The Earth's mantle is divided into two layers, the upper mantle and the lower mantle, based primarily on their physical properties, particularly their rheology (how they deform under stress). Here's a breakdown:

Upper Mantle:

* Lithosphere: The uppermost layer, composed of the crust and the rigid, brittle part of the upper mantle. This is where tectonic plates reside.

* Asthenosphere: A soft, partially molten layer below the lithosphere. It's characterized by low viscosity and is responsible for the movement of tectonic plates.

Lower Mantle:

* Mesosphere: The bulk of the mantle, extending to the Earth's core. This layer is much denser and more rigid than the asthenosphere.

Why the Division?

* Mineralogical Differences: The upper and lower mantle have different mineral compositions, particularly in the presence of specific minerals like olivine and pyroxene.

* Pressure and Temperature: The immense pressure and temperature of the lower mantle cause the minerals to behave differently.

* Rheology: The different mineral composition and pressure conditions lead to a significant difference in how the layers deform. The asthenosphere behaves like a very viscous fluid, while the mesosphere acts like a very strong solid.

Key Points:

* The division isn't sharp: The boundary between the upper and lower mantle is not a clean, distinct line. There's a transition zone with gradual changes in mineralogy and behavior.

* Convection: The difference in density and rheology between the upper and lower mantle is crucial for the process of mantle convection, which drives plate tectonics.

Let me know if you have any other questions about the Earth's mantle!