Desert formation

A desert forms when there has been a shortage of rain for a long time. It may have different geological conformations – mainly due to the effect of the wind (wind erosion). There are sand deserts, called erg, rock deserts, called hammada, and pebble deserts, the serir.
The history of a desert can be studied through palaeontology. During the Pleistocene (1 million years ago), where there are deserts now, rainy periods followed each other during the glaciations, while dry periods followed each other in warmer times. This is proven by the stratigraphical sequence of the land and the variations of the level of the waters of some lakes that still exist today. For instance, Lake Chad in the Sahara desert used to be much larger and 120 m deeper than it is today.
In more recent times, at the end of the last glaciations, the climate of some areas which were constantly submitted to high pressures determined the spreading of the deserts.
In other instances (the Patagonian medium desert in the Andes), it was the presence of mountain ridges that acted as a barrier against the wet currents coming from the oceans and that determined the conditions that favoured the development of the desert. Clouds are actually stopped by mountains and therefore they let rain fall on the first side they bump into, while “at the back” it rains very little.
The cold oceanic currents have also originated some dry areas: they generate cool and constant winds carrying little humidity which can at most condense into fog without causing any real rain. For instance, the coastal desert of Peru and northern Chile washed by the cold Antarctic current of the Hudson and the Namib desert, washed by the Antarctic current of the Banguela.
In addition, winds are responsible for the ongoing reshaping of the landscape: they play a key role in that there’s virtually no vegetation at all and the ground is easy to attack. In addition, their erosive effect is enhanced by the sand acting as an abrasive agent. The most spectacular results of the action of the winds are eroded and polished rocks in the most bizarre shapes.
Dunes are also generated by the wind that builds and “moves” these sandy mountains around; the draughts lift the grains and settle them down again when friction increases. The shape of the dunes mostly depends on the direction and variability of the wind; there may be parabolic, dome-shaped, boat-shaped, crosswise , straight, opposing or star-shaped.

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