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Geodynamics in Planetary Science

Geodynamics in Planetary Science

It is a question that humankind has been asking for thousands of years:

Are we alone in the Universe or are there other worlds like our own?

As of today, it is unknown whether or not inhabited planets exist outside of our own solar system. With the discovery of the extrasolar planet 51 Peg b in 1992, it was confirmed that our sun is not the only star that hosts planets and therefore the search for extraterrestrial life has expanded beyond our own solar system.
However, before we look for an inhabited exoplanet, we must understand what makes a planet habitable.
Of course, the best example of an inhabited (and hence habitable) planet is our Earth and therefore it is a reasonable approach to first look for Earth-like planets. So, the question we should ask is

What makes Earth habitable?

  • The planet should be in the so-called habitable zone: the zone where the planet contains liquid water on its surface. One usually calculates this zone assuming an Earth-like atmosphere.  [e.g. Lammer et al., 2009]
  • The planet also needs to have an atmosphere that protects it from radiation but also keeps the planet warm with greenhouse gases. [e.g. Seager, 2013]
  • The planet should be made of rock and should have a molten core. A convective outer core gives rise to a magnetic field that protects the planet from solar winds and cosmic rays. [e.g. Shahar et al., 2019]

Interestingly, we can couple all three points: greenhouse gases in the atmosphere can heat a planet that is too far away from its host star and therefore make it habitable. On the other hand, they can also heat a planet too much such that it becomes inhabitable.
The third point (a planet made of rock with a molten core) brings geodynamics into play: plate tectonics and volcanic outgassing contribute to burial and recycling of atmospheric gases [Seager, 2013].
In our solar system, Earth is the only inhabited planet, and it is also the only planet we know of that exhibits plate tectonics (including exoplanets).
For example, Venus, our neighbouring sister planet, is very similar to Earth in terms of size, mass and composition. Some studies even suggest that Venus might have been the first habitable planet of our solar system [Way et al., 2016].
But present-day Venus is an inhospitable planet with a very thick carbon dioxide atmosphere (90 times denser than that of Earth) and an extremely hot surface temperature (up to 750K) which is mainly because of runaway greenhouse gases. But why did Earth become habitable and Venus did not?
To explain their different evolutionary paths, plate tectonics might play a major role. Through plate tectonics, Earth can efficiently recycle carbon back into its surface (deep carbon cycle) and this may help to prevent a runaway Greenhouse effect.

The importance of plate tectonics on the habitability of a planet is still being studied, and it is not yet fully understood how efficient this recycling is.

Plate tectonics also influences the generation of a magnetic field. Plate tectonics efficiently cools the mantle by subducting cold slabs into the deep interior, which leads to high heat flow out of the core. Therefore, the style of mantle convection controls the convection in the outer core. This then generates the magnetic field of a planet. The magnetic field acts as a protective shield from the solar winds, which otherwise might erode the planet’s atmosphere. As discussed above, the atmosphere controls the climate mainly through greenhouse gases. The resulting climate influences the tectonic regime: cool climates are favourable for plate tectonics because they facilitate the formation of weak shear-zones in the lithosphere [Foley et al., 2016].
This coupling between the climate, mantle and the core is called the “whole planet coupling” [Foley et al., 2016] and as a whole, it might explain why Earth and Venus have evolved so differently.

Whole planet coupling“: The atmosphere controls the climate which influences the tectonic regime. Subducting slabs cool the mantle which leads to high heat flow out the core. Therefore, the mantle convection controls the type of convection in the outer core which can generate a magnetic field. The magnetic field protects the atmosphere from solar winds and cosmic rays.

To understand the habitability of exoplanets, we therefore need to investigate all the components of the whole planet coupling. Most interestingly for geodynamicists, it is the interior dynamics of a planet’s mantle that couples all these different components!

In the past years, astronomers have discovered many exoplanets, and we expect many more to join this list. For some of them, astronomers and astrophysicists can measure its size, mass, and sometimes even the atmospheric composition and/or surface temperature.
This is very different from studying the Earth, where we can gather a lot of information about the interior through, for example, seismology. Geophysicists, Astronomers, Astrophysicists and many other research disciplines have to collaborate such that they can understand an exoplanet’s whole planet coupling and potential habitability. For geodynamicists the challenge will be to infer the exoplanet’s interior dynamics from a limited amount of data only.

References:
Foley, B. J. and Driscoll, P. E.: Whole planet coupling between climate, mantle, and core: Implications for the evolution of rocky planets, Geochemistry, Geophysics, Geosystems, Vol. 17, 2016.
Lammer, H., et al.: What makes a planet habitable?, The Astronomy and Astrophysics Review, Vol. 17, 2009.
Seager, S.: Exoplanet Habitability. Science, Vol. 340, 2013.
Shahar, A., Driscoll, P., Weinberger, A. and Cody, G.: What makes a planet habitable?, Science, Vol. 364, 2019.
Way, M. J., et al.: Was Venus the first habitable world of our solar system?, Geophysical Research Letters, Vol. 43, 2016.

Introducing the blog team!

Introducing the blog team!

It’s time for another proper introduction of the blog team! As you will probably know, things have been a bit silent on the blog front lately. This is because all the blog editors were very busy and also: it’s hard to upload 52 times a year. You come up with some great blog ideas! (if you do: e-mail us, please!). Luckily, we used the EGU General Assembly to find some fresh blood for the blog team. Together with the seasoned blog team members and a new blog strategy, we are buzzing to give you regular content once again. Expect the usual blog posts on Wednesday at 9:00 am and in the future, maybe expect a little extra on Fridays… But who are these great people providing you with your weekly dose of geodynamics news?

The Blog Team

Iris van Zelst
I am a PhD student in the Seismology and Wave Physics group at ETH Zürich, Switzerland. I am right at the seismology border of geodynamic research, as I am combining geodynamic modelling with dynamic rupture modelling to look at earthquakes in subduction zones on the entire timescale relevant to the process. I also occasionally look at some data, because you should always keep it real. I am in the final year of my PhD (oh help!), so my aim as Editor-in-Chief is to make sure everyone else is organised and uploading regularly, while I will be mostly pulling the strings behind the scenes and writing an occasional blog post. Such as this one! In my spare time, I love to read lots of books in all kinds of genres, go to the theatre, and play a little bit of theatre myself. I recently enrolled in an improv class and it is so much fun! All the world’s a stage. You can reach my via e-mail.

Luca Dal Zilio
I am a postdoctoral researcher in Mechanical Engineering and Geophysics at the California Institute of Technology (Caltech). My research is primarily aimed at understanding the relationship between crustal deformation and earthquakes in mountain belts, such as the Alps and Himalaya. By combining theoretical, computational, and observational approaches, I attempt to understand the interplay between geodynamic space–time scales of millions of years of slow and broadly distributed regional deformation with seismic space–time scales of rapid and localised earthquake processes. My passion lies in democratising science communication via innovative and accessible tools in order to spread scientific research and discovery. And yes, I like coffee. Espresso. You can reach me via e-mail.

Anne Glerum
I am a postdoctoral researcher at GFZ Potsdam, Germany. With numerical models, I investigate the link between local stress and strain observations and far-field forcing in the East African Rift System. Other modelling interests include magma-tectonic feedback and surface evolution during continental extension. Outside of research, I love to go on walks with my dog, to explore my new home Berlin and to read books on all possible topics. I’m excited to show you the variety of geodynamics and its overlap with other disciplines as an editor of the GD blog team. You can reach me via e-mail.

Anna Gülcher
I am a PhD student at the Geophysical Fluid Dynamics group at ETH Zürich, Switzerland. With the use of numerical modelling, I study the interior dynamics of the Earth and other planets. For my research, I am trying the put geophysical, geological, and geochemical observations in a geodynamically coherent framework (with an emphasis on trying). I found a passion for windsurfing early on while still living in my flat home country (the Netherlands). Yet, since moving to mountainous Switzerland, I have traded in my windsurfing equipment for hiking boots or snowboarding gear and try to spend my free time in the Alps to seek some adrenaline. I’ve very recently started to learn how to play the guitar, and am very proud to say that I can now play my very first complete song. I am excited to be part of the GD team as an Editor! You can reach me via e-mail.

Diogo Lourenço
I am a postdoctoral researcher at the Department of Earth and Planetary Sciences at the University of California Davis, USA. My research aims at understanding the evolution and interior dynamics of the Earth and other rocky planets, primarily through the use of numerical models. When I am not working on theoretical geodynamics, I like to keep things theoretical. I like reading and playing music. Sometimes I also exercise by walking around museums and looking at things. With my work as an editor in this blog, I hope to bring geodynamics to the reader in a friendly and exciting way. I also hope to help building a more involved and integrative geodynamics community. You can reach me via e-mail.

Tobias Meier
I am currently a PhD student at the Center for Space and Habitability (CSH) at the University of Bern. My research focuses on understanding the interior dynamics of rocky exoplanets, particularly planets that are partly molten. At the CSH, Earth and planetary scientists and astrophysicists work side-by-side to understand the formation and evolution of solar system bodies and exoplanets. As an editor of the GD blog I will nurture the link between geodynamics and terrestrial planet evolution and foster interactions between related disciplines.
As an undergraduate I worked in the field of cosmology, so it was necessary for me to downsize from thinking about the vast scales of the universe to zooming in on individual planets when I transitioned to my PhD work. At the time of writing, there has not been a confirmation of an inhabited exoplanet where we could possibly travel to. So, on our own wonderful planet, I enjoy hiking in the beautiful Swiss mountains and I also (almost) never say no to a game of table tennis. You can reach me (also for table tennis!) via e-mail.

Antoine Rozel
I am a senior researcher in ETH Zürich. After studying physics (nobody is perfect), I have been working on numerical simulations of mantle convection involving absurd rheologies for quite a while now, I am getting old. I am also interested in crust and craton production in all solar system planets. To make life even more beautiful, I have also finished the conservatory in classical piano and I organised some painting exhibitions in the last years (you can find my gallery here). I have also found recently that -when I do not play pinball or videogames- I can save time by doing both music and sport at the same time by playing Japanese drums (taiko)! You can reach me via e-mail.

Grace Shephard
I am a Researcher at the Centre for Earth Evolution and Dynamics (CEED) at the University of Oslo, Norway. My research links plate tectonics,​ palaeogeography, and deep mantle structure and dynamics. I spend much of my time hunting for evidence to constrain the opening and closure of ocean basins, particularly around the Arctic, Atlantic and the Pacific. I think GPlates is an excellent Tardis with which to time travel. Geodynamics offers a lot of interdisciplinary and creative avenues to explore – and why not follow up your idea with a blog post! You can reach me via e-mail or find a sporadic tweet at @ShepGracie.

The Sassy Scientist
I am currently employed at a first tier research institute where I am continuously working with the greatest minds to further our understanding of the solid Earth system. Whether it is mantle or lithosphere structure and dynamics, solid Earth rheology parameters, earthquake processes, integrating observations with model predictions or inversions: you have read a paper of mine. Even if you are working on a topic I haven’t mentioned here, I still know everything about it. Do you have any problems in your research career? I have already experienced them. Do you struggle with your work-life balance? Been there, done that. Nowadays, I have only one hobby: helping you out by answering the most poignant questions in geodynamics, research, and life. I am waiting for you right here. Get inspired.

The geodynamic processes behind the generation of the earliest continents

The geodynamic processes behind the generation of the earliest continents

The earliest continents played a fundamental role on Earth’s habitability. However, their generation is still not understood, and it requires an integrated approach between petrology and geodynamic modelling. In a new study, Piccolo and co-workers developed a method to handle the effects of chemical evolution on the geodynamic processes. They show that the production of the earliest felsic crust triggers a self-feeding chain of events that leads to the generation of the first proto-continents.

The Archean Eon (4.0-2.5) is the first act of the evolution of life and represent one of the first steps of Earth towards its present-state. Only few remnants have reached us representing different space-time windows that are difficult to reconcile in a unique interpretative framework. Many questions are still unsolved, and all the answers may be interconnected. One of them is related to the generation of the continental crust, which played a fundamental role on the developing of the biosphere as we know now. Although the present-day sites of production are all most understood, we still try to grasp the dominant process that were generating its constituents (e.g. granitoids) during the Archean. Herein the main “hot” topic is the felsic components of the continental crust,  (i.e. felsic crust/melts), and their generation during the Archean, following the recent results from Piccolo et al. (2019) [1].

Felsic melts cannot be directly produced from a partially molten mantle source; therefore, silica-rich magma generation must occur via a multistage process [2] that comprises at least two main steps: i) extraction of raw mafic materials; ii) differentiation via fractional crystallization or via partial melting of the hydrated basalts. Both processes come at costs, and, in fact the generation of felsic melts implies the production of large volume of complementary mafic/ultramafic dense residuum (the solid fraction of a partially melted rock), which is not observed in geological records. Such residual rocks have a different chemical composition with respect to the parental magma, being more mafic, and thus potentially producing denser minerals. During the Archean the most widely accepted recipe to produce felsic crust is to bake hydrated meta-basalts at high pressure until they partially melt [3]–[5].

Available Archean geological records are not enough to provide a coherent picture. Therefore, it is natural to assist the interpretation of available data with indirect means such as numerical and petrological modelling. Moreover, it is necessary to employ an integrated approach between geodynamic and petrology to study the effect of the chemical evolution on the overall dynamics of the lithosphere. In Piccolo et al. (2019), the authors accepted this challenge by conjugating petrological phase diagrams with geodynamic simulations to handle a simple — but petrologically robust — chemical evolution of the mafic protolith. They produce a set of genetically linked phase diagrams for both mantle and mafic crustal compositions — exploiting the recent advances on thermodynamic modelling for mafic/ultramafic systems (for further details, [5]–[8]) — to simulate the chemical evolution as a function of the melt extracted. Then, they modify their finite element code (MVEP2) to change the compositional phase as a function of the principal mineralogical transformation.

The base line scenario comprise a lithosphere (composed by hydrated effusive basalts, intrusive rocks and a residual lithospheric mantle) underlaid  by a fertile asthenosphere featuring high temperature and radiogenic heat production consistently with Archean conditions [9], [10]. During the initial stage of the experiments, asthenosphere self-heats above solidus, resulting in the genesis of more mafic melts that are readily extracted and converted into mafic hydrated basalts and dry intrusions. The latter are emplaced within the lower crust and heat the overlying hydrated units that start to partially melt. Such two-steps processes have two main consequences: the production of the first granitoids that are emplaced at middle-crust depths and the production of large amount of dense mafic residuum. Such rocks are not so common in the geological record, as stated above. Therefore, the inevitable question is: what are the processes that assists their disposal? The solution of this conundrum relies on the density contrast between the underlying mantle and complementary mafic residuum. If the density contrast is sufficient high, any geometrical and thermal perturbation between them can trigger Rayleigh-Taylor drip instabilities. The foundering of such drip into the mantle forces the weak asthenosphere to upwell causing more mafic melts and thus inducing more felsic crust production and consequent gravitational instabilities. After a few million years the mantle is quiescent because its temperature significantly drops because it mixes with the dripped mafic while the crust start being more enriched of felsic crustal components. Several parameters have been tested, and the main chains of events have been replicated in a plethora of conditions, especially at lower mantle temperature, suggesting that the generation of felsic crust is associated to mantle cooling events, that may buffer the upper mantle temperature.

Melt coming from the astenosphere percolates through the lithosphere, heating and both plastically and thermally weakening it. Part of the mafic melts are emplaced in the lower crust, while the remnants are erupted. As soon as a critical amount of residuum is reached, RTIs are triggered, and asthenosphere fills the space left by the delaminated lithosphere. This results in a feedback between mantle melting and new felsic crust production. From ref. [1].

The removal of the residuum implies production of more mafic melts, and thus more felsic crust that leads inevitably to again to gravitational instabilities yielding a self-feeding. Moreover, any vertical tectonic setting fuelled by mantle magmatic processes is suitable to produce proto-continents thank to these feedbacks. Such findings are consistent with many independent line of research (e.g., [11], [12], [13]) and the novelty of this study relies in a first attempt to conjugate realistic petrological constraint with the chemical evolution induced by melt extraction.

References:

1. A. Piccolo, R. M. Palin, B. J. P. Kaus, and R. W. White, “Generation of Earth’s early continents from a relatively cool Archean mantle,” Geochemistry, Geophys. Geosystems, 2019.
2. R. L. Rudnick and S. Gao, “4.1 – Composition of the Continental Crust,” in Treatise on Geochemistry, 2014, pp. 1–51.
3. J.-F. Moyen and G. Stevens, “Experimental constraints on TTG petrogenesis: implications for Archean geodynamics,” Archean Geodyn. Environ., pp. 149–175, 2006.
4. R. P. Rapp, N. Shimizu, and M. D. Norman, “Growth of early continental crust by partial melting of eclogite,” Nature, vol. 425, pp. 605–609, Oct. 2003.
5. R. M. Palin, R. W. White, and E. C. R. Green, “Partial melting of metabasic rocks and the generation of tonalitic???trondhjemitic???granodioritic (TTG) crust in the Archaean: Constraints from phase equilibrium modelling,” Precambrian Res., vol. 287, pp. 73–90, 2016.
6. R. M. Palin, R. W. White, E. C. R. Green, J. F. A. Diener, R. Powell, and T. J. B. Holland, “High-grade metamorphism and partial melting of basic and intermediate rocks,” J. Metamorph. Geol., vol. 34, no. 9, pp. 871–892, 2016.
7. E. C. R. Green, R. W. White, J. F. A. Diener, R. Powell, T. J. B. Holland, and R. M. Palin, “Activity–composition relations for the calculation of partial melting equilibria in metabasic rocks,” J. Metamorph. Geol., vol. 34, no. 9, pp. 845–869, 2016.
8. R. W. White, R. M. Palin, and E. C. R. Green, “High-grade metamorphism and partial melting in Archean composite grey gneiss complexes,” J. Metamorph. Geol., vol. 35, no. 2, pp. 181–195, 2017.
9. C. T. Herzberg, K. C. Condie, and J. Korenaga, “Thermal history of the Earth and its petrological expression,” Earth Planet. Sci. Lett., vol. 292, no. 1–2, pp. 79–88, 2010.
10. J. Ganne and X. Feng, “Primary magmas and mantle temperatures through time,” Geochemistry, Geophys. Geosystems, vol. 18, no. 3, pp. 872–888, Feb. 2017.
11. J. H. Bédard, “A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle,” Geochim. Cosmochim. Acta, vol. 70, no. 5, pp. 1188–1214, 2006. 12. T. E. Zegers and P. E. van Keken, “Middle Archean continent formation by crustal delamination,” Geology, vol. 29, no. 12, pp. 1083–1086, 2001. 13. E. Sizova, T. V. Gerya, K. Stüwe, and M. Brown, “Generation of felsic crust in the Archean: A geodynamic modeling perspective,” Precambrian Res., vol. 271, pp. 198–224, 2015.

Travel log – The Kenya rift

Travel log – The Kenya rift

Topographic map of the Kenya rift and surroundings. Dark red lines indicated faults from the GEM database. Dotted blue lines separate the northern, central and southern Kenya rift. In green circles the discussed locations.

A little over a year ago, I was lucky enough to join a field trip to the Kenya rift organized by Potsdam University and Roma III. This rift is part of the active East African Rift System, which I introduced in a previous blog post. With a group of 25 enthusiastic participants from Roma Tre, Potsdam University, Nairobi University and GFZ Potsdam (we somehow always managed to make the 20-person bus work), we set out to study the interaction between tectonics, magmatism and climate and their link to human and animal evolution. Based on several pictures, I’ll take you through the highlights.

 

 

 

 

Basement foliation and fault orientation

Two numerical modellers looking at rocks… Gneisses of the Mozambique belt with steeply dipping foliation – I think. Courtesy of Corinna Kallich, Potsdam University.

Although this first picture might not look so impressive (I promise, more impressive ones will come), this road outcrop shows the structure of the basement that is responsible for the orientation of the Kenya rift’s three western border faults. Here in particular, we are slightly west of the Elgeyo escarpment, the scarp of the major east-dipping Elgeyo fault. It reactivated the steep foliation of the Mozambique belt gneisses that formed during the Pan-African orogeny (550-500 Ma; Ring 2014). Changes in foliation orientation are mirrored by changes in fault orientation from NNE to NW upon going from the Northern to the Southern Kenya rift (see map). The Elgeyo fault itself displaced the 14.5 My old massive extrusion of phonolite lavas that can be seen throughout the Kenya rift area, marking the start of the current rift phase. From the differences in basement level between the western shoulder and the rift centre, the total offset along the fault is ~4 km!

Rift axis volcanism

Lunch overlooking the Menengai caldera that collapsed 36,000 yr ago. Courtesy of Corinna Kallich, Potsdam University.

With on-going rifting, the tectonic and magmatic activity localised in the centre of the Kenya rift. One massive central volcano is the Menengai volcano, whose view we enjoyed over lunch. This 12 km wide caldera collapsed 36,000 yr ago; the ash flows of the eruption can be found throughout the whole of Kenya. Within the caldera, diatomite layers alternating with trachyte lava flows indicate the presence of lakes 12 and 5 ky ago. These lakes were fed by the neighbouring Nakuru basin overflowing into the Menengai crater. The volcano itself was responsible for the earlier compartmentalization of the larger Nakuru-Elmentaita basin. At the moment, freshwater springs are being fed by the groundwater, and 40 geothermal wells are being constructed to benefit from the groundwater being heated by the magma chamber at 3-3.5 km depth.

Lunch at Hell’s Gate

Looking along Hell’s Gate Gorge – cut into the white diatomite and pyroclastic layers – towards feeder dikes of the remaining core of a volcano. Courtesy of Corinna Kallich, Potsdam University.

Watching the wildlife and beautiful scenery is usually the reason people visit Hell’s Gate National Park, but we studied the flow structures in a highly viscous, silica-rich lava flow. We then scrambled our way through Hell’s Gate Gorge that cut into mostly diatomite lake sediments (these algae are very helpful) alternated with pyroclastic layers. Most impressive however, were the crosscut basaltic intrusions that we could trace back to the centre of an otherwise eroded volcanic dome. The well-deserved lunch was a rather frustrating affair, as Vervet monkeys took every chance at stealing our food, not even shying away from distracting us with their adorable babies.

Monkey enjoying my lunch. Courtesy of Corinna Kallich, Potsdam University.

 

 

 

 

 

Wishing the lake was back

The white diatomites of the Olorgesailie Formation, indicating the presence of a lake. Courtesy of Corinna Kallich, Potsdam University.

The Olorgesailie basin is where paleoanthropologist Louis Leakey and his wife palaeontologist Mary Leakey (Wikipedia) unearthed a score of Acheulean hand axes in the 1940s. The 600-900 ky old tools were used to dig for roots, cleave, hammer and scrape meat and can be seen in the Kariandusi museum site. Besides the hand axes (made from all the trachyte found in the area), we marvelled at the Olorgesailie Formation that contains them, which was deposited between ~1.2-0.5 Ma. The formation consists of repetitions of wetland, river and lake sediments and paleosols (fossil soils, indicating dryer conditions). As we stand baking in the sun on top of the dusty, white diatomite, the vision of a lake sure is very alluring.

A not-so-fresh lake

On our way to a tiny hotspring along the edge of the slightly pink waters of Lake Magadi. In the foreground the white evaporates the lake is mined for. Courtesy of Corinna Kallich, Potsdam University.

While we mostly stayed in resorts, our only campsite (proper “glamping” with a shower and bathroom in the tent) was close to Lake Magadi, one of the lakes along the rift axis. This saline, alkaline lake gave its name to magadiite, a sodium hydro silicate, that when dehydrated forms chert (i.e. flint). The lake is also mined for its sodium carbonate, known as trona. During the African Humid Period (15,000-5,000 yr ago; Maslin et al. 2014), Lake Magadi was about 40 m higher, a lot fresher and connected to Lake Natron further south. Fun fact from Wikipedia: elephants visit the Magadi Basin to fill up on their own salts supplies as well. From my own experience, I can tell you, it does not taste very good.

 

 

My trusted companions for over a decade did not survive Kenya’s heat and volcanics… Serves me right for not taking them out often enough!

And then there were the hippos, neptunic dikes, dancing Maasai, a boat trip to the hydrothermal vents on Ol Kokwe Island, giraffes outside our cabin, midnight stargazing… too much to capture in one blog post. I had a wonderful time in Kenya exploring the geology, admiring the wildlife and getting to know its people. My only regret? Losing my shoes…

 

 

 

 

References:

Maslin, M. A., Brierly, C. M., Milner, A. M., Shultz, S., Trauth, M. H., Wilson, K. E. (2014). East African climate pulses and early human evolution, Quaternary Science Reviews 101, 1-17.

Ring, U. (2014). The East African Rift System, Austrian Journal of Earth Sciences, 107, 1.

Strecker, M. R., Faccenna, C., Wichura, H., Ballato, P., Olaka, L. A. and Riedl, S. (2018). Tectonics, seismicity, magmatic and sedimentary processes of the East African Rift Valley, Kenya, Kenya Field School Field Guide.

Personal communication with Strecker, M. R., Wichura, H., Olaka, L. A. and Riedl, S.