The mechanism of pressure solution, a source of controversy for years, must be understood before we can evaluate the effectiveness of pressure solution during geological processes. The water film diffusion (WFD) mechanism proposed by Weyl (1959) and Rutter (1976, 1983) is believed by many to be the primary mechanism responsible for intergranular pressure solution (IPS) in non-porous metamorphic rocks as well as porous sedimentary rocks. Tada and Siever (1986), experimenting with halite single crystals, suggested the new plastic deformation plus free-face pressure solution (PD + FFPS) mechanism.

The effectiveness of PD + FFPS as an IPS mechanism is theoretically evaluated for porous quartzose sandstone and compared with WFD. The result suggests that, though the driving force of the reaction (relative activity increase) is 4 to 5 orders of magnitude larger in WFD, the ease of diffusion (diffusion path width times the diffusion coefficient) is 7 to 9 orders of magnitude larger in PD + FFPS. Consequently. PD + FFPS yields diffusion rates 2 to 5 orders of magnitude faster than WFD.

In WFD, diffusion is always the rate-controlling process, whereas either dissolution at IPS contacts or precipitation on free grain surfaces may be the rate-controlling process in PD + FFPS, when temperatures are low and/or grain sizes are small. The dissolution or precipitation rate of PD + FFPS is faster than the diffusion rate of WFD except when the total free grain surface area is very small. In final stages of compaction, when the total free grain surface area has become very small, WFD replaces PD + FFPS.