Unit 38


Unit Overview

This unit explores several major types of weathering processes. The main sections are as follows:

  • Chemical weathering
  • Physical weathering
  • Biological weathering
  • Geography of weathering

Weathering is the first stage of the degradation process. Mechanical, or physical, weathering involves the destruction of rocks through the imposition of certain stresses; this type of weathering is controlled primarily by frost and temperature fluctuations. Chemical weathering involves the dissolution of a rock¿s minerals. The three types of chemical weathering are hydrolysis, oxidation, and carbonation. Biological weathering involves mostly the forcing open of bedding planes and joints by plant roots. The most intense weathering can be found in equatorial and tropical areas, where chemical and biological weatherings prevail.

Unit Objectives

  • To differentiate the major categories of weathering - mechanical, chemical, and biological
  • To introduce and briefly discuss common weathering processes
  • To note the general environmental controls over weathering processes

Glossary of Key Terms

Biological weathering The disintegration of rock minerals via biological means; earthworms and plant roots are important in the development of soil, lichens contribute to the breakdown of rocks, and humans, of course, play various roles in the disintegration of rocks and the operation of soil-formation processes.
Carbonation The reaction of weak carbonic acid (formed from water and carbon dioxide) with minerals; carbonic acid, in turn, reacts with carbonate minerals such as limestone in a form of chemical weathering that can be quite vigorous in certain humid areas, where solution and decay lead to the formation of karst landscapes.
Chemical weathering The disintegration of rock minerals via chemical means; in any rock made up of a combination of minerals, the chemical breakdown of one set of mineral grains leads to the decomposition of the whole mass.
Frost action A form of mechanical weathering in which water penetrates the joints and cracks of rocks, expands and contracts through alternate freezing and thawing, and eventually shatters the rocks.
Hydrolysis A form of chemical weathering that involves moistening and the transformation of rock minerals into other mineral compounds; expansion in volume often occurs in the process, which contributes to the breakdown of rocks.
Mechanical weathering Involves the destruction of rocks by physical means through the imposition of certain stresses, such as freezing and thawing or the expansion of salt crystals; also known as physical weathering.
Oxidation The chemical combination of oxygen and other materials to create new products (biologists call this process respiration).
Rock sea The area of blocky rock fragments formed when weathered rocks particularly from frost wedging - remain near their original location; also known as a blockjield orfelsenmeer, and a boulder field when the rock fragments are dominated by large boulders.
Spheroidal weathering A product of the chemical weathering process of hydrolysis; in certain igneous rocks such as granite, hydrolysis combines with other processes to cause the outer shells of the rock to flake off in what looks like a small- scale version of exfoliation.
Talus cone A steep accumulation of weathered rock fragments and loose boulders that rolled downslope in free fall; particularly common at the bases of cliffs in the drier climates of the western United States (also called a scree slope).

Unit Outline

  • Physical weathering
    • Destruction of rocks through stress
      • frost action
    • Rock seas are produced when large pieces of rock accumulate near their origin
      • also known as a blockfield or felsenmeer
    • Talus cones form when small rock fragments roll downslope
      • also known as scree slope
    • Stone nets are created when ice forms under rocks and wedges them together
    • Salt wedging in arid regions
  • Chemical weathering
    • Rocks vary in their ability to resist weathering
    • Hydrolysis occurs when minerals are moistened, changed chemically, and expand in volume
      • spheroidal weathering is hydrolysis and other processes combining to flake off outer layers of rock
    • Oxidation
      • minerals react with oxygen in the air, producing iron and aluminum
    • Carbonation
      • water is converted to a mild acid, which is the weathering agent
        • carbonic acid
  • Biological weathering
    • Soil mixed by burrowing animals and worms
    • Action of lichens (a combination of algae and fungi)
    • Action of humans
      • pollution
      • quarrying, mining
      • farming and fertilization
  • Geography of weathering
    • More intense weathering in lower latitudes with higher heat, rainfall, and humidity
    • Stone nets only form under certain conditions, particularly in Arctic areas
    • Wedging effect of frost action only in higher latitudes

Review Questions

  1. List some of the factors that make weathering processes more intense in the lower latitudes.
  2. What is a talus cone? A rock sea? A salt wedge?
  3. In what ways do humans contribute to the process of biological weathering?