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Chapter 8-I: Weathering Weathering is the break-up of rock due to exposure to the atmosphere When rocks that formed deep underground are moved to the surface by Earth's forces, they are subjected to water, oxygen and lower temperatures. Types of Weathering: Mechanical Weathering (disintegration) takes places when rock is split or broken into smaller pieces of the same material without changing its composition. Chemical Weathering (decomposition) takes place when the rock's minerals are changed into different substances (usually by water) Types of Mechanical Weathering Common mechanical weathering processes are frost action, wetting and drying, action of plants and animals, and the loss of overlying rock and soil. Water takes up about 10% more space when it freezes. Water held in the cracks or rocks wedges the rocks apart when it freezes --> ice wedging Ice wedging occurs mostly in porous rocks, and in rocks with cracks Repeated wetting and drying is effective at breaking up rocks that contain clay. Clays swell up when wet and shrink when dry, causing rocks that contain clay to fall apart. Small plants such as lichens grow on rocks and wedge tiny roots into pores and crevices, when the root grows, the rock splits. Granite is formed deep underground. It is exposed when large masses of rock are lifted to form mountains and the rocks above the granite are worn away. Upward expansion of the granite leads to long curved breaks, or joints. This is known as sheet jointing. From time to time large sheets of loosened rock break away from the outcrop, in a process called exfoliation. Chemical Weathering • Results mainly from the action of rainwater, oxygen, carbon dioxide, and acids of plant decay. • The chemical reaction of water with other substances is called hydrolysis. - ex. Feldspar, hornblende, augite form clay • The chemical reaction of oxygen with other substances is called oxidation. - iron-bearing minerals are the ones most easily attacked by oxygen • Carbon dioxide dissolves easily in water, forming carbonic acid. Carbonic acid attacks many common minerals. - Carbonic acid has the greatest effect on calcite, dissolving it completely. Resisting Weathering • Quartz does not react very much to water, oxygen, or acids. It is almost unchanged by chemical weathering. • Quartz is hard and does not have cleavage, so it also resists mechanical weathering. In time, quartz is broken into pebbles and sand grains. • Feldspar, hornblende, biotite mica, augite, calcite, and gypsum are all affected by mechanical and chemical weathering. • Most igneous rocks and many metamorphic rocks weather more rapidly in wet climates than in dry ones. • Sedimentary rocks are only as resistant as the cements that hold them together. The Rate of Weathering • Rocks weather at different rates: • Less resistant rocks weather more quickly, more resistant rocks (granite and gneiss) take much longer to weather away. • When rocks of different resistance are in the same place, spectacular landscapes are sometimes formed. • An important factor that affects the rate at which rock weathers is the amount of rock surface that is exposed. • Smaller pieces have greater surface area exposed, and more surface is exposed to weathering, increasing the rate. • Climate is an important factor in rock weathering: warm, wet climates favour chemical weathering processes. Cold or dry climates favour mechanical weathering processes. • Weathering is a very slow process. Limestone dissolves as little as 1/20 of a cm in 100 years.