Core within the core?

[Dorfl]

I could say the same, since I learned a couple of things from them.

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 coffee break or beer time.

I had a talk with my mentor today, and yes: resolution turns out to be the problem. I hadn't really appreciated that we only have an angular resolution of about ten degrees, which isn't likely to let us do much more than verify that the core is actually there.

The project itself isn't actually seismology-related. I'm working with the IceCube neutrino telescope, which mainly looks at neutrinos passing through the Earth. (If something hits the telescope from above, it could be any random gunk, but if something has made it through the Earth, we know it's a neutrino). The question turned up whether we could use our data to actually decrease the uncertainties in the model of the Earth, instead of just treating them as sources of error in the results we're actually trying to find. It's looking like the most we'll be able to do is to see that the core-mantle boundary is in fact there, though.

 

[Dinwar]

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

That's it exactly. Uraninite, a common uranium ore, is UO2, or uranium dioxide. There are others as well. My minerology professor used it as a way to get us to understand the necessity of thinking in terms of minerals, rather than in terms of elements. Heavy elements can make surprisingly light minerals, and light elements can form some remarkably dense ones.

The other thing to consider is the solubility of these minerals. Uranium ores tend to be fairly soluble, so they'd be brought to the surface with hot water (such as you'd find in subduction zones). Iron, on the other hand, wouldn't--its minerals tend to be insoluble. So once uranium's AT the surface it'll STAY at the surface, where as iron doesn't necessarily do so.

 

[Ziggurat]

We need to go deeper.

 

[rjh01]

That's it exactly. Uraninite, a common uranium ore, is UO2, or uranium dioxide. There are others as well. My minerology professor used it as a way to get us to understand the necessity of thinking in terms of minerals, rather than in terms of elements. Heavy elements can make surprisingly light minerals, and light elements can form some remarkably dense ones.

The other thing to consider is the solubility of these minerals. Uranium ores tend to be fairly soluble, so they'd be brought to the surface with hot water (such as you'd find in subduction zones). Iron, on the other hand, wouldn't--its minerals tend to be insoluble. So once uranium's AT the surface it'll STAY at the surface, where as iron doesn't necessarily do so.

The density of uranium dioxide is 10.97 g/cm³. The density of iron oxide is 5.24 g/cm³. So density alone does not account for a lack of a uranium core. It would not dissolve either as there would be no liquid water in a molten Earth.

The only explanation that does make sense so far is that there is not enough of it to make the charts.

On the other hand others have had the same thought as the OP. One of the implications is a natural nuclear reactor at the core. See these references for more
http://www.nature.com/news/2008/080515/full/news.2008.822.html
http://www.spacedaily.com/news/earth-03k.html

 

[Dinwar]

The density of uranium dioxide is 10.97 g/cm³. The density of iron oxide is 5.24 g/cm³.

Unless the core is iron oxide, the second is irrelevant. As for the uranium thing, it was merely a uranium mineral for which I could remember the chemical formula. It was meant to illustrate the difference between uranium's density and uranium ore's density, not to be an exhaustive list of uranium ores.

It would not dissolve either as there would be no liquid water in a molten Earth.

I have already addressed that issue. I have not said that it would disolve while the Earth was molten; rather, it would disolve after, and therefore would be less likely to be transported into the core via subduction. The time when the Earth was liquid isn't the only time material has been transported from one layer to another, and when you've got 4.5+ billion years to work with you have to consider post-molten factors as well as the ones impacting the molten planet in order to develop an adequate explanation.

The only explanation that does make sense so far is that there is not enough of it to make the charts.

Are you serious? Or are you completely unfamiliar with geologists?

 

[rjh01]

Unless the core is iron oxide, the second is irrelevant. As for the uranium thing, it was merely a uranium mineral for which I could remember the chemical formula. It was meant to illustrate the difference between uranium's density and uranium ore's density, not to be an exhaustive list of uranium ores.

I have already addressed that issue. I have not said that it would disolve while the Earth was molten; rather, it would disolve after, and therefore would be less likely to be transported into the core via subduction. The time when the Earth was liquid isn't the only time material has been transported from one layer to another, and when you've got 4.5+ billion years to work with you have to consider post-molten factors as well as the ones impacting the molten planet in order to develop an adequate explanation.

Are you serious? Or are you completely unfamiliar with geologists?

The % of Uranium in the Earth is very small. Here are the %
http://wiki.answers.com/Q/What_is_the_composition_of_the_Earth

Earth's solid mass is about 32% iron, 30% oxygen, 15% silicon, 14% magnesium, 3% sulfur, 2% nickel, 1.5% calcium, and 1.4% aluminum. Much of the iron and nickel are in the planetary core, which is 89% iron and 6% nickel.

So what are you talking about?
Also I am sure that when the Earth was liquid that would have been plenty of time for any heavy elements to have sunk to the core. Or are you suggesting that at some stage elements from the core rose back to the crust? I am afraid the more you write the less sense you are making.

 

[Dinwar]

So what are you talking about?

I'm unaware of what the percentage of uranium has to do with the conversation at hand. The percentage of gasoline in crude oil has no bearing on [eta: the temperature at which] it is distilled--only on how much you'll get. Similarly, the percentage of uranium on Earth has no bearing on where it will be--only how much we can mine. The reasons are the same.

And if you can't figure out what I'm talking about given the context, I'd suggest you may want to brush up on geology. I've flat-out stated it numerous times.

Also I am sure that when the Earth was liquid that would have been plenty of time for any heavy elements to have sunk to the core.

If we were dealing with elements in isolation, sure, this would likely be true (this is geology, it's all plus or minus). Simple fact is, though, we're not. So what the elements would do is largely irrelevant (except, obviously, for native elements that would have been present). What the minerals they formed would do is the relevant factor. Look up xenolith sometime--just because something's liquid, doesn't mean there aren't chunks floating in it. If your interpretation of what happeend was true, we'd have no oxygen in the mantle (it's a light element, after all). Instead, we find ample oxygen in the form of silica tetrahedrons within minerals. So there's obviously more going on than "This is a heavy mineral, it goes to the core; this is a light one, it goes to the crust". It's those complications that I'm attempting to discuss.

Or are you suggesting that at some stage elements from the core rose back to the crust?

Quite obviously not. I was saying that AFTER THE EARTH COOLED processes existed to MAINTANE URANIUM AT THE SURFACE. I put those in bold because you've missed them twice already, and I don't have a great deal of patience just now. I've stated my reason for bringing up the solubility of uranium ore twice in plane English--I honestly don't know how to make it any more clear than the statement above. Again, we have 4.5 BILLION YEARS after the Earth cooled during which elements can travel all over the place. Subduction and antimountains and other processes show quite clearly that in the modern world material can be transported from the edge of the crust to the outer core, at least. Anything times ten to the nineth is significant, so it's not merely sufficient to explain how uranium got to the surface, you also need to briefly discuss how it remained there (particularly considering the solubility issue--and if I have to explain that, again, brush up on geology).

That said, there may be processes that bring material from the core to the crust. We know there are processes that bring material from the mantle to the crust, so anything that brings material from the core to the mantle could potentially bring it, ultimately, to the crust. Mantle plumes may also be a candidate for doing so, though I suspect they originate at shallower depths most of the time. My point is, I doubt that too much goes down without something coming back up--the Earth doesn't have a black hole at the center, so mass should roughly ballance over time.

I am afraid the more you write the less sense you are making.

You've misunderstood most of what I've said, often taking my words to apparently mean the precise opposite of what I've said. I'm not surprised you're getting confused.

 

[Dinwar]

Also, to be clear: My crack about not knowing geologists was intended as a bit of a joke. I always say that if you ask two paleontologists you'll get three opinions, and geologists are the same. There's almost NO question which has only a single viable explanation. There are geologists who will construct a new viable explanation just to annoy people (some rather widely respected ones, too). I highly, HIGHLY doubt that in an area as open to interpretation as the composition of the core there's only one answer that makes sense. More likely, there's one answer that makes sense TO YOU. That's certainly not a bad thing; experts are supposed to have opinions. But as I don't share your view I think it's incumbant upon me to point out the difference between what makes sense to you (or me, or any individual) and what makes sense period. They ARE two different concepts, after all.

 

[MarkCorrigan]

I'd like to interrupt the science for just a moment to say something.

This thread is nice. REALLY nice. It's a genuine science question from someone who is educated in science wanting real answers about something they were unsure of. There isn't a "hidden" motive in play. No one is having to answer tired old boilerplate questions about what a theory is or what proof means. There isn't any "I don't do maths" or "Well my theory is [insane crackpottery]".

This thread is a breath of fresh air. I've learned something, the OP has learned something, and everyone is getting a meaningful, light hearted discussion. There isn't any lying, or accusations, or paranoia that I've come to expect from threads I click on here. I am genuinely happy to see that there are threads here where lunatics don't reign free and genuine science is asked for, given and absorbed.

Thank you everyone.

 

[shadron]

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.

That is correct. The rule is that a spherical body can be considered a point source of gravity until the the measuring body is inside the outer edge of the actual body. The gravity at that point is equal to a point source with mass equal to the sum of the mass which is closer to the center than the measurement body; the mass outside that distance is ignored (it all cancels itself out).

For example, the gravity on a ball which falls into the proverbial well to China, and is 100 miles from the center, is the gravity exerted by all the matter that is within 100 miles of the center. This also says that the gravity at the center is zero.

 

[shadron]

It might also be amusing to note that early in the Earth's accretion phase, at a point when the entire planet was molten, the actual sinking of heaver metals to the center of the planet brought on a period of time known as the iron catastrophe the sinking of these metals to the center released tremendous potential energy, which in turn defined the planet structure as we know it. See this.

 

[Libra]

Thank you for starting this interesting thread and everyone else's contributions.

I always understood that the geophysical work that's been done on the Earth's core, utilised seismic events as the energy source to track the propagation of various waves and the results of this, in combination with aspects like thermal and moment of inertia, was used to infer the model.

Is this still the case ?

I have only worked with very very shallow (up to 50m) geophysical data (mainly for Geotechnical purposes in foundation and stability design of large structures) and have seen some major differences in terms of seismic velocities and the inferred expected material properties and distribution from such geophysical work and then the actual material properties and distribution upon examining the area more in terms of either confirmatory Geotechnical Investigations in positions of concern and/or exposure and subsequent mapping in the event of dry construction techniques.

I am assuming that just as much, or perhaps maybe more, variation could be expected between the current inferred model and the actual ?

Far off the mark ?

 

[rjh01]

A few points to answer Dinwar
- Minerals are composed of elements
- I agree that once the Earth cooled then any uranium (or anything else for that matter) that was on the surface would stay on the surface (ignoring plate tectonics)
- Minerals are made up of elements. Any time I have mentioned elements it also includes minerals
- Uranium minerals would have to have a huge % of other elements to make it less dense than iron oxide (would these even be stable at high temperatures?). Uranium oxide is not one of them, so should sink into the centre while the core is still molten. What happens after the Earth has cooled is not important as anything that could sink would would have already sunk.
- If the % of uranium in the core is very small it would not be mentioned by most references.
- Oxygen and other lighter elements could be in the core because it came down with other elements as part of various minerals, like iron oxide.
- Once iron oxide has got to the core due to the temperature and pressure it could turn into iron and oxygen. If it was iron that went to the core then it would have been uranium also that went to the core for the same reasons.

 

[Dinwar]

- Minerals are composed of elements

Gee, I've taken innumerable geology classes and have five years' experience in the field and no one EVER told me that!!!!!

Yes, minerals are composed of elements. However, when dealing with density THAT DOES NOT MATTER. Ships are composed of iron. Iron sinks. Ships, unless someone puts a hole in them, do not. Gee, I wonder why that is. Volcanic glass sinks, but for some reason pumice floats! Oxygen and hydrogen are gasses, so H2O should have slightly more boyancy than oxygen, right?

Unless we are dealing with native elements, the density of the element is only part of the story. The density of the mineral it's in is the rest of it--the part you persist on ignoring. And if you're going to persist in assuming that only one element is important in that calculation you will look progressively more ridiculous.

- I agree that once the Earth cooled then any uranium (or anything else for that matter) that was on the surface would stay on the surface (ignoring plate tectonics)

You'd be wrong. Particularly the "anything else" part and the "ignoring plate tectonics" part. Huge volumes of material have been subducted well into the mantle, sometimes coliding with the outer core. If you want to argue otherwise, please find me a Frasnian-aged piece of oceanic crust that hasn't been thrust onto a continent.

- Minerals are made up of elements. Any time I have mentioned elements it also includes minerals

You can redefine the words to your heart's content--just don't say that I'm the one that doesn't make sense. Elements are elements. Minerals are composed of one or more elements. If you use "elements" to mean "minerals" you are wrong by every definition of the words.

- Uranium minerals would have to have a huge % of other elements to make it less dense than iron oxide

You obviously know very little about minerology. Here's a hint: silica tetrahedrons can form minierals of varying density. Yes, yes, I know uranium ores aren't usually silicates, but it's the principle I'm driving at here, not the specifics.

Or, go back to the ship idea. Iron sinks. Steel ships float. Obviously, elemental composition is not the only factor here. It gets even more complex when you're talking about crystalization (because now you're not just talking the boyancy of a single element, but of minereal SYSTEMS [ie, if one mineral is captured within another one, something not uncommon in igneous rocks, the density of the whole system must be considered, not just any one crystal and especially not any one element]).

As for uraninite, again (and for the last time), I merely chose a uranium ore I could remember the composition of. I did not intend that to be an exhaustive listing of uranium ores.

- If the % of uranium in the core is very small it would not be mentioned by most references.

Define "most references". And you're ignoring half the equation. There's a large amount of uranium at the surface. Here's some light reading.
While normally I shun Wiki for technical discussions, this article isn't bad and it hits a number of points that you're ignoring. For example, uranium is more abundant than silver (a lighter element) in the crust, and it's highly mobile in alkaline pH solutions (not as rare as people assume in the oceans).

The issue isn't "What happened to the uranium when the Earth was formed?" We know what happened--a lot of it (everyone I've talked to argues most of it) is at the surface. The issue you need to address is "Why is there so much at the surface?" Because even if it's not most of the uranium there's a remarkable amount up here. I may be wrong about the density thing, but simply pretending that there's not a large amount of uranium is simply bad science. It requires you to ignore data. I've proposed a mechanism; you've proposed flaws. Okay, hot shot--what mechanism do YOU propose? Because there's got to be one; the uranium's here somehow.

- Oxygen and other lighter elements could be in the core because it came down with other elements as part of various minerals, like iron oxide.

I need to find that smiley that shows a guy bashing his head against a wall....If you understand how LIGHT elements can be brought DOWN due to the difference between a mineral and an element, you can apply the same principle to explain why HEAVY elements were brought UP. It actually is precisely the same principles, merely different applications. You've already accepted what I'm arguing, you simply refuse to acknowledge the necessary implications of your own statements.

Also, to play by your rules I should now consider ALL minerals that contain oxygen to be iron oxide. Nothing else--just iron oxide. There are no other oxides, because you've only mentioned iron oxide. I can assume that silicates don't include oxygen, because you've only mentioned iron oxide.

Do you see where that can get extremely frustrating?

If it was iron that went to the core then it would have been uranium also that went to the core for the same reasons.

And ya backslid. If you can prove that all the uranium was UO2 while the Earth was molten, I'll consider agreeinig with you. Good luck with that.

I have now answered every one of your petulant "points to answer". Can we now have a grown-up discussion, or are you going to continue to refuse to read what I've written and make assinine assumptions about what I've said that contradict what I've said? I'm not asking you to agree with me--I'm merely asking you to acknowledge my points as written. I'm also not asking you to respect me--anyone who hands another person a list of "points to answer" has no respect for the other person. I'm just asking for an honest hearing, which I thus far have not gotten.

I always understood that the geophysical work that's been done on the Earth's core, utilised seismic events as the energy source to track the propagation of various waves and the results of this, in combination with aspects like thermal and moment of inertia, was used to infer the model.

Is this still the case ?

My understanding is that experimental minerology is also used. It's one thing to estimate the density and temperatures; it's another entirely to estimate composition. The latter requires some good old-fashioned lab experiments. Had a friend that switched from paleo to doing exactly that (only for the mantle, not the core). Phase diagrams and the rest are wonderful tools, but there are more things in Heaven and all that.

I am assuming that just as much, or perhaps maybe more, variation could be expected between the current inferred model and the actual ?

The structural stuff is at the 1:1,000,000,000 scale.

 

[Cylinder]

...IceCube neutrino telescope...

 

[rjh01]

Dinwar I think your post is very funny. It is not worth any more comment than that.

 

[Dinwar]

Dinwar I think your post is very funny. It is not worth any more comment than that.

I'm sorry you feel that way. Obviously you're not interested in discussing these concepts; it wouuld have saved us both a great deal of time if you'd simply said so at the beginning.

 

[rjh01]

I have been thinking about what would happen if uranium had fallen to the centre of the earth. There would have been a lot more U235 than at present and it would have ignited. A lot of the U238 would have been converted into plutonium and that would have ignited too. Result - very little uranium left. What is left are the radioactive waste products plus a lot of heat.

 

[Dinwar]

You still need to address why there's so much at the surface. We know what happens when uranium gets highly concentration--natural nuclear reactors (well, one that I know of for a fact, and I recall hearing about at least one more) have been found. They don't explain why uranium is present in the crust.

For that matter, your mechanisms do not explain why iron is in the crust. Obviously there's a lot more going on then simply "Heavy elements fall inward", whatever definition of "elements" you use.

 

[rjh01]

You still need to address why there's so much at the surface. We know what happens when uranium gets highly concentration--natural nuclear reactors (well, one that I know of for a fact, and I recall hearing about at least one more) have been found. They don't explain why uranium is present in the crust.

For that matter, your mechanisms do not explain why iron is in the crust. Obviously there's a lot more going on then simply "Heavy elements fall inward", whatever definition of "elements" you use.

That is a good question. Here is a web page that has the answer though

During the initial formation of the Earth's crust, an event known as the Iron Catastrophe occurred, where the denser elements of the Earth's composition, such as iron and nickel, sank to its core, while the lighter elements, like silicon, formed a crust at the top.

The crust began to cool when the Earth was at least 40% of its current size, possessing enough gravity to hold down an atmosphere containing water vapor. Much of this early water vapor would have come from comets. This era in the Earth's history, extending from the Earth's birth to about 3.8 billion years ago, is known as the Hadean era, after the Greek Hell, Hades, for the difficult conditions on the planet at the time. Scientists believe the Hadean era was lifeless.

Around 4.0 to 3.8 billion years ago, towards the end of the Hadean era, the planet underwent the Late Heavy Bombardment, a period of time with many large asteroid impacts.

http://www.wisegeek.com/what-is-the-history-of-the-earths-crust.htm

Hard to believe that most of the water on Earth came from comets.