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Exogenous processes

 

The processes of weathering

Rocks on the surface exposed to the damaging effects of water, gases (oxygen and carbon dioxide), organisms and their metabolic products, temperature fluctuations. The set of processes of mechanical crushing (disintegration) of rocks and to chemical decomposition of the minerals composing them is called weathering or supergene . Geological work of wind in these processes plays a very minor role. The most important source of energy processes of hypergenesis is the Sun. In this sense – hypergenesis – a concept inextricably associated with exogenous processes. Development of hypergenesis got the name of the hypergenesis zone.

The term “weathering” (hypergenesis) combined two qualitatively different, although inextricably related, process – physical and chemical. The result of physical processes (mechanical crushing) is breaking rocks into fragments of various sizes, and, ultimately, to its constituent mineral grains; chemical composition of rocks does not change. During the chemical decomposition of primary minerals are replaced by secondary change. Thus, depending on the factors affecting the rock, and the results of weathering processes are divided into physical and chemical weathering.

Physical weathering

Physical weathering is invoked by a variety of factors, most important of which is the daily fluctuation of temperature. The larger amplitude of diurnal temperature fluctuations characteristic of tropical deserts (70 ºC, 90 ºC and sometimes). This occurs due to temperature weathering of rocks. In terms of temperature fluctuations in the crystalline rocks occur internal stresses. Most characteristic postroennyh polymineral rocks. Dark minerals are heated stronger light and ceteris paribus stronger expand. However, and equally colored minerals due to the different coefficients of linear expansion increase in volume in different ways. In the end, on the borders of mineral grains occur stress, formation of cracks and, ultimately, monolithic crystalline rock is divided into separate mineral grains. For example, granite is the result of physical weathering breaks up into grains of quartz, feldspars, biotite and hornblende, turning into a crumbly mass of loose material – sand. In this way, is the formation of sand in sandy deserts.

In monomineral rock stresses occur due to the difference in coefficients of linear expansion in different directions in the crystal. So, in quartz in various directions, the coefficient of linear expansion is different in two times. Coarse-grained rocks, ceteris paribus dissolved faster than crystalline. The rapid destruction of rocks also contributes to their poor thermal conductivity. The day heats up, only the most superficial part of Skalnik, but the deeper parts remain cold. Similarly, night is cooling. As a consequence, there is a tension between the inner and outer parts of the rock. As a result the outer portion of rock breaks off and falls off from internal and exposed fresh rock exposed to the same processes. The so-called “peeling” of the breed. The term “peeling” stresses that the destruction process is subjected to a thin layer of the monolith, and the resulting fragments are thin plates of tiles, scales. Because the process is continuous, the monolith turns into rubble. By physical weathering, homogeneous rocks (basalts) often formed the so-called ball-joint.

Temperature weathering is most intensive in tropical deserts, as mentioned above, due to the large diurnal amplitudes of temperature. In addition, physical weathering in deserts favored by the absence of vegetation cover. The second area where widespread physical processes, including temperature weathering, is the area of the highlands. Weathering in the zone of high mountains contribute to the great insolation and high rate of transparency of the air. The resulting debris lay stone rivers – screes, slowly moving down the slopes. In Arctic and subarctic latitudes and at high altitudes, above the snow line, rocks are subjected to frost weathering. Frost weathering occurs intensively in spring and autumn, when temperatures are around 0 ºC (freezing point of fresh water). In the afternoon the snow and ice melts and the water seeps into pores and cracks of rocks. At night the water freezes and increases in volume by 9 %. Occurs when this pressure is sufficient to fracture rocks weakened by cracks. Formed in the rocks rasklinivat or ice causes frost heaving.

In arid climates role of a wedges can perform growing in pores and cracks across the salt crystals that fall when evaporation is suitable to the land surface groundwater. The same mechanical effect on the rocks has the root system of trees. Products of physical weathering are: boulders, rubble, gravel, sand.