The primary difference between the upper and lower mantle lies in their physical state; the lower mantle is significantly more solid than the upper mantle.
While both the upper and lower mantles are primarily composed of silicate rocks, the differing temperatures and pressures at their respective depths result in distinct properties. The upper mantle contains Olivine, compounds of silicon dioxide, and peridotite.
Here's a breakdown of the key differences:
- Physical State: The upper mantle is partially molten, exhibiting plasticity, while the lower mantle is mostly solid.
- Pressure: The lower mantle experiences far greater pressure than the upper mantle due to the weight of the overlying material. This immense pressure contributes to its solid state despite high temperatures.
- Composition: While both are largely composed of silicate minerals, the extreme pressures in the lower mantle cause these minerals to adopt different crystal structures compared to the upper mantle.
- Seismic Waves: Seismic waves travel at different speeds through the upper and lower mantle, providing evidence of the distinct physical properties of each layer.
In essence, the increased pressure in the lower mantle makes it more rigid and solid compared to the partially molten and more pliable upper mantle, even though both layers consist largely of similar materials.
