Convergent plate boundaries and subduction zones
As an oceanic plates collides with another plate, it is subducted and generates volcanism on the overlying plate.

Krakatoa explodes
In the afternoon of 17 Oct, a particularly violent explosion occurred at Anak Krakatau, blasting away a portion of the southern crater rim. A similar event might have been the final trigger for the catastrophic landslide that let the cone collapse and cause a devastating tsunami on 22 Dec 2018.