GED Science TestEarth and Space ScienceHard
A geologist is analyzing seismic wave data from an earthquake. She observes that S-waves are detected at seismograph stations located on one side of the Earth, but not on the opposite side, creating a 'shadow zone'. What does this observation primarily indicate about Earth's interior?
- AThe Earth's mantle is composed of molten rock.
- BThe Earth's inner core is entirely solid.
- CThe Earth's outer core is liquid.
- DThe Earth's crust is uniformly thick globally.
Show answer & explanationAnswer & explanation
Correct answer: C. The Earth's outer core is liquid.
S-waves (secondary or shear waves) cannot travel through liquids. The existence of an S-wave shadow zone, where S-waves are not detected on the opposite side of the Earth from an earthquake, is direct evidence that Earth's outer core is liquid.
Why the other options are wrong
- A. The mantle is mostly solid, though plastic-like in its upper layers, and S-waves travel through it.
- B. S-waves do travel through the solid inner core, but their absence in the shadow zone is due to the liquid outer core.
- D. Seismic data indicates the crust varies significantly in thickness, but this is not the primary conclusion from the S-wave shadow zone.
S-wave Shadow Zone
An area on Earth's surface, opposite an earthquake's epicenter, where S-waves are not detected due to their inability to travel through the liquid outer core.
- S-waves are shear waves, only propagating through solids.
- The S-wave shadow zone spans more than 103 degrees from the epicenter.
- Provides crucial evidence for the liquid nature of Earth's outer core.
Memory trick: Seismic Shadows Show the Secret of the Core.