Learning Objectives

1. Weathering creates sediment and soil by either mechanical or chemical processes. Erosion is the pick-up and removal of weathering products (transportation).

2. Mechanical weathering causes physical disintegration without compositional change. Frost wedging, frost heaving, and pressure release cause most mechanical weathering. Sheeting and exfoliation domes develop as a result of pressure release. Plant growth, burrowing organisms and salt crystal development are additional mechanical weathering processes. Differential weathering is also a common result of mechanical weathering

3.Chemical weathering causes rock decomposition and new mineral formation that reflects mineral instability because conditions of formation of the original minerals are significantly different from those of the earth's surface. Oxygen and slightly acidic rainwater (carbonic acid) are the agents of chemical weathering. The feldspars and ferromagnesian minerals weather to clay minerals, while quartz does not weather chemically. Calcite dissolves adding calcium and bicarbonate ions to ground water, while some silica is produced by chemical weathering of feldspars. This dissolved load may eventually be carried to the ocean.

4. Soil is the layer of unconsolidated weathered material on top of bedrock. Clay minerals make important contributions in holding water and nutrients on their surfaces because they are negatively charged.

5. Soil horizons develop as it matures. The O horizon is the top layer and consists of plant litter and other organic material. The A horizon is the next layer and is characterized by leaching downward. The B horizon is the zone of accumulation for material leached from the A horizon. The C horizon is transitional from soil to un-weathered bedrock. Most soils are residual, but transported soils can be deposited by ice, wind, and running water. Soils thicken with time.

6. The character of the soil depends on the parent material. Soils forming on granite are sandy, while those forming on basalt are never sandy. Soil types reflect climate. Soils containing large amounts of aluminum and iron oxides are found in wet climates. Soils formed in arid climates are thinner and contain higher concentrations of calcite. Hardpans form in either wet or dry climates and have thick B horizons. Laterites form in tropical regions, have thick A horizons and may be mined for aluminum. The Soil Conservation Service utilizes a detailed classification of soils provided in Table 5.3.