Compared with continental crust, oceanic crust is

• A. Thinner and more dense
• B. Thicker and more dense
• C. Thinner and less dense
• D. Thicker and less dense

Answer: (A)

The difference in crustal thickness and density is what sets oceanic crust apart. Oceanic crust forms from basaltic, mafic rocks that are denser than the lighter granitic rocks typical of continental crust. Because of that denser composition, oceanic crust is much thinner—roughly 5 to 10 kilometers—whereas continental crust is thicker, commonly about 30 to 50 kilometers. The higher density means it sinks lower in the mantle’s buoyancy system, so it sits as the ocean floor, yet it remains floating on the mantle. This combination—thin and dense—also explains why oceanic crust tends to subduct beneath less-dense continental crust (or older, dense oceanic crust) at convergent boundaries, forming trenches and volcanic arcs. In short, oceanic crust is thinner and more dense because of its basaltic, mafic composition, compared with the thicker, less dense continental crust.