When two oceanic tectonic plates converge, one is subducted or pulled under, the other. This convergent activity allows for magma from the earth's core to erupt at the point of contact. The cooled erupted lava and debris build up over time. When the pile of debris and lava build up high enough, land is formed above sea level. This land is called an island. Often, these island volcanoes stay active, continuously growing.