HR: 14:25h
AN: H33F-04    [Abstracts]
TI: Teflon peaks: the evolution of high local relief in glaciated mountain ranges
AU: * Anderson, R S
EM: Robert.S.Anderson@colorado.edu
AF: Department of Geological Sciences and INSTAAR, UCB 450, University of Colorado, Boulder, CO 80309 United States
AB: It is commonly stated that efficient attack of mountains by glaciers amounts to an erosional buzzsaw. High local relief exists in the cores of the world's great mountain ranges. Photographs of high alpine landscapes highlight the rock walls that attract the world's best climbers into these landscapes. I hypothesize a chain of events that produce this high local relief when glaciers erode into massive rock in the range cores. As glaciers consist of a viscous fluid, they occupy valleys. When not frozen to their rock beds, glaciers efficiently erode the landscape over which they slide at rates dictated by the long-term ice discharge. Preferential erosion of valley floors leads to steepening of the valley walls, which promotes avalanching of snow and ice off the walls. That snow and ice remaining on the walls tends to be either thin or frozen to the bed (polar). The efficient glacial erosion mechanism is disallowed on the valley walls because they are too slippery. The snow that avalanches off the walls accumulates on the valley floor, further increasing the ice discharge required to achieve balance. The rock exposed on the walls by avalanching is then subjected to periglacial processes. In massive rock, periglacial processes are inefficient, leaving the rock walls intact while the valley floor is glacially deepened. The only mechanism remaining for erosion of the valley walls is failure by deep-seated landslides. The height of the walls is therefore set by the rock mass strength, which in turn is dictated by the spacing of flaws in the rock. While tectonic processes that increase crustal thickness lead to the pervasive fracturing of rock, syn-orogenic and post-orogenic igneous intrusive rocks are not subjected to this strain history. As the highest peaks of many of the world's ranges consist of igneous intrusive rocks, it is the spacing of flaws in these rocks that is important. New work on intrusive mechanisms suggests that progressive piecewise emplacement of plutons can lead to a punctuated cooling history that allows annealing of incipient cracks, leading to rocks with only widely spaced joints. It is these flaw-free walls that stand tall and steep above the glaciers. The glacial impact on the high alpine landscape is therefore not well described as a buzzsaw, but is better characterized as a non-uniform scooping of the landscape between high hard slippery Teflon peaks.
DE: 0720 Glaciers
DE: 1815 Erosion
DE: 1827 Glaciology (0736, 0776, 1863)
DE: 1859 Rocks: physical properties
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: Fall Meeting 2005