In which layer of the Earth do tectonic plates float on top?

Tectonic plates float on top of the asthenosphere, which is a semi-fluid layer of the upper mantle. The asthenosphere is characterized by its plasticity, allowing the rigid plates of the lithosphere above it to move and shift. This movement is driven by convection currents within the mantle, which arise from the heat generated by the decay of radioactive elements and the residual heat from Earth's formation. The ability of the tectonic plates to drift over this partially molten and deformable layer is fundamental to the theory of plate tectonics, leading to various geological phenomena such as earthquakes, volcanic activity, and the formation of mountain ranges. Understanding this relationship between tectonic plates and the asthenosphere is crucial for grasping Earth’s geological processes.