Volcanic islands form over hot spots, which occur when tectonic plate movement allows magma from the Earth's core to erupt. As the volcano continues to erupt and cool, an island is formed.
The surface of the Earth is made of moving pieces of land called tectonic plates. When the plates collide, a process known as subduction occurs. Subduction happens when the lighter continental plate or crust pushes across the dense oceanic crust. The collision pushes the oceanic crust into the Earth's hot layer, called the mantle. The crust melts and forces its way back up to the surface to form volcanoes that, in turn, allow the flow of magma to erupt. The magma flows out of the newly formed volcano in the form of lava and cools as it comes into contact with ocean water. In time, the volcano becomes large enough to break the surface of the ocean and islands form from the cooled lava.
Tectonic plates move regularly and as such, so do the volcanoes that form. When undersea volcanoes become extinct before breaking the surface, they form mounds that are known as seamount chains. When moving seamounts encounter active hotspots, volcanoes can become active above them to produce linear hotspot tracks. As the active volcanoes move, island chains, like Hawai'i and the Galapagos are formed.