What is basaltic crust is and what type of rock is compositionaly similar to it?
I can answer just the first part. Basaltic crust is the crust that is formed by seafloor spreading at the mid-ocean ridges. The major component is the igneous rock, basalt, which is more fine-grained and more dense than the granite which makes up continental crust. Thus the continental crust floats higher than the oceanic crust. This is significant, because at trenches, it is the more dense oceanic (basaltic) crust which sinks below the continental crust in a process called subduction.
