The immediate cause is the removal of vegetation. Unprotected, dry soil surfaces then blow away with the wind or are washed away by flash floods, leaving infertile lower soil layers that bake in the sun and become an unproductive hardpan. Overgrazing destroys valuable plant species, leaving mostly unpalatable ones. Losses of vegetation and biodiversity threaten habitat for other species.
Mountains often bordering a desert create what is in geographic terms a "rain shadow." The mountains stop most of the rain from entering the soon to be desert. There also has to be an overall shortage of water in the area in the form of shallow aquifers, otherwise it won't become a desert. Also more often then not the soil becomes poor in nutrients because of the lack of rain fall. Thus an area meeting most of these requirements will undergo a process of "desertification." Deserts can also be man made by draining the local water sources in area of low rainfall and high drainage of soil nutrients through excessive harvesting. Cyclical climate changes and ocean currents also affect an area's susceptibility to desertification. Also water scarcity deforestation and natural or man - made disasters.