Update on Tectonic Plate Dynamics

[Astronuc]

This caught my attention the other day, and I don't yet have time to review/explore the topic.

A discovery about the movement of tectonic plates will have scientists rewriting (or update?) textbooks
http://www.businessinsider.com/what-moves-tectonic-plates-2017-1

During their observations, they concluded that the movement of the East Pacific Rise could not be completely explained by subduction — when one plate moves under the other — and other forces had to be at play. In the paper, they state that buoyancy is created by heat rising up from deep within the Earth's core.

The estimate is 50% of plate tectonic movement is driven by this heat, and about 20 terawatts of heat flows between the core and the mantle.

20 TW is pretty impressive.

Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling
http://advances.sciencemag.org/content/2/12/e1601107

Abstract
Earth’s tectonic plates are generally considered to be driven largely by negative buoyancy associated with subduction of oceanic lithosphere. In this context, mid-ocean ridges (MORs) are passive plate boundaries whose divergence accommodates flow driven by subduction of oceanic slabs at trenches. We show that over the past 80 million years (My), the East Pacific Rise (EPR), Earth’s dominant MOR, has been characterized by limited ridge-perpendicular migration and persistent, asymmetric ridge accretion that are anomalous relative to other MORs. We reconstruct the subduction-related buoyancy fluxes of plates on either side of the EPR. The general expectation is that greater slab pull should correlate with faster plate motion and faster spreading at the EPR. Moreover, asymmetry in slab pull on either side of the EPR should correlate with either ridge migration or enhanced plate velocity in the direction of greater slab pull. Based on our analysis, none of the expected correlations are evident. This implies that other forces significantly contribute to EPR behavior. We explain these observations using mantle flow calculations based on globally integrated buoyancy distributions that require core-mantle boundary heat flux of up to 20 TW. The time-dependent mantle flow predictions yield a long-lived deep-seated upwelling that has its highest radial velocity under the EPR and is inferred to control its observed kinematics. The mantle-wide upwelling beneath the EPR drives horizontal components of asthenospheric flows beneath the plates that are similarly asymmetric but faster than the overlying surface plates, thereby contributing to plate motions through viscous tractions in the Pacific region.

I'm starting this thread for discussion on this topic (primarily on tectonic plate dynamics, but also on the EPR) since we apparently don't have a relevant one open.

 

[Astronuc]

Related to plate tectonics -

Radioisotope studies show the continental crust formed 3 billion years ago

(Phys.org)—New research sheds light on how and when the modern day continents began to form. Researchers from the University of Bristol analyzed radio isotope abundances in 13,000 samples of continental crust of varying age and found that the continents began to form around 3 billion years ago. This date may coincide with when plate tectonics began. Their research appears in Nature Geoscience.

Read more at: https://phys.org/news/2015-07-radioisotope-continental-crust-billion-years.html#jCp

This relates to the use of strontium and rubidium isotopes in determining the age of a rock bearing these isotopes. "Rubidium (87Rb) becomes strontium-87 (87Sr) through radioactive decay. Strontium-87 has a half-life of 48.8 billion years. Strontium-86, the most abundant isotope of Sr, is stable and does not undergo radioactive decay. Therefore, the abundance of 87Sr in the continental crust can be traced to the radioactive decay of 87Rb, and based on its half-life, one can determine the age of a rock sample."

https://www.physicsforums.com/threads/radioisotopic-dating-with-isotopes-of-different-masses.902975/

The above Phys.org article leads to another, somewhat more sensationally entitled one - "Earth's crust slowly being destroyed", or it's not growing as fast.

The researchers say the Earth regulates the net amount of new crust produced by destroying and recycling existing crust. This was achieved as tectonic plates moved and collided against each other and one plate was forced below the other and deep into the Earth. This process is known as subduction, which also generates new crust in the form of large volumes of magma above the subduction zone and results in chains of volcanoes such as in the present day Andes. This process also destroys existing crust by eroding and transporting older crust back down to within the Earth’s mantle.

Read more at: https://phys.org/news/2012-03-earth-crust-slowly.html#jCp

 

[DrDu]

20 TW is pretty impressive.

Yes, but given that big power plants already reach several GW, and we are talking about the forces moving whole continents, I find this number not surprising.

 

[billiards]

Very interesting subject.

My criticism would be that it seems bit circular. They use their model to show that the Earth is not as expected if the preexisting hypothesis is true. Then they use the same model to support their alternative hypothesis. At least that's my understanding.

Furthermore, they concede that similar models from other groups give a different picture.

p.12: In this other analysis of Pacific mantle flow, Conrad et al. (47) argue that the EPR is spatially disconnected by some 30° from the larger-scale mantle upwelling that is centered in their calculations on the Southwest Pacific Superswell (Fig. 12A) and therefore that EPR spreading is independent of the large-scale mantle flow, contrary to the conclusions presented above.

 

[Ratman]

Actually that's not a completely new thing. The mid-ocean ridges were considered as active participants of the tectonic cycle decades ago, only later it's been shown that most ridges exhibit features indicating extension instead of compression – hence the notion of 'slab pull' as the motor of the plate tectonics. It's nice to know that some people have done the maths to reexamine the old notion of 'ridge push' as well. And 20 terawatts does not look so impressive if you notice that this is a sum over ~153 million square kilometers of the core–mantle boundary. It corresponds to only 0,13 W/m2, just a bit more than the mean heat flux through the oceanic crust (0,10 W/m2).
Although it is probably not uniform due to inhomogeneities in the lowest mantle.