Which type of evidence is used to study Earth's internal structure?

• A. Direct evidence from rock samples
• B. Indirect evidence of seismic waves
• C. None of the above
• D. Both direct and indirect evidence

Answer: (D)

Studying Earth's interior relies on both direct samples we can bring up and indirect clues from how seismic waves travel through the planet. Direct evidence comes from rocks we can actually examine—crustal samples from drilling and mantle material carried to the surface by volcanic activity or trapped in xenoliths. But we can’t drill deep enough to reach the mantle or core, so direct samples are limited in scope. Indirect evidence comes from seismic waves produced by earthquakes or explosions. By tracking how P-waves and S-waves speed up, slow down, bend, reflect, or disappear, scientists infer where materials are solid or liquid and how dense they are, revealing the layers and boundaries inside Earth, like crust, mantle, outer core, and inner core. Since direct data are sparse at depth and seismic data provide a global picture, combining both types of evidence gives the most complete model of Earth’s internal structure.