Travelers from Marco Polo to Lord Curzon, the British viceroy of India, have marveled at the phenomenon of singing sands. Found in deserts and a few beaches around the world, singing sand dunes emit booming sounds- -sometimes carrying for miles--when buffeted by the wind. The sounds have been likened to anything from approaching cavalry to a foghorn, but no one has known how they are made. Now a group of Canadian researchers think they’ve solved the mystery. In order to sing, sand grains need to be coated with a layer of silica gel.
Marcel Leach, a physicist at Laurentian University in Sudbury, Ontario, had been working on the singing sand puzzle in his spare time for a few years. He had reached an impasse and decided to ask a university colleague, Douglas Goldsack, for advice. We’re a small university, says Goldsack, so he and I would see each other in the pub and talk about what we were working on. As a chemist, the first thing I suggested was a composition analysis of the different sands he had gotten from around the world.
Goldsack and Leach soon discovered that booming sand had an unusually high silica content. While most sands typically consist of about 50 percent silica and a mixture of other minerals, booming sands were 95 percent silica and also tended to be slightly moist from night air. Goldsack noticed that grains from singing sands had a pearly, lustrous quality, suggesting that something about their surface was unique.
They examined the sand under an infrared spectrometer, which measures the characteristic wavelength of light emitted by elements or compounds when they’re subjected to infrared radiation. They found that singing sand, unlike normal sand, contained water mixed with silica, forming a silica gel. The gel is used as a drying agent, often seen in packaging for electronics equipment, and is common in chemistry labs. When Goldsack shook some pure silica gel in a glass jar, the gel made a thrumming sound.
Goldsack and Leach think silica gels make dunes sing. The gel loosely glues sand particles together. When air blows through the gel, the grains shake in unison, transforming ordinary air patterns into coherent vibrations, like those produced with a tuning fork. Goldsack says other labs should find it easy to check his and Leach’s idea since silica gel is cheap and readily available. Everyone in the world can buy it.