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

Question

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?

Answer 1

Oceanic tectonic plates are primarily composed of basalt which contains relatively dense minerals, whereas continental tectonic plates are primarily composed of rocks with minerals that are less dense than those found in basalt. Because of this, dense oceanic plates will subduct when colliding with less dense continental plates.

This difference in density is also what causes ocean basins (large scale surface depressions) to persist on Earth's surface. All tectonic plates "float" on the mantle and denser plates (oceanic plates) will float deeper than less dense plates (continental plates). This concept is called isostacy.

Therefore, it is not subduction that causes ocean basins to form, but isostacy; though, the two concepts are related.