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?
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.
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.