A fault is a fracture surface along which motion has take place. Mountain building begins with the deposition of sediments in a sinking area, followed by tectonic movement and erosion. The crust and the upper part of the mantle make up a shell of hard rock called the lithosphere . Below the lithosphere is a layer of hot, soft rock called the asthenosphere . According to the theory of plate tectonics , the lithosphere is divided into seven huge plates and a number of smaller ones.
Continental drift occurs because of plate motion.
According to the principle of uniform change , geologic processes in the past were the same as those in the present. An unconformity , which is a buried surface of erosion, indicates that tectonic uplift, erosion, and sedimentation have occurred in that order. Radionuclides and the decay products in rocks make it possible to date geologic formations. Geologic time is divided into Precambrian time and the Paleozoic, Mesozoic , and Cenozoic Eras. Eras are subdivided into periods and the periods into epochs .