The Philippine Sea plate is tectonically unusual in that almost all the boundaries are convergent. The Pacific plate is subducting beneath the Philippine Sea plate to the east while the west/northwestern part of the Philippine Sea plate is subducting beneath the continental Eurasian plate. The areas surrounding the Philippine Sea plate are very active, producing numerous local earthquakes, tsunamis, and volcanic activity. 

The evolution of the Philippine Sea plate originated lower than the equator about 55Ma ago and was followed by several tectonic events resulting in its present day location. Ultimately it was a change in the movement, to a westward motion, of the Pacific plate that converted the N-S transform fault on the Philippine Sea plate to the subduction zone currently present along the boundary between these two plates.

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.

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).