Flexi Says: Many types of volcanoes can be found under the ocean. In fact, the whole ocean floor is formed by volcanic activity at mid-ocean ridges. Magma continually erupts at these divergent plate boundaries, and then cools to form the ocean floor. Underwater volcanoes also form at convergent plate boundaries between two oceanic plates. When these volcanoes grow large enough to poke above the water, they form island arcs. Hotspot volcanoes can also form on the ocean floor away from any plate boundaries. These volcanoes can grow large enough to form islands like Hawaii, but if they are fully underwater they are called seamounts.