Here's why:
* Lithosphere: This is the rigid, outermost layer of Earth. It includes the crust and the uppermost part of the mantle. It's relatively cool and solid.
* Asthenosphere: This is a layer of the upper mantle located beneath the lithosphere. It's characterized by its partially molten state. This partial melting is due to high temperatures and pressures, creating a semi-solid, viscous state. This makes the asthenosphere behave like a very viscous fluid over long periods.
The partially melted rock in the asthenosphere is crucial for plate tectonics. The movement of the lithospheric plates is driven by convection currents within the asthenosphere.
