Original: The crust forms in part by aggregation of island arcs including granite and metamorphic fold belts, and it is preserved in part by depletion of the underlying mantle to form buoyant lithospheric mantle.
Simplified: The crust forms in part by the coming together of island arcs including granite and metamorphic fold belts. They are kept together partly by the using up of the mantle below the crust, which makes a mantle on which the crust can float.