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Soil Genesis and Development, Lesson 2 - Processes of Weathering

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2.1 - Processes of Weathering - Introduction

Have you ever considered how rock becomes soil?  What would happen to human civilization, food and fiber production if the processes we call weathering ceased to occur?  The processes of weathering are critical to soil formation.Goal: Students will understand the weathering process and its influence on soil formation.

Objectives

  1. Describe how climatic factors influence the weathering of rocks and minerals.
  2. Define and distinguish physical, chemical, and biological weathering processes.

How do rocks become soil?  How does the climate in places such as Peru, Alaska, and Algeria influence the weathering of rock?  These questions are critical in understanding the roles weathering processes and climate play in the disintegration of rocks and minerals to begin the process of soil formation.

 
Figure 1.  Long-term monthly temperature and precipitation in Iquitos, Peru; Barrow, Alaska; and Tondoof Algeriia.  Image courtesy of http://www.uwsp.edu/geo/faculty/ritter/interactive_climate_map/climate_map.html

Weathering is the process of disintegration of rock from physical, chemical, and biological stresses. Weathering is influenced by temperature and moisture (climate). As rock disintegrates, it becomes more susceptible to further physical, chemical, and biological weathering due to the increase in exposed surface area. During weathering, minerals that were once bound in the rock structure are released.


Figure 2. The influence of the interaction of temperature and rainfall on processes of physical and chemical weathering. Notice that as annual rainfall and temperature increase, chemical weathering dominates over physical weathering. On the contrary, notice that as the temperature lowers, physical weathering begins to dominate over chemical weathering. Image courtesy of UNL, 2005

 

The degree of weathering that occurs depends upon the resistance to weathering of the minerals in the rock, as well as the degree of the physical, chemical, and biological stresses. A rule of thumb is that minerals in rocks that are formed under high temperature and pressure tend to be less resistant to weathering, while minerals formed at low temperature and pressure are more resistant to weathering. Weathering is usually confined to the top few meters of geologic material, because physical, chemical, and biological stresses generally decrease with depth. Weathering of rocks occurs in place, but the disintegrated weathering products can be carried by water, wind, or gravity to another location (i.e., erosion or mass wasting).

 

Figure 3.  A conceptual diagram showing how weathering breaks down rocks and minerals; eventually, soil formation begins in place. Erosion or mass wasting involves removal of particles offsite. Note: One, two or all three processes can be present to weather rock depending on the environment. Image courtesy of UNL, 2005

For more information about rock types, go to: Principles Lesson 1 - Rocks, Minerals, and Soils

For more information about soil genesis and development, go to:  Principles Lesson 3 - Soil Forming Factors and Principles Lesson 4 - Soil Profile Development

 

 

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