What do plate tectonics ride on?

Answer 1

The Earth's crust comprises a series of interlocking plates which are less dense than the materials which comprise the underlying mantle. The crustal plates therefore float on the mantle and move with the mantle as it undergoes a cooling convection driven by the heat of the core. This movements means that some plate boundaries are being squashed together (a destructive boundary) while others are being moved apart (a constructive boundary).

Answer 2

Plate tectonics ride on the asthenosphere, which is a semi-fluid layer of the Earth's mantle. The movement of the plates on the asthenosphere is driven by the heat generated from the Earth's core, causing the plates to slowly drift and interact with one another along their boundaries.