Question

Irons normal melting point is 1535°C, yet Earth's inner core temperature is greater than 4000°C. Why does the inner core remain solid?

Answer 1

one word Pressure

Explanation:

The composition of the core is thought to be an iron-nickel alloy with minor amounts of oxygen, silicon, and sulfur—elements
that readily form compounds with iron.

The pressure acting on the inner core is very high forcing its molecules to come closer to each other and transform from the liquid to the solid state.

That same concept is why in some fish markets they wrap big ice blocks tightly so they take longer time to melt.

That's all.