How do mid-ocean ridges form new crust? Spreading mid-ocean ridges form the longest mountain ranges in the world. New oceanic crust is created at this boundary when basalt magma, formed in the mantle, rises into fractures in the crust and solidifies. Spreading ridges are high elevation because the young oceanic plate at the ridge crest is hot and less dense than the older, colder and more dense plate on the flanks of the ridge. As the plates move away from the boundary, they thicken because unde