The earth's inner core reaches temperatures that are at times greater than 9000°f. what causes this layer to remain solid despite the high temperature?

The earth's inner core remains solid due to high amounts of pressure, so even though it should be liquid due to an extremely high temperatures, it acts as a solid. More specifically, solid iron-nickel, or pure crystalised iron. The pressure at the earth's core is almost three and a half million times higher than on the surface, and it is this pressure that causes the atomic structure of iron to actually change.