Final answer:
The layer directly below Earth's plates is called the asthenosphere. It is located in the upper part of the mantle and plays a crucial role in plate tectonics.
Step-by-step explanation:
The layer directly below Earth's plates is called the asthenosphere. The asthenosphere is located in the upper part of the mantle, and it is partially molten and able to flow, making it more plastic in nature. It plays a crucial role in plate tectonics, as it is the layer where the plates float and move on.