the oceanic ridge system, producing a steplike plate margin (Figure 2.20A). Notice that the zigzag shape of the Mid- Atlantic Ridge in Figure 2.11 roughly reflects the shape of the original rifting that caused the breakup of the supercon- tinent of Pangaea. (Compare the shapes of the continental margins of the landmasses on both sides of the Atlantic with the shape of the Mid-Atlantic Ridge.)
Typically, transform faults are part of prominent linear breaks in the seafloor known as fracture zones, which
include both active transform faults and their inactive extensions into the plate interior (Figure 2.20B). Active transform faults lie only between the two offset ridge segments and are generally defined by weak, shal- low earthquakes. Here seafloor produced at one ridge axis moves in the opposite direction of seafloor pro- duced at an opposing ridge segment. Thus, between the ridge segments, these adjacent slabs of oceanic
crust are grinding past each other along a transform fault. Beyond the ridge crests are inactive zones, where the frac- tures are preserved as linear topographic depressions. The trend of these fracture zones roughly parallels the direc- tion of plate motion at the time of their formation. Thus, these structures are useful in mapping the direction of plate motion in the geologic past.