ACT (Enhanced)ScienceHard
A seismologist is studying the internal structure of a newly discovered planet. They detect two types of seismic waves, P-waves and S-waves, traveling through the planet's interior. They observe that S-waves completely disappear at a certain depth, while P-waves continue to propagate but with a change in speed and direction. What is the most likely inference about the planet's interior at that specific depth?
- AThere is a solid-liquid boundary, with the outer layer being solid.
- BThe planet's core is entirely solid, as P-waves continue to travel.
- CThere is a liquid layer, as S-waves cannot travel through liquids.
- DThe material at that depth is extremely rigid, causing S-waves to stop.
Show answer & explanationAnswer & explanation
Correct answer: C. There is a liquid layer, as S-waves cannot travel through liquids.
S-waves (shear waves) can only propagate through solids, while P-waves (compressional waves) can travel through both solids and liquids. The disappearance of S-waves at a specific depth, coupled with the continued (though altered) propagation of P-waves, is strong evidence for the presence of a liquid layer at that depth.
Why the other options are wrong
- A. While it suggests a solid-liquid boundary, the disappearance of S-waves specifically points to the *presence* of a liquid, not just that the outer layer is solid.
- B. The fact that P-waves continue doesn't imply an *entirely* solid core; it only means P-waves can travel through the material present. The S-wave disappearance is the critical piece of information.
- D. If the material were extremely rigid (solid), S-waves would propagate even better, not stop.
Seismic Wave Properties
Characteristics of P-waves (primary, compressional) and S-waves (secondary, shear) that reveal the physical state (solid/liquid) of Earth's interior.
- P-waves travel through solids, liquids, and gases.
- S-waves travel only through solids.
- Changes in speed and direction indicate changes in material density or state.
Memory trick: P-waves Push, S-waves Shake, Liquids Make S-waves Break.