Outline
Weathering is rock decomposition (or purification) and has both a physical and chemical aspect.
It is the geologic process most closely linked to the health and welfare of humans.
We depend on weathering for food (soil & nutrients), energy (fossil and nuclear) and Earth materials (Fe, Al, etc.)
The type and rate of weathering is influenced by:
a) climate,
b) nature of the rock (mineralogy and structure),
c) topography,
d) vegetation, and
e) time.
Because rocks exhibit varying resistance to weathering, even hand specimens may display differential weathering.
Both mechanical and chemical weathering usually operate together, however, depending upon climate, one usually dominates.
Mechanical weathering is enhanced by dry, cold climates and chemical weathering is enhanced by humid, warm climates.
Mechanical (= Physical) Weathering leads to the development of greater surface area.
Examples: a) abrasion, b) frost wedging, c) organisms (plant roots and burrowing animals, d) unloading (produces sheeting).
Chemical Weathering or dissolution is aided by high surface area and is normally the dominant weathering process in most environments.
-H20 is a good solvent and is aided by H2C03, HN03 and H2/s04
-As the time and distance of transport increases, pH increases, the concentration of dissolved material increases and the oxidation potential decreases resulting in a tremendous regularity during natural separation and concentration of chemical elements.
Elements concentrated according to:
a) Resistance to dissolution or abrasion (Si, Ti, Au, Pt)
b) hydrolysis (Al, Fe, Si, K)
c) oxidation (Fe, Mn, S)
d) precipitation (Ca, Mg)
e) extreme evaporation and precipitation (Na, K, Ca, Mg)
Chemical Weathering results in
a) finer grain size,
b) increase in bulk (volume),
c) lower density material and
d) more stable minerals.

Soil is the most significant result of rock weathering
-good soil consists of 25% air; 25% H20; 5% organic matter and 45% mineral matter.
-consists of 1 or more layers parallel to Earth's surface.

Soil Profile:
A horizon - zone of leaching; humus layer on top
B horizon - zone of accumulation
C horizon - contains weathered bedrock

Soil Types:
Pedocals - arid environments; contains smectite
Pedalfers - humid temperate environments; contains illite
Laterites - tropical environments; contains kaolinite

In the U.S. there is > 4 billion ton of soil eroded per year (= 4.8 tons acre-1); soil is mined because it is disappearing at a faster rate than its being produced.
-soil erosion may produce significant stream and lake pollution.

Review Questions Weathering & Soil Formation
1. Explain the roles of rock type, rock structure, slope, climate, and time in the weathering process.

2. Describe mechanical and chemical weathering and cite one example of each. Briefly explain the interrelationship between mechanical and chemical weathering. Why is chemical weathering more rapid in warmer and wetter climates than in cooler and drier climates?

3. Explain how sheeting (or exfoliation) develops in granitic plutons. How do joints affect rock weathering? What is frost wedging and why is it most effective at temperatures between 5o and -15oC?

4. Rank the following rocks in order of their rapidity of weathering in a humid climate: granite, quartz, sandstone, limestone and bedded salt (halite). Briefly explain the reasons for your ranking.

5. Draw a completely labeled diagram of a well-developed soil profile and point out where material is leached as well as where it is accumulated.

6. Briefly explain the relative influence of parent material and climate in the development of a soil.

7. How do pedalfer, pedocal and laterite soils differ? What climatic conditions are conducive to the formation of each of these soil types?

8. Explain how chemical weathering can result in the concentration of chemical elements far higher than their average concentration in crustal rocks (Hint: Consider the roles of hydrolysis, oxidation, precipitation and extreme evaporation).