Krakatoa, more accurately spelled Krakatau, was a stratovolcano, the classic steep-sided cone built from alternating layers of lava, ash, and rock fragments over thousands of years. Situated in Indonesia’s Sunda Strait between Java and Sumatra, it owed its existence to the same tectonic engine that has spawned roughly a hundred active volcanoes along the region’s arc. The famous 1883 eruption destroyed most of the original volcanic island, but the stratovolcano classification still applies to both the pre-1883 edifice and to its modern successor, Anak Krakatau, which has been rebuilding in the same caldera ever since.
What Makes a Stratovolcano
Stratovolcanoes, sometimes called composite volcanoes, are the tall, roughly symmetrical cones that most people picture when they hear the word “volcano.” They form where one tectonic plate dives beneath another, a process called subduction. Water trapped in the descending plate lowers the melting point of rock in the mantle above, generating magma that is relatively rich in silica. That silica-rich magma is thick and sticky, which means gas pressure builds up inside it rather than escaping gently. The result is a volcano prone to explosive eruptions interspersed with quieter lava flows, and the alternating deposits of hardened lava, pumice, and ash are what give the volcano its layered internal structure.
Krakatau fits this profile precisely. It sits along the Sunda Arc, where the Indian-Australian Plate subducts northward beneath the Eurasian Plate at a rate of roughly 6.8 to 7.2 centimeters per year.1Journal of Volcanology and Geothermal Research. Evidence for high fluid/melt content beneath Krakatau volcano (Indonesia) from local earthquake tomography That ongoing collision has produced around a hundred active volcanoes stretching from Sumatra through Java and beyond. Krakatau is one node in that long chain, and its chemistry, eruption behavior, and steep profile all reflect the subduction-zone conditions that created it.
The Magma That Built Krakatoa
A volcano’s classification is not just about shape. The composition of its magma matters because it determines how the volcano erupts. Krakatau’s magma ranged across several types, from relatively silica-poor andesite to silica-rich rhyodacite. During the 1883 eruption, about 90 percent of the roughly 12.5 cubic kilometers of erupted magma (measured as dense-rock equivalent) was rhyodacite, with smaller proportions of mafic dacite and andesite making up the rest.2Journal of Volcanology and Geothermal Research. Magma mixing, fractional crystallization and volatile degassing during the 1883 eruption of Krakatau volcano, Indonesia The melt compositions spanned a range of at least 13 weight percent silica, a wide spread that reflects a complex plumbing system beneath the volcano.
What produced that range? Researchers have modeled the magma chemistry as a process where andesitic parent magma slowly crystallized minerals like plagioclase, pyroxene, and iron-titanium oxides, progressively enriching the remaining liquid in silica until it reached the rhyodacite composition that dominated the eruption.2Journal of Volcanology and Geothermal Research. Magma mixing, fractional crystallization and volatile degassing during the 1883 eruption of Krakatau volcano, Indonesia This kind of chemical evolution is typical of stratovolcanoes in subduction zones. It also explains why Krakatau could produce both relatively mild, lava-rich eruptions during quiet periods and catastrophically explosive ones when gas-charged, silica-rich magma finally broke through.
The high silica content is what made the 1883 eruption so violent. Silica-rich magma traps dissolved gases more effectively because it is viscous. When that magma ascends and the confining pressure drops, the dissolved gas expands explosively, shattering the magma into fine ash and pumice. A volcano built on basalt, the fluid, low-silica magma typical of places like Hawaii, rarely behaves this way. Krakatau’s chemistry made catastrophic explosions not just possible but almost inevitable given enough time.
The 1883 Eruption and the Caldera It Left Behind
The eruption that began in May 1883 and climaxed on August 26 and 27 was one of the largest in recorded history. It rated a VEI 6 on the Volcanic Explosivity Index, placing it among a handful of eruptions of that scale since the mid-1800s, alongside Katmai in Alaska in 1912 and Pinatubo in the Philippines in 1991.3Geosphere. Anticipating future Volcanic Explosivity Index (VEI) 7 eruptions and their chilling impacts VEI 6 means the eruption ejected more than ten cubic kilometers of material, enough to reshape the landscape and affect global climate.
The climactic phase involved a series of colossal explosions that sent pyroclastic flows racing across the sea surface and generated tsunamis that killed tens of thousands of people on surrounding coastlines. The energy released was staggering. Analysis of the atmospheric pressure wave that circled the globe multiple times after the explosion suggests the blast was equivalent to roughly 100 to 150 megatons of TNT.4Oxford Academic. The Krakatoa Air—Sea Waves: An Example of Pulse Propagation in Coupled Systems For context, that is thousands of times more powerful than the atomic bomb dropped on Hiroshima.
The eruption emptied enough of Krakatau’s underground magma chamber that the overlying rock could no longer support itself. Most of the volcanic island collapsed into the void, forming a caldera, a broad, roughly circular depression partly submerged beneath the sea. Caldera formation is a hallmark of large stratovolcano eruptions. The volcano builds itself up over centuries, then partially destroys itself in a single cataclysm when the magma reservoir empties faster than it can be refilled. Before 1883, the Krakatau island group consisted of three main islands arranged around an older caldera from a previous prehistoric collapse, so the cycle of construction and destruction had already played out at least once before.
How the Eruption Changed the World Around It
The sheer volume of material Krakatau threw into the sky had consequences far beyond Indonesia. Billions of tons of volcanic ash entered the atmosphere, where fine particles and sulfur aerosols scattered sunlight to produce vivid, multicolored sunsets visible across the globe for months afterward.5Anglica Wratislaviensia. “The wrathful sunset glared…”: The Krakatoa Sunsets in Victorian Science and Art In England and elsewhere in Europe, the blood-red skies were so dramatic they alarmed the public and inspired painters and poets. The aerosol veil also cooled global temperatures by about a degree for a year or more, a phenomenon common after very large eruptions.
Closer to the volcano, the effects were more absolute. The islands were effectively sterilized. The combination of thick, hot pyroclastic deposits, ashfall, and tsunamis stripped away all visible plant and animal life. What happened next made Krakatau one of the most famous case studies in ecology: scientists documented how life recolonized the barren islands from scratch over the following decades, tracking the arrival of ferns, grasses, and eventually forest trees by wind, waves, and animals.6Ecological Monographs. Plant Recolonization and Vegetation Succession on the Krakatau Islands, Indonesia The Krakatau islands became a natural laboratory for understanding how ecosystems rebuild after total destruction, and researchers continue to study them for that purpose today.
Anak Krakatau, the Volcano That Rose from the Caldera
Stratovolcanoes that collapse into calderas do not necessarily stay quiet. Ongoing magma supply from the subduction zone below keeps feeding the system. In 1927, fishermen noticed steam and ash rising from the sea inside the 1883 caldera. A new volcanic island was breaking the surface. Named Anak Krakatau, meaning “Child of Krakatau,” it has been erupting intermittently and growing ever since, building itself into a cone that by the early 2000s stood several hundred meters above sea level.
Anak Krakatau is geologically continuous with the older Krakatau system, drawing magma from the same deep source and producing similar andesitic to dacitic compositions. Researchers have documented its growth from 1919 to the present using a combination of archival records, satellite imagery, and drone surveys.7Bulletin of Volcanology. Forecasting future instability hazards at Anak Krakatau volcano, Indonesia, using archival reconstructions of edifice evolution That record shows a young volcano building itself rapidly, which introduces its own set of risks distinct from the explosive hazard of the parent.
The 2018 Collapse and What It Revealed
On December 22, 2018, Anak Krakatau demonstrated a danger that stratovolcanoes share with other steep-sided cones: flank collapse. During a period of eruptive activity that had begun the previous June, the volcano’s southwestern flank gave way without recognized warning. The relatively small landslide, estimated at less than about 0.2 cubic kilometers of material, slid into the 250-meter-deep caldera and generated a tsunami that struck the coasts of Sumatra and Java with wave heights up to 13 meters.8Scientific Reports. Modelling of the tsunami from the December 22, 2018 lateral collapse of Anak Krakatau volcano in the Sunda Straits, Indonesia More than 400 people were killed.9Geology. Reconstructing the Anak Krakatau flank collapse that caused the December 2018 Indonesian tsunami
What made this event particularly concerning for hazard scientists was that the landslide did not produce the strong short-period seismic waves that conventional earthquake-based tsunami warning systems are designed to detect.10Science Advances. The 22 December 2018 tsunami from flank collapse of Anak Krakatau volcano during eruption In other words, the ground shaking that might have given coastal residents time to evacuate simply was not there in the usual frequency range. The collapse registered instead as a slow, long-period seismic signal. This highlighted a gap in early warning capabilities for volcanic tsunamis as opposed to those triggered by submarine earthquakes.
The flank failure also changed the volcano itself. The collapse removed the summit and altered the eruption style, with subsequent explosive eruptions partly rebuilding the lost flank over the following months and years.9Geology. Reconstructing the Anak Krakatau flank collapse that caused the December 2018 Indonesian tsunami Researchers tracking the volcano’s growth trajectories since then have been assessing how quickly the edifice might reach an unstable configuration again, a pressing question given how rapidly Anak Krakatau rebuilds itself.7Bulletin of Volcanology. Forecasting future instability hazards at Anak Krakatau volcano, Indonesia, using archival reconstructions of edifice evolution
Why Stratovolcanoes Are Particularly Dangerous
Krakatau’s history illustrates several hazards that are specifically tied to the stratovolcano type. The steep slopes that make these volcanoes visually imposing are the same slopes that can fail catastrophically, as Anak Krakatau demonstrated in 2018. The silica-rich magma that builds the layered cone traps gas under enormous pressure, setting the stage for explosive eruptions like 1883. And the cycle of construction and caldera collapse creates an ongoing hazard rather than a one-time event: the volcano can destroy itself, then begin rebuilding toward the next crisis within decades.
Shield volcanoes, by comparison, tend to erupt fluid lava that flows away from the summit rather than exploding. Their slopes are gentle, their collapses less violent, and their eruptions, while sometimes damaging, rarely produce the hemisphere-altering clouds of ash and aerosol that Krakatau’s 1883 event did. Cinder cones are typically small, short-lived, and limited to localized eruptions. Stratovolcanoes sit at the dangerous intersection of size, chemical composition, and structural instability. They are the source of virtually every historically catastrophic volcanic eruption.
How Krakatau Fits into Indonesia’s Volcanic Landscape
Indonesia has more active volcanoes than any other country, a direct consequence of its position along multiple subduction zones. The Sunda Arc alone, where Krakatau sits, hosts around a hundred of them.1Journal of Volcanology and Geothermal Research. Evidence for high fluid/melt content beneath Krakatau volcano (Indonesia) from local earthquake tomography Most are stratovolcanoes for the same fundamental reason Krakatau is: subduction produces the kind of magma that builds steep, explosive cones. Familiar names like Merapi, Tambora, and Agung are all stratovolcanoes on the same arc, each with its own history of destructive eruptions.
Krakatau stands out among them partly because of the 1883 eruption’s unusually well-documented global impact, and partly because the birth and growth of Anak Krakatau gave scientists a rare opportunity to watch a stratovolcano build itself essentially from sea level. Most stratovolcanoes are studied as mature systems thousands of years old. Anak Krakatau has been under observation from essentially day one, which makes it an unusually valuable window into how these structures grow, destabilize, and recover from catastrophic failures.
The Air-Pressure Wave That Circled the Globe
One of the more remarkable aspects of the 1883 eruption, and one that speaks to just how powerful a large stratovolcano eruption can be, is the atmospheric pressure wave it generated. The explosion created a pulse in the atmosphere that propagated outward in all directions, traveling at roughly the speed of sound. Barographs around the world recorded the wave as it passed, and it was detected circling the globe at least three or four times before it finally dissipated.
The physics of this wave are interesting for what they reveal about how the eruption interacted with the ocean. The atmospheric pulse traveled faster than open-ocean waves, but it was efficient at transferring energy to the sea surface. Researchers have shown that air waves with phase velocities close to the natural wave speed of the ocean can excite sea waves effectively, and that these atmospheric disturbances could “jump” over land barriers and re-excite ocean waves on the other side.4Oxford Academic. The Krakatoa Air—Sea Waves: An Example of Pulse Propagation in Coupled Systems This explains an otherwise puzzling observation: tide gauges far from Krakatau, including some on opposite sides of landmasses that should have blocked any direct ocean wave, still recorded unusual water-level changes after the eruption. The atmosphere carried the signal where the ocean could not.
This coupling between atmospheric and oceanic waves has become relevant again in the modern era. When the Hunga Tonga-Hunga Ha’apai volcano erupted in the South Pacific in January 2022, it produced a similar globe-circling pressure wave that excited measurable sea-level changes in harbors thousands of kilometers from the eruption site. The mechanism Krakatau’s 1883 eruption first demonstrated is now recognized as a real hazard pathway for any sufficiently large volcanic explosion near water.