Subduction zone beneath Sumatra, Indonesia
Block-diagram showing a cross-section of the Sunda Megathrust. The Indo-Australian tectonic plate (on the left) dives under the Eurasian plate (on the right).
When the Indo-Australian plate reaches a depth greater than 100km, the water it contains lowers the fusion point of the surrounding rocks, which creates magma. This magma is hotter and less dense than the rocks around and therefore starts to migrate towards the surface, where it eventually generates volcanism typical of subduction zones on Sumatra. The Sumatran chain of volcanoes is more or less always at the same distance (around 300km) from the trench, which is the surface limit between Indo-Australian and Eurasian plates.
Credits: Yves Descatoire.