Academic Reading — Test 147

Reading passage

Among the many forms that windblown sand can assume, the star dune is perhaps the most striking. Rising in some places to more than three hundred metres, these pyramidal mountains of sand carry several arms, or radiating ridges, that spread outwards from a central peak like the points of a star when viewed from above. They are found in several of the world's great sand seas, but some of the finest examples occur in the Namib Desert of south-western Africa, a narrow strip of arid land that runs for roughly two thousand kilometres along the Atlantic coast. The Namib is among the oldest deserts on the planet, and its star dunes are counted among the largest and most stable on Earth. The shape of any dune is dictated chiefly by the wind, and the distinctive geometry of the star dune reveals the conditions under which it forms. Where a desert is dominated by a wind blowing steadily from a single direction, sand tends to gather into long crescent-shaped ridges known as barchans, which march downwind in an orderly fashion. Star dunes, by contrast, are the product of a more complicated wind regime. They develop only where winds arrive from three or more directions over the course of a year, with no single direction strong enough to dominate the others. Because the sand is pushed first one way and then another, it cannot escape downwind as it would under a constant breeze. Instead it piles ever higher around a fixed point, and each prevailing wind sculpts a fresh arm onto the growing mass. The result is a tall, multi-armed structure that behaves quite differently from its simpler cousins. This difference is most apparent in the way star dunes move. Barchans can travel many metres in a single year, but a star dune typically creeps forward only a few centimetres in the same period, and some appear scarcely to shift their position at all over a human lifetime. The reason lies in the balance of competing winds: because sand is transported in several directions that roughly cancel one another out, the dune as a whole has little net tendency to migrate. Its individual arms, however, are far from motionless. The arm that faces the season's dominant wind will lengthen and advance while the others wait their turn, so that the dune slowly reshapes itself from within even as its centre stays put. A star dune is therefore best understood not as a single travelling object but as a slowly turning assembly of smaller, more active forms. Establishing how old these dunes are, and how quickly they grow, has long challenged researchers, because sand offers few obvious clues to its history. A technique known as luminescence dating has proved especially valuable here. When a grain of quartz is buried and shielded from sunlight, it gradually accumulates a faint store of energy from the natural radioactivity of the surrounding ground. By measuring how much energy a buried grain has absorbed, scientists can estimate how long ago it was last exposed to daylight, and thus when it was covered over. Studies using this method suggest that the largest star dunes in some sand seas took many thousands of years to reach their present height, and that their cores are considerably older than their outer flanks. The surface sand may be reworked by every passing storm, but the heart of the dune is ancient and seldom disturbed. The conditions that allow star dunes to form are surprisingly particular, which helps to explain why they make up only a small fraction of the world's dunes despite their size. A generous and continuing supply of sand is needed, together with the multidirectional winds already described and, crucially, enough space and stability for the dune to remain in roughly the same place for a very long time. Many of the world's star dunes sit at the margins of sand seas, near mountains or other barriers that disturb the airflow and bend the wind into the shifting pattern they require. Where any of these ingredients is missing, simpler dune forms take their place. The rarity of star dunes is thus not an accident but a direct consequence of the demanding circumstances they depend upon. Beyond their scientific interest, star dunes carry a wider significance. Their layered interiors preserve a record of past winds and climates, allowing researchers to read former environmental conditions much as the rings of a tree record the seasons of its growth. They also serve as a useful guide to geologists studying ancient sandstones, where fossilised dune structures hint at the deserts of the distant past. As instruments capable of reading the atmosphere across millennia, these quiet giants of the Namib are valued for far more than their remarkable appearance.