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Transcript
Earth Science Chapter 7: Weathering, Erosion, and Soil Chapter Overview Section 1: Weathering The process by which rocks on or near Earth’s surface break down and change is called weathering. The removal and transport of weathered material from one location to another is known as erosion 1. Mechanical Weathering The process by which rocks and minerals break down into smaller pieces is mechanical weathering, also called physical weathering. Mechanical weathering does not involve any change in a rock’s composition, only changes in the size and sometimes the shape of the rock. A variety of factors are involved in mechanical weathering • Temperature Temperature plays a significant role in mechanical weathering. When water freezes its volume increases by about 9%. So when liquid water seeps into cracks in rocks or between rock layers and then freezes, its volume will increase and put pressure or the rock. When the temperature increases the ice will melt. The repeated thawing and freezing of water in the cracks of rocks is called frost wedging • Pressure Pressure is another factor in mechanical weathering. Rock layers that are buried at great depth are under a great deal of pressure. When the overlying layers are removed the pressure on the buried rock layer is reduced allowing the rock to expand, and long, curved cracks can form. Over time, the outer layers of rock are stripped away. The process by which outer rock layers are stripped away is called exfoliation. Exfoliation often results in dome shaped formations 2. Chemical Weathering The process by which rocks and minerals undergo changes in their composition as the result of chemical changes is called chemical weathering. Significant agents of chemical weathering include water, oxygen, carbon dioxide, and acids. • Water Water is an important agent in chemical weathering because it can dissolve many kings of minerals and rocks. The reaction of water with other substances is known as hydrolysis • Oxygen The chemical reaction of oxygen with other substances is called oxidation. The oxygen in Earth’s atmosphere will react with certain metals in minerals causing those minerals to undergo a chemical change • Carbon Dioxide When carbon dioxide combines with water in the atmosphere a weak acid, carbonic acid, is formed. Carbonic acid can fall to Earth’s surface as precipitation and react with minerals such as calcite in limestone and marble to dissolve rocks. Carbonic acid can also react with silicate minerals to form clay minerals. In high enough concentrations, carbonic acid can aid in the formation of caves • Acid Precipitation Human activity can also contribute to chemical weathering. Through the combustion of fossil fuels, tremendous quantities of sulfur and nitrogen oxides are released into Earth’s atmosphere. Once in the atmosphere, these oxides will react with water to form sulfuric and nitric acids. These acids can fall as rain and have a significant negative impact on the environment and on man-made structures. Acid rain can damage large areas of forests, increase the acidity of lakes and streams making it impossible for certain species of amphibians and fish to survive, react with the limestone and marble in structures and statues, and increase the rate of corrosion of metals. 3. What Affects the Rate of Weathering The natural weathering of Earth materials occurs very slowly. It may take 2000 years to weather 1 cm (less than half an inch) of limestone, and most rocks weather at even slower rates. • Climate The climate of an area is a major influence on the rate of physical and chemical weathering of Earth’s materials. Variables of climate include precipitation, temperature, and evaporation, and the interaction between temperature and precipitation has the greatest effect on a region’s rate of weathering. Chemical weathering occurs readily in climates with warm temperatures, abundant rainfall, and lush vegetation. Physical weathering occurs readily in cool, dry climates. Physical weathering rates are highest where water undergoes repeated freezing and thawing • Rock type and composition The characteristics of rocks, including how hard or resistant they are to being broken down, depend on their type and composition. In general, sedimentary rocks are more easily weathered than harder igneous and metamorphic rocks. • Topography and other variables Materials on level areas are likely to remain in place as they undergo changes, whereas materials on slopes have a greater tendency to move as a result of gravity. As material moves down a slope, it exposes underlying rock surfaces and provides more opportunities for weathering to occur. Another factor that influences the rate of weathering is the presence of organic matter. Decaying organic matter releases carbon dioxide, which reacts with water to form carbonic acid. Section 2: Erosion and Deposition Erosion is the process that transports Earth materials form one place to another. A number of different agents transport weathered materials on Earth. At some point the movement of transported materials will slow down and the materials are dropped in another location in a process known as deposition 1. Gravity’s Role in Erosion Gravity is associated with many erosional agents. Gravity tends to pull all materials down slope, so is responsible for mass movements such as landslides, mudflows, and avalanches 2. Erosion by Running Water The erosion of running water in small channels on the side of a slope is called rill erosion. When a channel becomes deep and wide it can evolve into gully erosion • Coastal Deposition and Erosion Each year streams and rivers carry billions of tons of sediments and weathered materials to coastal areas. In the ocean weathering and erosional processes continue. The work of ocean currents, waves, and tides carves out cliffs, archers, and other features along t he continents’ edges 3. Glacial Erosion The erosional effects of glaciers are large scale and dramatic. Glaciers scrape and gouge out large sections of Earth’s landscape, and have the capacity to carry huge rocks and piles of debris over great distances. The landscape features left in the wake of a glacier include valleys and variously shaped deposits of sediments 4. Wind Erosion Wind is a major erosional agent in areas on Earth that experience both limited precipitation and high temperatures. Such areas typically have little vegetative cover to hold the soil in place. Section 3: Soil 1. Soil Formation Soil – the loose covering of weathered rock particles and decaying organic matter overlying the bedrock of Earth’s surface, and serves as a medium for the growth of plants • Soil is the product of thousands of years of chemical and mechanical weathering and biological activity 2. Soil Layers During the process of soil formation, layers develop in the soil. Most of the volume of soil is formed from the weathered products of a source rock. • Residual soil – a soil whose parent material is the local bedrock. • Transported soil – soil that develops from parent material that has been moved for from its original location • Soil profile – a vertical sequence of soil layers. It can take tens of thousands of years for distinct layers to form in a soil • Soil horizon – a distinct layer within a soil profile. There are typically four major soil horizons in mature soils, designated as O,A,B, and C O-horizon – the top layer of organic material which is made of humus and leaf litter A-horizon – a layer of weathered rock combined with a rich concentration of dark brown organic material. B-horizon – called the zone of accumulation, is a red or brown layer that has been enriched over time by clay and minerals deposited by water flowing the layers above or percolating upward from layers below C-horizon – contains little or no organic matter, and is often made of broken-down bedrock 3. Factors of Soil Formation Five factors influence soil formation: climate, topography, parent material, biological activity, and time. These factors combine to produce different types of soil.