the slow but erratic aging of the earth.
The cause for the development and growth of all things, including minerals, was the logos (Greek: the word, the reasonable speech, the reason), a general metaphysical ordering principle that permeates the entire cosmos . In the philosophical school of the Stoa , the concept of Logoi spermaticoi , the "seed-like grounds", was developed from this . These contain, it was assumed, the ideas that determine the final form of the individual things. In Neoplatonism , which would later have a significant influence on Christian theology, these ideas sprang from the divine spirit. Today we would rather think of the atomic binding forces that force the individual atoms of a mineral into a crystal lattice , or the genes that determine the development of a living organism.
The ancient "stone books"
Such views were summarized by Theophrastus , the student and successor of Aristotle, in his work On the Stones . After that, they were considered generally binding until well into modern times. In the later stone books, however, these theories were increasingly mixed with ideas from the Orient, about the magical, astrological and medicinal properties of metals and precious stones , but also with practical recipes for counterfeiting gold and for the artificial production of glass and dyes. Here you can see the origins of technical chemistry .
The last great summary of all this, by now very extensive and contradicting, material was undertaken by Pliny the Elder in his encyclopedic Naturalis historia , the last five books of which dealt with the mineral kingdom. When Vesuvius erupted , which destroyed the city of Pompeii , Pliny ventured too close to the volcano out of willingness to help but also out of curiosity and suffocated on the escaping gases. Due to the very detailed eyewitness report of his nephew Pliny the Younger , such explosive eruptions are still called Plinian eruptions today.
Otherwise only a few geological observations were made in antiquity. The lack of interest was mainly based on the general disdain for dirty manual labor. That left particularly in the field of applied geology , such as mining and ore geology , exclusive domain of slaves and craftsmen who verbally passed on their practical knowledge at best. Only in the biblical book Job ( Hi 28.1-19 EU ) is there a brief description of the miners' (ultimately unsatisfied) curiosity .
The basics of the Christian image of the earth
In late Christian antiquity, many old ideas about the nature of the earth were already lost. Theophilus of Antioch (115–181) rejected the old Greek ideas about the eternity of the world, or about millennia-old cycles of the creation and destruction of the earth. Instead, he tried, following the Jewish model, to calculate the age of the earth from the information in the Bible , using a date of 5529 BC. Came. Lactantius Firmianus (approx. 240-320), on the other hand, denied the spherical shape of the earth and favored a flat-earth theory , as suggested by his interpretation of the Old Testament .
middle age
While mining stagnated for a long time after the collapse of the Roman Empire in Western Europe, in the Arab-Muslim cultural area the ancient ideas about the origin of ores and rocks were further developed. Ibn Sina (Latinized: Avicenna , around 980-1037) made particular use of Aristotle, whose doctrine of the transformation of metals, however, he rejected. In addition, it provided a modern-looking classification of the mineral kingdom into salts, sulfur, metals and stones. From the layered form of rocks he deduced that they were formed by sedimentation, and he attributed the formation of mountains to the effects of earthquakes. Incidentally, in his ideas about the effects of water, Ibn Sina was close to an order ( tariqa ) of Sufi mystics who called themselves the brothers of purity . These taught that the oceans filled with sediment from the mountains and rivers over long periods of time. Eventually the seas overflow and new material is deposited on the mainlands.
Such ancient and Arabic ideas reached Western Europe in the 12th and 13th centuries, where they inspired the occidental alchemists . These explained the formation of metals through the concentrated radiation of all planets on the center of the earth, which was imagined as a huge, fiery furnace. Albertus Magnus (1200–1280) described the formation of ore veins as a distillation process. Due to the heat of the earth's interior, the finer components of the moist vapors are driven into the natural pores and cracks in the earth's crust. There they are cooled, excreted and concentrated, much like in the neck of a retort. This essentially corresponds to the modern theory of hydrothermal vein deposits .
Otherwise, when people in the Middle Ages wondered about the state of the world, they looked more at the sky than at the ground beneath their feet. In heaven he suspected, depending on the level of education, either an all-ruling Lord God, or the forces of attraction and radiation of the planets that lift the mountains, retreat the seas, or caused the growth of minerals, plants and animals.
In the late Middle Ages (especially through the incorporation of Aristotelian philosophy into Christian theology) the first doubts about the short, biblical chronology arose ; so with Jean Buridan (approx. 1328–1358), who postulated an eternal world with cycles of “perhaps a hundred thousand million years” , even if this seemed incompatible with Christian faith. However, the Reformation , with its programmatic return to the wording of the Bible, again supported the biblical chronology.
Renaissance
Leonardo da Vinci (1452–1519) rediscovered the organic nature of fossils and described them in the Codex Leicester , clearly denying the significance of the biblical flood for the process. He also rejected the short age of the earth, calculated from the Bible, and observed the different sedimentation of grains of sand in flowing water. Since Leonardo never published his notebooks , his findings were practically ineffective.
Miners ...
The work of Georgius Agricola (1494–1555) is therefore considered to be the beginning of modern geology . The main part of his work De re metallica libri XII consists of detailed descriptions of the mining and engineering art of the time, the construction of smelting furnaces, the production of soda , saltpeter , sulfur and alum , the transport of ores, wind and water power, but also legal and administrative Concerns. In the first chapters, however, he also gives many practical tips for locating deposits ( exploration ) on the basis of natural features. His De natura fossilium is considered the first manual of mineralogy , as the classification of minerals is based on external characteristics such as color, luster and taste. In his work De ortu et causis subterraneorum (1546) Agricola describes his views on the formation of minerals. Here he assumed the effect of a "petrifying juice" (succus lapidescens) , which is created by the joint heating and thickening of dry and moist substances in underground waters, and which decomposes (literally: licks) the surrounding rocks. In this concept one can see an early forerunner of the "mineral-containing solutions" ( fluids ) in modern deposit science, although Agricola attributed the various different mineral formations, following the example of Aristotle, but somewhat unsatisfactorily, to the mere effects of heat and cold. Agricola not only rejected the biblical thesis that all minerals were formed at the moment of divine creation, but also the alchemical theory about the transformation of metals. Nevertheless, these should hold up for a long time. An important drive for the European voyages of discovery overseas was z. B. the idea that the 'sun metal' gold is found particularly in the hot, tropical regions of the world.
The Swiss naturalist Conrad Gessner (1516–1565) collected in his work De rerum fossilium, lapidum et gemmarum (1565), in the spirit of the ancient stone books, a wealth of (often incorrect) information about fossils, minerals and precious stones.
... and alchemists
The speculations of the humanistic scholar Paracelsus (1493–1541) represent a further example of the (fairly free) return to the ancient tradition . In his opinion, the divine, immaterial spirit (Iliaster) was divided into the four elements fire, water and air and earth, each of which could serve as a matrix for the formation of various substances. The minerals grow according to a metaphysical principle of order (Archeus) in the earth, very similar to plant seeds. Its matrix is the water that runs through the entire earth body in a network of underground watercourses. In addition, based on the Christian Trinity, he also subdivided nature into the basic elements salt, sulfur and mercury, which give it its properties. In doing so, however, he resorted to Arabic traditions rather than Greek ones.
The idea of minerals as “seeds” that grow in the earth until they are finally transformed into “mature” metals can be found not only among alchemists, but also in the folklore of miners and metallurgists of many peoples.
In the writings of the German alchemist and mining engineer Johann Joachim Becher (1635–1682) Paracelsus' principles of salt, sulfur and mercury developed into "glassy earth" (the mother of stones), the "fat earth" (mother of ordinary earth) and "mercury earth" (mother of metals). While the mercury earth soon disappeared in the later development of the theory, the glassy earth was associated with the mineral quartz , which is actually a major component of rocks and often forms the matrix for other minerals. In theory, the fat earth was transformed into phlogiston , a hypothetical substance that was supposed to make the substances flammable. The transfer of phlogiston from one substance to another has long been used to explain combustion reactions and metallurgical processes that we would now call reduction and oxidation . In the course of the 18th century such concepts developed into ideas about special earths such as lime , silica , magnesia and alum, which one began to view as quasi-chemical components of minerals.
The discovery of the earth's history
What sets geology apart from most other natural sciences is v. a. the historical approach. The minerals could easily be classified by a chemist, the fossils by a biologist. A physicist could describe the properties of the earth's body, its shape a geographer. The geologist not only asks the question: "What is it?", But above all: "How did it get what it is?"
The Danish doctor and naturalist Niels Stensen, latinized: Nicolaus Steno (1638–1686) , took the first steps in the direction of a geological history . In 1669 he designed the first geological profile in Tuscany that was really meant to be historical. With the fundamental knowledge that the lower rock layers are also the older ones, and the layers above them, successively younger and younger, Stensen discovered the stratigraphic principle . The arrangement in space corresponds in reality to a sequence in time. In addition, Stensen postulated that all layers were originally deposited horizontally and that the layers can only be adjusted, broken and folded later by earthly forces. Likewise, Stensen again understood the organic nature of fossils. If the fossils had only subsequently formed within the rock, as Aristotle believed, then they would have been deformed by the surrounding rock, like tree roots growing into a crevice. In fact, however, the surrounding rock adapted to the fossils, which made it clear that they must be older than the surrounding rock. Stensen was the first crystallographer to recognize the law of constant angles in quartz .
Stensen's contemporaries continued to concern themselves with the problem of why the fossils were embedded deeply in the rocks rather than on the surface. One way out was to simply deny the organic origin of the fossils and to dismiss them as spontaneous formations and curious "nature games", as is the case e.g. B. Martin Lister (1638-1711) did. Robert Hooke's (1638–1703) brainstorm that a temporal sequence of changing environmental conditions could be reconstructed from the fossil content of the rocks was not pursued further for the time being.
Such geological approaches were hindered for a long time by adherence to the biblical time scale. The best known example is the calculation of the Archbishop of Armagh (Ireland), James Usher (1580–1656), who wrote the creation of the world on Monday, October 23, 4004 BC. Dated. The only event that could have significantly changed the shape of the earth after creation was the flood. She was held responsible not only for the existence of fossils far away from the sea, but also for the widespread bed debris . These rocks, which occur in large parts of northern and central Europe, were only recognized in the 19th century as evidence of the last cold ages. Because of the similarity of the coastlines of Africa and South America, a theologian named Lilienthal even blamed the flood in 1736 for the breaking up of these continents.
Geology as a modern science
In the course of the Enlightenment , belief in the biblical timescale was gradually lost, and attempts were made to build a bridge between the traditional practical knowledge of miners and metallurgists and the purely theoretical speculations of Descartes , Leibniz or Kant about the origin of the earth. With this, geology made the transition from a descriptive to an explanatory science. Collecting fossils and minerals became a veritable fad among bourgeois circles, and knowledge of geological peculiarities was an important part of general education.
The first to put Hooke's idea about a possible geological history into practice were the Prussian mountain ridge Johann Gottlob Lehmann (1719–1767) and the princely personal physician Georg Christian Füchsel (1722–1773). In doing so, they consulted the different formation of the rocks ( lithology ) rather than the fossil content. In the middle of the 18th century they made the first profile sections and geological maps that represented the rock layers in the mining districts of Thuringia.
The Tuscan mine director Giovanni Arduino (1735–1795) also made a profile of the Italian Alpine foothills. He divided the rocks of the earth's crust into 'primary', 'secondary', tertiary and quaternary . The last two terms are still in use today, the first two roughly correspond to today's Paleozoic and Mesozoic Era . He also realized that the fossils in the younger strata are becoming more and more similar to the organisms living today.
However, the breakthrough in the basic working method of geological mapping was achieved by the surveyor and canal builder William Smith (1769–1839). In 1815 he published his monumental, full color map of the geology of England and Wales taking both fossil contents and lithology into account. Smith had recognized that certain rock sequences are also characterized by a very specific, unmistakable sequence of fauna . In 1827, Leopold von Buch (1774–1853) coined the term reference fossil for fossils that allowed relative dating . Smith's map continued to guide all subsequent national agency projects . With the help of such maps it is not only possible for the geologist to show the distribution of certain rocks on the surface, but also to predict their location in the subsurface. The more one became aware that the rock layers were also units of time , the more the geological map became a complex representation of four dimensions (the three of space and time) in two dimensions.
The development of geology subsequently took place in a series of, sometimes extremely violent, scientific controversies.
A couple like fire and water: Plutonism and Neptunism
The first of these controversies was the so-called "basalt dispute" between Plutonists and Neptunists. Ostensibly scientifically conducted, the basalt dispute was also a fundamental discussion of various religious views with regard to the biblical story of creation.
The Neptunism has dating back roots to Thales of Miletus. According to this, the rocks are formed exclusively through sedimentation from aqueous solutions. His main representative was the head of the newly founded Bergakademie in Freiberg, Abraham Gottlob Werner (1749-1817). He explained volcanic phenomena as insignificant, local earth fires, and the resulting rocks were merely melted sediments.
One of Werner's opponents was the Scottish “gentleman farmer” James Hutton (1726–1797). The Plutonismus believes that the origin of all the rocks to look for in magmatic and volcanic processes. All of these ideas are ultimately based on the “dry” and “wet vapors” of Aristotle. Melted masses from the interior of the earth find their way upwards from time to time and can even break through to the surface. Erosion exposes these rocks and removes them again to be deposited on the mainland as soils and in the oceans as sediments. Due to the weight of new layers of sediment, the older layers are solidified more and more and finally, under the enormous pressure, heated again and transformed until they finally melt again. This idea of the rock cycle is generally accepted today.
Various exaggerated views of the Neptunists could subsequently be refuted, such as B. the formation of granites and basalts as chemical precipitates from the waters of a hot primordial ocean. That is why it is often claimed in Anglo-Saxon literature that the Plutonists won the controversy. One must not forget, however, that various basic assumptions by Hutton could not be upheld, such as the total denial of the existence of chemically precipitated sediments, the explanation of the salt domes as igneous intrusions, and especially the assumption that silicates are insoluble in water . On the contrary, water plays an indispensable role in all igneous and metamorphic processes. At this point Werner's superheated, mineral-saturated solutions ( brines ), under the name fluids , have returned to theory.
Werner's contribution was also that the mining academies not only did research, but also taught systematically. Many important contemporaries, such as Alexander von Humboldt , Novalis or Goethe (who carried out important experiments on the solubility and precipitation of silica gel ) attended the lectures and spread interest in geological problems all over the world.
At the beginning of the 19th century, the various loose ends began to join together. On their extensive travels, Werner's students made the acquaintance of undoubtedly volcanic formations, such as the Auvergne in France or the Eifel, and modified their views accordingly. On the other hand, attempts were made to correlate the various “mountain formations ” with the neighboring stratigraphic sequences, as observed in Thuringia, in the Paris Basin or in England, in the manner of Werner. Using William Smith's methods, these could be clearly illustrated in geological maps and profiles. Increasing use was made of key fossils.
An eternally current world, or a universe of catastrophes?
The study of key fossils led to another, long-lasting controversy about the role that can be ascribed to catastrophic events in the history of the earth. The main proponent of the cataclysm theory is Georges de Cuvier (1769–1832). From the often dramatic differences in the fossil record of the individual formations, he concluded that huge upheavals must have taken place in the course of the earth's history, which in certain areas would have wiped out all living beings. Afterwards these were replaced by new organisms, either immigrated from outside or completely newly created. The biblical flood was only the very last of these catastrophes.
The concept of actualism was developed by Sir Charles Lyell (1797–1875). His major work Principles of Geology first appeared in 1830. Based on the ideas of James Hutton, Lyell concluded that the geological timescale is very long compared to human history. He also assumed that the processes that led to the formation of certain rocks are essentially identical to the processes that can still be observed today. ("The present is the key to the past") Lyell explained the changes in the fossil record by constant, slow uplifts and subsidence of the earth's crust, as Aristotle had already imagined. The layer boundaries at which the living beings apparently changed in leaps and bounds correspond simply to the times in which no sediments would have deposited on the prominent mainland.
It was Charles Darwin (1809–1882) who largely helped topicalism to break through. He received formal, if brief, training as a geologist in his youth, and his explanation of how atolls were formed is still accepted today. His greatest achievement, however, the theory of evolution , is based essentially on Lyell's actualistic principle. It was only through the comparative study of organisms living today that he put paleontology on a solid theoretical basis. With his theory of natural selection, Darwin provided the tool with which one can explain the slow change in organisms in the course of the earth's history without having to postulate completely unknown, arbitrary, if not supernatural, forces. One of the last paleontologists who, as a supporter of catastrophism, metaphysically interpreted biodiversity and traced it back to a creative god, was Louis Agassiz .
Even so, it was premature to announce the final victory of the actualists. In fact, Lyell of all people found it very difficult to accept Darwin's theory of evolution. Lyell's prediction that vertebrate remains would also be found in the oldest strata, however, never came true. The late appearance of man, as well as the increasing signs of a global ice age at that time , contradicted his view that the earth would never have changed significantly in its history. In more recent times, catastrophism, which had already been believed dead, has experienced a renaissance. The idea of long, stable geological epochs in which practically nothing changes does not ultimately rule out the possibility of one-off, sudden, catastrophic upheavals (such as meteorite impacts ).
First global hypotheses on mountain formation
In the course of the 19th century, more and more individual pieces of information were gathered around the world. Gradually, a generally accepted, relative geological time scale emerged. The various states founded their respective geological institutes, which were particularly concerned with the production of national maps and the exploration of deposits.
The catastrophist Léonce Élie de Beaumont (1798–1874) developed the first comprehensive theory of orogeny . According to this, the global mountain belts are created by the cooling of the earth's body accompanied by cataclysmic volcanic eruptions, similar to the shrinking skin of a cold baked apple.
In the Swiss Jura , and especially in the coal fields of the Appalachian Mountains in North America, more and more indications were actually discovered that pointed to significant lateral narrowing of rock layers. These movements had apparently led to the formation of extensive folds and tectonic thrusts there. In 1873, the American actualist James Dwight Dana (1813–1895) summarized such observations for his geosynclinal theory . This remained the main tectonic explanatory model well into the 20th century. In Europe, Eduard Suess (1831–1914) helped such ideas to break through with his work on the Alps. The distinction between the worldwide mountain building phases also goes back to Suess. The best known are the Caledonian , Variscan and Alpid mountain building eras. Hans Stille (1876–1966) was a very successful advocate of the contraction hypothesis , according to which mountain formation v. a. caused by the shrinking of the earth's body ( Stille cycle ).
The problem with this hypothesis is that it can not explain certain expansive phenomena, such as the subsidence of rift fractures or crevice volcanism , in a satisfactory manner. In addition, it remains unclear how a continuous cooling process should lead to cyclically recurring phases of mountain formation, which are separated from one another by long periods of tectonic calm. It was only the discovery of natural radioactivity that provided a plausible source of energy that could counteract the unstoppable cooling and shrinking process of the earth's body, which was previously assumed. But even then, the mountain building cycle phenomenon remained a mystery.
The search for the firmly anchored supercontinent
In the second half of the 19th century, more and more similarities between the deposits and fossils were discovered on different continents, especially in South America, Africa and India. It was therefore postulated the existence of land bridges that would have connected the continents in the past, just as the Isthmus of Panama today connects North and South America. Suess, however, assumed that large parts of the originally contiguous Gondwanaland had sunk and turned into ocean floors. Incidentally, it was precisely this idea that was very well received in occult and esoteric circles around Madame Helena Blavatsky . Not only the sinking of Atlantis , but also of ' Lemuria ' (the presumed original home of the lemurs) in the Indian Ocean, and of Mu in the Pacific, was imagined in the episode of 'Medien', and with the theory of the oceanization of continental Crust explained.
Various geotectonic hypotheses were proposed until the middle of the 20th century, such as the pulsation hypothesis , which assumes alternating phases of contraction and expansion of the earth, or the oscillation hypothesis , which increasingly relies on vertical, isostatic compensatory movements in the earth's crust. Like their predecessors, what all these hypotheses have in common is that they assume a firm fixation of the earth's crust on its basis.
Italian and later German geophysicists in particular began to construct seismographs with which the propagation waves of earthquakes could be recorded in the earth's body. Around 1900, Emil Wiechert (1861–1928) concluded from seismic data the shell structure of the earth, with the earth's core , earth's mantle and earth's crust .
The discovery of the drifting continents
From around 1930, instead of the models of fixism , those of mobilism and a movable earth crust increasingly prevailed. The theory of contraction and its opposite, the theory of expansion of the earth, emerged. Both had numerous arguments for themselves, but could not explain all of the phenomena. The final paradigm shift came with insights from deep drilling and oceanographic research vessels .
When the first submarine telephone cables were laid from the British Isles to North America at the end of the 19th century, the mid-Atlantic ridge was discovered . However, for a long time no conclusions were drawn from the fact that it runs parallel to the coast from north to south across the entire ocean, instead of, as would have been expected, connecting the mainlands on both sides of the Atlantic in an east-west direction.
The first mobilistic ideas about the possibility of significant lateral movements of mainland masses can be found in Alfred Wegener's (1880–1930) continental drift hypothesis from 1915. Wegener assumed that the relatively light, granitic rocks of the continental crust ( sial ) on the denser but viscous subsoil made of basaltic material ( sima ) float like icebergs on water. An original supercontinent ( Pangea ) could break into pieces and drift apart due to relatively weak forces. This would not only explain the parallel course of the eastern and western coasts of the Atlantic, but also the similarities of the fossils and climate evidence , as well as certain ancient mountain ranges in Gondwana. Wegener's theory met with widespread rejection during his lifetime, as he could not plausibly explain the forces at work. Only Arthur Holmes (1890–1965) proposed a mechanism in 1930 that could explain the movement of continental plates: convection currents of hot magmas in the earth's mantle.
However, the breakthrough in mobilist theories did not come until three decades later in the 1960s. It was recognized that the global system of the mid-ocean ridges is seismically active and that there, along volcanic crevices, new material continuously emerges from the earth's mantle to the surface. In the case of Iceland, which lies exactly on the mid-Atlantic ridge, paleomagnetic measurements of the rocks on the ocean floor have shown that the two symmetrical sides of the ocean floor move a few centimeters apart every year. This phenomenon is today, with a not entirely happy translation from English, referred to as ocean floor spreading (see: Sea-Floor-Spreading ), ocean floor spreading would probably be more appropriate. From an abundance of geophysical , oceanographic , paleontological and petrographic observations, the now generally accepted theory of plate tectonics developed . The cyclical alternation of phases of the breakup of continents and the renewed collision of these plates provides a plausible explanation for the recurring, global orogeny phases ( Wilson cycle ) as well as for a number of other geological phenomena.
The change in working methods in the 20th century
Geologists began to use chemical and physical methods to examine rocks and minerals as early as the 18th and 19th centuries. Particularly noteworthy here are the soldering tube trials , which can be traced back to Axel Frederic Cronstedt , and the wet chemical analysis, which gained in importance in the 19th century. But until the beginning of the 20th century, descriptive research methods dominated geology. In the 20th century, geology turned into an analytical natural science: with the discovery of X-ray diffraction , it was possible to determine the mineralogical composition of fine crystalline rocks, and with the development of geophysics , knowledge about the interior of the earth was gained for the first time. With the help of computer modeling , geological processes can be better understood. An ever greater proportion of geological research moved from the site to the desk and into the laboratory. This change in methods turned the previously purely qualitative geology into a quantitative science and thus represents the second quantum leap in the scientific history of geology after the abandonment of metaphysical ideas in the early modern era.
literature
- François Ellenberger : History of Geology, 2 volumes, Balkema, 1996, 1999
- Helmut Hölder : Brief history of geology and paleontology , Springer Verlag, 1989, ISBN 3-540-50659-4 .
- David R. Oldroyd : Thinking about the Earth , Harvard Press, 1996, ISBN 0-674-88382-9 ; dt .: The biography of the earth. On the history of science in geology , Frankfurt am Main 1998.
- Alan Cutler: The shell on the mountain - About Nicolaus Steno and the beginnings of geology. Albrecht Knaus Verlag, Munich 2004, ISBN 3-8135-0188-4 .
- Gabriel Gohau A history of Geology , Rutgers University Press 1990 (French original edition La Decouverte 1987).
- Martin Rudwick Bursting the Limits of Time: The Reconstruction of Geohistory in the Age of Revolution , University of Chicago Press 2005.
- Martin Rudwick Worlds Before Adam. The Reconstruction of Geohistory in the Age of Reform , University of Chicago Press 2008
- Anthony Hallam Great Geological Controversies , Oxford University Press 1983, 2nd edition 1989.
- Bernhard Hubmann The Great Geologists , Marix Verlag 2009.
- Karl Alfred von Zittel History of geology and paleontology up to the end of the 19th century , Munich: Oldenbourg 1899, archive .
- Max Pfannenstiel How did you do geology a hundred years ago? , Communications of the Alpine Geological Association, Volume 34, 1941, Vienna 1942, pdf .
Web links
- Olaf Otto Dillmann: People and data on the history of geology and paleontology. In: Geoscientific Service. February 21, 2017, accessed November 17, 2020 .
- Gottfried Zirnstein: natural scientist for and against the flood, the ice sheet and continental drift. (PDF) How the formation of the earth's surface was elucidated. Retrieved November 17, 2020 .
Individual evidence
- ↑ Martin Kemp : Leonardo , CH Beck, Munich 2005, p. 186 ff. ISBN 978-3-406-53462-1
