The Earth's outer solid and rigid skin is called its lithosphere (for strength). This skin, some 10 to 100 km in thickness, consists of a dozen major lithospheric plates (map below - left) and many more smaller micro plates moving in a variety of directions and at different velocities with respect to each other and in reference to the more or less stationary deep interior of the planet. Each plate maintains its rigidity without any significant internal deformation. Practically all tectonic activity, both past and present, is and has been located along the boundaries that separate the plates. Thus there is a striking spatial correspondence between the plate boundaries and the location of earthquakes. The 52 individually colored plates shown in the Earth's Tectonic Plates map and shown below (right) are from the compilation of Bird (2003).
The scientific paradigm known as "Plate Tectonics" originated in 1967 as a theory to explain the pattern of seafloor spreading revealed by symmetric ocean floor magnetic anomalies, the orientation and configuration of fracture zones, the occurence of earthquakes in narrow belts, and the various lines of compelling evidence indicating that today's continents had been joined previously into a supercontinent as recently as 200 my ago.
Although there are two types of lithosphere - thicker continental and thinner oceanic - plate boundaries are not necessarily located along continent/ocean boundaries.
Earth's plates are akin to separate caps on a sphere. Consequently their motions with respect to each other can be explained and quantified as rotations around virtual poles that scientists call Euler poles after Leonhard Euler, an eighteeth century Swiss mathematician who pioneered the concepts of spherical trigonometry used today by geologists and geophysicists. Boundaries between adjacent plates meet each other at what are called "triple junctions".
The rigid lithosphere rides over, and with very little evidence of frictional resistance, a much weaker and ductile mantle called the asthenosphere. The movements of the plates are essentially determined by gravitational potential. Plate motion is propelled by the higher elevation of the plates at the axis of mid-ocean ridges. Although this driving force has been called "ridge push" it is actually a form of gravitational sliding. The lithosphere also sinks (essentially falls) into the mantle below by a force termed "slab pull". Lithosphere may descend all the way to the core/mantle boundary and eventally recycle to the surface in mantle plumes.
Plate tectonics has been active since shortly after the birth of our planet and contributed to the early formation of continents. Plate tectonics is the primary method by which the earth releases its interior heat. Contrary to popular misconception, the movement of the plates and the return flow of asthenosphere to the spreading centers actually stirs the convection in the interior mantle and not vice versa. The plates are not carried piggy-back on the top of a convecting mantle, but instead are a main driving force for mantle convection.
Data Source:
Bird, P. (2003) An updated digital model of plate boundaries, Geochemistry Geophysics Geosystems, 4(3), 1027, doi:10.1029/2001GC000252.
Further Reading:
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