Question

In which one of the following layers inside the Earth, the velocity of the P-wave exceeds 13 km/s?

• A. Upper mantle
• B. Outer core
• C. Lower mantle
• D. Inner core

Hint:

P-wave speed rises through the solid mantle and peaks just before the core mantle boundary, then drops on entering the liquid outer core.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to identify in which of the Earth's internal layers the P-wave (primary, compressional seismic wave) speed goes above 13 km/s.

Step 2: Key Concept:
P-wave speed in the Earth depends mainly on the rigidity, incompressibility and density of the material it travels through, and it changes with depth as pressure and composition change. As a general trend, P-wave speed rises steadily through the solid mantle with depth, then drops sharply at the boundary into the liquid outer core because a liquid cannot support shear the way a solid can, and then rises again gradually through the outer core before reaching the solid inner core.

Step 3: Detailed Explanation:
In the upper mantle, P-wave speed is around 8 to 10 km/s, well under 13 km/s.
Moving down through the lower mantle, the increasing pressure keeps pushing the P-wave speed higher, and it climbs to about 13.5 to 13.7 km/s just above the boundary with the core, so the lower mantle is where the speed actually crosses past 13 km/s.
At the core mantle boundary, the material changes from solid rock to liquid iron alloy, and the P-wave speed drops abruptly to around 8 km/s at the top of the outer core, then increases only gradually to about 10 km/s by the base of the outer core, staying below 13 km/s throughout.
In the solid inner core, the P-wave speed is around 11 km/s, higher than the outer core but still short of 13 km/s.

Step 4: Final Answer:
The lower mantle is the layer where P-wave velocity exceeds 13 km/s, so the correct option is (C). The upper mantle, option (A), stays near 8 to 10 km/s. The outer core, option (B), drops sharply at the core mantle boundary and stays near 8 to 10 km/s. The inner core, option (D), sits around 11 km/s, still below 13 km/s.