The question of when Krakatoa will erupt again focuses on a complex of islands in the Sunda Strait. Krakatoa is a volcanic system, and the active threat today is the cone that grew from its ruins. Scientists cannot provide an exact date for the next eruption, but they assess the probability and potential scale of future events. The risk is managed by understanding the geological forces and continuously monitoring the active cone.
The Tectonic Setting: Why Krakatoa Exists
The volatile nature of the Krakatoa region is a direct consequence of its location on a major tectonic boundary. The volcano sits above the subduction zone where the Indo-Australian tectonic plate is sliding beneath the Eurasian Plate. This process, known as subduction, drives the entire Indonesian island arc system. As the oceanic plate descends, intense heat and pressure release water from the rock, lowering the melting point of the surrounding material. This generates magma that rises through the crust, feeding the chain of volcanoes in Indonesia’s portion of the Pacific “Ring of Fire”.
The location in the Sunda Strait, between Java and Sumatra, is a particularly active segment of this boundary. The continuous motion of these massive crustal plates ensures a perpetual supply of magma to the area. This tectonic stress makes the region highly susceptible to significant earthquakes and frequent, explosive volcanic activity.
From Catastrophe to Creation: The Rise of Anak Krakatau
The current geological activity echoes the catastrophic 1883 eruption, which destroyed the original Krakatoa island. That massive event emptied the underlying magma chamber, leading to a huge caldera collapse that submerged the bulk of the island. The eruption killed tens of thousands of people, mostly from resulting tsunamis, and radically reshaped the strait.
The submerged caldera remained dormant until a new volcanic cone began to emerge from the water in 1927. This new island, named Anak Krakatau, or the “Child of Krakatoa,” has been rapidly growing and frequently erupting ever since. It typically grew at an average rate of several meters per year, constantly building up new volcanic material.
Anak Krakatau’s rapid construction on the unstable slopes of the old caldera makes it fragile. This weakness became apparent in December 2018 when a normal eruption triggered a massive flank collapse. A large section of the volcano’s southwestern side slid into the sea, generating a deadly tsunami that struck the coastlines of Sumatra and Java without the typical earthquake warning.
The 2018 event demonstrated that the greatest hazard is not the eruption itself, but the resulting landslide into the ocean. This collapse removed over two-thirds of the cone’s volume, significantly reducing its height. Since then, the volcano has been actively rebuilding. New eruptions continue to reshape the remnant structure, indicating that the magmatic system remains highly pressurized and active.
Monitoring the Activity and Current Status
Volcanologists utilize a multi-parameter monitoring system for continuous, real-time assessment of Anak Krakatau’s internal processes. The system deploys seismometers, which detect magma movement by recording tremors and volcanic earthquakes. Analyzing the frequency and intensity of these seismic signals indicates whether magma is rising toward the surface. A change from routine, low-level activity to continuous, high-amplitude tremor often signals an imminent eruption.
Ground deformation is measured using GPS receivers and tiltmeters, which track microscopic changes in the volcano’s shape. Swelling or bulging of the flanks indicates that a magma body is inflating beneath the surface, placing stress on the edifice. Satellite-based radar imagery complements ground measurements, allowing for the detection of subtle ground shifts over large areas. This proved useful in identifying pre-collapse movement before the 2018 event.
The chemical composition of gas emissions is a significant data stream, with sensors analyzing the output of sulfur dioxide and carbon dioxide. A sudden increase in the ratio of these gases suggests that fresh, gas-rich magma is nearing the surface and degassing rapidly. For instance, a spike in carbon dioxide emissions was reported during increased activity in 2022. These multiple data streams are integrated to create a comprehensive picture of the volcano’s stability and eruptive potential.
Volcanic Forecasting and Warning Systems
The data collected from monitoring instruments is immediately relayed to the Indonesian Center for Volcanology and Geological Hazard Mitigation (PVMBG). This agency uses the information to determine the volcano’s official alert level within a four-tiered system. The tiers range from Level I (Normal) to Level IV (Eruption), and they inform public safety measures.
Forecasting does not mean predicting a precise day or hour for the next major event. Instead, it involves providing a probability window and defining a hazard zone based on the intensity of the observed unrest. For example, when activity heightens, the alert level may be raised to Level III (Standby), prompting the immediate establishment of an exclusion zone.
These exclusion zones prohibit public and tourist activity within a specified radius, typically set at two to five kilometers from the active crater. The system’s goal is to translate complex scientific data into actionable public safety protocols. While the threat of a sudden flank collapse remains a unique challenge for tsunami warning, the established alert tiers and exclusion zones are the foundation of public protection for Anak Krakatau’s next eruption.