Final answer:
The asthenosphere is the layer beneath Earth's lithosphere where tectonic plates float, driven by convection currents within the mantle, which is integral to the theory of plate tectonics.
Step-by-step explanation:
The layer where the plates float is called the asthenosphere. It is located beneath Earth's outer layer, the lithosphere, to which the crust is attached. The asthenosphere is a semi-fluid layer that allows tectonic plates to move over it. This movement is driven by convection currents within the mantle, where heat from Earth's interior causes the warmer material to rise and cooler material to sink. This process is fundamental to the theory of plate tectonics, affecting the Earth's surface and contributing to geological phenomena such as earthquakes, volcanic activity, and the creation of mountain ranges.