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Plate tectonics  is a scientific theorythat describes the large-scale motion of Earth's lithosphere. This theoretical model builds on the concept of continental driftwhich was developed during the first few decades of the 20th century. The geoscientific community accepted the theory after the concepts of seafloor spreading were later developed in the late 1950s and early 1960s.The lithosphere, which is the rigid outermost shell of a planet (on Earth, the crust and upper mantle), is broken up intotectonic plates. On Earth, there are seven or eight major plates (depending on how they are defined) and many minor plates. Where plates meet, their relative motion determines the type of boundary; convergent, divergent, or transform. Earthquakes,volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative movement of the plates typically varies from zero to 100 mm annually.[2]Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories (that still have some supporters) propose gradual shrinking (contraction) or gradual expansion of the globe.[3]Tectonic plates are able to move because the Earth's lithosphere has greater strength than the underlying asthenosphere. 
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Plate tectonics is the scientific theory that attempts to explain the movements of the Earth's lithosphere that have formed the landscape features we see across the globe today. By definition the word "plate" in geologic terms means a large slab of solid rock. "Tectonics" is a part of the Greek root for "to build" and together the terms define how the Earth's surface is built up of moving plates.The theory of plate tectonics itself says that the Earth's lithosphere is made up individual plates that are broken down into over a dozen large and small pieces of solid rock. These fragmented plates ride next to each other on top of the Earth's more fluid lower mantle to create different types of plate boundaries that have shaped the Earth's landscape over millions of years.
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