what are plate tectonics

Plate tectonics is the scientific theory that Earth's lithosphere comprises a number of large tectonic plates which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. Earth's lithosphere, the rigid outer shell of the planet including the crust and upper mantle, is fractured into seven or eight major plates and many minor plates or "platelets." Where the plates meet, their relative motion determines the type of plate boundary (or fault): convergent, divergent, or transform. The relative movement of the plates typically ranges from zero to 10 cm annually. Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Plate tectonics explains the movement of Earth's surface, the creation of major landforms, and geological activities such as earthquakes and volcanic eruptions