Which layer of the Earth's upper mantle is described as relatively plastic?

The asthenosphere is the layer of the Earth's upper mantle that is characterized as relatively plastic. This property of plasticity means that the asthenosphere is capable of flowing slowly over geological time, allowing for the movement of tectonic plates situated above it in the lithosphere. The asthenosphere is located beneath the lithosphere, extending to about 700 kilometers deep into the Earth's mantle.

This plastic nature is critical for plate tectonics as it provides a semi-fluid medium that supports the rigid plates above, making movement possible due to processes such as convection. In contrast, the lithosphere is rigid and brittle, while the outer core and inner core are primarily liquid and solid iron, respectively. Understanding the asthenosphere's properties helps explain many geological processes, including earthquakes and volcanic activity.