1

2 Engineering Geology and Seismology Lecture#07 Department of Civil Engineering CECOS University of Engineering and Technology, Peshawar

3 Weathering and Erosion Outlines of the Presentation

4 Weathering is the process that produces changes in the surfaces of rocks exposed to the atmosphere. The rocks break and undergo decay under the influence of the atmospheric agencies like wind, sun, frost, water and organisms and produce soil. OR Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere. Erosion involves the transportation and removal of weathered materials from the place of their formation. OR When rock and soil are moved from one place to another by natural forces, this is called Erosion. The agents of Erosion are Water (rain water runoff, streams, rivers, groundwater, and waves Ice (Glaciers) Etc, Weathering

5 Weathering is of two types: 1.Physical weathering or disintegration 2.Chemical weathering or decomposition Physical weathering or disintegration: The mechanical breaking of rocks to form particles of smaller size, without change in composition is called physical weathering. The principal agents of physical weathering are as follows: Frost action: Weathering

6 a) Frost Action: The alternate freezing and thawing of water in the cracks of rocks tend to disintegrate them because water on freezing expands and therefore, exert pressure on the walls of the cracks. By this process, angular fragments of rocks are broken off from the high mountain ranges. These fragments of rock roll down the hill slope and accumulate at the bottom. b)Heating and cooling: The heating and cooling of rock masses occur due to daily and seasonal temperature changes. The heat causes them to expand and cooling causes them to contract. The repeated expansion and contraction tend to develop cracks in the rocks. In desert areas, the coarse grained rocks, like granite, disintegrate soon into their constituent crystals, and become desert sands as a result of temperature variation Physical Weathering

7 c)Plants. Plants disintegrate rocks when it grow in the cracks of the rocks. d)Organisms. such as worms, ants etc also contribute to the disintegration of rocks. Weathering

Physical Weathering Role of Physical Weathering 1)Reduces rock material to smaller fragments that are easier to transport 2) Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering 8

9 Chemical Weathering or decomposition is the process in which rocks are broken down by chemical decay of minerals. The main agents of chemical weathering as follows: Water. The chemical weathering of rocks is done mainly by rain water. The process involved are: Oxidation :The chemical reaction of oxygen with other substances is called oxidation. The oxygen, in the presence of water, readily units with the iron present in the minerals such as pyroxenes, amphiboles and olivine and convert it to iron oxide (hematite) or to hydroxides (limonite). The oxidation of pyrite leads to the formation of limonite and weak solution of sulphuric acid. This acid is a powerful chemical agent which attacks the rocks and develop solution pits. The iron oxides and hydroxides are the very common products of oxidation which impart the red and yellow colures to soils. Chemical Weathering

10 Hydration. Hydration is the process in which water molecules combine chemically with the minerals to produce new compounds. The formation of gypsum (CaSO 4. 2H 2 O) from anhydrite (CaSO4) is a good example of hydration. Carbonation frequentley occurs together with hydration. Thus orthoclase feldspar, a mineral which is abundant in granite, is decomposed and converted into kaolin by hydration and carbonation. Feldspars to clay + H 2 CO 3 (acid) Carbonation. When minerals containing calcium, magnesium and sodium or potassium ions react with water containing carbon dioxide, carbonates and bicarbonates are formed which are taken into solution. This process of decomposition is called carbonation. Chemical Weathering

11 Solution. The process of solution and carbonation goes on together. The limestones, dolomite, rock salt and gypsum are particularly susceptible to solution when they are attacked by water containing carbon dioxide. Vegetation. Decay of organic matter releases certain organic acids which increase the solvent power of water. For example, the solubility of silica, alumina and iron is much greater in the presence of organic acids. Some plants tend to extract certain chemical elements from rocks and thus they contribute to their decomposition. Chemical Weathering

12

13 Exfoliation. The process in which rocks, exposed to weathering chip off in thin sheets or layers concentric with the surface, thereby producing a round mass. It is sometimes formed when the pressure on a rock from overlying rock is released. The rock then expands and cracks. The exfoliation is also caused by alternate heating and cooling of rocks by weather conditions. OR Rock breaks apart in layers that are parallel to the earth's surface; as rock is uncovered, it expands (due to the lower confining pressure) resulting in exfoliation. Exfoliation

14 Rates of weathering will be influence by surface area exposed. More surface exposed, the faster the weathering will occur Rate of weathering

15

16 Some minerals are more resistant than others. For example Quartz is resistant to chemical and physical weathering. Mineral Composition