La France et ses montagnes ! (Géologie) - C'est pas sorcier [Intégrale]

Where are they? Higher, Jamy. Look higher. Very funny. There's no one. No, Jamy. Higher. I took altitude. I'm in the Alps. At the top of the Midi needle. Join me. No. Join me. I'm also in the mountains. But in the Pyrenees. The Pyrenees are nice too. Come on, Marcel. We're going down south. I mean, we're going to take altitude. So, Marcel and I are here, halfway between the Pyrenees and the Alps. That way, we won't be jealous. Right? And the Midi needle, where Fred is, is located at the top of his voice in the Mont Blanc massif. I'm getting dizzy. The landscape is magnificent. Look. I'm above the Chamonix valley, on the Midi needle, at an altitude of 3,842 metres. And from here, we have an exceptional panorama of the highest peaks of the Alps. We can see the Drus and the Green Needle all the way down. The Servin, closer to us. The Grandes Jaurasses and the Dents du Géant, which dominate the White Valley. All these peaks rise to more than 4,000 metres in altitude. And of course, the highlight of the show is the Mont Blanc, which culminates at an altitude of 4,807 metres. That's it for the scenery. That's a good start, isn't it? Me, Jamy, I think you should have joined me in the Pyrenees. I'm at the top of Bigorre's Midi needle, at an altitude of 2,877 metres. And from up there, I'm sure I'll have an exceptional view of the entire mountain range. Tell me, Jamy, how did the Pyrenees form? Good question. And the Alps? It seems that the Pyrenees, like the Alps, are young mountains. Does that mean there will be old mountains? Old mountains? That's weird, isn't it? And yet, they do exist. The Armorican Massif, the Central Massif, the Vosges, or the Ardennes are very old dams next to the Alps or the Pyrenees. And for good reason. These massifs are the remains of an ancient chain, the Hercynian chain, whose formation ended 300 million years ago and culminated, believe me, at an altitude of 6,000 metres. Tell me, Jamy, our planet had a funny face at the time, didn't it? That's true. However, the continents didn't always occupy the same place. In fact, the Earth is covered with plates that fit into each other like pieces of a puzzle. Each plate is often made up of a continental crust, like here, and an oceanic crust. In addition, they move because of convection movements that form in the Earth's mantle. So when a continental crust meets an oceanic crust, the denser oceanic crust slides under the continental crust. However, when two continental plates meet, they end up facing each other. The Hercynian chain formed in this way the Pyrenees and the Alps as well. I'm sorry, Sabine, but in my opinion, the Alps formed before the Pyrenees. No, Fred! The Pyrenees formed before the Alps! You don't agree? The Alps first? No, no, no! The Pyrenees first! Stop! You're both right. The Pyrenees and the Alps formed in parallel, but the Pyrenees stopped growing before the Alps. Great! It's my turn! So Sabine is in the Pyrenees, at the top of the Millitbigor peak, at an altitude of 2,877 m. It's the little blue dot. Look! A natural border between France and Spain, this mountain chain extends over 450 km between the Mediterranean Sea to the east and the Atlantic Ocean to the west, on the French side. The highest peak of the Pyrenees is at the tip of my finger. It's the Vignemal. I know it. It's at an altitude of 3,298 m. However, if the summit is French, a part of the base is Spanish. So, the tourists prefer to say that the highest French summit of the Pyrenees is the Long Peak. Very good! So this one is 3,192 m high. And without a doubt, the highest summit of all the Pyrenees is in Spain, 53 km from here. It's the Aneto. No problem. With its 3,404 m, it's the highest. The highest. The formation of the Pyrenees can be summarized in three acts. The first act is about 125 million years ago. At the time, the continents were still far from occupying the place they have today and their contours are very different. Look, France is not even drawn yet. As for Spain, it is still well embedded in the future European continent. 125 million years ago, under the effect of convection that animates the Earth's mantle, the American plate and the European plate separate, giving birth to the North Atlantic. This movement will also lead to the detachment of the Iberian plate which will shift to the East. Second act, 80 million years ago, the African plate which moves to the North interrupts the drift of the Iberian plate to the East and pushes it back to the North. Third act, 70 million years ago, the Iberian plate collides with the European plate. In the end, the Iberian plate will slide under the European plate. Only the meeting of the two will generate a huge storm the Pyrenees' chain which at the time could probably reach 5,000 meters in altitude. And seen from the sky, the Pyrenees' chain is magnificent. More than 200 peaks exceed 3,000 meters in altitude. All these mountains, less high than the Alps, have many reliefs with soft slopes. It is in these areas that the vast herds of sheep and sheep come to take their summer quarters during the transhumance. It is with their milk that the famous Pyrenean cheeses are made. All these herding herds also maintain the beauty of the landscapes. To discover these reliefs, all you have to do is walk. It's a bit of a climb, but at the end of the trail, the view is magnificent. At the heart of the mountains, magnificent lakes are the witnesses of ancient glaciation. On the side of the valley of Osso, the visit can be made on board the small train of Artouste, the highest railway in Europe which climbs at 2,000 meters in altitude. Not far from here, the peak of the Midi d'Osso is the vestige of an ancient volcano that appeared long before the Pyrenees. By forming, this mountain range raised the old Ercinian base which is among the highest. At the heart of this granite, a row of talc was discovered in the 19th century. This exploitation, located in Ariège, is the largest talc quarry in the world. Jamy, since we are in the Pyrenees, I absolutely have to show you an exceptional site. Oh yes, a bridge! First, it's a pretty bridge. Then, raise your eyes, the little voice, and admire the Gavarni Circus, classified as a World Heritage Site by UNESCO. Just that! Ah yes, the Gavarni Circus is located at the heart of the National Park of the Pyrenees, on the Franco-Spanish border. And yes, on the other side, it's Spain! It was worth the detour, wasn't it? The Gavarni Circus is a kind of natural monument, a semicircle whose diameter measures 6 km, and its walls reach 3,000 m high. On average! The culminating point is the peak of the Marboré, 3,248 m. And at the bottom, you will find the highest waterfall in Europe, 423 m. The whole thing is essentially composed of limestone rocks. It was formed during the growth of the Iberian plate. So, aren't they beautiful, my Pyrenees? Yes, they are very beautiful, your Pyrenees, Sabine, but here the scenery is even more majestic. I am currently flying over the Mont Blanc massif, above the Chamonix valley. And under these eternal snows, this massif hides granite that formed 300 million years ago at the same time as the Ercinian chain. There, just below us, are the Grandes Jorasses, a huge granite wall very popular with alpinists. It measures 1,200 m high and 1,000 m wide. And there, well, it is the tooth of the giant that is also part of the mythical summits. But the Alps go well beyond the Mont Blanc massif and extend over about 1,000 km long between the Mediterranean Sea and Austria. And now, the highlight of the show, we arrive above the Mont Blanc. Here, just below us, this roof of Europe attracts every year more than 10,000 alpinists who attempt the ascent. To understand how the Alps were formed, we must go back to the previous animation and focus on the Apulia plate, a plate located in the extension of the African plate. About 80 million years ago, the African plate goes up to the north and pushes the Apulia plate. This plate will get closer to the European plate. Little by little, the ocean that had formed between Europe and Apulia closes. And as Africa keeps pushing, well, the Apulia plate will end up colliding with the European plate. Well, the Alps are born from this collision between the European plate and the Apulia plate. That's it for the big lines. Ah, that's it for the big lines, because it's a bit flat here, Jamy. Precisely. Let's see what it looks like in relief and detail. You recognize here the Apulia plate and here the European plate. Between the two, the ocean we just talked about. Under the pressure of Africa, the Apulia will go up to the north and get closer to the European plate. The ocean crust, being denser than the Apulia plate, the European plate plunges under the Apulia plate, while the Apulia continues to rise to the north by raking the ocean floor and accumulating large amounts of sediment. So, in general, only the ocean crust, denser as we said, plunges under the continental crust. But here, exceptionally, the ocean crust will drag the continental crust, which will also plunge under the Apulia plate. The continental crust will plunge up to a hundred kilometers in depth. This portion of continental crust, and I repeat, less dense than the ocean crust, a piece, will detach and rise to the surface, a bit like a plug rising to the surface of water. Once this portion of crust to the surface, well, the movement of rapprochement will continue, then, under the pressure, the rocks will straighten up and, little by little, the Alps will appear in the landscape. The gigantic cliff of the African continent will move large amounts of rocks and sediments on the European continent. The limestones and clay that once covered the bottom of the ancient ocean have been raised. This is how its massive element of the Chartreuse and the Vercors, or some limestone cliffs, reaches 2,000 meters in altitude. In the Alps of Haute-Provence, the limestone cliffs were simply raised vertically. Elsewhere, the cliffs lay under the forces of compression. And there, they took the form of a gigantic cycling track. This place was nicknamed the Velodrome. Even more surprising, these rocks are the remains of volcanic activity that reigned at the bottom of the ancient ocean. The lava, cooled by the water, froze in the form of a cushion called the Pilo Lava. Today, it is located at more than 2,500 meters in altitude in the Chenaillet Massif, near Brienzon. The Mont Viseau, located at the Franco-Italian border, is also a part of the ancient ocean that now culminates at 3,841 meters. As for this peak, the Servin, it is a piece of the African continent in the middle of the Swiss Alps. But do the Alps continue to grow? The question divides scientists. It would seem that some areas always grow by a millimeter per year, like the Belledonne Massif, while others disappear. What is certain is that these mountains are not eternal. The Alps will one day be what is called a geological peleplain. After the birth of a mountain chain, a mountain chain will live for 200 million years between sedimentation in the sea, the collision between two continents, the day the collision will stop, the mountain chain will erode and we will end up with a plain. It is the life and death of a mountain chain. Don't panic, we still have some time to enjoy the show. I am still in the Mont Blanc Massif, where the most spectacular and grandiose glaciers in Europe are located. To name just a few, there is the Besson Glacier, which is famous for its steep slope and its large crevasses, but there is also the Argentière Glacier, the Tour Glacier, the Giant Glacier and of course the famous Mer de Glace, the largest glacier in France which spans 12 kilometers. You should know that in total, glaciers in the Alps cover an area of about 3,000 square kilometers. To give you an idea, the Alps are about the equivalent of a French department. Of course, the size of the glaciers varies depending on the climatic periods and currently, we see a decline of the Alpine glaciers. Yes, during geological times, the climate has not always been the same. It has regularly oscillated between cold glacial periods and softer interglacial periods. For two million years, the Alps have undergone about twenty glaciations during which huge glaciers covered the valleys. Twenty-six thousand years ago, a huge glacier more than one kilometer thick was located just above the current city of Grenoble. Imagine that 50,000 years ago, the Rhône Glacier descended into the Lyonnaise Valley near the Saint-Exupéry airport. All these large glaciers have completely shaped the Alpine landscape. The valleys were cut by the rivers and torrents and the glaciers flowed into these valleys, enlarged and excavated. When the glaciers left, they abandoned large lakes filled with sediment. These lakes were filled for 10,000 to 15,000 years. At the beginning of the glaciers, there were large collapses. That is, the valleys' walls were pressed against the ice, like the Arve Valley and the Isère Valley in Grenoble. Once the glaciers disappeared, the walls decompressed. These decompressions generated large collapses. These collapses occurred for millennia since the glaciers disappeared 20,000 to 15,000 years ago. By withdrawing, the glaciers abandoned large amounts of dust produced by the action of the ice on the rocks. This is called leuce. It was transported by the winds several hundred kilometres away. Today, this leuce forms the fertile land found in Burgundy, Dordogne or Normandy. Thank you for the wind! See you later! Do you know what I learned, Jamy? It seems that the Alps shaped all of France, from the southeast to the northwest. It's incredible! Hey! The Pyrenees also shaped all of France, from the southwest to the northeast. And that's also incredible! Hey, easy! Let's calm down. In fact, the crust on the Earth's mantle reacts a bit like a tablecloth. If I push this tablecloth, a series of folds form behind each other, folds whose amplitude is less and less large as we move away from where I pushed. Well, the formation of the Pyrenees and the Alps had exactly the same effect on the French landscape. Thus, when we read our landscape in the southeast-northwest direction, we see that the Alps form a hump. At the level of Lake Leman and Lake Neuchâtel, there is a hollow, then a new hump at the level of Jura, a hollow that extends from La Bresse to the Rhône Valley, a new hump from the central massif to the Vosges through Bourgogne and the Morvan, and finally a depression that extends from Poitou to Champagne through Sologne. Now, starting from the Pyrenees and going up to the northeast, we find the same pattern. A crest at the level of the Pyrenean chain, a depression at the level of the Aquitaine Basin, then a line of crests from the central massif to the Armorican massif, a hollow at the level of the Parisian Basin, and it goes up on a line that extends from the Boulonnais to the Ardennes. The Alps and the Pyrenees have also modeled Corsica, which was once linked to the European continent. And yes, they form with the Pyrenees a huge mountain chain. Corsica has been detached for 25 million years and has taken with it part of these mountains, including the highest, Mount Chinto, culminating at 2,710 metres. It is possible to discover all these massifs on foot by following the GR 20, which crosses the island from part to part at the heart of the regional park. All these high mountains end on the west coast in huge cliffs that plunge directly into the sea, like the Calanques de Piana or the Scandola reserve in the region of Porto. By detaching, Corsica has also embarked on part of the Alps, which can be found in the northeast of the island. But the mountains there are less elevated than in the centre. The splendid cliffs of Bonifacio, south of the island, were formed by the accumulation of marine sediments at a time when Corsica was partly underwater. This island of beauty is in fact a concentrate of all the geological landscapes that can be found on continental France. So if the Alps have largely modified the French geological landscape, another very surprising thing is that the formation of the Alps would also have been at the origin of the Auvergne volcanoes. Let's take a closer look. Papa Delta Charlie, do you copy? This is Libellule. Sorry Fred, but I'm already in the helicopter with Sabine. Too late! We are flying over the volcanoes of the Chêne des Puys, which is at the door of Clermont-Ferrand in the department of the Puy de Dôme. The Chêne des Puys is a massif composed of 80 small volcanoes stretching on a north-south axis 40 km long. These volcanoes are monogenic volcanoes, that is to say that they appeared after a single eruption. An eruption that lasted from a few days to a few months. There are several types of monogenic volcanoes. The first is the Scorry cone, like here the Puy des Goules. Here, just below, we have the Jumeaux, the Puy de la Vache and the Solace. They have a particular shape. Their crater is not closed. It is said that they are hollow because a lava flow took part of their flank. The second type of monogenic volcano is the Dôme. We recognize it by its typically rounded shape. The most spectacular of them in Auvergne is the Puy de Dôme. With its 1,465 meters of height, it dominates the entire chain of the Puy. This is why telecommunications But tell me, Jamy, how did the Alps cause the birth of these volcanoes? Without going into detail again, the Alps were formed following the slide of the European plate under the Apulian plate. By diving under its own weight, this European plate stretched and shrank. Faults, as you can see, appeared, which then caused collapses. We find these collapses today in Auvergne, at the level of the Limagne plain. This European plate, exerting less pressure at the level of the finest areas, the rocks of the mantle that were below started to melt and rose to the surface passing through the faults that were above and formed volcanoes. Volcanoes that are quite old since they were formed between 25 and 15 million years ago. But the story does not stop there. By diving, the European plate brought the rocks of the terrestrial mantle towards it. It pushed them back under its finest area. The pressure at this place being less important, well, these rocks also started to melt and rose to the surface passing through the faults that were just above. This phenomenon is at the origin of more recent volcanoes. The Mont d'Or, for example, the volcanoes of the Cantal or the Chêne des Puys. The landscapes of Auvergne were entirely modeled by volcanism. These plateaus are old lava flows whose very hard volcanic rocks are better resistant to erosion. In the Monts-du-Saint-Cy region, we find magnificent volcanic orgs. The lava cooled down, retracted and took the form of long prisms. These orgs were used to make tiles covering the roofs of the region. As for this chapel of the Puy-en-Velay, it was built on a huge rock which is in fact the chimney of an old volcano. We also find volcanism on the walls of houses and the cathedral of Clermont-Ferrand which was entirely built with volvique stone taken at the heart of a lava flow. Volcanoes are also at the origin of many lakes, such as the Pavin, which formed in the crater of the youngest volcano in Auvergne which erupted just 7,000 years ago. If all these volcanoes are now extinguished, they could one day wake up. However, we don't really know when. The mantle at depth is abnormally hot. It's hotter than under the Paris basin or in the Languedoc. So we have heat here which explains the eruptions we had earlier and tells us that we will have other eruptions. It could be tomorrow, it could be in 1,000 years, 10,000 years, we don't know. The only thing we know is that we will be warned by earthquakes The volcanoes of Auvergne may not have finished writing their history. Where are our mountaineers? What's that? What's going on? I don't know. I think the mountain is shaking! It's moving here too! It's shaking! What's going on? Don't worry, Marcel and I decided to create a new channel. Not a TV channel, a mountain channel. The Alpine or the Pyrenees. It's the same. Stop that right now! Stop! If you want to see this show again and have more information, you can find us on the C'est Pas Sorcier website at www.cepasorcier.com