Page 486 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

450 part III The earth–Atmosphere Interface
mass movement and erosion of those materials. Slopes that form the boundaries of landforms have several general com- ponents: waxing slope, free face, debris slope, and waning slope. Slopes seek an angle of equilibrium among the operat- ing forces.
denudation (p. 424)
differential weathering (p. 424) dynamic equilibrium model (p. 425) geomorphic threshold (p. 425)
slope (p. 428)
1. Define landmass denudation. What processes are included in the concept?
2. What is the interplay between the resistance of rock structures and differential weathering?
3. Describe what is at work to produce the landform in Figure 14.1.
4. What are the principal considerations in the dynamic equilibrium model?
5. Describe conditions on a hillslope that is right at the geomorphic threshold. What factors might push the slope beyond this point?
6. Givenalltheinteractingvariables,doyouthink
a landscape ever reaches a stable, old-age condition as originally speculated? Explain.
7. Whatarethegeneralcomponentsofanidealizedslope?
8. Relative to slopes, what is meant by an angle of equilibrium? Can you apply this concept to the
photograph in Figure GIA 14.3?
■ Define weathering, and explain the importance of parent rock and joints and fractures in rock.
Weathering processes disintegrate both surface and sub- surface rock into mineral particles or dissolve them in water. On a typical hillside, loose surface material over- lies consolidated, or solid, rock called bedrock. In most areas, the upper surface of bedrock undergoes continual weathering, creating broken-up rock called regolith. The unconsolidated, fragmented material that is carried across landscapes by erosion, transportation, and deposi- tion is sediment, which along with weathered rock forms the parent material from which soil evolves.
Important in weathering processes are joints, the frac- tures and separations in the rock. Jointing opens up rock surfaces on which weathering processes operate. Factors that influence weathering include the character of the bed- rock (hard or soft, soluble or insoluble, broken or unbroken), climatic elements (temperature, precipitation, freeze–thaw cycles), position of the water table, slope orientation, surface vegetation and its subsurface roots, and time.
weathering (p. 428) bedrock (p. 428) regolith (p. 428)
parent material (p. 429) joint (p. 429)
9. Describe weathering processes operating on an open expanse of bedrock. How does regolith develop? How is sediment derived?
10. Describe the relationship between climate and weathering at microscale levels.
11. What is the relationship between parent rock, parent material, regolith, and soil?
12. What role do joints play in the weathering process? Give an example from this chapter.
■
Describe the physical weathering processes of frost ac- tion, salt-crystal growth, and pressure-release jointing.
Physical weathering, or mechanical weathering, refers to the breakup of rock into smaller pieces with no alteration of mineral identity. The physical action of water when it freezes (expands) and thaws (contracts) causes rock to break apart in the process of frost wedging. Working in joints, ex- panded ice can produce joint-block separation through this process. Another physical weathering process is salt-crystal growth (salt weathering); as crystals in rock grow and en- large over time by crystallization, they force apart mineral grains and break up rock.
Removal of overburden from a granitic batholith re- lieves the pressure of deep burial, producing joints. Exfo- liation, or sheeting, occurs as mechanical forces enlarge the joints, separating the rock into layers of curved slabs or plates (rather than granular disintegration that occurs with many weathering processes). The resulting arch- shaped or dome-shaped feature is an exfoliation dome.
physical weathering (p. 430) frost wedging (p. 430) exfoliation (p. 432)
13. What is physical weathering? Give an example.
14. Why is freezing water such an effective physical
weathering agent?
15. What weathering processes produce a granite dome?
Describe the sequence of events.
■ Explain the chemical weathering processes of hydra- tion, hydrolysis, oxidation, carbonation, and dissolution.
Chemical weathering is the chemical decomposition of minerals in rock. It can cause spheroidal weathering, in which chemical weathering that occurs in cracks in the rock removes cementing and binding materials, so that the sharp edges and corners of rock disintegrate and become rounded.
Hydration occurs when a mineral absorbs water and expands, thus changing the mineral structure. This process also creates a strong mechanical (physical weathering) force that stresses rocks. Hydrolysis breaks down silicate miner- als in rock through reaction with water, as in the chemical weathering of feldspar into clays and silica. Oxidation is a chemical weathering process in which oxygen reacts with certain metallic elements, the most familiar example being the rusting of iron to produce iron oxide. The dissolution of materials into solution is also considered chemical weath- ering. An important type of dissolution is carbonation, resulting when carbonic acid in rainwater reacts to break down certain minerals, such as those containing calcium, magnesium, potassium, or sodium.
chemical weathering (p. 432) spheroidal weathering (p. 432) hydration (p. 433)
hydrolysis (p. 434)
oxidation (p. 434) carbonation (p. 434)
16. What is chemical weathering? Contrast this set of processes to physical weathering.