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Author Topic: How did tectonic plates originally form?  (Read 14776 times)

Offline DoctorBeaver

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How did tectonic plates originally form?
« on: 30/12/2008 23:02:34 »
I've seen lots of programs, and read lots of literature, that talk about the movement of tectonic plates. I understand all that. But how did the plates form in the first place?

I know that some plates are denser than others, but how did that happen? If the Earth formed by accretion of debris and was molten for much of its early history, I would have expected the material to have mixed rather than forming areas of differing density.


 

Offline RD

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How did tectonic plates originally form?
« Reply #1 on: 31/12/2008 00:26:43 »
Homogeneous systems going "lumpy" is called symmetry breaking.

Earth isn't exactly a symmetrical sphere: it's an oblate spheroid. So Earth's surface would not have cooled and solidified symmetrically.


The gravitational effect of the moon may alson be a factor: its crust is not uniform either ...

Quote
Curiously, the Moon's center of mass is offset from its geometric center by about 2 km in the direction toward the Earth.
Also, the crust is thinner on the near side.
http://www.nineplanets.org/luna.html
 

Offline DoctorBeaver

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How did tectonic plates originally form?
« Reply #2 on: 31/12/2008 00:26:59 »
I didn't say anything about symmetry. I asked whether the incoming space debris that formed the Earth would have mixed into the existing molten planet. Surely, each, or at least most of, the incoming meteors or asteroids would have similar composition if they were all formed at more-or-less the same distance from the sun. So, why/how did some plates become more dense than others?

I can understand that possibly there was a centrifugal effect that could cause a difference in composition between the equatorial region and the poles. But tectonic plates do not come in latitudinal sections.
 

Offline RD

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How did tectonic plates originally form?
« Reply #3 on: 31/12/2008 01:43:47 »
A possible explanation for the differences in density is that over time the denser elements have sunk to the Earth's core,
 so the areas of the surface which solidified first would be more dense than areas which solidified later.

Even though plates are slowly recycled this uneven density distribution could persist to present day.
« Last Edit: 31/12/2008 01:53:48 by RD »
 

Offline Bass

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How did tectonic plates originally form?
« Reply #4 on: 31/12/2008 03:25:15 »
RD is on the right track-  igneous rocks fractionate as they cool, that is the heavier minerals that form first sink deeper into the magma chamber, and the lighter, more volatile minerals and elements tend to rise toward the top.  For a simplified explanation, check out the Bowen Reaction Series  http://en.wikipedia.org/wiki/Bowen's_reaction_series

Lighter rocks would stay at the surface (sort of like foam on water) and eventually coalesce.
 

Offline RD

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How did tectonic plates originally form?
« Reply #5 on: 31/12/2008 03:38:11 »
Quote
In general the earth's interior has been sorted by Gravity. Heavier elements like iron tend to sink toward the center or core of the earth. Lighter materials, the silicates, oxygen compounds and water have risen to become part of the crust. This action has created distinct layers within the earth and is still in process today.

Oceanic crust - The crust under the oceans is about 10 km thick and is generally made up of rock rich in iron and magnesium. These are primarily basalt formed by volcanic action at the mid ocean ridges. The oceanic crust is denser than continental crust.

Continental crust (continental cratons) - Where there are continents the crust is about 30 to 50 km thick. It is made up of igneous, metamorphic, and sedimentary rocks. The continental crust is less dense than the oceanic crust. When the continental crust collides with oceanic crust through plate movement the continental crust rides over the top of the oceanic crust while the oceanic crust is pushed back down towards the mantle.
http://www.rocksandminerals4u.com/earths_interior.html



« Last Edit: 31/12/2008 03:48:03 by RD »
 

Offline DoctorBeaver

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How did tectonic plates originally form?
« Reply #6 on: 31/12/2008 23:17:44 »
RD is on the right track-  igneous rocks fractionate as they cool, that is the heavier minerals that form first sink deeper into the magma chamber, and the lighter, more volatile minerals and elements tend to rise toward the top.  For a simplified explanation, check out the Bowen Reaction Series  http://en.wikipedia.org/wiki/Bowen's_reaction_series

Lighter rocks would stay at the surface (sort of like foam on water) and eventually coalesce.

I was wondering about that; but wouldn't that mean that the density at the surface would be more uniform than it is?
 

Offline JimBob

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How did tectonic plates originally form?
« Reply #7 on: 01/01/2009 21:57:22 »
No!
« Last Edit: 01/01/2009 22:02:41 by JimBob »
 

Offline JimBob

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How did tectonic plates originally form?
« Reply #8 on: 01/01/2009 21:58:21 »
The crust is still all less dense than the mantle. Pressure alone would determine this.
 

Offline DoctorBeaver

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How did tectonic plates originally form?
« Reply #9 on: 01/01/2009 22:21:17 »
Jim, I appreciate that; but tectonic plates are crust not mantle, aren't they? My point is that it seems to me that the crust would have formed with fairly uniform density. Subsequent subduction would have caused the plates to mix with mantle rock, but if the plates were originaly fairly uniform, how did they start moving and what caused the subduction?

I was wondering if very early vulcanism may have been a cause; but then I thought that any vulcanism would have occurred after the crust hardened (prior to which everything was molten so mixing would have occured easily) and consequently the newly erupted lava would not mix with the crust but simply spread out on top of it.

Bear in mind my knowledge of this subject is almost non-existent so I may have got totally the wrong idea of how the crust formed.
« Last Edit: 01/01/2009 22:23:07 by DoctorBeaver »
 

Offline JimBob

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How did tectonic plates originally form?
« Reply #10 on: 02/01/2009 03:15:24 »
Well, where to start is the next question.

Lets skip all of the detail about what came first. The gas, the elements, the formation of the solar system, yada, yada, yada ... Originally the earth was PROBABLY not molten but as the gravitational attraction of the material that would eventually form the earth increased, the inner part became molten, allowing more and more compression and an increase in pressure and temperature. As this molten material continued to cool and differentiated, (in general) the more complex minerals cooled out of the molten material first. As the crystals solidify they take up more space as the crystal lattice is formed. This makes the hardened material lighter than the magma. Oceanic volcanoes, mid-ocean rifts and rift basalts eject magma that form crust. And there are magma chambers where subduction material returns upward towards the surface of the earth that often break through and form volcanos but also cool under the surface, forming granites and dikes, sills and pagmatites.

All of this crystalline material is less dense than the underlying mantle material. It floats like the skim on milk or the crust on custard. And relative to the entire volume of the earth, it is about the same thickness. You can even mix this skim back in but the skim that forms later as the hot milk or custard cools is still floating on top. It may be the same material that is underneath but is in a less state of excitement and thus a more stable form that is more structured.

I hope this helps. Perhaps Bass can explain it better. He is the one who specializes in minerals and mineral differentiation. I just sift sand. DOH! This has been work!


« Last Edit: 02/01/2009 03:17:02 by JimBob »
 

Offline DoctorBeaver

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How did tectonic plates originally form?
« Reply #11 on: 02/01/2009 11:55:33 »
Oceanic volcanoes, mid-ocean rifts and rift basalts eject magma that form crust. And there are magma chambers where subduction material returns upward towards the surface of the earth that often break through and form volcanos but also cool under the surface, forming granites and dikes, sills and pagmatites.


But there weren't any oceans when the crust was originally forming.

You've mentioned subduction. But if the crust were of uniform density, how could subduction occur? As far as I am aware it is the difference in density between the plates being pushed together that causes subduction.
 

Offline Bass

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How did tectonic plates originally form?
« Reply #12 on: 02/01/2009 21:33:56 »
But there weren't any oceans when the crust was originally forming.

You've mentioned subduction. But if the crust were of uniform density, how could subduction occur? As far as I am aware it is the difference in density between the plates being pushed together that causes subduction.

There is overwhelming evidence for plate tectonics at 3.0 b.y.  There is fairly convincing evidence that plate tectonics is at least as old as 4.0 b.y.  Analysis of zircon crystals suggest water may have covered the earth as early as 4.3 b.y., and that plate tectonics may have been active at that time.

Even today, subduction occurs at oceanic plate to oceanic plate convergence zones- the crust on each side of the subduction zones should be close to equal density. 

What causes subduction?  There are several hypotheses and not all of them require density differences between plates.

Also, as igneous magmas fractionated, they would create igneous rocks of differing densities- everything from dense peridotite/basalt to relatively light granite.

To me, the more intriguing questions involve the difference between the processes we see active today and those that created the ancient cratons- for example, the great archean greenstone belts, of which we have no modern analogues.
 

Offline DoctorBeaver

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How did tectonic plates originally form?
« Reply #13 on: 02/01/2009 21:38:58 »

There is overwhelming evidence for plate tectonics at 3.0 b.y.  There is fairly convincing evidence that plate tectonics is at least as old as 4.0 b.y.  Analysis of zircon crystals suggest water may have covered the earth as early as 4.3 b.y., and that plate tectonics may have been active at that time.

Water was really around that early? I thought it was too hot.

Quote
Even today, subduction occurs at oceanic plate to oceanic plate convergence zones- the crust on each side of the subduction zones should be close to equal density. 

What causes subduction?  There are several hypotheses and not all of them require density differences between plates.

OK, thanks. That's something else I didn't know.

Quote
Also, as igneous magmas fractionated, they would create igneous rocks of differing densities- everything from dense peridotite/basalt to relatively light granite.

To me, the more intriguing questions involve the difference between the processes we see active today and those that created the ancient cratons- for example, the great archean greenstone belts, of which we have no modern analogues.

Whatever "the great archean greenstone belts" are  [:I]
 

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How did tectonic plates originally form?
« Reply #13 on: 02/01/2009 21:38:58 »

 

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