plate tectonics definition by American Heritage Dictionary
(used with a sing. verb) A theory that explains the global distribution of geological phenomena such as seismicity, volcanism, continental drift, and mountain building in terms of the formation, destruction, movement, and interaction of the earth's lithospheric plates.
(used with a sing. or pl. verb) The dynamics of plate movement.
In geology, a theory that explains the distribution of continents, earthquakes, volcanoes,, mountains and other geologic phenomena in terms of the formation, movement, and destruction of tectonic plates. These plates move in response to forces deep within the Earth. Because continents, such as North America, often ride piggy-back on plates, their movement is referred to as continental drift.
Earthquakes and volcanoes tend to occur at the boundaries between plates: the San Andreas Fault is on such a boundary.
New plate material is constantly created by the process of sea floor spreading, and old material is destroyed when two plates collide and one plate moves under the other.
In geology, a theory that the Earth's lithosphere (the crust and upper mantle) is divided into a number of large, platelike sections that move as distinct masses. The movement of the plates is believed to result from the presence of large convection cells in the Earth's mantle which allow the rigid plates to move over the relatively plastic asthenosphere. The theory of plate tectonics was developed in the 1960s in an effort to explain the jigsawlike pattern of the Earth's continents. See Note at fault. See more at tectonic boundary.
A Closer Look Although German physicist, meteorologist, and explorer Alfred Wegener proposed the theory of continental drift in 1912, suggesting that the continents were once joined as one large landmass, the explanation for the movement of such large landmasses into their current positions was not developed for several more decades. According to the theory of plate tectonics, which was proposed in the 1960s, the continents (and ocean floors) ride atop about a dozen semirigid plates—huge slabs of Earth's lithosphere—that are much larger than the continents themselves. The plates' constant movement is powered by huge convection currents of molten rock in Earth's mantle, thought by many geologists to be heated by the decay of radioactive elements deep within Earth. Although the plates move only a few inches per year, over the hundreds of millions of years of geological time, the continents are carried thousands of miles. Along their margins, the independently moving plates interact in three main ways. Where plates pull apart, new crust is formed. Where they collide, one plate is submerged beneath the other, and material from the bottom one returns to Earth's mantle. If the converging plates have land masses on them, the boundaries crumple, forming mountains. Plates also slide past each other, creating the faults that produce earthquakes. The six major plates are the Eurasian, American, African, Pacific, Indian, and Antarctic. See more at note at Wegener.