# Physicists/Geologists - Evolution of Earth Internal Layers
**URL:** https://boards.straightdope.com/t/physicists-geologists-evolution-of-earth-internal-layers/698066
**Category:** Factual Questions
**Created:** [September 10, 2014, 2:00pm UTC](https://boards.straightdope.com/t/physicists-geologists-evolution-of-earth-internal-layers/698066 "2014-09-10T14:00:40Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@am77494](https://boards.straightdope.com/u/am77494)
#### Post date: [September 10, 2014, 2:00pm UTC](https://boards.straightdope.com/t/physicists-geologists-evolution-of-earth-internal-layers/698066/1 "2014-09-10T14:00:40Z")
I remember reading about the Chandrasekhar limit : as to how a star of a certain mass goes on to become a black hole or explodes in a supernova - depending on its starting mass.
Got me wondering - do we know how the[internal layers](http://en.wikipedia.org/wiki/Structure_of_the_Earth#mediaviewer/File:RadialDensityPREM.jpg)of the earth came about ? Is it correct to assume that Earth started out originally with homogeneous structure and over time developed the mantle, crust, etc ? If so - whats the timeline of the current structure ?
Bonus questions : Do all planets in our solar system have internal layers like the earth ? How about stars - do all stars have these layers?
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### Author:  [@njtt](https://boards.straightdope.com/u/njtt)
#### Post date: [September 10, 2014, 2:08pm UTC](https://boards.straightdope.com/t/physicists-geologists-evolution-of-earth-internal-layers/698066/2 "2014-09-10T14:08:01Z")
I think all the major planets probably have layers, but not all like the Earth. Some of them are mostly gas. I believe all the solid planets went through a stage of being completely molten, and the layers you are talking about fractionated out in the liquid mix under the influence of gravity. Thus most of the heavy stuff (mostly iron) went to the core.
Of course, most of the layers, the strata, we see in crustal rocks are sediments, formed much later, when things were cool, and most of them under water. (Some are lava flows, and the like.)
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### Author:  [@Xema](https://boards.straightdope.com/u/Xema)
#### Post date: [September 10, 2014, 2:24pm UTC](https://boards.straightdope.com/t/physicists-geologists-evolution-of-earth-internal-layers/698066/3 "2014-09-10T14:24:16Z")
> [@am77494](#):
>
> a star of a certain mass goes on to become a black hole or explodes in a supernova - depending on its starting mass.
“Or” isn’t correct here: a mass greater than the Chandrasekhar limit leads to a supernova, the remnant of which is a black hole.
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### Author:  [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)
#### Post date: [September 10, 2014, 3:41pm UTC](https://boards.straightdope.com/t/physicists-geologists-evolution-of-earth-internal-layers/698066/4 "2014-09-10T15:41:59Z")
> [@am77494](#):
>
> Is it correct to assume that Earth started out originally with homogeneous structure and over time developed the mantle, crust, etc ? If so - whats the timeline of the current structure ?
Eh, pretty much. The earth (like other planets) formed when little bits of stuff smacked into other little bits of stuff and started clumping together due to gravity. As time went on, more and more bits clumped together, and the earth started getting bigger and bigger.
As the earth grew, heating from compression and from radioactive materials caused it to get hotter and hotter, eventually resulting in what is called the “iron catastrophe”. This is when it got hot enough to melt things like iron and nickel, and that’s when the earth started forming layers. Now that the materials were molten and could move, the heavier stuff like iron and nickel sank, while the lighter materials like silicon and carbon were forced up towards the surface. As the earth cooled, these lighter elements solidified and formed the crust.
> [@am77494](#):
>
> Bonus questions : Do all planets in our solar system have internal layers like the earth ? How about stars - do all stars have these layers?
Yes, all planets have layers, though as was already mentioned their composition isn’t necessarily at all like the earth’s. Jupiter’s core for example is a dense mass of several different elements surrounded by liquid hydrogen and helium.
Stars also have layers. Stars are great big fusion reactors, fusing hydrogen into helium, and also fusing helium into heavier elements. The heavier elements sink deeper into the core. Like planets, stars also vary in their layers, depending on how big and how old the star is. Older stars will have more heavier elements, as their fusion reactions have been burning for much longer. At the center of a star will be its core, where fusion occurs. Outside of this is a radiation layer, then a convection layer, which are both named for how the energy of the star flows through them. Outside of that you have the photosphere, where photons are generated, then the chromosphere, which is kinda like the star’s atmosphere, and then a super hot region called the corona.
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### Author:  [@MrDibble](https://boards.straightdope.com/u/MrDibble)
#### Post date: [September 10, 2014, 4:11pm UTC](https://boards.straightdope.com/t/physicists-geologists-evolution-of-earth-internal-layers/698066/5 "2014-09-10T16:11:13Z")
> [@am77494](#):
>
> do we know how the[internal layers](http://en.wikipedia.org/wiki/Structure_of_the_Earth#mediaviewer/File:RadialDensityPREM.jpg)of the earth came about ?
We have some good guesses.
> [@](#):
>
> Is it correct to assume that Earth started out originally with homogeneous structure and over time developed the mantle, crust, etc ?
“Homogeneous” isn’t _quite_ the word I’d use for a conglomeration of protoplanets which may, themselves be differentiated, but I think I know what you mean, and yes, the structure developed over time. There was a whole lot of melting going on - radioactive-driven and impact-driven melting.
> [@](#):
>
> If so - whats the timeline of the current structure ?
The [Rain-out model](http://en.wikipedia.org/wiki/Rain-out_model) suggests just a few dozen millennia after the Earth accumulated, you’d have [a core-mantle differentiation](http://en.wikipedia.org/wiki/Iron_catastrophe). The crust would take longer to form - it requires igneous processes in the mantle that take time - to differentiate out lighter elements - say, 100 Ma.
> [@](#):
>
> Bonus questions : Do all planets in our solar system have internal layers like the earth ?
That’s the theory. Even some planetoids do, like [Vesta](http://en.wikipedia.org/wiki/4_Vesta#Geology) and [Ceres](http://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29#Internal_structure)
> [@](#):
>
> How about stars - do all stars have these layers?
It’s not quite the same with stars - structure is driven less by compositional differences and more by zones of reaction/energy transfer types, I think. But I’m no expert.
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### Author:  [@am77494](https://boards.straightdope.com/u/am77494)
#### Post date: [September 10, 2014, 5:10pm UTC](https://boards.straightdope.com/t/physicists-geologists-evolution-of-earth-internal-layers/698066/6 "2014-09-10T17:10:59Z")
> [@engineer\_comp\_geek](#):
>
> As the earth grew, heating from compression and from radioactive materials caused it to get hotter and hotter, eventually resulting in what is called the “iron catastrophe”. This is when it got hot enough to melt things like iron and nickel, and that’s when the earth started forming layers. Now that the materials were molten and could move, the heavier stuff like iron and nickel sank, while the lighter materials like silicon and carbon were forced up towards the surface. As the earth cooled, these lighter elements solidified and formed the crust.
Reading through the history of Geology has left me very skeptical of the claims that seem so obvious. I have a few questions on this and hopefully I can understand better if answered :
> [@engineer\_comp\_geek](#):
>
> As the earth grew, heating from compression
When you say grew, do you mean gained mass by other objects joining it - or grew as in expanded due to temperature rise ? Compression would result in shrinking not growing - right ?
> [@engineer\_comp\_geek](#):
>
> heating from compression and from radioactive materials
Do we know which type of heat dominated ? Radioactive or Compressive ? Did we have enough radioactive material to sustain fusion reactions ? Was this like U-235 or Pu-239 - what elements were they ? Also how much heat is produced by compressive heating of the magnitude on earth - we did not approach a Plasma state or fusion temperatures / densities - did we ?
> [@engineer\_comp\_geek](#):
>
> This is when it got hot enough to melt things like iron and nickel, and that’s when the earth started forming layers
How do we know that a molten planet will start forming layers ? Is this due to gravity alone - or does the earth’s rotation play a part like a centrifuge ? Also - it would take a lot of conductivity to conduct the heat presumably from the earth’s center to the outside to melt the outer elements. Also, wouldn’t a molten core slow down the radioactive reactions / heat by attenuating neutrons/diluting radioactive pockets ?
> [@engineer\_comp\_geek](#):
>
> Now that the materials were molten and could move, the heavier stuff like iron and nickel sank
Wouldn’t that mean that all the nickel and iron mined on earth/seafloor will roughly occur in the same proportions (since they were well mixed as liquids) in all ores ? We know that is not the case. What gives ?
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### Author:  [@MrDibble](https://boards.straightdope.com/u/MrDibble)
#### Post date: [September 10, 2014, 6:01pm UTC](https://boards.straightdope.com/t/physicists-geologists-evolution-of-earth-internal-layers/698066/7 "2014-09-10T18:01:31Z")
> [@am77494](#):
>
> Reading through the history of Geology has left me very skeptical of the claims that seem so obvious.
What? Why?
> [@](#):
>
> When you say grew, do you mean gained mass by other objects joining it - or grew as in expanded due to temperature rise ?
Accreting other objects
> [@](#):
>
> Do we know which type of heat dominated ? Radioactive or Compressive ?
It likely varied from time to time. And don’t forget impact energy.
> [@](#):
>
> Did we have enough radioactive material to sustain fusion reactions ?
Probably some (similar to [Oklo](http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor)) but that wasn’t the main driver. Radioactive elements are just plain hot -as in, physically give off heat - without actually fissioning.
> [@](#):
>
> Was this like U-235 or Pu-239 - what elements were they ?
[Al[sup]26[/sup]](http://en.wikipedia.org/wiki/Aluminium-26)is considered the major contributor.
> [@](#):
>
> Also how much heat is produced by compressive heating of the magnitude on earth - we did not approach a Plasma state or fusion temperatures / densities - did we ?
No-one’s suggesting anything near that hot. Just hot enough to melt rocks - and that happens right under your feet, right now.
> [@](#):
>
> How do we know that a molten planet will start forming layers ?
We know ours did, we know the Moon did, we know Mars did…and we have the physics of it down pat.
> [@](#):
>
> Is this due to gravity alone - or does the earth’s rotation play a part like a centrifuge ?
Gravity is enough. Density differences [can do wonders](http://en.wikipedia.org/wiki/Salt_tectonics), even when stuff isn’t molten.
> [@](#):
>
> Also - it would take a lot of conductivity to conduct the heat presumably from the earth’s center to the outside to melt the outer elements.
Nickel-iron is plenty conductive. And the melting wasn’t initially only centre-outwards, quite a lot of it happened on the surface first.
> [@](#):
>
> Also, wouldn’t a molten core slow down the radioactive reactions / heat by attenuating neutrons/diluting radioactive pockets ?
Well, yes, notice how the planet\* isn’t\* a molten cueball these days? That’s because most of the radioactive elements are dissolved in the NiFe core.
> [@](#):
>
> Wouldn’t that mean that all the nickel and iron mined on earth/seafloor will roughly occur in the same proportions (since they were well mixed as liquids) in all ores ?
No, why would they? Iron ores aren’t derived from the core, they’re derived mostly from dissolved iron that’s precipitated out, or iron oxides in igneous rocks. There’s whole cycles of redistribution and beneficiation at work before you arrive at iron ores. A lot of it geared towards making iron a lot more common than nickel as an ore mineral.