In the previous part I wrote that we would be investigating some of the largest sub-aerial volcanism on the planet as we took a closer look at this the largest known sub-aerial volcanic system in the world. And now it is time to look at volcanism on a stupendous scale.
But before we do that I suggest that you read the previous parts in this series in order if you have missed them since I am writing them in a geographical timeline:
The Volcanic Dome of Tibesti
The bedrock under the Tibesti Mountains is 500 to 600 million years old and consists of an intrusive metamorphic base overlaid by sedimental overburden. As such it is an old and hard piece of continental crust. It is situated next to the West Saharan Craton, an even older and harder piece of continental crust.
The Tibesti Mountains covers 100 000 square kilometers, or roughly the same area as Iceland and we will spend time comparing these two volcanic centers.
The first difference is age and magma origin, Iceland is an approximately 14 million year old volcanic center based on a localized mantleplume originating from the surface burrowing down into the mantle to a depth of 270 and 540 kilometers. The Tibesti is instead a 40 million year old broad mantle upwelling originating from a layer between 80 and 150 kilometers deep.
Iceland is a part of the Mid Atlantic Rift and is as such a part of a continental divide that is fully formed whereas The Tibesti Mountains is part of a formative continental divide that yet has to succeed.
Due to the shallow and highly ductile crust under Iceland the crust can’t hold enough magma to create large eruptions, instead we see frequent small to medium sized eruptions ranging from 0.01 to 30 cubic kilometers per eruptions that occur between once every three to seven years.
The 120 kilometer thick and hard crust under The Tibesti Mountains can hold and constrain vast amounts of magma and this causes eruptions to be infrequent but very large in scale when they occur. Normal eruptions are thousands of years apart and ranges from medium sized to small trap formations, with sizes ranging from 1 cubic kilometers (to be seen as a minimum) and the maximum known eruption is in the 15 000 cubic kilometer range.
The ductile and thin crust under Iceland can’t constrain a large doming so the average uplift of Iceland is 50 meters whilst the doming at The Tibesti is about 700 meters. Now, ponder the amount of forces that is needed to lift an area that is 100 000 square kilometers and 120 kilometers thick an average of 700 meters, the mind shudders. In the end that much uplift and shear energy could not be contained and the crust started to rip apart.
The origin of the volcanism
As the mantle upwelling lifted the crust above fractures started to emerge that the magma could enter. Due to the extreme thickness of the crust this process took a long time and the magma melted first the intrusive metamorphic rock above it. And that rock contained a lot of metals and rare earth mineralization’s.
At this stage you had hot basalts being pushed upwards with enriched re-melted crust riding above it and then the slightly colder re-melt entered the water rich sedimental rock. The result was tremendously explosive as large scale kimberlite pipes and diatremes erupted at supersonic speeds and large ignimbrite sheets formed prior to onset of basaltic volcanism. This is the moment where the bulk of the ore bodies formed that we discussed in part 1.
Another way to think about the opening stage is as a VEI-8 sized industrial disaster spreading heavy metals and radiogenic material all over the local area and into the atmosphere. It is not something that I would like to see in my lifetime.
Six phases of volcanism
A while ago I was asked how you know if a mountain is a volcano or not and I jokingly answered that if you pick up a rock and you find Clive Oppenheimer happily grazing it is a volcano. I do not understand how he finds the time to write about every single volcano on Earth, but this time I am very happy that he grazed upon The Tibesti Mountains.
In a paper he wrote with Permenter they divide volcanism in the Tibesti Mountains into six phases. These phases are probably not that distinct, but they create a good enough timeline of the evolution of volcanism there.
- During this stage the uplift and extension of the crust is believed to have reached its maximum and large scale basaltic eruption centers formed in the central parts of the Tibesti Volcanic Complex. Volcanoes that formed during this period are Tarso Tamertiou, Tarso Tieroko, Tarso Toon, Tarso Yega and Ehi Yéy (volcanic plateau).
- Mainly at the Eastern TVC: Diatreme eruption and large ignimbrite sheet flows as the fissure that would form Emi Koussi opened up, initial formation of Emi Koussi via trap-forming basalt floods.
- Central TVC: Effusive and explosive eruptions at Tarso Yega and Tarso Tieroko as they went caldera and birth of Tarso Voon. Eastern TVC: Formation of the elongated fissure shield volcano part of Emi Koussi and formation of the large shield volcano of Emi Koussi.
- Western TVC: Formation of Tarso Toussidé and Tarso Tôh. Central TVC: Extensive tephra eruption as Tarso Voon goes caldera. Eastern TVC: Continued formation of the Emi Koussi fissure shield and the Emi Koussi central shield volcano, both at diminished speed of effusion.
- Western TVC: Tarso Toussidé becomes a nested caldera in a series of eruptions and birth of Ehi Sosso. Central TVC: Minor eruption at Ehi Mousgou. Eastern TVC: Formation of nested calderas on Emi Koussi.
- Western TVC: Formation of Pic Toussidé, Trou au Natron and Doon Kidimi. Eastern TVC: Pyroclastic flows down the flanks of Emi Koussi, intra-caldera eruptions and formation of vents and cones inside the nested caldera.
Here it is important to notice that all of the phase 6 volcanism has happened during the last 2 500 years as witnessed by the Toubou people. Even the name of Toussidé means “The Mountain that kills the Toubou”.
Even though that “minor” eruption of Pic Toussidé is far from the largest the resulting trachytic to trachyandesite lava flow covers an impressive 200 square kilometers and resulted in the collapse of the 8 kilometer wide and 1 kilometer deep Trou au Natron caldera that dissects the older 14 kilometer wide ignimbrite caldera of Yirrigue. It may be the only time in my life I will call a 45 cubic kilometer eruption “minor”, but at the TVC it is a run of the mill small eruption.
For those who enjoy spending time searching for lost eruptions during the Holocene in various ice records I think this eruption from Pic Toussidé may be a fruitful ground.
I am not going to write extensively about the active volcanoes in the Tibesti Volcanic Complex, instead I will just put it into context by a few not so well known records. The Emi Koussi complex is built up by more than 15 000 cubic kilometers of lava and it is both the largest fissure volcanic shield volcano on Earth and the world’s largest continental shield volcano. With fissure volcanic shield I mean a shield volcano that has formed along the entire length of volcanoes fissure swarm. And yes, it is the only known example of this type of volcano.
At Emi Koussi the Era Kohor natron filled crater is a powerful reminder that even though this brute of a volcano is not any longer at the peak of its power it is still fully able to produce notable eruptions. Era Kohor is an unusually large crater that is 3 kilometer wide and 350 meters deep that formed during a VEI-6 eruption. There are also similar sized Maars and numerous pyroclastic cones inside the two 15 kilometers wide nested calderas at the summit of Emi Koussi.
The range has numerous hydrothermal fields and the latest to form is located at Trou au Natron at Pic Toussidé that is nowadays happily percolating rich mineral deposits. The numerous and large calderas are topped by Tarso Yega that has a 19 by 20 kilometer caldera that is so large that you can fit the entire Katla Volcano inside the caldera.
As time went by volcanism has switched from extremely explosive over to highly effusive and back towards more explosive events. At the same time the size of the eruptions has diminished.
Even though the Tibesti Volcanic Complex suffers from smaller eruptions it is still active and the TVC will erupt sooner or later. Even though volcanism here has generally lost in size it is still amply capable of producing VEI-6 eruptions or effusive eruptions in the order of tens of cubic kilometers. As such it is a volcanic region in dire need of both studies and monitoring.
In the next installment about the forgotten volcanoes of Africa we will visit an even grander location of volcanism. And this time we will actually look at volcanoes that you yourself can go and visit as a tourist, time to go to one of my favourite parts of the Sahara!
Here is a selection of sources used for this article and series. As always I am thankful for the works of Haraldur Sigurdson for The Book Of All Things Volcanic.
Permenter & Oppenheimer: Volcanoes of the Tibesti massif (Chad, northern Africa)
Burke: Origin of the Cameroon line of volcano-capped swells. J Geol 109:349–362
Derouelle et al: The Cameroon Line; a review. In: Magmatism in extensional structural settings, the Phanerozoic African Plate
Furon: Geology of Africa
Gèze B, Hudeley H, Vincent P, Wacrenier P (1959) The volcanoes of the Tibesti (Sahara of Chad) (in French). Bull Volcanologique 22:135–188
Goodell PC (1992) Uranium potential of the Tibesti and Hoggar massifs, north–central Africa. Geol Libya 7:2627–2637
Vachette M (1964) Radiometric ages of crystalline formations of equatorial Africa (Gabon, Central African Republic, Chad, Middle Congo) (in French). Ann Fac Sci Univ Clermont 25:1–31