Here's why:

* Solid but flowing: The asthenosphere is a layer of the upper mantle that is solid but behaves like a very viscous fluid over long periods of time. This is because the immense pressure and heat cause the rock to deform and flow, even though it's not actually melted.

* Plasticity: The word "plastic" refers to this ability to deform and flow without breaking.

* Above the lithosphere: The asthenosphere lies beneath the lithosphere, which is the rigid outer layer of the Earth that includes the crust and uppermost mantle.

The asthenosphere is crucial for plate tectonics, as it allows the tectonic plates to move on top of it.