What would happen to the oceans if continental and oceanic crust had the same density?

The oceanic crust is formed by partial melting of the mantle at mid-ocean ridges. The continental crust is formed even more cycles of partial melting over time, resulting less dense rocks.

Explanation:

The mantle, oceanic crust, and continental crust all have different compositions due to a process called partial melting.

This is where you start to melt a rock, but don't melt it all the way.

When you partially melt a rock, certain chemical elements tend to stay in the solid rock while others tend to go into the melted part.

As a result, the rock that forms from that melt is less dense than the original rock.

If you then partially melt that rock, you get a rock that is even lighter.