What process forms most volcanoes at convergent plate boundaries?

• A. Upwelling mantle plumes beneath continental crust
• B. Meteorite impacts
• C. Subduction of oceanic crust beneath continental crust
• D. Rifting at mid-ocean ridges

Answer: (C)

Subduction of an oceanic plate beneath another plate at convergent boundaries is what generates most volcanoes there. When the oceanic slab sinks, it carries water and other volatiles into the overlying mantle. The presence of these fluids lowers the melting point of the mantle rock, causing partial melting. The resulting magma, being buoyant, starts to rise and feeds magma chambers that erupt at the surface, forming volcanic arcs along the overriding plate. This process also tends to produce intermediate to felsic magma, which makes volcanoes that are often highly explosive and build stratovolcanoes, like those in the Andes or the Japanese arc.

Other scenarios don’t fit this setting: hotspots from mantle plumes create volcanoes away from plate boundaries, not specifically at convergent margins; meteorite impacts don’t involve magmatic processes that form volcanoes; and rifting at mid-ocean ridges occurs at divergent boundaries, not convergent ones.