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billiards

Subduction Zones -- the key?

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BASICS:

 

Convergent margins: where one plate goes under another and is "destroyed". Subduction zone is the three dimensional picture of a convergent margin. The plate goes down sinking into the mantle, it isn't destroyed, it just goes underground (deep).

 

MAJOR IMPORTANCE:

 

1) Subduction is the primary mode of mantle convection. Plumes (if they exist) are of secondary importance.

 

2) Subduction drives plate tectonics.

 

3) Subduction controls the chemical and thermal evolution of the planet. -- positive spinoff: conditions for life, minerals for industry and our enjoyment -- negative spinoff: killer volcanoes and earthquakes.

 

 

PROBLEM:

 

Dynamics of subduction not understood. You can solve this one, then you get massive kudos. Effectively you turn plate tectonics from a kinematic theory to a dynamic theory. Nobel prize in geology (if it existed) coming your way.

 

DISCUSSION:

 

I strongly believe that if we understand subduction zones, then we can find the key to understanding plate tectonics. Mid ocean ridges are important too, but less so. Plumes are probably not that important either, although will help explain some anomalies.

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3) Subduction controls the chemical and thermal evolution of the planet. -- positive spinoff: conditions for life, minerals for industry and our enjoyment -- negative spinoff: killer volcanoes and earthquakes.

 

 

PROBLEM:

 

Dynamics of subduction not understood. You can solve this one, then you get massive kudos. Effectively you turn plate tectonics from a kinematic theory to a dynamic theory.

 

I was just thinking about this a few days ago ...after hearing this tidbit on the news about (essentially isostatic) rebound, due to water loss.

 

http://www.sciencedaily.com/releases/2014/08/140821141542.htm

 

“Scientists have used GPS data to discover that the growing, broad-scale loss of water is causing the entire western US to rise up like an uncoiled spring.”

“From the GPS data, they estimate the water deficit at nearly 240 gigatons (62 trillion gallons of water), equivalent to a six-inch layer of water spread out over the entire western U.S.”

 

“Scripps researchers ...found that the water shortage is causing an "uplift" effect up to 15 millimeters (more than half an inch) in California's mountains….”

 

 

It seems too easy to imagine a ratchet-like mechanism, whereby the continent (and its 'wedge-shaped' root) rises up away from the ocean crust (not pulling ocean crust up with it, but letting ocean crust fill in below), and then sinks back down (with increasing hydrological mass), dragging down the ocean crust with it.

...or more simply:

 

It seems too easy to imagine a ratchet-like mechanism, whereby the continent rises up away from the ocean crust

and then sinks back down, dragging down the ocean crust with it

 

...as a part of some yearly or other climate-related, continental-scale, hydrological cycle.

 

~

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1) Subduction is the primary mode of mantle convection. Plumes (if they exist) are of secondary importance.

 

 

 

 

Do you not need an agent or source of disturbance, in short an energy source to power the action?

 

If not plumes then what?

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Plumes make sense for the ocean crust and seafloor spreading, but as for the continents and subduction....

 

 

 

Do you not need an agent or source of disturbance, in short an energy source to power the action?

 

If not plumes then what?

 

...some periodic redistribution of mass?

 

~ :confused:

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Conventionally, slab drag from descending plates and push from mid-ocean ridge vulcanism.

 

Billiards, are you conflating plumes with any form of thermally convective circulation in the mantle?

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I was just thinking about this a few days ago ...after hearing this tidbit on the news about (essentially isostatic) rebound, due to water loss.

 

http://www.sciencedaily.com/releases/2014/08/140821141542.htm

 

 

It seems too easy to imagine a ratchet-like mechanism, whereby the continent (and its 'wedge-shaped' root) rises up away from the ocean crust (not pulling ocean crust up with it, but letting ocean crust fill in below), and then sinks back down (with increasing hydrological mass), dragging down the ocean crust with it.

...or more simply:

 

It seems too easy to imagine a ratchet-like mechanism, whereby the continent rises up away from the ocean crust

and then sinks back down, dragging down the ocean crust with it

 

...as a part of some yearly or other climate-related, continental-scale, hydrological cycle.

 

~

Interesting bit of research. New to me. Thanks for sharing.

 

I think its significance is in tracking the water resource. If the uplift and downlift(?) cycle (which is pretty subtle, why it wasn't noticed before) has an effect on plate tectonics it is surely incidental. Furthermore every subduction zone is different. Many subducting plates are not subducting beneath a continent. Often subduction is of one ocean plate under another (ocean arc subduction). So this could not possibly be a global explanation.

 

It is clear that subduction is driven by the negative buoyancy of the slab. It is buoyant (wants to float) to start off with, when it is young and hot (born at mid-ocean ridge). It cools down over 10s to 100s of millions of years, making it dense (negatively buoyant -- wants to sink). Then it subducts. This is convection in action.

 

However, it is more complicated than considering the Stokes sinking of the slab. Bending of the plate, moving trenches, and the effects of lateral viscosity variations complicate matters immensely (http://www.ees.lehigh.edu/ftp/retreat/for_retreat/Becker&Faccenna%20review%20subduction.pdf). We still do not understand all the complexity we can observe.

Edited by billiards

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It is clear that subduction is driven by the negative buoyancy of the slab. It is buoyant (wants to float) to start off with, when it is young and hot (born at mid-ocean ridge). It cools down over 10s to 100s of millions of years, making it dense (negatively buoyant -- wants to sink). Then it subducts. This is convection in action.

It is not clear at all. It is clear that this is a plausible and certainly partial explanation. It is not clear that it is the only explanation.

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It is not clear at all. It is clear that this is a plausible and certainly partial explanation. It is not clear that it is the only explanation.

Ophiolite, I am talking about subduction. By that I mean a plate sinking down into the mantle. How else do you propose subduction could work?

Conventionally, slab drag from descending plates and push from mid-ocean ridge vulcanism.

I assume you are listing the forces that drive plate motions?

 

Billiards, are you conflating plumes with any form of thermally convective circulation in the mantle?

I consider subduction to be a form of thermal convection. So I guess I am not.

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A key question for subduction dynamics is whether the mantle around the slab gets dragged down with the slab, or whether the slab is decoupled from the surrounding mantle.

 

There is some pretty cool evidence that some times the mantle does decouple. For example there are toroidal patterns of anisotropy at some slab edges (Calabria, West USA, Kamchatka) that really looks like what you expect if the mantle is decoupling and flowing laterally parallel to the trench underneath the slab.

 

 

In other words. When the slab sinks the mantle below it seems to be getting out of the way (by moving around the sides of the slab) to let this happen.

Edited by billiards

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Conventionally, slab drag from descending plates and push from mid-ocean ridge vulcanism.

 

Billiards, are you conflating plumes with any form of thermally convective circulation in the mantle?

 

I was sloppily using the 'plume' terminology to indicate a convecting mantle; but they are different, so thanks for helping us relearn/recall the difference.

 

This was news to me:

"The geological record suggests that until three billion years ago the earth's crust was immobile...."

"...and tectonic plates did not become cold and dense enough to spontaneously sank."

===

 

I would have thought the effects of various 'snowball earth' phases, and the 'recent' effects of continuous cold deep ocean water

--especially flowing into and along the deep trenches--would have been an obvious cooling mechanism; but apparently not!

 

091814_lw_tectonic.jpg

LaboratoryEquipment.com/news

 

"The mystery of what kick-started the motion of our earth's massive tectonic plates across its surface

has been explained by researchers at the Univ. of Sydney."

Wow!

 

Spreading continents kick-started plate tectonics

http://www.nature.com/nature/journal/v513/n7518/full/nature13728.html

 

~

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I was sloppily using the 'plume' terminology to indicate a convecting mantle; but they are different, so thanks for helping us relearn/recall the difference.

 

This was news to me:

"The geological record suggests that until three billion years ago the earth's crust was immobile...."

"...and tectonic plates did not become cold and dense enough to spontaneously sank."

===

 

I would have thought the effects of various 'snowball earth' phases, and the 'recent' effects of continuous cold deep ocean water

--especially flowing into and along the deep trenches--would have been an obvious cooling mechanism; but apparently not!

 

091814_lw_tectonic.jpg

LaboratoryEquipment.com/news

 

"The mystery of what kick-started the motion of our earth's massive tectonic plates across its surface

has been explained by researchers at the Univ. of Sydney."

Wow!

 

Spreading continents kick-started plate tectonics

http://www.nature.com/nature/journal/v513/n7518/full/nature13728.html

 

~

 

Wow hot off the press, that only came out yesterday!

 

I wouldn't be so quick to say that the mystery of the starting mechanism has been explained. That's a very bold statement. It's a model (or set of models) and scientists will enjoy kicking it around for a while until it breaks. Whether in truth this is how plate tectonics started remains to be established.

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This is as good a place as any -and better than some- for this new information on plate tectonics. Enjoy. :)

 

Geophysicists Find Source Behind Sudden Tectonic Plate Movements

January 20, 2015

Geophysicists from Yale University reveal that the combination of crustal plugs with weakening causes abrupt slab detachment in a few million years, which can account for observed precipitous changes in plate tectonic motion and rapid continental uplift.

 

Yale-led research may have solved one of the biggest mysteries in geology namely, why do tectonic plates beneath the Earths surface, which normally shift over the course of tens to hundreds of millions of years, sometimes move abruptly?

...

Traditionally, scientists believed that all tectonic plates are pulled by subducting slabs which result from the colder, top boundary layer of the Earths rocky surface becoming heavy and sinking slowly into the deeper mantle. Yet that process does not account for sudden plate shifts. Such abrupt movement requires that slabs detach from their plates, but doing this quickly is difficult since the slabs should be too cold and stiff to detach.

...

Abstract of originating study; study behind paywall. Abrupt tectonics and rapid slab detachment with grain damage

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Hi Acme, do you care to comment further on this?

Erhm...it's kewl? Science is always amendable? Just ran across it, found it interesting, and looked for somewhere to put it as it didn't seem to deserve its own thread.

 

PS Per your OP, as far as how plate tectonics got started there is the matter of large Earth impactors and antipodal focusing. I think I put something up here on it in a thread on impacts. If your interested I can try & root it out.

Edited by Acme

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I think you guys need to consider when and how did the oceans get there, before discussing 'oceanic crust'.

 

Also what was meant by "the crust was immobile?"

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I think you guys need to consider when and how did the oceans get there, before discussing 'oceanic crust'.

 

I don't have accurate number but liquid water in large amounts did appear back in Hadean and probably as far back as 4.4 billion years ago. Although the oceanic crust appeared is a whole different story.

 

 

Also what was meant by "the crust was immobile?"

 

I think it means just that... immobile. There was no plate tectonics and so no part of crust was moving relative to another.

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Erhm...it's kewl? Science is always amendable? Just ran across it, found it interesting, and looked for somewhere to put it as it didn't seem to deserve its own thread.

 

PS Per your OP, as far as how plate tectonics got started there is the matter of large Earth impactors and antipodal focusing. I think I put something up here on it in a thread on impacts. If your interested I can try & root it out.

Definitely cool..

 

To be clear though this paper is about slab detachment. Potentially an important process in rapid plate motion changes (Hawaii-Emperor chain anyone?) I imagine also very important in the eventual death of a subduction zone.

 

It is a nice example that shows how important processes at subduction zones are for the global scale plate motions.

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