Final answer:
The asthenosphere is the uppermost part of the mantle, made of partially molten rock that can deform and flow, which supports the movement of tectonic plates.
Step-by-step explanation:
The uppermost part of the mantle, known as the asthenosphere, is composed of partially molten upper mantle material that behaves plastically and can flow. This behavior allows the tectonic plates of Earth's crust to move and shift. The asthenosphere's composition includes solid rock similar to the rest of the mantle, primarily made up of elements such as silicon, oxygen, iron, aluminum, and magnesium, which are also found in crustal rocks like basalt and granite. However, due to the higher temperatures and pressures at this depth, the rock can deform and is less rigid compared to the overlying lithosphere, allowing for slow convection currents to facilitate the movement of the tectonic plates.