what is plate tectonic theory

Plate tectonic theory is a scientific theory that explains how Earths lithosphere, the rigid outer shell of the planet including the crust and upper mantle, comprises a number of large tectonic plates which have been slowly moving since about 3.4 billion years ago. The theory builds on the concept of continental drift, which was developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s. The lithosphere is fractured into seven or eight major plates and many minor plates or "platelets". The relative motion of the plates typically ranges from zero to 10 cm annually, and where the plates meet, their relative motion determines the type of plate boundary (or fault): convergent, divergent, or transform. Faults tend to be geologically active, experiencing earthquakes, volcanic activity, mountain-building, and oceanic trench formation. Plate tectonics has become the unifying theory of geology, explaining the earth’s surface movement, current and past, which has created the tallest mountain ranges and the deepest oceans.