Lithium isotopes allowed her to trace recycling in crusts and other diffusional processes in earth. Operation of a diffusional thermal cooling CPC is depicted on the right. The time is selected to limit diffusional movement of bismuth. This approximate equation becomes exact when used to define the diffusional mean free path. Until this point, substitutional and ring methods were the dominant ideas for diffusional motion. It includes both the convectional mass flux and the diffusional mass flux. Nevertheless, diffusional creep can dominate in very cold or deep parts of the upper mantle. Diffusional creep leads to grain-boundary separation, that is, voids or cracks form between the grains.