The Earth's inner core is solid because despite the enormous temperature in this region, there is also enormous pressure there, which in turn raises the melting point of iron and nickel to a value above the Earth's core temperature. read more
At some point of depth in the Earth's interior, around 5150 km, is the transition from the liquid outer core and the solid sphere of the inner core, a ball of incredibly hot iron-nickel alloy. read more
Best Answer: It is a phase change due to the effects of greater pressure at the outer core/inner core boundary. The core is composed of iron and nickle. The phase change is due to increased pressure forcing liquid into solid, despite the higher temperature in the inner core compared to the outer core. read more