asthenosphere
Dense oceanic crust slides under less dense continental crust
The weight of the rocks above the mantle results in high pressure.
Diamonds need high temperatures and pressure to form from carbon. Diamonds form in the mantle in certain zones where conditions are right for them to do so.
There just happens to be the right critical temperature-pressure-mineral environment in the mantle in certain zones for their formation.
Continents cannot cut through the ocean floor.
Reverse
Its Thrust!
The continental crust is up to 70 km thick.
At ocean ridges