This article is part of The Purple Report September 2026 | Disaster Forecasting and Predictions and focuses specifically on volcano and lahar risk.
Volcano forecasting is fundamentally different from earthquake forecasting.
Many volcanoes provide measurable evidence when magma, gas or fluids begin moving. Seismicity can change. The ground can deform. Gas chemistry can shift. Thermal output can rise. Crater lakes can change. Satellite data can reveal uplift.
That makes short-term forecasting more plausible than it is for tectonic earthquakes.
But one boundary remains essential:
Volcanic unrest is evidence that the volcanic system has changed. It is not proof that eruption will occur.
Quick Read
The September baseline tracks ten volcanic systems:
- Campi Flegrei / Naples
- Svartsengi–Sundhnúkur / Grindavík
- Mayon
- Kanlaon
- Taal
- Merapi / Yogyakarta
- Popocatépetl
- Nevado del Ruiz
- Cotopaxi
- Mount Rainier
Six of them enter September with live unrest or official alert states strong enough to justify elevated attention. The other systems remain long-term high-consequence watches.
What Volcano Observatories Actually Watch
No single signal proves eruption.
Observatories combine multiple evidence streams.
- Volcanic earthquakes: brittle fracture, fluid movement and changing pressure can alter earthquake rates and types.
- Ground deformation: GNSS and InSAR can reveal inflation, deflation or intrusion.
- Gas: sulphur dioxide, carbon dioxide and other gases can change with magma movement and degassing.
- Thermal observations: satellites, cameras and ground instruments can detect heating.
- Visual activity: ash, lava, crater glow, rockfall and dome growth matter.
- Hydrology: crater lakes, snow and glaciers can transform an eruption into a flood or lahar problem.
- Acoustic and infrasound signals: explosions can sometimes be detected beyond visual range.
The forecast is strongest when independent signals converge.
It is weaker when one noisy variable changes by itself.
Campi Flegrei: Unrest Inside a City
Campi Flegrei is one of the clearest examples of why volcanic hazard cannot be separated from exposure.
Italy’s Civil Protection currently keeps the system at yellow alert. The caldera is experiencing bradyseism—ground uplift and associated seismicity.
On 31 July 2026, a magnitude-4.7 earthquake caused injuries, damage and temporary displacement around Pozzuoli and nearby areas.
That event matters even though it was not a volcanic eruption.
It demonstrates that volcanic unrest can create an earthquake and building-safety problem before eruption becomes the dominant question.
The Civil Protection red zone also reveals the consequence scale: Pozzuoli, Bacoli, Monte di Procida, Quarto and parts of Naples sit inside the evacuation planning system.
The Purple Forecast therefore classifies Campi Flegrei HIGH-CONCERN for September 2026.
That label means live unrest + enormous exposure.
It does not mean an eruption has been forecast for September.
Italy Civil Protection — Campi Flegrei
Italy Civil Protection — 31 July 2026 earthquake update
Reykjanes: Where Ground Deformation Changes the Near-Term Forecast
The Icelandic Meteorological Office reported on 18 August 2026 that uplift and magma accumulation continued beneath Svartsengi.
Its modelling estimated around 29.1 million cubic metres of accumulated magma since the start of the previous eruption cycle, and the office said the most likely scenario remained a new magma intrusion towards the Sundhnúkur crater row, potentially followed by another eruption.
Crucially, previous events in this system have sometimes provided warning measured in tens of minutes to a few hours.
This is a completely different forecasting problem from Nankai or Cascadia.
At Reykjanes, live deformation can materially change the near-term eruption scenario. At a megathrust fault, similar exact timing inference is not available.
Icelandic Meteorological Office — Reykjanes situation
Mayon and Kanlaon: Volcano Plus Tropical Rain
PHIVOLCS lists both Mayon and Kanlaon at Alert Level 2 at the end of August 2026.
The volcanic signals matter.
So does the weather.
Volcanic material deposited on steep slopes can be remobilised by heavy rain. That means a volcanic system may become a lahar problem even when the most dramatic eruptive phase has passed.
This is why the Purple Forecast refuses to model volcanoes as isolated cones.
The real hazard system can include:
- crater and dome;
- ash and pyroclastic deposits;
- river channels;
- rainfall;
- roads;
- farms;
- evacuation centres;
- air quality;
- downstream settlements.
PHIVOLCS — current volcano bulletins
Taal: Moderate Alert, Extraordinary Exposure
Taal is at Alert Level 1 in the September baseline.
Why track it at all?
Because the consequences of renewed activity are shaped by a densely inhabited lake-caldera setting and proximity to the Greater Manila region.
This creates a useful rule:
Low current unrest can coexist with high structural consequence.
Taal should be downgraded or upgraded when PHIVOLCS changes its evidence-based alert status.
It should not disappear from the ten-year structural watch merely because one month is quiet.
Merapi: Activity, Pyroclastic Flows and the River Afterward
Indonesia’s BPPTKG reported elevated effusive activity and SIAGA status during 2026.
Merapi has a recurrent pattern in which dome growth, collapse and pyroclastic flows are only one part of the hazard.
Rain can later move unconsolidated volcanic material into river channels as lahars.
That creates a multi-clock forecast:
- volcanic unrest changes;
- eruption/deposition occurs;
- material remains stored on slopes;
- monsoon rainfall arrives;
- lahar risk rises in channels.
The event is not over when lava stops moving.
BPPTKG — Merapi monitoring and hazard information
Popocatépetl: Alert Level Is an Operating State
CENAPRED keeps Popocatépetl at Yellow Phase 2 in the August 2026 monitoring record.
The expected scenarios at this level include smaller-to-moderate explosions, variable tremor, ashfall and incandescent fragments inside the exclusion zone.
CENAPRED also warns people to stay away from ravines during strong rain because of mud and debris flows.
Again, the volcano and weather systems cannot be cleanly separated when we consider the people and places exposed downstream.
CENAPRED — Popocatépetl monitoring
Nevado del Ruiz and Cotopaxi: Why Ice Matters
Nevado del Ruiz remains at yellow activity in Colombia’s late-August 2026 bulletins.
Its significance cannot be understood from ash alone.
Snow and ice can turn eruption heat into meltwater. Meltwater can entrain sediment. Narrow valleys can transform that mixture into fast-moving lahars far downstream.
Cotopaxi in Ecuador belongs in the same broad hazard family: a glaciated volcano where downstream valleys matter as much as the summit.
The two should not be treated as equivalent current-alert states.
The shared mechanism is what matters to the ten-year watch.
Servicio Geológico Colombiano — weekly volcano bulletins
Mount Rainier: The Volcano Whose Greatest Threat May Be a Mudflow
USGS classifies Mount Rainier as a Very High Threat volcano.
The most important part of that statement is not simply that Rainier could erupt again.
USGS says lahars are the greatest threat Rainier poses to people and property downstream.
Some of the relevant valleys are now densely populated and contain highways, bridges, ports and pipelines.
Rainier also contains hydrothermally weakened rock and large glacier/ice systems.
This creates an important Purple Forecast rule:
The highest-consequence volcanic pathway may not be the pathway most people imagine when they hear the word “eruption”.
USGS — Volcanic hazards at Mount Rainier
September 2026 Volcanic Ledger
| System | September state | Main reason |
|---|---|---|
| Campi Flegrei | HIGH-CONCERN | yellow alert + bradyseism + recent damaging M4.7 earthquake |
| Svartsengi–Sundhnúkur | HIGH-CONCERN | continued uplift and magma accumulation; possible new intrusion/eruption |
| Mayon | HIGH-CONCERN | PHIVOLCS Alert Level 2 + lahar-sensitive tropical rainfall environment |
| Kanlaon | HIGH-CONCERN | PHIVOLCS Alert Level 2 |
| Taal | ELEVATED | Alert Level 1 + very high exposure |
| Merapi | ELEVATED | high effusive activity / SIAGA context in 2026 |
| Popocatépetl | HIGH-CONCERN | Yellow Phase 2 + ongoing exhalations/ash |
| Nevado del Ruiz | ELEVATED | yellow activity + lahar consequence |
| Cotopaxi | WATCH | structural glaciated-volcano/lahar exposure |
| Mount Rainier | WATCH | Very High Threat; major lahar exposure despite background state |
How the Volcano Watch Will Move
A volcano can move between states much faster than a tectonic long-term probability model.
Each monthly update will therefore look for:
- official alert-level changes;
- new deformation;
- changes in seismicity;
- gas and thermal changes;
- dome growth or collapse;
- new ash or lava;
- rainfall loading of recent deposits;
- changes in exclusion or evacuation zones;
- recovery and infrastructure repairs.
A volcano that becomes quieter must be downgraded.
A volcano that stays at elevated unrest for months without eruption is not a forecasting embarrassment. It is evidence about how often unrest does and does not lead to eruption.
That is exactly the calibration information the ten-year project needs.
Final Rule
A volcano forecast becomes useful when it connects changing geophysical evidence to the people and places at risk without turning unrest into certainty or the absence of unrest into permanent safety.