what is the special feature of the upper mantle?

The special feature of the upper mantle is that it contains the asthenosphere, a soft, weak, and partially molten layer. This semi-fluid nature of the asthenosphere allows it to flow slowly over time, providing lubrication that enables the movement of tectonic plates. This feature is crucial for plate tectonics, as it allows the rigid lithospheric plates above it to move. The upper mantle extends from the base of the crust down to about 410 to 670 kilometers deep and is composed mainly of solid rock minerals like olivine and pyroxene, but the asthenosphere within it behaves plastically and can deform, facilitating geological processes such as continental drift, earthquakes, and volcanic activity.