How do the outer core and inner core differ?

• A. Outer core is liquid iron-nickel; inner core is solid iron-nickel
• B. Outer core is solid; inner core is liquid
• C. Both are liquid but differ in magnetic properties only
• D. Outer core is gaseous; inner core is solid

Answer: (A)

The main idea is that the outer core is a liquid layer while the inner core is a solid sphere. In Earth's deep interior, the pressure is high and the temperature is extremely hot, but the iron-nickel alloy in the outer core remains above its melting point, so it behaves as a liquid that can flow. The inner core, however, experiences even greater pressures that keep iron-nickel in a solid state despite the high temperature. This difference in state—liquid outer core and solid inner core—explains why the outer core can convect and sustain Earth’s magnetic field, while the solid inner core stays put and influences the field through its growth and interaction with the circulating liquid.