Page 464 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

428 part III The earth–Atmosphere Interface
occurs in a steady-state equilibrium) to a point of abrupt change that takes it to a new system state (as occurs in a dynamic equilibrium)—such as when a flood establishes a new river channel or a hillslope adjusts after a land- slide. Such a threshold can also occur at the precise mo- ment when force overcomes resistance within a system; for example, when slope stability fails and movement en- sues in a downhill direction (as during a landslide). After crossing this threshold, the system establishes a new set of equilibrium relationships. (Please review Figure 1.9 in Chapter 1.)
The dynamic equilibrium model encompasses a se- ries of steps that usually follow a sequence over time. First is equilibrium stability, in which the system fluc- tuates around some average. Next is a destabilizing event, followed by a period of adjustment. Last is the development of a new and different condition of equilib- rium stability. Slow, continuous-change events, such as soil development and erosion, tend to maintain a near- equilibrium condition in the system. Dramatic events such as a major landslide require longer recovery times before equilibrium is reestablished. Figure GIA 14.1 pro- vides an example, in which the failure of saturated slopes caused a landslide that brought sediment and debris into a river and introduced a disequilibrium condition.
slopes
Material loosened by weathering is susceptible to ero- sion and transportation. However, for material to move downslope, the forces of erosion must overcome other forces: friction, inertia (the resistance to movement), and the cohesion of particles to one another (GIA). If the angle is steep enough for gravity to overcome frictional forces, or if the impact of raindrops or moving animals or even wind dislodges material, then erosion of particles and transport downslope can occur.
Slopes, or hillslopes, are curved, inclined surfaces that form the boundaries of landforms. The basic com- ponents of a slope, illustrated in Geosystems in Action, Figure GIA 14, vary with conditions of rock structure and climate. Slopes generally feature an upper wax- ing slope near the top (waxing means increasing). This convex surface curves downward and may grade into a free face, a steep scarp or cliff whose presence indi- cates an outcrop of resistant rock.
Downslope from the free face is a debris slope, which receives rock fragments and materials from above. The condition of a debris slope reflects the local climate. In humid climates, continually moving water carries ma- terial away, lowering the angle of the debris slope. But in arid climates, debris slopes accumulate material. A debris slope grades into a waning slope, a concave sur- face along the base of the slope. You can identify these slope components and conditions on the actual hillslope shown in GIA.
Slopes are open systems and seek an angle of equilib- rium among the forces described here. Conflicting forces
work simultaneously on slopes to establish a compromise incline that balances these forces optimally. When any condition in the balance is altered, all forces on the slope compensate by adjusting to a new dynamic equilibrium.
In summary, the rates of weathering and breakup of slope materials, coupled with the rates of mass movement and material erosion, determine the shape and stability of the slope. A slope is stable if its strength exceeds these denudation processes and unstable if its materials are weaker than these processes. Why are hillslopes shaped in certain ways? How does slope anatomy evolve? How do hillslopes behave during rapid, moderate, or slow uplift? These are topics of active scientific study and research.
CRitiCALthinking 14.1
Find a slope; Apply the Concepts
Locate a slope, possibly near campus, your home, or ex- posed in a local road cut. Using GIA, can you identify the different parts of the hillslope? What forces act on the hillslope, and what is the evidence of their activity? How would you go about assessing the stability of the slope? Do you see evidence of slope instability near your campus or the region in which you are located—perhaps at a con- struction site or other disturbed area? •
Weathering Processes
Weathering is the process that breaks down rock at Earth’s surface and slightly below, either disintegrat- ing rock into mineral particles or dissolving it into water. Weathering weakens surface rock, making the rock more susceptible to the pull of gravity. Weathering processes are both physical (mechanical), such as the wedging action of frost in the cracks of a rock surface, and chemical, such as the dissolution of minerals into water. The interplay of these two broad types of weath- ering is complex; in many cases, the suite of processes combine synergistically to produce unique landforms such as Delicate Arch in Figure 14.1.
On a typical hillside, loose surface material such as gravel, sand, clay, or soil overlies consolidated, or solid, bedrock. In most areas, the upper surface of bedrock undergoes continual weathering, creating broken-up regolith. As regolith continues to weather, or is transported and deposited, the loose surface mate- rial that results becomes the basis for soil development (Figure 14.3). In some areas, regolith may be missing or undeveloped, exposing an outcrop of unweathered bedrock.
As a result of this process, bedrock is known as the parent rock from which weathered regolith and soils develop. Wherever a soil is relatively young, its parent rock is traceable through similarities in composition. For example, in the canyon country of the U.S. South- west, sediments derive their colour and character from the parent rock that is the substance of the cliffs seen