The asthenosphere is weaker and lies below the uppermost mantle. This means it is _______.

• A. Weaker than the lithosphere
• B. Stronger than the lithosphere
• C. Liquid at surface
• D. Solid iron core

Answer: (A)

The key idea is that the asthenosphere is a weak, ductile layer of the upper mantle that lies beneath the rigid lithosphere, allowing the plates to move on top of it. Because it is hot and partly capable of melting, rocks there flow slowly and deform under stress, giving it much lower strength (viscosity) than the overlying lithosphere. That’s why it behaves as a soft, floating substrate for the tectonic plates rather than a strong, rigid layer. It isn’t liquid at the surface—it’s still solid rock that can flow on geologic timescales. It also isn’t the Earth’s core, which is a much deeper and compositionally different layer. So the description that fits best is that the asthenosphere is weaker than the lithosphere.