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Difficulty: MediumEarth's Interior and Plate Tectonics

During major earthquakes, primary waves (P-waves) travel through the entire interior of the planet, but secondary waves (S-waves) cannot penetrate beyond the mantle boundary at a depth of roughly 2,900 kilometers. What fundamental property of Earth's outer core does this seismic observation demonstrate?

  1. A
    It exists in a highly compressed solid state with high iron content.
  2. B
    It is composed of semi-fluid plastic rock that drives mantle convection.
  3. It exists in a liquid state that cannot transmit transverse shear waves.Answer
  4. D
    It is composed of porous rock layers that absorb seismic energy.

Answer

Earth's outer core exists in a liquid state that cannot transmit transverse shear waves.
Transverse seismic waves (S-waves) rely on the shear strength of materials to propagate. Because liquids have no shear strength, S-waves cannot travel through liquids. When S-waves abruptly stop at the core-mantle boundary (2,900 km depth), it proves that the outer core is liquid.

Step-by-Step Solution

1
Analyze the mechanical properties of primary (P) and secondary (S) seismic waves.
P-waves are longitudinal compressional waves that can pass through solids, liquids, and gases, whereas S-waves are transverse shear waves that can only propagate through solid media.
Understanding wave physics determines what material states permit wave propagation.
2
Evaluate the seismic boundary observation at 2,900 km depth.
The abrupt termination of S-waves at the core-mantle boundary creates an S-wave shadow zone beyond that depth.
The sudden inability of shear waves to travel past the mantle indicates a transition from solid rock to liquid.
3
Deduce the physical state of the outer core.
The outer core must be liquid.
Only a liquid layer prevents transverse shear waves from propagating while permitting compressional P-waves to continue through.

Key Concept

Seismic wave propagation and the physical state of Earth's internal layers
Estimated Time:1m 0s
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Topics

Earth's Interior and Plate Tectonics

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