Which layer lies directly below the crust and is characterized by relatively low resistance to plastic flow, where convection is thought to occur?

Which layer lies directly below the crust and is characterized by relatively low resistance to plastic flow, where convection is thought to occur?

• A. Asthenosphere
• B. Lithosphere
• C. Mantle
• D. Outer Core

Answer: (A)

Rocks right beneath the crust that behave in a ductile, slowly flowing manner form a layer that lets convection occur. This region, the asthenosphere, is hot and partially molten enough that rocks can deform and flow like a viscous paste rather than crack as a rigid solid. That low resistance to plastic flow is what allows mantle convection currents to rise and fall, which in turn moves the overlying rigid lithosphere. The lithosphere itself is the stiff outer shell, so it doesn’t flow easily. The deeper mantle includes more rigid and more plastic zones, but the specific feature described—low resistance to flow with convection—is characteristic of the asthenosphere. The outer core is a separate, much deeper layer of liquid metal, not the layer directly under the crust.

Rocks right beneath the crust that behave in a ductile, slowly flowing manner form a layer that lets convection occur. This region, the asthenosphere, is hot and partially molten enough that rocks can deform and flow like a viscous paste rather than crack as a rigid solid. That low resistance to plastic flow is what allows mantle convection currents to rise and fall, which in turn moves the overlying rigid lithosphere. The lithosphere itself is the stiff outer shell, so it doesn’t flow easily. The deeper mantle includes more rigid and more plastic zones, but the specific feature described—low resistance to flow with convection—is characteristic of the asthenosphere. The outer core is a separate, much deeper layer of liquid metal, not the layer directly under the crust.