The asthenosphere is a layer within the Earth's mantle, located beneath the lithosphere. It starts at a depth of approximately 100 to 200 kilometers and extends to a depth of around 660 kilometers. However, the precise depth of the asthenosphere can vary depending on location and geological conditions.

Here's a breakdown:

* Upper boundary: The asthenosphere begins at the base of the lithosphere, which is the rigid outer layer of the Earth. This depth is generally considered to be around 100 kilometers, but it can be as shallow as 50 kilometers in some regions.

* Lower boundary: The asthenosphere transitions into the lower mantle at a depth of around 660 kilometers. This depth is marked by a significant change in mineral composition and density.

It's important to note that the asthenosphere is not a solid layer like the lithosphere. It's a semi-solid, partially molten zone with a weaker, more ductile behavior. This allows tectonic plates to move over it.

Here are some key characteristics of the asthenosphere:

* Partially molten: The asthenosphere contains pockets of magma, which contributes to its ability to flow.

* Low viscosity: The asthenosphere has a relatively low viscosity compared to the lithosphere.

* Ductile behavior: The asthenosphere can deform under pressure without breaking.

The asthenosphere plays a crucial role in plate tectonics. It allows the lithospheric plates to move and interact, leading to phenomena like earthquakes, volcanoes, and mountain formation.