The 1883 eruption of Krakatoa (also spelled Krakatau) in the Sunda Strait between Java and Sumatra was one of the most violent volcanic events in recorded history. Its climactic explosion on August 27 generated tsunamis that killed more than 36,000 people, blasted an atmospheric pressure wave that circled the planet multiple times, and injected so much material into the stratosphere that global temperatures dropped and sunsets turned vivid red for years. The eruption destroyed most of the island itself, leaving behind a submarine caldera. What followed, both in the surrounding ocean and across the globe, reshaped scientific understanding of how volcanoes can alter Earth’s climate, biology, and even art.
The Eruption Sequence
Krakatoa did not erupt all at once. Activity began months before the catastrophic finale. Starting in May 1883, a series of lesser explosions signaled that the volcano was waking up. Analysis of the erupted material shows that the earliest blasts tapped a shallow pocket of chemically distinct, highly evolved magma sitting near the surface, which was expelled during this preliminary phase.1ScienceDirect. The magmatic and eruptive evolution of the 1883 caldera-forming eruption of Krakatau: Integrating field- to crystal-scale observations As the eruption intensified into August, the magma feeding it changed character, drawing from deeper and less evolved sources.
The climactic phase arrived on August 26–27. A series of four enormous explosions occurred, the largest at roughly 10:02 a.m. local time on August 27. This blast was heard thousands of kilometers away, with reports from places as distant as Rodrigues Island in the Indian Ocean, roughly 4,800 kilometers from the source. The eruptions sent pyroclastic density currents, superheated avalanches of gas and rock, racing across the sea surface in different directions. Stratigraphic evidence records a shift in the transport direction of these flows from northeast to southwest, coinciding with the deposition of thick layers of rock fragments. The eruption culminated in the collapse of the volcanic edifice into the emptied magma chamber, forming a caldera. This final collapse phase is preserved in the geological record as a second layer of coarse rock debris overlying the pyroclastic deposits.2ScienceDirect. The magmatic and eruptive evolution of the 1883 caldera-forming eruption of Krakatau: Integrating field- to crystal-scale observations
How Pyroclastic Flows Traveled Across Open Water
One of the most remarkable and deadly features of the 1883 eruption was the ability of pyroclastic flows to travel enormous distances across the sea. Normally, when a superheated avalanche of volcanic material hits water, you might expect it to slow down and stop. At Krakatoa, the opposite happened. Researchers have proposed that steam generated at the boundary between the scorching flows and the seawater created a cushion of vapor beneath the flow, reducing the rate at which particles settled out. This effectively delayed the point at which the flow lost momentum and became a rising column of buoyant gas and ash, allowing it to skim across the sea surface far beyond what would otherwise be possible.3Bulletin of Volcanology. Pyroclastic flows and surges over water: An example from the 1883 Krakatau eruption
This mechanism helps explain both the range of destruction and the tsunami generation. Pyroclastic flows slamming into and racing across the water displaced massive volumes of seawater. The resulting tsunamis were catastrophic, with waves striking the coasts of Java and Sumatra and wiping out entire coastal communities. The official death toll from the eruption is generally placed above 36,000, with the overwhelming majority of fatalities caused by the tsunamis rather than by the eruption’s direct blast or ashfall.
A Pressure Wave That Circled the Globe
The 1883 explosion produced atmospheric pressure waves so powerful that barometric stations around the world registered them as they passed, not just once but repeatedly. The wave radiated outward from Krakatoa, traveled to the opposite side of the Earth, converged, and then radiated back, completing multiple circuits over the following days. Barometric readings from dozens of stations documented at least seven passes of the wave.
For well over a century, the Krakatoa pressure wave stood as the benchmark for volcanic atmospheric disturbance. That changed only in January 2022, when the eruption of Hunga Tonga–Hunga Ha’apai in the South Pacific generated atmospheric waves of comparable magnitude. Analysis of the timing of multiple wave passages at barometric stations found the two events comparable in the scale and duration of propagation of the atmospheric waves they produced.4Pure and Applied Geophysics. TIMING Analysis of the Multiple Passages of the Pressure Wave Generated by the 2022 Hunga Tonga-Hunga Ha’apai and Comparison with the 1883 Krakatoa Pressure Wave Independent seismoacoustic analysis confirmed the same conclusion: as measured by the amplitude of Lamb waves, the Hunga explosion was comparable in size to Krakatoa’s climactic blast.5PubMed. Atmospheric waves and global seismoacoustic observations of the January 2022 Hunga eruption, Tonga The fact that it took 139 years for another volcanic event to rival Krakatoa in this specific metric gives a sense of how extreme the 1883 eruption truly was.
Climate Cooling and an Oceanic Fingerprint That Lasted Decades
Krakatoa blasted an estimated 20–25 cubic kilometers of rock and ash into the atmosphere, with fine particles and sulfur dioxide reaching the stratosphere. The sulfur dioxide converted into sulfate aerosols that spread around the globe, reflecting sunlight back into space and cooling the planet. Global average temperatures dropped by roughly 0.5–1°C in the year or two following the eruption, and the cooling effect lingered for several years. When the eruption was sufficiently large, the direct injection of water vapor into the stratosphere may have further contributed to the atmospheric disruption, as modeling suggests that a climatically significant amount of water vapor can remain in the lower stratosphere after aerosol formation runs its course.6Atmospheric Chemistry and Physics. The climatic effects of the direct injection of water vapour into the stratosphere by large volcanic eruptions
What surprised researchers much later was how long Krakatoa’s cooling signature persisted in the ocean. An analysis of twelve modern climate models showed that volcanically induced cooling of the ocean surface penetrated into deeper layers, where it persisted for decades after the eruption. The effect was large enough to substantially reduce twentieth-century ocean warming and sea-level rise, offsetting a significant fraction of the warming caused by human greenhouse gas emissions. In other words, the deep ocean was still “remembering” Krakatoa well into the twentieth century, long after the atmospheric aerosols had cleared.7Nature. Krakatoa’s signature persists in the ocean That finding reshaped how climate scientists account for volcanic forcing in long-term models: ignoring deep-ocean thermal inertia after eruptions can lead to overestimates of how fast the ocean should have warmed during the industrial era.
Vivid Sunsets and the Birth of Atmospheric Optics
In the months following the eruption, people around the world witnessed spectacular twilight displays. Skies turned brilliant shades of red, orange, and purple, sometimes so intense that fire brigades were called out by citizens who mistook the glow for distant blazes. These colors were caused by sunlight diffracting through the fine volcanic particles and sulfate aerosols that had spread through the upper atmosphere.
The German scientist Johann Kiessling recognized these twilights as a scientific opportunity. He studied the extraordinary optical appearances and concluded that diffraction through particles in high-altitude condensation and dust clouds, carried around the globe by stratospheric winds, was the primary cause. His work on the Krakatoa twilights substantially advanced the understanding of how aerosols and fog form in the atmosphere and laid groundwork for the modern field of atmospheric optics.8Royal Society Publishing. Johann Kiessling, the Krakatoa event and the development of atmosheric optics after 1883
The visual impact reached art as well. A study analyzing hundreds of sunset paintings by well-known artists from 1500 to 1900 found a statistically significant correlation between the red-to-green color ratios in the paintings and the known dust levels in the atmosphere from volcanic eruptions. Estimated aerosol optical depths derived from the paintings ranged from about 0.05 during clean background conditions to roughly 0.6 after the Krakatoa and Tambora eruptions, the highest values in the dataset.9Atmospheric Chemistry and Physics. Atmospheric effects of volcanic eruptions as seen by famous artists and depicted in their paintings The lurid skies in works by artists of the 1880s and 1890s were not romantic exaggeration; they were faithful records of an altered atmosphere.
How Life Returned to the Islands
The 1883 eruption stripped the Krakatoa archipelago down to sterile rock and ash. Every tree, every animal, every microbe on the remaining islands was wiped out, or at least that was the prevailing assumption. The islands became one of the most closely watched natural experiments in ecological history: how does life recolonize a blank slate?
Plants arrived first, carried by ocean currents, wind, and birds. Coastal species colonized the shorelines, and over decades a progression of inland forest developed. But even fifty years after forest canopy closed over the interiors, the forests remained species-poor, dominated by typical early-succession trees. On Rakata, the largest remnant of the original volcano, the inland forest was led by species like Neonauclea calycina and Ficus pubinervis up to about 550 meters altitude, with a mossy scrub forest above. On the smaller islands of Sertung and Rakata Kecil, young stands of Timonius compressicaulis dominated, with older patches of Dysoxylum gaudichaudianum forming a second forest type.10Ecological Monographs. Plant Recolonization and Vegetation Succession on the Krakatau Islands, Indonesia
The paths of recovery diverged across islands. Rakata, sheltered from ongoing volcanic disturbance, followed a relatively steady successional trajectory. But Sertung and Rakata Kecil faced repeated setbacks from volcanic activity after Anak Krakatau emerged in 1927. Eruptions in 1930, the mid-1930s, 1952–53, and 1961 deflected the vegetation on those islands onto a different developmental pathway than the one Rakata followed. On Anak Krakatau itself, volcanic action repeatedly wiped the slate clean, preventing any successful colonization away from the growing coastal deposits.11Ecological Monographs. Plant Recolonization and Vegetation Succession on the Krakatau Islands, Indonesia
Animals followed a similar but slower arc. Surveys of vertebrates showed continuing colonization and species turnover well into the late twentieth century. Nonmigrant land birds and reptiles were only approaching equilibrium species numbers a century after the eruption, with rates of new species arriving lower than they had been in the first forty years. Actual extinction rates on the islands turned out to be far lower than theoretical models predicted, suggesting that the islands were still in an active colonization phase rather than a stable equilibrium.12PubMed Central. Colonization of the Krakatau Islands by vertebrates: equilibrium, succession, and possible delayed extinction
Anak Krakatau and the Cycle of Growth and Collapse
The caldera left by the 1883 eruption did not stay quiet. In 1927, submarine eruptions signaled the birth of a new volcanic island, Anak Krakatau, meaning “Child of Krakatoa.” By 1930 it had risen above the waves and began steadily growing through ongoing eruptions. For nearly a century, Anak Krakatau built itself up, reaching a peak height of over 300 meters above sea level.
Then, on December 22, 2018, during a period of heightened eruptive activity that had been ongoing since June, the southwestern flank of Anak Krakatau collapsed into the sea. The landslide was relatively small by geological standards, less than about 0.2 cubic kilometers of material, but it struck the water of the 250-meter-deep caldera with enough force to generate a tsunami that arrived on the coasts of Sumatra and Java with runups of up to 13 meters.13PubMed Central. The 22 December 2018 tsunami from flank collapse of Anak Krakatau volcano during eruption The event killed at least 437 people and injured thousands, making it the deadliest volcanically induced tsunami since the 1883 Krakatoa eruption itself and the 1888 collapse of Ritter Island.14PubMed Central. Modelling of the tsunami from the December 22, 2018 lateral collapse of Anak Krakatau volcano in the Sunda Straits, Indonesia
The 2018 collapse removed roughly two-thirds of the island’s above-water volume in minutes. But Anak Krakatau began rebuilding almost immediately. Analysis of the volcano’s growth record from 1919 to 2023, using archival records combined with satellite and drone surveys, reveals that post-collapse regrowth has mimicked historical development trends but on a considerably accelerated timescale.15Bulletin of Volcanology. Forecasting future instability hazards at Anak Krakatau volcano, Indonesia, using archival reconstructions of edifice evolution The island is rebuilding itself faster than it grew the first time around, which raises an uncomfortable question about how soon it might become unstable again.
The Warning Problem for Volcanic Tsunamis
The 2018 Anak Krakatau tsunami exposed a critical gap in disaster preparedness. Indonesia’s tsunami early warning system was designed primarily for earthquake-triggered tsunamis, which are detected by seismometers and confirmed by deep-ocean buoys. A flank collapse produces a very different seismic signature, one that does not trigger the same automatic alerts. The 2018 tsunami arrived at the nearest coasts within about 30 minutes, striking at night with no official warning.
Post-disaster analysis emphasized that modification and enhancement of tsunami early warning systems are necessary to anticipate volcanic tsunamis in the future, given that Anak Krakatau will remain active and continues to pose threats to the surrounding area.16Natural Hazards and Earth System Sciences. The 22 December 2018 Mount Anak Krakatau volcanogenic tsunami on Sunda Strait coasts, Indonesia: tsunami and damage characteristics One response has been the development of a network of automatic water level stations installed around Anak Krakatau. These stations use ultrasonic sensors to measure water level changes every minute, with data sent in real time to Indonesia’s meteorological agency for processing. The network was designed to complement seismometer data, providing a way to verify and detect tsunami waves that seismic instruments alone might miss.17Journal of Physics: Conference Series. Design of Sunda Straits automatic water level station network
The challenge remains substantial. Volcanic tsunamis are rarer than earthquake-generated ones, which means fewer resources are typically allocated toward detecting them. The proximity of populated coastlines to Anak Krakatau also leaves very little lead time, even with faster detection. And the accelerated rebuilding of the volcanic edifice means the window before the next potential instability event may be shorter than historical patterns would suggest. It is a problem where the geology moves faster than the institutional response.
Krakatoa’s Place Among Great Eruptions
In terms of sheer volume of ejected material, Krakatoa ranks below the 1815 eruption of Tambora, also in Indonesia, which expelled roughly four times as much material and caused the famous “Year Without a Summer” in 1816. But Krakatoa’s combination of explosive intensity, tsunami generation, global atmospheric impact, and detailed contemporary documentation made it the defining volcanic event of the modern scientific era. It occurred at a time when telegraph networks and global shipping lanes meant that news traveled fast and observations were systematic. Barometric stations, tide gauges, and shipping logs provided a dense web of data that scientists could analyze in ways that were impossible for earlier eruptions.
The 2022 Hunga Tonga eruption offered the first real atmospheric comparison. Both events produced Lamb waves of comparable amplitude that circled the globe multiple times.18PubMed. Atmospheric waves and global seismoacoustic observations of the January 2022 Hunga eruption, Tonga But Hunga Tonga’s eruption was notably different in other respects: it was submarine, much shorter in duration, and injected massive amounts of water vapor rather than sulfate aerosols into the stratosphere, which produced a warming rather than cooling effect. Krakatoa remains in a class largely its own when it comes to the combined scale of its climatic cooling, oceanic thermal imprint, and human toll.
The ongoing activity at Anak Krakatau ensures that the story is not a historical curiosity. The same geological system that produced the 1883 catastrophe is still active, still building, and still capable of generating destructive events. Research projecting the volcano’s future growth trajectories suggests that assessing potential instability conditions will be an ongoing necessity as the edifice regrows.19Bulletin of Volcanology. Forecasting future instability hazards at Anak Krakatau volcano, Indonesia, using archival reconstructions of edifice evolution The Sunda Strait remains one of the most volcanically hazardous places on Earth, and the lessons of 1883 are still being applied to the risks of the present.