You might be familiar with Pangaea: the supercontinent composed of most of the continents we know today when they were crammed up together and the rest of the world was a continuous huge ocean. Pangaea still existed in the early Mesozoic Era (the dinosaur one), when it began breaking apart into the continents we know today. But have you ever wondered HOW we got to have freshwater lakes in the most inland places after the separation of Pangaea? Was it all just “trapped ocean water”?
Sure, we have rain and other forms of precipitation, but this is not always enough to carve up a basin and fill it up with fresh water. So how are those basins formed and filled? Let’s follow the natural history of some of the most famous lakes in the world: the African Rift Valley lakes.
I’ve explained the cauldron (caldera) lakes before, and this time, within the Lake Types Series, I will introduce you to a new type: tectonic lakes.
You might recall from school sciences some of the main tectonic movements – or maybe not, so let’s quickly refresh. Tectonic plates can rub against each other (transform movement), move towards each other (convergent) or move in opposite directions (divergent).
The most common type of lakes formed by tectonic movements are the ones located in the so-called “Rift Valleys”. These rifts are formed due to the divergent movement (or rift) of two plates. This forms a depression in between them. These depressions usually fall so deep that the water table is very close to the surface, so underground water can play an important role in these basins. They can then be filled with fresh water as it floods the valleys through precipitation.
Most tectonic lakes existing today are amongst the oldest and deepest in the world, such as Lake Baikal in Siberia (the oldest and biggest with 25 million years (Ma) and over 1,600 m deep!).
One of the most famous continental rift valleys is the East African Rift Valley, where we can find more than 30 tectonic lakes, such as Lake Naivasha. This rift is “opening” at a rate of about 3 mm per year! The Western Branch of the Rift, the Albertine Rift, contains the second largest lake in the world: Lake Tanganyika (1,470 m deep), which in some parts even goes below sea level.
These lakes hoard amazing biodiversity and very high endemism. This is mainly because whatever was there during their formation was afterward geographically isolated for approximately 40 Million years already. Because of this, they are an awesome living laboratory for ecological and evolutionary processes.
What’s interesting here as well is that there are not only freshwater lakes, but also “salty” (“soda”) lakes.
“Saltwater” lakes, more properly referred to as brackish water, are usually in endorheic basins (meaning they are enclosed, with no outlet). Inflowing rivers carry with them materials that they erode during their journey before reaching the lake. In arid zones, where evaporation exceeds precipitation, the constant evaporation of water leaves behind all the salts that the inflowing rivers brought with them. Since we’re talking now about lakes without a river flowing out of them (or any other type of outlet), these salts remain in the lake, thus increasing its salinity and conductivity. These two parameters will vary throughout the dry and rainy seasons in this region.
This also explains why we have freshwater lakes.
You are probably used to the idea of a river going into a lake and another river flowing out of it. The latter carries on the salts and other dissolved components out into the next basin or finally into the ocean (this is also why the ocean is so salty!).
In February 2017 I had the opportunity to do an exchange trimester in Kenya.
I will tell you now about a really nice example where I had the opportunity to do some sampling at and hope this gives you a better image of how these lakes work.
Lake Naivasha in Nakuru, Kenya, is one of the freshwater lakes in the East African Rift valley formed by tectonism. There is no apparent outlet going out of it, so it’s likely a saltwater (brackish) lake. Right? Well, its chemical characteristics are fully freshwater. So what’s going on here?
By now it is known that the lake has an underground outlet! The water flows out of the lake through the underground, carrying with it other inorganic material (e.g. salts). On the other side of it, water found another smaller basin where we can find a brackish lake: lake Oloidien (pictured below). The smaller size of this basin, the lack of an outlet, and the high aridity of the region give lake Oloidien the proper conditions for it to be a salty-lake.
To give you a quantitative idea, according to my field notebook from those incredible days in Africa, the salinity in Lake Naivasha was of 0.13 g/L and for lake Oloidien of 1.5 g/L. The conductivity was 257 uS/cm for Naivasha and 2530 uS/cm for Oloidien.
There are other types of tectonic lakes that are not formed by plates diverging
I could not possibly finish this blog post without giving you some examples from Guatemala. In Guatemala, we actually have a complex set of tectonic lakes that are not that directly related to divergent movements. We do have our biggest lake, Lake Izabal, originating from the divergence of the North-American and Caribbean plates, but let’s talk about a different tectonic process that occurred in the lowlands of northern Guatemala.
The Petén region of Guatemala is swamped with lakes of all shapes and sizes (pun intended). In fact, some archeologists sustain the hypothesis that during the early years of its inhabitation by the Maya civilization the region was still a huge wetland complex. Even after all the human impacts in the region for the past 5000 years, we still have a lot of different types of lakes in this region surrounded by a beautiful -and endangered- tropical rainforest.
So if it was not tectonic plates diverging, how did this happen?
Back in Pangaea, part of Central America had already formed and around 200 Ma these pieces of continental crust started moving to their current position. About 75 Ma the continental platform containing what is now Northern Guatemala had completely “drowned”. It was completely underwater forming sections of shallow seas. Between 35 and 12 Ma, this now-continental crust was being uplifted. The upward movement of this big chunk of land basically caused some cracks in the terrain, creating the basins necessary for lake formation. This explains the long shape of these lakes. As the continental crust kept rising above sea level, these basins were isolated, leaving a beautiful, almost linear trace of over 30 lakes that you can go check out today.
(If you want more details on the formation of Central America go check out my story highlights on Instagram!)
One of them is lake Petén Itzá. Lake Petén Itzá is the deepest and largest one in the Guatemalan lowlands (about 165 m deep). Besides the archeological interest of Petén, which contains a lot of Maya cities and the largest rainforest in Mesoamerica, the Lake Petén Itzá also provides an interesting setting for paleo-climatological studies.
How come?
During the last glacial period, as things got extremely cold in temperate regions, the climate in the Neotropics was getting arid. Evaporation in the Petén lakes exceeded the precipitation of that time, and everything fell dry. Everything, except Lake Petén Itzá. Scientists realized this until 1999 after a bathymetric survey of the lake was finally done, which pointed paleo-climatologists’ interest to this region of the world.
In lay terms, this is important because, although the lake’s volume greatly reduced, some deeper sections remained covered with water. This provided better conditions than dry areas for good preservation of organic material and some inorganic chemical forms that can be now traced back to have a better idea of how climate, forest, and human activity have changed over long periods of time.
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References
- Ring, U. 2014. The East African Rift System. Austrian Journal of Earth Sciences. 1(107):132-146.
- Tiercelin, J.J. & K.E. Lezzar. 2002. A 300 Million Years History of Rift Lakes in Central and East Africa: An Updated Broad Review. In: Odada, E.O., Olago, D.O. (eds) The East African Great Lakes: Limnology, Palaeolimnology and Biodiversity. Advances in Global Change Research, vol 12. Springer, Dordrecht.
- Mueller, A.D., G.A. Islebe, F.S. Anselmetti, D. Ariztegui, M. Brenner, D.A. Hodell, I. Hajdas, Y. Hamann, G.H. Haug, & D. J. Kennett. 2010. Recovery of the Forest Ecosystem in the Tropical Lowlands of Northern Guatemala after distintegration of Classic Maya Polities. Geology 6(38):523-526.
- Hodell, D., F. Anselmetti, M. Brenner, D. Ariztegui & the PISDP Scientific Party. 2006. The Lake Petén Itzá Scientific Drilling Project. Scientific Drilling. 3: 25-29.
- Pérez, L., R. Bugja, J. Massaferro, P. Steeb, R. van Geldern, P. Frenzel, M. Brenner, B. Scharf, & A. Schwalb. 2010. Post-Columbian Environmental History of Lago Petén Itzá, Guatemala. Revista Mexicana de Ciencias Geológicas 3(27):490-507.
- Dodds, W. K. & Whiles, M.R. 2010. Freshwater Ecology: Concepts and Environmental Applications of Limnology. 2nd ed. Academic Press, Elsevier
- Castañeda Salguero, C. 1995. Sistemas Lacustres de Guatemala: recursos que mueren. Editorial Universitaria. 196 pp.