Crater formation in Moon's highland areas, illustration. At top, a flat crust of lighter rocks (grey) overlies subsurface rocks, with molten rock (orange) in the mantle deep underground. At centre, a large meteor or asteroid impact forms a large crater and basin structure. Ejected debris falls back to the surface as ejecta which piles up to form what is known as 'mega-regolith'. At lower left, changes in the mantle rocks affect the overlying crust. Dense mantle rocks sink downwards (purple areas and arrows), while light mantle rocks float upwards (yellow areas and arrows). This pushes up the crust from beneath.