Notion of dynamics
Gravitational potential energy and gravitational force
The magnitude of the Gravitational Potential Energy (GPE, in Pa) for a given lithospheric column is given by the surface area of the region delimited (i.e. bounded) by i) its ρ.g.z function (lithostatic pressure), ii) the vertical axis (z), and iii) the compensation level. For column B this is the orange region, and for column C the yellow region which extends underneath the orange region. The gravitational force (Fg in N.m-1) that C and B applies on each other is equal to the difference of their respective GPE (difference between the two colored surface areas, ie the thin yellow wedge on the graph below). An excess in GPE in column C (Fg>0) will drive extension, whereas a deficit (Fg<0) will drive contraction.
Fg = GPEC - GPEB

 

This movie shows a 65 km thick crust adjacent to a 40 km thick crust (brown), in green the upper mantle. It starts at a stage when thermal relaxation has produced 20 to 25% of melt (dark pink) in the lower crust. The only force acting is the gravitational force. It is the same physics that is responsible of the spreading of camembert, or that of warm butter.