When Will Anak Krakatau Erupt Next? Here's What Experts Say - Tempo.co English

TEMPO.CO, Jakarta - Mount Anak Krakatau currently lies in an "incubation" period, after a series of major eruptions on September 4-6, 2026. Based on the magma composition, subsurface structure such as magma chamber depth, and eruption patterns, Anak Krakatau has returned to the early stage of the volcanic maturation phase.

A researcher at the Geological Disaster Research Center of the National Research and Innovation Agency (BRIN), Aditya Pratama, explained that Anak Krakatau's magma contains a small amount of sulfur dioxide (SO2) during this incubation phase, which builds up as it approaches maturation. According to him, at this phase, Anak Krakatau will erupt frequently, albeit with lower intensity.

Models created by Aditya and his research team, as well as other researchers, show shallow magma pockets beneath Anak Krakatau. Upon maturation, these pockets will merge into a large magma chamber at shallow to deep depths. "Thus, in contrast to the incubation phase, a large-scale eruption occurs when it reaches maturation," he told Tempo on September 18, 2026.

Mount Anak Krakatau's shallow magma chamber is located at a depth of less than 10 kilometers from the summit. The medium depth ranges from 7-15 kilometers, while the deep magma chamber is located at 22-25 kilometers.

From discussions with other volcanologists, Aditya revealed that their concern does not lie with the major eruption itself, but with the potential landslide from this underwater volcano that could trigger a tsunami, similar to what happened in 2018.

Based on the latest modeling results conducted by a team at BRIN in collaboration with ITB, PVMBG, University of Birmingham, Penn State University, British Geological Survey, and University of Oxford, a similar landslide is estimated to occur in 2036.

The calculations are based on the history of volcanic activity in the Sunda Strait, which experienced an extreme eruption in 1883, and other estimates requiring additional evidence, such as the large eruption of Krakatau in 416.

Additionally, there has been a shift in the speed of Anak Krakatau's growth after the 2018 eruption. "After the major eruption on September 4–6, this change might accelerate, leading to an earlier eruption with a landslide before 2036," said Aditya.

He added, "Our modeling did not accommodate potential large-scale eruptions akin to the recent eruptions."

Mount Anak Krakatau, currently building its body through eruptions and lava formation, was also mentioned by another researcher at the BRIN Geological Disaster Research Center, Dini Nurfiani. "The eruptive history of Anak Krakatau shows that this volcano evolves through cycles of growth, eruption, collapse, and reconstruction," she said on the BRIN website on September 22, 2026.

Dini said the journey began when Anak Krakatau emerged in the ocean in 1927 after the collapse of Krakatau during the 1883 eruption. In the early period, the eruptive vent remained submerged or very close to sea level, so the interaction of hot magma with sea water resulted in Surtseyan eruptions, consisting of explosive bursts of steam, ash, and volcanic material.

Continuous accumulation of these deposits eventually formed a permanent landmass of Anak Krakatau, which emerged above sea level in August 1930. Until 1959, the activity was dominated by phreatomagmatic eruptions due to the interaction of magma with water. "Repeated eruptions with material ejections and ash columns gradually enlarged the body of Anak Krakatau."

Entering the period of 1959–2018, when the crater became increasingly isolated from the sea, the activity shifted to predominantly Strombolian–Vulcanian and effusive styles. Eruptions produced incandescent material, volcanic bombs, ash, and lava flows. Decades of these types of eruptive products allowed the Anak Krakatau cone to grow to a height of about 330 meters.

A significant change in growth occurred on December 22, 2018, when the southwestern sector of Anak Krakatau collapsed into the sea, with a volume of around 0.116 cubic kilometers of material. Most of the mountain body above the sea surface disappeared, reducing the height of the mountain to 120 meters, and triggering a tsunami in the Sunda Strait.

Dini said the eruptive system of Anak Krakatau also shifted due to reduced pressure in the magma conduit, while sea water entered the open crater. These conditions caused a rapid shift in activity from Strombolian eruptions to more explosive Surtseyan eruptions. The renewed interaction between magma and sea water also reshaped the morphology of the Anak Krakatau crater.

As of September 21, Mount Anak Krakatau's volcanic activity has been downgraded into Alert Level II, from the previous higher alert status since July 2.