It is located on the destructive plate boundary between the Oceanic Philippine plate and the Continental Eurasian plate. The two plates were moving towards each other, they collided and jolted, causing an earthquake. As the Oceanic Philippine plate is an oceanic plate and the Continental Eurasian plate is a continental, the oceanic plate is denser and thus subducted below the less dense continental plate. As this happened the two plates jolted and ground causing earthquakes such as magnitude 7.8 earthquake on the richter scale 100km northeast of mount Pinatubo. The Oceanic Philippine plate eventually reached the mantle, and got melted down and destroyed.The excess of magma in the magma chamber, causing a buildup of pressure. Then, extra magma was forced up the vents, causing the volcanic eruption.