I readily admit to enjoying large scale volcanism. Yes, small scale volcanism like Tor Zawar may be a fun diversion, but it is the majestic scale that volcanism can take that get my juices flowing. By now I have touched upon most of the worlds large volcanic features at least in the passing, but there are still quite a few I have not touched upon yet.
Lake Turkana is one of those that I have pondered upon writing about for a long time. And there are two reasons for my interest, one is that it is caused by the largest mantleplume on earth and the second one is the effect that the volcanism has had on humanity during the Dawn of Man.
I would also like to point out that Lake Turkana is one of the most beautiful places I have had the honor of visiting. Going there is in so many ways to finally come home to where it all started.
Geological setting in brief
Most who are interested in volcanoes have read about the Great African Rift system that stretches from North of Afar down into Kenya where it divides into the East and West African Rift systems.
Lake Turkana is situated on the East African Rift and as with all parts of the Great African Rift system it is oldest to the North and youngest to the South. As such the volcanism will slowly continue to move southwards with time.
As the effects of the African super plume moved southwards a process of continental splitting started and it is this secondary effect that causes the continuing development of Lake Turkana. Technically the Lake Turkana volcanism is both spread-center volcanism and mantleplume volcanism; this explains the large volumes that occasionally are involved. Especially one eruptive sequence has been very large, The Turkana-Omo Basin eruption that lasted from 4.18 to 3.99 million years ago.
The Turkana Basin that holds Lake Turkana is a Graben of unusual size. At the northern end it is 320 kilometers wide and at the southern end it is 170 kilometers wide evidencing that the spread is continuous from north to south and that we may expect it to continue both in extension and in distance.
The original bedrock is dated to around 510 to 522 million years old. What is interesting here is that we can date the onset of spreading very exactly, it started with the Nabwal Hills eruption 34.8 million years ago in the northern end. That gives a very precise rate of the spreading at 0.92 centimeters per year. The rate of spread is higher as you get towards the northern parts of the Turkana depression compared to the southern.
The Dawn of Man
It may not come as a surprise that it is not until after the great Turkana-Omo eruption that what would in the end become Homo sapiens moved into the budding valley. And back then it apparently was as close to the Garden of Eve as is possible as evidenced by the large amount of hominid skeletons that have been found there.
During the next 4 million years the area was lush and it was open enough to induce developments and refinements to our upright gait. It is here that the our ancestors truly became bipedal, so much so that Homo ergaster was better constructed at walking upright than we are.
It is also here that we developed proto-speech and the Broccas Center in our brains. One theory is that close-living groups of more arboreal hominids did not need to convey messages over distance, but that it was a good thing out on a lush grass savanna where people was modestly separated from each other during hunting and gathering. It is also here that the human brain became larger in volume and we learned to control our opposable thumbs enough to manufacture complexly manufactured tools like stone-axes and stone knives.
It is here that the most famous skeleton of all was found by Kamoya Kimou while working for the Leakey’s. I am of course talking about the 1.6 million year old skeleton of the Turkana Boy. But, there is also a pretty much unbroken record of various hominids ranging for all of the 4 million years that the place has been inhabited.
I have so far mentioned that the area was a lush and green garden of Eve, this did though take a dramatic and tragic turn 10 000 years ago. At this time the Lake Turkana was at its peak, 3 or more rivers flowed in to the area and a great river flowed out of the lake, it was also the most densely populated place on earth. Then utter and almost total catastrophe struck.
It was though not volcanism that killed all the people, instead volcanism had made the ground some of the most fertile on earth concentrating humanity there. No, it was the end of the ice age causing a massive change in climate. The amount of rain in the catchment area diminished and the level of the lake fell more than 75 meters cutting off completely the outflow from the lake. And since the lake is volcanic with numerous fumaroles the water rapidly became saline.
And it is here that humanity turned from being a whimsical and slightly comic side-note in the order of things. It is at that exact moment we invented organized warfare. As harvests failed and the animals to hunt died off the humans banded together to kill each other over ever scarcer sources of food until a very small number remained, all of which is evidenced by numerous skeletons bearing signs of being killed in a very short time span. It is a sobering thought that the first known war was caused by scarcity and lack of water and food instead of the search of power and wealth. It is mainly sobering because at the beginning of the history of war we also see the likely end to the history of war, since we are likely to sooner or later start a great war for food and water. Know your history to know your future.
Today the area is populated even though the lake has lost its former glory. Today it is the world’s fourth largest saline lake and the area is dreadfully inhospitable. But, nature is resilient and fish and crustaceans have rapidly evolved from fresh water varieties into saltwater varieties that are surprisingly abundant. The area is very hot and highly arid and it is classified as a desert.
Volcanism at present Lake Turkana
Thanks to the Leakey’s the area is surprisingly well investigated from a volcanological standpoint. In some instances it is even at apex of research. I will write below about the 4 currently active volcanic centers at Lake Turkana, most of them are single volcanoes, but in the southern end we find a real candy. Remember that the volcanism is spreading from north to south, so it is not surprising that the fireworks are down south. To the north there are several long dormant or dead volcanoes, but the first active volcano is to be found slightly to the north of the lake center.
Welcome to North Island situated at the northern central end of Lake Turkana. It is constructed mainly out of tuffs ejected from a series of tuff cones constructed out of trachyandesite. On the island there are 3 young lava flows of undetermined Holocene age. Two of them are likely to have been erupted at the same time, one from the dominant tuff cone on the island and the other from a flank vent of the cone. There is also a younger lobate lava flow extending to the coast of the island. We know that these are young since they have not been covered by sand from the frequent sandstorms, but we do see a marked difference between the two slightly older flows and the most recent that is still jet black. The lava flows are made out of basalts.
The island is still active with fumarolic activity to the north and southwest and there is no reason to exclude future eruptions.
The English are if nothing else highly consequent with their naming practices so our next stop is Central Island. Well, at least almost constant. The island is also aptly known as Crocodile Island from the abundant Saltwater Nile Crocodiles that galumph around oblivious to the fact that they should be freshwater crocodiles and that since their species is so old they should not be able to become a subspecies in such a short time. Well, just to confound everyone they did it, and here is probably a clue to how crocodiles survived the test of time, rapid adaptability.
Central Island is mostly explosive during eruptions, and there is evidence that the island is affected by water/magma interaction. The island is littered with explosive craters and the largest crater is larger than 1 kilometer across.
The island is a smorgasbord of varying lavas ranging from basic basalt all the way up trachydacite. That does not matter here, if it comes up it will do so explosively due to the magma/water and lava/water interaction.
The island has erupted many times during the Holocene, but no eruption has directly been witnessed in historical times. In the thirties the island was very actively producing steam and sulphurous gases and in 1974 it did a version of Kawah Ijen as it erupted molten sulphur.
The largest risk at Central Island is a medium sized explosive eruption, or that yet another kilometer wide Maar will detonate into existence. Not a happy prospect for the shore-dwelling local fishermen.
As we reach the Southern Island (Hohnel Island) the morphology of the volcanism has changed dramatically into an 11km long fissure volcano that is rife with young lava flows that has effused along the entire length of the fissure.
The lavas here are mainly unevolved basalt, but with some highly evolved inclusions or explosively erupted magmas such as phono-tephrite/tephra-phonolite and trachyandesite. It is most likely a sign that even here the volcanism has started to evolve away from basic basalt riftvolcanism towards a more explosive existence like its northern brethren.
The last dated eruption occurred in 1888 and it was a rifting fissure eruption quite like what we can see in Iceland. With that we leave the islands and get ready for some volcanic candy.
The Barrier Volcanic Complex
The ancestral Barrier Volcano is a large basalt shield that is heavily eroded. This ancestral volcano suffered a subsidence caldera formation in a large eruption 92 000 years ago. It is unclear if the northern flank of the volcano suffered a fatal collapse into Lake Turkana at this point or if this happened later. The summit caldera is circular with a distinct horseshoe opening towards the lake and it is 3.8 kilometers across.
Volcanism in The Barrier volcano has continued up until recently infilling most of the caldera floor and causing larger lava flows that has reached all the way down to Lake Turkana. The Barrier has had intra caldera eruptions during the Holocene, and some eruptions may have occurred within the last 2000 years.
What makes The Barrier into a real volcanologic gem is that it turned itself into a Somma volcano of sorts with distal flank volcanoes, something that is not common for large shield volcanoes and that may indicate that in the future a second larger caldera may form. It is though good to remember that this is not a true somma volcano, instead it is a basaltic shield caldera with both distal and radial fissures. In some ways it may be more correct to look towards a volcanic complex such as Grimsvötn for comparison instead of the classic image of somma-volcanism.
Now let us turn our eyes to the other volcanoes in the complex since they have stood for most the action in the last couple of thousand years.
After the 7710 Abili Agituk eruption from The Barriers northern part of the caldera and the southern flanks a range of subsidiary volcanoes has erupted on the distal and radial fissures.
On the northern side The Teleki Volcano grew down at the edge of Lake Turkana forming both monogenetic and polygenetic cones and vents. As Teleki erupted it created new land extending out into Lake Turkana causing the eruptions to be more explosive than the effusive volcanism you otherwise find at The Barrier. The last known eruption from Teleki occurred in 1921 and as such it was the latest eruption at The Barrier Volcanic Complex.
Next to Teleki we find Likaiu with one known eruption in the Holocene. It is a polygenetic cone of tephra and it last erupted in 1897.
One of the most active volcanoes around The Barrier is Andrew’s Volcano (also known as Nagaramasainia). It is a mainly basaltic volcano prone to suffer both from top vent and flank vent eruptions. Its last confirmed eruption occurred in 1917.
Volcanism at The Barrier Volcanic Complex may be cyclical in nature, but one must realize that the eruptive record is too short to ascertain that with any high degree of certainty. The first known eruptive cycle occurred around 1030AD to 1070AD and the next round of eruptions occurred between 1871 and 1921. This would spuriously put the cyclical rate to 800 years or so.
It is a bit problematic to talk about cycles at a place where you could easily loose an eruption or two. But, if we look towards Iceland where we also have spreading center volcanism combined with mantleplume activity we also see distinct cyclical patterns emerging there, so it would after all not be farfetched to believe that the same could be happening at Lake Turkana and The Barrier.
Turkana, volcanism and dating
In volcanology we tend to look towards ice cores for everything, but there is another very good way to date things and that is from lake sediments.
For a long time it was believed that the VEI-8 eruption of Toba almost wiped out humanity. The first “evidence” of this was that the ash should have produced a long term and extensive climate change due to ash hanging around in the upper atmosphere. The second evidence turned almost ludicrous as they took the inbred genome of Europeans and determined that very few people (around 25) had survived on all of earth.
For some reason they forgot to mention the vast genome variations that exist in Africa. After all, it was not a million man strong exodus that meandered out of Africa to colonize the planet, it was just a couple of hundred and they most likely came from the same tribe. So, the genome was limited to begin with.
That wandering band of humans had reached quite near Toba as it blasted off about 71 000 years ago (Lake Malawi sediment core), so the toll on them was quite naturally high. That notwithstanding there is evidence that they survived the ordeal surprisingly well as evidenced in the archaeological records directly on top of the Toba ash in Southeast Asia.
Now I hear people who are not readers of Volcanocafé start mumbling: “Hey, it was a supereruption, all that ash must have killed everyone! The Daily Fail says so!” Not really, first of all we must realize that 90 percent of the ash will come out as pyroclastic basesurges that will inundate everything within a circle of 100 to 250 kilometers. This is an absolute death zone. Nothing will survive here. Another 9 percent will fall down as distal ashes and only 1 percent will be injected so far up into the atmosphere that it will linger. Question is how long will it linger? This amount will halve every 3 months, so within a year the effect will be pretty much gone.
Aren’t volcanoes gassy? Yes they are, but the same goes for the sulphates that the volcano produces, so after about two years even the largest eruptions have had their sulphates washed out sufficiently to not be a problem.
Now you are probably wondering why I am yabbering about Lake Toba when I am writing about Lake Turkana?
There are two reasons for this. One is that the place has been inundated by paleontologists looking for skeletons for the last 50 years, so we have a very good skeleton-history of Lake Turkana vis-à-vis human mass extinctions. And one thing that is abundantly clear is that there was no mass dying there during the Toba eruption. As many skeletons was produced before as there was after, thus proving that the procreating humans needed for the skeleton production had not died out or gone through an existential “eye of the needle” moment.
The other reason is of course that Lake Turkana (and Lake Malawi) is filled with wonderful deep undisturbed lake sediments. So, scientists took core samples and looked for all that doomsday ash that supposedly should be there. Obviously they did find Toba ash, but the amount is more indicative of a temporary cooling for a year or two and not widespread famine. Today the volcanic winter theory is rather soundly disproven and the previous given figures of 3.5 to 4 degrees Celsius temperature drop has now been moderated to around 2C for around two years time.
So, what then happened to all that Toba Ash? Well, Toba is on an island so all the ash ended up unceremoniously out in the Southeast Asian sea. Both the pyroclastic base surges and the distal ash. The latter was evidently blown out by the monsoon into the sea where.
Grand scale volcanism and geology does not always need to come attached with the prefix of super- to be interesting. Few places have such a rich coexistence between humans and volcanoes as Lake Turkana. In many ways all of us have this remote spot on earth to thank for us becoming us. It is here we developed the ability to walk straight, talk less straight and beat ourselves to death. It is a microcosm of human prehistory in the shadow of the volcanoes that formed humanity.