SURFACE PROCESSES

15.5. SURFACE PROCESSES

Surface geological features are formed by upward movement of materials from earth’s crust. With exposure to water, oxygen, freeze-thaw cycles, organisms, and other influences on the surface, surface features are subject to two processes that largely determine the landscape—weathering and erosion. As noted earlier in this chapter, weathering consists of the physical and chemical breakdown of rock and erosion is the removal and movement of weathered products by the action of wind, liquid water, and ice. Weathering and erosion work together in that one augments the other in breaking down rock and moving the products. Weathered products removed by erosion are eventually deposited as sediments and may undergo diagenesis and lithification to form sedimentary rocks.

One of the most common surface processes that can adversely affect humans consists of landslides that occur when soil or other unconsolidated materials slide down a slope. 6 Related phenomena include rockfalls, mudflows, and snow ava- lanches. As shown in Figure 15.5 , a landslide typically consists of an upper slump that is prevented from sliding farther by a mass of material accumulated in a lower flow. Figure 15.5 illustrates what commonly happens in a landslide when a mass of earth moves along a slip plane under the influence of gravity. The stability of earthen material on a slope depends upon a balance between the mass of slope material and the resisting force of the shear strength of the slope material. There is a tendency for the earth to move along slip planes. In addition to the earthen material itself, water, vegetation, and structures constructed by humans may increase the driving force leading to a landslide. The shear strength is, of course, a function of the geological material along the slip plane and may be affected by other factors as well, such as the presence of various levels of water and the degree and kinds of vegetation growing on the surface.

The tendency of landslides to form is influenced by a number of outside factors. Climate is important because it influences the accumulation of water that often pre- cedes a landslide as well as the presence of plants that can also influence soil stabil- The tendency of landslides to form is influenced by a number of outside factors. Climate is important because it influences the accumulation of water that often pre- cedes a landslide as well as the presence of plants that can also influence soil stabil-

Contour of surface prior to slide

Upper slump

Lower flow

Figure 15.5. A landslide occurs when earth moves along a slip plane. Typically, a landslide consists of an upper slump and lower flow. The latter serves to stabilize the slide, and when it is disturbed, such as by cutting through it to construct a road, the earth may slide farther.

As discussed in Section 15.13, landslides can be very dangerous to human life and their costs in property damage can be enormous. In addition to destroying structures located on the surface of sliding land or covering structures or people with earth, landslides can have catastrophic indirect effects. For example, landslides that dump huge quantities of earth into reservoirs can raise water levels almost instantaneously and cause devastating waves and floods.

Subsidence occurs when the surface level of earth sinks over a significant area. The most spectacular evidence of subsidence is manifested as large sinkholes that may form rather suddenly, sometimes swallowing trees, automobiles, and even whole buildings in the process. Overall, much more damage is caused by gradual and less extreme subsidence, which may damage structures as it occurs or result in inundation of areas near water level. Such subsidence is frequently caused by the removal of fluids, such as petroleum, from below ground.