Tectonic plates are segments of which rigid outer layer, and what are the main differences between continental and oceanic crust?

• A. The asthenosphere; continental crust is basaltic, thin; oceanic crust granitic, thick.
• B. The mantle; continental crust is metallic; oceanic crust is silicate.
• C. The lithosphere; continental crust is granitic, thick and buoyant; oceanic crust is basaltic, thinner and more dense.
• D. The crust only; both types have identical composition.

Answer: (C)

Tectonic plates are segments of the lithosphere, the rigid outer shell that includes the crust and the uppermost mantle and that sits atop the weaker asthenosphere. The key differences between continental and oceanic crust come down to composition, thickness, and density. Continental crust is granitic in composition, thicker, and less dense, which makes it buoyant and elevated. Oceanic crust, by contrast, is basaltic in composition, thinner, and more dense, so it sits lower and can readily sink or be forced beneath continents at convergent boundaries. These traits explain why continents protrude higher and ocean basins are underlain by thinner, denser crust, and why oceanic crust tends to subduct beneath continental crust.