Weathering processes cause deserts to emerge when substantial temperature differences between day and night impose strain on the rocks, causing them to shatter into fragments. The wind smoothes down the rocks and arranges the sand into homogeneous deposits. The granules eventually become flat sheets of sand or pile up in billowing sand dunes. Weathering also occurs at a slower rate where there are no strong winds to blow away debris that would otherwise cover the surface of the land. In this case, new material is added to the surface from below, forming layers of rock that become weathered over time. Deserts form either when snow melts at lower altitudes during hot summers or when rain falls at higher elevations then runs down steep slopes without being absorbed into the soil. Deserts can also appear after glaciers melt at the end of an ice age. The most extensive desert in the world is the Sahara, which covers roughly one-fifth of Africa. Other major deserts include the Sonoran, Mojave, and Great Basin deserts in North America; the Gobi and Takla Makan deserts in Asia; and the Kalahari, Sahel, and Chihuahuan deserts in Africa. The term "desert" is used to describe any area of dryness and barrenness caused by lack of precipitation. Deserts can be divided up into two general categories: hyperdeserts and arid deserts.