why are volcanos often within a few hunderd kilometers of subduction zones?

Question

why are volcanos often within a few hunderd kilometers of subduction zones?

Answer 1

It's because as the slab descends, it stays fairly cold (because rock doesn't conduct heat very well!), and the pressure increases. As the pressure increases, certain minerals change their shape to more compact arrangments, typically losing water and other "volatiles" (this is called slab dehydration). When these volatiles are lost, they move upwards into the overlying "mantle wedge", where they lower the melting point of the rock. It's from here that magmas arise. The reason for the 100km or so distance from the zone of subduction is due to the slab "dehydration" not occuring until a certain depth, and because the slab typically descends at an angle of ~60deg

Answer 2

crust goes down, melts, comes back up again = volcanoe.