Here's why it's hard to give a definite thickness in miles:
* Depth Variability: The depth of the asthenosphere varies depending on location and geological conditions. It can be as shallow as 60 miles (100 km) in some places and as deep as 400 miles (650 km) in others.
* Transitional Zone: The asthenosphere transitions gradually into the lithosphere above and the mesosphere below. There's no distinct boundary.
Therefore, it's more accurate to say the asthenosphere is a zone spanning approximately 60 to 400 miles (100 to 650 km) in depth.
