Unless you are more interested in volcanoes than is technically healthy, it is likely that you have missed out a lot on the classic Monty Python skit ‘The twin peaks of Kilimanjaro’. Most people laugh at it since they believe that there is only one peak, but for us with an un-checked interest in volcanoes, we know that there are three peaks at Kilimanjaro.
It is actually even more complicated than that. Kilimanjaro is a mountain chain, and not a single mountain. Time to take a look at the individual mountains at bit closer, a lot closer in fact.
The volcanic mountain of Shira is 2.5 million years old. It is normally said that Kilimanjaro is a stratovolcano, but Shira is more related to shield-volcanism than stratovolcanism. The last major basaltic phase was 1.9 million years ago, with minor more silicic activity continuing for another 200 000 years.
It has a large flat plateau at the top believed to be an infilled caldera, not uncommon for large basaltic shield volcanoes. Erosion had set in prior to the formation of the caldera, so it is likely that it formed due to shrinkage in a large shallow magma chamber as the magma cooled down and solidified.
Shira is slightly taller than 3 800 metres with the peak being a doleritic extrusion at 4 005 metres in the middle of the caldera that could be the remnant of the last silicic eruption. It is believed that the volcano was 1 200 metres higher prior to caldera formation, but a more reasonable height curve for a basic volcanic shield would imply 4 500 metres as the peak height.
Today Shira is the carcass of times past, and it is completely dead as volcanoes go.
The 5 149 metres high Mawenzi came into existence after a 700 000-year long period without volcanism. 1 million years ago eruptions started, and this time it was more silicic products that came out from the start, and a classic stratovolcano was constructed.
Today the volcano is heavily weathered with an exposed intense dyke swarm and exposed internal ring faults. The original peak has been worn down and horse-shoe shaped caldera-like structure has formed, but not through volcanic activity.
The youngest volcanic rock seems to indicate that the volcano stopped erupting 448 000 years ago.
Kibo is what most people associate with being Kilimanjaro. The last reliable height measurement put it at 5 899 metres in 2014. But this is a variable and not a constant. I will be getting back to that below.
The top has a 2.5 by 3.5-kilometre small caldera with an intra-caldera cone present. The actual peak is named Uhuru Peak and is on the caldera wall. Inside of the caldera is a fresh-looking low cone with a pristine looking crater that is kept ice free from intense geothermal activity.
Kibo is at best lightly weathered, and to some extent almost pristine, and that is problematic. Even highly problematic, and here is why. The youngest rocks at Kibo are K-Ar dated to be 170 000 years old. And that is odd for a volcano that has been subject to a long-time exposure to weather, wind and glacial erosion.
And Kibo is quite tall enough to have been constantly eroded by glaciers throughout the last glacial period throughout to this day. In fact, it is only in the last couple of decades that the glaciers have diminished to make it relatively simple to climb the mountain. Simple being quite up to your stamina and the weather.
On top of that, few stratovolcanoes can go without erupting for 170 000 years and still have geothermal fields powerful enough to overcome glacial build up inside a crater.
I will return to this oddity below.
The official word is that after activity waned at the summit caldera, a chain of more than 250 parasitic cones formed on the northeast and southwest flanks, this indicates that the entire volcano is formed on top of a NE/SW trending fissure swarm.
The fissure swarm is quite sizeable, reaching to the northeast all the way to the Lengurumani Plain, and to the southwest to Lake Chala and Taweta.
The fissure swarm cones are according to conventional wisdom 200 000 to 150 000 years old. But it is at the boarder between Kenya and Tanzania that things take a turn toward being a Monty Python skit on its own.
Lake Chala is stated as being a crater lake on a 150 000-year-old monogenetic parasitic cone. If true it is both remarkably large with a surface area varying between 4.2 and 4.5 square kilometres depending on the season.
It is also known for its happy tourist eating Nile-crocodiles. The Kenyan police calls it “Heavily crocodile infested” and recommend tourists to not offer themselves as a late-night snack. Something that tourists tend to do anyways.
Problem seems to be the monogenetic part. Locals usually try to point out to any volcanologist that ambles by that it erupted 200 years ago, and that a village containing 2 000 people was wiped out in that eruption.
A couple of decades ago a volcanologist did indeed amble out there, looked at the typical Tanzanian greenery and said, I do not see it, and wrote it off. Same with a couple of itinerant archaeologists that could not find the village.
Problem here is that volcanologists are not particularly familiar with green things, so they believe that volcanic residue will be pristine at a lowland lake, after all it would be that in Iceland. And the archaeologists are familiar with green things, but not heavy ash layers covering a village (unless it is called Pompeii). If both had been there at the same time the result might have been different.
Understandably the locals are quite miffed at this. A couple of friends from the area in unusually strong tones explained that they would be happy to take me to the place of the former village and dig up samples for me as I happened to utter the conventional wisdom.
I plan to take them up on their offer since I am quite convinced that since everyone from the area states the same thing, something rather momentous has happened. It will also give me an opportunity to sit in the middle of the jungle imbibing the brilliant local booze called Konyagi while dodging galumphing crocodiles.
Many age-related problems
The disparity between the pristine summit and the dating must be solved. I think that the problem is that the samples taken has missed the younger eruptions due to the ashes having been carried away by glacial ice movement.
Anything that lands on a mountain top glacier tends to end up quite a bit further down the sides. There is though a spot where there should be young ashes and lavas, and that is at the bottom of the crater in the middle of the caldera.
A few years ago, I would have gone up myself and taken a sample at the bottom of the crater but having become fat and old prohibits this. I would honestly need two years of exercise before even attempting that feet. It is driving me bonkers that I am no longer fit and able to run up a mountain any longer.
Thankfully I know an irritatingly fit mountain climber named Paul Schreilechtner who is going to climb Kibo during the Christmas period. He has promised to grab samples for me after I explained the problem, if the weather and conditions permit. I will then hand them over in turn for laboratory testing.
Above I mentioned that the current height is 5 899 metres, but that has changed even during the time of accurate GPS-measurements. And quite a bit to boot.
In 1999 the height was 5 892 metres, it then increased over the next 9 years to 5 902 metres in 2008, before dropping 3 metres in the next six years. Since all of these measurements are accurate to within a few centimetres something must be afoot.
This is where Kibo solidly ends up as a dormant volcano, and not even that dormant, since this means that magma is intruding at depth before it moves out (probably into the fissure swarms).
What I have written above is to be seen as an “ahem”, and not as a statement that all that is known is wrong. Instead I am stating that we need new samples from the top that is from the sweet spot, and that someone should get off their lazy arse and go and dig at Lake Chala (me).
The game is afoot!