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Scientists have been puzzled for a long time about how a huge star-shaped sand dune in the Sahara Desert formed. This massive dune, called a "star dune," has arms that stretch out in different directions, unlike regular dunes that just move one way. After lots of research, scientists figured out that these dunes are shaped by winds blowing from multiple directions over time. The changing wind patterns cause the sand to build up in a star-like shape instead of forming a single ridge. It’s been a tough mystery to solve because the winds in the Sahara can be really tricky. But now that scientists have cracked it, they understand how these unique dunes are made! Credit:
Man Carecter Walking: by Juwel_Mahmud, CC BY 4.0 https://creativecommons.org/licenses/by/4.0/, https://skfb.ly/o7PVn
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Transcript
00:00You're flying over the Sahara Desert in a small light aircraft, when you spot something
00:05truly weird.
00:07It seems to belong to a very different part of the world, the ocean.
00:11But what ocean can be huge enough for this monstrous sea star to dwell there?
00:17You descend and realize that it's not a sea star.
00:21It's a towering dune with a pretty unique shape, and it's not alone.
00:27From afar, these magnificent sand structures look a bit like pyramids.
00:32They have pointed tops and radiating arms.
00:35Called star dunes, they're the most complex and largest desert dunes in the world.
00:40They're the tallest of their kind, but that's not the coolest thing about them.
00:45Researchers have found similar formations on Mars, and even on one of Saturn's moons,
00:51Titan.
00:52Star dunes aren't super rare, but they're especially widespread in southeastern Morocco
00:57near the border with Algeria.
00:59That's where you're flying at the moment, and the breathtaking dune you're marveling
01:03at is Lella Lallia, probably the most famous of them all, a giant 330 feet tall.
01:10Scientists believe that it formed in less than 1,000 years.
01:14How about doing some research to find out more about these unusual formations?
01:18There, I see a great spot for landing!
01:22Star dunes got their name because of their multi-armed shapes.
01:26They form in areas where winds change direction throughout the year.
01:29But even though you can find them in different places on our planet, there is just one confirmed
01:34star dune in the rock record, the collective record of Earth's development reflected
01:39in preserved rocks.
01:41The star dune in question is in Scotland, and it's ancient, from about 250 million
01:47years ago.
01:48That's all we know about super old star dunes, probably because we don't know what
01:53to look for to identify one.
01:57Another problem, star dunes often form in remote locations, which makes them extremely
02:01difficult to study.
02:03Imagine reaching the nearest settlement, then somehow traveling who knows how many miles
02:08to the dune itself, and then slogging up several hundred feet of shifting sand.
02:14I'll pass.
02:15But you can go since you've already landed, and the dune is right here in front of you.
02:21Currently, you're in a dune field called Erg Shaby.
02:25Lucky for you, this area has long become a popular tourist stop.
02:30Look, in the distance, there are hotels and even a good road!
02:36Scientists who examined Lele Lalea before you used ground-penetrating radar to find
02:41out more about the sand formation.
02:44This equipment can detect the tiniest differences in sand grain sizes and water content under
02:49the surface of the dune.
02:51That's how you can create a picture of its interior layers.
02:57Look out!
02:58Phew!
02:59It was a close call.
03:00See this trench?
03:01The researchers dug such trenches to take samples of long-buried sands and then examined
03:06them.
03:07It's a pretty cool process.
03:09The thing is, while buried deep inside, the quartz in the sand accumulates radiation coming
03:14from natural sources within our planet.
03:17We can compare the grains of quartz to miniscule rechargeable batteries.
03:21They can store the energy they get from radiation.
03:24When researchers bring them to the laboratory, they can make the grains release that energy,
03:29which comes out in the form of light.
03:32Scientists can measure the brightness and say when the sand enjoyed sunlight for the
03:36last time.
03:38Now when those experts who dug the trenches looked inside Lele Lalia, they were shocked.
03:44The enormous formation turned out to be very young, for a dune of course.
03:49You'd expect a dune that is several hundred feet tall to be pretty old, like thousands
03:53or tens of thousands of years.
03:55But that wasn't the case, well at least not entirely.
03:59The upper part of the dune has turned 900 just recently.
04:05But look at the sand near the base of the dune.
04:07Yep, right there.
04:09Well it was buried there around 12 or 13 thousand years ago, representing ancient dunes in the
04:15area.
04:17Those good old dunes were active for a couple thousand years, and then something went wrong.
04:22Because for a whopping 8 thousand years, the sand wasn't accumulating there.
04:26A mystery?
04:28Scientists might have an explanation.
04:31You see, the first part of this quiet period occurred during the time when the climate
04:35became warm and wet.
04:37Yep, even in the Sahara.
04:39It happened about 11,700 years ago.
04:43It was the end of the last ice age and the beginning of a new era, the Holocene.
04:48And guess what?
04:49The Sahara went green!
04:51It started blooming like your local botanic garden.
04:54Plants started popping up all over the place, stabilizing the sand.
04:58If you had visited the Sahara during that period, you wouldn't have recognized this
05:02place.
05:03But you would have likely come across humans wandering marshy landscapes and hunting for
05:07food.
05:09Researchers have found pottery fragments and stone tools on one side of Lele Lalea.
05:16That's true.
05:17As sand dunes form and then shift throughout their lives, they swallow and protect ancient
05:22remnants of life.
05:24Just dig well enough, and you might find something that will provide you with a window into the
05:29past.
05:30It can be ancient fossils, anything from fossilized shells and corals to prehistoric plants.
05:37Those can offer precious insights into the history of our planet and the living beings
05:41once inhabiting certain regions.
05:44Now it might come as a shock, but in the Sahara Desert, scientists have uncovered well-preserved
05:50marine reptiles and fossilized fish within the layers of those ancient sand dunes.
05:56They are the remains from the time when the Sahara was covered with a sea millions of
06:01years ago.
06:03Anyway, it's time to leave that ancient paradise if you want to survive.
06:08Four thousand years ago, the wet period came to an end, and the Sahara dried out again.
06:13Interestingly, our dune didn't begin building up right away.
06:17For some time, sand probably blew through but didn't accumulate.
06:22Or the dune could have started growing in a different location.
06:25But it's a valid theory since scientists have discovered recently that Lelulalia shifts
06:30around 1.6 feet every year.
06:33It also accumulates about 6,400 tons of sand every year.
06:40Now if you've been inspired by your dune trip and are now eager to explore more of them,
06:44I've got some unpleasant news for you.
06:47Unfortunately, although there might be star dunes locked in sandstones around the globe,
06:52they're super tricky to detect.
06:55The problem is, dunes are huge, but they lack a single distinguishing feature.
07:00So you'd really need large exposed beds of rock if you want to get a wide enough view
07:05to identify a star dune.
07:07On the bright side, there surely are places where it might be possible to do such research,
07:12but you should be ready to know that it might be just a set of various features that look
07:17like other dunes.
07:19But if you combine them, you'll say, ah, it's likely to be a star dune!
07:25Comment below if you'd like to go on a trip in search of a star dune!
07:28Meanwhile, we'll continue our journey.
07:31You decide to have some rest and start climbing down the side of the dune.
07:35That's when you begin hearing it, terrible, haunting sounds.
07:40Are there ghosts nearby?
07:43Worry not, you're safe.
07:44You might have heard of a phenomenon called singing sands.
07:48As you walk down some sandy slopes, you disturb the grains of sand.
07:52They rub against each other, producing friction that creates those eerie melodies.
07:57With each step, the sand beneath your feet comes alive in a ghostly choir of whistles
08:02and whispers.
08:04The sounds range from low hums to loud, high-pitched wails, as if an otherworldly symphony is echoing
08:10through the dunes.
08:12But of course, scientists have their own very practical explanation.
08:16Attributing this phenomenon to the right atmospheric conditions combined with the unique size,
08:22shape, and moisture content of the grains.
08:25Now you've almost made it all the way to the bottom of the dune.
08:29But beware of slip faces!
08:31When a sand dune reaches a certain steepness, it develops a slope.
08:36That's the very slip face I was talking about.
08:38This slope is usually on the leeward side of the dune and sheltered from the prevailing
08:43wind.
08:44Sand grains start accumulating on the windward side, slowly creeping along the dune's gentle
08:50slope.
08:51At one point, they reach the top, gravity takes over, and they start sliding down, triggering
08:56a cascading effect.
08:58It looks like a miniature sand avalanche, and I wouldn't want to end up in its way.
09:03That's it for today!
09:07So hey, if you pacified your curiosity, then give the video a like and share it with your
09:11friends!
09:12Or if you want more, just click on these videos and stay on the Bright Side!

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