Page 482 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

446 part III The earth–Atmosphere Interface
(b) Note the subsurface effects on rock strata caused by surface creep.
Downslope breaking of rock outcrop
▲Figure 14.28 Soil creep and its effects. [(b) Bobbé Christopherson.]
In the freeze–thaw cycle, particles are lifted at right angles to the slope by freezing soil moisture, as shown in Figure 14.28. When the ice melts, however, the parti- cles fall straight downward in response to gravity. As the process repeats, the surface soil gradually creeps its way downslope.
The overall wasting of a creeping slope may cover a wide area and may cause fence posts, utility poles, and even trees to lean downslope. Various strategies are used to arrest the mass movement of slope material—grading the terrain, building terraces and retaining walls, plant- ing ground cover—but the persistence of creep often ren- ders these strategies ineffective.
In polar regions and at high elevations, freeze–thaw processes are critical for mass wasting. During the sum- mer when the upper layers of soil thaw and become saturated, slow downslope movement occurs, called solifluction (shown in Figure 14.24 and discussed with periglacial environments in Chapter 17).
Humans as a Geomorphic Agent
Any human disturbance of a slope—highway road cut- ting, surface mining, or construction of a shopping mall, housing development, or home—can hasten mass wasting. Large open-pit surface mines—such as the Bingham Canyon Copper Mine west of Salt Lake City, Utah, and the Kalgoorlie Super Pit in Western Australia
(Figure 14.29)—are examples of human impacts that move sediment, soil, and rock material, a process known as scarification. Extraction of oil in the Athabasca oil sands region of Alberta has so far been done mostly by open pit mining methods (see Geosystems Now at the be- ginning of this chapter).
At the Bingham Canyon Copper Mine, a mountain literally was removed since mining began in 1906, form- ing a pit 4 km wide and 1.2 km deep. In April 2013, a large landslide occurred within the open pit on an un- stable slope that was being closely monitored for safety reasons (Figure 14.29a). No fatalities occurred.
The disposal of tailings (mined ore of little value) and waste material is a significant problem at any mine, also discussed in Geosystems Now 14 at the beginning of this chapter. Such large excavations produce tailing piles that are unstable and susceptible to further weathering, mass wasting, or wind dispersal. Additionally, the leach- ing of toxic materials from tailings and waste piles poses an ever-increasing problem for streams, aquifers, and public health.
Where underground mining is common, land sub- sidence and collapse may produce mass movement on hillslopes. Homes, highways, streams, wells, and prop- erty values are severely affected. A controversial form of mining called mountaintop removal is done by re- moving ridges and summits and dumping the debris into stream valleys, thereby exposing the coal seams
Expansion with freezing—frost heaving
Subsidence as frost melts
(a) Typical soil-creep features.
Soil creep moving turf
Gravity
Broken retaining wall
Leaning poles
Curving growth pattern
Leaning fence
Regolith
Expanded surface