The part of the mantle that is involved in plate tectonics is called the asthenosphere. It's not technically "soft rock", but behaves like a very viscous fluid over long periods of time. Think of it like a very thick, gooey honey – it can move and flow, but it does so extremely slowly.

Here's a breakdown:

* Lithosphere: This is the rigid outer layer of the Earth, including the crust and the uppermost part of the mantle. It's broken into tectonic plates.

* Asthenosphere: This is the layer beneath the lithosphere. It's still solid, but behaves like a very viscous fluid due to the high temperatures and pressure. This allows the tectonic plates to move slowly on top of it.

* Mesosphere: This is the lower part of the mantle. It's solid and more rigid than the asthenosphere.

So, the plates (part of the lithosphere) move on top of the asthenosphere, which acts like a slow-moving fluid, allowing for the processes of plate tectonics like earthquakes and volcanic activity.