ACT (Enhanced)ScienceMedium
A geophysicist is studying seismic wave data to understand Earth's interior structure. They observe that P-waves (primary waves) can travel through both solids and liquids, but their speed changes significantly at certain depths. S-waves (secondary waves), however, are observed to travel only through solids and are completely absent in the outer core. What can be inferred about the state of matter of Earth's outer core based on this seismic data?
- AThe outer core is mostly liquid, as S-waves cannot propagate through it.
- BThe outer core is composed of a gas, which slows down P-waves significantly.
- CThe outer core is primarily solid, but with varying density.
- DThe outer core is a mixture of solid and molten rock, allowing some S-wave propagation.
Show answer & explanationAnswer & explanation
Correct answer: A. The outer core is mostly liquid, as S-waves cannot propagate through it.
S-waves (shear waves) cannot travel through liquids because liquids cannot support shear stress. The complete absence of S-waves in the outer core is direct evidence that the outer core is liquid.
Why the other options are wrong
- B. Earth's outer core is under immense pressure and composed of metals, not gas.
- C. This contradicts the absence of S-waves, which only travel through solids.
- D. If it were a mixture, some S-wave propagation would likely be observed, even if attenuated.
Seismic Wave Properties
Seismic waves are vibrations that travel through Earth's interior, providing information about its structure. P-waves are compressional and travel through all states of matter, while S-waves are shear and travel only through solids.
- P-waves (Primary): Fastest, compressional, travel through solids, liquids, gases.
- S-waves (Secondary): Slower, shear, travel only through solids.
- Wave behavior (speed, reflection, refraction) changes at boundaries between different materials.
Memory trick: Waves Reveal Deep Earth's State