Fig. 17. Models for lithospheric dilation

Class 2 ridges and bands (top row) are end members in a continuum of processes which form dilational lineaments. Intermediate dilational lineaments (middle), which have characteristics of both bands and ridges, represent intermediate forms created by such a process continuum. Morphologic elements that may vary in response to this process continuum are: (1) elevation of the lineament surface; (2) width of the lineament, although width is also a function of longevity; (3) bilateral symmetry; and (4) prominence of a central groove. Mixtures of the internal forcing which produces dilational ridges and the external forcing which produces bands can create these intermediates by causing variations in the intensity with which these elements are expressed. The key to these variations is the degree to which tidal compression is counteracted by external pulling forces. If external forcing is intermittent (bottom), doublet ridges (Class 1, Greenberg taxonomy) may grow at the active crack, only to be separated with later spreading. Repeated episodes may produce densely-ridged terrain.