The eruption of Mount Anak Krakatau in Indonesia has caused widespread disruption across western Indonesia, forcing airport closures, grounding thousands of flights and sending volcanic ash across Jakarta, Banten, West Java and parts of Sumatra.
The eruption began intensifying late Friday, September 4, and continued for roughly 25 hours, producing powerful explosions, lava fountains and ash plumes that reached altitudes of up to 50,000 feet. Although the most intense phase has eased, authorities continue to monitor the volcano and the movement of volcanic ash through the atmosphere.
By Monday, September 7, more than 2,300 flights had been disrupted and more than 270,000 passengers affected, according to authorities. Eight airports have been affected by the ash, with seven still closed as officials assess whether conditions are safe for aircraft operations.
Why Anak Krakatau Ash Reached Jakarta and Sumatra
At first glance, the spread of volcanic ash across Jakarta and parts of Sumatra may seem surprising. Anak Krakatau, however, is located in the Sunda Strait, between the islands of Java and Sumatra, putting it relatively close to some of Indonesia’s most populated and heavily traveled areas.
The volcano is roughly 130 to 150 kilometers from Jakarta and sits between the coastal regions of Banten and Lampung. That means the ash does not need to travel thousands of kilometers to affect major population centers.
The more important factor is how high the eruption column rose.
An ash plume reaching 50,000 feet can enter the upper part of the atmosphere where winds are considerably stronger than those experienced near the ground. Once fine volcanic particles become suspended at those altitudes, winds can transport them hundreds of kilometers in a relatively short period.
Weather conditions then determine where the ash travels. In this case, atmospheric winds carried volcanic material toward Banten, Jakarta, West Java and parts of Sumatra, creating problems far beyond the immediate vicinity of the volcano.
Why Even Small Amounts of Volcanic Ash Can Ground Aircraft
For aviation authorities, the problem is not simply whether passengers can see ash in the sky.
Volcanic ash is extremely hazardous to jet engines. The particles contain rock and volcanic glass that can be drawn into an engine, where extreme temperatures can melt the material. The molten particles can then stick to turbine components, potentially disrupting airflow and causing serious engine problems.
That is why airlines and aviation authorities take a highly conservative approach when volcanic ash is detected along flight routes.
A relatively thin ash cloud can therefore trigger flight cancellations, diversions and airport closures, even when there is no obvious danger at ground level.
The disruption can also spread rapidly through an airline network.
How One Airport Closure Can Disrupt Hundreds of Flights
Jakarta’s Soekarno-Hatta International Airport is one of Indonesia’s most important aviation hubs. When operations are suspended, the impact extends far beyond flights scheduled to land or depart from Jakarta.
An aircraft that cannot leave Jakarta may be unable to operate its next flight to another Indonesian city or an international destination. The same problem affects flight crews, aircraft positioning and connecting passengers.
This creates a cascade effect.
A single aircraft can operate several flights during one day. If the first flight is canceled, subsequent services may also have to be delayed, canceled or operated with a different aircraft. As the disruption continues, the backlog becomes increasingly difficult for airlines to clear.
That helps explain why the number of affected flights can rise quickly when an airport remains closed for multiple operating periods.
Seven Airports Remain Closed as Ash Threat Persists
The disruption has affected multiple airports in western Indonesia, including Soekarno-Hatta and Halim Perdanakusuma in the Jakarta area, as well as airports in Banten and Lampung.
Indonesia’s Directorate General of Civil Aviation has repeatedly extended or adjusted operating restrictions based on volcanic-ash observations and aviation safety assessments. On September 6, official monitoring confirmed volcanic ash at Soekarno-Hatta, prompting temporary suspension of operations.
The number of affected airports has changed as conditions evolved. Eight airports were affected overall, while seven remained closed on September 7, according to Reuters.
The aviation situation remains fluid because wind direction, ash concentration and visibility can change quickly.
The Eruption Has Eased, but the Hazard Has Not Disappeared
The most intense phase of the eruption has ended after approximately 25 hours of continuous activity, but that does not mean the situation has returned to normal.
Indonesia’s Geological Agency has warned that Anak Krakatau remains capable of further eruptions. Intermittent activity and continuing ash emissions mean authorities must keep monitoring both the volcano and the atmosphere around it.
Satellite observations have also been used to track remaining ash concentrations and their movement over affected areas.
For aviation authorities, this distinction is crucial: the eruption can weaken while the aviation hazard remains significant.
An airport does not necessarily reopen the moment explosive activity decreases. Authorities must first determine whether volcanic ash is still present in the relevant airspace and whether aircraft can safely operate from the affected airport.
No Tsunami Signal Detected in the Sunda Strait
The eruption has also raised concerns because Anak Krakatau was responsible for the 2018 Sunda Strait tsunami, which killed more than 400 people.
So far, however, Indonesia’s tsunami monitoring system has detected no sea-level anomaly indicating a tsunami associated with the latest eruption.
BMKG said its multi-sensor monitoring through the Indonesia Tsunami Early Warning System found no abnormal sea-level changes in the Sunda Strait as of September 6. Monitoring of tide gauges and other sensors continues around the clock.
That does not eliminate the need for monitoring. Instead, it means authorities have not detected evidence of a tsunami at the time of the latest assessment.
Authorities Expand Health and Safety Measures
The effects of the eruption are not limited to aviation.
Volcanic ash has affected communities in areas including Jakarta, Banten, West Java and Lampung. Authorities have advised people in affected areas to reduce unnecessary outdoor activity and use protective masks when exposure to ash is unavoidable.
Some schools have also moved classes online as a precaution, reducing the amount of time students and teachers spend outdoors while ash remains suspended or accumulates on the ground.
The Geological Agency continues to maintain a 3-kilometer exclusion zone around the crater, with residents and visitors prohibited from entering the designated danger area. Anak Krakatau remains at Level III, or Siaga, indicating a significant level of volcanic activity and the need for continued vigilance.
What Scientists Watch for Before Activity Escalates
Volcanologists do not rely on visual observations alone to determine whether an eruption is becoming more dangerous.
They monitor several indicators that can reveal changes beneath the volcano.
Seismic activity is one of the most important signals. Changes in the frequency or intensity of volcanic earthquakes can indicate that magma is moving beneath the surface.
Scientists also monitor ground deformation. Instruments can detect subtle changes in the shape or elevation of the volcanic edifice that may occur as magma accumulates underground.
Another key indicator is volcanic gas emissions, particularly sulfur dioxide. Changes in gas output can provide information about magma movement and degassing.
Finally, thermal cameras and visual monitoring systems help scientists detect changes in crater temperature, lava activity and other surface phenomena.
Taken together, these measurements give authorities a much clearer picture of whether volcanic activity is stabilizing or moving toward another major eruption.
What Could Trigger a Higher Alert?
If monitoring shows a significant escalation in volcanic activity, Indonesia’s geological authorities can raise the volcano’s alert status.
A higher alert level could lead to expanded exclusion zones, additional aviation restrictions and stronger warnings for nearby communities. BMKG would also continue monitoring the Sunda Strait for any unusual sea-level changes that could indicate a tsunami threat.
For now, officials are emphasizing continued monitoring rather than mass evacuation.
The Biggest Challenge Is Now the Aftermath
The most explosive phase of the Anak Krakatau eruption may have passed, but the disruption is far from over.
Airlines must reposition aircraft, recover delayed passengers, clear accumulated flight schedules and determine when affected routes can safely resume. Airports must also assess runways and surrounding airspace for continued ash contamination.
At the same time, authorities must monitor air quality and protect communities from continued ash exposure.
Indonesia has also deployed measures intended to help clear suspended ash from the atmosphere, including weather-modification efforts and water-mist operations in affected areas.
The episode demonstrates why volcanic eruptions can have impacts far beyond the volcano itself. Anak Krakatau may be a relatively small island in the Sunda Strait, but its position near Java and Sumatra and its proximity to one of Southeast Asia’s busiest aviation networks means a major eruption can quickly become a national transportation and public-health problem.
For travelers, airlines and residents across western Indonesia, the immediate priority remains the same: wait for aviation and geological authorities to confirm that conditions are safe before normal operations resume.
