The fishermen of Pandeglang, on the western tip of Java, do not need a geology lecture to understand what Anak Krakatau can do. Many of them were on the coast the night of December 22, 2018, when the volcano’s southwestern flank slid into the Sunda Strait without warning and sent a tsunami crashing into beachfront hotels, fishing villages, and a concert stage where an Indonesian band was performing for hundreds of fans. By the time the water receded, 437 people were dead and more than 14,000 were injured, according to Indonesia’s National Disaster Mitigation Agency (BNPB).
Now the volcano is restless again. Fresh explosive activity at Anak Krakatau through early 2026 has drawn renewed attention from volcanologists studying how the island has been rebuilding itself since that catastrophic collapse. A study published in the Journal of Asian Earth Sciences in June 2026 analyzed ash samples collected near the crater and found that the volcano has been alternating between two distinct eruption styles, a pattern that complicates forecasting and keeps hazard planners on edge.
A volcano born from catastrophe
Anak Krakatau, which translates roughly to “Child of Krakatau,” owes its existence to one of the deadliest volcanic events in recorded history. In August 1883, the original Krakatau island tore itself apart in a series of colossal explosions that killed more than 36,000 people, according to a U.S. Geological Survey retrospective. The vast majority of those deaths were caused not by lava or ash but by tsunamis that radiated across the Indian Ocean, some reaching coastlines thousands of kilometers away. NOAA’s historical records note that the atmospheric shockwave from the eruption circled the globe multiple times, and fine volcanic dust altered sunsets worldwide for years afterward.
The eruption left behind a submerged caldera. In 1927, a new volcanic cone broke the ocean surface inside that caldera and began growing through intermittent eruptions. By 2018, Anak Krakatau had built itself into a steep-sided island roughly 330 meters tall. Then, on that December night, the southwestern flank gave way.
The 2018 collapse and what it revealed
A peer-reviewed reconstruction published in Nature Communications pieced together the sequence using satellite imagery, bathymetric surveys, and seismic records. The study found that the flank failure removed roughly two-thirds of the cone’s above-water volume in a matter of minutes, displacing enough rock and debris into the strait to generate the tsunami that struck western Java and southern Sumatra. The collapse scar now faces southwest, directly toward heavily trafficked shipping lanes and the densely populated coasts of Banten and Lampung provinces.
The disaster also reshaped the volcano’s internal plumbing. With the cone’s upper structure gone, a new crater opened at or near sea level, allowing seawater to interact directly with hot rock and rising magma. That interaction is central to understanding what has happened since.
Two eruption styles, one unpredictable volcano
The June 2026 study in the Journal of Asian Earth Sciences examined tephra samples collected from sites near the crater to classify the types of explosions Anak Krakatau has produced in the years following the collapse. Under a microscope, the researchers found two distinct populations of ash grains. Some were vesicular and glassy, characteristic of magmatic eruptions driven by gas-rich magma rising from depth. Others were blocky and intensely fractured, the signature of phreatomagmatic explosions triggered when seawater meets molten or superheated volcanic material and flashes violently to steam.
The distinction is not academic. Phreatomagmatic blasts tend to produce finer, more widely dispersed ash clouds and can escalate with little warning. The post-2018 deposits show that Anak Krakatau has switched between these two modes over short timescales. When seawater can easily reach the conduit, explosions tend to be water-driven and more violent per unit of magma erupted. When fresh lava builds barriers around the vent, eruptions shift back toward a more purely magmatic character. A single lava flow sealing a crack, or a minor slump opening a new pathway for seawater, can change the eruption style within days.
What scientists still cannot answer
For all the progress in understanding the volcano’s recent behavior, several critical questions remain open. No publicly available real-time seismic or gas emission data from Indonesia’s Center for Volcanology and Geological Hazard Mitigation (PVMBG) has been cited in the recent scientific literature to confirm the precise nature of the current unrest. Without that observatory-level information, it is difficult to determine whether Anak Krakatau is building toward a significant eruption or simply continuing the low-level explosive activity it has displayed intermittently since 2019.
The tsunami risk from future flank instability is also hard to quantify. The remaining edifice is lower and less steep than it was before 2018, which should in principle limit the volume of material that could fail in a single event. But the actual hazard depends on how quickly the cone rebuilds, how fractured and unconsolidated the new deposits are, and how steeply they accumulate above the surrounding seafloor. No recent high-resolution bathymetric surveys have been widely released to show how much loose material now drapes the submarine slopes.
Whether the crater floor currently sits above or below sea level, a factor that strongly influences which eruption style dominates, has not been confirmed in any source reviewed for this report. And persistent explosions can oversteepen the crater rim and deposit loose tephra on already unstable flanks, while shallow magma intrusions can pry open fractures that later serve as failure planes. The Nature Communications reconstruction found evidence that deformation and small collapses preceded the main 2018 sector failure, but whether similar precursory patterns would necessarily repeat remains unknown.
Why the Sunda Strait cannot afford complacency
More than 20 million people live in the coastal provinces that frame the Sunda Strait. The 2018 disaster exposed gaps in Indonesia’s tsunami warning infrastructure: the event was triggered by a volcanic landslide rather than an earthquake, which meant it did not register on the seismic-based detection systems designed for tectonic tsunamis. Tide-gauge buoys that might have provided minutes of additional warning had been offline due to vandalism and budget shortfalls, a problem Indonesian officials acknowledged publicly in the disaster’s aftermath.
Since then, Indonesia has worked to expand its monitoring network, including deploying new ocean-bottom sensors and improving communication links to coastal communities. But the fundamental challenge remains: a volcanic flank collapse in the Sunda Strait can send a tsunami to nearby shores in as little as 20 to 30 minutes, leaving almost no margin for delayed alerts or confused evacuation orders.
The historical record and recent research converge on one uncomfortable point. Anak Krakatau does not need to replicate the 1883 cataclysm to be deadly. The 2018 event proved that a partial collapse of a relatively small volcanic island can kill hundreds of people and displace tens of thousands. With the volcano actively rebuilding and cycling between eruption styles that are difficult to predict, the communities along the Sunda Strait are living with a hazard that demands sustained attention, not just during dramatic eruptions, but in the quieter intervals when the next flank, the next lava flow, or the next steam-driven blast is taking shape beneath the surface.
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*This article was researched with the help of AI, with human editors creating the final content.
