What is the role of the asthenosphere in the Earth's structure?

The asthenosphere plays a crucial role in the Earth's structure, particularly in plate tectonics. It is located beneath the lithosphere and extends from about 100 kilometers (62 miles) to about 350 kilometers (217 miles) into the mantle. The key characteristic of the asthenosphere is that it behaves plastically, meaning it can flow and deform over geological time. This plasticity allows the rigid lithospheric plates resting on top of it to move and interact with one another.

The movement of the lithospheric plates is facilitated by the soft, ductile nature of the asthenosphere, which creates a dynamic environment that can accommodate the stresses generated by tectonic forces. Such movements are responsible for geological phenomena such as earthquakes, volcanic activity, and the formation of mountain ranges.

In contrast to this, other layers and features are distinct from the asthenosphere. The asthenosphere is not a fluid layer; rather, it is solid but behaves like a viscous fluid over long periods, which is different from a true liquid state. Additionally, while the Earth's crust above consists of various rocks, the asthenosphere does not contain the oldest rocks; those are typically found in the continental crust. Lastly, the asthenosphere is