Around the core is a partially molten boundary layer with a radius of about 500 km (310 mi). This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon’s formation 4.5 billion years ago. Crystallization of this magma ocean would have created a mantle rich in magnesium and iron nearer to the top, with minerals like olivine, clinopyroxene, and orthopyroxene sinking lower.