Weathering

Weathering is the chemical decomposition and mechanical disintegration of rock materials. We can think of this as the wearing away of earth materials through time. Different weathering processes have relative degrees of importance in different environments and are partially functions of climate. Many mechanical weathering processes require temperature and moisture fluctuations. These changes exert a mechanical stress on the rock (such as frost wedging), which causes it to disintegrate along joint planes or between individual mineral grains (Fig. 2.1). Chemical weathering, like all chemical processes, is a function of temperature. With the presence of water, at higher temperatures chemical weathering will take place more rapidly.

Thus, we see different types and rates of weathering of particular rock types (such as granite) in low-, middle-, and high-elevation climates of the White-Inyo Range. Weathering rates in the range have been quantified by Denis Marchand (1968, 1970, 1971, and 1974) in his studies of weathering and soil development at Sage Hen Flat and the Cottonwood Basin. He estimated that weathering and accompanying erosion (removal of weathered materials) of 0.3 to 1.2 in (1 to 3 cm) of material occur in 1,000

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Figure 2.1
Weathering of granitic rocks may result in the formation of angular fragments along joint
planes or rounded forms with pressure release. Here a combination of mechanical and
chemical weathering processes has resulted in the formation of a very large weathering
pit (note person lying in it for scale) in Mt. Barcroft Granodiorite below White Mountain
Peak. Felsenmeer surrounds this unusual boulder.

years. In conjunction with this, Valmore LaMarche (1968) has measured weathering and erosion on slopes inhabited by Bristlecone Pine (Pinus longaeva ). Tree roots are exposed through time as material is removed around them. Using dendrochronology (tree-ring dating), LaMarche was able to quantify the amount of material removed for a given unit of time. Elliott-Fisk (1987) has investigated the weathering of glacial till boulders deposited by a series of glaciers over time; her results show that weathering is slow but progressive in the White Mountains. All of these researchers have shown that weathering is a relatively slow process in the dry climate of the White Mountains. Because precipitation (available moisture) increases with altitude (to the crest), weathering may be thought to be more rapid at higher elevations, but it must be remembered that temperature decreases with altitude as well, and frost and ice are weathering agents.

It may prove fruitful to look at the relative degree of soil development as an indication of weathering along an altitudinal gradient through the range. Soils form as the result of weathering and the decomposition of organic matter and are functions of climate, organisms, parent material, relief, and time (Jenny, 1941). Thus, one must try to hold these factors constant to assess the relative importance of climate (or altitude) to soil formation and weathering. It is possible to find the same geologic formation (for example, a particular granitic pluton) at different altitudes in the range. The relief, or slope position, can also be held constant. If we can determine

that the surface of the landscape has not been exposed to disturbance through various erosional or depositional agents, time can also be held constant. However, it has been difficult to locate a sequence of landforms of the same age that span the range's entire elevational gradient. Elliott-Fisk (1987) has studied soils formed on glacial deposits of different ages (i.e., allowing time to vary) and shown that soils do progressively develop through time. It is also difficult to hold the organisms, especially the vegetation, constant as one goes up an altitudinal gradient. Sagebrush (Artemisia spp.) communities span virtually all altitudes of the range, but, as their productivity varies, they do not allow soil development to be evaluated only as a function of climate.