What is the role of silicon in the Earth's core?

[1oldman2]

The missing element mystery in the Earth's core may have been solved.
http://www.bbc.com/news/science-environment-38561076
Lead researcher Eiji Ohtani from the University of Tokyo told BBC News: "We believe that silicon is a major element - about 5% [of the Earth's inner core] by weight could be silicon dissolved into the iron-nickel alloys."

 

[rootone]

I would have expected heavy elements like Gold and Lead would sink into the core of early Earth more so than relatively light Silicon.

 

[1oldman2]

I would have expected heavy elements like Gold and Lead would sink into the core of early Earth more so than relatively light Silicon.

Same here, thus the thread title. I'm still trying to figure out if it's because the silicone is trapped in suspension with the Iron/Nickle or if I'm missing something really strange.

 

[Baluncore]

A really interesting model.
But I do not think there needs to be a single element to satisfy that requirement. I would expect roughly equal amounts of both silicon and aluminium to be held in the matrix of the solid inner core. Any Si and Al in the liquid Fe-Ni outer core should have floated up into the Si-Al mantle long ago.

 

[davenn]

The missing element mystery in the Earth's core may have been solved.
http://www.bbc.com/news/science-environment-38561076
Lead researcher Eiji Ohtani from the University of Tokyo told BBC News: "We believe that silicon is a major element - about 5% [of the Earth's inner core] by weight could be silicon dissolved into the iron-nickel alloys."

interesting, I will have to do some reading

silicone

silicone is a man made substance ... sealers, fake chest mounds etc

stick with the silicon (no "e")

Dave

 

[1oldman2]

stick with the silicon (no "e")

This problem originates from a life in construction vs. a life in general.

 

[Ophiolite]

I would have expected heavy elements like Gold and Lead would sink into the core of early Earth more so than relatively light Silicon.

The composition of core, mantle and crust, while partly determined by density are also strongly influenced by chemical affinity. Likewise, we would expect that the speed of the iron catastrophe (core formation) would have inhibited complete unmixing of the silicon component.

My curiosity centres on what the silicon is bound to .

Edit: From the paper abstract here it appears they postulate the silicon is alloyed with the iron, or iron-nickel. Not an obvious alloy under crustal conditions, but that's the core for you.

 

[1oldman2]

From the paper abstract here

Thank you,

but that's the core for you

It gets stranger yet.
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/146476

 

[1oldman2]

http://www.nature.com/news/long-awaited-mathematics-proof-could-help-scan-earth-s-innards-1.21439
"The problem is called the boundary-rigidity conjecture. It belongs to the field of Riemannian geometry, the modern theory of curved spaces with any number of dimensions. Albert Einstein built his general theory of relativity - in which mass warps the geometry of space-time - on this branch of mathematics."

 

[1oldman2]

I've heard of "Zealandia" before, this on the other hand caught me by surprise. I'm curious to hear what Dave makes of it. (Could NZ actually have its own Craton? )
http://www.geosociety.org/gsatoday/archive/27/3/abstract/GSATG321A.1.htm
A 4.9 Mkm2 region of the southwest Pacific Ocean is made up of continental crust. The region has elevated bathymetry relative to surrounding oceanic crust, diverse and silica-rich rocks, and relatively thick and low-velocity crustal structure. Its isolation from Australia and large area support its definition as a continent—Zealandia. Zealandia was formerly part of Gondwana. Today it is 94% submerged, mainly as a result of widespread Late Cretaceous crustal thinning preceding supercontinent breakup and consequent isostatic balance. The identification of Zealandia as a geological continent, rather than a collection of continental islands, fragments, and slices, more correctly represents the geology of this part of Earth. Zealandia provides a fresh context in which to investigate processes of continental rifting, thinning, and breakup.
https://phys.org/news/2017-02-zeala...e=menu&utm_medium=link&utm_campaign=item-menu
http://www.bbc.com/news/world-asia-39000936

 

[flatwaterguy07]

The composition of core, mantle and crust, while partly determined by density are also strongly influenced by chemical affinity. Likewise, we would expect that the speed of the iron catastrophe (core formation) would have inhibited complete unmixing of the silicon component.

My curiosity centres on what the silicon is bound to .

Edit: From the paper abstract here it appears they postulate the silicon is alloyed with the iron, or iron-nickel. Not an obvious alloy under crustal conditions, but that's the core for you.

My question to everyone here is how we know exactly what the core is made of? You all speak with such confidence of the make up of the core of the earth, with the acception of this missing element. How did we know it was missing or unidentified? To my knowledge, the deepest man has ever drilled into the Earth is approximately 7.5 miles? Nowhere near the center of earth. I am not highly educated in this subject (save the obvious snide replies to that fact). So I'm honestly asking by what means have we determined the composition of the core? I know sticking to topic is important to a lot of you.. To me, this does relate, but may be a different discussion to have on another thread. And that's fine. I'm learning... Thank you.

 

[Drakkith]

My question to everyone here is how we know exactly what the core is made of? You all speak with such confidence of the make up of the core of the earth, with the acception of this missing element. How did we know it was missing or unidentified? To my knowledge, the deepest man has ever drilled into the Earth is approximately 7.5 miles? Nowhere near the center of earth. I am not highly educated in this subject (save the obvious snide replies to that fact). So I'm honestly asking by what means have we determined the composition of the core? I know sticking to topic is important to a lot of you.. To me, this does relate, but may be a different discussion to have on another thread. And that's fine. I'm learning... Thank you.

We don't know for absolute certainty what the Earth's inner core is made of, but we know that within a reasonable degree of accuracy that it must be composed of mainly Iron and Nickel with about 3-5% of its mass made up of lighter elements. This is based on observations via seismic waves, the known density of the Earth as a whole vs the density of the crust, the relative abundance of elements throughout the solar system, and many other factors. Silicon is believed to make up the bulk of the lighter elements making up the core primarily because it is by far the most abundant element in the Earth's crust capable of staying mixed with iron and nickel. Oxygen is more abundant, but it boils too easily.

 

[flatwaterguy07]

We don't know for absolute certainty what the Earth's inner core is made of, but we know that within a reasonable degree of accuracy that it must be composed of mainly Iron and Nickel with about 3-5% of its mass made up of lighter elements. This is based on observations via seismic waves, the known density of the Earth as a whole vs the density of the crust, the relative abundance of elements throughout the solar system, and many other factors. Silicon is believed to make up the bulk of the lighter elements making up the core primarily because it is by far the most abundant element in the Earth's crust capable of staying mixed with iron and nickel. Oxygen is more abundant, but it boils too easily.

Thank you very much for your clear and concise reply. I do have a question, however. How do we know that it must be made up of mainly Iron and Nickel? With all due respect, it sounds like what you're saying is that this all is one big guess. Granted, clearly there have been years of study invested in this subject but regardless of that, ultimately, at the end of the day we don't know what Earth's core is made up of. Nor do we truly know that there is a "core" as it were. Do we? We're just guessing, right?

 

[berkeman]

Thank you very much for your clear and concise reply. I do have a question, however. How do we know that it must be made up of mainly Iron and Nickel? With all due respect, it sounds like what you're saying is that this all is one big guess. Granted, clearly there have been years of study invested in this subject but regardless of that, ultimately, at the end of the day we don't know what Earth's core is made up of. Nor do we truly know that there is a "core" as it were. Do we? We're just guessing, right?

Maybe read through the Wikipedia article about the Earth's Structure, and follow some of the references out of the article to some of the peer-reviewed journal articles:

https://en.wikipedia.org/wiki/Structure_of_the_Earth#Core

That should help you understand what Drakkith said in his reply, about how science approaches a question like this.

 

[flatwaterguy07]

Maybe read through the Wikipedia article about the Earth's Structure, and follow some of the references out of the article to some of the peer-reviewed journal articles:

https://en.wikipedia.org/wiki/Structure_of_the_Earth#Core

That should help you understand what Drakkith said in his reply, about how science approaches a question like this.

I will do that. Thank you.

 

[Drakkith]

I do have a question, however. How do we know that it must be made up of mainly Iron and Nickel?

I'm not a geologist, so take the following with a grain of salt:

We rely on a combination of different methods from different fields. For example, we can look at the composition of meteorites, which are believed to be representative of the elements making up the rocky planets, and infer from them the composition of the Earth and other planets. We also know enough about stellar nucleosynthesis (creation of different elements inside stars and supernovas) to know that iron and nickel are very abundant compared to heavier elements and should form a substantial fraction of the non-gaseous material available for planetary formation.

Note that the average density of the Earth compared to the Earth's crust places great constraints on what elements could possibly make up the internal structure of the Earth. Since the density of the Earth as a whole is greater than the density of the crust, this means that heavier elements must be making up a substantial amount of the internal structure of the Earth. This fits with our current models of planetary formation in that when proto-planets are still hot and molten, heavier elements tend to sink inwards, which should lead to a core composed mainly of the heavy elements.

So just based off of the composition of meteorites, known reactions from nuclear fusion, the density of the Earth vs the Earth's crust, and some simple physics regarding buoyancy, we can already say that the Earth's internal structure must be composed of something similar to iron and nickel.

But that doesn't guarantee that the core itself is composed of iron and nickel. Further evidence comes from the aforementioned seismic waves. If, say, the mantle was composed mainly of iron and nickel, instead of silicon and oxygen, seismic waves would behave much differently than we observe. We'd also have massive amounts of molten iron and nickel popping out of volcanoes, but we don't see that either.

The details get very, very complicated. Just try reading this paper for example. Or this one. You'll find that unless you've been trained in geology and whatever else may be required, you will have a lot of difficulty understanding the details underlying all of our models. Unfortunately there is no way to teach this in a forum. You'd need to be formally educated (or devote a substantial amount of personal time) to learn it all.

With all due respect, it sounds like what you're saying is that this all is one big guess.

None of the definitions found here of the word 'guess' seem to apply. While there may some guessing involved during the initial formation of a theory or model, the entire process evolves from a guess into a (hopefully) sound model or theory based on months or years of work. I see no reason to consider the end product of this process a "guess".

Granted, clearly there have been years of study invested in this subject but regardless of that, ultimately, at the end of the day we don't know what Earth's core is made up of. Nor do we truly know that there is a "core" as it were. Do we? We're just guessing, right?

No, I wouldn't say that at all.

 

[flatwaterguy07]

I'm not a geologist, so take the following with a grain of salt:

We rely on a combination of different methods from different fields. For example, we can look at the composition of meteorites, which are believed to be representative of the elements making up the rocky planets, and infer from them the composition of the Earth and other planets. We also know enough about stellar nucleosynthesis (creation of different elements inside stars and supernovas) to know that iron and nickel are very abundant compared to heavier elements and should form a substantial fraction of the non-gaseous material available for planetary formation.

Note that the average density of the Earth compared to the Earth's crust places great constraints on what elements could possibly make up the internal structure of the Earth. Since the density of the Earth as a whole is greater than the density of the crust, this means that heavier elements must be making up a substantial amount of the internal structure of the Earth. This fits with our current models of planetary formation in that when proto-planets are still hot and molten, heavier elements tend to sink inwards, which should lead to a core composed mainly of the heavy elements.

So just based off of the composition of meteorites, known reactions from nuclear fusion, the density of the Earth vs the Earth's crust, and some simple physics regarding buoyancy, we can already say that the Earth's internal structure must be composed of something similar to iron and nickel.

But that doesn't guarantee that the core itself is composed of iron and nickel. Further evidence comes from the aforementioned seismic waves. If, say, the mantle was composed mainly of iron and nickel, instead of silicon and oxygen, seismic waves would behave much differently than we observe. We'd also have massive amounts of molten iron and nickel popping out of volcanoes, but we don't see that either.

The details get very, very complicated. Just try reading this paper for example. Or this one. You'll find that unless you've been trained in geology and whatever else may be required, you will have a lot of difficulty understanding the details underlying all of our models. Unfortunately there is no way to teach this in a forum. You'd need to be formally educated (or devote a substantial amount of personal time) to learn it all.
None of the definitions found here of the word 'guess' seem to apply. While there may some guessing involved during the initial formation of a theory or model, the entire process evolves from a guess into a (hopefully) sound model or theory based on months or years of work. I see no reason to consider the end product of this process a "guess".
No, I wouldn't say that at all.

That's fair. I truly appreciate your information, patience and willingness to debate with me rather than shutting me down. That may change after this post... No, I'm kidding. Maybe hypothesis is a more appropriate term than guess?

I did do a little digging into the Wiki on Earth's core and the corresponding references. Although, as you said, it is a bit heavy with formally educated type vocabulary, I managed to fumble my way through it. To see that analysis of how seismic waves traveled through the Earth as the way we first determined there was a core is interesting to me.

It would seem to me, especially back in 1936, technology was not that which would provide reliable data for the purpose of understanding the precise make up of our planet. I can just see her looking at the paper tape with the squiggly lines on it when out of nowhere... "Wait! These squiggles are more squiggly than the last reading... That must mean there's a solid iron core in the center of the Earth reflecting these waves back to the surface." I'm way over-simplifying, but how can we without doubt know that we are observing waves reflecting back off the core of the earth?

What if it's a wave who's source is the original point of seismic activity? Wouldn't you need to know the precise source, direction and intensity of the originating wave to be able to trace it to the core of the Earth and back? How did she do that back in 1936? How would we accurately do that now? Couldn't other factors cause these waves to bend in the way Ms Lehmann says she observed? This just doesn't seem like irrefutable evidence to me. Sounds like another guess, albeit an educated guess or hypothesis for that matter.

Another thing that stuck out to me in both the wiki and the reference material was the abundance of words and phrases like "it is believed to be" "it is thought to be" "it's probable" theoretical, estimated, implied, inferred... all words of uncertainty. I'm not sold on this one.

 

[rootone]

In most cases, anywhere in science, there is no absolute certainty.
In many cases though there is a model that fits very well to experimental data.
If there is no other theory that fits that data then it is reasonable to assume that the model is good.
Especially if the model predicts stuff which is later confirmed by experiment.

 

[berkeman]

Although, as you said, it is a bit heavy with formally educated type vocabulary, I managed to fumble my way through it.

If you can quote the parts that you didn't understand, I think we can help you out. There is some great math behind imaging technologies (much of which existed back in the years you refer to), and with some help from us, you can probably start to understand it.

Saying "I don't believe it because I don't understand the math" is not really accepted here. We are happy to help you understand the math, as long as you are willing to put in the effort.

 

[Drakkith]

It would seem to me, especially back in 1936, technology was not that which would provide reliable data for the purpose of understanding the precise make up of our planet. I can just see her looking at the paper tape with the squiggly lines on it when out of nowhere... "Wait! These squiggles are more squiggly than the last reading... That must mean there's a solid iron core in the center of the Earth reflecting these waves back to the surface." I'm way over-simplifying, but how can we without doubt know that we are observing waves reflecting back off the core of the earth?

Without doubt? You can't. Ever. It's impossible.

What if it's a wave who's source is the original point of seismic activity? Wouldn't you need to know the precise source, direction and intensity of the originating wave to be able to trace it to the core of the Earth and back? How did she do that back in 1936? How would we accurately do that now?

If you'd like to know how seismic waves work and how we detect them, you're going to have to spend a lot of time getting into the technical literature. I certainly don't know the details. But the great thing is that you can find a huge amount of information online, and if not there, then you can almost always order a book off of amazon or somewhere else on a topic. For example, this book would seem to be excellent starting point if you're looking into learning more about seismic theory: https://www.amazon.com/dp/0521708427/?tag=pfamazon01-20

This book provides an approachable and concise introduction to seismic theory, designed as a first course for undergraduate students. It clearly explains the fundamental concepts, emphasizing intuitive understanding over lengthy derivations. Incorporating over 30% new material, this second edition includes all the topics needed for a one-semester course in seismology. Additional material has been added throughout including numerical methods, 3-D ray tracing, earthquake location, attenuation, normal modes, and receiver functions. The chapter on earthquakes and source theory has been extensively revised and enlarged, and now includes details on non-double-couple sources, earthquake scaling, radiated energy, and finite slip inversions. Each chapter includes worked problems and detailed exercises that give students the opportunity to apply the techniques they have learned to compute results of interest and to illustrate the Earth's seismic properties. Computer subroutines and datasets for use in the exercises are available at www.cambridge.org/shearer.

If you question the material in that book, then you'd need to get into the underlying physics supporting geology and seismic waves, which, unsurprisingly, is extremely complicated.

Couldn't other factors cause these waves to bend in the way Ms Lehmann says she observed? This just doesn't seem like irrefutable evidence to me. Sounds like another guess, albeit an educated guess or hypothesis for that matter.

The initial idea was a hypothesis, but the subsequent development of that idea turned it into a theory. Note that just because a theory exists does not mean it is correct. The Earth's core could very well be made out of some strange type of matter never seen before. We just have little reason to believe so.

Another thing that stuck out to me in both the wiki and the reference material was the abundance of words and phrases like "it is believed to be" "it is thought to be" "it's probable" theoretical, estimated, implied, inferred... all words of uncertainty. I'm not sold on this one.

Those phrases are found throughout all of science, so if you're basing your trust on science on whether or not it uses those words then you're going to be sorely disappointed everywhere you look. In the end it comes down to whether you trust that scientists generally know what they're doing or not.

 

[billiards]

how can we without doubt know that we are observing waves reflecting back off the core of the earth?

Because we make a model of the Earth that includes a solid core and predict when and where the energy from an earthquake would arrive. Then we look at seismic recordings of all the earthquakes that have ever got recorded and we see that our model fits the data (it predicts what we observe). Then some time in the future an earthquake happens and we can tell you exactly where to look to find that little squiggle that was caused by the wave that bounced off the core. You look at the seismic recordings for a station in Mozambique and sure enough at 12.45 on the 18th of January GMT there's that little squiggle in the recording which has a wavelet with exactly the size and shape we predicted.

There are other, independent, observations of the so-called "PKJKP" phase --- a shear wave passing through the inner core --- that would necessitate that inner core is solid.

Wouldn't you need to know the precise source, direction and intensity of the originating wave to be able to trace it to the core of the Earth and back?

You get this sort of info by looking at data from multiple recording stations. There are people who do this sort of thing for a full time job. See here for a database of loads of earthquakes: http://www.globalcmt.org/CMTsearch.html

 

[davenn]

@billiards

great response

 

[1oldman2]

@billiards

great response

That says it all.
Here are some abstract links to papers from the meeting I pulled the original post from. The BBC article I cited was a spin off from these. They are rather technical by my standards but they give an idea of how the concept was developed.
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/153611
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/146476
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/160185
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/146572
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/128201
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/136411
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/174036
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/196402

 

[billiards]

In terms of the "light" element in the core. It is important to reiterate that we are talking about ~5%.

Nobody doubts that 95% is made of iron and nickel.