The Moho (Mohorovicic discontinuity) marks the boundary between which two Earth layers?

• A. Mantle and Crust
• B. Outer Core and Mantle
• C. Crust and Core
• D. Lithosphere and Asthenosphere

Answer: (A)

The concept being tested is the identification of a boundary defined by a change in rock type revealed by seismic speeds. The Mohorovicic discontinuity occurs where seismic waves speed up as they pass from crustal rocks into mantle rocks, indicating a transition from crust to mantle. Since the crust is the outer, less dense shell and the mantle lies beneath it, this boundary is the crust–mantle boundary. The depth of this boundary varies: it sits deeper beneath continents (roughly tens of kilometers) and much closer to the seafloor beneath the oceans. This boundary is distinct from the boundary at the top of the outer core, and it’s not a purely mechanical division like lithosphere vs. asthenosphere.