Fig. 6: Schematic cross-sections of the classes of ridges defined here, showing structure expected on the basis of our model of their formation. (a) Class 1 forms by repetitive diurnal working as shown in Fig. 2. (b) Incomplete extrusion of ice debris allows a crack to ratchet open during the diurnal working, creating a Class 2 ridge structure. If there is crustal downwarping due to the load, parallel side cracks may form. (c) Diurnal working may activate some of the side cracks along all or part of their lengths, building small lateral ridges analogous to formation of Class 1 or 2, but becoming part of an interbraided complex characteristic of Class 3. (d) If the weight of ridge complex warps the crust enough that part of the surface is below the water line, liquid may seep through the porous ridges and flood the margins, modifying and coloring the surface, as observed on Europa.