October 23, 2021
Plate tectonics, plate tectonics or plate tectonics is the scientific theory that explains the geographic location of continents, oceans, mountains and other structures on the Earth's surface, as well as the geological structures in the Earth's crust and where earthquakes and volcanism occur. According to this theory, the lithosphere (the outermost layer in the Earth, on average about 100 km thick) is divided into tectonic plates or plates, which move independently of each other over the Earth's surface by "currents" in the underlying asthenosphere. Although the asthenosphere is not liquid, it has a relatively low shear strength, which, when measured on the geologic time scale, makes it a slow flowing viscous liquid. There are seven or eight major plates on Earth (depending on how they are defined) and a large number of smaller plates. The plates typically move relative to each other at a rate of several centimeters per year, which can actually be measured with geodetic measurements with satellites and VLBI since the 1990s. There are three types of boundaries: convergent (where plates move toward each other), divergent (where plates move away from each other), and transforme (where plates move past each other). At a divergent plate boundary, new lithosphere is formed by solidification of magma, this process is called oceanic spreading. At a convergent plate boundary, one of the two plates will slide under the other (subduction). In practice, due to its low density, continental crust will not subduct well. Usually the subducting plate therefore consists of oceanic crust. If two continents move towards each other, neither plate will subduct, and mountain formation will occur. Because the movement in the lithosphere is concentrated along these boundaries, earthquakes, volcanism and the formation of mountain ranges and oceanic troughs occur here. The theory of plate tectonics was developed in the 1960s as a successor to Alfred Wegener's continental drift hypothesis. Contrary to Wegener's hypothesis, plate tectonics soon became widely accepted in the scientific community.