A quarter of our world is made up of deserts. The region is very dry and hot that only few plant species survive and animals retreat underground during the day, hunting at night. Soil temperature in deserts gets higher. This is because in the dry, sparsely vegetated desert environment with its clear cloudless skies, nearly all the sun’s radiation comes through to heat the bare ground and air above it.In many deserts, cliffs are stained with streaks of color ranging from tan to dark red to black. Even rocks on the desert floor are covered by a thin, lacquer like film. This coating on stable rock surfaces is called desert varnish which was a mystery to desert travellers before.
The desert varnish is actually a thin layer of metallic oxides that have been deposited on the rock surfaces and then burnished by windblown sand. It is combination of clay particles along with iron and manganese oxide. Clay then combines with additional substances which react with each other chemically when the temperature starts to rise in the day time.
Wetting of rocks by dew also plays an important role. It is believed that dew dissolves the metallic elements from the rocks and they remain as a surface coating when the dew evaporates. Study suggests that microorganisms may also play a role in creation of desert varnish.
Different rocks have different abilities to retain the varnish on their surface. Dense black varnishes form on rocks like basalt, quartz, shales and the concentration of manganese is found to be quite high in black desert varnish. Also some rocks cannot retain the varnish as they may be water soluble.