Chapter 8. Weathering, Sediment, and Soil

Introduction

Learning Objectives

After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

  • Explain why rocks formed at depth in the crust are susceptible to weathering at the surface
  • Describe the main processes of mechanical weathering, and the types of materials that are produced when mechanical weathering predominates
  • Describe the main processes of chemical weathering, and common chemical weathering products
  • Explain the characteristics used to describe sediments, and what those characteristics can tell us about the origins of the sediments.
  • Discuss the relationships between weathering and soil formation, and the origins of soil horizons and some of the different types of soil
  • Describe and explain the distribution of some of the important soil types in Canada
  • Explain how changing rates of weathering affect the carbon cycle and can lead to climate change
The Hoodoos, near Drumheller, Alberta, have formed from the differential weathering of sedimentary rock that was buried beneath other rock for close to 100 Ma [SE photo]

Figure 8.1 The Hoodoos, near Drumheller, Alberta, have formed from the differential weathering (weaker rock weathering faster than stronger rock) of sedimentary rock that was buried beneath other rock for close to 100 Ma [Steven Earle CC-BY 4.0]

What Is Weathering?

Weathering occurs when rock is exposed to the “weather” — to the forces and conditions that exist at Earth’s surface. Except for volcanic rocks and some sedimentary rocks, most rocks form deep within the crust. There they experience relatively constant temperature, high pressure, no contact with the atmosphere, and little or no moving water. Once a rock is exposed at the surface, which is what happens when the overlying rock is eroded away, conditions change dramatically. Temperatures vary widely, pressure is much lower, oxygen and other gases are plentiful, and in most climates, water is abundant.

Weathering includes two main processes that are quite different. Mechanical weathering refers to the physical breaking of rock into smaller pieces. Chemical weathering refers to chemical reactions that change minerals into forms that are stable[1] in the surface environment. Mechanical weathering provides fresh surfaces for attack by chemical processes. Chemical weathering weakens the rock so that it is more susceptible to mechanical weathering. Together, these processes create the particles and ions that can eventually become sedimentary rock.  They also create soil, which is necessary for our existence on Earth.

 


  1. Stable means that they won't undergo further chemical weathering.

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