<p>Not all the iron sank to the core. Have a look at my answer to a previous question:</p>

<p><a href="https://earthscience.stackexchange.com/a/7076/725">https://earthscience.stackexchange.com/a/7076/725</a></p>

<p>Your question is similar, but coming the other way. You're asking - why is there iron in the mantle/crust?</p>

<p>The answer is that there was enough oxygen left in the Earth after oxidising the other lithophile elements (Si, Mg, Al, Ca, etc) to oxidise some of the iron. And when iron is oxidised - it is not a metal anymore, but instead incorporated into rock forming minerals like olivine and pyroxenes.</p>

CC BY-SA 3.0
2018-02-09T13:31:00.847Z
So all the siderophile elements attach themselves to iron before iron oxidizes?
AtmosphericPrisonEscape
CC BY-SA 3.0
2018-02-09T21:14:59.787Z
@AtmosphericPrisonEscape yes. It's less about time, more about chemistry. Siderophile elements partition to metallic iron and do not partition into oxidised iron (silicates and oxides to be exact).
Gimelist
CC BY-SA 3.0
2018-02-12T17:23:49.213Z
Ok, makes sense. So Iron is indeed a leftover of the iron catastrophe, and the reason why that fraction didn't sink is because it was oxidized. Thanks!
Camilo Rada
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2018-02-12T19:54:53.243Z
@CamiloRada exactly.
Gimelist
CC BY-SA 3.0
2018-02-09T09:20:36.600Z
2018-02-09T09:20:36.600Z
Gimelist
<p>When pondering the hypothesis of the <a href="https://en.wikipedia.org/wiki/Iron_catastrophe" rel="noreferrer">Iron catastrophe</a>, that seem to be widely accepted nowadays. It surprises me that currently the crust still contain a significant amount of Iron (<a href="https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Crust" rel="noreferrer">5.6% in weight</a>).</p>

<p>I wonder if that much Iron is what didn't sink in the Iron catastrophe (just because the process wasn't perfect), or it can be explained by slow reloading with iron transported from the outer core and mantle to the crust trough convection and volcanic activity.</p>

<p>Or perhaps it cam from other sources like meteorites?</p>

<p><a href="https://i.sstatic.net/pSpKE.png" rel="noreferrer"><img src="https://i.sstatic.net/pSpKE.png" alt="enter image description here"></a>
Picture from <a href="http://astro.hopkinsschools.org/course_documents/earth_moon/earth/geologic_time/iron_catastrophe.htm" rel="noreferrer">astro.hopkinsschools.org</a></p>

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2018-02-08T18:57:19.697Z
Don't forget meteor impacts during the late heavy bombardment.  (not an answer, just a possibility).    I'd also add that the early oceans held some dissolved Iron that wouldn't sink.   Much of the Iron we mine today is from the early oceans.
userLTK
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2018-02-08T18:58:26.177Z
@userLTK Good points, I'll add those considerations to the question.
Camilo Rada
CC BY-SA 3.0
2018-02-08T19:11:30.513Z
@userLTK Thinking it again. Were there oceans during the iron catastrophe? I would think it was way too hot for liquid water on the surface.
Camilo Rada
CC BY-SA 3.0
2018-02-08T19:17:28.843Z
Yes, that's a bit of a sticky subject and not one I really wanted to get into in detail.   You asked Iron currently being 5.6% of the mass of the crust - some of that 5.6% came from Iron that was dissolved in the oceans prior to oxygen from photosynthesis pulling it out.   I really just wanted to say - look at this possibility, I don't know enough to try to make an answer.
userLTK
CC BY-SA 3.0
2018-02-09T04:22:44.673Z
considering 35%of the planet (on average) is iron and the crust is only about 5% iron you're not looking at a huge retention, keep in mind the iron is not always going to be heavier, it depends on if it is bonded to anything.
John
<geophysics><geochemistry><volcanology><core><iron>
Is the iron on Earth's crust a leftover of the Iron catastrophe, or it was brought back by volcanoes?
Camilo Rada
# Is the iron on Earth's crust a leftover of the Iron catastrophe, or it was brought back by volcanoes?

(Asked by: username_0 on 2018-02-08)

When pondering the hypothesis of the [Iron catastrophe](https://en.wikipedia.org/wiki/Iron_catastrophe), that seem to be widely accepted nowadays. It surprises me that currently the crust still contain a significant amount of Iron ([5.6% in weight](https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Crust)).

I wonder if that much Iron is what didn't sink in the Iron catastrophe (just because the process wasn't perfect), or it can be explained by slow reloading with iron transported from the outer core and mantle to the crust trough convection and volcanic activity.

Or perhaps it cam from other sources like meteorites?

[![enter image description here](https://i.sstatic.net/pSpKE.png)](https://i.sstatic.net/pSpKE.png)
Picture from [astro.hopkinsschools.org](http://astro.hopkinsschools.org/course_documents/earth_moon/earth/geologic_time/iron_catastrophe.htm)

**Tags:** geophysics, geochemistry, volcanology, core, iron

> **Comments:**
> **username_1**: Don't forget meteor impacts during the late heavy bombardment.  (not an answer, just a possibility).    I'd also add that the early oceans held some dissolved Iron that wouldn't sink.   Much of the Iron we mine today is from the early oceans.  
> **username_0**: @username_1 Good points, I'll add those considerations to the question.  
> **username_0**: @username_1 Thinking it again. Were there oceans during the iron catastrophe? I would think it was way too hot for liquid water on the surface.  
> **username_1**: Yes, that's a bit of a sticky subject and not one I really wanted to get into in detail.   You asked Iron currently being 5.6% of the mass of the crust - some of that 5.6% came from Iron that was dissolved in the oceans prior to oxygen from photosynthesis pulling it out.   I really just wanted to say - look at this possibility, I don't know enough to try to make an answer.  
> **username_2**: considering 35%of the planet (on average) is iron and the crust is only about 5% iron you're not looking at a huge retention, keep in mind the iron is not always going to be heavier, it depends on if it is bonded to anything.  

---

## 1 answers

### Answer by username_3 on 2018-02-09. Score: 9 | Accepted answer

Not all the iron sank to the core. Have a look at my answer to a previous question:

<https://earthscience.stackexchange.com/a/7076/725>

Your question is similar, but coming the other way. You're asking - why is there iron in the mantle/crust?

The answer is that there was enough oxygen left in the Earth after oxidising the other lithophile elements (Si, Mg, Al, Ca, etc) to oxidise some of the iron. And when iron is oxidised - it is not a metal anymore, but instead incorporated into rock forming minerals like olivine and pyroxenes.

> **Comments:**
> **username_4**: So all the siderophile elements attach themselves to iron before iron oxidizes?  
> **username_3**: @username_4 yes. It's less about time, more about chemistry. Siderophile elements partition to metallic iron and do not partition into oxidised iron (silicates and oxides to be exact).  
> **username_0**: Ok, makes sense. So Iron is indeed a leftover of the iron catastrophe, and the reason why that fraction didn't sink is because it was oxidized. Thanks!  
> **username_3**: @username_0 exactly.  

---

