PDA

View Full Version : Discussion: Earth's Core Rotates Faster Than ...



lswinford
2005-Aug-25, 10:06 PM
In LiveScience, Ker Than writes a summary of a study that questions the relative spin of the Earth's core to the surface: http://www.livescience.com/forcesofnature/..._earthcore.html (http://www.livescience.com/forcesofnature/050825_earthcore.html). The piece presents how University of Illinois geologist Xiaodong Song studied shockwave timing to descover that the core turns a little bit faster than the crust. In relative sense its, "We're talking about 50,000 times that of plate tectonic motion."

om@umr.edu
2005-Aug-25, 11:46 PM
Originally posted by lswinford@Aug 25 2005, 10:06 PM
In LiveScience, Ker Than writes a summary of a study that questions the relative spin of the Earth's core to the surface: http://www.livescience.com/forcesofnature/..._earthcore.html (http://www.livescience.com/forcesofnature/050825_earthcore.html). The piece presents how University of Illinois geologist Xiaodong Song studied shockwave timing to descover that the core turns a little bit faster than the crust. In relative sense its, "We're talking about 50,000 times that of plate tectonic motion."
Thanks, lswinford.

That is an interesting story, and it identifies another poorly understood part of the Earth's internal structure.

A faster spinning core seems difficult to imagine if produced by geochemical separation of iron from other elements and allowing it to sink.

We have other reasons to doubt that the Earth's core formed in this manner.

I look forward to reading the article and watching for its impact on others working in this field.

With kind regards,

Oliver
http://www.umr.edu/~om

cran
2005-Aug-26, 07:26 AM
Yes, lswinford, it is an interesting story and an excellent piece of data analysis... :D
Xiaodong actually broke the story (announced his initial findings) a few years ago (around 02/03, I think... gonna have to sift through my archives to find out); he and others were continuing with high resolution tomography to get a clearer idea of what the 'solid' core was all about...
the first thing they found was the faster rotation... then they resolved the question of whether the 'solid inner core' behaved as if it were a single crystal of iron - turns out the answer is "no" ... and that the 'solid' inner core is further layered (differentiated) and inhomogenous and anisotropic (the 'density variations' referred to elsewhere), it's outer boundary ('surface') is horizontally heterogenous (ie, has its own topography) and moves independently of the sub-structure (the 'inner-inner core), in other words, it has an analog of 'plate tectonics'... :)

An unresolved question (as far as I am aware) is why the core rotates slightly faster ... it may be that the vertically dynamic fluid 'outer core' encourages this extra motion in much the same way as magma encourages plate tectonic motion at the surface ... the transfer of heat energy between the inner and outer cores may also be a factor... :unsure:

lswinford
2005-Aug-26, 05:16 PM
I'm wondering whether there is a relation to the center of gravity between the earth and moon, if that isn't turning the center (since the center of earth-lunar mass is off-center to earth's mass). On the other hand, what if the center were turning the wrong way, perhaps providing sufficient churching forces that are the cause of crustal fractionation. I'm assuming that the core is spinning in the same direction of the earth's turn, only faster, but assumptions are sometimes fun (which is why I'm questioning) and embarrassing (why I'm not more specific).

antoniseb
2005-Aug-26, 05:54 PM
Originally posted by lswinford@Aug 26 2005, 05:16 PM
I'm wondering whether there is a relation to the center of gravity between the earth and moon, if that isn't turning the center (since the center of earth-lunar mass is off-center to earth's mass).
Hmmm. that's an interesting question... Is it possible that the core is spinning at a faster rate because the moon's tidal friction is working on the rigid outer part of the Earth but not so much on the solid core floating in the liquid inner core?

cran
2005-Aug-26, 10:36 PM
Originally posted by antoniseb+Aug 27 2005, 01:54 AM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (antoniseb &#064; Aug 27 2005, 01:54 AM)</td></tr><tr><td id='QUOTE'><!--QuoteBegin-lswinford@Aug 26 2005, 05:16 PM
I&#39;m wondering whether there is a relation to the center of gravity between the earth and moon, if that isn&#39;t turning the center (since the center of earth-lunar mass is off-center to earth&#39;s mass).
[/b][/quote]
:huh: yes, lswinford, that is a really interesting question >thinks< rats&#33; why didn&#39;t I think of that? <_<


Hmmm. that&#39;s an interesting question... Is it possible that the core is spinning at a faster rate because the moon&#39;s tidal friction is working on the rigid outer part of the Earth but not so much on the solid core floating in the liquid inner core? That, and possibly where Iswinford might be coming from, that a bias is introduced to the inner core by the &#39;off-centre&#39; centre of gravity between the Earth and the Moon&#33; :blink:

You know, antoniseb, I think Iswinford might just be on to something... ? :)
In fact, as I think about it, you may both be on to something...&#33; :D
I had to do an assignment on the core (hence the memories of reports, etc) ... and gravitional influence from the Moon never showed up (at least not in that respect) ... wow, guys, a potential contribution to the body of knowledge&#33; B)
Now, who wants to research whether anything has already been published on this?
And, how can we test these ideas...? :unsure:

antoniseb
2005-Aug-26, 10:51 PM
Originally posted by cran@Aug 26 2005, 10:36 PM
I had to do an assignment on the core (hence the memories of reports, etc) ... and gravitional influence from the Moon never showed up
The core should NOT be off center because of the Moon&#39;s gravitational influence. Why would the core be influenced in a way that the rest of the planet is not?

cran
2005-Aug-26, 11:18 PM
Sorry, antoniseb, I wasn&#39;t suggesting that the core is, or should be, off-centre ... but Iswinford is correct by pointing out that the &#39;common centre of gravity between the Earth and the Moon&#39; is not at the centre of the Earth (ie not at the centre of the inner core) ... and that started me wondering if it might introduce a bias to the spin ... encouraging the density variations found in the inner core and the slight rotational velocity difference between the inner core and the mantle... :huh:

antoniseb
2005-Aug-26, 11:58 PM
Originally posted by cran@Aug 26 2005, 11:18 PM
the &#39;common centre of gravity between the Earth and the Moon&#39; is not at the centre of the Earth (ie not at the centre of the inner core) ... and that started me wondering if it might introduce a bias to the spin
Since the Earth, and the core are essentially in free-fall around this center of mutual gravity, there should be no effect... not any more than there should be an effect from the movement around the Earth-Sun barycenter.

cran
2005-Aug-27, 12:15 AM
Originally posted by antoniseb+Aug 27 2005, 07:58 AM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (antoniseb @ Aug 27 2005, 07:58 AM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-cran@Aug 26 2005, 11:18 PM
the &#39;common centre of gravity between the Earth and the Moon&#39; is not at the centre of the Earth (ie not at the centre of the inner core) ... and that started me wondering if it might introduce a bias to the spin
Since the Earth, and the core are essentially in free-fall around this center of mutual gravity, there should be no effect... not any more than there should be an effect from the movement around the Earth-Sun barycenter. [/b][/quote]
Not even from &#39;tidal effects&#39; on the fluid outer core? :huh:

antoniseb
2005-Aug-27, 12:24 AM
Originally posted by cran@Aug 27 2005, 12:15 AM
Not even from &#39;tidal effects&#39; on the fluid outer core?
For one thing, the tidal effects should slow it down, not speed it up. What&#39;s going on is probably that the greater flexibility of the outer part of the planet is making it more vulnerable to the tidal breaking than the core is. So the cores is spinning down more slowly (i.e. at any moment looks faster).

cran
2005-Aug-27, 09:07 AM
Yeah, the more I thought about it, the more I realised the effect, if any, would be &#39;drag&#39;... so, the best possibility then is the one you suggested... tidal drag on the crust... just had a day in the country... does wonders for clearing the head :D
So, maybe a sniff through the literature to what has been written up about Lunar (and for that matter, solar (at 0.46 Lunar - in combination, should produce something of a &#39;sine-wave&#39; effect... sometimes combined, sometimes opposed...) tidal drag on the crust? :)

cran
2005-Aug-28, 02:53 AM
Just to bring things up to speed, the following was reported from Champaign IL (SPX) Aug 25, 2005 on SpaceDaily.com

"The similar seismic waves that passed through the inner core show systematic changes in travel times and wave shapes when the two events of the doublet are separated in time by several years," [Xiaodong] Song said. "The only plausible explanation is a motion of the inner core."

The most likely explanation for why the inner core is rotating at a different speed, Song said, is electromagnetic coupling. "The magnetic field generated in the outer core diffuses into the inner core, where it generates an electric current. The interaction of that electric current with the magnetic field causes the inner core to spin, like the armature in an electric motor."

The fluid outer core decouples the solid inner core&#39;s movement from the mantle. Because the fluid outer core is not very viscous, frictional drag is small.

"Differential rotation is a fundamental dynamic process that goes to the heart of the origin of our planet and how it has evolved," Song said. "There is still much to learn about the inner Earth."

The full article is here (http://www.terradaily.com/news/earth-05t.html)

... no mention of &#39;drag effects&#39; on the crust, though antoniseb... you might still win a lollipop... :)

piersdad
2005-Aug-28, 07:06 AM
Tidal drag.
thios would be accentuated if earths population decided to use massive tidal generation.
this would slow down the crust over a few 1000 years and and may be create stronger magnetic fields in the earth due to the core rotation at a greater rate than it is at the moment

cran
2005-Aug-28, 08:34 AM
Interesting, piersdad... how large-scale would the energy off-take need to be to make that sort of difference... and would it be a good thing, or a bad thing, to have a stronger magnetic field (assuming that is the consequence)? :huh:

lswinford
2005-Aug-29, 03:56 PM
Thanks for the discussion. I had long ago heard that the tidal drag about our barycenter was slowing down the earth. That was why I wondered "which way?". But then too, the picture that the instability that might result makes me wonder if that was part of the driving force (that and the Mid-Atlantic Ridge) of plate techtonics.

BTW: As far as &#39;being onto something&#39;, when I was in high school (a looong time ago), I thought I may have solved one of those "impossible" geometric problems--mechanically trisecting an angle. My geometry teacher was amazed at how well my method (long since forgotten and lost) was working as we put it through several tests. Then he said, "Well, now you just have to prove it&#33;" Then he described how I essentially had to present the geometric proofs like a lawyer preparing his precedent briefs at court. My father showed me his notes on the proofs that his graduate school prof had given that trisection was impossible. After looking at those, I said, "it was fun while it lasted", tossed my notes in the trash and rarely gave it another thought since. So, Cran, if you can make the idea fly I&#39;m not going to gripe if you put your name on it, especially since you&#39;d have to do all the work to prove it. :D

Fraser
2005-Aug-29, 05:29 PM
SUMMARY: According to new research from geologists, the Earth&#39;s core rotates just a little bit faster - about 1 degree per year - than the crust of the planet. The scientists took advantage of historical records for "earthquake twins" near the South Sandwich Islands. These are quakes that occurred in virtually the same spot with the same magnitude, but were years apart. As the seismic waves passed through the Earth, they were bent as they passed through the Earth&#39;s iron core. The shape of this bending has changed over time, indicating the core&#39;s faster rotation.

View full article (http://www.universetoday.com/am/publish/core_rotates_faster_than_crust.html)

What do you think about this story? Post your comments below.

cran
2005-Aug-29, 06:21 PM
Me? :huh: Not me, lswinford... I&#39;m only in it for the fun :D
No, we&#39;ll let antoniseb take the lollipop, and we can sit back and bask in all that reflected glory... B)

Oh, I tripped over a uni assignment with refs to the Xiadong Song reports while I was packing up some loose files ... the earliest mention I found was in an article "Earth&#39;s Core Spins Independently" (Kobus J, July 22, 1996); another, "Earth&#39;s Inner Core Has Distinct Layers, Geologists Report in Earthquake Study" (Columbia Earth Institute, October 29, 1998) ... Xiang-Dong Li was mentioned with Romanowizc and Durek in a 1996 article "Earth&#39;s core not monolithic iron crystals, scientists claim" (Environmental News Network, November 15, 1996) ... I wrote the research assignment in 2000 ... earlier than I thought >thinks< rotten memory&#33; :P

Guest_Mike
2005-Aug-29, 09:58 PM
A faster rotating core may be further evidence that our day has been shorter far into the past. Devonian age coral fossils have long been known to suggest a much shorter day. Recent research also seems to support a slowing earth, due to tidal friction, and it may well take time for this delay to be transferred to the deepest core of earth&#39;s interior. I wouldn&#39;t be surprised if this work leads to eventual discovery of a delay between the crust and the mantle below the MOHO, though the difference, here could be expected to be orders of magnitude less than the one described in the article.

cran
2005-Aug-30, 12:59 AM
Hi Guest_Mike, welcome...

A faster rotating core may be further evidence that our day has been shorter far into the past. Devonian age coral fossils have long been known to suggest a much shorter day. Recent research also seems to support a slowing earth, due to tidal friction, and it may well take time for this delay to be transferred to the deepest core of earth&#39;s interior. Yes, the diurnal lineations indicated more days/per year - I don&#39;t have my refs handy, but my unreliable memory tells me it was ~380 in the Devonian ... and the only to account for that (given a slightly shorter annual cycle as well...) is that the days were about 21 hours long... there are similar indications of even shorter days in exceptionally well preserved Precambrian stromatolites ... :)
I wouldn&#39;t be surprised if this work leads to eventual discovery of a delay between the crust and the mantle below the MOHO, though the difference, here could be expected to be orders of magnitude less than the one described in the article. I would, Guest_Mike, because the evidence for that would be more than lost in the continuing convection of the mantle ... as you pointed out, such a variation would be that much smaller ... and easily absorbed into the advective flow of the asthenosphere ... <_<