Under the influence of the low-intensity , long-term stresses that drive tectonic motion , the lithosphere responds essentially as a rigid shell and thus deforms primarily through brittle failure , whereas the asthenosphere ( the layer of the mantle below the lithosphere ) is heat-softened and accommodates strain through plastic deformation . The boundary between the lithosphere and the underlying asthenosphere is defined by a difference in response to stress : the lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure , while the asthenosphere deforms viscously and accommodates strain through plastic deformation . The lithosphere is broken into tectonic plates .