Tectonic uplift is the geologic uplift of Earth's surface that is attributed to plate tectonics. While isostatic response is important, an increase in the mean elevation of a region can only occur in response to tectonic processes of crustal thickening (such as mountain building events), changes in the density distribution of the crust and underlying mantle, and flexural support due to the bending of rigid lithosphere. Tectonic uplift results in denudation (processes that wear away the earth's surface) by raising buried rocks closer to the surface. This process can redistribute large loads from an elevated region to a topographically lower area as well – thus promoting an isostatic response in the region of denudation (which can cause local bedrock uplift). The timing, magnitude, and rate of denudation can be estimated by geologists using pressure-temperature studies. Crustal thickening Crustal thickening has an upward component of motion and often occurs when continental crust is thrust onto continental crust. Basically nappes (thrust sheets) from each plate collide and begin to stack one on top of the other; evidence of this process can be seen in preserved ophiolitic nappes (preserved in the Himalayas) and in rocks with an inverted metamorphic gradient. The preserved inverted metamorphic gradient indicates that nappes were actually stacked on top of each other so quickly that hot rocks did not have time to equilibrate before being thrust on top of cool rocks. The process of nappe stacking can only continue for so long, as gravity will eventually disallow further vertical growth (there is an upper limit to vertical mountain growth). Density distribution of the crust and underlying mantle Although the raised surfaces of mountain ranges mainly result from crustal thickening, there are other forces at play that are responsible for the tectonic activity. All tectonic processes are driven by gravitational force when density differences are present. A good example of this would be the large-scale circulation of the Earth's mantle. Lateral density variations near the surface (such as the creation, cooling, and subduction of oceanic plates) also drive plate motion. The dynamics of mountain ranges are governed by differences in the gravitational energy of entire columns of the lithosphere (see isostasy). If a change in surface height represents an isostatically compensated change in crustal thickness, the rate of change of potential energy per unit surface area is proportional to the rate of increase of average surface height. The highest rates