How is the composition of rock on the seafloor different than rock on the land?

Oceanic crust is dominated by mafic and ultramafic intrusive igneous rocks whereas continental rocks are dominated by granitic (felsic) intrusive igneous rocks.

The difference in density has an impact on isostacy of crust floating on the semi-fluid upper mantle (asthenosphere), with continental crust (about 2.7g/cm3) rising or floating above oceanic crust (about 3.5 g/cm3).  

The continental crust is by far the older of the two types of crust.

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

Because continental crust is less dense than oceanic crust it floats higher on the mantle, just like a piece of Styrofoam floats higher on water than a piece of wood does.

Explanation: