OVERVIEW

This is the first chapter of Part III. It begins the study of the matter and energy flows that shape the surface of the land. This chapter examines how rock material is broken down by weathering and how weathered material moves downhill under the influence of gravity.

• A variety of weathering processes cause rock to break down into smaller particles at the earth's surface.
• Physical weathering is the disintegration of rock into smaller fragments of the same mineral composition by processes such as frost action, salt-crystal growth and unloading.
• Chemical weathering is the decomposition of rock resulting from mineral alteration processes such as hydrolysis, oxidation and solution.
• Over much of the land surface, the underlying bedrock is covered by a layer of weathered material called regolith.
• Regolith is the source of sediment carried by wind, water and glacial ice, and the parent material for soil development.
• Mass wasting is the spontaneous downhill movement of soil, regolith and rock under the influence of gravity.
• Regolith and soil are more susceptible to mass wasting than bedrock.
• Soil creep is the gradual downhill movement of particles as they are rearranged by wetting and drying, freezing and thawing and other processes.
• Water-saturated regolith can move quickly down a slope in an earthflow.
• Mudflows can develop when intense rains fall on exposed soil surfaces.
• A large mass of bedrock which breaks free from a slope can slide rapidly downhill as a landslide. The rock mass usually disintegrates as it moves.
• Earthflows, mudslides and landslides can be induced both by natural processes and by human activities. They can have a major impact on the environment and present a serious hazard to humans.
• The periglacial system refers to the distinctive landforms and geomorphological processes of arctic and alpine tundra environments which have a strong annual temperature cycle and extremely cold winters.
• Much of the tundra environment has a layer of permafrost (perennially frozen ground) beneath a surface active layer (seasonally thawed ground).
• Ground ice in permafrost can occur as ice wedges and pingos.
• Intense frost action can generate patterned ground features such as ice-wedge polygons and stone polygons.
• Summer thawing of the active layer can produce saturated soils that flow downhill to form solifluction lobes and terraces.

KEY TERMS

weathering         alluvium           periglacial        
mass wasting       soil creep         permafrost         
bedrock            earthflow          ground ice         
chemical           mudflow            ice wedge          
weathering         landslide          patterned ground   
physical           scarification      solifluction       
weathering                                               
sediment                                                 

STUDY QUESTIONS

1. What is weathering? What is the distinction between physical and chemical weathering?

2. What are regolith, bedrock and sediment? How are they related to one another?

3. Sketch and label a cross-section of a hill slope to show the following features: bedrock, outcrop, regolith and soil.

4. What is frost action and how does it break rock into smaller fragments? Give two examples of landforms produced by frost action.

5. What is salt-crystal growth and why, under natural conditions, is it most effective in dry climates?

6. What is an exfoliation dome and how is it produced?

7. Limestone outcrops often show surface forms produced by chemical weathering. What chemical weathering process produces these forms and why is limestone especially vulnerable to this process?

8. Define mass wasting and discuss why regolith is more susceptible than bedrock to this process.

9. Soil creep occurs when a soil particle repeatedly lifts perpendicular to a slope and settles vertically under the influence of gravity. Sketch a diagram to show how this can move a soil particle down a slope. What mechanisms might cause lifting of the soil particle?

10.Compare earthflows and mudflows in terms of their behavior, causes and potential hazard to humans.

11. What are landslides and where do they usually occur?

12. What are some of the most common ways in which human activity causes mass wasting?

13. What is the periglacial system?

14. Sketch a diagram to show how the change of ground temperature with depth varies between summer and winter in permafrost. Label the active layer and permafrost layer on your diagram.

15. Describe the formation of ice-wedge polygons.

16. How do solifluction lobes develop?