Core formation
The Earth formed through accretion, absorbing planetisimals (lumps of rock and ice) through collisions. Did the Earth accrete undifferentiated material that then separated into shell and core – in which case, did the planet reach its present mass before differentiating, or was it a more gradual process? Alternatively, core formation might have happened rapidly inside growing planetisimals, so the Earth’s core is combination of these previously formed cores (photo and text credit: Bill Minarik 2003 Nature News and Views Nature 422 126)