*Once the subducted oceanic plate makes its way to the inner core which indeed is burning like hell compliments of Iron's radioactive decay, the plate then gets melted whereby some of them a.k.a. "lava" makes their way back up the surface.

Unfortunately, a volcanoes interior is like that of a tight cylinder which not only offers the least resistance to the upward motion of the lava but more destructively, enhances this upward motion much like how the cylindrical shape of a hose enhances the squirt of water (or that of a cannon whose cylindrical shape enhances the thrust of its ordnance).

The end result then is a violent explosion which Indonesia has had since time-immemorial thereby killing tens of thousands of its people!