The branch of Earth Science that deals with the structure, evolution, relative motion, and deformation of the outer part of the Earth, or the lithosphere, on a regional to global scale, for time scales ranging from very short up to thousands of millions of years. The term active tectonics refers to tectonic movements that occur on a time scale of up to thousands of years, and neotectonics to tectonic movements on a scale of thousands of years to a few million years. The lithosphere includes the Earth's crust and part of the Earth's upper mantle and averages about 100 km in thickness. Plate tectonics is a theory of global tectonics in which the Earth's lithosphere is divided into a number of spherical plates that undergo dominantly rigid-body translation on the sphere. The relative motions of these plates cause earthquakes and deformation along the plate boundaries and in adjacent regions