Touchez du doigt la fin des dinosaures – Sous Nos Pieds #3


This little dot, right here, is Earth. Every human being who changed the course of history did it on this small planet, which seems so fragile when captured from space. And this fragility is exactly what we should thank for our existence, and for the diversity of our mammal cousins. For our ancestors, after having spent 200 million years in the shadow of terrible lizards, finally had the chance, 66 million years ago, to take over the niches left vacant by the extinction of the dinosaurs. And we could think that such a shocking mass extinction requires the impact of a gigantic meteorite. But in reality, a small rock no bigger than the little Isle of Ré was enough to precipitate the end of these incredible creatures. Almost everyone knows this by now, even the younger ones. But how did we come up with this idea ? You may not know it, but the answer is right behind you when you’re tanning on the beaches of Basque country. Take your magnifying glass, your hammer, and let’s go head to the field. If we want to know what happened 66 million years ago, we need to find rocks of this age. On the geological map of France, we spot in the southwestern region Cretaceous rocks, which formed when dinosaurs were still alive. We can also find younger rocks, in which you will not find any dinosaur remains. If we want to understand what happened between these 2 periods, we need to study the boundary between these layers. Let’s go, we’re heading to Basque country. We are here on a beach near Bidart. All of these rocks are sedimentary, which means that they were formed by the accumulation of sand, mud, or small fossils. When these rocks were formed, they were still horizontal. Every layer was once the bottom of an ocean roamed by marine reptiles. These layers were then covered by younger ones, before being tilted by the formation of the Pyrenees’s mountains. Now that they are found at Earth’s surface, we can thus study the sea floor of that time. To the South, we have Cretaceous rocks, which are thus older than 65 million years. On top of them, we have Paleogene rocks, which are younger than 66 million years. If we come closer, we find a darker layer, a layer of clay. This is the boundary between the Cretaceous and the Paleogene. When Cretaceous layers settled, dinosaurs, ammonites, or flying reptiles were still alive. But when you dig through Paleogene rocks, you will not find any such fossil. These species no longer existed. It has now been a while since we noticed that this boundary is characterized by a mass extinction, but for a long time we had no idea why. It’s only when the 70s were coming to an end that scientists started to find something interesting in this boundary. When we measure the amount of chemical elements in these rocks, here’s what we find. There is a lot of Iridium. And this spike, this layer rich in Iridium, we find it all over the world. But apart from this anomalous, thin layer, Iridium is rare on Earth’s surface. If Earth’s crust contains so little Iridium, it’s because it’s a heavy element, and it sunk, like Iron and Nickel, towards Earth’s core when the layers of our planets were formed. But in meteorites, which neither possess a mantle nor a crust, Iridium can still be found in great quantities. For Alvarez father and son, for surface layers to contain such high amounts of Iridium, there had to be a lot of dust coming from a pulverized asteroid, which would then have spread Iridium around the world. This dust would have then fall unto the ocean, forming around the world a very thin layer of sediments, such as clays. But they still did not have a culprit. For an asteroid of the size of the Isle of Ré, there would be a crater the size of Brittany (western France). But for 66 million years, it had enough time to be eroded by streams or covered by sand. This is exactly in times like this that communication between scientists is important. In Mexico, geologists are starting to find strange things. During the 70s, two geologists are tasked with understanding the structure of rocks underneath the water that fills the gulf of Mexico. They start to notice small density differences in the crust, which can be explained by changes in thickness, which are arranged as a semi-circle. In order to understand what is causing this shape, They search in old documents to see if similar types of work were done on land. And they finally stumble upon the answer to their question. In results that petroleum companies were keeping secret, they find the second half of their circle. This is the first time that this shape is revealed to mankind, but another decade would pass before this work was published by a Canadian geologist. We finally have the culprit. The scar left by the impact of a large meteorite. When this crater is drilled, geologists find between the young sediments and the fractured rocks, rocks that are made of fragments resulting from the ejection of molten crust. When the meteorite collided with the crust, the energy released was strong enough to melt the crust, sending in the sky drops of molten rocks. These small drops will quickly cool in the cold atmosphere. But when molten rocks cool rapidly, meaning that the chemical elements won’t have time to arrange themselves into minerals, they will form glass. An amorphous material, without any organisation between the atoms. In the few minutes that followed the impact, a rain of small glass balls fell upon the dinosaurs, who had no idea that an enormous tsunami was heading their way. A gigantic wave, travelling at 800 km/h and which leaves behind layers of sand containing these small glass balls. Some of these small balls have even be uncovered in some truly incredible rocks. In North Dakota, paleontologists have found, just underneath the Iridium layer, the deposits left by a river flowing during day. In these deposits, they found an enormous amount of fishes buried in the same position. But when they looked closer, they found these small glass balls in their gills, as well as in small bits of amber. These small pieces of molten rocks, ejected from the crust on which Mexico stands today, fell on North America like a rain of small glass balls. Some of them fell on tree sap, and they remained trapped until this day in amber. For a while, fishes continue to swim for food, unaware of the fact that the small balls coming in through their gills are the first sign of their incoming doom. During that time, the seismic waves that were generated by the impact are moving through the crust. And when these waves reach the river, they will unleash a seiche, a back and forth movement in the water. This phenomenon, we cans till observe it today during earthquakes. But if we think about the fact that this meteorite probably caused an earthquake of a magnitude between 10 and 12, it’s hard to imagine the power of this seiche. A back and forth movement of an extreme strength, which is going to shake the mud and sand that once draped the river bed, and it will bury its inhabitants with the small glass balls. After a while, the dust will settle, leaving around the world a very thin layer of Iridium. On the beaches of Basque country, you can place your finger on one of the most incredible event in the history of our planet. The impact of a meteorite as big as the Isle of Ré, which left a gigantic scar buried under the gulf of Mexico. Within a few seconds, this meteorite unleashed a devastating tsunami, pulverized Earth’s crust and sent flying drops of glass balls thousands of kilometers around. It also produced a lot of Iridium-rich ash. This dust blocked the rays of the Sun, which caused a drop in temperatures that eventually lead to the extinction of 75% of our planet’s species. If you’re looking for a memorial to remember the fallen dinosaurs, The beaches of Basque country are the best spot in France to do so. You can share the video if you enjoyed this french trip to the Cretaceous. I hope to see you for the next episode, and don’t forget to watch where you put your feet.

6 thoughts on “Touchez du doigt la fin des dinosaures – Sous Nos Pieds #3

  1. J'adore toujours, la voix posée, les petits éléments de mise en scène, les animations… Une vulgarisation à la portée de tous et toutes ^^ Vraiment du très bon boulot, continues comme ça !

  2. Je viens de m'abonner et une nouvelle vidéo est là. Super travail encore une fois, j'ai appris plein de choses que j'ignorais.

  3. Saurez-vous retrouver à qui appartiennent les visages qui défilent au début de l'épisode ?
    Un indice : ils sont loin d'être étrangers à la science …

  4. C'est incroyable ça! Je vais allez voir ça moi.. MERCI, au top , de super petits reportages quel boulot! vivement les prochains

  5. Super boulot !
    On parle bien de la disparition des dinosaures non aviaires ?
    Bonne continuation !!!

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