How do tectonic plates move

Plate tectonic theory has developed slowly and progressively since it was developed in the 1960s. It is a theory that truly has the entire world as its experiment.According to the theory of plate tectonics, the Earth's crust and upper mantle are broken into moving plates of "lithosphere." The lithospheric plates are solid rock. There are several very large plates, each consisting of both oceanic and continental portions.There are a dozen or more smaller plates. The plates average about 80 kilometers (50 miles) in thickness.All of the plates are moving. They are slow, moving at speeds of centimeters to tens of centimeters per year. They slide along on top of an underlying mantle layer called the asthenosphere, which contains a little magma (molten rock). Many types of evidence indicate that the plates move.All objects on and in the Earth are pulled towards its center by the force of gravity. This may affect the plates at converging plate boundaries in areas called subduction zones, where one plate sinks into the mantle.At the midocean ridges, the plates are separating, pulled apart by the motion of convections cells beneath the plates. Lava then comes up to fill in the resulting gap and cools into solid rock when it comes into contact with cold ocean water,becoming new, young ocean floor. The ocean floor cools more as it ages (making it contract) and more and more sediments pile on top of it, so it becomes more and more dense with time. As the ocean floor moves away from the mid-ocean ridge (“sea-floor spreading”), it either pushes a continent (“continental drift”) or runs intoanother plate, leading to earthquakes.The movements of plates over millions of years resulted in the opening and closure of oceans and the formation and disassembly of continents.There are three principal types of plate boundary (divergent, convergent, and transform).

Plates move apart at divergent plate boundaries such as the oceanic ridge system that separates the North American and Eurasian plates in the north Atlantic Ocean. Plates crash into each other along convergent plate boundaries marked by volcanoes and mountain belts. Finally, plates slide past each other along a transform plate boundary such as the San
Andreas Fault, California, that separates the North American and Pacific plates.