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Mike2220 t1_jb2ridj wrote

Realistically, whatever plate is following behind the sinking plate would just become the new subduction zone with whichever plate was initially going over. Also there would be a lot of mountainous terrain formations and seismic activity as islands/continents would literally have been shoved together during this process

This has happened before if you'd like to read more specifics

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sth128 t1_jb326bd wrote

Is it possible the ruins of an entire ancient civilization got subducted and are lost forever?

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sciencedthatshit t1_jb332xy wrote

If that civilization was on the ocean floor, sure.

The "what if geological processes erased a truely ancient civilization" question comes up frequently and the most rigorous treatment of this thought experiment is contained in a paper called The Silurian Hypothesis...check out the wikipedia article on it and the paper as well if you're interested in that sort of thing.

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[deleted] t1_jb373no wrote

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[deleted] t1_jb3fvbj wrote

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CrustalTrudger t1_jb4huld wrote

This presumes that the plate that "follows" is subductable or if it is subductable (i.e., it's oceanic) that the boundaries between the fully subducted plate and the following plate are such that allow for continued subudcution. None of those conditions are guaranteed and in fact with reference to the latter consideration, we very often see cessation of subduction when a mid-ocean ridge (i.e., the boundary between two oceanic portions of a plate) approaches or reaches a subduction zone.

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LordNoodles t1_jb57qjt wrote

What happens then? Like there’s the massive momentum, where does it go?

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CrustalTrudger t1_jb5hcc8 wrote

The force is driven by the negative buoyancy of the slab. If the slab detaches, there is no more driving force for the portion of the plate on the surface whereas the slab continues to sink. A simple analogy would be a weight clipped to the edge of a floating mat. If the mat rips, the portion attached to the weight will sink but the rest of the mat will just sit there (assuming it is buoyant). This is expanded on in much more detail in a top level answer I made within this thread to try to address the relative incompleteness of the specific top level comment that everyone is upvoting.

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[deleted] t1_jb39dd3 wrote

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NightOfTheLivingHam t1_jb49lsm wrote

I was about to say the farallon plate and there it is.

Its why nevada is a big stretch mark too.

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DisillusionedExLib t1_jb44z8w wrote

I hope someone who actually knows their stuff can fill in the details but here's something amazing: it's possible the remains of a subducted plate (known as a "slab") to stick around as a distinct entity within the mantle for an extremely long time.

A diagram on that page shows a model of the "Farallon Slab" believed to lie beneath North America.

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stickylava t1_jb4fwkh wrote

Can't find the ref but a piece of it was detected deep under New York recently.

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HeartwarminSalt t1_jb4jqv4 wrote

The D” (D double prime) layer, the lowermost zone of the mantle, was describe to me in grad school as the “subducting slab graveyard”. This layer was also hypothesized to insulate the core enough to cause heat anomalies large enough to create break thru hotspots in some places that give rise to features like the Hawaiian or Yellowstone hotspots.

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forams__galorams t1_jb6j3ew wrote

> The D” (D double prime) layer, the lowermost zone of the mantle, was describe to me in grad school as the “subducting slab graveyard”.

It’s still fairly unexplained what exactly the D” prime layer is — whether it’s made from a build up of old semi-molten slabs or if it’s even compositionally different from the rest of the lower mantle at all is not yet settled. Even the idea of what the whole lower mantle in general is composed has evolved a lot since the discovery of the D” layer; in part due to new types of high pressure minerals being proposed as important parts of the mineralogy but also due to the ever increasing heterogeneity of the mantle as it gets probed at slightly higher resolutions with time.

>This layer was also hypothesized to insulate the core enough to cause heat anomalies large enough to create break thru hotspots in some places that give rise to features like the Hawaiian or Yellowstone hotspots.

I’m not sure that a slab-graveyard interpretation of the D” layer would provide thermal insulation at all — subducted slabs are colder than surrounding mantle material, even by the time they reach that depth; this would have the effect of increasing the thermal gradient (and thus heat loss) at the core mantle boundary rather than insulating; though in a roundabout way this can cause Rayleigh-Taylor instabilities (ie. thermally buoyant regions) elsewhere at the core-mantle-boundary. Seismic tomography makes a convincing case that the Hawaiian hot-spot has origins at the core-mantle-boundary, possibly from such a mechanism (or maybe because the physics of the fluid outer core just happen to create hotter and ‘colder’ regions of the CMB). The origins of the Yellowstone Hotspot are even more enigmatic, seismic methods employed by Yuan & Dueker, 2005 traces what is likely the Yellowstone plume down to only 500 km depth (over 2000 km higher than the CMB). Either a lower mantle counterpart to this plume existed in the past but doesn’t today, or the origin was/is at some point in the upper mantle.

It looks increasingly like the two huge continent scale structures known as LLSVPs which rise up from either side of the CMB and extend hundreds of kilometres through the mantle could be providing the sort of insulating process that you describe — whereby rising plumes get temporarily stuck underneath them and build up heat and/or material before leaking around the LLSVP edges to continue towards the surface. The whole thrust of the research from Torsvik et al, 2006 was establishing how surface expressions (in particular large igneous provinces) of plumes can be traced back to the margins of LLSVPs. The Yellowstone plume does not fit in with this model, but then that would make sense with it not having a deep mantle origin as origins at or near the CMB would be the ones to get ‘stuck’ underneath LLSVPs.

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HeartwarminSalt t1_jb7a891 wrote

Wow you must tectonophysics! Great clarifications! I’m glad to hear the updates on my now ancient knowledge of the D”.

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PedroDaGr8 t1_jb5j14k wrote

This video lecture from Nick Zentner featuring Karin Sigloch has some great tomographic imagery of subducted plates in the mantle!

Skip to around 56 min in if you just want the imagery: https://www.youtube.com/live/l0z3p8ypZKY?feature=share

This paper from Sigloch & Mihalynuk has some great imagery/models of subducted slabs, including some of the ones used in Karin Sigloch's presentation above.

Edit: Attached this to the correct comment this time!

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Dangerous_Ad_6831 t1_jb5bktk wrote

I can’t say exactly why, but I love this fact while also finding it a little unsettling. Geology is the best!

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velociraptorfarmer t1_jb5d3hn wrote

There's a confirmed slab sitting under eastern mainland Asia that is the source of a hotspot volcano. I can't remember which one though.

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forams__galorams t1_jb6jmkz wrote

Atlas of the Underworld would be interest to you — a research project which integrates seismic tomography datasets to produce an atlas of the mantle all the way down to the edge of the core, and thus all the known subducted slabs.

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raducu123 t1_jb5vu2s wrote

> distinct entity within the mantle for an extremely long time.

Why don't they just melt?

Are there fossils burried in the mantle?

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tomtom5858 t1_jb6d12g wrote

>Why don't they just melt?

Pressure is too high to let them. That said, "melt" isn't a well defined term in conditions like this; at what point has ice cream melted?

>Are there fossils buried in the mantle?

Yep. If fossiliferous rocks are subducted, the fossils will be buried in the mantle until eventually, those fossils are somehow transformed beyond being recognizable as fossils (i.e. they're mixed enough, melted or not).

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forams__galorams t1_jbc9f59 wrote

> Yep. If fossiliferous rocks are subducted, the fossils will be buried in the mantle until eventually, those fossils are somehow transformed beyond being recognizable as fossils (i.e. they're mixed enough, melted or not).

Your use of ‘eventually’ is kinda misleading here. Any fossiliferous rocks would be at the top of a subducting slab and so if they didn’t get scraped off onto the overriding plate they would already be sheared upon entering the mantle; not to mention right at the slab-mantle interface where it won’t be long at all before the heat finishes them off.

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Big_Reply5848 t1_jb6lho4 wrote

The Juan De fuca plate which is under the North American plate is a small part of the Farallon plate of which the JDF is actually tearing 93 miles down below the surface. That plate is actually dying, and they don't know what happens to a tectonic plate when it comes to its end.

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Keejhle t1_jb3a5so wrote

Look at the geography of the western United States because that is exactly what happened. The Farallon plate was subducted under the north American plate a couple million years ago. The rift zone from the Farallon plate is actually stretching the continental crust of the North american plate creating features such as the Baisin and Range, Colorado Plateau, or even possibly the Yellowstone volcanic chain.

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CrustalTrudger t1_jb4j0iv wrote

There is no single answer and it depends on what the nature of the lithosphere of the "following" plate is and/or the geometry of the boundaries between the subducting plate and the "following" plate. Some options are:

  1. Plate that follows has oceanic lithosphere with a mid-ocean ridge between the subducting and following plate and subduction ceases before the ridge reaches the subduction zone. Effectively the idea is that subduction is driven by the negative buoyancy of the subducted slab, which is a function of the age/temperature of the slab. The piece of lithosphere adjacent to an active ridge is pretty warm, young, and positively buoyant so it will resist subducting. Depending on the relative competition of forces what may happen is that subduction slows down as this young lithosphere approaches the ridge (resisting subduction) and then the slab rips off (i.e., it detaches) because the slab pull force overcomes the strength of the slab nearer the surface. This can effectively terminate subduction (no slab pull = no subduction). As to what happens from there, it will depend on the specific forces, but most likely the ridge might die and there will be a general reorganization. That reorganization might see a wholly different set of plate boundary kinematics or the subduction zone might "jump", keeping effectively similar broad scale kinematics but with the subduction zone in a different place. It might also jump and reverse polarity. Or it might transition into a new type of boundary depending on the kinematics of the plates that meet. This option is quite common if the ridge is roughly parallel to the subduction zone (e.g., Burkett & Billen, 2009). Semi-parallel ridge subduction does happen though, and for it to happen, usually some amount of complicated geometries and "3D effects" are required (e.g., Burkett & Billen, 2010).
  2. Plate that follows has oceanic lithosphere with a mid-ocean ridge between the subducting and following plate and subduction ceases after the ridge subducts. A geodynamically unlikely option, but assuming the ridge is roughly parallel to the subduction zone, this would also lead to slab detachment and cessation of subduction and reorganization depending on the kinematics of the two plates that now meet.
  3. Plate that follows has oceanic lithosphere with a mid-ocean ridge between the subducting and following plate and subduction continues after the ridge subducts. This is relatively common if the ridge is very oblique or orthogonal to the subduction zone. In this scenario, the ridge will subduct and in many cases a "slab window" will open along the subducted segment of the ridge. You can picture the ridge effectively unzippering down the length of the subduction zone, kind of like this. This makes some specific predictions about what you would see in the upper plate, specifically a gap in normal arc volcanism and instead magmatism that is more indicative of direct mantle interaction with the upper plate rocks.
  4. Plate that follows has continental lithosphere. This would largely require a plate with subduction zones "across" from each other and at the moment that the two subduction zones meet, the result will depend on the nature of the adjacent section of the other overriding plate (is it continental or oceanic) and the relative motion between the two plates that meet. If the other overriding plate is oceanic and the kinematics favor it, subduction might continue via a polarity flip where the formerly overriding plate becomes the subducting plate. Instead, subduction might cease and the boundary might change kinematics (e.g., become a transform boundary).
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comparmentaliser t1_jb4wqbl wrote

This video is about an unusual volcano that straddles the border between China and North Korea, and has some really, really good animations drawn from tomography mapping of the plate dynamics around that area:

https://m.youtube.com/watch?v=3C2HVOB-g5s

Skip to the ten minute mark, but the whole darn thing was really well put together.

Basically, the plates can either be forced all the ay through to the core, or slip between the mantle and the plate above it.

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Ridley_Himself t1_jb5weu9 wrote

If the last of a plate is subducted, the result is a boundary between the overriding plate and whatever was on the other side of the subducted plate. For instance, the west coast of North America was once a subduction zone between the North American Plate and the Farallon Plate. Eventually, the Farallon Plate was almost completely subducted and North America met the spreading center between the Farallon and Pacific Plates, forming the current boundary, which includes the San Andreas Fault.

If, instead of a spreading center reaching the trench, the plate includes continental crust, then a continental collision results, as in the case of India colliding with Eurasia.

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Allfunandgaymes t1_jbarp4g wrote

First you need to realize that subduction happens over periods of time that the human mind can scarcely comprehend. Hundreds of millions of years. Over periods of time this large, and under immense pressure and temperature, crustal rock that subducts can be considered to act in a ductile or plastic manner similar to the mantle it descends into. Think of soil and soil creep - if you grabbed a handful of soil you'd call it solid material, but over decades or centuries, soil acts in a fluid manner as it creeps laterally under gravity and large stationary objects sink into it. Less than 0.1% of the mantle is thought to be molten, but this is enough to allow its lithic material to act in a ductile manner.

Then you need to understand that as a plate subducts, it is not at all rapidly dissolved or rendered into magma. Some of it does convert to magma and collect in magma chambers that slowly rise due to their buoyancy, which is how you get subduction-related volcanism. Think the suite of volcanoes at the perimeter of the Pacific "Ring of Fire". The immense pressure and heat generated by the spreading and subducting Pacific plate grinding beneath more buoyant crustal plates - with the addition of water and other volatile substances from the ocean - is what generates the magma which eventually rises and produces those volcanoes. Eventually, and over the course of hundreds more millions of years, the subducting plate sinks into the asthenosphere - the uppermost region of the mantle - where it may homogenize with the surrounding lithic material. The ancient Farallon plate, which subducted under the west edge of the North American plate ~50-100 million years ago, is believed to be currently undergoing this process. The plate itself can still be detected with seismological technology.

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