The buoyancy of tectonic plates is explained by their density relative to the mantle. Which statement is accurate?

• A. It is less dense than the mantle
• B. It is more dense than the mantle
• C. It is identical density to the mantle
• D. It has a higher temperature than the mantle

Answer: (A)

Buoyancy is determined by density relative to the material around it. An object floats when it is less dense than its surroundings. The lithosphere, which makes up tectonic plates, sits on the underlying mantle and is less dense than that mantle. Because of this density difference, the plates ride on top rather than sinking, giving them their buoyant behavior. That’s why the accurate statement is that the plates are less dense than the mantle. If a plate were more dense, it would tend to sink; if densities were identical, it would be neutrally buoyant, and temperature effects aside, the key factor here is the relative density.