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Why do many volcanoes form near subduction zones?

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volcanology — Earth sciences Geologist using a rock hammer to sample active pahoehoe lava for geochemical analysis on the Kilauea volcano, Hawaii, June 26, 2009.

Volcanoes form near subduction zones primarily due to the process of partial melting triggered by the descent of an oceanic plate into the Earth's mantle.12

As an oceanic plate is forced beneath an overriding plate, it carries seafloor sediments rich in water and carbon dioxide into the hotter depths of the mantle. When the subducting slab reaches a depth of approximately 100 km (60 miles), these volatile components are driven off. These compounds act as fluxes, which significantly lower the melting temperature of the mantle rocks in the "mantle wedge" above the subducting plate.123

This process generates magma, which is typically basaltic in composition. Because this magma is buoyant, it rises through the overriding plate to form a line of volcanoes known as a volcanic arc. As the magma ascends, its composition can be further altered through fractional crystallization or by melting and assimilating the surrounding crustal rocks, often resulting in the highly viscous and explosive nature characteristic of many subduction-zone volcanoes.12