Before and after satellite images taken by Japan’s space agency showed that a two square kilometre chunk of the volcanic island had collapsed into the water.
The volcano, whose name means Child of Krakatoa, was a new island that emerged around 1928 in the crater left by Krakatoa, whose massive 1883 eruption killed at least 36,000 people.
The crater’s status has been raised to high alert, the second-highest warning on Indonesia four-point danger scale.
The exclusion zone has been extended from two to five kilometres (1.2 to three miles).
A week after the tsunami, thousands of Indonesian Muslims attended a mass prayer on Saturday to remember the victims and pray for the safety of their tsunami-prone hometown.
Residents of Pandeglang regency, which was hit the hardest by the disaster, gathered in the early morning, some in tears as they chanted their prayers.
“I prayed for the victims and I also pray for the safety of the people who live in the tsunami affected area,” Dadan Suryana, a tsunami survivor, told AFP.