Chapter 14 - Weathering and Erosion
Weathering is the breaking down of rock into smaller particles. Mechanical (also called
physical) weathering does not change the chemical composition of the particles. Examples
include ice wedging and abrasion. Chemical weathering, which does changes the particles’
chemical composition, includes oxidation, hydrolysis, carbonation, and acid precipitation.
Erosion is the movement of weathered material from one place to another.
Rocks weather at different rates due to differences in composition, surface area exposed,
presence of joints or cracks, climate, and topography.
Soil is a mixture of minerals, gases, water, and organic matter. If a soil forms “in place”,
weathering from the parent rock beneath it, it is called residual. If soil is found in a different
place from its parent rock, the soil is referred to as transported.
The texture of the soil is determined by the percentages of clay (less than .004mm diameter),
silt (from .004mm to .06mm), and sand (from .06mm to 2.0mm). Larger sand particles drain
well but hold moisture poorly. Tiny clay particles drain poorly but hold water well. A
combination of the three sizes is a texture called loam.
The side view of the different zones or layers (also called horizons) is a soil profile. Soil profiles
are seen along stream banks, road cuts, and obtaining a cylindrical core using an auger.
Soils can be classified by climatic conditions as tropical, temperate, desert, and arctic. In
Virginia we have a temperate climate and receive more than 65 cm rain per year. This creates a
pedalfer soil which contains clay, quartz, and iron compounds.
To lessen the erosion of valuable topsoil, various conservation practices as used such as
contour plowing, strip-cropping, terracing, and crop rotation.
The movement of particles down a slope is called mass movement. These include rockfalls,
landslides, mudflows, slump, solifluction, and creep.
Landforms produced by erosion include plains, mesas, plateaus, buttes, and talus.