Divergence is due to upwelling mantle thinning and extending the crust when the mantle decompressed and melts basalt is formed. Basalt and other mafic rocks from oceanic crust while continental crust is made up of mostly felsic (but some mafic) rocks. Upwelling mantle does thin continental crust and sometimes this thinning leads to new oceanic basins. Thus oceanic crust is formed from divergent oceanic plates (at mid oceanic rifts) and continental crust can become oceanic crust when upwelling mantle rifts continents into new oceanic basins
