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Three quarters of Earth’s volcanic activity occurs beneath the sea, predominantly along the extensive mid-ocean ridge system that winds its way through the major ocean basins. The style of eruptive activity along mid-ocean ridges has been extensively studied and well characterized. Submarine eruptions at mid-ocean ridges are dominated by effusive production of pillow and sheet-flow lavas at water depths of several thousands meters. The other major style of submarine eruptions occurs along island arcs where subduction of oceanic crust triggers melting of mantle rocks by the release of volatile components, such as water and carbon dioxide. Submarine volcanism constitutes an important component of active island arc systems, although a significant part of arc volcanism can also occur subaerially.