Page 466 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

430 part III The earth–Atmosphere Interface
orientation is especially noticeable in the middle and
higher latitudes.
• Subsurface water. The position of the water table and
water movement within soil and rock structures influ-
ence weathering.
• Vegetation. Although vegetative cover can protect
rock by shielding it from raindrop impact and pro- viding roots to stabilize soil, it also produces organic acids, from the partial decay of organic matter, that contribute to chemical weathering. Moreover, plant roots can enter crevices and mechanically break up a rock, exerting enough pressure to force rock seg- ments apart, thereby exposing greater surface area to other weathering processes (Figure 14.5). You may have observed how tree roots can heave the sections of a sidewalk or driveway sufficiently to raise and crack concrete.
Weathering processes occur on micro- as well as macroscopic scales. In particular, research at microscale levels reveals a more complex relationship between cli- mate and weathering than previously thought. At the small scale of actual reaction sites on the rock surface, both physical and chemical weathering processes can occur over a wide range of climate types. At this scale, soil moisture (hygroscopic water and capillary water) ac- tivates chemical weathering processes even in the driest landscape. (Review types of soil moisture in Figure 9.9.) Similarly, the role of bacteria in weathering is an impor- tant new area of research, with these organisms poten- tially affecting physical processes as they colonize rock surfaces and chemical processes as they metabolize cer- tain minerals and secrete acids.
Keep in mind that, in the complexity of nature, all these factors influencing weathering rates are operat- ing in concert, and that physical and chemical weather- ing processes usually operate together. Time is the final critical factor affecting weathering, for these processes require long periods. Usually, the longer the duration of exposure for a particular surface, the more it will be weathered.
physical Weathering processes
Physical weathering, or mechanical weathering, is the disintegration of rock without any chemical alteration. By breaking up rock, physical weathering produces more surface area on which all weathering may operate. For example, breaking a single stone into eight pieces ex- poses double the surface area susceptible to weathering processes. Physical weathering occurs primarily by frost action, salt-crystal growth, and exfoliation.
Frost Wedging When water freezes, its volume expands as much as 9% (see Chapter 7). This expansion produces a powerful mechanical force that can overcome the ten- sional strength of rock. Repeated freezing (expanding) and thawing (contracting) of water is frost action, or freeze–thaw, which breaks rocks apart in the process of frost wedging (Figure 14.6).
The work of ice begins in small openings along ex- isting joints and fractures, gradually expanding them and cracking or splitting them in varied shapes, de- pending on the rock structure. Sometimes frost wedg- ing results in blocks of rock, or joint-block separation (Figure 14.7).
Frost wedging is an important weathering process in the humid microthermal climates (humid continental and subarctic) and the polar climates, and in the highland climates at high elevations in mountains worldwide. At high latitudes, frost action is important in soils affected
▲Figure 14.6 Physical weathering by frost wedging. Ice expansion involved in freeze–thaw processes broke this marble (a metamorphic rock) apart. [Bobbé Christopherson.]
▲Figure 14.5 Physical weathering by tree roots. [Bobbé Christopherson.]
Animation
Physical Weathering