Stress at Plate Boundaries vs. Pure Elastic Stress: A Comparison

[curious_ocean]

Hi PF!

I'm teaching a little unit on tectonic plates and have a question about the projected pattern of future plates.

I know that plates can be broken into 2 (or more) new plates as the result of a continental rift.

Our textbook also says that "The Atlantic seafloor may eventually become old and dense enough to form subduction zones around its margins, like the present-day Pacific plate". I'm assuming this is another way that a plate can be broken apart into new plates.

Is there a way that 2 tectonic plates can merge into a single plate? (I know that they can come together at convergent boundaries, but is there a way that they can fuse into a single plate?)

If there is a not a mechanism for merging plates, but there are mechanisms for breaking a single plate apart, would we expect this to create a pattern in how future plates evolve?

I realize that things can be reset as ocean floor is created and subducted, but continental lithosphere doesn't sink into the mantle, so ultimately will we get more and more (smaller and smaller) pieces of continent on each plate?

Thanks for your help!

 

[BillTre]

The Indian sub-continent was a separate plate until it slammed into Asia, maybe 30 million years ago.
Now they seem to be on plate.

IIRC, N. America has had several plates fused on to the west coast.

There have been a lot of similar fusions.

 

[curious_ocean]

I think the Indian sub-continent is on the Australian-Indian plate, slamming into the the Eurasian plate.

But good point that the continents were built by being fused together. I have much more to learn here, but I'm under the impression that the mechanism for that is still an active area of research?

When and how is a new plate formed?

 

[curious_ocean]

Edit: When and how is a new single plate formed from two (or more) previously existing plates?

 

[snorkack]

Edit: When and how is a new single plate formed from two (or more) previously existing plates?

By a plate boundary fault zone becoming inactive and decelerating to zero speed.
Note that acceleration of faults does not necessarily result in permanent zero speed... a fault at divergence might accelerate to transform fault, or convergent fault.

Continents contain a lot of ancient, long inactive seams that were plate boundaries once. Can you point at faults which were active plate boundaries 5 or 10 million years ago and which have come to full stop now?

 

[Astronuc]

I think the Indian sub-continent is on the Australian-Indian plate, slamming into the the Eurasian plate.

But good point that the continents were built by being fused together. I have much more to learn here, but I'm under the impression that the mechanism for that is still an active area of research?

When and how is a new plate formed?

Apparently the India-Australia plate separated about 3 millions years ago. "Once fused with the adjacent Australian Plate to form a single Indo-Australian Plate, recent studies suggest that India and Australia have been separate plates for at least 3 million years and likely longer."
https://en.wikipedia.org/wiki/Indian_Plate

(CBS News, 2012) A new study suggests that two recent earthquakes may indicate a literal seismic shift in our understanding of tectonic plate movements.
https://www.cbsnews.com/news/earthquakes-suggest-new-tectonic-plate-is-forming/

https://www.nationalgeographic.org/media/plate-tectonics/https://www.forbes.com/sites/davidb...upercontinent-will-look-like/?sh=4f2201a21df1
Dr. Christopher Scotese has made video simulations from 240 million years ago to 250 million years in the future. He offers some explanation. His simulation has Africa crashing into Europe and Middle East, Australia crashing into Japan, Taiwan and ultimately China, and Antarctica crashing into India. We won't be around to see it.

 

[DaveC426913]

I think the Indian sub-continent is on the Australian-Indian plate, slamming into the the Eurasian plate.

On a diversionary note, I have never understood why India is often called a "sub-continent". It's tiny!

I mean, heck, Greenland or the Middle East are as good a match for a continent as India!

My assumption is that it must have something to do with India being almost completely cut off from Asia by mountains - but that's really more an issue of climate than gross landmass considerations.

 

[BillTre]

On a diversionary note, I have never understood why India is often called a "sub-continent". It's tiny!

I mean, heck, Greenland or the Middle East are as good a match for a continent as India!

My assumption is that it must have something to do with India being almost completely cut off from Asia by mountains - but that's really more an issue of climate than gross landmass considerations.

When I was taught continents in grade school (1960's), India was a sub-continent because of its mountain based isolate (with maybe some deserts to the west). I think the Urals are (were) supposed to separate Europe and Asia. Its separate and recent origin could also provide a basis for such a claim.
It also seems to be somewhat culturally different from its neighbors, due to the mountains isolating the area.

I think Greenland is part of the North American plate.

Its my guess that what amounts to a continent has changed over the years, because:

• Our knowledge of geology has changed a lot (tectonics), providing a deeper understanding of these things.
• Not everyone is equally aware of these issues of geology
• I think in the popular mind (layman knowledge level) people don't think of geology/geography in equally highly resolved ways. Many people would not be interested in knowing about more that 7 or so continents. They have better things to think about.
• This lowers the threshold for successfully communicating ideas in the non-layman culture (to non-layman people).

 

[curious_ocean]

@snorkack Thank you! That makes sense. I hadn't thought of an answer that simple - which almost certainly means it is true! I thought there must be some sort of melting/welding of plates into each other or something.

I don't know the names of many faults so I am not sure if there are particular faults you are hoping I would mention as candidates for extinct plate boundaries.

@Astronuc Thanks for the fun facts/ cool video!

 

[Astronuc]

One might consider acquiring the textbook: Earth as an Evolving Planetary System (Third Edition) by
Chapter 6 concerns Crustal and Mantle Evolution (Kent C. Condie)

Chapter 7 is entitled The Supercontinent Cycle

This chapter reviews methods of supercontinent reconstruction and the processes involved in supercontinent assembly and breakup. It reviews problems associated with identifying the first supercontinent in the late Archean, and briefly reviews the history of later supercontinents Nuna, Rodinia, and Gondwana-Pangea. It discusses the episodicity of U/Pb zircon ages and its relationship to supercontinent formation and breakup and to large igneous province (LIP) events. The relationships of the supercontinent cycle to mineral deposits, Sr isotopes of seawater, sea level variations, and organic evolution are also summarized. Mantle LIP events are discussed as are possible relationships and feedbacks between various climatic regimes, the carbon cycle, and the supercontinent cycle.

One can read the front matter https://www.sciencedirect.com/book/9780128036891/earth-as-an-evolving-planetary-system

Chapter 5, The Mantle, "summarizes new and exciting information on the mantle and how it operates. It includes a summary of the seismic structure of the mantle and a discussion of large low S-wave velocity provinces (superplumes) and geoid anomalies, as well as constraints on temperature distribution in the mantle. In addition, it discusses both the oceanic and continental lithosphere, including composition, structure, thickness, subductability, plate driving forces, and age. New information from high-pressure experimental work is included, which relates to the origin of seismic discontinuities in both the upper and lower mantle and sinking of subducted slabs. New ideas on the role of water in the mantle are also summarized. Mantle plumes are discussed and seismic tomographic results are presented to constrain both the structure of the mantle and the tracking of plumes into the deep mantle. It also includes a discussion of mantle geochemical domains as monitored by radiogenic isotopes, and lastly, an up-to-date discussion of mantle convection."

It may help answer the many questions regarding volcanoes and tectonic plate evolution.

 

[snorkack]

Is a plate boundary a place where Earth is under bigger stress than inside a plate, or a place where Earth has lower failure strength?

 

[Astronuc]

Is a plate boundary a place where Earth is under bigger stress than inside a plate, or a place where Earth has lower failure strength?

There are different kinds of boundaries, and three types of stress correlate with the three types of plate boundaries. https://www.e-education.psu.edu/earth520/content/l7_p3.html

1. Compressive stress happens at convergent plate boundaries where two plates move toward each other.
2. Tensional stress happens at divergent plate boundaries where two plates are moving away from each other.
3. Shear stress is experienced at transform boundaries where two plates are sliding past each other.

I was watching a video on dangerous places, and one of the places is the Danakil Depression in Ethiopia/Eritrea -

The Danakil Depression is the northern part of the Afar Triangle or Afar Depression in Ethiopia, a geological depression that has resulted from the divergence of three tectonic plates in the Horn of Africa.

The Danakil Depression lies at the triple junction of three tectonic plates and has a complex geological history. It has developed as a result of Africa and Asia moving apart, causing rifting and volcanic activity. Erosion, inundation by the sea, the rising and falling of the ground have all played their part in the formation of this depression. Sedimentary rocks such as Sandstone and Limestone are unconformably overlain by Basalt which resulted from extensive lava flows.

https://en.wikipedia.org/wiki/Danakil_Depression

Unconformable (adjective) - (of rock strata in contact) marking a discontinuity in the geological record, and typically not having the same direction of stratification.

In the Afar region of Africa (border area of Ethiopia and Eritrea), "The Arabian Plate is rifting away from the African plate along an active divergent ridge system, to form the Red Sea and Gulf of Aden. The rifting then extends southwards where the African Plate is itself becoming stretched along the line of the East African Rift Valley and is splitting to form two new plates; the Nubian and Somalian Plates."
https://www.geolsoc.org.uk/Plate-Tectonics/Chap3-Plate-Margins/Divergent/Triple-Junction

So, the future of that place it to move apart, or diverge.

https://en.wikipedia.org/wiki/Asthenosphere
https://www.geolsoc.org.uk/Plate-Te...ical-properties-lithosphere-and-asthenosphere

the lithosphere-asthenosphere boundary is defined thermally, with a gradual transition from the cold conductively cooling lithosphere to the warmer, convecting asthenosphere beneath. Overall, lithospheric thickening with age is observed beneath the oceans and toward the continental interiors suggesting that temperature and conductive cooling play a first-order role in controlling lithospheric thickness.

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JB016463

Crustal rocks on Earth have an average density between 2.7 - 3.0 g/cm³, and mantle has a density about ~3.37 g/cm³. Thickness also varies.
http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-1002/HTML.Lect1/sld016.htm

The effects of lithospheric thickness and density structure on Earth′s stress field
https://academic.oup.com/gji/article/188/1/1/632058

The Earth is composed of several lithospheric plates that float atop the asthenosphere and are in constant motion. Plate Tectonics is essentially the study of the movement of the lithospheric plates and the consequences of that movement
https://ww2.odu.edu/~tmmathew/geolgreece/plate.shtml

Driving Forces of Plate Motions
http://www.columbia.edu/~vjd1/driving_forces_basic.htm

 

[snorkack]

There are different kinds of boundaries, and three types of stress correlate with the three types of plate boundaries. https://www.e-education.psu.edu/earth520/content/l7_p3.html

Yes. My question is how does the size of stress at boundaries compare with the size of pure elastic stress away from boundaries.