The Earth's Inner Core 

I came across THIS ARTICLE and now know more about the inner core than I did before - and some of it is very surprising!

The inner core is solid but is very hot and under great pressure. Previously it was even hotter and was liquid. When did the temperature drop low enough to allow solid iron to appear? Between 500 and 1,500 million years ago. People discovered its existence in 1936.

Its radius increases by 1mm per year. This implies 8,000 tonnes of iron solidify every second! Eventually all the core will solidify and the Earth will not have a magnetic field.

The inner core is lop-sided. The eastern part, under Indonesia, is growing faster than the western part, under Brazil. But, thankfully, gravitational forces sort things out and the core remains spherical.

The article tells us how all this is surmised - none of this can be found first hand! Read the page - there is lots of good stuff therein.

Seismic waves have suggested Earth’s solid iron core is asymmetrical. Sanne Cottaar, Author provided