On 29 December 1927, Anak Krakatau first emerged from the caldera formed in 1883 by the explosive volcanic eruption that destroyed the island of Krakatoa.

There has been sporadic eruptive activity at the site since the late 20th century, culminating in a large underwater collapse of the volcano, which caused a deadly tsunami in December 2018.

On 11 August 1930 the volcanic island permanently rose above sea level and was locally named Anak Krakatau (Child of Krakatoa).

The entire island comprises a Somma-stratovolcano system of the late Holocene epoch, and features a pyroclastic cone.

The major rock-type components of Anak Krakatau include andesite, dacite, and basalt; with minor indications of trachyte.

Scientists had modeled the possibility six years before the event, and had identified the western flank as the section of the volcano most likely to fail.

On 23 December, this was confirmed by satellite data and helicopter footage, with the main conduit seen erupting from underwater, producing Surtseyan-style activity.

[11] Satellite radar observations showed that by 10 January 2019, the volcano had continued to form, with further eruptions beginning to re-model the remnant structure.

[23] In May 2019, phreatomagmatic activity was observed around the newly reconstructed crater as the volcano continued to increase in height and remodel the areas destroyed in 2018.