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Plate tectonics (from Greek τέκτων, ''tektōn'' "builder" or "mason") is a Theory of Geology that has been developed to explain the observed evidence for large scale motions of the Earth 's Lithosphere . The theory encompassed and superseded the older theory of Continental Drift from the first half of the 20th century and the concept of Seafloor Spreading developed during the 1960s. The outermost part of the Earth's interior is made up of two layers: above is the Lithosphere , comprising the Crust and the rigid uppermost part of the Mantle . Below the lithosphere lies the Asthenosphere . Although solid, the asthenosphere has relatively low Viscosity and Shear Strength and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again. This is, however, not due to cooler temperatures but due to high pressure. The lithosphere is broken up into what are called ''tectonic plates''—in the case of Earth, there are seven major and many minor plates ( or collision boundaries, Divergent or spreading boundaries, and Transform boundaries. Earthquake s, Volcanic Activity , Mountain -building, and Oceanic Trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 0.66 to 8.50 centimeters per year (the speed at which human nails grow). SYNOPSIS OF THE DEVELOPMENT OF THE THEORY In the late ninteenth and early twentieth centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement, as explained by Geosynclinal Theory . The discovery of the Americas showed that the opposite Coasts of the Atlantic Ocean — or, more precisely, the edges of the Continental Shelves — have similar shapes and seem once to have fitted together. Since that time many theories were proposed to explain this apparent coincidence, but the assumption of a solid earth made the various proposals difficult to explain. The discovery of Radium and its associated heating properties in 1896 prompted a re-examination of the apparent Age Of The Earth , since this had been estimated by taking its temperature and assuming that it radiated like a Black Body . Such calculations assumed that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years. With this new heat source, it was credible that the Earth was much older, and also that its core was still sufficiently hot to be liquid. Plate tectonic theory arose out of the hypothesis of Continental Drift first proposed by Alfred Wegener in 1912 and expanded in his 1915 book ''The Origin of Continents and Oceans'', which suggested that the present continents once formed a single land mass which had drifted apart, floating on the molten rocks of the Core . But without detailed evidence and calculation of the forces involved, the theory remained sidelined. The Earth might have a solid crust and a liquid core, but there seemed to be no way that portions of the crust could move around -- although later science proved theories proposed by English geologist Arthur Holmes in 1920 that their junctions might actually lie beneath the Sea . The first evidence that crust plates did move around came with the discovery of variable and magnetic field reversals by Harry Hess and Ron G. Mason Ron Mason's key work [http://www.tos.org/oceanography/issues/issue_archive/issue_pdfs/16_3/16.3_spiess.pdf] pinpointed the precise mechanism which accounted for new rock upwelling. Following the recognition of Magnetic Anomalies defined by symmetric, parallel stripes of similar magnetization on the seafloor on either side of a Mid-ocean Ridge , plate tectonics quickly became broadly accepted. Simultaneous advances in early Seismic imaging techniques in and around Wadati-Benioff Zone s collectively with numerous other geologic observations soon solidified plate tectonics as a theory with extraordinary explanatory and predictive power. Study of the deep Ocean floor was critical to development of the theory; the field of deep sea Marine Geology accelerated in the 1960s. Correspondingly, plate tectonic theory was developed during the late 1960s and has since been accepted all but universally by scientists throughout all geoscientific disciplines. The theory revolutionized the Earth sciences, explaining a diverse range geological phenomena. KEY PRINCIPLES
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