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Ocean plates subduct beneath continental plates. Does this make the oceanic plates stretch and become thinner so that the elevation of the sea flow is lower? Is this why oceans form where they do - when water collects in these low areas?

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    $\begingroup$ Sort of yes but not in the way you think. $\endgroup$ – bon Feb 21 '17 at 17:58
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    $\begingroup$ I have edited the question for clarity. Original poster please re-edit if this is not what you meant. I assume the question is from a non-geologist and I think it is a reasonable one. It relates to why the mid-oceanic ridges are at higher elevation than other parts of the oceans and how the forces driving plate tectonics and seafloor spreading actually operate. $\endgroup$ – haresfur Feb 22 '17 at 5:16
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Ocean plates subduct beneath continental plates. Does this make the oceanic plates stretch and become thinner so that the elevation of the sea flow is lower?

When a continental plate collides with an oceanic plate, or when two oceanic platers collide, the result is subduction of an oceanic plate. This subduction makes oceanic crust recycle rather than build up over time. When two continental plates collide, the result is the Himalayas and the Tibetan plateau. Continental crust isn't recycled to anywhere near the extent that oceanic crust is. Excluding the rather rare ophiolites, the oldest continental crust is about 20 times older than is the oldest oceanic crust.

This quick recycling of oceanic crust versus the slow build up of continental crust is the primary reason that oceanic crust currently is considerably thinner than is continental crust. The very first continents would have had have had fairly thin crusts. Continental crust has become thicker and thicker through the ages, while oceanic crust has not and cannot thanks to subduction.

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No.

The oceanic crust is basically the same material as the upper mantle, but its cooled under lower pressure into a solid. Continental crust is much less dense felsic material that tends to stay on top of oceanic crust. There isn't enough continental crust to cover the planet so the rest gets filled in with cooled mantle products (oceanic crust). As the plate moves, it creates a divergent boundary (e.g. the mid-Atlantic ridge) through which mantle material is forced out and cools into basalt. The oceanic crust is not stretched thin. Instead it is originally thin when it forms at the divergent boundary because it is only cooled crust of a more ductile semi-molten solid.

Oceanic crust is made of three layers: a layer of sediment on top and two layers of mantle sourced rock. The layers of mantle sourced rock differ structurally because they cooled at different rates. So, there is fast cooled basalt on top of ultramafic slow cooled material. This bottom most layer also gets thicker as the plate gets older since more material cools, causing that portion of the plate to sink further down.

As for why there is the extreme difference in height, think of oceanic crust as the crust on cooled soup while continental crust is more like styrofoam sitting on top. The styrofoam sticks up higher because it is much lighter and thicker, and rock under high pressure and heat behaves enough like a liquid for buoyancy to matter. Oceans form over the lower oceanic crust because water runs downhill.

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    $\begingroup$ Wrong. The upper mantle is not liquid. The oceanic crust is tholeiitic basalt, and the upper mantle is (solid) peridotite. The basalt forms by partial melting of a very tiny section of the mantle, at mid ocean ridges. $\endgroup$ – Gimelist Feb 21 '17 at 22:32
  • $\begingroup$ @Michael Yes the asthenosphere is technically solid but it behaves so plastically for the layman it is better to think of it as a liquid. as for the difference between the mantle and oceanic crust both are highly mafic and are far more similar to each other than they are the felsic continental crust, hence the term "basically" $\endgroup$ – John Feb 22 '17 at 2:11
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    $\begingroup$ @John this is a similar discussion to what I had with someone here on the comments a while ago. I will repeat it again, just so people will stop spreading this misconception. The mantle is not liquid. The asthenosphere (btw, does not equal "upper mantle") is more ductile than the lithosphere, but it is not liquid at all. The crust is not the same material. It is chemically and mineralogically different. Referring to the upper mantle or asthenosphere as liquid is not a "layperson simplification", it's misleading. This perpetuates the idea (among laypersons) that... $\endgroup$ – Gimelist Feb 22 '17 at 2:22
  • $\begingroup$ ...the crust is floating on top of a magma ocean and volcanoes are the result of this magma ocean. This is wrong. The mantle is solid, period. $\endgroup$ – Gimelist Feb 22 '17 at 2:23
  • $\begingroup$ If you are concerned about explaining it to laypeople then use the chocolate analogy. Cold chocolate is solid and rigid. Warm chocolate is solid and plastic. Melted chocolate is a liquid. $\endgroup$ – bon Feb 22 '17 at 8:24

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