What caused the supercontinent to break up

[verdigris]

When the Earth first formed there was a single supercontinent.
Why did the ocean ridges appear and break up this continent into smaller units?

 

[HallsofIvy]

That's not true. Plates of the Earth's crust have been in motion ever since the formation of the earth. The "supercontinent" you are referring to, Pangea, was a fairly recent (compared to the age of the earth) incident in which a number of areas happened to come together. It was necessarily unstable, just like todays continent formations.

 

[verdigris]

HALLSOFIVY said:

"a number of areas happened to come together"

Did these areas contain all the land mass on Earth?

 

[billiards]

The fact that the Earth is differentiated suggests that either it accreted that way (unlikely, and pretty much a dead theory), or that it has gradually evolved. Therefore plates weren't always there, geochemistry tells us that the mantle (sampled in its bastardized MORB (Mid-Ocean Ridge Basalt) form) is depleted in 'incompatible' elements, and that the crust is enriched in them, suggesting that at some point the crust and mantle separated and the crust took more than its fair share of these elements.

The formation of ocean ridges cannot be fully explained, they are necessary for preserving a mass balance, this is where mantle material is upwelled; matter is downwelled at subduction zones. Essentially your question -which is one that troubles many eartn scientists - is why do we have plate tectonics? Why do we not have a stagnant lid like Venus?

Most people think it has something to do with the moon forming impact early on in Earth's history, there is currently no mechanism proposed that I am aware of, but watch this space...

 

[Andre]

There are ideas that Venus is not about stagnant lids, which is only a hypothesis. It might be that the thermal gradient of Venus is too low to require plate tectonics in the first place.

There are also ideas that the generated heat inside the planet is related to friction heat due to mantle core friction caused by the orbital perbutations on which inner core and mantle react differently. Since the rotation of Venus is negliglible, it could be that Venus is no longer generating significant amounts of heat. The total lack of internal magnetic field, thought to be caused by outer core convection cells appears to support that idea. Most of Venus heat is already outside.

 

[verdigris]

A lot of the water in the oceans came from comets.Maybe a big impact from a comet played a part in the fracturing of the Earth's surface.

 

[billiards]

A big impact (with an object the size of mars) would have melted the surface...

 

[verdigris]

The surface of the Earth may have been intially smooth.But if J Marvin Herndon is right about there being a nuclear fission reactor at the Earth's core,then perhaps there was an explosion in the core at one time ( Wigner energy caused the fire at Windscale nuclear reactor because pressure built up in the moderating material) that sent S and P waves to the Earth's surface and cracked the crust.The heat released and pressure waves would have pumped magma through the cracks and formed the continental land mass.The explosion must have been on one side of the core or else continents could have formed elsewhere.So,in this scenario,at one time the core of the Earth must have had an inhomogenous chemical composition.
Perhaps shear wave anisotropy in the Lehman discontinuity - which happens under the continents but not under the oceans - has something to do with
energy coming from an explosion at the Earth's core.

 

[Andre]

A big impact (with an object the size of mars) would have melted the surface...

or would have caused another asteroid belt. I think we could do better than that. Suppose that we put a planet identical to Earth but about 90% size in Venus orbit but without moon. What if we mix the geophysical thoughts of Jim Vanyo about core-mantle coupling and orbital pertubations http://www.me.ucsb.edu/dept_site/vanyo.htm and stir it with some ordinary basic physics like gyroscopic effects, and then let it simmer for a couple of billion years, what would be cooking?

 

[billiards]

The surface of the Earth may have been intially smooth.But if J Marvin Herndon is right about there being a nuclear fission reactor at the Earth's core,then perhaps there was an explosion in the core at one time ( Wigner energy caused the fire at Windscale nuclear reactor because pressure built up in the moderating material) that sent S and P waves to the Earth's surface and cracked the crust.

First, there is no evidence that suggests that he is right, that paper is old (1993) if I remember correctly, deep Earth geophysics has advanced A LOT since then. The most recent work seems to suggest that the inner core is composed of iron with perhaps some light element alloys, there is nothing to suggest that there is a sufficient composition of uranium/thorium down there to do the things suggested in that paper.
Second, I think you might be overplaying the capacity of S and P waves to crack the crust - they are elastic waves. Then again it is not entirely impossible (as far as I am aware) that the rarefaction which follows a shock wave might be capable of cracking the crust (see: shatter cones), however the explosion would have to have been MASSIVE (think millions to billions of nukes all at the same time) because shock waves cannot travel that far.

The heat released and pressure waves would have pumped magma through the cracks and formed the continental land mass.

No, it simply doesn't work like that.

The explosion must have been on one side of the core or else continents could have formed elsewhere.So,in this scenario,at one time the core of the Earth must have had an inhomogenous chemical composition.

How exactly are you proposing the continents formed? EDIT: I just reread your pumping magma through cracks idea so I guess that answers my question! Let me ask you this instead: where exactly did the cracks you propose actually happen?

Perhaps shear wave anisotropy in the Lehman discontinuity - which happens under the continents but not under the oceans - has something to do with
energy coming from an explosion at the Earth's core.

I fail to see your point here. The anisotropy occurs under the oceans and not under the continents because there is difference between oceanic and continental crust. Shear wave anisotropy will result in so-called shear wave splitting: normally this involves the horizontally polarized s wave arriving a little before the vertically polarized s wave (depending on the nature of the anisotropy). This is just a property of the material down there, and it enables us to constrain (work out within limits) what that material might be; as far as I am aware it has nothing to do with energy coming from the core.

 

[verdigris]

BILLIARDS said:
"there is no evidence that suggests that he is right"

Evidence could be hard to come by because he says that there is only a five mile radius core of uranium - would that resolve as a detail in a s - wave,p - wave analysis done at the surface? But the fact that venus and other planets radiate more heat than they receive from the sun could be supporting evidence,although some people attribute this heat excess to stored gravitational energy.

"the explosion would have to have been MASSIVE (think millions to billions of nukes all at the same time)"

I agree - the explosion would have been massive but an atomic bomb needs less than 1 kg of uranium 235 and a five mile radius could have at least 10 to the power of 12 kg given that u235/u238 ratio in nature is 1%.And there is also the possibility that a fission reaction involving hydrogen started a fusion reaction which is more energetic and created helium.This link shows that there is up to 1 % by mass of hydrogen in the Earth's core:http://www.nature.com/nature/journal/v268/n5616/abs/268130a0.html.

and this link that helium can come from the lower mantle(perhaps the core?)
to the upper mantle:
http://camb.demonhosting.co.uk/JConfAbs/5/768.pdf[/URL]

There may not have been an explosion just a rapid and uneven release of a large amount of heat - it depends on what the full chemical core composition was at the time.

"I fail to see your point here. The anisotropy occurs under the oceans and not under the continents because there is difference between oceanic and continental crust. Shear wave anisotropy will result in so-called shear wave splitting: normally this involves the horizontally polarized s wave arriving a little before the vertically polarized s wave (depending on the nature of the anisotropy). This is just a property of the material down there,"

But did the material get that property from the pressure of an explosion - whatever the property is.

 

[Andre]

Verdigris,

When we were doing that puzzle, thinking itwas a nuclear reactor, pieces did not fit pretty soon. you can have individual nuclear molecules decaying and producing heat but there is not the beginning of a ghost of a chance that there is a near critical mass of fission material that sustains a nuclear chain reaction. Everything about that is a very long shot and two long shots together is about the same change that you hit the jackpot of two lotteries on the same day.

 

[verdigris]

I looked at the thread from 2004 on a nuclear reactor at the Earth's core.
If I was asked to bet on the cause of heating in the Earth I would say that it was down to residual heat from Earth's formation and radioactivity in the mantle.Then movement of large masses of mantle material affecting
Earth's axis tilt through gravitational effects and climate sometimes through volcanic effects.