Final answer:
The lithosphere-asthenosphere boundary marks the transition from the rigid upper mantle (part of the lithosphere) to the plastic, flowing upper mantle (the asthenosphere), typically at a depth of about 100 kilometers.
Step-by-step explanation:
The boundary between the lithosphere and the asthenosphere is important to understand in the context of Earth's layers. The lithosphere includes the crust and the uppermost part of the mantle, extending to a depth of about 100 kilometers. This layer behaves as a rigid, brittle solid. On the other hand, the asthenosphere lies beneath the lithosphere and reaches down to about 660 kilometers. This layer consists of partially molten upper mantle material that behaves plastically and can flow.
The crucial detail to note is that the asthenosphere is not within the crust but extends into the upper mantle. The lithosphere-asthenosphere boundary is, therefore, the division between rigid mantle and plastic mantle rather than simply a boundary within the crust. The depth of this boundary can vary but is around 100 kilometers deep, which is significantly deeper than 35 to 40 kilometers often mistakenly cited as the depth of the crust only.
The boundary between the lithosphere and the asthenosphere is the division between rigid mantle and plastic mantle. This boundary is a mechanical differentiation within the upper mantle, not restricted to the crust, and is approximately 100 kilometers deep.