Page 468 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

432 part III The earth–Atmosphere Interface
(a) Ancient cliff dwelling in a niche formed partially by
salt weathering, Canyon de Chelly, Arizona. The dark streaks on the cliff are thin coatings of desert varnish, composed of manganese that is taken up and metabolized by microbes and transformed into oxide minerals.
▲Figure 14.9 Physical weathering in sandstone. [Bobbé Christopherson.]
built entire villages in these weathered niches, as in Mesa Verde in Colorado and Arizona’s Canyon de Chelly (pro- nounced “canyon duh shay”; Figure 14.9).
Exfoliation The process whereby rock peels or slips off in sheets instead of breaking up into grains is exfoliation, a term that generally refers to the removal or shedding of an outer layer. This process is also known as sheeting. Exfoliation creates arch-shaped and dome-shaped fea- tures on the exposed landscape. These exfoliation domes are probably the largest weathering features, in areal ex- tent, on Earth (Figure 14.10).
Exfoliation is thought to occur as pressure is released from the removal of overlying rock. Recall from Chapter 12 how magma that rises into the crust and then remains deeply buried under high pressure forms intrusive ig- neous rocks called plutons. These plutons cool slowly into coarse-grained, crystalline, granitic rocks that may
Chemical Weathering processes
Chemical weathering refers to the chemical breakdown, always in the presence of water, of the constituent miner- als in rock. The chemical decomposition and decay be- come more intense as both temperature and precipitation increase. Although individual minerals vary in suscepti- bility, all rock-forming minerals are responsive to some degree of chemical weathering.
A familiar example of chemical weathering is the eating away of cathedral façades and the etching of tombstones by acid precipitation. In Europe, where in- creasingly acidic rains resulted from the burning of coal, chemical weathering processes are visible on many buildings. An example is Saint Magnus Cathe- dral, in the Orkney Islands north of Scotland, which was built of red and yellow sandstone beginning around a.d. 1137. Almost nine centuries of chemical weathering have dissolved cementing materials in the sandstone, breaking the rock down and making the building’s in- tricate decorative sculpture look “melted” and “out-of- focus” (Figure 14.11).
Spheroidal weathering is chemical weathering that softens and rounds the sharp edges and corners of jointed rock (thus the name spheroidal) as water penetrates the joints and dissolves weaker minerals or cementing ma- terials (Figure 14.12). A boulder can be attacked from all sides by such weathering, shedding spherical shells of decayed rock, like the layers of an onion. Spheroidal weathering of rock resembles exfoliation, but it does not result from pressure-release jointing.
Sandstone
Shale
Niche
(cliff dwellings built in niche)
mechanical weath- ering that separates the joints into layers resembling curved slabs or plates, often thinner at the top of the rock struc- ture and thicker at the sides. Recent re- search suggests that exfoliation may also result from the force of gravity working a curved surface, creating tension be- neath a dome, that augments pressure- release processes.
then be uplifted and subjected to weathering and ero- sion. As the tremendous weight of overlying material is removed from a granite pluton, the pressure of deep burial is relieved. Over millions of years, the granite slowly responds with an enormous physical heave, ini- tiating a process known as pressure-release jointing, in which the rock cracks into joints. Exfoliation is the
(b) Water and an impervious rock layer helped concentrate weathering processes in a niche in the overlying sandstone.