Les scientifiques ont longtemps été perplexes sur la façon dont une immense dune de sable en forme d'étoile s'est formée dans le désert du Sahara. Cette énorme dune, appelée « dune étoilée », a des bras qui s'étendent dans différentes directions, contrairement aux dunes ordinaires qui se déplacent simplement dans une seule direction. Après de nombreuses recherches, les scientifiques ont découvert que ces dunes sont façonnées par des vents soufflant de multiples directions au fil du temps. Les variations des schémas des vents font que le sable s'accumule sous une forme étoilée au lieu de former une seule crête. Cela a été un mystère difficile à résoudre car les vents du Sahara peuvent être vraiment compliqués. Mais maintenant que les scientifiques ont trouvé la solution, ils comprennent comment ces dunes uniques se forment ! Animation créée par Sympa. ---------------------------------------------------------------------------------------- Musique par Epidemic Sound https://www.epidemicsound.com Pour ne rien perdre de Sympa, abonnez-vous!: https://goo.gl/6E4Xna ---------------------------------------------------------------------------------------- Nos réseaux sociaux : Facebook: https://www.facebook.com/sympasympacom/ Instagram: https://www.instagram.com/sympa.officiel/ Stock de fichiers (photos, vidéos et autres): https://www.depositphotos.com https://www.shutterstock.com https://www.eastnews.ru ---------------------------------------------------------------------------------------- Si tu en veux encore plus, fais un tour ici: http://sympa-sympa.com
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00:00You fly over the Sahara desert aboard a small light aircraft when you see something really strange.
00:07It seems to belong to a world very different from the one that is under your eyes.
00:12To the ocean.
00:13But what ocean can be big enough to shelter such a monstrous starfish?
00:18You go down and you realize that it is not a starfish,
00:22but an imposing dune whose shape is quite unique.
00:26And there are others.
00:27From afar, these magnificent sand structures look a bit like pyramids.
00:32They have a pointed top and star-shaped branches.
00:35With the name of starfish, they are the largest and most complex dunes in the world.
00:41Researchers have discovered similar formations on Mars and even on Titan, one of Saturn's moons.
00:48Starfish are not very rare.
00:50They are particularly common in the southeast of Morocco, near the border with Algeria.
00:56It is this region that you are flying over.
00:59And the dune that amazes you is called Lala Lalia.
01:04It is probably the most famous of them all.
01:07A giant 100 meters high, scientists think it formed in less than a thousand years.
01:13What would you say to go explore these singular formations?
01:17This is an ideal place to land.
01:20Starfish, of course, owe their name to their shape.
01:24They are found in regions where the winds change direction throughout the year.
01:29They are present in different parts of our planet.
01:32However, there is only one official starfish in the rock record,
01:37where all geological data relating to the evolution of the Earth are assigned.
01:41The starfish in question is in Scotland, and it is very old.
01:46It is about 250 million years old.
01:49That's all we know about these strange formations.
01:53Because we don't know what to look for to identify them.
01:56Another problem, starfish often form in remote places,
02:01which makes them extremely difficult to study.
02:04Imagine that you reach the nearest village,
02:07that you then travel, I don't know how many kilometers, to the dune itself,
02:11and that you then have to climb several hundred meters of slippery sand.
02:15Would you try the adventure?
02:17But since you have already landed and the dune is right in front of you,
02:20why not?
02:21After all, you have arrived in a dune field called Erg Shebi.
02:25Fortunately for you, this area has long been a very popular place for tourists.
02:30Look, in the distance, there are hotels and even a practical road.
02:35Scientists who have examined Lala Lalia before you,
02:39have used a georadar to find out more about this sand formation.
02:44This radar allows you to detect the slightest size differences
02:47between the grains of sand and the water content under the surface of the dune.
02:51This is how it is possible to create an image of the interior layers of the dune.
02:56Be careful, it was not easy.
02:58Do you see this trench?
03:00Researchers have dug it to take samples of the sand buried there and analyze them.
03:05A really interesting process.
03:07Indeed, when it is buried deeply,
03:09the quartz contained in the sand absorbs the radiation
03:12from the natural sources of our planet.
03:15We can therefore compare the grains of quartz to tiny rechargeable batteries.
03:19They can store the energy they draw from these radiations.
03:22Once in their laboratory,
03:24the researchers make sure that the grains release this energy,
03:27which manifests itself in the form of light.
03:30Scientists then measure this luminosity,
03:33which tells them when the sand was last exposed to sunlight.
03:39When the experts who dug these trenches looked inside Lala Lalia,
03:44they were amazed.
03:46The huge formation was actually very young.
03:49For a dune, of course.
03:51We would expect a dune several hundred meters high to be quite old.
03:55That is, it dates back several thousand or tens of thousands of years.
03:59But this is not the case here.
04:01The upper part of the dune was 900 years old very recently.
04:04But look at the sand near the base of the dune.
04:07Yes, right there.
04:09It was buried about 12,000 or 13,000 years ago,
04:12like the oldest dunes in the region.
04:15These old dunes were active for several thousand years,
04:18then something went wrong.
04:20For 8,000 years, the sand did not accumulate in this place.
04:24A mystery?
04:25Scientists may have an explanation.
04:28A first period of calm took place at a time
04:31when the climate was warm and humid.
04:34Yes, we are talking about the Sahara.
04:36It dates back to about 11,700 years.
04:39It was the end of the last glacial period
04:42and the beginning of a new era, the Holocene.
04:45And guess what?
04:47The Sahara then became green.
04:49It started to bloom like a pretty botanical garden.
04:52Plants began to grow everywhere, stabilizing the sand.
04:56If you had visited the region at this time,
04:58you would not have recognized it.
05:00However, you would probably have met humans
05:03wandering from swamp to swamp.
05:05And hunting for food.
05:07Researchers found fragments of pottery
05:09and stone tools on one side of the Alalaliya.
05:13Yes, the sand dunes form and move throughout their life,
05:17swallowing and protecting the remains
05:19that we leave on our way.
05:21You just have to dig enough
05:23and you will find something
05:25that will open a window to the past.
05:27It can be ancient fossils,
05:29shells, corals,
05:31or prehistoric plants.
05:33These fossils provide us with precious information
05:36on the history of our planet
05:38and on the beings who inhabited these regions.
05:41This may seem surprising,
05:43but in the Sahara desert,
05:45scientists discovered marine reptiles
05:47and fossilized fish
05:49well preserved in the lower layers
05:51of these ancient sand dunes.
05:53These are the remains of the time
05:55when the sea covered the Sahara
05:57millions of years ago.
05:59Anyway, if you want to survive,
06:02it's time to leave this ancient paradise.
06:044,000 years ago,
06:06the wet period was over
06:08and the Sahara dried up again.
06:10Note that our dune
06:12did not start to form right away.
06:14For a while,
06:16the sand moved here and there,
06:18but did not accumulate.
06:20It is also possible
06:22that the dune began to develop
06:24in another place.
06:26It's an interesting theory.
06:28Scientists have recently discovered
06:30that Lala, the lia,
06:32moves about half a meter per year
06:34and that it accumulates
06:36about 6,400 tons of sand each year.
06:38If your trip to the dunes
06:40has inspired you
06:42and that you now want to explore others,
06:44I have bad news to announce.
06:46Alas,
06:48even if there are dunes
06:50starry all over the world,
06:52they are very difficult to detect.
06:54The problem
06:56is that they are huge,
06:58but that they have no distinctive sign.
07:00Therefore,
07:02you need very large stone beds
07:04exposed if you want to have
07:06a sufficiently wide view to identify
07:08a starry dune.
07:10And there are certainly places
07:12where it is possible to carry out such research.
07:14But you will need to be able to recognize
07:16a whole set of different characteristics.
07:18And being very attentive
07:20and studying a lot,
07:22you may one day come across
07:24one of these mysterious starry dunes.
07:26Comment below if you want
07:28to go in search of one of these wonders.
07:30In the meantime,
07:32let's continue our journey.
07:34You decide to rest
07:36and you begin to descend along the dune.
07:38It is then that you hear strange
07:40and disturbing noises.
07:42Are there ghosts around?
07:44Do not worry, you are safe.
07:46Have you ever heard of a phenomenon
07:48called singing sand?
07:50By descending a sandy slope,
07:52you move the grains of sand
07:54against each other,
07:56creating this sinister music.
07:58At each step, the sand under your feet
08:00is animated like a heart of ghosts
08:02whistling and whispering.
08:04These noises go from deaf rumbling
08:06to high rumblings.
08:08It is as if a symphony of another world
08:10resonated in the dunes.
08:12But, of course, scientists
08:14have their own explanation.
08:16They attribute this phenomenon
08:18to the right atmospheric conditions,
08:20as well as to the size, shape
08:22You have almost reached the foot of the dune,
08:24but be careful not to slip.
08:26When a sand dune reaches
08:28a certain degree of inclination,
08:30it forms a slope
08:32and it becomes very slippery.
08:34This slope is generally
08:36on the side under the wind of the dune
08:38and sheltered from the dominant wind.
08:40The grains of sand begin to accumulate
08:42on the exposed side to the wind,
08:44slowly climbing along
08:46the soft slope of the dune.
08:48At some point, they reach the top.
08:50Gravity takes over
08:52and they begin to slide down,
08:54triggering a cascade effect.
08:58It looks like a miniature avalanche.
09:00And it is better not to be
09:02on its way.