Answer to Question 1
Answer: Some rocks seen at Earth's surface formed as much as 50 kilometers or (rarely) as far as 150 kilometers below the surface. These rocks reveal mostly felsic continental crust and mafic oceanic crust, both of which overlie peridotite mantle. The rocks found near the surface cannot account for the density of the whole Earth. Along with wobbles in Earth's rotation, the high overall density of the planet implies a central core of material that is much denser than mantle peridotite.
Answer to Question 2
Answer: Waves move very slowly through the air or water that fills the pore spaces between the solid sediment grains. Seismic waves in igneous rocks are generally fast, and they move faster in mafic gabbro than felsic granite and even faster in ultramafic rocks; these differences between rock types indicate the importance of mineral types on seismic velocity. Seismic velocity also varies with changes in temperature and pressure. As an analogy, consider what would happen if you were holding onto one end of a rubber pole in one hand and one end of a steel pole in the other hand while someone struck the opposite ends of the poles with a hammer. You would feel much stronger vibrations through the steel pole than through the rubber pole. This happens because the body waves initiated by the hammer blows cause more squeezing and twisting within the rubber than within the steel, so the waves travel more efficiently, and thus faster, through the steel.
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