Which layer is the thickest part of Earth's interior?

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

Answer: (D)

This question tests understanding of how thick Earth's interior layers are relative to each other. The crust sits at the surface and is relatively thin—oceanic crust is only about 5–10 km thick, continental crust up to roughly 30–50 km. The mantle lies beneath the crust and stretches from about 35 km depth down to roughly 2,900 km, making its thickness about 2,900 km. The outer core is a fluid layer about 2,200 km thick, and the inner core is a solid sphere with a radius near 1,220 km. Since the mantle spans from just below the crust to the boundary with the outer core, it encompasses the largest portion of Earth's interior by distance and volume, making it the thickest layer.