How did the continental plates came above the oceanic plates?
Too long for a comment, so I'll post here, but it's far from a complete answer.

Basalt can't become continental crust because the elements are different.  Iceland is made up of primarily basalt rock.  (80% per this article).  So it's made up of a different material than continental crusts.   Basalt rock is also less sturdy and less enduring than granite, so it's unlikely that a continental crust could be formed out of Icelandic eruptions or any similar eruptions.   My best guess is that Iceland will break apart over time, preventing the formation of a permanent land-mass.  

Granite, the primary material in continental crust, has more Aluminum and Potassium in it.    Basalt, the heavier stuff, has more Magnesium.  Source.    They are made from distinctly separate processes and material, so one can't simply be turned into the other.

While I have no doubt that some mixing goes on within the earth's mantle, I don't believe a significant amount of new continental crust is regularly being formed because to form new crust, you would need free aluminum ready to bind with Oxygen and Silicates and I believe most of the aluminum has already done that.    That's my layman's answer anyway.  I'm open to being corrected.