What is the weaker, hotter zone beneath the lithosphere that allows for motion of Earth's rigid outer shell?
1 Answer
That is called the asthenosphere.
Explanation:
The asthenosphere is part of the upper mantle and has the same composition as the rigid upper mantle rock above (mostly magnesium and iron silicates).
The asthenosphete is like warm taffy that's soft enough for you to twist and pull, whereas the lithosphere above it is colder and, like cold taffy, it's hard and brittle. With rock as with taffy, tenperature is more important than the specific chemistry, at least in the crust and upper mantle.
But unlike taffy, "warm" for the rock in the upper mantle is about 1300°C. The isotherm at that temperature inside the mantle is conventionally taken to be the boundary between the "cold-taffy" lithosphere and the "warm-taffy" asthenosphere (https://en.wikipedia.org/wiki/Asthenosphere).
