Indonesia’s Mount Anak Krakatau erupted again this week, sending plumes of volcanic ash into the sky and disrupting hundreds of flights. More than 300,000 passengers have been affected. Anak Krakatau, meaning “child of Krakatau” in Indonesian, has erupted intermittently for almost a hundred years. This volcano arose from the devastation caused when its predecessor, Krakatau (also known as Krakatoa), erupted between May and August 1883. It is difficult to overstate how large the 1883 eruption was and the catastrophic and often bizarre impacts that followed. The Moon turned blue, sunsets turned green, and global temperatures fell. Around 35,000 people lost their lives, almost all in tsunamis caused by the eruptions. The sound of one of the 1883 explosions remains the loudest sound recorded in history; could be heard “like the distant roar of heavy weapons” near Mauritius, almost 5,000 kilometers away. So what caused the 1883 eruptions? Could they happen again? What caused the 1883 eruption? Most volcanoes are located on the edges of the Earth’s tectonic plates. Krakatau lies above what is called a subduction zone, where the Indo-Australian plate is forced beneath the Eurasian plate. This produces molten rock (or magma) with a large amount of dissolved gas, which helps explain why Indonesian volcanoes can erupt so explosively. There is still some debate about the main triggers at different stages of the eruption. The 1883 eruption was once thought to have been caused by seawater entering the volcanic system and interacting with magma, causing an explosive eruption. However, the deposits of volcanic material laid down in May 1883 contain two distinct layers: light-colored pumice beneath darker gray ash. The two layers are also chemically distinct, suggesting that different magmas were involved. One scientific interpretation of the initial trigger is that new, hot, gas-rich magma surged from deep within the Earth and mixed with more viscous magma already stored beneath Krakatau, adding heat and dissolved gases and destabilizing the system. This process, known as magma recharge or mixing, may also have been a trigger or contributor to large eruptions such as: The real danger in 1883 came from the location of Krakatau. Had it been on land, the main dangers would have been the fall of volcanic material (tephra) and the spread of pyroclastic flows (avalanches of gas, hot ash and rocks). But because Krakatau was in the ocean, pyroclastic flows and the collapse of the volcanic edifice generated tsunamis, which killed more than 90% of those who died. What were the impacts in Australia and around the world? In Australia, strange rumbling noises, described as “rock explosions” and “distant artillery,” were recorded in Western Australia and the Northern Territory, about 3,000 kilometers from Krakatau, in August. Tsunamis generated by the eruption reached the coasts of Australia and New Zealand at around the same time, and waves over two meters high were recorded at several locations in Australia. No human deaths were reported, but some livestock drowned in Western Australia. In Geraldton, Western Australia, locals reported seeing the sea suddenly recede “over 100 yards” one August afternoon, leaving fish stranded on the shore. Unaware of the impending danger, the people scooped up the sinking fish, before the water suddenly returned with a loud noise. Fortunately, no deaths were reported. An earthquake was also reported due to the eruption in the Northern Territory on the afternoon of 26 August. The effects of the eruption lasted long after the event. Large quantities of pumice floated along the coasts of Australia for almost a year, and pumice rafts (where fragments of pumice stick together to form thick floating platforms) traveled thousands of kilometers across the Indian Ocean. By July 1884, pumice had arrived in Zanzibar, on the east coast of Africa. A report for the Royal Society described how schoolchildren on the coast of Zanzibar found stones that could float in water, as well as several “fairly clean” human skulls and bones next to them. Krakatoa also had a noticeable effect on the atmosphere. Volcanic gases and fine particles were injected high into the atmosphere, where they remained suspended for months. These small particles scattered red light across the spectrum, while allowing blue light to pass through, making the Moon appear blue. The large amount of sulfur dioxide released was oxidized in the atmosphere to form sulfate aerosols. These reduced the amount of sunlight reaching the Earth’s surface, causing near-surface temperatures to drop by around 0.3°C for several years after the eruption. Could it happen again? It is difficult to say whether Anak Krakatau will produce devastation similar to that of 1883 in the future. But we do know that the volcano is still very active. This could increase the risk because eruptions can cause volcanic instability, which could lead to collapse. On the other hand, it could also reduce the risk of a major rash in the long term; These intermittent eruptions can relieve the accumulation of large volumes of viscous magma within the volcano. In 2018, a large eruption caused most of the existing building about 300 meters high to collapse and produced a deadly tsunami that killed more than 400 people. Since then, the volcano has remained relatively small. According to Indonesia’s National Disaster Management Agency (known locally by the acronym BNPB), it does not currently have the potential to collapse on a scale that could trigger a tsunami. What’s up with the latest eruption? The main danger from the current eruption is airborne ash, made up of small abrasive fragments of volcanic rock. It is dangerous to inhale and can irritate the eyes and skin. It can also scratch airplane windshields, making it difficult for pilots to see. It can cause airplane engines to stall. Partial satellite image of the erupting Anak Krakatau on September 5, 2026. Copernicus Sentinel-2 Satellite, CC BY-SA Indonesia’s disaster management agency has urged all those affected to remain calm and seek information from government authorities. The agency has been conducting weather modification operations after the eruption to remove ash from the atmosphere. This includes spraying water mist from airplanes in an attempt to make it rain.
