Have you ever noticed how a desert looks like a sea of sand, with waves of sand going up and down?
That’s not by accident. The mounds of sand that rise and fall are called sand dunes, and are naturally occurring geological structures that are usually found on beaches or in the desert. But how are they formed and why are they only found in these specific areas? Let’s find out.
Sand dunes are formed by the wind carrying grains of sand and soil and depositing them together. The wind transports sand in three ways: suspension, saltation, and creep. Saltation is the main way by which dunes are formed, where the grains of sand are pushed along the ground by the wind picking them up and dropping them every few inches. Suspension is when the grains are picked up by the wind and carried a certain distance with it, while creeping refers to the wind causing grains to knock into each other and thus, pushing each other ahead.
Sea breeze and wind often transport sand off beaches and deposit them near the inlands, often against obstructing objects like pieces of wood or rocks, making dunes. Even in deserts, due to unhindered winds and sand storms, large amounts of sand is transported from one place to another creating sand hills or dunes that can go as high as several hundred feet.
Now, you may be wondering how the wind neatly piles the sand in smooth waves, when it can carry the sand in any direction. The science behind this is simple. Sure, the wind may take the sand particles in multiple directions, but at any given instance, it will always blow in any one direction only. So the sand travels in one direction, slowly growing into a pile till it’s too unstable and then it collapses. Most dunes are formed in an asymmetrical heap, with the windward side (the side from where the wind is blowing) having a larger slope than the other side. They are usually formed at a 30-40 degree angle. When the sand continues to pile, pushing the dune higher, it becomes more unstable as dry sand doesn’t form a strong base, and forms a crest at the very top. The top or peak part of the dune is called a ‘ridge,’ where the crest is often formed before the dune collapses.
That part in the movies when they show characters walking across the desert on top of what looks like a heap of sand? They’re walking on the ridges of huge sand dunes!
Sand dunes can be found in different regions of the world — deserts, beaches and even wetlands. There are different types of dunes, depending on the force of the wind, the shape the dune takes, and its height.
Barchan dunes or Crescent dunes, are piles of sand that form in a ‘C’ shape, with its ends or two ‘arms’ or ‘horns’, pointing outward, away from the wind. They are formed by winds that are blowing at different speeds in the same direction.
Transverse dunes are long sand dunes that are formed due to strong winds carrying a lot of sand. They are parallel to each other and usually have wavy ridges that run at a 90-degree angle to the wind. This is also why they tend to have steep sides.
Star dunes are huge piles of sand that are made by winds blowing in different directions. They often look like peaked icing on cakes and have multiple arms. If you were to see them from above, they would look like star-shaped sand clouds. They also tend to be higher than other types of dunes.
Linear dunes are the most common type of sand dunes. They’re long stretches of narrow sand piles, with ridges that run parallel to each other. They tend to be very long, often stretching across 20 kilometers in the desert.
There are also composite dunes, which are a combination of two or three kinds of sand dunes. The most common combination is that of transverse and linear dunes, which in turn form gigantic dunes called ‘draas.’ A field of sand dunes is called an ‘erg,’ and they can span around 6,00,000 square kilometers.
Sand dunes aren’t just random piles of sand in nature. They form an integral part of the ecosystem as protectors. Sand dunes act as barriers, especially along coastal areas that are prone to flooding and storms. They also prevent erosion of vegetation and soil and protect coastlines from shrinking due to catastrophic winds and tides, by stopping water entering the inlands.
Not to mention, sand dunes are a great avenue for those who enjoy adventure sports. Many attempt skiing on the slopes of large dunes, some even sand board and sled down those slopes.
So, would you jump off the ridge of a sand dune? Let us know in the comments.
When she's not scrolling through endless cat and food videos, Madhavi Pothukuchi loves to read, paint, and binge on K-dramas and chips. A former journalist and an education enthusiast, she believes that asking more questions only makes life more interesting.