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 . 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 .