The outer core is composed of which of the following?

• A. It is a semisolid rock
• B. It is crust
• C. It is liquid iron-nickel
• D. It is a solid iron-nickel

Answer: (C)

This question tests understanding of the outer core’s state and composition. The outer core is a molten layer, made primarily of iron with some nickel, located beneath the mantle. Its liquid state arises because temperatures are high enough to melt the metals even under the immense pressures present there, and this liquid metal can circulate in convection currents. That motion drives Earth’s magnetic field through the geodynamo, which is why the outer core’s liquid nature is so important.

Seismic evidence supports this: s-waves do not travel through liquids, so they don’t propagate across the outer core, helping reveal a liquid layer. P-waves slow and bend when they cross the boundary into the outer core, which is consistent with a distinct, fluid layer rather than a solid one. In contrast, the inner core is solid iron-nickel, and the crust is a separate, much less dense rocky layer above the mantle.

So the best description is a liquid iron-nickel outer core.