1. Pre-eruption: Krakatau was a complex volcanic island with three main peaks: Rakata, Danan, and Perboewatan. These peaks were formed by eruptions over thousands of years.
2. The 1883 Eruption: This eruption, one of the most powerful volcanic events in recorded history, was a culmination of several smaller eruptions. It began in May 1883 and culminated in a series of massive explosions on August 26-27.
3. The Explosion and Collapse: The final and most violent explosions were caused by the interaction of magma with seawater, creating superheated steam and triggering a chain reaction. This massive eruption blew apart Danan and Perboewatan, and the resulting pressure wave significantly eroded Rakata.
4. Caldera Formation: The loss of volcanic material from the eruptions led to a massive collapse of the magma chamber beneath the island. The caldera formed in the resulting void, with the remaining parts of the island collapsing into the sunken area. The caldera's dimensions are approximately 6 kilometers (3.7 miles) in diameter.
5. Aftermath: The 1883 eruption left a significant impact on the surrounding area, including the formation of Anak Krakatau (child of Krakatoa) - a new volcanic cone that emerged from the caldera in 1927.
In summary, the caldera on Krakatau was formed due to a series of powerful eruptions that emptied the magma chamber, leading to the collapse of the volcanic structure and the creation of a large depression or caldera. This event marked a dramatic and devastating transformation of the island.
