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Vous êtes dans le forum : BAC +2 - Langues  
Presentation orale/correction
Envoyé par: Muma (Adresse IP conservée)
Date: mar. 3 novembre 2009 18:01:00

Bonjour,


vendredi je passe un oral, et voici le texte que j'ai rédigé. Je ne suis pas très fort en anglais, donc si quelqu'un saurait m'aider en le corrigeant, ca serait très sympathique. Je vous remercie d'avance.




The extinction of the dinosaurs


A lot of people thinks that dinosaurs disappeared when a asteroid hit the earth in Mexico about 65 millions years ago. It’s the most widespread explanation. The crater is there. It’s 180 km in diameter. Specialists suggest that the impact formed a cloud of dust called “nuclear winter”. This cloud would stopped the plants photosynthesizing. Then animals starved to death.
But it’s possible that it was false. Professor Sankar Chatterjee has found another crater called Shiva. Shiva is bigger than the Chicxulub crater in Mexico. Shiva is 500km in diameter. The explosion that caused it may have been 100 times the size of the one that created Chicxulub.
Dr Chatterjee presented his lasted findings on Shiva to the annual meeting of the Geological Society of America in Portland on October 18th. He explains that he discover an underwater mountain called Bombay High of the coast of Mumbai. This mountain measures 5km from sea bed to peak. It is surrounded by Shiva crater rim. It was formed when the dinosaurs disappeared. Sankar Chatterjee says that no force other than the rebound from an impact could have produced this mountain so quickly. And this powerful impact certainly smashed the ecosystems around the world.


In fact, paleontologists have really never been agree on the cause of the mass extinction of the Cretaceous. One confounding factor is that the late Cretaceous was also a period of great volcanic activity. In India, was discovered large fields of basalt called the Deccan Traps. These fields were created by eruptions. Before the discovery of Chicxulub, specialists thought that these eruptions were the cause of the dinosaurs extinction. After its discovery, we thought that these eruptions didn’t cause the death of the dinosaurs, but they weakened the biosphere before the Chicxulub impact.
There are also anomalies in the pattern of extinction. The first of these is that the sensitive animals like birds, turtles and frogs survived at this horrible impact. Moreover, studies show that many Cretaceous species disappear both before and after the impact.
Ironically, it was while he was studying the Deccan Traps that Sanker Chatterjee came across the evidence for Shiva. First, he discovered dinosaur nests between lave flows. It was the proof that they were acclimated at the volcanic activity. Then, about 65 millions years ago, a layer of lava nearly 2km thick suddenly appears. He wonders then what have caused this sudden volcanic surge. After he was searching in different documents, he found that India was the site from a cosmic collision.


Further examinations show that the crater is rich in shocked quartz and iridium, minerals that are commonly found at impact sites. Most important, the rocks above and below Shiva date it to 65 millions years ago. Sanker think that an object 40km in diameter hit the earth in Shiva. This Shock would also have caused the breaking of Seychelles away from India. These islands have long looked anomalous. They more look like parts of continents rather than real oceanic islands.
But there is now another question. Why there is but a single ejecta layer of iridium and shocked quartz around the world ? One answer might be that the two impacts were simultaneous. The objects which created Shiva and Chicxulub would be the daughters of a comet that had broken in space. Other smaller craters in Ukraine were discovered, these impacts could therefore be connected with Shiva.
Extensive dating research at Chicxulub now suggests that the crater was created 300 000 years before the death of the dinosaurs, meaning really should be two ejecta layers. That there are not could be explained by the fact that the accumulation of sediment in most rocks is so slow that the two layers are, in effect, superimposed.
The final result of these different studies is of a strange set of coincidences. A period from 300 000 years in geological history is like a eyeblink for us. The fact that two biggest impacts happened in this period is incredible. Furthermore, this period coincided with one of the largest periods of vulcanicity int the pas billion years.


Finally, we can say that is thus just a string of the most bad luck that really killed the dinosaurs.





Encore merci à ceux qui pourront m'aider!




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