Which boundary type typically forms an island arc?

• A. Oceanic-Oceanic Convergent
• B. Oceanic-Continental Convergent
• C. Continental-Continental Convergent
• D. Divergent

Answer: (A)

Island arcs form at convergent boundaries where one oceanic plate sinks beneath another. As the subducting plate descends, it carries water into the mantle, which lowers the melting point of the overlying rock. This partial melting generates magma that is less dense than surrounding rock, so it rises to feed volcanoes. The magma builds a chain of volcanoes that emerge as islands, curling along the trench to form a curved arc.

This boundary type is the best fit because the key driver is subduction of oceanic crust, which routinely produces the magma supply and tectonic setup needed for an island arc. If an oceanic plate subducted beneath continental crust, you’d get a continental volcanic arc on the margin rather than a chain of islands. Continental-continental convergence creates mountains with little volcanism, and divergent boundaries mainly form new crust and mid-ocean ridges, not island arcs.