What is weathering and how is it controlled? Rocks are broken down at Earth’s surface by chemical weathering—the chemical alteration or dissolution of minerals—and by physical weathering—the fragmentation of rocks by mechanical processes. Erosion dislodges the products of weathering, which are the raw materials of sediments, and moves them away from their source. The properties of the parent rock affect weathering because different minerals weather at different rates and have differing susceptibilities to fracturing. Climate strongly affects weathering: warmth and heavy rainfall speed weathering; cold and dryness slow it down. The presence of soil accelerates weathering by providing moisture and acids secreted by organisms. The longer a rock weathers, the more completely it breaks down.
What are the processes of chemical weathering? The weathering of feldspar, the most abundant silicate mineral, serves as an example of the processes that weather most silicate minerals. In the presence of water, feldspar undergoes hydrolysis to form kaolinite. Carbon dioxide (CO2) dissolved in water promotes chemical weathering by reacting with the water to form carbonic acid (H2CO3). The slightly acidic water dissolves away potassium ions and silica, leaving kaolinite. Iron (Fe), which is found in ferrous form in many silicate minerals, weathers by oxidation, producing ferric iron oxides. These processes operate at varying rates, depending on the chemical stability of the minerals involved under various weathering conditions.
What are the processes of physical weathering? Physical weathering breaks rocks into fragments along preexisting zones of weakness or along joints and other fractures in massive rock. Physical weathering is promoted by frost wedging and by burrowing and tunneling by animals and tree roots, all of which expand cracks. Microorganisms contribute to both physical and chemical weathering. Patterns of breakage such as exfoliation probably result from interactions between chemical weathering and temperature changes.
What factors are important in soil development? Soil is a mixture of rock particles, clay minerals, and other products of weathering, as well as humus. It develops through inputs of new materials, losses of original materials, and modification through physical mixing and chemical reactions. The five key factors that affect soil development are parent material, climate, topography, organisms, and time.
What are mass movements, and what kinds of materials do they move? Mass movements are slides, flows, or falls of large masses of material downslope in response to the force of gravity. The movements may be imperceptibly slow or too fast for a human to outrun. The masses may consist of consolidated material, including rock and compacted or cemented sediments; or unconsolidated material. Mass movements of rock include rockfalls, rockslides, and rock avalanches. Mass movements of unconsolidated material include creep, slumps, debris slides, debris avalanches, earthflows, mudflows, and debris flows.
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What factors are responsible for mass movements, and how are such movements triggered? The three factors that have the greatest bearing on the predisposition of material to move down a slope are the nature of the slope material, the water content of the material, and the steepness of the slope. Slopes made up of unconsolidated material become unstable when they are steeper than the angle of repose, the maximum slope angle that the material will assume without cascading downslope. Slopes made up of consolidated material may also become unstable when they are steepened or denuded of vegetation. Water absorbed by slope material contributes to instability by lowering internal friction and by lubricating planes of weakness in the material. Mass movements may be triggered by earthquakes, heavy rainfall, or gradual steepening of a slope due to erosion.