A self-assembling stone shield
Across vast, flat expanses of the Gobi Desert in Alxa League, the ground is covered by a tight mosaic of interlocking stones. This surface, known as desert pavement, is barren and static; however, it is the product of multiple geologic processes acting over thousands of years. The formation is complex, with geologists identifying several contributing mechanisms that sort the desert floor, leaving a stable armor of pebbles and cobbles on top. Once established, this stone layer protects the finer sediments underneath from further erosion.
The classic theory for pavement formation is deflation. Aeolian processes, the action of the wind, selectively remove smaller, lighter particles like dust and sand. These fine materials are carried away, the larger, heavier stones, or clasts, are left behind. They gradually settle and jostle into a stable, interlocking arrangement. However, wind action alone is not the full story. As the pavement covers more of the surface, it increasingly shields the underlying soil from the wind; deflation becomes nearly ineffective once stones cover about 50 percent of the area.
The slow upward movement of stones
A more complete explanation involves processes happening within the soil itself. One primary mechanism is linked to the shrink-and-swell properties of clays in the soil beneath the surface. When infrequent rains wet the ground, the clay absorbs water and expands. As it dries, it contracts and cracks. Over many cycles, fine particles wash into these cracks. When the soil swells again, it pushes larger stones upward, preventing them from sinking back into the tightening cracks. This slow, ratcheting process, a type of granular convection, gradually moves larger clasts to the surface.
Another related hypothesis suggests that windblown dust continuously accumulates on the surface, filtering down between the existing rocks. This slowly builds up the soil layer from below, effectively lifting the pavement over millennia. These soil-based mechanisms explain why many desert pavements sit atop a layer of fine, stone-free silt and clay, a feature that deflation alone cannot fully account for. The entire process, from a mixed-sediment surface to a stable pavement, can take an estimated 1,500 years or much longer, depending on local conditions.
The stones themselves are also altered by their long exposure. Many develop a dark, sometimes shiny coating called desert varnish. This coating is composed of windblown clay minerals cemented to the rock by iron and manganese oxides. The concentration of manganese is often 50 to 60 times greater than in the Earth's crust, a process aided by manganese-oxidizing microbes living on the rock surface. This varnish forms incredibly slowly, with growth rates measured in micrometers per thousand years, and helps protect the rock from further weathering.