What is the innermost layer of the Earth, a solid ball made primarily of iron?

• A. Inner core
• B. Outer core
• C. Mantle
• D. Crust

Answer: (A)

Earth’s interior is organized by both composition and how the rocks behave under extreme pressure. The deepest region is a solid ball mainly made of iron (with some nickel), called the inner core. It stays solid because the pressures deep inside Earth are enormous, raising iron’s melting point so high that it remains solid even though temperatures are extreme.

Surrounding the inner core is the outer core, which is liquid iron-nickel. Above that lies the mantle, a thick layer of hot rock that behaves like a very slow-moving solid, and finally the crust, the thin outer shell. This arrangement explains why the innermost layer is a solid iron sphere, distinct from the liquid outer core and the rocky mantle and crust.