The eruption of the Krakatoa is caused by a subduction between the indian-australian plate and the eurasian plate. This zone had high volcanic and seismic avtivity wich makes it very dangerous to be there.The volacano is situated just outside of a place called Sumatra and a place called Bandar Lampung wich belongs to the land Indonesia, wich is a country well know for it's high number in volcanic activity as well as earthquakes. The sea surronding the volano is the Sunda Strait. It is The volcano was formed with subducting plates, wich means that if two plates collide one of the plates goes underneath the other and causes major friction. Below you can see the sattelite foto of where the valcono is situated. The place I got it from was a dutch website so it doesn't say Krakatoa but Krakatau.

During the eruption, the volcano produced very, very much lava and the lava made an empty space under the island (Rakata Besar). The island was pulled into the sea, into the empty space made by the lava.