Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 40

they have buoyant, less dense crustal roots that extend deep into the upper mantle sup-
porting them from below. Analogously, any floating body creates the addition of mass
and loading. For example, increased glacial ice or a new lake from a dammed river
will cause subsidence of the crust, whereas removal of mass through deglaciation or
massive erosion of rock will cause isostatic uplift of a mountain mass. If a high plateau
undergoes overall erosion without significant dissection, isostasy will uplift the whole
plateau, but not quite to its original level. If, however, erosion of the plateau carves
many deep canyons and leaves only a few mountain ridges and peaks behind, the entire
regional block will have less mass and will float isostatically upward. Its average sur-
face level, falling somewhere between ridge and peak top and canyon valley bottom, will
rise toward the original plateau surface level, but this will carry the residual mountain
peaks considerably higher than they were before, when they were part of the plateau
(Montgomery 1994). In this fashion, dissection of a high plateau will generate mountain
peaks well above the original plateau surface. This seems to have happened in the east-
ern Himalaya, where deep glacial erosion during the Pleistocene seems to have raised
the peaks above their pre-Pleistocene tectonic levels.