Answer:
Oceanic and continental plates are colliding.
Step-by-step explanation:
The collision of the ocean and continental plates makes the oceanic plates denser by overriding them by thicker and less thick continental plates. The ocean plates are forced to move down to the mantle through a seduction process. The melting, therefore, produces magma above the sub ducting ocean plates.
The magma chambers are usually not concentrated like the mantle material within the surrounding. The depth and size of the magma can be determined by mapping the earthquake activities happening around.