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This question has puzzled me for a while. I know that earth's mantle is made of different minerals, metals and rocks etc. and that has always made complete sense to me. But why is the inner core made of an iron-nickel alloy? The only reasons that I could come up with (they are kind of crazy) is that:

A. The iron-nickel alloy that composes the inner core is heavier than other elements in the mantle and outer core, causing it to sink to the center of earth.

B. Since both iron and nickel and attracted to magnets earth's magnetic field drew the to metals into the inner core mixing them into the alloy.

C. Earth started as a giant asteroid composed of an iron-nickel alloy (which many are), and other materials from space built upon it causing earth to have an iron-nickel core (I told you they were crazy!).

But these explanations don't really make much sense to me.

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    $\begingroup$ A is correct, essentially. This happened when Earth formed. $\endgroup$ – Neo Apr 25 '14 at 18:28
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    $\begingroup$ It happened shortly after the Earth formed, not when. That planetary differentiation represented a huge drop in potential energy. It's the principle of minimum potential energy again, the same reason why the Earth is an oblate spheroid. $\endgroup$ – David Hammen May 2 '14 at 1:40
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The first thing you should think about is how the accretionary disk cooled and the cosmochemical constraints this put on Earth (But I am not going into details here). From studying meteorites it is apparent that the oldest meteorites don't show signs of chemical differentiation (e.g. melting, ...) and are thought to represent the solids that formed from the accretionary disk. Because of their characteristic round structures, chondrules, they are called Chondrites. The most chemically primitive Chondrites (e.g. https://en.wikipedia.org/wiki/Allende_meteorite) roughly have the same composition as Earth. This is why it is thought that Earth formed by accretion of these smaller chondritic objects.

There are also other meteorites called "achondrites", meaning "no chondrules". They show signs of chemical differentiation (we can find stony-, stony-iron-, and iron-achondrites). For this chemical differentiation to happen it is necessary to think of the meteorite as having a parent body, on which this differentiation took place. These parent bodies differentiated similar to earth into an iron-nickel core (iron-achondrites), an olivine-rich mantle (stony and stony-irons) and a silicate crust (stony-achondrites). Because differentiation takes time, it is somewhat unlikely that a parent body formed, was destroyed and the iron-core was recycled to nucleate Earth (Earth has roughly the same radiometric age as many meteorites). So your suggestion C would take quite some explaining. Also suggestion B is not temporally possible, because the magnetic field needs a liquid iron core to work and therefore only came into existence after the iron migrated to the core.

http://www.nature.com/nature/journal/v491/n7422/fig_tab/nature11565_F4.html - Current thoughts about differentiation

A few words about iron-nickel. This has something to do with the abundance of elements in the accretionary disk (http://upload.wikimedia.org/wikipedia/commons/e/e6/SolarSystemAbundances.png). Iron and Nickel are very common elements. They are also siderophile, which means, that when a chondrite melts, the iron will try to separate from the sulfide- and silicate-melt. Because of the larger density of this melt, it will try to move towards the core of a planet. But it is very likely that other elements form a certain percentage of the core's chemical composition and it can be reckoned that this will be similar to some of the compositions of iron-meteorites.

If you get more interested in this I can fully recommend "McSween, Harry Y. (1999). Meteorites and their parent planets (2. ed. ed.). Cambridge [u.a.]: Cambridge University Press. ISBN 978-0521583039." which is very enjoyable to read and because of its descriptive approach not outdated. The newer book is also very good "Huss, Harry Y. McSween, Jr., Gary R. (2010). Cosmochemistry. Cambridge: Cambridge University Press. ISBN 978-0521878623.".

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  • $\begingroup$ This appears to be a bit outdated of an answer. The most widely accepted hypothesis regarding the formation of terrestrial planets is formation of planetesimals followed by runaway growth and then oligarchic growth. Those protoplanets / oligarchs most likely were at least partially differentiated (think iron meteorites). The question then is what happens when two oligarchs collide: Do their cores remain intact and then merge, or is at least one of the cores disrupted, making the merged protoplanet have to start all over again to form a new core (at least to some extent)? $\endgroup$ – David Hammen May 2 '14 at 14:58
  • $\begingroup$ Can you post your citations for that. I am still relying on my Uni-course, which is some years ago. $\endgroup$ – tobias47n9e May 2 '14 at 16:27
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    $\begingroup$ @DavidHammen your comment may relate to the moon-forming hypothesis, where another Mars-sized planet hit the earth, causing the cores of both planets to merge. Both cores had Fe-Ni cores to begin with anyway. $\endgroup$ – Gimelist Nov 11 '14 at 11:31
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Your guess A is correct. Iron, nickel, and other heavy metals like gold, are what makes up the core of the earth because they are the heaviest of the metals and so sank toward the center of the earth.

Also, since the earth is believed to have been made out of loose material (asteroids) out in space, it would make sense that it would be made out of much of the same materials as the asteroids themselves. Which, I guess, would make C partially correct also.

B is not correct because, (as pointed out in Spießbürger's answer):

The magnetic field needs a liquid iron core to work and therefore only came into existence after the iron migrated to the core.

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  • $\begingroup$ As a note, there are other processes that prevent some heavy metals from sinking - mostly based on their chemical affinity. For example, uranium is much denser than iron, but is mostly concentrated in the crust. $\endgroup$ – Luaan Mar 9 '16 at 16:05
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From what I found after doing a little research on this question, your answer C is the most accurate explanation.

Scientists believe that the overall chemical composition of the Earth is very similar to a kind of meteorite called chondrites, which formed at the same time the Earth was formed. We know a lot about the composition of the Earths crust and mantle, because we can observe those rocks that have been brought to the surface by geologic processes. By comparing the composition of rocks from the Earths crust and mantle to the composition of chondrites, we can see what elements are missing, and therefore must be found in the core. Theories about how Earths magnetic field is formed, as well as experiments done at high temperature and pressures give clues as to the actual composition of the Earths core. Based on all these theories and observations, scientists know that the Earths core is mostly iron with some nickel and lighter elements such as oxygen or sulfur.

Reference: http://scienceline.ucsb.edu/getkey.php?key=2618

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    $\begingroup$ that doesnt mean C is the best case, it just means that we use Chondrites as analogs for Earth's composition. $\endgroup$ – Neo Apr 27 '14 at 1:00

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