Historical background
continental drift
At the end of the century and the beginning of the century, geologists assumed that the main features of the Earth were fixed and that most geological features, such as the development of basins and mountain ranges, could be explained by the vertical movement of the crust, described in what is called geosynclinal theory. Generally, this was placed in the context of a contracting planet Earth due to heat loss over the course of a relatively short geological time.[4].
As early as 1596 it was noted that the opposite coasts of the Atlantic Ocean (although it is more accurate to speak of the edges of the continental shelves) have similar shapes and appear to have fit together at some time in the past. Many theories have since been proposed to explain this apparent complementarity, but the assumption of a solid Earth made these various proposals difficult to accept.
The discovery of radioactivity and its associated heating properties in 1895 prompted a reexamination of the apparent age of the Earth. This had previously been estimated by its cooling rate under the assumption that the Earth's surface radiated as a black body. Those calculations had implied that, even if it started out red hot, the Earth would have fallen to its current temperature within a few tens of millions of years. Armed with the knowledge of a new heat source, scientists realized that the Earth would be much older and that its core was still hot enough to be liquid.
In 1915, after having published a first article in 1912, Alfred Wegener presented serious arguments in favor of the idea of continental drift in the first edition of The Origin of the Continents and Oceans. In that book (reissued in four successive editions until the last one in 1936), he pointed out how the east coast of South America and the west coast of Africa seemed to fit together (which Benjamin Franklin, among others, had previously realized).[5] Wegener was not the first to notice this (Abraham Ortelius, Antonio Snider-Pellegrini, Eduard Suess, Roberto Mantovani and Frank Bursley Taylor" preceded him, just to name a few), but he was the first to assemble important fossil, paleo-topographic and climatological evidence to support this simple observation (and was supported in this by researchers such as Alex du Toit). He also took into account the similarity of the fossil fauna of the continents. northern regions and certain geological formations, Wegener conjectured that the set of present-day continents were united in the Earth's remote past, forming a supercontinent, called Pangea.[6] Furthermore, since the rock strata on the margins of separate continents are very similar, it suggests that these rocks were formed in the same way, implying that they were united originally. Newfoundland and New Brunswick. Additionally, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology.[7].
However, his ideas were not taken seriously by many geologists,[8] who noted that there was no apparent mechanism for continental drift. In his original thesis, Wegener proposed that the continents moved on the Earth's mantle in the same way that one moves a rug on the floor of a room. However, this is not possible, due to the enormous force of friction involved, which led to the rejection of Wegener's explanation, and the putting on hold, as an interesting but unproven hypothesis "Hypothesis (scientific method)"), of the idea of continental displacement until the appearance of Plate Tectonics. More to the point, they didn't see how continental rock could break through the much denser rock that makes up oceanic crust. Wegener could not explain the force that drove continental drift, and his claim did not come until after his death in 1930.[9].
Floating continents, paleomagnetism and seismic zones
As it was early observed that although granite existed on the continents, the seafloor appeared to be composed of denser basalt, the predominant concept during the first half of the century was that there were two types of crust, called "sial" (continental-type crust). and "sima" (oceanic-type crust).[10] In addition, it was assumed that there was a static layer of strata beneath the continents. Therefore, it seemed evident that a layer of basalt (sial) underlies the continental rocks.
However, based on anomalies in the plumb deviation of the Andes in Peru, Pierre Bouguer had deduced that less dense mountains must have a downward projection into the denser lower layer. The concept that mountains had "roots" was confirmed by George B. Airy a hundred years later, during a study of the gravitation of the Himalayas, and seismic studies detected corresponding density variations. Therefore, in the mid-1950s the question remained unresolved as to whether the roots of the mountains were packed into the surrounding basalt or floated on top of it like an iceberg.
During the century, improvements and greater use of seismic instruments such as seismographs allowed scientists to understand that earthquakes tend to concentrate in specific areas, especially along oceanic trenches and ridges. By the late 1920s seismologists were beginning to identify several prominent earthquake zones parallel to trenches that typically dipped between 40 and 60° from the horizontal and extended several hundred kilometers into the Earth's interior. These zones later became known as Wadati-Benioff zones, or simply Benioff zones[11], after the seismologists who first recognized them, Kiyoo Wadati") of Japan and Hugo Benioff") of the United States. The study of global seismicity advanced enormously in the 1960s with the establishment of the World Standard Seismograph Network (WWSSN) to monitor compliance with the 1963 treaty banning aerial testing of nuclear weapons. The greatly improved data from WWSSN instruments allowed seismologists to accurately map earthquake concentration zones around the world.
Meanwhile, debates developed around the phenomenon of polar drift. Since the first debates about continental drift, scientists had discussed and used evidence that polar drift had occurred because the continents appeared to have moved through different climate zones during the past. Furthermore, paleomagnetic data had shown that the magnetic pole had also shifted over time. Reasoning the other way around, the continents could have moved and rotated, while the pole remained relatively fixed.[12] The first time evidence for magnetic polar drift was used to support the movements of the continents was in a paper by Keith Runcorn in 1956, and successive papers by him and his students Ted Irving (who was actually the first to be convinced of the fact that paleomagnetism supported continental drift) and Ken Creer.
Mid-ocean ridge spreading and convection
The first map of the ocean floor was created in 1956 thanks to advances in sonar technologies. The Atlantic Ocean was investigated and it was discovered that:.
For these reasons, in 1960 Harry Hess and in 1961 Robert Dietz") suggested that the ocean floor expands. In 1963 this hypothesis was proven when Vine and Matthews identified lines of magnetism of different polarities, that is, that the Earth's magnetic field is inverted.[13].
In 1974, within the international FAMOUS project, a team of scientists from the Woods Hole Oceanographic Institution (USA) and the French Center Oceanologique de Bretagne (Brest, France) used surface research vessels, as well as various advanced instruments that included magnetometers, sonar and seismographs, as well as two submersibles: the Alvin (USA) and the Archimède (France). Investigations confirmed the existence of an elevation in the central Atlantic Ocean and discovered that the bottom of the seabed, beneath the sediment layer, consisted of basalt, not granite, which is the main component of the continents. They also found volcanic and seismic activity and that the oceanic crust was much thinner than the continental crust. All of these new findings raised important and intriguing questions.[14].
The new data collected on the ocean basins also showed particular characteristics in terms of bathymetry. One of the main results of these data sets was that a mid-ocean ridge system was detected worldwide. An important conclusion was that a new ocean floor was being created along this system, leading to the concept of the "Great Global Rift." This was described in Bruce Heezen's crucial article (1960) based on his work with Marie Tharp, which would trigger a true revolution in thought. A profound consequence of seafloor spreading is that new crust is created and continues to be created along mid-ocean ridges. Heezen therefore defended S. Warren Carey's supposed "Expanding Earth" hypothesis (see above). So the question still remained: how can new crust be continually added along mid-ocean ridges without increasing the size of the Earth? In fact, this question had already been resolved by numerous scientists during the 1940s and 1950s, such as Arthur Holmes, Vening-Meinesz, Coates and many others: the excess crust disappears along the so-called oceanic trenches, where the process known as subduction occurs. Therefore, when several scientists in the early 1960s began to reason about the data they had at their disposal about the ocean floor, the pieces of the theory quickly fell into place.
The question particularly intrigued Harry Hammond Hess, a Princeton University geologist and rear admiral in the Naval Reserve, and Robert S. Dietz, a U.S. scientist. National Geodetic Survey, who first coined the term ocean floor spreading. Dietz and Hess (the former published the same idea a year earlier in Nature, but priority belongs to Hess, who had already distributed an unpublished manuscript of his 1962 paper in 1960) were among the small handful who really understood the broad implications of seafloor spreading and how it would eventually agree with the then unconventional and unaccepted ideas of continental drift and the elegant, mobilist models proposed by earlier researchers such as Holmes.[15].
Magnetic inversions and magnetic banding
Beginning in the 1950s, scientists such as Victor Vacquier, using magnetic instruments (magnetometers) adapted from aerial devices developed during World War II to detect submarines, began to recognize strange magnetic variations on the ocean floor. This finding, although unexpected, was not entirely surprising because basalt, the iron-rich volcanic rock that forms the ocean floor, was known to contain a strongly magnetic mineral (magnetite) and can locally distort compass readings. This distortion was recognized by Icelandic sailors already at the end of the century. More importantly, because the presence of magnetite gives basalt measurable magnetic properties, these newly discovered magnetic variations provided another means of studying the deep ocean floor. When the newly formed rock cooled, such magnetic materials recorded the Earth's magnetic field at that time.
As more and more of the seafloor was mapped during the 1950s, magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern: one stripe with normal polarity and the adjacent stripe with reversed polarity. The general pattern, defined by these alternating bands of normally and inversely polarized rock, became known as magnetic bands and was published by Ron G. Mason and his collaborators in 1961, who did not, however, find an explanation for these data in terms of seafloor spreading, as did Vine, Matthews and Morley a few years later.[16].
The discovery of the magnetic stripes required an explanation. By the early 1960s, scientists such as Heezen, Hess, and Dietz had begun to theorize that mid-ocean ridges mark structurally weak zones where the ocean floor was splitting in two along the crest of the ridge. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. This process, originally called the "conveyor belt hypothesis" and later "ocean floor spreading", operates for many millions of years and continues to form new ocean floor throughout the 64,000 km long ocean ridge system.[17]
Only four years after the "zebra pattern" magnetic stripe maps were published, the link between ocean floor spreading and these patterns was established, correctly and independently, by Lawrence Morley, Fred Vine, and Drummond Matthews, in 1963, known today as the Vine-Matthews-Morley hypothesis.[18] This hypothesis linked these patterns to geomagnetic reversals and was supported by several lines of evidence. evidence:.
Explaining both the zebra-like magnetic stripes and the construction of the ocean ridge system, the ocean floor spreading hypothesis quickly gained adherents and represented another important advance in the development of the theory of plate tectonics. Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" of the history of geomagnetic field reversals of that of the Earth. Extensive studies are currently devoted to calibrating normal inversion patterns in oceanic crust, on the one hand, and known time scales derived from the dating of basalt layers in sedimentary sequences (magnetostratigraphy), on the other, to arrive at estimates of past propagation rates and plate reconstructions.[16].
The plate tectonics revolution
After all these considerations, plate tectonics (or, as it was initially called "new global tectonics") was quickly accepted in the scientific world, and numerous articles followed that defined the concepts involved:
The plate tectonics revolution was the scientific and cultural change that developed from the acceptance of the theory of plate tectonics and represented a paradigm shift and a scientific revolution that transformed geology.