Subduction zone beneath the Philippines
Block-diagram showing a cross-section of the subduction zone beneath the Philippines. The Philippine tectonic plate (on the right) dives under the Eurasian plate (on the left).
When the Philippine 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. The chain of volcanoes is more or less always at the same distance (around 300km) from the trench, which is the surface limit between the Philippine and the Eurasian plates.