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Great extinctions

  1. Jun 11, 2003 #1
    what do you know about the permian extinction event. the creataceous extinction was probably caused due to an asteroid. but what about the larger permian extinction?all views welcome.
     
  2. jcsd
  3. Jun 12, 2003 #2
    Current ideas point to several causes:
    Forming and break-up of Pangea,
    Eruption of the Siberian trap
    A comet/meteor (but this is always included in an extinction event!)

    Global sea level was doing nothing dramtic at the time and temperatures were on the increase after the end Carboniferous Ice Age in the southern hemisphere (Geological History of Britain & Ireland, Woodcock and Strachan, 2000). So I doubt these two could be causes.

    There's is a new book by Mike Benton (Professor at Bristol University) called "The day life nearly ended", which is all about this event. Anyone read it yet? He gave a talk in Edinburgh a few months ago as part of the Science festival, but unfortunately I was in Cambridge at the time - typical!
     
  4. Jun 12, 2003 #3

    Phobos

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    Good question. Don't know. A quick google search adds this to the list of possibilies...

    glaciation on Gondwana

    rapid warming and severe climatic fluctuations produced by concurrent glaciation events on the north and south poles

    basaltic lava eruptions in Siberia (I assume this is what rdjon meant)

    http://hannover.park.org/Canada/Museum/extinction/permcause.html
     
  5. Jun 12, 2003 #4

    LURCH

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    I tend to lean towards the collision theory. I believe that collision events occur periodically as the Earth passes through the thick of the gallaxy, and that all the major extinction events are caused by these collisions. It just makes the most sense to me.
     
  6. Jun 27, 2003 #5
    The collision theory is the favourite - but it's very interesting to note that there's also a mass volcanic event at that time, in Siberia, when the Cretaceous extinction of 65Myrs ago also coincided with the eruption of the Deccan Traps in India.

    In fact, I'm sure I just read of an entirely new proposed asteroid collision about 125 millions years after the Permian.
     
  7. Jun 27, 2003 #6

    LURCH

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    Yes, I had heard of that evidence. However, I must say that before hearing about it, I had already assumed that there would be vulcanism associated with any major impact event. As a youngster, I used to walk along the banks of a certain river in the springtime, to watch the sheets of ice float down. I would sometimes throw large rocks at these sheets of ice. I noticed that, when the ice was struck by a rock, liquid water from underneath the sheet would shoot up through any holes in the ice. I am sure that in a similar fashion, when the frozen rock floating on the surface of our planet is struck with an impact of sufficient energy, liquid rock from underneath shoots up through the holes.
     
  8. Jun 28, 2003 #7
    Given that the Permian extinction is (it would seem) the largest we know of there most probably were several contributary causes. One possibility (can't remember the source) is that the formation of Pangaea wiped out a lot of the continental shelf.

    With regard to the K/T extinction 65 million years ago surely the Chicxulub asteroid or comet impact was the main contributary factor. The misfortune for the dinosaurs was that the comet or asteroid hit what is now Mexico's Yucatan Peninsula; an area rich in sulphur.
    (See also http://www.xtec.es/recursos/astronom/craters/chicxulube.htm)
     
  9. Jun 28, 2003 #8

    LURCH

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    I've also wondered if we might be able to find an "exit wound" for Chicxulube. The impact should have sent a shockwave right through the mantle to the far side of the planet. Volcanos imediately sarounding the impact site would have erupted violently, while those further away would display less force as the wave spread out and weakend.

    However, there should be a point on the opposit side of the globe where these concentric circles of magma displacement reconverged, forming interferons less deisruptive than the original impact, but more disruptive than the wave propogation. And of course this site would not be Exactly opposite, as the rotation of the planet during the time the waves were passing through the mantle would offset the location slightly.

    However, I have not heard anything about such a feature being found, or even looked for. This leads me to believe that I have misjudged the dynamics involved; e.g., the incoherence of the interferons caused by rotational offset, the amount of damping effect the mantle would have on the force of the wave, etc.
     
  10. Jun 28, 2003 #9
    Actually, I don't at all accept that the Chicxulub crater impact is a sole cause for the Cretaceous extinction. There simply isn't the evidence to suggest that a there is room for a sole cause.

    Different studies highlight different aspects of the extinction - firstly, that there's is no sudden extinction, and secondly, that any sudden extinction is local-applicable only, ie, North America - which is precisely where all the supporting data for a sole-cause impact extinction comes from.

    It's an essential point of note that the impact body *cannot* in dynamic terms actually provide the sufficient transfer of energy to cause of global extinction event. So US scientists "fudge" the issue by claiming that an undefined and entirely speculative "threshold" point was reached. As to the nature of the rocks in the Chicxulub crater - it's entirely speculative and unsupportable that this would have any bearing on the issue. Especially in comparision to the amount of sulphir churned out frmo the Deccan traps.

    There's nothing wrong with including the impact event as part of a general long-term "period of extinction" which has multiple causes. One of the more interesting theories is that Chicxulub itselt could have been caused by a fragment of a larger extra-terrestrial impact event. But, again, there's a very real danger that even this is nothing more than a fudge to give an easy "one cause fits all" answer to an incontravertibly complex issue.

    I took this issue to task in my own forum in this thread here - which has some useful links.
     
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