They are impressive, mysterious structures that loom out of deserts on the Earth and are also found on Mars and on Saturn’s biggest moon, Titan.
Experts from universities including Aberystwyth in Wales have now pinpointed the age of a star dune in a remote area of Morocco and uncovered details about its formation and how it moves across the desert.
Prof Geoff Duller of the department of geography and earth sciences at Aberystwyth said: “They are extraordinary things, one of the natural wonders of the world. From the ground they look like pyramids but from the air you see a peak and radiating off it in three or four directions these arms that make them look like stars.”
The team, which was also made up of University of London academics, travelled to the south-east of Morocco to study a 100-metre high and 700-metre wide dune in the Erg Chebbi sand sea known as Lala Lallia, which means the “highest sacred point” in the Berber language.
They discovered that the very base of the dune was 13,000 years old but were surprised that the upper part of the structure had only been formed in the last 1,000 years or so. “It turned out to be surprisingly young,” said Duller.
The base continued to build up until about 9,000 years ago. “Then the surface stabilised. We think it was a bit wetter than today. We can see traces of old plant roots, suggesting the dune was stabilised by vegetation. It seems to have stayed like that for about 8,000 years. Then the climate started to change again and this star dune started to form.”
Duller said the dune was formed because the wind blows in two opposing directions – from the south-west and the north-east – leading to the sand building up. A steady third wind that blows from the east is shifting the dune slowly to the west at a rate of about 50cm a year.
“That’s important when you’re thinking about building roads, pipelines or any sort of infrastructure,” said Duller. “These things actually do move.”
Luminescence dating techniques developed at Aberystwyth were used to discover the last time minerals in the sand were exposed to sunlight to determine their age.
Duller said: “We’re not looking at when the sand was formed – that’s millions of years ago – but when it was deposited. The grains of quartz have a property like a mini rechargeable battery.
“It can store energy that it gets from naturally occurring radioactivity. When we bring it back to the laboratory, we can get it to release that energy. It comes out in the form of light. We can measure that and the brightness tells us the last time the sand grain saw daylight.”
One catch is that they have to collect the grains of sand without exposing them to light. They did this by cutting pits into the dunes and hammering in an old piece of drainpipe to gather the samples.
“That part isn’t terribly hi-tech,” said Duller. The work in the lab – which is much more hi-tech and sensitive – has to be carried out in the same sort of conditions as a photographic darkroom.
The same luminescence technique was used to date remnants of what is thought to be the world’s oldest known wooden structure, an arrangement of logs on the bank of a river bordering Zambia and Tanzania that predates the rise of modern humans.
The dune discoveries are published in the journal Scientific Reports.
