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Underground continent info needed

  1. Apr 17, 2007 #1
    Underground "continent" info needed...

    Hello, I am new here and I am looking for about 15 years to get more information from a discovery published in April 1993 that says to have found a high density mass of about 320km long by 130km thick at 3200km deep under the North-American continent... I have made numerous research online, and it look like that nobody's have every heard of this... :cry:
    I would be very greatful if somebody could help me get more information about what was this phenomenon.

    Thanks in advance!
  2. jcsd
  3. Apr 17, 2007 #2


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    Staff: Mentor

  4. Apr 17, 2007 #3

    This is not the same thing, as what I am talking is situated under our Continent, and is much deeper at about 2000miles deep!! So if it was a molten Earth crust like described in the article it could place this piece of Earth as old as 500 million years!! Which should have got plenty of time to melt away!!
  5. Apr 17, 2007 #4


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    This is in North America.
  6. Apr 18, 2007 #5
    Sorry, was refering to the Beijing anomaly.

    The information I got were made in 1993, and the information at that time, was pretty much different that what is described here.
    Here's my reference (in french, sorry) : Science et vie No907 Avril 1993)
  7. Apr 18, 2007 #6


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    Are you sure about the depth? That would put it in the outer core. I've not studied it in great detail but had the impression the core was fairly homogeneous and well mixed. Something like that in the mantle on the other hand wouldn't be at all surprising.
  8. Apr 18, 2007 #7
    Yup I am sure of the depth, it is actually situated at the delimitation of the outercore and the mantle, which is indeed very unusual. It is why I do look for more information about this. I have contacted USGS and they promised me to come back with more info about this, and since THEY are the ones who discover it, I was expecting an answer... I got a reply that they could not find any info in their database, but the guy that replyied me, did remember it.
    Look like I am alone stuck with this one...:confused:
  9. Apr 18, 2007 #8
    At that depth how would you determine it was "high density"? I can think of no geophysical method that would tell you this (especially as gravity cannot easily distinguish small shallow objects from large deep objects - unless perhaps you developed some kind of 'gravity-gradiometry'?), and as I am about to graduate I would appreciate it if you could fill me in so I don't feel like a complete klutz in my voce viva. Cheers.

    EDIT: I'm graduationg for a masters degree in geophysics.
    Last edited: Apr 18, 2007
  10. Apr 18, 2007 #9

    I have quite a few lecture notes on the CMB (which for the record is at ~2890 km), I might be able to find something beneath America for you. From memory I recall that there is some kind of anomaly there which is thought might act as the path for one of the magnetic poles as it reverses. I believe it may have been detected by seismic tomography as an anomalously fast region, and is often quoted as being "cold".
  11. Apr 18, 2007 #10
    It is quite simple, in this case the high density mass as been detected by calculating the path of seismic waves after the explosion of a nuclear bomb in the goby desert. The explosion was strong enough (0.66Mt) to create such seismic waves and the way (propagation) they have travelled through the planet.
    You can find much more detailled information's about this by surfing on the USGS geophysic website... They are the one's who are the pioneer in this field.
    By the way, when I talk about high density, I just litterally translate it from French, so it might not sound like what you are used to hear... Sorry...
    Last edited: Apr 18, 2007
  12. Apr 18, 2007 #11
    I think that you are on the right path!! When you say cold, it can mean some sort of solid metal?? Actually, I also remember about the news on TV talking about this too, and if I remember right, they were talking about the fact that this mass was made of solid metal. But of course, I cannot rely on my memory for those facts, as if it was true (at that time I did not have enough knowledge in the field), it would mean that this mass could be made of solid metal at 3200km deep in the Earth, which is REALLY an anomaly!! And it is what I need to confirm!
    Last edited: Apr 18, 2007
  13. Apr 19, 2007 #12
    I am not convinced that seismology can tell you density, even if you use Birch's Law which sayas that velocity is proportional to density, and you invert your travel-time data perfectly so that you resolve the seismic velocity in that material, you still need to know what material you are looking at before you can infer the density.

    The D" layer which is a couple of hundred km thick layer at the base of the mantle is very heterogeneous, it contains ultra low velocity zones (ULVZ) (which are smaller than the thing you're after), it also contains phase changes - in cold regions I believe you get perovskite (the same as the lower mantle), in hot regions you get a phase change to post-perovskite and if it's hot enough it may even change back to perovskite!
    Some people think slabs end up there, one elegant mechanism to explain ULVZs is that perhaps they are subducted banded iron formations (BIF), incidentally some people also think ULVZs are where hotspots originate from. As for a slab of metal down there, never heard of it, this is because no one knows what's down there so they just have to guess, it just so happens that the people I know that study the Deep Earth either haven't made that guess, or perhaps they just haven't told me if they have. Any lateral heterogeneity will only be detected as a seismic anomaly, and until people know what it is for sure, they should only really describe it for what it is seismically to avoid confusion.
  14. Apr 19, 2007 #13
    You are right about this, and it may explained why the two scientists of the USGS that have claimed this discovery have not gone public with further study about this phenomenon. If this really solid metal, it may explained the abnormal oscillations in the Earth magnetic field... But wether or not this anomaly is a consequence of the abnormal oscillations or, more troubling is causing them, will be even more difficult to figure out.

    So the fact that you were the only one responding to this question, even among senior geophysicists that I know, mean that you are a genius of some sort! So I guess your master degree will be succesful! CONGRATULATION!!:wink:
    Good luck and thank you for your input!!
  15. Apr 26, 2007 #14


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    So what do they do then? And why can't they "guess and check?"

    In elementary school I was taught the core was guessed to be made of heavy metals and solid, from the pressure of the world squeezing on it. Is that right?

    But the core spins faster than the rest of the Earth, doesn't it?
    http://www.geology.uiuc.edu/~xsong/papers/zhangetal05_science.pdf [Broken]
    So that would mean that the slab's location translated to the Earth's surface could never be pinpointed to "under North America" because of the spinning core. I assume in the core's spinning it would drag material with it.

    Does that make sense? Tell me what you think.
    Last edited by a moderator: May 2, 2017
  16. May 3, 2007 #15
    Well, I must admit I don't know enough about it, but there are ways in which geophysicsts can constrain density at these depths, although I believe that the seismology alone cannot be used. Difference in s and p wave arrivals can tell you something about the bulk and shear moduli, but this alone does not tell you density. Birch's Law (which is only really approximate) cannot tell you density if you don't know the composition. I believe that thermodynaimc and ab initio simulations are used to constrain density, exactly how it works is beyond me, and besides, there remains a high degree of uncertainty. It's quite easy to guess, but it is technically impossible to check, the deepest borehole is what, 14 km? We're talking depths of 2800 km or more here, so for obvious reasons you can't just check.
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