Chapter 5
Weathering and Soils
Weathering and Soils begins with a brief examination of the external processes of weathering, mass wasting,
and erosion. The two forms of weathering, mechanical and chemical, are investigated in detail, including the
types, conditions, rates, and net effect of each. The soils section of the chapter begins with a description of the
general composition and structure of soil. After examining the factors that influence soil formation,
development, and classification, soil erosion, as well as some ore deposits produced by weathering, are
presented.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
• Explain the difference between the Earth’s external and internal processes.
• Discuss the various processes involved in physical weathering.
• Discuss the various processes involved in chemical weathering.
• List and briefly explain those factors which control rates of weathering.
• Define soil and explain the difference between soil and regolith.
• Discuss soil formation, including those factors that control soil formation.
• Explain the concept of the soil profile and how it relates to soil formation.
• Discuss the characteristics of the soil horizons in a typical soil profile.
• Briefly explain soil classification.
• Briefly discuss soil erosion.
• Relate the process of weathering to the formation of some ore deposits.
Chapter Outline___________________________________________________________________
I. EARTH’S EXTERNAL PROCESSES
A. Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or
near Earth’s surface
B. Mass wasting – the transfer of rock and soil downslope under the influence of gravity
C. Erosion – the physical removal of material by mobile agents such as water, wind, or ice
II. WEATHERING
A. Two types of weathering
1. Mechanical weathering
a. Breaking of rocks into smaller pieces
b. Four processes
1. Frost wedging
2. Unloading
3. Thermal expansion
4. Biological activity
2. Chemical weathering
a. Breaks down rock components and internal structures of minerals
b. Most important agent is water
c. Major processes
1. Dissolution
a. Aided by small amount of acid in the water
b. Soluble ions are retained in the underground water supply
2. Oxidation
a. Any chemical reaction in which a compound or radical loses electrons
b. Important in decomposing ferromagnesian minerals
3. Hydrolysis
a. The reaction of any substance with water
b. Hydrogen ion attacks and replaces other positive ions
d. Alterations caused by chemical weathering
1. Decomposition of unstable minerals
2. Generation or retention of materials that are stable
3. Physical changes such as the rounding of corners or edges
B. Rates of weathering
1. Advanced mechanical weathering aids chemical weathering by increasing the surface area
2. Other important factors include
a. Rock characteristics
1. Marble (calcite) readily dissolves in weakly acidic solutions
2. Silicate minerals weather in the same order as their order of crystallization
b. Climate
1. Temperature and moisture are the most crucial factors
2. Chemical weathering is most effective in areas of warm temperatures and abundant
moisture
3. Differential weathering
a. Masses of rock do not weather uniformly
b. Creates many unusual and spectacular rock formations and landforms
III. SOIL
A. A combination of mineral and organic matter, water, and air – that portion of the regolith (rock and mineral
fragments produced by weathering) that supports the growth of plants
B. Controls of soil formation
1. Parent material
a. Residual soil – parent material is the bedrock
b. Transported soil – forms in place on parent material that has been carried from elsewhere
and deposited
2. Time
a. Important in all geologic processes
b. Amount of time to evolve varies for different soils
3. Climate
a. Most influential control of soil formation
b. Temperature and precipitation are the elements that exert the strongest impact on soil
formation
4. Plants and animals
a. Organisms influence the soil’s physical and chemical properties
b. Furnish organic matter to the soil
5. Slope
a. Steep slopes often have poorly developed soils
b. Optimum terrain is a flat-to undulating upland surface
c. Slope orientation is also important
C. The soil profile
1. Soil-forming processes operate from the surface downward
2. Vertical differences are called horizons – zones or layers of soil
a. Horizons in temperate regions
1. O – organic matter
2. A – organic and mineral matter
a. High biological activity
b. Together the O and A horizons make up the topsoil
3. E – little organic matter
a. Eluviation
b. Leaching
4. B – zone of accumulation
5. C – partially altered parent material
b. O, A, E, and B together called the solum, or “true soil”
D. Classifying soils
1. Variations in soil formation over time and distances has led to a great variety of recognized soil
types
2. Groups have been established using common characteristics to facilitate analysis and better
understanding
3. In the United States a system is used called the soil taxonomy
a. Emphasis is placed on physical and chemical properties of the soil profile
b. Six hierarchical categories ranging from order, the broadest category, to series, the most
specific
c. Names are descriptive, using syllables derived from Latin or Greek
d. 12 basic soil orders are recognized in the soil taxonomy
E. Soil erosion
1. Recycling of Earth materials
2. Natural rates of soil erosion depend on
a. Soil characteristics
b. Climate
c. Slope
d. Type of vegetation
3. In many regions, the rate of soil erosion is significantly greater than the rate of soil formation
4. Sedimentation and chemical pollution
a. Related to excessive soil erosion
b. Occasionally soil particles are contaminated with pesticides
IV. WEATHERING AND ORE DEPOSITS
A. Secondary enrichment – concentrating metals into economically valuable concentrations
1. By downward-percolating water removing undesirable materials
2. By carrying desirable elements to lower zones and concentrating them
B. Bauxite
1. Principal ore of aluminum
2. Forms in rainy tropical climates from chemical weathering and the removal of undesirable elements
by leaching
C. Other deposits, such as many copper and silver deposits, result when weathering concentrates metals that
are deposited through a low-grade primary ore