Final answer:
Volcanoes are primarily formed at divergent boundaries, such as the Mid-Atlantic Ridge, where Iceland and the Azores are located, and at subduction zones.
Step-by-step explanation:
The two types of plate boundaries that form volcanoes are rift zones (divergent boundaries) and subduction zones (convergent boundaries). At rift zones, like the Mid-Atlantic Ridge, plates pull apart allowing molten rock to rise and form new crust. At subduction zones, one plate dives beneath another, which can melt and generate magma that feeds volcanoes. Along the Mid-Atlantic Ridge, the two countries where this underwater mountain range emerges above the water are Iceland and the Azores (an autonomous region of Portugal).