Which of the following describes the Transition Zone?

• A. The layer between the crust and the mantle
• B. The outer boundary of the core
• C. The layer between the upper and lower mantles
• D. The interface between the mantle and inner core

Answer: (C)

The Transition Zone is a mantle region located between the upper mantle and the lower mantle, roughly from about 410 to 660 kilometers below the Earth's surface. It’s defined by seismic discontinuities that arise from mineral phase changes: olivine transforms to wadsleyite near 410 km, and a later transition near 660 km helps mark the boundary to the lower mantle. These changes alter how seismic waves travel, which is how scientists identify this zone.

It’s not the layer between the crust and mantle (that boundary is the Moho). It’s not the outer boundary of the core (that’s the core–mantle boundary). And there isn’t a direct mantle–inner-core interface, since the inner core is separated from the mantle by the outer core.