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Introducing Volcano Mount Karthala

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  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

My intention is to produce a series of articles (depending on interest) which introduces ideas about volcanic and earthquake activity. I want to go slightly off the beaten track to explore oddities, volcano hazards, analysis methods, Wonders and Mankind’s impacts. Keep in mind I am not expert (corrections gratefully received), but hopefully these will at least give a flavour of some different places in the world and provide a few minutes escape from people’s troubles.

Introducing Volcano Mount Karthala


Reason for Interest
This is a very active volcano with a history of collapse, lava flows reaching the sea, going through a step change in activity in a tectonic environment that can create new large volcanoes. For me this is a very dangerous volcano which will cause issues in the near future.

The Setting
Named after the Arabic word for moon the Comoros Islands are volcanic islands between the coast of Mozambique and the island of Madagascar consisting of four islands – Ngazidja (Grande Comore), Moheli, Anjouan and Mayotte. Originally the Comoros were known as the ‘Perfume Islands’ due to their fragrant flowers, like the yellow ylang-ylang, used in perfumes which where imported and cultivated by French colonialists. While Mayotte remains under French Administration the rest of the islands became an independent nation in 1975.



Grande Comore is the largest of the Comoros Islands with the capital city (Moroni) of the nation located there. The whole western side of Grande Comore is full of very highly rated beaches with Ndroudé being the longest and widest and Mitsamiouli Beach being considered one of the top beaches in the world.




In the far northern part of the island is the small crater lake 'Lac Sale' which the locals call the bottomless lake probably due to some professional divers who tried to explore the lake but never came back.


South of Moroni is Iconi which was once the capital of the Sultanate of Bambao and the first capital of Comoros. 



 Moroni has a distinctly relaxed Arabian pace of life with the labyrinthine medina at the center which is crisscrossed by cobbled alleyways that are lined with intricately carved doorways, ornate mosques and charming Middle Eastern-style cafes. Moroni’s principal landmark is the two-storied and gleaming white 15th-century Old Friday mosque ( Nouvelle Mosquee de Vendredi Mosque dating back to around 1427)  restored in the 1920s by France to pay tribute to those Comorians killed fighting for them during the First World War.


The islands have a tropical climate with two clearly marked seasons: a cooler, dry period between May and October and a warmer, humid season between November and April. In November the summer monsoon (kashkazi) brings the highest afternoon temperatures—about 33 °C. The highest monthly rainfall occurs in January with about 11–15 inches. Dry season daily maximum temperatures fall to their lowest 29 °C in July. The average annual rainfall varies between 40 to 100 inches being highest on the windward northeast sides of the islands.

 The island has a coastal zone of mangroves followed inland by a zone of coconut palms, mangoes, and bananas up to about 1,300 feet. Above this a forest zone of Mohogany and orchids rises to about 5,900 feet where they give way to broom, heather, and lichens. Additional aromatic plants such as frangipani (Plumeria), jasmine, and lemongrass lend a delightful fragrance to the islands.

The Tectonic environment
The Comoros islands are roughly W-NW aligned and extend over 270 km in the Western Indian Ocean, on the northern edge of the Mozambique channel. The channel formed by the opening of the Somali and Mozambique Basins which dislocated the micro continent Madagascar to the south, away from the African continental plate. It was thought that a hotspot migrated under the chain of islands propagating to the northwest creating the volcanic Islands. This chain of volcanic islands and seamounts becomes progressively older from the still-active volcanism at Grande Comoros toward the older volcanic areas in northern Madagascar.

Volcanic activity history and aging are not really consistent with a migrating hotspot as older volcanoes don't seem to be becoming less active. It is now proposed that East Africa consists of smaller sub plates and it is the twisting between these sub plates that has given rise to the islands. The clockwise rotation of Somalia relative to Lwandle creates a right-lateral shear zone which causes ripping and rifting along the sub plate boundary where the Comoros Islands are. A zone of broad deformation extends from the eastern boundary of the Rovuma microplate, across the Comoros Islands, and including parts of central and northern Madagascar.


This does not explain volcanic rock analysis from the islands which still suggest a deep mantle source for Magma, so we might expect some refinement of tectonics in the area over time.

2018 Earthquake Mystery

On 10 May 2018, an unprecedented long and intense seismic crisis started offshore, east of Mayotte, the easternmost of the Comoros volcanic islands. The population felt hundreds of events. Seismic activity began with a swarm of thousands of seemingly tectonic earthquakes then changed around June to a completely new form of earthquake signal which was so strong that it could be recorded up to a thousand kilometers away. These 20 to 30 minute long signals are characterized by particularly harmonic, low frequencies, similar to a large bell or a double bass, and are called Very Long Period (VLP) signals. 

French researchers have now identified the source of the puzzling activity as the birth of a submarine volcano some 31 miles off Mayotte's eastern shore. Sitting about two miles underwater, the new volcano stretches nearly half a mile high and extends up to three miles across.

The Volcano
Mount Karthala is an active volcano and the highest point of the Comoros Nation at 2,361 m  above sea level. It is the southernmost and larger of the two shield volcanoes forming Grande Comore island (the other volcano being La Grille volcano). The Karthala volcano is very active, having erupted more than 20 times since the 19th century and is considered the youngest volcano in the island group. Frequent eruptions have shaped the volcano's summit caldera with two well developed rift zones intersecting at the summit caldera complex.


  Mount Karthala features eight or more overlapping calderas, nested pit craters and several terraces which are aligned in a sort of diamond shape. The center of the caldera complex is occupied by the crater “Choungou Chahalé”, the older main crater, which consists now of two inner pits. Another crater is “Choungou Changouméni”, a small circular pit crater of 220 m diameter in the northern lobe of the caldera, next to a small scoria cone from 1972. Around my inner crater, fumaroles emit superheated steam with a sulphurous odour. Elongated rift zones extend to the NNW and SE from the summit. The lower SE rift zone forms the Massif du Badjini, a peninsula at the SE tip of the island. A sharp landslide scarps constitutes the northern limit of the Badjini massif at the SE end of Grande Comore. This landslide scarp is evidence of a large failure of the E flank of Mt. Karthala.  

Past Eruptions
Two strong eruptions in 1972 and 1977 did significant damage as lava flows reached the ocean. In 1977, the coastal village of Singani was partly destroyed by lava flows. In 1860, a lava flow even reached the coast close to the island capital where you will find a number of lava tubes. The 1991 eruption marks the beginning of a style change in Karthala’s behaviour. The eruption followed three months of increasing seismicity and inflation. After several hours of relative calm a sizeable phreatic explosion further enlarged Choungou Chahalé and lowered its floor. In a visit to the summit two weeks after the 11 July explosion people heard a “fountaining” sound, but no lava fountains or any other source were visible. It turned out later that this would have been the sound of the forceful arrival of water into the new crater, forming a crater lake for the first time. Scientists believe now, that this explosion had changed the hydrothermal system below the volcano, as all following eruptions were phreato-magmatic in nature.


Recent Activity
2005 eruption
  During April inhabitants heard a rumbling coming from the volcano and observed an ash column above the summit. The first ash-fall deposits began to form soon afterwards on the island's eastern side. According to the firsts reports, ash deposition increased and continued accompanied by a strong smell of sulfur. Observers saw a very dark plume spreading into a mushroom shape and accompanied by lightning flashes. Some inhabitants panicked and fled the island's eastern villages.  
 During the next few days ash continued to fall on the eastern part of the island and were heavy enough to require inhabitants to use umbrellas to get about. A pilot for Comoros Aviation, flew over the west flank and observed a large plume in the direction of the Chahalé crater and observed airborne molten ejecta. As the eruption continued authorities decided to evacuate some 30,000 residents from eastern villages. The following days saw ash falls on the island's western and northern parts, notably, on the country's capital city of Moroni and on the Hahaya airport. Lahars damaged roads and hundreds of houses, affecting thousands of inhabitants at the foot of Karthala volcano. Eruptions continued through to later in the year when investigators ascended Karthala and observed a molten-surfaced lava lake inside Chahalé crater. 


2006 activity
The eruption started during June with the first signs being smoke from the volcano. Rather than being explosive with ash emissions this eruption was more effusive with lave flowing down the side of the mountain. Within a few days the eruption stopped and lava flow stopped before it could reach coastal areas.

2007 Activity
  Scientists from the KVO reported that an eruption occurred during the evening of 12 January. A local news article noted Moroni residents observing jets of red flames above the summit. An ash plume was observed above the volcano and Aerial observations later in the month showed a lava lake forming within the crater.

2012  Activity 
Observers in the villages of Mde and Mkazi, on the W flank of Karthala, reported increased incandescence from the summit during the night of 9-10 May.

It is certainly a dangerous volcano, but what perhaps catches my attention is the risk of Lahars and Landslides. Maybe it is just the hotspot contention or the mysterious new volcano created under water near Mayotte. Certainly worth a bit more investigation and conversation.


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  • Location: on a canal , probably near Northampton...
  • Weather Preferences: extremes n snow
  • Location: on a canal , probably near Northampton...

Thanks for all these, fascinating look into how different volcanoes can be, not only from each other, but also different types of eruption in subsequent events from previously expected.

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