Featured Rethinking the Earth's core

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

 

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.

 

interesting, I will have to do some reading

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

stick with the silicon (no "e")

Dave

 

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 alot 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.

 

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?

 

I will do that. Thank you.

 

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 with out 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.

 

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.

 

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

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/Introduction...TF8&qid=1489199657&sr=8-2&keywords=seismology

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.

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.

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.

 

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.

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

 

@billiards

great response

 

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.