Subduction typically proceeds at rates of mere centimeters per year, but it is associated with intense, deep-seated seismic activity and volcanic activity (volcanic island arcs). ◙ subduction zone magmas ◙
Wherever continental and oceanic plates collide, the denser oceanic plate subducts underneath the less dense continental plate, sinking toward the asthenosphere where the oceanic crust is consumed.
Subduction zones are typically arcuate, with the convex arc directed toward the oceanic plate. More steeply inclined subduction plates create less pronounced arcs than do shallow subduction plates. Back-arc basins develop in the concave area between the subduction zone and the continent.
At subduction zones, the oceanic lithosphere, layers of marine sediments, and trapped water are recycled into the deep mantle. When the oceanic plate sinks to about 100 km in depth, peridotite is metamorphosed to eclogite, becoming denser and sinking farther into the mantle.
Magmatism begins at depths of 100 km or deeper in the Wadati-Benioff zone, which is typically 100-300 km inland from the subduction trench.
Island arcs contribute about 25% of the total volume of magma produced each year on Earth (~30-35 km³), which is much less than the volume produced at mid-oceanic ridges. At the spreading centers of the mid-oceanic ridge systems, convection currents within the mantle carry basaltic magmas to the surface and regenerate oceanic crust. Island arc volcanic lavas originate in a variety of magmas.