How Scientists Predict Volcanic Eruptions and Warn Communities in Advance

Why this is in the news

Anak Krakatau, the volcanic island that rose in the Sunda Strait between Java and Sumatra after the 1883 Krakatoa eruption, produced a major eruption on Saturday, 6 September 2026, followed by at least three more eruptions on the morning of Tuesday, 8 September, according to Al Jazeera.

The ash blanketed critical airport infrastructure across Java and Sumatra. Five airports, including Jakarta's main Soekarno-Hatta International Airport, closed and then reopened on the Tuesday once crews had cleared ash from runways and facilities, Al Jazeera reported. Indonesia's transport minister, Dudy Purwagandhi, said cargo planes resumed first while passenger aircraft awaited checks.

Al Jazeera reported the disruption affected more than 340,000 travellers and roughly 2,900 flights. The head of Indonesia's Geology Agency, Lana Saria, said the impact was smaller than previous Anak Krakatau eruptions but that monitoring would continue.

Reading a volcano's warning signs before it erupts

The U.S. Geological Survey's Volcano Hazards Program describes two complementary approaches to forecasting. Long-term assessments catalogue the size, frequency and style of a volcano's past eruptions to establish its baseline hazard. Short-term forecasting then watches for real-time deviations from how that same volcano behaves when it is quiet.

• Seismicity: swarms, frequency changes or shifting locations of small earthquakes can signal magma or fluid moving underground, per USGS.
• Ground deformation: swelling or subsidence as magma accumulates can be detected on the ground and from above, USGS says.
• Gas emissions: changes in the character or rate of gas output point to degassing magma, according to USGS.
• Thermal and satellite imagery: heat anomalies and crater hotspots are also tracked remotely between ground visits, as seen in Anak Krakatau's own monitoring record.

USGS says volcanoes typically give a longer warning phase before an eruption than many other natural hazards. Forecasters use probabilistic 'event trees' — branching diagrams of possible unrest scenarios and their likelihoods — which are updated continuously as new monitoring data arrive, alongside physical and numerical models of how ash, pyroclastic flows or lahars might spread.

Indonesia's four-level alert system in practice

Indonesia's Geology Agency (PVMBG) rates volcanoes on four alert levels, as described by Liputan6: Level I, Normal, covers ordinary fluctuations that do not show increased activity; Level II, Waspada, means initial signs of rising activity have been observed or recorded, with any eruption threatening only the area near the crater; Level III, Siaga, applies when an observable increase in activity or an eruption threatens the area around the crater, without yet threatening settlements; and Level IV, Awas, is used when an observable increase in activity or an eruption threatens settlements around the volcano. Liputan6 notes identical levels can mean different real-world hazards at different volcanoes, and that warnings are issued by the Ministry of Energy and Mineral Resources and conveyed through regional governments.

Anak Krakatau's own record shows the system in motion. According to the volcano-monitoring site The Watchers, PVMBG raised the volcano from Level II to Level III on 2 July 2026 at 16:30 local time (09:30 UTC), after an eruption that day sent an ash plume about 200 metres above the summit. In the period from 16 June to 2 July, instruments had logged 740 exhalation earthquakes, 247 low-frequency earthquakes and 520 hybrid or multiphase earthquakes, while satellites picked up rising sulfur dioxide output and thermal anomalies from 1 June and crater hotspots from 10 June, The Watchers reported. Authorities set a 3-kilometre exclusion zone around the active crater.

The Watchers also notes why Anak Krakatau draws extra scrutiny: a flank collapse on 22 December 2018 triggered a tsunami that killed 437 people along the coasts of western Java and southern Sumatra, so eruptions at this particular volcano trigger separate tsunami-hazard checks in addition to the standard ash and ejecta warnings.

Keeping aircraft out of the ash

Ground-based alert levels protect people near a volcano, but ash plumes travel far beyond any exclusion zone. According to Wikipedia, nine regional Volcanic Ash Advisory Centres (VAACs) — in Anchorage, Buenos Aires, Darwin, London, Montreal, Tokyo, Toulouse, Washington and Wellington — divide the world between them and issue advisories whenever an eruption threatens aviation, drawing on satellite imagery, ground and pilot reports, and ash-dispersion models.

Japan's Meteorological Agency, which runs the Tokyo VAAC, describes the process in more detail: forecasters feed wind data from JMA's global weather model into a volcanic ash transport model to project where a plume will be 6, 12 and 18 hours ahead, then issue advisories at least every six hours — normally at 00, 06, 12 and 18 UTC — while ash remains visible on satellite, more often if conditions change quickly. Those advisories reach airlines, aviation authorities and meteorological watch offices, which use them to prepare the flight-planning warnings pilots rely on, JMA says.

That international aviation-warning layer is separate from, but runs alongside, PVMBG's ground alert levels — together they explain why an eruption on a sparsely populated island can still ground hundreds of thousands of travellers at airports hundreds of kilometres away, as happened at Jakarta's Soekarno-Hatta in September 2026.