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I recently read that the earth's core rotates faster than the surface.
Well, firstly, it's easier to digest the concept of planetary bodies, stars, galaxies in rotation and/or orbital motion.

But, what makes a planet's core rotate? And in the earth's case, faster than its surface?

Secondly, I am aware that the core's rotation is what lends the earth its magnetic field but.. what keeps it going in the first place?

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The same thing that makes the surface rotate: the fact that it always rotated. Because angular momentum is (almost) conserved our planet has no other option than to always rotate. (Of course in reality this picture is complicated by interactions with rest of the universe, but those are just small corrections). You can ask then why did it rotate in the first place and this has to do with the origin of the Solar system.

So, the main question is not what makes Earth rotate but what layers of Earth rotate at what speed. If Earth were a solid body then it would rotate with same angular speed everywhere. But the interior of the Earth is liquid, so the picture is quite complicated. Only thing that is obvious is that every part of Earth must rotate (at some speed or other) because of friction: if one layer (say crust) were rotating and other (say mantle) weren't then after a little while it would start to rotate anyway because the atoms of the moving layer would drag the atoms of the static layer with them.

To say anything more than this one would have to consider precise materials and fluid equations in the interior of Earth. Hopefully someone else will come along and complete the picture.

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The earth's rotation is slowing down, due primarily to tidal interaction with the moon (transfering angular momentum to the moon). This frictional loss occurs near the surface. On this basis, if the rotational coupling of the inner core to the mantle is imperfect, it would be expected to lag with respect to the slowdown. I think precessional changes (cycle length roughly 23000 years) are probably more important for generating rotational mistmatches. Then it all gets mixed up with/by thermal convection and magnetic fields as well.

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The moon was formed in relatively low earth orbit, and the day was probably only a few hours long during the early period of our planets history. This slowdown of the planets rotation is not such a small thing as people might imagine. The moon gets a couple of cm further away every year. –  Omega Centauri Nov 28 '10 at 4:15
    
I had only heard the moon's rotation helps stabilize the earth's rotation didn't imagine it might be slowing it down too.. –  Robin Maben Nov 28 '10 at 14:31
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According to our current understanding of the formation of planets, they are created from dust which originates in a previous supernova (or other large) explosion. This is called a nebula.

http://en.wikipedia.org/wiki/Nebular_hypothesis

As this nebula contracts into planets, there are generally two possible cases: that the dust that is eventually going to form a planet is spinning, overall, or not. The most general and likely case is that it is spinning in some form. There is no overall reason why it shouldn't, short of some "magical" combination of canceling factors.

What happens when this spinning dust cloud contracts under gravity? It spins faster (think of an ice skater spinning), due to the conservation of angular momentum.

Therefore, you end up with planets with a relatively fast spinning core.

Now, what keeps this spin going? Fundamentally, conservation of angular momentum. Nothing is keeping it going, and it's actually spinning down. It should eventually stop - in million or billions of years. That's how much momentum is stored in it :-)

Why does the crust spin slower than the core? For two reasons: since things spin faster as they contract, you naturally end up with a variation of speed between the outer layers and the inner core; secondly, the core is actually dragging the crust around, but the Earth is not really a solid so the drag cannot be thought of as a rigid motion.

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"the conservation of the moment of inertia"... the conservation of angular momentum. The moment of inertia is changing, that is way there is a spin up. –  Vagelford Nov 27 '10 at 16:56
    
@Sklivvz: Oh. All that massive rotation is just conservation? interesting, I almost believed there might be something contributing. Never really heard or read of planets that eventually stopped rotating. Neither old nor newly discovered ones. hmmmm.. –  Robin Maben Nov 27 '10 at 17:44
    
@Vagelford, you are right of course, I stand corrected. –  Sklivvz Nov 27 '10 at 21:00
    
@Sklivvz Obviously, "that is way" should have been "that is why". I think my typo was worse than yours. lol –  Vagelford Nov 27 '10 at 22:01
    
A planet that is too close to its star (or a moon too close to its planet) can lose rotational energy through tidal effects until –  Omega Centauri Nov 29 '10 at 2:31
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On a planetary scale the earths surface is a good insulator and we do not lose significant net heat to space or gain net heat from the sun. So heat generated may build up over time. Radioactive decay heat and gravitationally produced friction would tend to melt the interior of a sufficiently large rocky planet. Heavy elements would sink toward the center and thus conservation of angular momentum would cause the resulting core to spin faster than the surface. It might take the heavy elements a long time to sink to the core. This relative motion core vs magma might help generate the earths magnetic field. (Lots of ideas but no one seems to have a verifiable theory on what causes the earths magnetic field.) Friction between the core and the magma would tend to reduce the difference in rotation rate over time. Only thing we know for sure is that earths magnetic field has continued to decline over the last few centuries we have been making measurements.

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Precisely the inner core is rotating a bit faster than the planet surface. The outer core, which separates it from the mantle and crust, is a liquid with quite a complex dynamics --mostly probably rotation, convection plus creation of Earth magnetic field. While this all is governed by magnetohydrodynamics it very hard even to simulate the phenomenon, and the obvious lack of experimental data is not helping. In a light of this there is no wonder it does not directly transfers angular speed from inner core to the surface.
The question what is the origin is even harder; most probably the whole energy of Earth interior is derived from the times of Solar System creation, but as I wrote before how this energy manifests is a highly nonlinear and difficult to predict.

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protected by Qmechanic Apr 7 '13 at 5:22

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