Answer to Question 1
Answer: Laboratory experiments relate seismic-wave velocities to different types of rocks, under temperature and pressure conditions comparable to those in the deep interior of Earth. Seismic waves move more slowly in unconsolidated sediment than through sedimentary rocks, and they move even faster in igneous and metamorphic rocks. Within igneous rocks, felsic rocks transmit waves more solely than mafic rocks, and ultramafic rocks display the most rapid velocities.
Answer to Question 2
Answer: Earthquakes produce surface waves and two distinct body waves, called primary (P) and secondary (S) waves, which travel through the planet. P and S waves reflect and refract when encountering boundaries between materials. Seismograms are records of earthquake waves detected by seismometers located around the globe. The time required for different earthquake waves to reach these instruments reveals the velocity at which the waves move through Earth. Travel-time curves reveal that, in general, seismic-wave velocity increases at deeper levels in the mantle. Shadow zones where S waves, P waves, or both are not recorded reveal that Earth has a liquid outer core, beginning at 2900 kilometers below the surface, surrounding a solid inner core at 5155-6371 kilometers depth.
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