Here's why:

* Convergent Plate Boundary: This is where two tectonic plates collide.

* Subduction Zone: In this type of convergent boundary, one plate (the denser one) slides beneath the other. The plate that slides beneath is subducted.

* Krakatoa's Location: Krakatoa sits on the boundary between the Eurasian Plate and the Indo-Australian Plate. The Indo-Australian Plate is denser and is being subducted beneath the Eurasian Plate. This subduction process is what causes the volcanic activity that led to the famous 1883 eruption of Krakatoa.

The subduction zone is responsible for the formation of the Sunda Arc, a chain of volcanoes that stretches along the western edge of Indonesia, including Krakatoa.