 The mechanically rigid outer layer of Earth , the lithosphere , is broken into pieces called tectonic plates . These plates are rigid segments that move in relation to one another at one of three types of plate boundaries : convergent boundaries , at which two plates come together , divergent boundaries , at which two plates are pulled apart , and transform boundaries , in which two plates slide past one another laterally . Earthquakes , volcanic activity , mountain @-@ building , and oceanic trench formation can occur along these plate boundaries . The tectonic plates ride on top of the asthenosphere , the solid but less @-@ viscous part of the upper mantle that can flow and move along with the plates . 
