Nobody can give a date, and the honest answer is that Krakatoa’s successor volcano, Anak Krakatau, is already erupting on a semi-regular basis. The question most people are really asking is whether another catastrophic event on the scale of the famous 1883 explosion or the deadly 2018 flank collapse is coming. Researchers studying the volcano’s rapid regrowth and underground plumbing have outlined scenarios ranging from decades to roughly a century before certain hazard thresholds are reached again, but a precise forecast remains out of reach for reasons rooted in the physics of volcanic systems themselves.
Anak Krakatau Is Not Dormant
A common misconception is that Krakatoa blew itself apart in 1883 and has been quiet since. In reality, a new volcanic island called Anak Krakatau (“Child of Krakatau”) broke the ocean surface in 1927, rising from the caldera left by the 1883 eruption. It has been intermittently active ever since, producing lava flows, Strombolian eruptions, and ash plumes throughout the twentieth and twenty-first centuries. The volcano’s magma is basaltic andesite in composition, broadly consistent with what it has erupted for decades, and analysis of tephra from 2018 showed nothing chemically unusual compared to past activity.1PubMed Central. Complex hazard cascade culminating in the Anak Krakatau sector collapse In other words, the system beneath Anak Krakatau has not gone to sleep. It continuously feeds material to the surface, and significant eruptions are part of its baseline behavior rather than rare surprises.
What Happened in December 2018
The most dramatic recent event was the flank collapse on 22 December 2018, when a chunk of Anak Krakatau’s southwestern side slid into the Sunda Strait and triggered a tsunami that killed more than 430 people along the coasts of western Java and southern Sumatra.2PubMed Central. The 22 December 2018 tsunami from flank collapse of Anak Krakatau volcano during eruption The landslide volume was relatively small, with estimates ranging from roughly 0.1 to 0.3 cubic kilometers depending on the method used.2PubMed Central. The 22 December 2018 tsunami from flank collapse of Anak Krakatau volcano during eruption What made it so dangerous was that the collapse did not generate the kind of strong, short-period seismic waves that conventional earthquake-based tsunami warning systems detect. There was no warning shake felt on land, so the wave arrived on nearby coastlines without any organized alert.
Satellite radar data collected in the months before the collapse revealed that the volcano’s southwestern flank had been creeping outward and downward since at least January 2018. The movement was slow at first, roughly four millimeters per month, but accelerated to about ten millimeters per month after volcanic activity ramped up in late June 2018.1PubMed Central. Complex hazard cascade culminating in the Anak Krakatau sector collapse That acceleration pattern is a textbook precursor to a sector collapse, but it was identified clearly only in hindsight. The lesson for future hazard assessment is that the volcano signaled its intentions for months; the challenge is reading those signals in real time.
How Fast the Volcano Is Rebuilding
After losing a large portion of its edifice in 2018, Anak Krakatau immediately began rebuilding. A recent study reconstructing the island’s growth from archival records, satellite imagery, and drone surveys spanning 1919 to 2023 found that post-collapse regrowth has followed the same general development pattern as the pre-collapse island, but on a much faster timescale.3Bulletin of Volcanology. Forecasting future instability hazards at Anak Krakatau volcano, Indonesia, using archival reconstructions of edifice evolution Two growth scenarios give very different timelines for when the volcano could reach a size and shape comparable to its pre-collapse 2018 form:
- Long-term average growth: If the volcano grows at the rate it averaged between 1960 and 2018, it would reach a morphology equivalent to its 2018 shape, but with larger overall dimensions, by around the year 2100.
- Accelerated recent growth: If the much higher growth rates observed between 2019 and 2023 continue, a comparable morphology could be reached as soon as the 2030s.
That range matters because the size and shape of the edifice are key factors in whether another flank collapse can occur. A taller, steeper island with more mass perched above sea level is inherently less stable. So the regrowth rate is not just an academic curiosity; it is one of the primary controls on when the next collapse-and-tsunami scenario becomes physically possible again.3Bulletin of Volcanology. Forecasting future instability hazards at Anak Krakatau volcano, Indonesia, using archival reconstructions of edifice evolution
Why the Volcano Rebuilt So Quickly After 2018
Bathymetric surveys after the collapse showed that the seafloor around Anak Krakatau shallowed by about 25 meters in the collapse zone, meaning enormous volumes of volcanic debris were deposited on the surrounding seabed.4IOP Conference Series: Earth and Environmental Science. Study of Bathymetry and Seabed Morphology Changes Around Anak Krakatau Waters of 2018 Post-Eruption Satellite imagery tracking the island’s outline in the two years after the collapse showed a slow-fast-slow pattern of morphological change, with the southeastern part of the volcano expanding significantly while the western collapse scar gradually filled in.5MDPI Remote Sensing. Revealing the Morphological Evolution of Krakatau Volcano by Integrating SAR and Optical Remote Sensing Images
The rapid rebuilding is partly a consequence of the volcano’s plumbing. Local earthquake tomography has identified a zone of high fluid and melt content beneath the Krakatau complex, extending down to at least four to six kilometers. This anomaly appears to be split into two levels, consistent with the idea that multiple stacked magma storage zones feed the volcano.6Journal of Volcanology and Geothermal Research. Evidence for high fluid/melt content beneath Krakatau volcano (Indonesia) from local earthquake tomography With a well-fed magma supply sitting just a few kilometers down, the system can push fresh material to the surface on a near-continuous basis.
Has the 2018 Collapse Stabilized the Volcano for Now?
Counterintuitively, the answer is probably yes, at least in the medium term. A study of the collapse mechanics concluded that the failure was controlled by the internal structure of the island, and the reconfiguring of Anak Krakatau’s vent network following the collapse likely re-stabilized it for the time being.7Geology. Reconstructing the Anak Krakatau flank collapse that caused the December 2018 Indonesian tsunami In simple terms, the collapse removed the oversteepened, weak portion of the flank and rearranged the pathways through which magma and gas reach the surface. The volcano is currently shorter, less steep, and its internal conduits are positioned differently than before. All of that buys time.
But “medium term” in geology is not a reassuring phrase. Regrowth rate is the primary control on when another flank failure becomes possible. If the accelerated growth continues, the window of relative stability could close within a decade or two. The interplay between how fast the island grows and how its internal geometry evolves will determine whether the next collapse hazard arrives in the 2030s or closer to 2100.
Why Nobody Can Give You a Date
Volcanic eruption forecasting has improved enormously over the past few decades, but it operates under constraints that weather forecasting does not face. A review of twenty-first-century forecasting methods describes three categories of uncertainty that apply to every volcano, Anak Krakatau included.8Natural Hazards and Earth System Sciences. A unified probabilistic framework for volcanic hazard and eruption forecasting First, volcanic systems are naturally variable. Even with perfect data, the same starting conditions can produce different outcomes. Second, our understanding of how the interior of a volcano works is incomplete, so models rely partly on expert judgment. Third, there is always the possibility that something happens that scientists had not anticipated at all and therefore could not build into their probability estimates.
Long-term forecasts for any volcano typically rely on its past eruption record and geological history. Shorter-term forecasts use real-time monitoring data: seismicity, ground deformation, gas emissions, thermal anomalies. These can be combined using statistical frameworks to produce probabilistic estimates, but as one comprehensive review noted, eruption forecasts may never be as reliable as weather forecasts, and caution is necessary when predicting complex volcanic behavior.9Journal of Geophysical Research: Solid Earth. Partly Cloudy With a Chance of Lava Flows: Forecasting Volcanic Eruptions in the Twenty‐First Century
For Anak Krakatau specifically, the difficulty is compounded by the fact that the most dangerous scenario is not a classic magmatic eruption but a flank collapse, and collapses are notoriously harder to forecast than eruptions. The 2018 collapse was preceded by months of measurable ground movement, but that movement fell within a range that could also have simply continued indefinitely without a catastrophic failure. Identifying the threshold at which slow creep becomes sudden collapse remains one of the hardest problems in volcanology.
What About Another 1883-Scale Eruption?
The 1883 eruption of Krakatoa was one of the most powerful volcanic events in recorded history, generating a tsunami with run-up heights reaching roughly 35 meters and killing more than 36,000 people.10IOP Conference Series: Earth and Environmental Science. Coastal Hazards in the Sunda Strait: Mitigation Strategy and Coastal Management The eruption ejected enormous volumes of material, and geochemical analysis of that magma showed it came from a compositionally zoned chamber: evolved, silica-rich magma sitting above hotter, more mafic material at depth, with the whole system containing about four percent dissolved water by weight.11Journal of Volcanology and Geothermal Research. Magma mixing, fractional crystallization and volatile degassing during the 1883 eruption of Krakatau volcano, Indonesia That volatile-rich, silica-rich upper zone was a key ingredient in the explosive power of the eruption.
Could that happen again? In principle, yes, but the current system looks quite different. Anak Krakatau has been erupting basaltic andesite for decades, a less evolved and generally less explosive composition than the rhyodacite that dominated in 1883. For an 1883-scale event, the magma chamber would need to accumulate a large volume of more evolved, gas-rich magma without erupting it piecemeal. Given that Anak Krakatau has been leaking material steadily, building up that kind of reservoir would take a long time. The geological record suggests that caldera-forming eruptions at Krakatoa are separated by thousands of years, not decades or centuries.
There is also a persistent myth that a massive eruption occurred at or near Krakatoa around AD 535, supposedly causing a global climate catastrophe. Radiocarbon dating of marine sediments around the Indonesian arc found no evidence for such an eruption and concluded it is sufficiently unlikely to have occurred to effectively rule it out.12Radiocarbon. Planktonic Foram Dates from the Indonesian Arc: Marine 14C Reservoir Ages and a Mythical AD 535 Eruption of Krakatau The 1883 event appears to stand on its own in the historical record rather than being part of a frequent cycle of mega-eruptions.
What Monitoring Exists Today
Indonesia’s Center for Volcanology and Geological Hazard Mitigation (CVGHM) maintains a monitoring network around Anak Krakatau that includes seismometers, tiltmeters, and visual observation posts. Satellite-based methods have become increasingly important since the 2018 collapse. Synthetic aperture radar (SAR) from satellites like Sentinel-1 and ALOS-2 can track changes in the island’s shape, detect ground deformation on the order of millimeters, and monitor growth trends even through cloud cover.5MDPI Remote Sensing. Revealing the Morphological Evolution of Krakatau Volcano by Integrating SAR and Optical Remote Sensing Images Photogrammetric methods using ground-based or drone cameras have also been explored to estimate eruption column dynamics and mass discharge rates in something closer to real time.13Bulletin of Volcanology. Eruption dynamics of Anak Krakatau volcano (Indonesia) estimated using photogrammetric methods
The 2018 disaster exposed a critical gap: Indonesia’s tsunami early warning system was designed primarily for earthquake-generated tsunamis, not volcanic ones. Earthquake-triggered tsunamis produce strong seismic signals that travel faster than the wave itself, giving coastal communities minutes to evacuate. The Anak Krakatau landslide produced only weak short-period seismic signals, so the warning system effectively missed it.2PubMed Central. The 22 December 2018 tsunami from flank collapse of Anak Krakatau volcano during eruption Since then, there has been significant discussion about expanding the system to include offshore pressure sensors, coastal radar, and dedicated volcanic tsunami detection, though the Sunda Strait’s geography and the short travel time of any wave from Anak Krakatau to shore make this an extraordinarily difficult engineering and logistics challenge.
Communities Living in the Shadow
Coastal populations in Banten (western Java) and Lampung (southern Sumatra) are the most directly exposed. Research on disaster resilience in communities near Anak Krakatau has found that preparedness depends on integrating scientific monitoring with local knowledge, including traditional observations of natural signs that sometimes precede eruptions or unusual sea behavior.14PubMed Central. Integration of knowledge and local wisdom for disaster resilience in Anak Krakatau volcano In practice, the 2018 event revealed that infrastructure on these coastlines, including roads, bridges, and evacuation routes, was not designed with volcanic tsunamis in mind. The run-up heights were much smaller than in 1883, yet the death toll was still in the hundreds partly because there was no warning and no rehearsed evacuation response.
The practical takeaway for anyone living in or visiting the Sunda Strait region is that Anak Krakatau does not need to produce a cataclysmic eruption to be deadly. The 2018 tsunami came from a modest landslide during a routine eruptive phase. Awareness of this risk and access to rapid communication systems matter more than predicting a specific eruption date.
Coral Reefs and the Ecological Clock
One measure of how Krakatoa’s activity shapes its surroundings over time is the state of coral reefs in the archipelago. The 1883 eruption obliterated marine ecosystems around the islands, but surveys of Rakata Island, part of the Krakatoa Islands Nature Reserve, show that coral cover has recovered to levels ranging from good to poor depending on depth and site, with the best areas reaching about 50 percent live coral cover.15IntechOpen. A Review of Volcanic Activity and the Evolution of Coral Reefs That recovery has taken well over a century, and it depends on geological stability as much as water quality and temperature. Each significant eruption or collapse event sets the reef clock back, and the ongoing activity of Anak Krakatau means the marine environment around the volcano exists in a perpetual cycle of destruction and slow recolonization. For marine biologists, Krakatoa serves as one of the clearest natural laboratories for studying how reef ecosystems respond to sudden, total disturbance and what controls the pace of their return.