Describe the relationship between the lithosphere and the asthenosphere and how this relationship influences plate motion.

• A. The lithosphere sits on the ductile asthenosphere, and mantle convection drives plate motion
• B. The asthenosphere sits above the lithosphere in rigid contrast
• C. The lithosphere is liquid
• D. Mantle convection occurs only in the outer core

Answer: (B)

The main idea is that the lithosphere is a rigid outer shell that rides on a weaker, ductile layer called the asthenosphere. Because the asthenosphere can flow slowly, it acts like a conveyor belt for the plates. Mantle convection in this softer layer transfers heat and creates horizontal flow that exerts shear at the base of the lithosphere, driving the movement of tectonic plates.

This describes why plates move: the strong contrast in strength between the rigid lithosphere and the pliable asthenosphere lets the plates be carried along by the mantle’s convection currents.

The other statements don’t fit with what we observe. The asthenosphere is below the lithosphere, not above, and it is not rigid—it's the weak, ductile layer that enables motion. The lithosphere is not liquid; it’s rigid. And mantle convection relevant to plate tectonics happens throughout the mantle, not only in the outer core.