Describe and explain weathering in desert environments (15 Marks)

Physical weathering processes are considered to be important in arid environments as they weather the rock at a very fast rate. This is because the high diurnal temperature ranges results in lots of insolation weathering. For example, Frost shattering and exfoliation. However, due to such small amounts of precipitation in deserts, Frost Shattering is not such an important process in deserts as Exfoliation and Block separation.

                        Figure 1:

                          The Peltier Diagram

Physical Weathering requires a large diurnal range.

There are also three main types of Chemical weathering that occur in desert environments. These include; Salt crystallization, Hydrolysis and the processes that form duri-crusts and desert varnish. The main type of chemical weathering found working in desert environments is salt crystallization. This is the process of breaking down rocks by the growth of salt crystals in cracks. Salt crystallization is caused by saline water, which comes from rainfall or from the groundwater that is drawn up to the desert surface by evaporation. This saline water then enters into pores or cracks in the desert rocks. The high temperature in hot deserts causes the water to evaporate, forming salt crystals. As the salt crystals form they expand, exerting pressure on the rocks. As more evaporation occurs, the salt crystals expand even more. This consequently increases pressure in the rock, causing pieces of the rock to fall off. Another chemical process that weathers hot desert environments is hydrolysis. This occurs if rocks containing salt minerals allow water to percolate, causing the salt crystals swell in the

presence of Water. The rocks are therefore put under internal pressure and are vulnerable to future break down. For example, rocks containing anhydrite are changed to gypsum. Finally, Chemical weathering is also seen working in deserts in the formation of duri-crusts and desert varnish. A Duri-crust is a thin hard layer on or near the surface of soil, usually a few mm to a few cm thick.It is typically formed by the accumulation of soluble materials like salts which are deposited by water that moves upward by capillary action, commonly assisted in arid settings by high rates of evaporation. Desert varnishes are also formed by chemical weathering. A desert varnish is another form of crust, a thin, hard glazed surface on exposed rocks. It is formed of a coating of iron and manganese oxides and possibly bacteria. The longer a rock is exposed, the darker the desert varnish becomes.

Traditionally, Geologists have said that chemical weathering is of less significance than Mechanical weathering. This is because most forms of chemical weathering require moisture and water and as deserts have very small amounts of precipitation, there is no water to create chemical reactions. However, More recently geologists have come to accept that chemical weathering is becoming increasingly recognized in deserts. This is because that it has been found that deserts have high relative humidity levels at night-time and in the form of due, water is present for chemical processes, such as hydrolysis. Also, forms of chemical weathering, such as salt Crystallization occur as there are very high rates of evapo-transpiration, due to the high temperatures found in hot deserts.

Finally, weathering in deserts also depends on what type of desert it is. For example, In Reg and Hamada deserts, which are stony and rocky, most of the weathering tends to be physical. For example, disintegration due to granular disintegration or exfoliation. This is as there are lots of rocks, which means that insolation weathering take place. However, despite low levels of precipitation, chemical weathering is noted to be significant too, for example, hydrolysis, which is caused by high levels of moisture at night. Inversely, In Erg deserts, which are very sandy, most of the landforms, like Barchan dunes are formed as a result of deposition, rather than weathering.