What does the term 'asthenosphere' refer to?

The term 'asthenosphere' refers specifically to a plastic layer within the Earth's upper mantle. This region is characterized by its ability to flow slowly over geological time scales, which allows tectonic plates situated above it in the lithosphere to move. The asthenosphere is significant because it provides the necessary conditions for the movement and interactions of tectonic plates, which is fundamental to processes such as continental drift, earthquakes, and volcanic activity. Unlike the rigid lithosphere above it, the asthenosphere is less viscous, enabling the plates to glide over it. This distinction in physical properties is crucial for understanding plate tectonics and the dynamic nature of the Earth's interior.