Wind and water can form sandy ripples in a matter of minutes. Most will be erased, but some can grow to meter-scale and beyond. What distinguishes these two fates? Researchers used a laser scanner to measure early dune growth in the Namib Desert to see. They found that the underlying surface played a big role in whether sand gathered or disappeared from a given spot. Surfaces like gravel, rock, or moistened sand were better for starting a dune than loose sand was. Each of these surface types affected how much sand the wind could carry off, as well as whether grains bounced or stuck where they landed. Every trapped sand grain made the surface a little rougher, increasing the chances of trapping the next sand grain. Over time, the gathering sand forms a bump that affects the wind flow nearby, further shaping the proto-dune. As long as the wind isn’t strong enough to scour the surface clean, it will keep gathering sand as the process continues. (Image credit: M. Gheidarlou; research credit: C. Rambert et al.; via Eos)
Tag: sand dunes
La Grande Dune du Pilat
Southwest of Bordeaux in France stands Europe’s tallest sand dune, La Grande Dune du Pilat. Some 2.7 kilometers long and over 100 meters high, this dune took shape here over thousands of years. It moves inland a few meters every year as winds blowing from the Atlantic push sand up its shallow seaward side to the dune’s crest. There, sand will avalanche down the steeper leeward side, advancing the dune little by little. The dune’s accumulation has not been steady; during cooler and drier times, sand has collected there, but it took warmer and wetter climes to grow the forests that have helped stabilize the soil and build the dune higher. Humanity has played a role as well, at times introducing new tree species to stabilize the dune. (Image credit: W. Liang; via NASA Earth Observatory)
Fediverse Reactions
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Junggar Basin Aglow
The low sun angle in this astronaut photo of Junggar Basin shows off the wind- and water-carved landscape. Located in northwestern China, this region is covered in dune fields, appearing along the top and bottom of the image. The uplifted area in the top half of the image is separated by sedimentary layers that lie above the reddish stripe in the center of the photo. Look closely in this middle area, and you’ll find the meandering banks of an ephemeral stream. Then the landscape transitions back into sandy wind-shaped dunes. (Image credit: NASA; via NASA Earth Observatory)
Field of Dunes
Barchan dunes collide in this astronaut image of Brazil’s southern coastline. Barchan (pronounced “bar-kahn”) dunes are crescent-shaped; their tips point downwind into their direction of travel. When many barchan dunes overlap, they coalesce into a dune field like the one seen here. A dune’s speed depends on many factors, including the wind speed, dune size, and its proximity to other dunes. In experiments, dunes have even chased one another and changed speeds to avoid collision. (Image credit: NASA; via NASA Earth Observatory)
Changing Climes on Mars
China’s Zhurong rover explored Utopia Planitia on Mars from May 2021 to December 2022. During that expedition, the rover uncovered evidence of a major shift in climate that took place some 400,000 years ago. Originally, the area was covered in crescent-shaped barchan dunes formed by winds from the northeast. But after Mars exited its last ice age — courtesy of a shift in its rotational axis — the winds shifted around 70 degrees, coming from the northwest. Those shifted winds eroded the barchan dunes and caused new transverse ridges to form atop them. (Image credit: NASA/JPL-Caltech/UArizona; research credit: J. Liu et al.; via Gizmodo)
Complex Dunes
Sometimes landscapes have a beauty that’s hard to see from the ground. This astronaut’s photo shows a dune field in the sand seas of Saudi Arabia. Vast linear dunes line up along the direction of prevailing winds. Atop these dunes are more complex formations, star dunes, that are built up in the wake of changing winds. Built from three or more intersecting arms, the star dunes are steeper than the linear dunes they sit atop. Such complex dune fields — with multiple types of dunes — form in areas with especially abundant sands. (Image credit: NASA; via NASA Earth Observatory)
Racing Dunes
The deserts of Namibia are home to some of the fastest and most consistent winds in the world. As a result, they’re also home to some of the fastest-moving dunes on Earth. Dunes are shaped and moved by the wind, which pushes sand up the dune’s windward side and dumps it down the leeward side. As the process repeats, the entire dune moves. The bigger a dune is, the slower it moves.
Animation of Landsat images showing dune movement between April 2013 and April 2022. In this animation, showing dune motion from 2013 to 2022, the largest dunes move about 9 meters per year. In contrast, the smallest dunes move as fast as 83 meters a year! Check out the right side of the image, and you’ll see the dark specks of small dunes racing up and past their bigger brethren. (Image credit: top – E. Böhtlingk, animation – J. Stevens; via NASA Earth Observatory)
Dune Fields From Space
An astronaut captured this image of the Oyyl Dune Field in Kazakhstan from the International Space Station. To the south and east of the dune field (right and lower parts of image) there are fluvial floodplains, sources of sediment that feed the dunes. With sufficient wind and sand sources, the dune field has grown in a topographic low spot roughly 90 meters lower than the surrounding steppes. Dark specks scattered across the sands are clusters of vegetation, a sign that the dunes may get anchored rather than continue to shift in the wind. (Image credit: NASA; via NASA Earth Observatory)
How Dunes Form
On its face, the idea that sand and wind can come together to form massive mountainous dunes seems bizarre. But dunes — and their smaller cousins, ripples — are everywhere, not just on Earth but on other planetary bodies where fine particles and atmospheres interact. In this video, Joe Hanson gives a great overview of sand dynamics, beginning with what sand is, how it moves, and what it can ultimately form. It’s well worth a watch, even if you know a little about dunes already; I know I learned a thing or two! (Image and video credit: Be Smart)
Martian Flyover
Fly over a Martian crater in this incredibly detailed 8K video built from Mars Reconnaissance Orbiter imagery. Like Earth’s deserts, Mars is largely shaped by wind, and we get some fantastic views of sand ripples in this flyover. For reference, the vertical scale covered in the video image is roughly 1 kilometer. It’s pretty astounding to see this kind of detail from a spacecraft 250 kilometers away! (Video and image credit: S. Doran/NASA; via Colossal)
