Here's a breakdown:
* Mantle: The thickest layer of the Earth, sandwiched between the crust and the core. It's mostly solid rock but behaves like a very viscous fluid over long periods.
* Upper Mantle: The top portion of the mantle, extending from the crust down to about 670 km deep.
* Lower Mantle: The bottom portion of the mantle, extending from 670 km down to the outer core boundary at about 2,900 km.
The lower mantle is under immense pressure and temperature, making it very dense and rigid. It's this part of the mantle that plays a crucial role in plate tectonics, as heat from the core drives convection currents within the mantle.
