This is a very good question. It is one of the great chain of volcanoes in the Sumatra Java line. Google "volcanoes of Indonesia" and you will startled by the number of volcanoes there are. Krakatoa is a volcano that is sticking up out of the sea in the strait between the two islands and it does not take too much imagination to realise that the division of the island into two by that strait, was the work of Krakatoa. There is also a beautiful satellite photo of Krakatoa on the net somewhere. Google Map Krakatoa.
Because of the Northward movement of the Australian Plate, it is anticipated that the area above that line of volcanoes will one day be another Himalaya type mountain.