Toba, like all volcanic features, owes its origin to the eruption of magma. Along the axis of Sumatra, Java, and many other Indonesian islands, magma has erupted periodically to build classic stratovolcanoes such as Sinabung, Krakatau, Semeru, Merapi, and Tambora. These volcanoes, as well as silicic calderas, all have a common source of their magmas, and plate tectonics provides a well-established model for understanding how this magma is generated.