The Mohorovičić discontinuity, or Moho, marks the boundary between which two Earth's layers?

• A. Boundary between the mantle and outer core.
• B. Boundary between the crust and atmosphere.
• C. Boundary between the crust and mantle.
• D. Boundary between the outer core and inner core.

Answer: (C)

The Moho marks the boundary where seismic waves speed up as they pass from the crust into the mantle, signaling a change in rock type and elastic properties. Crustal rocks are lighter and different in composition from the mantle rocks, so the transition produces a noticeable jump in seismic velocities—this is how geophysicists identify the crust–mantle boundary. It’s not a boundary at the surface between the crust and the atmosphere, and it’s not the boundary within the core (the mantle–outer core boundary is known as the Gutenberg discontinuity, and the inner–outer core boundary is the Lehmann discontinuity).