Does subduction of plates make them stretch and form the oceans?
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.