If the fracture is rough or wavy, this growth direction may not be parallel the opening trajectory. If the ridges are distinct and sharp, they may lock a grain boundary and this boundary will subsequently follow the ridge and hence the opening trajectory. If the fracture is smooth, grain boundaries do not get locked or to a lesser extent, and grain boundaries then do not or only partially follow the opening trajectory. Movie of grains growing by repeated crack-sealing. Growth is isotropic as envisaged in the model of Urai et al.
Grains grow outward from the surface and thus converge on each other in embayments between ridges.
Eventually, only grain boundaries are left that are growing vertically towards a ridge on the opposite side of the crack. To further investigate the model of Urai et al. Hilgers et al. In the 2-dimensional model, grains are defined by nodes that are linked by straight boundary segments. An initial horizontal surface fracture is created with a user-defined roughness.