The ‘Shadow Zone’ for P-waves exists because of which physical process at the Core-Mantle Boundary (CMB)?
P-waves slow down significantly when entering the liquid outer core from the solid mantle. This sudden velocity drop causes the waves to bend (refract) toward the center of the Earth. This bending creates a “gap” on the surface between 103° and 143° from the epicenter where direct P-waves cannot be recorded. ANSWER: (B) Refraction due to a sharp decrease in velocity
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When P-waves hit the Gutenberg Discontinuity, the change from solid silicate rock to liquid iron-nickel causes a dramatic change in wave speed. Following Snell’s Law, the waves are refracted inward. Consequently, they are diverted away from the 103°–143° region, creating the P-wave shadow zone. Unlike S-waves, which are blocked entirely by the liquid, P-waves eventually emerge beyond 143°, but the refraction pattern leaves a definitive “silent zone.” Analyzing this specific shadow zone was crucial for scientists to calculate the exact size and depth of the Earth’s core.