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Author Topic: Why is there a difference between the rotation rate of the Earth's core and man  (Read 3620 times)

Offline Bill S

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It seems that the Earth's core is rotating faster than the mantle.  why would this be?


 

Offline RD

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The moon's braking (tidal) effects would have a greater effect on the outer layers of Earth, slowing their rotation more than the core.
« Last Edit: 29/07/2012 09:08:28 by RD »
 

Offline Bill S

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Thanks, RD. I was looking down for an answer. :)
 

Offline CliffordK

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If that is the case, then we may attribute Earth's magnetic field to the moon. 
Another reason why our moon has been important on Earth.
 

Offline Bill S

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If that is the case, then we may attribute Earth's magnetic field to the moon.

Would it go any way towards explaining why the Earth's magnetic field varies with time, and why there are periodic magnetic reversals?
 

Offline RD

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... Would it go any way towards explaining why the Earth's magnetic field varies with time, and why there are periodic magnetic reversals?

The collective gravitational forces of other bodies in the solar system on Earth cause Earth's motion to vary in a periodic manner ..

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Precession is the trend in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of roughly 26,000 years. This gyroscopic motion is due to the tidal forces exerted by the Sun and the Moon on the solid Earth
https://en.wikipedia.org/wiki/Milankovitch_cycles#Axial_precession

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The Earth's eccentricity varies primarily due to interactions with the gravitational fields of Jupiter and Saturn.
https://en.wikipedia.org/wiki/Milankovitch_cycles#Earth.E2.80.99s_movements

If their gravitational influence  can periodically change Earth's eccentricity, tilt and precession rate then they are candidates for the cause of polar reversal.
« Last Edit: 01/08/2012 08:11:55 by RD »
 

Offline Boogie

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The moon's braking (tidal) effects would have a greater effect on the outer layers of Earth, slowing their rotation more than the core.

So would it be safe for me to assume that the earth (before the moon was present), rotated at a much faster speed, which began slowing down once the moon was introduced? Probably very quickly at first? So once upon a time, the crust and the core were rotating at the same speed perhaps? If that is the case, as Clifford points out, there may not have been any magnetic field to protect the earth from solar wind and such. I wonder what kind of life would have evolved due to no magnetic field shield.

I also wonder what speed the earth was rotating prior to the moon joining the team? Could it be possible that the earth was spinning too fast to hold an atmosphere, if not for the moon putting the brakes on? Wow! I ask a lot of questions... :P

I've always taken the moon for granted and only appreciated it for it's beauty. Only recently, have I began thinking about the importance of the moon and it's quite an awakening. It's also very fascinating.
 

Offline RD

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So once upon a time, the crust and the core were rotating at the same speed perhaps?

Even without the moon there are tidal forces from the sun, (much weaker than those from the moon though).
 

Offline RD

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I also wonder what speed the earth was rotating prior to the moon joining the team? Could it be possible that the earth was spinning too fast to hold an atmosphere

Earth existing and rotating too fast to retain an atmosphere are incompatible.
If the day length was less than 85 minutes centrifugal force would cancel out gravity at the equator.
However at that speed the largely liquid Earth would deform (oblate) and probably disintegrate at that speed.

Wind would be more severe on an Earth which rotated quicker, e.g. the wind speeds in hurricanes/tornadoes would be higher, ( maybe more frequent too ).
 

Offline CliffordK

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You wouldn't necessarily have to have Zero-G at the surface of the Earth to loose the atmosphere.  I've seen discussions that Hydrogen and Helium can escape from our atmosphere (to where?)  Neon probably also escapes from the atmosphere.  Perhaps water vapor would also escape if there wasn't the "cold trap".  So, one would only have to have a rotational velocity high enough that the oxygen and nitrogen would behave like neon.
« Last Edit: 02/08/2012 09:36:57 by CliffordK »
 

Offline evan_au

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The report of super-rotation of the Earth's core was printed in Science in 2005 (paid subscription).
They studied 18 pairs of earthquakes which occurred in the same locations, but years apart. A free summary is available here: http://news.nationalgeographic.com/news/2005/08/0825_050825_earthcore.html

It is thought that the core takes about 900 years to rotate once compared to the surface of the Earth.

The progressive crystallisation of iron at the surface of the solid core would release latent heat, which drives convective motion in the mantle, which in turn is thought to produce Earth's magnetic field. The core itself probably can't produce a magnetic field, as it is above the Curie temperature of iron.

What is not clear is whether the rotation of the core is a cause or an effect. Convection would occur even without the rotation of the core. But there is a hint that perhaps the rotation of the core is driven by the convection of the mantle.

More detailed study of earthquake waves is slowly producing a more detailed image of convection patterns within the mantle, by measuring arrival times of earthquake waves which arrive in the same direction as flows in the mantle, vs those that travel against it.
 

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