National Geographic : 1996 Nov
Circles in the Sands "The most striking feature we can see from orbit is the belt of desert that stretches, nearly unbroken, from northwest Africa to China, extending almost to Beijing," Apt says. Space travelers distinguish the deserts they pass over by watching for such land marks as the Richat structure (right) in the sands of central Mauritania. A geologic curiosity 25 miles wide and 300 feet deep, it consists of layered rock ridges sandblasted into view by fierce Saharan winds. Hundreds of millions of years ago the rock layers formed from sediments deposited by advancing and retreating seas. Under pressure from deep within Earth, a dome formed. As sandy winds gnawed away at the dome, concen tric rings of harder rock endured. Some 3,600 miles to the east, green circles-each encompassing 200 acres of farmland-dot the desert of Saudi Arabia (above). Center-pivot irrigation began transforming Saudi sands in the 1980s. By 1990, when this photograph was taken, astro nauts saw a tripling of cultivated areas. In the center of each circle is a well drawing water from an aquifer thousands of feet below. No one knows how long this water will last.