Question

Answer 1

A surface plates or subducting slab is a broken part of lithosphere. A thermal boundary layer is on the other hand a thin layer of viscous fluid in contact with a moving or non moving solid surface. The main difference is three fold. First is that the plate viscosity is much higher compared to the thermal boundary layer. Second is the velocity within a plate is generally constant whereas within a boundary layer velocity changes with depth or height. Lastly, thermal boundary layer is thermal in nature whereas a plate is thermo-mechanical in nature.

Descending plumes in 2D convection are essentially fuild with much lower viscosity whereas plates are solid with higher viscosity. Descending plumes have trace amount of water with it but subducting slabs contains very high water content.