How hove volcanoes played a role in the formation of land above or below the water?

Volcanoes play an important role in creating land since they are the sources of magma, which once above ground normally cools to create new land. In the ocean, this land normally forms at divergent and convergent boundaries or hot spots. At divergent boundaries (where two plates move apart), magma constantly erupts along a trench deep below the ocean's surface. This magma rarely piles upward and instead is pushed to both sides of the trench. This is how new seafloor forms. Convergent boundaries can create island arcs like Indonesia as magma erupts bit by bit. Hot spots occur in the middle of plates. They are instances where the mantle pierces through the crust and begins to erupt directly onto to seafloor. Over time, these eruptions will pile up and create underwater volcanoes until they potentially make it above sea level, thereby forming an island. This is how Hawaii was and continues to be formed.