As we have previously discussed, volcanism is associated with both accretionary plate margins and subduction zones. The worldwide distribution of active volcanoes is shown in Figure 1–52. Clearly, most volcanoes are associated with plate margins. Continuous volcanism occurs along the worldwide ocean ridge system, where it forms the 6-km-thick basaltic oceanic crust. Only a small fraction of this volcanism is included in the distribution of active volcanoes given in Figure 1–52; the remainder occurs on the seafloor, where it cannot be readily observed. The linear chains of active volcanoes associated with ocean trenches are clearly illustrated in Figure 1–52. How- ever, significant gaps in the chains do occur even when active subduction is going on. Examples are in Peru and central Chile.
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