FIG. P-5. (a) Plate-tectonic model. A=rift zone where plates P1 and P4 are moving apart (such as the Mid-Atlantic Ridge) where new crust is being created. C=ocean trench resulting from overriding of plate P3 over P1 (or P2 over P4); B=Benioff-Wadati zone of earthquakes dipping along the contact of plates P1 and P2 which are colliding. T=transform faults where plates are sliding by each other. (After Isaacs et al., 1968.) (b) There are three kinds of plate boundaries. The junction of three plates constitutes a triple junction (q.v.); See also Fig. T-16.