Modern active island arcs and their surroundings consist of two main physiographic zones-a volcanic ridge or arc and a submarine trench. Rocks in island arcs fall into three groups, the origin of which can be interpreted in terms of plate tectonics and of geological processes in the two zones and surrounding ocean floor. The groups are (1) volcanic arc deposits, consisting of undeformed subaerially erupted volcanic rocks, block-faulted volcanic and volcaniclastic rocks, diorites, granites, and low-pressure/high-temperature meta-morphic rocks; (2) submarine trench deposits, consisting of trench-fill sediments, represented by thick successions of quartz-deficient turbidites and pelagic sediments, and high-pressure/low-temperature meta-morphic rocks and melange deposits indicating a Benioff zone, (3) ocean-floor oceanic crustal rocks consisting of tholeiitic lavas, spilites, gabbros, ultrabasic rocks, and pelagic sediments, and including fine-grained turbidites rich in tholeiitic detritus. Most island arcs o...