How did the Earth's Solid Core Form?

It is widely accepted that the Earth's inner core formed about a billion years ago when a solid, super-hot iron nugget spontaneously began to crystallize inside a 4,200-mile-wide ball of liquid metal at the planet's center.

Age
Theories about the age of the core are necessarily part of theories of the history of Earth as a whole. This has been a long-debated topic and is still under discussion at the present time. It is widely believed that the Earth's solid inner core formed out of an initially completely liquid core as the Earth cooled down. However, there is still no firm evidence about the time when this process started.[3]

Two main approaches have been used to infer the age of the inner core: thermodynamic modeling of the cooling of the Earth, and analysis of paleomagnetic evidence. The estimates yielded by these methods still vary over a large range, from 0.5 to 2 billion years old.

Age estimates from different studies and methods

Date Authors - - - Age - - - - Method
2001 Labrosse et al.[53] 1±0.5 T(N)
2003 Labrosse[54] ~2 - - - - -  T(R)
2011 Smirnov et al.[55] 2–3.5 - P
2014 Driscoll and Bercovici[56] 0.65 T
2015 Labrosse[57] < 0.7 - - - - - T
2015 Biggin et al.[58] 1–1.5 - - . P
2016 Ohta et al.[59] < 0.7  - -  T
2016 Konôpková et al.[60] < 4.2 T
2019 Bono et al.[61] 0.5  - - - -  P


T = thermodynamic modeling, P = paleomagnetism analysis, (R) = with radioactive elements, (N) = without them.