by Dunne’s films remind us how beautiful, mysterious, vast and changeable dunes can be. For centuries these amazing landscapes have been filled with awe – and in some cases fear and dread – due to the apparent remoteness and risks associated with the deserts with which they are synonymous.
That’s what first attracted me to researching deserts and dunes more than 40 years ago, and I’ve been investigating them ever since. Here are five things I’ve learned that might surprise you:
Not all dunes are made of sand
Ash, snow and even gypsum can build dunes. Dunes develop when small particles are mobilized on dry, bare surfaces by moderate wind, accumulating when a barrier or surface undulation slows the movement. If the particles are blown by the wind they can create a small mound that gathers other particles against it, eventually leading to dunes.
“Sand” isn’t really a substance – it’s a particle size, somewhere between 0.06mm and 2mm in diameter. Dunes in the desert and on the coast are mainly made of grains of quartz and feldspar, the most common minerals on earth.
But in volcanic regions, such as the interior of Iceland, dunes can be made from ash, and in the center of Antarctica, the driest and windiest continental land, dunes can be formed from ice crystals and snow. In New Mexico, USA, the very soft and bright mineral gypsum forms the sand dunes – the place is aptly called the White Sands.
Dunes can record the history of climate changes
Sand dunes may appear soft and variable, but there is often older sand beneath their active surface that tells the story of long-term development.
The way the wind direction changes throughout the year affects the shape of the dunes: some dunes, such as the crescent-shaped barchan dunes, move forward under fairly consistent winds, with the sand turning over on a regular basis. Others develop, like linear and star dunes, where the wind direction is more variable, accumulating sand up to tens and even hundreds of meters.
Using a technique called luminescence dating, we can measure how long dune sand has been hidden from sunlight, identifying periods when the dunes even stopped forming and soils, themselves buried under more sand more now, developed on dune surfaces in wetter climates.
In the Arabian desert of Rub’ al Khali, for example, huge linear sand dunes were formed during several dry periods over the past 130,000 years. The dunes may be much older still, as it was not possible to drill all the way to the bedrock and establish the full history of the accumulation.
Only a fifth of deserts are covered with sand dunes
Only about a fifth of desert areas have the right conditions for dune formation: a supply of fine loose sediment, sufficient wind energy and a lack of protective vegetation. Other common features of the desert landscape include mountains, rock slopes, gravel surfaces and dry lake beds.
But we can go beyond today’s deserts and find evidence of more extensive dune landscapes, for example under the grasslands and woodlands of some of Africa’s Savannah regions such as the Kalahari and even under tropical rainforests in parts of South America. These dunes reflect different desert and climate patterns in the past.
The ancient dunes of Scotland changed history
In the 1780s, the Scottish geographer James Hutton realized that the well-bedded and distinctive red sandstones at Sicar Point on the east coast of Scotland were the preserved remains of ancient desert sand dunes. At this point an old Devonian red sandstone, as it is now called, abruptly overlies fine gray mudstones.
Hutton realized that a long period – we now know it to be over 65 million years – must have passed between the green rocks being laid down, ready to erode, and the red sand being placed on top.
His careful theory established the foundations of modern geology and our understanding that the earth was much older than the history calculated from biblical texts. Further developments in the 20th century enabled us to explain why rocks formed under desert conditions are found in the unlikely context of Scotland – we now know that it is due to the movements of the earth’s crust, or plate tectonics.
Coastal dunes protect against storms
Sand dunes border large areas of the world’s coastlines, built of wind-blown sand derived from the drying zone of an intertidal beach and captured by coastal vegetation. While only 7% of Britain’s coastline has dunes, 40% of Australia and 60% of Portugal have dunes.
These dunes play a vital role in protecting low-lying land from tidal surges and storms. But in some areas human pursuits and sand extraction for construction have degraded the dunes by damaging stabilizing vegetation and creating erosion, with sea level rise posing an additional threat.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
David Thomas receives funding from NERC and The Leverhulme Trust. It is also affiliated with the University of Witwatersrand, South Africa.
