The diagrams showing the forces associated with the subduction process generally take the form as shown in Fig 9. The calculations generally follow a similar path in that the resultant forces causing tectonic plate movement are stated as:
F (Ridge-Push): the force created by the hot magma either via a plume or convection as it breaks through to the seafloor
F (Slab-Pull): the downward force of the heavy cold slab as it moves towards the mantle
F (Viscous-drag): the force opposing the downward motion of the slab. This force is calculated as a function of the velocity of descent of the slab
F (Buoyancy): The lithosphere is buoyant at the beginning of any subduction process due to the density difference between the lithosphere and upper asthenosphere
The above-mentioned forces are generally stated as the following equation
F (ridge-push) + Force (slab-pull) - Force (viscous-drag)- F (Buoyancy) =0.
This notation is used as all the forces acting on a body must balance out to zero.
In solving for the numerical values of the forces involved in the above general equation many factors need to be considered. These include upward buoyancy forces, pressure, and temperature gradients to describe the density and hence the slab weight in the asthenosphere, isostatic considerations, and the variable water content and its effect on viscous drag coefficients. As both the approaches and the actual values vary between different authors, the simplified format of the equations will be considered in this section of this treatise.