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Unformatted text preview: 1 Weathering and Surface processes Types of surface processes: 1- Weathering: Is the in situ disintegration of rocks at the surface of the earth. 2- Erosion: Is the transportation of the products of weathering by some transporting agent (e.g. wind, water, ... etc.). 3- Mass movement: Is the movement of rock material down-slope by gravity. Weathering Weathering in an area and the amount and nature of weathering products will depend on: 1) the type of rocks exposed on the surface 2) the nature and number of planes of weakness in these rocks 3) the relief of the area 4) the climate Weathering is of two kinds, mechanical and chemical . Mechanical weathering is the physical breakup of rocks into smaller pieces that have the same chemical composition as the original rock (i.e. disintegration without any changes in chemical composition). Chemical weathering is the breakdown of rocks into smaller particles through one or more chemical reactions that take place between minerals and weathering agents as water or solutions. Each of these two types involves a number of processes. A- Mechanical weathering: Mechanical weathering is greatly facilitated if the rock originally had one or more planes of weakness . These planes of weakness could be joints (particularly when they occur in sets or systems), bedding planes (in sedimentary rocks) or foliation planes (planes of preferred orientation in metamorphic rocks). By breaking up the rock into smaller particles, mechanical weathering will lead to an increase in the surface area of the total rocks exposed to surface conditions (Fig. 1), and will therefore facilitate the process of chemical weathering. Joints are particularly important for mechanical weathering because (a) they effectively cut large blocks of rock into smaller ones thus increasing the surface area where chemical reaction can take place, and (b) they provide channelways through which water can pass, thus increasing the chance of frost wedging (see below) or chemical weathering. Therefore, while mechanical weathering will always lead to an increase in the surface area of the rocks affected, weathering in general will slow down if the rocks have the smallest surface area. For a given volume, the smallest surface area is achieved if the rock is spherical in shape. Therefore, weathering will always try to change rocks with many edges into rounded or spherically shaped masses (Fig. 2). 2 Types of mechanical weathering: 1) Frost wedging: If water percolates into a joint or fracture in a rock where it gets trapped, and the surface temperature then drops below freezing, this water will change to ice. Because ice occupies a larger volume compared to water (freezing water involves an increase in volume of 9%), freezing is accompanied by expansion which exerts a significant force on the rock, widening the fracture. When the temperature rises, the ice melts, and more water is allowed to occupy the same fracture. When the temperature drops again below freezing, the water is converted to ice with yet a larger volume, which again exerts more...
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This note was uploaded on 02/27/2012 for the course GEOLOGY 110 taught by Professor Staff during the Fall '08 term at Marshall.
- Fall '08