Thursday, 22 September 2011

Rift valley geology and soils

Many visitors to East Africa are looking forward to seeing the rift valley, but often aren't quite sure what they're seeing when they get there, especially here in Tanzania, where it's a remarkably complicated feature and doesn't show the typical east and west escarpments of a rift valley - only some obvious western edges. This means, of course, that here in Tanzania it's impossible to point out exactly when you enter the edge of the rift valley, which is a bit confusing and disappointing to some travelling from Arusha to, say, Manyara for the first time - you're definitely in the rift valley at Manyara, and the western escarpment is obvious - but when did you actually arrive there?! Now, I'm not a geologist and am not going to go into huge detail about the rift's formation here, just the general idea should do. But I am an ecologist, and the presence of the rift valley has huge consequences for the ecology of East Africa too, so I might go into more detail about that!
Rift valley scarpment above Lake Manyara

Firstly, what is the rift valley? Well, some very readable details are available here, if you want the full thing. In summary, it's a great series of cracks in the earth's crust that can be traced right from eastern Turkey, through the Middle East and down trhough Ethiopia, Kenya, Tanzania, Uganda, Rwanda, Burundi, etc., as far south as Mozambique. Here in East Africa there are two parts to it - the western, or Albertine Rift, than runs through Uganda, Rwanda and Burundi back to Tanzania, and the eastern rift, running through western Kenya and the middle of Tanzania. These cracks are around 20-30 million years old (oldest in Ethiopia) and are believed to form because there's a large plume or two of magma (molten rock) beneath the earth's crust that pushes up on the crust, creating large bulges (the Ethipian Highlands, and the Kenyan/Tanzanian higlands are both pushed up from below) and, in places on top of the bulge, cracking the crust and leaving a rift valley. Some times, of course, the magma has burst through as volcanos, with erruptions still fairly regular in various locations. The older volcanos associated with this (such as Monduli, and the Crater Highlands) date from 20-30 million years ago, though the major faults (big escarpments) are only about 2 million years old. It's a divergant fault, splitting the African contient in two and gradully moving even now - eventually it seems likely that this fault will split Africa in two - but I don't think we'll be seeing that for the next few years at least...
Oldonyo Lengai is spectacular from the air!

So, that's (very briefly) what it is. The important things from an ecological persepective are it's incredibly recent geological age - compared to most of Africa, these mountains and plains are new - even the oldest are only 20-30 million years old, and volcanic activity has probably been pretty much constant since then. Now, this age is important, because (as a first approximation), material recently thrown out of the earth is full of unusual chemicals that, over several million years, will be washed away. Consequently, soils derived from new rocks are usually nutrient rich, whilst older soils derived from older rocks have been washed clean (leached) and are generally rather nutrient poor. And as we know, nutrient availability is one of the big four processes driving savannah ecology. I couldn't find any very fine-scale pictures of this, but I've found a global map of nutrients available to plants in soils here that I've included below.

The important thing to note is that in general, soils in Africa are incredibly nutrient poor (yellow and orange on the map), but that there's a clear green bit associated with the rift valley. That's the consequence of volcanic activity in this part of the world. This large scale doesn't show too much, but focussing in on the underling rocks will give us some idea about nutrients too - so here's a map I've edited from here that shows the geology of East Africa.

In this map you can see the fault lines creating the escarpments nicely, but you can also see how the only areas with relatively recent rocks are those associated with the rift, from northern Tanzania up through Kenya, except for the Tana river area, where the recent rocks have other origins. Note especially that Serengeti/Mara only has recent bedrocks in the southern, short grass plains, and Tarangire only just gets into that new complex right in the northern tip. In both places these nutrient rich soils explain in part the migrations we see, with calving always happening on nutrient rich grasslands. It's also obvious why wildlife densities in the rift valley are so much higher than elsewhere in Tanzania - and indeed Africa. The scarcity of nutrient rich soils, and hence food availability, probably limits animal populations in many of these other areas, right down to South Africa. No doubt we'll come to this in more detail in the future, but for now, what's probably enough - as well as being a spectacular geological feature, the nutrient rich grasslands associated with the volcanic activity help explain both the number of animals up here, and their seasonal migrations. Geology matters!

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