![]() ![]() Steep mountain streams can move large boulders during spring flood but these boulders will never be transported out into a placid lowland river. ![]() Sediments are transported only when there is enough energy in the transporting medium, for example, when a stream is flowing rapidly enough to carry a given size of sedimentary particle. ![]() In addition to decreasing the particle size, as sedimentary material is transported it is also sorted into similar sized particles as a result of changing energy (velocity) in the transporting medium (water or wind), and rounded by continued abrasion. Other transporting agents include wind which blows dust and sand, glaciers, which carry large amounts of gravel and huge boulders in addition to smaller particles, and mass wasting on hillslopes. Water also carries dissolved minerals, such as silica and cations downstream as well as in the groundwater. Boulders and smaller rock fragments continue to be broken up and chemically altered as they tumble downstream. Water carries or rolls particles in rivers, from the smallest suspended clay particles to the largest boulders. The most important transporting agent is water. Quartz is itself an agent of mechanical weathering in the form of blowing dessert sand.Īs the process of weathering proceeds the products are carried off. Not only is quartz the most stable of the common rock forming minerals in chemical weathering, its high hardness and lack of cleavage make it quite resistant to mechanical weathering. Rock fragments will also remain where the rocks are not completely weathered. Some of the dissolved CO 2 reacts with the water forming the chemical compound carbonic acid.Ĭomplete weathering of silicate rocks will yield:Ģ) quartz sand (if the rock originally contained quartz) Most natural surface waters are slightly acidic because carbon dioxide from the air dissolves in the water. Hydrolysis is the reaction of minerals in weakly acidic waters. Others, especially silicate minerals, are altered by a chemical process called hydrolysis. Some minerals like halite and calcite may dissolve completely. In chemical weathering minerals are changed into new minerals and mineral byproducts. Mechanical weathering breaks rocks into smaller and smaller pieces but without otherwise altering the minerals. In mechanical weathering rocks are broken up into smaller pieces by frost-wedging (the freezing and thawing of water inside cracks in the rock), root-wedging (tree and other plant roots growing into cracks), and abrasion caused by, for example, sand-blasting of a cliff face by blowing sands in the dessert, or the scouring of water transported sand, gravel, and boulders on the bedrock of a mountain stream. When rocks (igneous, sedimentary, or metamorphic) are at or near the surface of the earth they are exposed to the processes of weathering. The latter two steps are called lithification. Sedimentary rocks are the product of 1) weathering of preexisting rocks, 2) transport of the weathering products, 3) deposition of the material, followed by 4) compaction, and 5) cementation of the sediment to form a rock. In that case, sedimentary rocks are derived rocks because they are formed from fragments of pre-existing rocks. Igneous rocks are sometimes considered primary rocks because they crystallize from a liquid. ![]()
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