Geology and Soils in Deserts of the Southwestern United States - Design with the Desert: Conservation and Sustainable Development - page 42

can also freeze, expand, and fracture rock. Cycles of wetting and drying can also disag-
gregate rocks that are rich in clay minerals. Water can be taken into the structure of certain
clay minerals, causing them to expand and exert stress on the surrounding rock fabric.
Another mechanism that may be involved in the spalling and reduction of large rock
masses to smaller fragments is that of pressure release. Rocks formed at depth, for example,
a mass of crystallized igneous rock such as granite are subject to confining pressures
from the surrounding rock, but when the granite is uplifted and exposed at the surface
as a hill or cliff face, the confining pressure is removed. The rock mass can now expand
along preexisting fractures, leading to the formation of large sheets or slabs of easily
detached material in a process known as exfoliation . Exfoliation can also occur at the scale
of individual boulders, but this is thought to be due to thermal expansion/contraction and
salt weathering processes.
Chemical weathering , or the chemical alteration of primary rock-forming minerals into
new minerals and chemical substances, is considered less active in deserts due to the
generally low availability of water but is still important in the breakdown of rock into
sediments and soil following precipitation and dew formation.* Water passing over or
through rock acts as a solvent, extracting various elements (Ca, Na, Mg, K, and to a lesser
degree Si, Al, and Fe) from surfaces it contacts and transporting those elements away,
resulting in changes to mineral structures and decreased rock strength. Hydration is
a process where water enters certain mineral structures leading to volume changes as
discussed in the previous section; water can also leave the mineral structure resulting in
further volume changes. Hydrolysis includes numerous chemical reactions between water
and rock-forming minerals, leading to the formation of secondary mineral species such
as clays with weaker crystal structures. Water can also combine with carbon dioxide in
soils (or the atmosphere) to form carbonic acid, which is an effective agent for dissolving
calcium from carbonate rocks like limestone or dolomite under humid conditions. In
semiarid to arid climates, this process is less effective, and limestone tends to be a resistant
and cliff-forming rock type. Oxidation involves the exchange of electrons between oxygen
and metallic elements in rock-forming minerals. This usually results in the formation of an
oxide mineral; a common example is the mineral hematite (iron oxide), which can impart
a reddish coloration to rocks, sediments, and soils.
Biological activity can also contribute to weathering of rock and formation of sediments
and soil. Lichens and algae are common in deserts and can exist both on and beneath the
rock surface. These organisms can create acids capable of etching minerals and can cause
flaking of rock by expansion and contraction of tissues attached to the surface, or in pore
spaces, or lodged in minute cracks. Large desert plants, such as saguaro cactus ( Carnegiea
gigantea ), can disturb and mix the local surface sediments if they topple. Burrowing
animals such as ants, lizards, and mice physically churn and mix the soil surface and
enhance erosion, a process known as bioturbation . Human recreational activities such as
off-road vehicle travel are another example of bioturbation.
Mechanical and chemical weathering, coupled with sediment-transport processes, has
lowered the mountain ranges of the Basin and Range Province and is still transforming
them into the thick sediments present in the adjacent valleys. The valleys that cradle the
Phoenix and Tucson metropolitan regions are filled with sediments derived from the adja-
cent mountain ranges. The following section presents the major mechanisms involved in
moving material from mountain slopes into adjacent valleys and river systems.
* A more detailed description of chemical reactions important to weathering can be found in Chapter 8 of
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