Page 465 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 14 Weathering, Karst Landscapes, and Mass Movement 429
Decaying vegetation
(a) A cross section of a typical hillside.
Soil development
▲Figure 14.3 Regolith and soil. [(b) Robert Christopherson.]
in Figure 14.4a, just as the sediments on Mars derive their characteriztics from weathered parent mate- rial seen in Figure 14.4b. Parent material is the con- solidated or unconsolidated material from which soils develop, ranging from unconsolidated sediments and weathered rock (the fragments in Figure GIA 14.3) to bedrock (the exposed strata in Figure 14.4a). We discuss soils in Chapter 18.
Factors influencing Weathering processes
A number of factors influence weathering processes.
• Rock composition and structure (jointing). The char- acter of the bedrock (hard or soft, soluble or insoluble, broken or unbroken) and its mineral composition (dif- ferent minerals weather at different rates) influence
(a) Reddish-coloured surfaces in the Pilbara region of Western Australia derive their colour from parent materials in the rock layers shown in the background.
(b) A cliff exposes hillside components.
Outcrop
Regolith
Bedrock
the rate of weathering. Joints are fractures or separa- tions in rock that occur without displacement of the rock on either side (in contrast with faulting). Joint- ing increases the surface area of rock exposed to both physical and chemical weathering.
• Climate (precipitation and temperature). Wetter, warmer environments speed up chemical weather- ing processes; colder environments have freeze–thaw cycles that cause physical weathering. Rocks that weather rapidly in warm, humid climates may be re- sistant to weathering in dry climates (an example is limestone).
• Slope orientation. Whether a slope faces north, south, east, or west controls the slope’s exposure to Sun, wind, and precipitation. Slopes facing away from the Sun’s rays tend to be cooler, moister, and more vege- tated than are slopes in direct sunlight. This effect of
(b) Weathered rocks and windblown sand (in approximate true colours) on Mars, looking toward the Columbia Hills, imaged by the Mars Exploration Rover Spirit in 2004.
▲Figure 14.4 Parent materials. [(a) Philip Giles. (b) Mars image courtesy of NASA.]