Africa in general is a place that is easy to fall in love with, in particular Tanzania is easy to become deeply enamoured with. Sometimes I think that there is a deep DNA-memory remnant in all of us that is telling us that this is our ancestral home.
When most people think about Tanzania, they think about the great national parks like Serengeti, Ngorongoro and so on, or perhaps Zanzibar Island and the allures of Stonetown. Some spend a day or two in the mega-city of Dar es Salaam, but few get to see the rest of the country. And it is a big rest indeed.
I have had the pleasure to travel there on 5 business trips in the last 7 months, due to time constraints I did not have the time to go to any of the obvious places that everyone goes to. More than a decade ago I got to spend a night in the Ngorongoro, but the company I was with was more interested in BBQing steaks on our campfire and imbibe on Konyagi than looking around. And we came late in the evening and flew onwards early morning with our post Konyagi pounding heads, so regrettably no chance to see that much.
If you go to Tanzania you must try out Konyagi, it is a local slightly sweet spiced spirit. It is not deadly strong at 32% (that would be 64-proof for the imperially confused). It goes down well mixed with pretty much anything, but it is especially good with Red Bull.
I was warned by the documentary film maker Michael Dalton-Smith that it would make me see blue baboon arses if I had too much. Something I can attest to after a night drinking with friends on the beach in Mtwara City, the full moon did indeed turn into a blue baboon arse.
Perhaps this is a good moment to move over to the volcanoes of Tanzania. After all, this is not a site for the scientific study of the Lost Blue Baboon Arse.
Up until a few years ago mantle plumes were a hotly (pun intended) contested subject. The reason for this was that the entire theory was based on the Hawaiian mantle plume and the volcanism caused by it.
This in turn led to problems as soon as you tried to use the mantle plume theory on other more diverse plumes, like for instance the Icelandic mantle plume, of the African plume derived volcanism.
In the end though it turned out that the initial plume theory was too simplistic and only fit a minor set of plume instances, and the theory was expanded to incorporate several types of mantle plumes.
And as science started to look for evidence for the theory, it came in leaps and bounds. In 2015 a research team used data from 273 large earthquakes and all of a sudden, we had tomographic pictures of two very large mantle plumes extending from the outer core boundary up to the surface.
One was in the pacific causing the Hawaiian volcanism, and the other far larger was under Africa. The data at this time was constrained by computational capacity (no high frequency data could be crunched), and the limiting factor of there being almost no seismic networks on the ocean floor leaving out roughly 70 percent of the surface of the globe.
Later more data was churned on more powerful computers using a larger earthquake data set, and smaller plumes turned up around the globe, either connecting to the core boundary, or starting mid-mantle.
So, what is a mantle plume? And how does a plume work? Well, here is where we run into a lot of “we do not know’s”.
The initial theory worked on the assumption that there was heat convection transferring heat from the core boundary upwards, and that this heat increase caused increased melt at shallow depth causing an increased volcanism above.
But what about matter convection, could not a plume theoretically move matter from depth upwards? The reason this question was asked was due to some rather annoying spots in the crust associated with plume tracks where you find inclusions of minerals that just should not be there, and that can’t be explained readily by metamorphosis of rocks in the crust.
As time went by the argument between the heat-conversionists and the matter-conversionists got a bit heated (yes, pun intended), since the heat conversionists demanded proof. The only thing we the materialists had going for us were the oddities, and the known speed of possible mantle material flow (2.5cm per year).
Let us now recapitulate, we had circumstantial evidence for mantle plumes in the form of tomographic pictures making plumes into a probably correct theory, but there was no direct evidence for mantle plume matter convection.
This changed on a particularly sunny day in Iceland. The answer had been around since 2002, but nobody had really understood what Kresten Breddam had done in the paper ‘Kistufell: Primitive Melt from the Icelandic Mantle Plume’.
Problem was that this was a paper in geochemistry about an unusually boring Icelandic volcano, and the paper turned into a dust-collector in the bottom drawers of science. Until it was pulled out during the weeks leading up to the Holuhraun eruption (by me).
Another reason this paper flew under the radar is that Kresten is not a volcanologist, instead Kresten works as a nuclear research scientist at the Danish Public Health Board (Sundhetsstyrelsen). Inter-disciplinary blindness is a bitch.
Kistufell is sitting at the centre of the Icelandic mantle plume, and in one of her samples Kresten found a crystal that had formed at 470 kilometres depth before being conveyed up to the surface. If there was a Nobel price in geophysics Kresten would by now have met the Swedish King.
Anyway, in my rambling ways I have now set up things for us to get acquainted with the African plume, or as it is called, The African Super-plume.
The African Super-plume
It got its super-moniker from its size and capacity. Even if you tally up the heat-convection capacity of all other plumes on Earth they do not come up to the heat transfer from the core boundary to the surface of the African Super-plume.
We do know that it is a fairly young plume, it roared into view roughly 30 million years ago as it caused a series of VEI-8 eruptions in Yemen, Saudi Arabia, Ethiopia and Eritrea. At the same time, it caused a number of flood basalts in the region, and it also started its career as a continent whacker as it separated Arabia from Africa by creating the Red Sea.
As Africa continued to move to the north it started to create the normal plume track, but it was just to large, and instead the plume divided into two distinct plume heads. The first one preferred the point of least resistance and continued to erupt through the Afar region. Lazy bugger!
The other one did something a bit more unexpected, it raced like the proverbial blowtorch out of hell south across the African continent cutting it apart into two continents. The part that is hard to explain is that the southern plume head moved at twice the speed of the continent.
As it moved south it created the Great Rift Valley in its wake, home of all of us. In a sense of it, we are all children of star stuff being birthed out of volcanoes in Africa.
In the end this happy giant blowtorch ran into a worthy opponent in the form of one of the densest, deepest and coldest parts of earths crust, The Dodoman Craton in Tanzania.
It turned into the true Rumble in the Jungle between two super-heavyweight champions. The ensuing blows has created among the largest volcanoes on the planet erupting incredibly weird lavas, a volcanic field containing hundreds of calderas ranging in eruptive size from VEI-6 to VEI-8, trap-formations, and even more continent busting.
In the beginning of this war the craton came up on top. It was just to deep, cold and hard to be greatly affected even by the jolly giant blowtorch. The plume head was though so large and powerful that it started to envelope the entire craton opening up battlefronts around it. One front marched relentlessly to the south-west, and the other one went towards the east and the ocean, pining to create an African Fiord.
The path towards the Indian Ocean has so far been tougher going since it too is quite thick and hard, but it will in the end succumb to the force and East Africa will be a continent separate from the rest of Africa.
And if that would fail, the Tanzanian plume head will just continue down the far weaker western side of the Dodoman Craton. The difference will just be that the Craton will then remain as a part of the future East African continent.
In the end though this fight is taking on even more of the characteristics of the Rumble in the Jungle. For those who are not familiar with the Rumble in the Jungle, it was the greatest of all boxing matches and involved George Foreman and Muhammed Ali dunking it out in the Democratic Republic of Congo.
Like Foreman the Dodoman Craton had the Tanzanian Plume against the ropes for tens of millions of years while it delivered endless series of death blows to it, but like Foreman against Ali, it had in the end expended its energy, and like Ali the plume started to hit back with blow after blow against the head. The difference is that this time there is no crowd shouting “Ali, Boma Ye!”.
The counter attacks started inside the edges of the cratons as volcanoes like Ol Doinyo Lengai erupted carbonatite lavas. No, Ol Doinyo Lengai is not unique, there are several dormant volcanoes sharing this feature in Tanzania, but I will get back to this.
And about 2 million years ago volcanism started to occur at the centre of the Dodoman Craton. A couple of weeks ago I visited Dodoma, the new capital of Tanzania. As the plane landed, I saw odd looking hill formations, and to my joy one of those was right behind my hotel.
As I ambled over to the hill, I discovered that it was a weather-worn rhyodacite dome roughly two million years old. This shocked me a bit since I did not know that there was mid-craton volcanism. After that I found something even more bizarre, but that is best left for another instalment.
Next article will be a brief list of the volcanoes in Tanzania as a reference for future more in depth articles about individual volcanoes.
To draw the analogy of the Rumble in the Jungle even one step further, in this match there is no referee. The level of monitoring is akin to being non-existent. Some might say that there has been comparatively little volcanic activity in Tanzania, but this is flat out wrong.
Roughly every ten years something will erupt, most often in the benign form of a bit of carbonatite ash from Ol Doinyo Lengai, but now and then something more dangerous will erupt.
Leaving volcanism on this scale un-checked and largely un-monitored is fool hardy. This is literally the only spot on the planet where a civilization decimating eruption could be building up and we would know nothing about it.
Obviously, there is no money in Tanzania to create a national network and a volcanic observatory, but something needs to be done. Question is just what?