Towering over the desert like ocean waves frozen in time are great mounds of shifting sands known as dunes. Forests, farms and even entire towns are smothered by these constantly moving mountains, which account in total for about 5 million square miles which is about 13 million square kilometres of the Earth’s surface, an area twice the size of the United States of America.
The dunes are blown by the winds, sometimes in violent sandstorms that scrape the paint from cars and slice through telephone lines. The sands move fast and far, depending on the strength and direction of the wind.
Of the two main types, crescent-shaped dunes form when the wind blows only in one direction, while “sief” dunes resemble the surface of a rough sea. Some dunes are shaped like elongated mounds, running in parallel lines and separated by rocky corridors; they may even stretch for 100 miles (160 km) or more.
The shallow troughs of the Sahara, known as ‘ergs’, have some of the largest of all dunes. Winds from many directions deposit their sand in great star-shaped heaps, sometimes more than 1000 ft (350 m) high. These huge peaks stay put for many years, serving as landmarks for desert travellers.
Dunes on the move
Most dunes, however, are constantly on the move. A dune creeps forward as sand blown up the gently sloping upwind side, spills over the crest and builds up at the top of the unstable steep slope. When this slope reaches a steep angle, the sand cascades down its face.
With crescent-shaped dunes, sand on the flanks moves faster than that in the centre, the sickle shape emphasised by higher wind speeds along the outer edges of the dune.
As long as the wind is blowing, the process continues. This continues to the extent that dunes can climb a slope to form a ramp leading up to a high plateau. The sand grains themselves are rounded and move easily in the wind.
Even deserts have “seasons”, since the formation and movement of sand dunes is dependent on seasonal wind pattens.
In the Wahibah Desert of Oman,a group of scientists found that sand movement is greater in the summer when the south-west monsoon winds blow the sand towards the north-east at a rate of about 10ft (3m) in ten days.
When the sand moves, it does not go piecemeal: instead, entire dunes retain their valleys and ridges and move in one great mass. In winter and spring, when the winds blow from different directions, the dune crests form and re-form, moving along at a rate of about 3ft (1m) each day.
Island of sand dunes
In some places, the sand is stopped in its tracks by a natural obstruction. Witsand, in the Kalahari Desert of southern Africa, is an island of white sand dunes, six miles (10km) long and two miles (3km) wide, in a sea of red sand. A row of quartzite outcrops of rock in an otherwise featureless landscape has trapped the sands blowing from the Kalahari and a range of low sand mountains has gradually accumulated.
Water, trapped in the quartzite and seeping up from it, bleaches the sand white or a grey-blue colour that contrasts with the oxide-rich sands of the surrounding desert.
Local superstition has it that the white sand will not mix with the red. The magic is enhanced by the presence of curious stalagmite-like formations. They occur where lightning has struck the sand, fusing the sand particles together into misshapen tubes known as fulgurites.
Sand dunes are not always the products of hot deserts. Coastal dunes made from rocky fragments, ground into a fine sand by the action of the sea may be pushed back from the coast by onshore gales.
The legacy of glaciers includes wind-blown fragments of shattered rock sculpted by cold winds after the Ice Age, into huge dune fields like those along Lake Michigan in North America and in parts of Germany and the Netherlands. A storm in 1775 buried the church of Skagen on the northernmost tip of Jutland in Denmark in sand, leaving only its tower poking through. In the hotter parts of southern Europe, dunes up to 100 ft (30m) high are sometimes swept up into the sky, burying green forests and ancient buildings all alike.
Dunes also make mysterious sounds. The sands squeak, rumble and roar. The sounds are created when accumulated heaps of sunbaked sand break up and tumble down the steep face of a dune. When sand grains are analysed under powerful electron microscopes, scientists have discovered that those from “booming”dunes or “singing” sands are more polished than those from other dunes.