What is the asthenosphere?

• A. The semi-fluid layer of the mantle beneath the lithosphere, allowing for the movement of tectonic plates.
• B. The rigid outer shell of the Earth that includes the crust and upper mantle.
• C. The innermost part of the Earth that remains molten.
• D. The upper mantle beneath the crust that behaves as a solid and does not flow.

Answer: (A)

The asthenosphere is a partially melted, ductile region of the upper mantle that can flow slowly, allowing the rigid lithospheric plates to move on top of it. This layer sits beneath the lithosphere, so its plastic behavior lets the plates glide and shift as convection currents in the mantle drive the motion. Because rock here behaves like a soft, malleable solid rather than a hard shell, it can deform and flow over geological timescales, which is why plate tectonics works.

The other descriptions refer to different parts: the rigid outer shell that includes crust and upper mantle is the lithosphere; the innermost molten part is the outer core; and the upper mantle that behaves as a solid and does not flow would be inaccurate for the asthenosphere, which is defined precisely by its ability to flow and deform.