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Les Conséquences d'un impact météoritique

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Les Conséquences d'un impact météoritique

  1. 2. LES CONSEQUENCES D’UN GROS IMPACT
  2. 3. Plan Introduction Energie et onde de choc Nuée ardente et grand incendie Modifications climatiques Conclusion Gaz nocifs Obscurité Incidences biologiques et crises Tsunami
  3. 4. Introduction Pour un bolide de 10 km de diamètre, pesant 10 12 tonnes et s’écrasant à la vitesse de 20 km/s, l’énergie libérée est de 10 24 joules
  4. 5. Les phénomènes associés à ce type d’ impact sont de plusieurs types : <ul><li>chaleur </li></ul><ul><li>explosion </li></ul><ul><li>effet de souffle </li></ul><ul><li>onde de choc </li></ul><ul><li>séismes </li></ul><ul><li>raz de marée </li></ul><ul><li>- embrasement de la végétation </li></ul><ul><li>émissions de vapeurs toxiques </li></ul><ul><li>nuée ardente </li></ul><ul><li>poussières et gaz </li></ul>
  5. 6. Les principales conséquences sont alors : <ul><li>vie détruite dans le rayon immédiat de l’impact </li></ul><ul><li>modifications climatiques </li></ul><ul><li>incidences biologiques ultérieures </li></ul>… et même crises …
  6. 7. Energie et onde de choc L’onde de choc (dans l’air) se propage de façon rayonnante et sa vitesse décroît progressivement de 20 km/s au point d’impact jusqu’à devenir nulle à 1000 ou 2000 km. Le vent s’inverse alors car les basses pressions exercées à la base du cratère rappellent les masses d’air. Forêts et animaux sont emportés et meurent dans un rayon de 500 à 1000 km.
  7. 8. Tunguska le 30 juin 1908 … en 1938, à 200 km … Végétation couchée
  8. 9. Nuée ardente Un impact en milieu continental donne lieu à une gigantesque nuée ardente dans laquelle se mêlent : - des gaz chauds issus de la vaporisation des roches de la météorite elle-même et des roches impactées; - des éjecta constitués de débris de verre provenant de la fusion des roches et une quantité considérable de fragments et de poussières de roches provenant de l'explosion des roches impactées.
  9. 10. Une fois en l’air, les éjecta retombent et transmettent leur énergie cinétique et interne, par rayonnement et par friction, aux gaz de l’atmosphère. Une partie de cette énergie se dissipe vers 60 km d’altitude, la moitié est irradiée vers l’espace, une petite proportion est absorbée par les gaz atmosphériques eux-mêmes et le reste atteint le sol, échauffant celui-ci à une température moyenne de 400°C.
  10. 11. Avec un tel échauffement du sol, des incendies d’échelle planétaire se déclarent (plus d’un tiers de la biomasse peut alors brûler) <ul><li>Vie détruite dans un rayon de 200 km autour du point d’impact </li></ul><ul><li>Modification du climat dans un rayon d’au moins 500 km </li></ul>Combustion d’arbres … Suie … Hydrocarbures aromatiques …
  11. 12. Gaz nocifs Les températures extrêmes régnant au sein de l’onde de choc provoquent la dissociation des molécules N 2 et O 2 qui se recombinent en oxydes nitriques NO et NO 2 hautement toxiques Conséquences : détérioration des feuilles et des graines chez les végétaux, grave troubles respiratoires chez les animaux, voile brunâtre autour de la Terre et bloquant la lumière
  12. 13. NO 2 se combine aussi à H 2 0 pour donner HNO 3 Conséquences : une acidification des eaux, le plancton est ravagé (et les chaines alimentaires qui en dépendent s’effondrent), la flore est dévastée … La mobilisation de O2 a également pour conséquence la chute du taux d’ozone permettant alors un passage accru des rayons ultraviolets …
  13. 14. Libération d’une grande quantité de CO 2 et de soufre sous forme SO 2 dans l’atmosphère (due à la vaporisation de certaines roches) : c’est ce que certains auteurs appellent l’effet de cible. SO 2 associé à la vapeur d’eau H 2 O produit de l’acide sulfurique H 2 SO 4 qu’on retrouve sous forme de cristaux glacés dans la haute atmosphère, renvoyant le rayonnement solaire vers l’espace pendant plusieurs mois, d’où un refroidissement climatique global.
  14. 15. H 2 SO 2 retombera au sol plus tard en pluies acides à cause du réchauffement du à l’effet de serre provoqué par le CO 2 dégagé.
  15. 17. Obscurité Les éjectas les plus fins restent en suspension dans l’atmosphère et forment un nuage opaque qui entoure la terre. L’obscurité ainsi créée dure quelques semaines à plusieurs mois : la photosynthèse en est dramatiquement bloquée.
  16. 18. Modifications climatiques Quand on voit les conséquences d’une « petite » éruption, comme celle du Pinatubo en 1991 …
  17. 19. … on imagine sans peine ce que peut provoquer un voile de poussières du à un impact … avec dérèglement climatique , chutes de températures de 0,1° à 1,5° (selon la latitude) pendant trois ans, dues aux aérosols et aux gaz …
  18. 20. <ul><li>Réchauffement brutal (quelques heures) </li></ul><ul><li>Refroidissement intense (quelques mois) </li></ul><ul><li>Réchauffement par effet de serre (quelques années à milliers d’année) </li></ul>Trois étapes :
  19. 22. Météorite Nuage de gaz et poussières Incendies Effet de voile autour de la planète Variations climatiques Ralentissement sévère de la photosynthèse Réchauffement local Refroidissement général Réchauffement général Extinction différentielle Disparition des végétaux Disparition des animaux inférieurs Disparition des animaux supérieurs Spores et graines en dormance Survie Mangés par petits mammifères Survie Pluies acides Modification des eaux océaniques
  20. 23. Tsunami Un impact en milieu océanique donne lieu à un gigantesque tsunami … simulation !
  21. 24. Il émet, en plus des poussières, un énorme volume de vapeur d’eau, entraînant un effet de serre car elle absorbe de nombreuses longueurs d’ondes des rayons infrarouges émis par la terre.
  22. 25. Incidences biologiques <ul><li>immédiates : effet de souffle, chaleur … </li></ul><ul><li>à court terme : gaz, empoisonnement … </li></ul><ul><li>à moyen terme : refroidissement … </li></ul><ul><li>à long terme : effet de serre … </li></ul>
  23. 26. Les effets induits … Enormes quantités de poussières, gaz et aérosols … Obscurité …  refroidissement  réchauffement Modification du chimisme des eaux  baisse de productivité cf.  18 O cf.  13 C cf.  CaCO 3  conditions anoxiques
  24. 27. Rupture de la chaîne trophique … Ralentissement sévère ( voire arrêt ) de la photosynthèse Extinctions différentielles … Les répercussions sur la vie … … et parfois crise biologique
  25. 29. La participation d’un (ou de plusieurs) impact(s) dans une crise est : - confirmée pour la crise K/T - probable pour la crise fini-triasique - envisagée pour la crise fini-dévonienne - envisagée pour la crise fini-permienne - envisagée pour une crise pré-édiacarienne
  26. 30. Crise K/T, impact de Chicxulub (Yucatan), 170 km
  27. 31. Crise fini-triasique, catena de Manicouagan, 180 km
  28. 32. Crise fini-dévonienne, Siljan en Suède, 52 km
  29. 33. Crise fini-permienne, impact en Antarctique, 480 km
  30. 34. Crise pré-édiacarienne, impact d’Acraman en Australie Diamètre : 90-150 km Age : 570-580 Ma Ejecta à plus de 500 km
  31. 35. Conclusion Mais tous les impacts n’ont pas donné lieu à des extinctions Et toutes les extinctions ne sont pas liées à un impact.
  32. 36. Références 1) Francis Lethiers (1998) - Evolution de la biosphère et événements géologiques, Gordon and Breach Science Publishers, 321 p., 224 fig.

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