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Plate Tectonics is the comprehensive theory that has been formulated to explain the surface distribution of motions and forces on Planet Earth. The “conveyor belt of plates at the Earth’s surface are destroyed through subduction at trenches and renewed by volcanism at mid-ocean ridges. Satellites have produced comprehensive images of the seafloor topography that allows us to see clearly that the ocean floor is younger and less complex geologically than the continents. The world-encircling mid-ocean ridge system dominates seafloor topography. Fracture zones offset the ridges, and at the other end of the ridge flanks, trenches mark the locations where lithosphere is subducted back into the mantle. The oldest seafloor is ~160 million years old, whereas the continents are up to 3.5 billion years old. Plate Tectonics holds that the continents ride on lithospheric plates. They, in turn, move relative to one another following the precise Euclidian laws of geometric motion on a sphere. Seafloor spreading occurs at the mid-ocean ridges in a tensional stress regime. Plate destruction., or subduction, occurs where two plates collide in zones of compression. Plates slide across one another at transform faults, where shear motion dominates. Plates are rigid and deform only at their edges. Plates overly a soft, partially liquid and convecting mantle.
Earthquakes reveals the physical state of the core, mantle., and lithospheric crust. The lithosphere is brittle and the asthenosphere of the upper mantle plastic. Earthquakes and volcanoes occur in linear belts which define the plate boundaries. Plate tectonics describes the surface mot ions of plates, but does not directly deal with mantle forces that cause the motions. Transform faults and fracture zones allow determination of poles of rotation between any two plates. Triple junctions occur where three plates intersect. Earthquake focal mechanisms and first motion studies define the nature of faulting at plate boundaries. Marine magnetic anomalies resulting from magnetic-field polarity reversals describe the seafloor spreading process and document past plate motions.
Volcanoes and earthquakes associated with subduction zones ring the plates. Earthquake hypocenters define the geometry of subducting lithospheric plates and image two parallel planes representing the top and bottom of the rigid portion of the plates. As the plates subduct deeper and heat, less remains rigid and the two seismic planes merge. Soft sediments are scraped off the subducting plate and accreted onto the overriding plate. Bound water lowers the melting point of rock, and escapes as free water with magma at depth producing volcanism at island arcs and on continental crust. B-value plots of the magnitudes versus numbers of earthquakes allow the discrimination of wet and dry rock environments at subduction zones. Island arcs are the early beginnings of continents. Magma diapirs occur at ~100 km spacing along the subducting slab, which results in a similar spacing of volcanos. Accretion and back-arc spreading, in addition to volcanism, contribute to continental growth.