Anak Krakatau Eruption Disrupts Flights in Indonesia: Why the Sunda Strait Is a Major Volcanic Zone
GS Paper I | Physical Geography | Geomorphology
GS Paper III | Disaster Management | Environment
Current Affairs | 6 September 2026
Why in News?
Indonesia’s Anak Krakatau volcano remained at Alert Level III (Siaga) on 6 September 2026 after a continuous eruption episode lasting nearly 25 hours.
Indonesia’s volcanology agency, PVMBG, said the continuous eruptive episode began at 11:07 PM on 4 September and ended at 12:04 AM on 6 September. A further short Strombolian eruption was recorded at 3:53 AM on 6 September. Authorities continued to warn that further eruptions, incandescent rock fragments, heavy ash fall and lava flows remained possible. A 3-km exclusion zone around the active centre continues to apply.
The volcanic ash also caused major aviation disruption on 6 September. Flights were suspended at six airports, including Jakarta’s Soekarno-Hatta International Airport, affecting 301 flights, including 58 international services.
The development is particularly important for UPSC because Anak Krakatau connects several core geography concepts:
Plate tectonics → Subduction → Volcanic arcs → Ring of Fire → Volcanic hazards → Tsunamis → Aviation disruption
Where is Anak Krakatau?
Anak Krakatau is located in the Sunda Strait between the Indonesian islands of Java and Sumatra.
Administratively, it lies in South Lampung Regency of Lampung Province.
The Sunda Strait connects the Java Sea with the Indian Ocean and separates:
Sumatra to the west/north-west
from
Java to the east/south-east
The volcano lies within the Krakatau volcanic complex.
Map memory
Sumatra → Sunda Strait → Anak Krakatau → Java
This location is worth remembering because UPSC frequently converts natural disasters into places-in-news and map-based questions.
What Does “Anak Krakatau” Mean?
“Anak Krakatau” literally means “Child of Krakatau.”
It developed within the volcanic caldera left after the catastrophic 1883 Krakatau eruption.
The new volcanic island began emerging above the sea in the early twentieth century and has continued to grow through repeated eruptions.
The European Union’s Copernicus programme notes that Anak Krakatau emerged in 1927 from the underwater caldera created by the 1883 eruption.
This makes Anak Krakatau an excellent example of how volcanic processes can both destroy and create landforms.
Why Is Indonesia So Volcanically Active?
Indonesia is situated along the Pacific Ring of Fire, one of the world’s most tectonically active regions.
But “Ring of Fire” should not be treated as just a memorised phrase. The underlying reason is plate tectonics.
Around Java and Sumatra, the Indo-Australian Plate converges with and subducts beneath the Sunda Plate.
The U.S. Geological Survey notes that convergence along the Sunda margin occurs at roughly 50–70 mm per year and produces the active Sunda volcanic arc.
How Does Subduction Produce Volcanoes?
The process can be understood step by step.
Step 1: Two plates converge
The denser oceanic portion of the Indo-Australian Plate moves towards the Sunda Plate.
Step 2: The oceanic plate sinks
Because it is denser, it descends beneath the overriding plate.
This is called subduction.
Step 3: Water enters the mantle
The descending slab carries water-rich minerals into deeper parts of the Earth.
Water lowers the melting point of the surrounding mantle material.
Step 4: Magma forms
Partial melting generates magma.
Because magma is less dense than surrounding rock, it rises towards the surface.
Step 5: Volcanoes develop
Repeated magma movement and eruptions create a chain of volcanoes parallel to the subduction zone.
This chain is known as a volcanic arc.
In Indonesia, this process has created the Sunda Arc.
Sunda Trench and Sunda Arc
Students should distinguish these two terms.
Sunda Trench
The Sunda or Java Trench is a deep oceanic trench associated with the subduction boundary.
It runs broadly parallel to Sumatra and Java.
Sunda Arc
The Sunda Arc is the volcanic chain generated by this subduction process.
It includes numerous active volcanoes across western Indonesia.
The USGS describes the Sunda-Java Trench as extending for roughly 3,000 km, while the broader Sunda convergent margin stretches much farther from the Andaman region towards eastern Indonesia.
What is the Pacific Ring of Fire?
The Pacific Ring of Fire is a broad belt surrounding much of the Pacific Ocean where:
- earthquakes,
- volcanic eruptions,
- ocean trenches, and
- tectonic plate boundaries
are highly concentrated.
It includes tectonically active regions such as:
- Indonesia,
- Japan,
- the Philippines,
- New Zealand,
- the western coasts of North and South America, and
- the Aleutian region.
Important Prelims point
The Ring of Fire is not a single tectonic plate.
It is a broad zone associated with the interaction of several tectonic plates, particularly at convergent boundaries and subduction zones.
What Type of Eruption Occurred?
PVMBG recorded a follow-up Strombolian eruption on 6 September.
What is a Strombolian eruption?
Strombolian eruptions typically involve relatively short, repeated explosive bursts caused by expanding gas bubbles in magma.
They may eject:
- incandescent lava fragments,
- volcanic bombs,
- lapilli,
- ash, and
- gas.
Lava fountains may also occur.
The September Anak Krakatau episode included activity resembling lava fountains, according to Indonesian geological monitoring.
Prelims distinction
A Strombolian eruption is generally more explosive than a quiet Hawaiian-type eruption, but usually less violent than the most powerful Plinian eruptions.
Why Did Volcanic Ash Disrupt Flights?
Volcanic ash is not the same as the soft ash produced by burning wood.
It consists of extremely fine particles of:
- pulverised volcanic rock,
- minerals, and
- volcanic glass.
At aircraft cruising altitude, these particles can create serious risks.
1. Jet-engine damage
Ash can enter jet engines, where high temperatures may soften or melt particles.
The material can then accumulate inside turbine components and interfere with airflow.
2. Engine failure
Severe ash encounters can cause compressor problems and even engine flameout.
The U.S. Federal Aviation Administration notes that aircraft encounters with volcanic ash have caused complete loss of power in jet engines and describes flying through volcanic ash as extremely dangerous.
3. Damage to aircraft surfaces
Ash is abrasive and can damage:
- windscreens,
- sensors,
- navigation equipment, and
- exterior surfaces.
4. Reduced visibility
Dense ash can also reduce visibility around airports and interfere with ground operations.
This explains why a volcanic eruption far from an airport can still cause widespread cancellation and diversion of flights.
Why Did Anak Krakatau Affect Jakarta?
The eruption produced an ash plume that reached high into the atmosphere and was carried by winds towards densely populated and aviation-sensitive areas.
Reuters reported volcanic ash reaching roughly 50,000 feet above Sumatra, leading authorities to suspend operations at several airports.
This illustrates an important disaster-management principle:
The geographical impact of a volcanic eruption is not limited to the immediate crater. Atmospheric circulation can spread ash hundreds of kilometres away.
Can Anak Krakatau Cause a Tsunami?
Yes, but the mechanism is important.
A volcanic eruption does not automatically cause a tsunami.
Volcano-related tsunamis can occur through processes such as:
- collapse of part of a volcanic island,
- submarine landslides,
- major underwater explosions,
- caldera collapse, or
- rapid displacement of seawater.
This distinction became tragically important at Anak Krakatau in 2018.
The 2018 Anak Krakatau Tsunami
On 22 December 2018, part of Anak Krakatau collapsed into the sea during an eruptive episode.
The collapse displaced seawater and generated a tsunami across the Sunda Strait, striking coastal areas of Java and Sumatra.
More than 400 people were killed.
The event was unusual because the tsunami was caused by a volcanic flank collapse, rather than by a major tectonic earthquake.
UPSC takeaway
Tsunamis may be generated by:
- undersea earthquakes,
- submarine landslides,
- volcanic eruptions or collapses, and
- rarely, meteor impacts.
Do not associate every tsunami only with earthquakes.
Why Is the 1883 Krakatau Eruption Famous?
The 1883 Krakatau eruption was one of the most powerful volcanic events in recorded history.
The eruption destroyed much of the original Krakatau island complex and generated destructive tsunamis across the Sunda Strait.
Indonesia’s Geological Agency itself cites the 1883 eruption as a major historical event that produced a tsunami.
The eruption also injected large quantities of ash and aerosols into the atmosphere, producing effects far beyond Indonesia.
Its historical importance makes the Krakatau complex useful for understanding:
- caldera formation,
- explosive volcanism,
- volcanic tsunamis, and
- atmospheric effects of major eruptions.
What is a Caldera?
A caldera is a large volcanic depression generally formed when the ground collapses after a major eruption empties part of the magma chamber beneath a volcano.
It is usually much larger than an ordinary volcanic crater.
Crater vs Caldera
| Feature | Crater | Caldera |
|---|---|---|
| Size | Usually smaller | Usually much larger |
| Formation | Around volcanic vent | Often caused by collapse |
| Trigger | Eruption/vent activity | Major evacuation of magma chamber |
| Example linkage | Summit crater | Krakatau volcanic complex |
Anak Krakatau subsequently grew within the broader area transformed by the 1883 eruption.
Why Does Indonesia Face Multiple Natural Hazards?
Indonesia’s tectonic setting creates a combination of risks.
Earthquakes
Plate boundaries accumulate and release enormous tectonic stress.
Volcanic eruptions
Subduction creates magma and active volcanic arcs.
Tsunamis
Undersea earthquakes, landslides and volcanic collapses can displace seawater.
Landslides
Steep volcanic terrain and heavy tropical rainfall increase slope instability.
This is why Indonesia is an excellent example of a multi-hazard environment.
Volcano Monitoring: What Does Level III Mean?
Indonesia uses a four-level volcanic alert system.
Broadly:
Level I — Normal
Level II — Waspada (Advisory/Alert)
Level III — Siaga (Standby/High Alert)
Level IV — Awas (Warning/Highest level)
Anak Krakatau remained at Level III on 6 September, even though the long continuous eruption episode had ended, because further volcanic activity remained possible.
Important lesson
The end of an eruption episode does not necessarily mean that the volcanic crisis is over.
Scientists monitor:
- volcanic earthquakes,
- gas emissions,
- deformation,
- temperature,
- ash emissions, and
- visual changes
before changing an alert level.
What is RSAM?
The Indonesian monitoring report referred to RSAM — Real-time Seismic Amplitude Measurement.
RSAM is used to track the average strength of seismic signals associated with volcanic activity.
A rise in RSAM can indicate increasing:
- magma movement,
- explosions,
- volcanic tremor, or
- other energetic activity.
PVMBG reported that RSAM rose on 5 September but showed a declining trend on 6 September.
This is a useful example of how modern volcanology combines visual observation with instrumental monitoring.
What is Ground Inflation?
PVMBG also reported signs of inflation at one monitoring station.
Volcanic inflation means the ground around a volcano expands or rises slightly.
It may occur when:
- magma,
- volcanic gases, or
- hydrothermal fluids
accumulate beneath the surface.
Scientists can monitor deformation using:
- GPS,
- tiltmeters,
- satellite radar, and
- other geodetic techniques.
Prelims connection
Inflation → possible pressure/magma accumulation
Deflation → possible withdrawal or release of magma/pressure
However, neither signal alone guarantees an eruption.
Disaster Management Lessons
Anak Krakatau shows why volcanic disasters require more than simply evacuating people near the crater.
1. Continuous scientific monitoring
Seismic activity, deformation and ash plumes must be tracked continuously.
2. Exclusion zones
Restricting access near active craters reduces exposure to sudden explosions and ballistic rocks.
3. Aviation coordination
Volcanic Ash Advisory Centres, airports and airlines must quickly exchange information.
4. Coastal preparedness
Because Anak Krakatau is an island volcano, nearby coastal communities also need tsunami preparedness.
5. Satellite monitoring
Earth-observation satellites can monitor:
- ash plumes,
- thermal anomalies,
- surface change, and
- lava flows.
The Copernicus Sentinel-2 image from July 2026 is an example of satellite-based monitoring of Anak Krakatau.
Prelims Focus
Remember these facts:
- Anak Krakatau is located in Indonesia.
- It lies in the Sunda Strait.
- Sunda Strait separates Java and Sumatra.
- Indonesia lies on the Pacific Ring of Fire.
- Anak Krakatau belongs to the Sunda volcanic arc.
- The Indo-Australian Plate subducts beneath the Sunda Plate in this region.
- The associated oceanic trench is the Sunda/Java Trench.
- Anak Krakatau remained at Alert Level III on 6 September 2026.
- A Strombolian eruption was recorded on 6 September.
- Volcanic ash can severely damage aircraft engines.
- The 2018 Anak Krakatau tsunami was associated with volcanic flank collapse.
- A tsunami does not have to be generated by an earthquake.
Prelims Traps
Statement: Anak Krakatau lies between Java and Sumatra.
Correct.
Statement: Sunda Strait separates Sumatra from Borneo.
Incorrect. It separates Sumatra and Java.
Statement: The Pacific Ring of Fire is a single tectonic plate.
Incorrect.
Statement: Indonesian volcanism is associated mainly with divergent plate boundaries.
Incorrect. Subduction at convergent boundaries is crucial.
Statement: Every volcanic eruption generates a tsunami.
Incorrect.
Statement: Volcanic ash is harmless to aircraft because it is extremely light.
Incorrect. It can cause severe engine and equipment damage.
Statement: A caldera is generally larger than a volcanic crater.
Correct.
Mains Perspective: Why Indonesia Is a Global Laboratory for Disaster Management
Indonesia’s location at the convergence of major tectonic plates gives it extraordinary geological dynamism.
This brings economic benefits such as:
- fertile volcanic soils,
- geothermal potential,
- mineral resources, and
- tourism.
But it also creates severe risks from:
- earthquakes,
- volcanoes,
- tsunamis, and
- landslides.
The Anak Krakatau episode demonstrates that disaster management must therefore combine:
scientific monitoring + land-use regulation + early warning + public communication + aviation safety + coastal preparedness.
It also shows why a natural hazard becomes a disaster only when it interacts with exposed populations, infrastructure and economic networks.
Possible UPSC Prelims Question
With reference to Anak Krakatau, consider the following statements:
- It is located in the Sunda Strait between Java and Sumatra.
- Its volcanism is associated with subduction along the Sunda tectonic margin.
- Its 2018 eruption-related tsunami was associated with the collapse of part of the volcanic island.
- The Sunda Strait connects the Persian Gulf with the Indian Ocean.
Which of the statements given above are correct?
A. 1 and 2 only
B. 1, 2 and 3 only
C. 2, 3 and 4 only
D. 1, 2, 3 and 4
Answer: B
Statements 1, 2 and 3 are correct. The Sunda Strait lies in Indonesia and separates Java and Sumatra; it has nothing to do with the Persian Gulf.
Possible Mains Question
“Indonesia’s vulnerability to volcanic disasters is fundamentally rooted in its plate-tectonic setting, but the scale of disaster depends equally on human exposure and preparedness.” Discuss.
Answer Approach
Begin with Indonesia’s location on the Pacific Ring of Fire.
Then explain:
- Indo-Australian–Sunda Plate convergence,
- subduction,
- Sunda Trench and volcanic arc,
- Anak Krakatau,
- earthquakes and tsunamis,
- volcanic ash and aviation,
- coastal population exposure,
- monitoring and warning systems,
- satellite technology,
- resilient infrastructure.
Conclude that tectonics determines the hazard, while governance and preparedness strongly influence whether that hazard becomes a major disaster.
30-Second Revision
Anak Krakatau → Indonesia → Sunda Strait → between Java & Sumatra → Sunda volcanic arc → Indo-Australian Plate subducts beneath Sunda Plate → Pacific Ring of Fire → Alert Level III → Sept 6 flight disruption → volcanic ash hazard → 2018 flank-collapse tsunami → 1883 Krakatau eruption → volcano + tsunami + aviation + disaster-management linkage.
