Agronomy and Horticulture Department

 

Title

Soil Genesis and Development, Lesson 2 - Processes of Weathering

Date of this Version

2019

Document Type

Article

Citation

Plant and Soil Sciences eLibrary (PASSeL) Lesson

Comments

Copyright © 2019 Plant and Soil Sciences eLibrary. Used by permission.

JNRLSE approved 2009

This lesson was developed by Martha Mamo, Timothy Kettler, and Dennis McCallister at the University of Nebraska–Lincoln; Jim Ippolito, Research Soil Scientist, USDA-ARS-NWISRL, Kimberly, Idaho, formerly at Colorado State University; Ron Reuter at Oregon State University; Christoph Geiss at Trinity College–Connecticut; and William Zanner at the University of Minnesota. Development of this lesson was supported by the National Science Foundation Course, Curriculum, and Laboratory Improvement Program (NSF-CCLI), Award Number DUE-0042603. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of NSF.

Abstract

This lesson identifies the factors of weathering processes and how they influence soil formation.

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, Algeria. 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.]

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