Which term refers to the layer between the upper and lower mantles?

• A. The boundary 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 where the mantle meets the inner core

Answer: (C)

The concept being tested is how Earth's mantle is structured and the specific name for the middle mantle region. Deep in the planet, the mantle isn’t a uniform shell; minerals transform as pressure rises. The layer between the upper mantle and the lower mantle is called the transition zone. It spans roughly from about 410 km to 660 km below the surface, and seismic waves show distinct changes there because mantle minerals change structure—olivine transforms to wadsleyite around 410 km, then to ringwoodite and other high‑pressure phases as you go deeper, culminating near the 660 km boundary that separates the transition zone from the lower mantle. This zone acts like a bridging region, separating the rigid upper mantle from the deeper, more ductile lower mantle.