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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 originated on oceanic crust and have resulted from intermittent descent of lithosphere along older Benioff zones. Since the Permian, some lithospheric plate margins have remained near present island arcs. Ancient island arcs from the Ordovician of the Lake District, Wales, and Newfoundland are discussed; oceanic crust and an ancient Benioff zone are described from the Ordovician of southern Scotland; a trench is postulated as the depositional environment of the Manx-Skid-daw slates; Wales is considered to have been a small ocean basin in Lower Paleozoic times.