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Karthala360by Discovermores

Geology and eruptions of Mount Karthala

A young and very active shield volcano: more than twenty eruptions in two centuries, some of which reached the sea, destroyed a village or covered Moroni in ash.

Formation

How Karthala was built

Karthala is a shield volcano, like those of Hawaii or Piton de la Fournaise on Réunion: thousands of fluid basalt flows, stacked on top of each other, have built a relief with long, regular slopes, about 30 km long and 15 km wide.

Eruptions do not only come out of the summit. The volcano is crossed by two fissure zones (“rifts”): one stretches 25 km to the north, the other 12 km to the south-east. Magma can force its way through them and open a fissure halfway down the slope, or even close to the coast. To the north, Karthala leans against another, older and less active volcano, La Grille, which forms the north of the island.

Why a volcano here? Geologists still debate the origin of the Comoros: a hotspot, as on Réunion, or fractures in the plate that let magma rise. One thing is certain: Grande Comore is the youngest and most active of the four islands of the archipelago.

The caldera and its craters

At the summit, the caldera measures about 3 km by 4. It is not a single crater but a set of nested collapses, which occurred when magma drained from beneath the summit. Two main craters open inside it:

  • the Choungou Chahalé, in the centre: about 1,300 m long, 800 m wide and 300 m deep. This is the one you see from the caldera rim;
  • the Choungou Changomeni, further north.

The floor of the Chahalé changes with every eruption: a lake appeared after 1991 and disappeared in April 2005, then lava lakes formed and solidified there in 2006 and 2007.

Mostly effusive eruptions

Most Karthala eruptions are effusive: lava fountains and flows, not very explosive, with a Volcanic Explosivity Index of 0 to 2. Two notable exceptions, in 1918 and November 2005, threw ash over much of the island. Another danger is less visible: when groundwater meets the heat of the volcano, it can cause a phreatic explosion, as in 1991.

Timeline

Two centuries of eruptions: the key dates

A selection, not a complete list: the volcano has erupted more than twenty times since 1828, and at least 34 times in the last 10,000 years. The latest alert was in 2022.

  1. The oldest dated eruption

    Dated to within ± 150 years. The next known eruptions date from the early 19th century.

  2. A 13 km lava flow

    Starting from the upper part of the northern fissure zone, a flow reached the west coast, very close to Moroni. In 1860, another flow again reached the shore near the capital.

  3. Deadly gases

    Gas emissions in August killed 17 people, according to accounts of the time.

  4. An explosive eruption

    With that of 2005, one of the two most explosive known eruptions of Karthala (Volcanic Explosivity Index 3).

  5. Fissures in the caldera

    From 8 September to 5 October, lava poured from four fissures in the caldera and covered about 2.5 km².

  6. Singani under the lava

    In April, a fissure opened on the south-western flank. The flow crossed the coastal village of Singani and destroyed about 300 houses.

  7. An explosion and a new lake

    A phreatic explosion in July, caused by groundwater meeting the heat of the volcano. A lake then formed on the floor of the Choungou Chahalé.

  8. The lake disappears

    A phreatomagmatic eruption from 16 April. Thousands of residents were evacuated, ash contaminated water and soil, and the crater lake disappeared.

  9. Ash over Moroni

    An explosive eruption from 24 November to 8 December. Ash fell over much of the island, including Moroni, and contaminated many rainwater tanks.

  10. A lava lake

    On 28 May, a magmatic eruption in the Choungou Chahalé: the glow was visible from the coast. The lava lake solidified in early June.

  11. The last confirmed eruption

    On 13 January, around 3 am, an eruption of about 12 hours, without strong explosions, formed a lava lake in the crater. A seismic crisis followed, with earthquakes above magnitude 4.

  12. A reported glow

    Residents of Mde and Mkazi reported a glow at the summit on the night of 9–10 May. No eruption was confirmed.

  13. The yellow alert

    From 15 July, the Observatory recorded abnormal earthquakes under the western flank, up to 50 a day. On 17 August, the Ministry of the Interior raised the yellow alert: climbing was banned. By early October, activity had clearly decreased. No eruption occurred.

Dates and figures come from the sources listed at the bottom of the page. The number of people evacuated in 2005 varies between sources (from 2,000 to more than 10,000 in April); we only keep what they agree on.

Monitoring

The Karthala Volcano Observatory

The volcano is monitored by the Karthala Volcano Observatory (OVK), created in 1988 by the National Centre for Documentation and Scientific Research of the Comoros (CNDRS), with the Institut de physique du globe de Paris (IPGP) and the University of Réunion. Its mission: to monitor the volcano’s activity, develop research, inform the public about volcanic risk and pass alerts on to the authorities in the event of an eruption.

The OVK works closely with the Piton de la Fournaise Volcano Observatory on Réunion, which provides technical support. It was thanks to this network that the slight increase in seismicity in early January 2007 announced the eruption of 13 January.

What the instruments measure

  • Earthquakes: a network of seismic stations around the massif records the small tremors caused by rising magma or moving fluids. It is the first sign of an awakening.
  • Ground deformation: tiltmeters installed in a lava tube, levelling benchmarks in the caldera and GPS measurements show the volcano “inflating” as magma accumulates.
  • Gases: since 2023, stations have measured carbon dioxide (CO2), an invisible gas that can escape from the volcano.

A network modernised since 2021

The European HATARI project (“danger” in Swahili), run from 2021 to 2023 by IPGP, the Piton de la Fournaise Observatory, the CNDRS and OVK, the University of Réunion and the Cité du Volcan, funded new seismic, GNSS (precision GPS) and CO2 stations, as well as training for Comorian and Réunionese students. In 2024, a relay station was built in the caldera, with the support of UNDP, the World Bank and the Directorate General of Civil Security, to transmit data in real time over 4G.

A fragile network. In 2022, only 4 of the 7 stations were working: cables, batteries, solar panels and GPS units had been stolen or destroyed. These instruments protect the island’s inhabitants. If you see a damaged station during the climb, do not touch it and tell your guide.

Alert

Karthala alert levels

The OVK informs the authorities; the alert is raised by the Ministry of the Interior, in charge of civil security, under the specialised Karthala emergency plan. The Directorate General of Civil Security then coordinates the response with the Comoros Red Crescent and partners.

  1. Green

    Normal volcanic activity. Trips go ahead depending on the weather.

  2. Yellow · vigilance

    Abnormal activity, with no certain eruption. People are asked to avoid any activity around the volcano. No trips.

  3. Orange

    Eruption possible in the short term: rescue services and evacuations are prepared. No trips.

  4. Red

    Eruption under way or imminent: evacuation of threatened areas. No trips.

Only the yellow level is described precisely in the public statements consulted; the orange and red levels are summarised from the Red Crescent’s 2022 action plan. In the event of an alert, follow official instructions only.

Hazards identified by the OVK

Lava flows

Slow, they leave time to evacuate, but destroy roads and houses, as in Singani in 1977.

Ash

Carried by the wind, it pollutes water in rainwater tanks, as in 2005.

Carbon dioxide

CO2, invisible and heavier than air, collects in hollows: a deadly danger near the volcano.

Earthquakes and lahars

Tremors that threaten buildings, and mudflows when rain washes away the ash.

Phreatic explosions

When groundwater meets the heat of the magma, as in 1991.

In 2022, the OVK estimated that four regions of Grande Comore were exposed, Mbadjini, Hambou, Bambao and Itsandra: 115 villages and nearly 300,000 inhabitants.

And our trips?

What this means when you climb

  • Yellow level or above: no trips. We follow the statements of the OVK and the Ministry of the Interior; the trip is postponed or refunded under the terms of sale.
  • Never a bivouac in the caldera. Carbon dioxide can collect there, especially at night and in hollows. The French authorities strongly advise their nationals against it; our camp is always set up outside the caldera.
  • At the caldera rim, we keep away from the walls, which can collapse, and we never go down into the craters.
  • No eruption has been confirmed since 2007: the risks of a trip are mostly those of the mountain: cold, fog, rain and falls.

Safety: the Go, Adapt, No Go rule

Walk on the volcano’s history

The full climb crosses old lava flows and the forests that have covered them; every tour stops at the caldera rim, facing the Choungou Chahalé.

See the tours See the GPS track

Sources

Sources checked in September 2026. Spotted a mistake or have something to add? Write to us: this page is updated as facts are checked.

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