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Core within the core?

Dorfl

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Jun 19, 2005
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If I understand basic geophysics correctly, the inner parts of the Earth can be divided into the mantle, consisting of rock, and the core, consisting of iron and nickel. The reason that the iron and nickel form the core is simply that they're a lot heavier, so they've sunk the deepest.

What I was wondering is if it's possible that there is a second core inside of the iron-nickel core, consisting of even heavier elements. It seems sort of reasonable to me that most of the heaviest elements present at the formation of the earth should end up sinking even deeper. It also wouldn't necessarily be detectable by the seismographic measurements we've done so far, if it's small enough.

Does anyone know enough geophysics to say if this is plausible, or if it's just stupid?
 
The core is divided into a solid inner core and a liquid outer core, but as I understand it the composition is fairly uniform - the heavier elements haven't separated out.
 
Near the center of the earth there would not be much gravity.

How is the inner core solid? Isn't it too hot?
 
Puppycow said:
Near the center of the earth there would not be much gravity.

How is the inner core solid? Isn't it too hot?
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Vast pressure.

eta: ~3.5 million atmospheres, apparently.
 
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Well, this is supposed to be the composition of the core:

An inner small sun shining over the nightless land of Agartha.

Outer core (behaves as liquid): Mostly Fe, plus 2% Ni and 9% to 12% Si and other light elements

Inner core (solid): 80%Fe, 20%Ni

Sure, there are minor and trace elements, and heavier elements quite likely are expected to concentrate down there, but not necessarily to diferentiate in a zone. Remember there are extra issues, their abundance and the possibility of building natural alloys.
 
Ok I am going to sound dumber than I normally do,

wouldn't the heavier elements be outside the core as at the centre there is more gravitational pull from the rest of the Earth, and also from the rotation of the zones?

I cannot brain today, Muscat Grape has given me the dumb
 
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Furi said:
Ok I am going to sound dumber than I normally do,

wouldn't the heavier elements be outside the core as at the centre there is more gravitational pull from the rest of the Earth, and also from the rotation of the zones?

I cannot brain today, Muscat Grape has given me the dumb
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No because the gravity from the opposite side would also be pulling. In the center the gravity would equalize but move a little out of center and the gravity would be a greater force behind you. Move behind the center and there'd be more gravity in front.
 
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Correa Neto said:
Sure, there are minor and trace elements, and heavier elements quite likely are expected to concentrate down there, but not necessarily to diferentiate in a zone. Remember there are extra issues, their abundance and the possibility of building natural alloys.
Click to expand...

Do you know whether it's known that they haven't differentiated in a zone, or if it's just that they're not particularly expected to have?
 
The thing to remember about the composition of the Earth is that it happened by and large during a time when the planet was, more or less, a liquid (the caveats are there because nearly any generalization regarding a planet usually hasa innumerable exceptions; this is a 1:10,000,000 view). Dense materials (not elements, but materials) fell for exactly the same reason that ice floats in water: boyancy. That's why the inner core is mostly iron and nickle, rather than uranium--uranium is more dense than iron, but uranium minerals are less dense than iron minerals. So they float, while iron sinks. When you're dealing with a sphere of liquid, you can only sink to the center. Gravity is only a part of the equation.

I haven't heard of any systematic differentiation in minerals in the core. That may be because I haven't looked (I deal with sedimentary rocks), but I get the sense that it's because there isn't any. Defects, yes--everything in geology has defects, due to thermodynamics (it's more stable to have defects than to not have them; counterintuitive, yes, but true). But they're likely to be relatively small-scale. If they were systematic, they'd almost certainly show up on seismic analyses.
 
Thanks!

I'm working on my master's thesis right now, and knowing this means I can avoid one blind alley at least.
 
Dinwar said:
The thing to remember about the composition of the Earth is that it happened by and large during a time when the planet was, more or less, a liquid (the caveats are there because nearly any generalization regarding a planet usually hasa innumerable exceptions; this is a 1:10,000,000 view). Dense materials (not elements, but materials) fell for exactly the same reason that ice floats in water: boyancy. That's why the inner core is mostly iron and nickle, rather than uranium--uranium is more dense than iron, but uranium minerals are less dense than iron minerals. So they float, while iron sinks. When you're dealing with a sphere of liquid, you can only sink to the center. Gravity is only a part of the equation.

I haven't heard of any systematic differentiation in minerals in the core. That may be because I haven't looked (I deal with sedimentary rocks), but I get the sense that it's because there isn't any. Defects, yes--everything in geology has defects, due to thermodynamics (it's more stable to have defects than to not have them; counterintuitive, yes, but true). But they're likely to be relatively small-scale. If they were systematic, they'd almost certainly show up on seismic analyses.
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I find it strange that uranium minerals would be less dense than the iron minerals when the Earth was molten. The density of uranium (18900 kg/cu.m) is a lot more than iron (7850 kg/cu.m) so there must be a lot of other elements in the molecule.

Ref: http://www.simetric.co.uk/si_metals.htm
 
Dorfl said:
Do you know whether it's known that they haven't differentiated in a zone, or if it's just that they're not particularly expected to have?
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First of all, you must note that there are lots of processes which generate anomalous (above background value) concentrations of elements. These concentrations may be of several different sizes. For these reasons, I honestly can't say if at the inner core there are or not anomalous concentrations of elements, heavy or light; I can't also say too much about their sizes. My gut feeling? Well, probably there are anomalous concentrations of elements, because we know lots of things probably happened while the Earth was differentiating and also due to heat transfer between the inner and outer cores. But don't ask me which ones, sizes, grades and exactly how they came to be. Note however that maybe some of these processes may also cause homogeneization.

Now, if these concentrations are or not detectable, well, the only thing I actually can tell you is that we already know the Earth's core is anisotropic. Exactly how and why... Well, I guess you'll find the links below useful. Note the rare Elsevier non-paywall text.

http://www.elsevierdirect.com/brochures/geophysics/PDFs/00132.pdf
http://www.itg.cam.ac.uk/people/heh/Paper130.pdf
http://211.144.68.84:9998/91keshi/Public/File/34/473-7347/pdf/nature10068.pdf
 
Correa Neto said:
First of all, you must note that there are lots of processes which generate anomalous (above background value) concentrations of elements. These concentrations may be of several different sizes. For these reasons, I honestly can't say if at the inner core there are or not anomalous concentrations of elements, heavy or light; I can't also say too much about their sizes. My gut feeling? Well, probably there are anomalous concentrations of elements, because we know lots of things probably happened while the Earth was differentiating and also due to heat transfer between the inner and outer cores. But don't ask me which ones, sizes, grades and exactly how they came to be. Note however that maybe some of these processes may also cause homogeneization.

Now, if these concentrations are or not detectable, well, the only thing I actually can tell you is that we already know the Earth's core is anisotropic. Exactly how and why... Well, I guess you'll find the links below useful. Note the rare Elsevier non-paywall text.
Click to expand...

Thanks!

Those links are very useful. The first one even says that

I. Sumita & M.I. Bergman said:
Some observations even suggest that the deepest part of the inner core may be distinctly different from other parts of the inner core (Ishii and Dziewonski, 2002, 2003; Beghein and Trampert, 2003), perhaps suggesting a phase transition from h.c.p. to b.c.c. iron.
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Do you know if such a phase transition would involve a discrete shift in density? Because the kind of analysis I'm thinking of doing would basically be sensitive to sudden density changes, to some extent to the density itself, and not much else.
 
I like big holes.

I like the big hole they drilled in Russia. They called it a super hole, didn't they ? Those funny Russians.

Anyway, why aren't I hearing of more hole digging going on ?

We need to go deeper. I don't know what the scientific reasons are for making a big hole in the earth but it just feels like the right thing to do.

Agreed ?
 
LandR said:
I like big holes.

I like the big hole they drilled in Russia. They called it a super hole, didn't they ? Those funny Russians.

Anyway, why aren't I hearing of more hole digging going on ?

We need to go deeper. I don't know what the scientific reasons are for making a big hole in the earth but it just feels like the right thing to do.

Agreed ?
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They made one in Florida. Guy's house fell into it.
 
anglolawyer said:
They made one in Florida. Guy's house fell into it.
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I see.

OK. On the one hand I'm happy to see the enthusiasm for holes. On the other we can't just have holes everywhere all willnilly like.

Because things like houses will fall into them and old people and people who aren't paying attention.

So, we may need a hole planning committee. They will plan where the holes will go and will probably put up signs like "Beware of Hole". Yes, that should absolve us of all legal responsibilities.

This is more complicated than I first thought.
 
Dorfl said:
Thanks!
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I could say the same, since I learned a couple of things from them.

Dorfl said:
Do you know if such a phase transition would involve a discrete shift in density? Because the kind of analysis I'm thinking of doing would basically be sensitive to sudden density changes, to some extent to the density itself, and not much else.
Click to expand...

I can only make an educated guess.

The Moho, outer core/mantle and inner core/outer core boundaries are sharp discontinuities. All of them are related to changes in the chemical composition; the outer core/mantle, inner/outer core are also related to changes in state (solid/"liquid").

Now, between the lower and upper mantle there are no changes in the chemical composition but crystal structures change (olivines will acquire the crystal structures of wadsleyite and then ringwoodite while pyroxenes will acquire garnet crystal structures). Its a broad ~100km transition zone.

So, based on the above, I would expect a smooth transition, with gradual density increase. Now, I have no idea on how thick such a zone would be neither if the density increase would be enough to make it detectable through increase in seismic waves' velocity - I guess math could give some answers to the last question, but those are waters too deep for me. This would be something nice to discuss with your mentor.

Note also there's another issue- resolution. Would this zone be big enough to be located with the available network of seismic stations? Once again, nice topic for a talk with you mentor during coffe break or beer time.
 
Dorfl said:
What I was wondering is if it's possible that there is a second core inside of the iron-nickel core, consisting of even heavier elements. It seems sort of reasonable to me that most of the heaviest elements present at the formation of the earth should end up sinking even deeper.
Click to expand...

In addition to what others have already said, an important question to consider is - what heavier elements? The Earth's composition is along the lines of - 32% iron, 30% oxygen, 15% silicon, 14% magnesium, 3% sulfur, 2% nickel, 1.5% calcium, 1.4% aluminium. Even if heavier elements did all sink to the middle, there still wouldn't actually be much there. How much of an element do you need before it could be plausible for it to form an entire separate core layer?
 
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