Question: Why do tectonic plates move and why do they exist?

Tim Craig answered on 12 Jun 2018:

Tectonics is essentially all driven by the loss of heat from the centre of the Earth. In the same way that if you pour cold milk into a hot cup of tea, the fluid ‘convects’ (hot things rise, cold/dense things sink), the Earths interior does exactly the same, just very, very slowly. The motions of tectonic plates across the Earths surface are just the outermost expression of the process, and the loss of heat from the Earths interior.