Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 236

as abundant water to enhance solution (Sharp et al. 1995). Silica fluxes and mean
silica concentrations increase with water discharge in glacial catchments. As erosion
increases, mineral surface area increases. With abundant meltwater, chemical weath-
ering becomes more common (Anderson 2005). Anderson et al. (1997) could not dif-
ferentiate between rates of chemical weathering in glacial and non-glacial catchments.
In the granitic and metamorphic mountains of the southern and central Rockies, rates
of chemical denudation are generally low, equivalent to 1 mm ka −1 (Caine 2001). In
Europe, rates of solute denudation are only about 2 mm ka −1 (Walling and Webb 1986).
Given the close correspondence with mechanical denudation, chemical weathering is
likely more significant than previously recognized. Periglacial chemical weathering is
generally slower compared to tropical areas; however, chemical weathering is often a
dominant agent of mass removal (Dixon and Thorn 2005). These chemical weathering
processes likely play an important role in mountains, though they are often overlooked
(Thorn et al. 2006).
Rock surfaces often present evidence of chemical weathering in their altered and
weakened exteriors. Chemical weathering may also be ascertained by measuring the
quantity of dissolved minerals being transported by streams. The presence of a weath-
ering rind, the chemically altered zone at the rock surface, can provide an estimate of
the amount of chemical weathering. The depth of weathering provides an index of the
age of the surface and the rate of the process under varying conditions. Although it pen-
etrates only a few millimeters, the weathered zone can easily be identified when the
rock is broken apart. The weathering rind commonly has a slightly reddish-brown color
because of the oxidation of iron and manganese silicates. The exterior is strengthened
at the expense of the interior, so that once the surface is breached, the underlying rock
deteriorates at a rate faster than the protective covering.
Much of the effect of chemical weathering is not immediately visible, since it involves
the removal of minerals through solution. When precipitation infiltrates rock, the water
soon becomes loaded with dissolved salts that are subsequently transported away. A
classic example of this is the solubility of limestone, which has created extensive cav-
erns in various parts of the world. This process is called hydrolysis; it involves not
merely absorption of water, as in a sponge (hydration), but a specific chemical change
that produces a new mineral. For example, the process begins when water absorbs car-
bon dioxide (CO 2 ) to form a weak solution of carbonic acid (H 2 CO 3 ). This then reacts
with the calcium carbonate (CaCO 3 ) in limestone and produces a soluble salt, calci-
um bicarbonate, Ca(HCO 3 ) 2 . Calcium bicarbonate is easily dissolved and transported
in solution. Other minerals are less susceptible to chemical reactions than the calcite
in limestone, but all undergo some reaction with water. Consequently, the quantity of
dissolved solids in a mountain stream provides a good index of the rate of chemical
weathering. However, some care must be taken to account for the quantity of particu-
late matter dissolved from the atmosphere and from the decomposition of organic ma-
terial. Mountains located in marine environments and those containing highly soluble
rocks display the most rapid rates of dissolution. In the limestone regions of the Alps,
the solution rate is about 0.1 mm/yr (Caine 1974). In dolomite (limestone containing
magnesium) areas of the White Mountains in eastern California, the rate is only about
0.02 mm/yr, since the climate is much drier and the dolomite is less soluble (Marchand
1971).