Data accumulated in recent years make it possible to compare two components of processes at depth, namely thermal expansion and lithostatic compression, or, strictly speaking, thermal and pressure-induced deformations or t-p deformations of chemical compounds and rocks under atmospheric conditions and the conditions of the earth's crust and mantle. The volume and density of rocks change in their downward movement within the earth primarily as a result of phase transitions and to a lesser degree because of t-p deformations. When rocks descend below the surface and are subjected to the opposing effects of temperature and pressure, they expand as a result of t-p deformations; this expansion decreases with depth as a result of the decrease in the temperature gradient with depth; the compression increment needed for compensation increases until complete cancellation of the pressure effects occurs in the roof of the upper mantle, and compression predominates at greater depths. These volume effects of deformation cause a monotonic change in the density of rocks with depth; but they are minor in magnitude compared with the effects of many phase transitions occurring at depth and consequently they govern density changes only in the absence of significant phase transitions.