Why is the Earth's inner core solid despite the high temperatures?

• A. Normally iron would melt at this temperature, but immense pressure keeps it solid.
• B. Low atmospheric pressure prevents melting.
• C. It is cooled by the surrounding mantle.
• D. Magnetic forces keep the core solid.

Answer: (A)

The key idea is that pressure dramatically raises the melting point of iron. The inner core is under immense pressure from the weight of the overlying Earth, which pushes the melting point of the iron-nickel alloy to well above the temperatures present there. So even though it’s extremely hot, the iron would only melt if the temperature reached even higher values; at these pressures it stays solid. In the outer core, the pressure is lower, and the temperatures do exceed the melting point there, so the material is liquid. Magnetic forces don’t decide whether the core is solid or liquid—the solid inner core is determined by the extreme pressure, not by magnetism.