By Will Dunham
RABAT, MOROCCO- They are among the wonders of our deserts: star dunes, the vaguely pyramid-shaped sand formations up to about 1,000 feet (300 meters) tall with arms stretching out from a central peak to give them a star-like appearance when viewed from above.
Scientists on Monday unveiled the first in-depth study of a star dune, revealing the internal structure of these geological features and showing how long it took for one of them to form - more quickly than expected but still a process unfolding over many centuries.
The study focused upon a star dune in eastern Morocco called Lala Lallia, meaning "highest sacred point" in the local Berber language, situated within the Sahara Desert in a small sand sea called Erg Chebbi about 3 miles (5 km) from the town of Merzouga, close to the border with Algeria.
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.”
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.”
Lala Lallia rises about 330 feet (100 meters) above the surrounding dunes and is approximately 2,300 feet (700 meters) wide, containing about 5-1/2 million metric tons of sand.
The researchers used ground-penetrating radar to peer inside the dune and employed luminescence dating to determine how long Lala Lallia has taken to form, a method based on the amount of energy trapped inside the grains of sand. The answer: about 900 years, accumulating roughly 6,400 metric tons annually as wind relentlessly blows sand through the desert.
Star dunes make up just under 10% of the dunes in Earth's deserts and are the tallest ones, surpassing other types such as crescent-shaped barchan dunes and straight and lengthy linear dunes. They also have been spotted on Mars and on Saturn's large moon Titan.
"I first encountered star dunes in Namibia 20 years ago, and was instantly amazed at the size of them. I have a vivid memory of the long climb to the top, struggling up very loose sand in the heat of the day," said geographer Geoff Duller of Aberystwyth University in Wales, co-author of the study published in the journal Scientific Reports.
"I find desert dunes very beautiful," Duller added. "The sight of the sinuous curves, and the way that the light and shadow changes with the sun mean that they always look different, whether that is in the cool of the morning, the midday sun or near sunset.
The different colours of sand in different deserts are also very striking, with yellow, white, red and even black dunes in different parts of the world."
The ground-penetrating radar revealed the layers within the Lala Lallia dune, showing how it was constructed over time through accumulating sand and how parts of its internal structure resembled other types of dunes.
"Star dunes are formed in areas with complex wind regimes, which means winds blowing from different directions, and net sand accumulation, points within the desert where big piles of sand can be blown around to form giant dunes," said Birkbeck University of London sedimentologist and study co-author Charlie Bristow.
The researchers also determined that Lala Lallia is moving westerly at a speed of about 1.6 feet (0.5 meters) annually.
While many star dunes are known today, only a single ancient one has been found preserved as sandstone in the geological record, dating to about 250 million years ago, in Scotland. By revealing their internal structure, the researchers said their findings provide a guide for geologists to identify more sandstone remnants of ancient star dunes.
Earth's largest star dunes are found in the Badain Jaran desert in western China. Star dunes also are found in places including the Namib Sand Sea in Namibia, large sand seas in Algeria such as the Grand Erg Oriental and Grand Erg Occidental, and Rub' al Khali in Saudi Arabia. In North America, Great Sand Dunes National Park in Colorado contains a series of them.
"They form extraordinary and awe-inspiring landscapes," Bristow said. "From the ground they can be intimidating, mobile mountains of sand."
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.
