How does a desert form?

Discover the world’s largest deserts and why these dry (but not barren) areas exist

Across vast expanses of thirsty land, silence is met with the occasional caw of a passing vulture or the howl of the wind. Our imaginations often conjure this typical scene when we think of our planet’s deserts. But deserts are not defined by the amount of sand they hold or how sun-baked their landscapes are. A ‘desert’ is classified as a region that receives little to no precipitation – typically less than 25 centimetres annually. Some of the world’s largest deserts are indeed hot stretches of sand, such as the Sahara Desert, but in fact the largest of them all is quite the opposite.

Spanning over 14 million square kilometres and the coldest place on Earth, the Antarctic wins the title of the world’s largest desert. Due to cold air’s poor ability to hold water vapour, the icy expanse doesn’t receive much precipitation.

What helps keep Antarctica a frozen wasteland is its snowy white appearance. Known as the albedo effect, the Sun’s warming rays are naturally reflected by light surfaces and sent back towards space. With such a large surface area, the Antarctic, as well as its Northern cousin, the Arctic  (the world’s second-largest desert), avoid being melted by the Sun’s heat, and so do not release water.

It is not the same for some of the other big deserts. Located around 30 degrees from the Earth’s equator, the subtropical deserts have been laid to waste, deprived of regular rainfall and dehydrated by the Sun over thousands or even millions of years. It’s a process known as aridification, and the Sahara Desert is the largest subtropical desert to be stripped of lush vegetation as a result of this process, around 7 million years ago. One theory suggests that during the breakup of the world’s supercontinents, which started around 250 million years ago, the African and Eurasian plates collided. As a result, the Alps were slowly formed, while a sea known as Tethys was lost. This body of water had played a vital role as a watering can for the surrounding land, including what is now northern Africa. The sea shrunk as the two plates collided, the precipitation gradually diminished and the Sahara’s dry spell began, lasting to the modern-day. High winds and little rainfall resulted in the vast expanse of desert we see today.

Deserts may appear to be lifeless expanses but are actually home to highly adapted survivalist species. Mammals such as the fennec fox, found in the Sahara, have evolved to take advantage of the desert’s cold nights. To avoid the Sun’s blasting daytime heat, these mammals are nocturnal hunters, feasting on invertebrates, small rodents, birds and their eggs.

Water is a hot commodity among the scorching sands, and some species have developed clever ways to find water. Some beetles in the Namib Desert utilise hydrophilic points on their back to collect water vapour from fog. As droplets form on the beetle, hydrophobic wax directs the water towards the beetle’s mouth.

In the shadows of giants

Deserts form as a result of many contributing factors, whether it’s in the deep freeze of the Arctic or because it simply misses the regular pathway of rainfall. However, living in the shadow of a mountain has forged some of the world’s largest colder deserts, such as the Great Basin Desert in the US or Asia’s Gobi Desert. Though still distinguished by a sandy surface, these are robbed of precipitation thanks to their towering neighbours.

Illustration by The Art Agency/Nick Sellers

This article was originally published in How It Works issue 124

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