And imagine such an enormous mass of countless particles of sand multiplied as often as there are leaves in the forest, drops of water in the mighty ocean, feathers on birds, scales on fish, hairs on animals, atoms in the vast expanse of the air
Where does desert sand come from?
Dr Pieter Vermeesch and colleagues had the same question about the sand in the Namib Sand Sea – one of the world’s oldest and largest sand deserts. We know little about where sand in the Namib, or other large deserts, comes from. How long the sand has been blowing around the 34,000 km square desert was also a mystery, although we know southwest Africa has been dry for at least five million years.
The team tracked the sand’s origins back to sediments in South Africa’s Orange River using natural uranium 238 clocks. Uranium 238 – the most common type of natural uranium – turns into lead over billions of years. Older sand contains more lead. In an area containing sands of varying ages, zircon sand grains can be linked to their source by the amount of uranium and lead they contain.
The next question was how long ago the sand left the Orange River bed and blew north into the Namib Sand Sea. The team again used natural clocks to retrace the sand’s path along the Namibian coastline. This time, they looked at radioactive beryllium (Be), aluminium (Al) and neon (Ne) in sands from quartz-containing rocks.
When sand is near the Earth’s surface, it’s bombarded by cosmic radiation – energetic particles from outer space. These particles collide with atoms of Be, Al and other elements in sand grains, causing them to become unstable. When the grains become buried in dunes, they stop being bombarded. The amount of unstable cosmogenic Be and Al declines over time – indicating how long ago the grains were buried.
