Plate Tectonics: On Again - Off Again

[Eclogite]

Here is the abstract from an article published 4th January in Science:

 

Intermittent Plate Tectonics?

Paul G. Silver and Mark D. Behn

Although it is commonly assumed that subduction has operated continuously on Earth without interruption, subduction zones are routinely terminated by ocean closure and supercontinent assembly. Under certain circumstances, this could lead to a dramatic loss of subduction, globally. Closure of a Pacific-type basin, for example, would eliminate most subduction, unless this loss were compensated for by comparable subduction initiation elsewhere. Given the evidence for Pacific-type closure in Earth's past, the absence of a direct mechanism for termination/initiation compensation, and recent data supporting a minimum in subduction flux in the Mesoproterozoic, we hypothesize that dramatic reductions or temporary cessations of subduction have occurred in Earth's history. Such deviations in the continuity of plate tectonics have important consequences for Earth's thermal and continental evolution.

 

This would help to explain why the rate of cooling of the Earth is less than that predicted by models employing continuously active plate tectonics.

 

If plate tectonics does stop then erosion of the continents would, in theory, eventually remove all land from the globe. One wonders if the thinning of the continental bodies, along with isostatic uplift and heat build up might contribute to the eventual initiation of new rifting zones and forced subduction.

[freeztar]

Great find Eclogite!

This would help to explain why the rate of cooling of the Earth is less than that predicted by models employing continuously active plate tectonics.

Perhaps, but if enough is understood about the history, then it should be able to be factored into the models.

If plate tectonics does stop then erosion of the continents would, in theory, eventually remove all land from the globe.

 

Yes, but the abstract doesn't make mention of complete cessation. It implies, to me, that it is a continual process, but fluxuates in quantity over time.

 

One wonders if the thinning of the continental bodies, along with isostatic uplift and heat build up might contribute to the eventual initiation of new rifting zones and forced subduction.

 

Have you read the entire article? I'm wondering if they identified mechanisms that cause the initiation of subduction zones.

[Turtle]

Great find Eclogite!

Agreed. ;)

 

 

Yes, but the abstract doesn't make mention of complete cessation.

Disagreed. :(

...temporary cessations ...

 

Cessation is by definition a bringing to an end, so saying 'complete cessation' is a double negative. Temporary cessation on the other hand says 'it' stopped, then started again. :D

 

I ran across an article somewhere about new found underwater volcano mounts that were neither on subduction zones or hotspots. :fly: I have been looking for 15 minutes now without success. :( Anyway, such a thing may provide a restart mechanism. ;)

 

Thanks for the topic Eclogite! :cup: :cup:

[freeztar]

Cessation is by definition a bringing to an end, so saying 'complete cessation' is a double negative. Temporary cessation on the other hand says 'it' stopped, then started again. ;)

 

I don't think so Turtle.

Dictionary.com gives the following definition:

a temporary or complete stopping; discontinuance

 

Hence I added the qualifier 'complete' to denote that it did not start up again, which is not what the abstract was saying. :cup:

I ran across an article somewhere about new found underwater volcano mounts that were neither on subduction zones or hotspots. :fly: I have been looking for 15 minutes now without success. ;) Anyway, such a thing may provide a restart mechanism. :cup:

Keep looking! I'd like to read that.

[Turtle]

I don't think so Turtle.

Dictionary.com gives the following definition:

 

___You're using the wrong dictionary. :cup: cessation - definition of cessation by the Free Online Dictionary, Thesaurus and Encyclopedia.

 

 

Keep looking! I'd like to read that.

 

___Don't hurt yourself rushing to beat me to it now! :fly:

___Something else I ran across but didn't save, a differential in the turning of the Hawaiian chain hotspot that is counter to previous plate speed/direction/duration movement calculations. Perhaps a shudder when subduction ceased? ;) ;) .....__......................:cup:

[Eclogite]

Yes, but the abstract doesn't make mention of complete cessation. It implies, to me, that it is a continual process, but fluxuates in quantity over time.

I am with Turtle on this one: cessation means a complete stop. A complete cessation is a redundancy. The authors state "we hypothesize that dramatic reductions or temporary cessations of subduction have occurred in Earth's history."

So they are offering an alternative. Either subduction drops dramatically from its present level, to a small fraction of that level, or it stops completely. If it stops completely, they theorise, this stoppage is a temporary one.

Have you read the entire article? I'm wondering if they identified mechanisms that cause the initiation of subduction zones.

Regretably not. No subscription to Science!

I did find this interesting:

http://www.dur.ac.uk/yaoling.niu/MyReprints-pdf/2003NiuEtAlJPet.pdf

They say, "Tonga and Mariana fore-arc peridotites, inferred to represent their respective sub-arc mantle lithospheres, are compositionally highly depleted (low Fe/Mg) and thus physically bouyant relative to abyssal peridotites representing normal oceanic lithosphere formed at ocean ridges. The observation that the depletion of these fore arc lithospheres is unrelated to and pre-dates the inception of present day western Pacific subduction zones demonstrates the compositional bouyancy contrast at the sites of these subduction zones. These observations allow us to suggest that lateral compositional bouyancy contrast within the oceanic lithosphere creates the favoured and necessary condition for subduction initiation."

While the authors are considering (mainly) compositional and density/bouyancy contrasts in oceanic crust I think the same principles might be applicable to continental crust. This mirrors nicely my speculation about the reinitiation of spreading zones below continental crust because of thickness (and therefore bouyancy) contrasts. [Or am I clutching at transform faults?;)]

[freeztar]

I am with Turtle on this one: cessation means a complete stop. A complete cessation is a redundancy. The authors state "we hypothesize that dramatic reductions or temporary cessations of subduction have occurred in Earth's history."

So they are offering an alternative. Either subduction drops dramatically from its present level, to a small fraction of that level, or it stops completely. If it stops completely, they theorise, this stoppage is a temporary one.

 

Ok gentlemen, to avoid further semantic discussions in this thread, allow me to humbly retract my phrase "complete cessation" and replace it with "cessation". Ok...moving on...

 

Regretably not. No subscription to Science!

Yeah, same here. This is a good one to add to the "check these out next time your at the library" list.

 

While the authors are considering (mainly) compositional and density/bouyancy contrasts in oceanic crust I think the same principles might be applicable to continental crust. This mirrors nicely my speculation about the reinitiation of spreading zones below continental crust because of thickness (and therefore bouyancy) contrasts. [Or am I clutching at transform faults?;)]

 

That sounds reasonable to me eclogite.