Question

With increasing depth in the Earth, the P-wave velocity shows a significant decrease across which one of the following boundaries?

• A. crust – mantle
• B. mantle – outer core
• C. outer core – inner core
• D. upper mantle – lower mantle

Hint:

Key fact: A sudden drop in P-wave velocity at the mantle–outer core boundary provided early geophysicists strong evidence that the Earth's outer core is liquid.

Answer 1

The Correct Option is B

Step 1: General trend of P-wave velocity. As depth increases, pressure and density generally cause P-wave velocity to increase inside the Earth. However, sharp changes occur at certain boundaries due to changes in the state of matter. Step 2: Crust–mantle boundary (Mohorovičić discontinuity). Here, velocity \emph{increases} because mantle rocks are denser than crustal rocks. No decrease is observed. Step 3: Mantle–outer core boundary. At this boundary, the medium changes from solid mantle to liquid outer core. Since P-waves travel more slowly in liquids than in solids, their velocity shows a sharp and significant \emph{decrease}. This is the main cause of the seismic P-wave shadow zone. Step 4: Outer core–inner core boundary. At this boundary, P-wave velocity actually \emph{increases} slightly again because the inner core is solid. Step 5: Upper mantle–lower mantle. P-wave velocity increases gradually with depth; there is no sudden decrease. Final Answer: The significant decrease in P-wave velocity occurs at the mantle–outer core boundary. \[ \boxed{\text{Mantle–Outer Core}} \]