GD
Geodynamics
Anne and Anna

Guest

Find out more about the blog team here.

Climate protests at the start of Global Week for Future

Climate protests at the start of Global Week for Future

For months, students have skipped school on Fridays to ask for more action against climate change. To kick off the Global Week for Future, last Friday saw thousands of demonstrations in many countries around the globe, with not only high school students joining the fray, but people from all walks of life. GFZ Potsdam’s Berhard Steinberger and Thilo Wrona share their experiences in Potsdam and Berlin.

Potsdam – Bernhard Steinberger

Scientists for Future at the Alter Markt, Potsdam.

Scientists for Future. “From knowing to acting. There are no alternatives for facts!” Courtesy of Bernhard Steinberger.

As part of the global climate strike on September 20, a 5,200-person-strong demonstration took place in Potsdam. To get to the demonstration, I and about 50 other employees of the institutions on Telegraph Hill – the GFZ German Research Centre for Geosciences, Alfred Wegener Institute (AWI) and the Potsdam Institute for Climate Impact Research (PIK) – participated in a “critical mass” bicycle ride from the Hill to the city centre.

Prof. Dr. Stefan Rahmstorf (PIK) speaking on behalf of Scientists for Future in Potsdam.

Prof. Dr. Stefan Rahmstorf (PIK) speaking on behalf of Scientists for Future in Potsdam. Find the view from the other side (different speaker though) here. The red and black sign translates to “Fly more, live less”. Courtesy of Bernhard Steinberger.

After several speakers briefly introduced the different organizations that supported the demonstration initiated by Fridays for Future, Prof. Dr. Stefan Rahmstorf from PIK addressed the crowds, speaking on behalf of Scientists for Future (S4F). Rahmstorf pointed out that even though the German Government has now decided what Climate Protection measures to take, this is not the time to relax. It is not sure whether these measures will be sufficient to reach the Government’s climate goals for 2030, and even if they are, those goals are not compatible with the Paris climate agreement, which would require an even larger drop in CO2 emissions. This is especially the case, since Germany’s per capita CO2 emissions are about twice the global average. So, in light of climate justice, Germany would need to reduce its emissions more strongly than other countries currently emitting less. Rahmstorf thanked Fridays for Future for putting the government under pressure to act and emphasized that the fight was far from over.

After the speeches, the protest march started moving through the Potsdam City centre towards the Brandenburg Gate (the one in Potsdam, not in Berlin). There, accompanied by the Queen-song Another one bites the dust, everybody lay down on the ground in a “die-in”. Then the demonstrators returned to Alter Markt for free rice and pea soup – of course, with reusable plates and spoons. For the grand finale, a band played live music on stage.

Protesters walking through Potsdam's city centre.

Protesters walking through Potsdam’s city centre. “Climate is like beer, it *** when it’s too warm!” Courtesy of Bernhard Steinberger.

Protesters walking through Potsdam's city centre.

Protesters walking through Potsdam’s city centre. “Respect existence or expect resistance.” Courtesy of Bernhard Steinberger.

 

Berlin – Thilo Wrona

On Friday, I went to the climate demonstration in Berlin organized by Fridays for Future. Arriving at the Brandenburg Gate, I was greeted by a large crowd of people (100.000 – 270.000 depending on whom you ask). I met people of all ages, from young pupils singing their own interpretation of Last Christmas to Rentner (senior citizens) For Future demonstrating that they too are worried about the future. There were speeches, songs and vegan soup at the Brandenburg Gate before the parade and, because it’s Berlin, a rave at Potsdamer Platz afterwards. The parade went on a loop through Berlin starting at the Brandenburg Gate and finishing in front of the Parliament, where the government had just agreed on a new piece of legislation on climate (Klimapaket)  the evening before. I later found out that there were many acts of protest on this day ranging from climate-conscious companies giving all their employees a day off to roadblocks across Berlin organized by the demonstrators.

Protestors near the Brandenburg Gate in Berlin.

Protestors on their way to the Brandenburg Gate in Berlin. “There’s no planet B.” Courtesy of Thomas van der Linden.

For more information, follow the UN climate summit this week and check out Scientists for Future, the global climate strike week and Nicolas Coltice’s recent blog.

Protestors on their way to the Brandenburg Gate in Berlin.

Protestors on their way to the Brandenburg Gate in Berlin. “All for the climate and the climate for all.” Courtesy of Thomas van der Linden.

Adaption

Adaption

As temperature records have continuously been broken all over the world, many of us scientist had to endure extreme conditions in our overheated offices. Climate change is happening, and faster than we’d like to think, but how does this play a role in the scientific community? In this week’s blog post, geodynamicist Nicolas Coltice (professor at Ecole Normale Superieure de Paris) shares his passionate opinion on the matter and sheds light on several important topics that may easily be overseen by enthusiastic scientists. 

 

Nicolas Coltice is a professor in the Laboratoire of Geology of Ecole Normale Supérieure de Paris.

I took my last flight for work in May 2017 to go to a meeting of the Deep Carbon Observatory in Moscow. I had started evaluating my carbon footprint a couple of years before, and flying to do science on carbon questioned me. The first flight I ever took was when I was about 14, to go to a place close to Chernobyl (in the U.S.S.R. at the time), a few years after the catastrophe. It seems that Russia brings me back to environmental questions. In the context of climate change today, many scientists give their opinions on the future. Civilization will collapse, or science will magically save us all. There is so much abstract agitation and noise in the debates. Both the exploitation of nature and pollution impact landscapes and the living so quickly. What will the world be like in 30 years? Who can guess rationally?

 

Climate countdown

When I started to write this blog post, it was June 13th. The temperature in Delhi was about 48°C and the Monsoon did not seem to start. Water shortages led to fights with people being killed. In Poland, temperatures reached over 30°C. But in France, temperatures were cooler and close to those occurring in Greenland. A few weeks later, we had the highest temperatures ever recorded in the South of France. Particles and pesticides are all over the place and we eat and drink litters of them every year. Climate will continue to change: even if we stop emitting carbon and methane, the ocean will keep on doing so for centuries.

 

Surface temperature of some European countries on June 27th 2019 (European Space Agency).

 

The IPCC and diverse agencies have given a very simple recommendation: by 2030, we have to lower our carbon emissions by 50%. That is 11 years from now, soon to be 10, soon to be 9… In the next 10 years, there is minimal chance that humanity has developed new energy sources for the billions of people all over the world. For example, building a nuclear power plant takes years, and it can only distribute energy about 10 years after completion. Besides blaming politics and big companies, what can one do here and now? Because the shift of society does not seem to start in comfortable offices, it has to start everywhere. It is now. What actions can we take as geodynamicists? In geodynamics we tackle problems with a large vision, including long-range dependencies, evaluating the forces at play.

Because the shift of society does not seem to start in comfortable offices, it has to start everywhere.

I started to think about our job as a scientist. Many carbon footprint tools help us identify how to mitigate our carbon emissions. The Tyndall Center for Climate Change Research proposes guidelines for low-carbon research, that are quoted here: ”

    • Monitor and reduce. I will keep track of the carbon emissions of my professional activities, and set personal objectives to reduce them in line with or larger than my country’s carbon emissions commitments (…).
    • Account and justify. I will justify my travel considering the location and purpose of the event, my level of seniority, and the alternative options available.
    • Prioritise, prepare and replace. For activities that I organise, I will choose the location giving high priority to a low carbon footprint of travel of the participants, and I will encourage, incorporate and technically support online speakers and webcasts to reduce unnecessary travel.
    • Encourage and stimulate. I will resist my own FOMO (Fear Of Missing Out) from not attending everything and work towards sensitizing others to the need of the research community to walk the talk on climate change.
    • Reward. I will work with my peers, Institute and Funders to value alternative metrics of success and encourage the promotion of low-carbon research as a realisable alternative to a high-carbon research career.”

The Tyndall Center for Climate Change Research also provides a Travel Strategy (link to PDF) that aims to help individual researchers to reduce their emissions through time.

Emit carbon or perish

This is essentially dealing with travel, clearly the main source of scientists’ carbon footprint. Let’s identify the forces at play that make us using tons of carbon every year on average. I identify a major shift in our practice between 2000 and 2010, with digital doping of the old “publish or perish”. It would be easy to blame computers. Private companies build publication databases with our work to compile performance indicators of individuals. Now a handful of business groups own most of the journals, questing increasing financial profit (Elsevier and Springer operated a better profit margin than Apple in 2014, 37% and 35% respectively). What was common (not state-controlled but controlled by the scientific community) became increasingly private and marketable. Growth of publication numbers generates billions of euros of dividends for stakeholders, at the expense of public money. Every year subscriptions cost more to the scientific community (see for instance the website of the University of Virginia Library). And we are now productive workers in a globalised science-market. It is for granted that competition is the source of good science… or in any case good money. Therefore, scientists have to publish more, be everywhere to “sell” their results or see which ones they “buy”, and hence travel all over the world.

Journal and articles figures for 2018 by the STM association. Profit data for the Scientific, Technical & Medical division of Reed-Elsevier only (Larivière et al., 2015) .

 

Competition-strategy modifies the science itself, introducing loss of integrity (Fanelli, 2010), and of course dramatically increasing our environmental impact. We are pushed to acquire the most powerful machines, generate the biggest datasets, do large-scale analysis, publish as many papers as we can, and travel the world to disseminate the results. Or perish. Bigger machines often require less energy to obtain the same performance as the old ones. However, the energy gain of new technology becomes more than compensated by accentuated use of it. This is the rebound effect. New technology is not a substitute but an addition. Hence we need more energy and more natural resources. Can we substitute instead of add? Can we identify when it is so easy to use machines instead of our brains, but somewhat irrelevant to do so?

Scientists have to publish more, be everywhere to “sell” their results, or see which ones they “buy”, and hence travel all over the world.

Although planes are getting more carbon-efficient, travel for science has become intensive. Some colleagues like their job because they can travel, which I understand. The number of conferences and workshops exploded. The increase in attendance of worldwide meetings like AGU (11,422 attendees in 2004 and 21,702 in 2012) questions their role in terms of scientific relevance and impact on the planet. What shall we do with all these gatherings? Mobility looks like a necessity today. However, research has shown that limiting the use of planes to travel has barely any impact on scientific careers (Wynes et al., 2019).

(Lower-carbon) Science as a common

Competing, publishing as much as possible, privatising science and transferring public money to the stakeholder of publishing groups constitute a dead-end for our job. This is a dead-end for science. This is a dead-end for knowledge and humanity. Exponential growths of h-index, publication rates, data collection and conference travel are not sustainable. The rebound effect often kills the gain of progress for production gain. Can we make a transition as a community, building our commons and collaborative organizations? Can we start to teach new research ethics and practices so the new generation will be ready to do this job in a sustainable way? We have 10 years, soon to be 9.

 

Larivière, V., Haustein, S., and Mongeon, P. (2015). The Oligopoly of Academic Publishers in the Digital Era. PLoS ONE 10(6): e0127502

Fanelli, D. (2010). do Pressures to Publish Increase Scientists' Bias? And Empirical Support from US States Data. PLoS ONE 5(4): e10271

Wynes, S., Donner, S. D., Tannason, S. and Nabors, N. (2019). Academic air travel has limited influence on professional succes. Journal of Cleaner Production 226: 959-967

What controlled the evolution of Plate Tectonics on Earth?

Great Unconformity - Immensity River, Grand Canyon
Stephan Sobolev

Prof. Dr. Stephan Sobolev. Head of the Geodynamic Modelling section of GFZ Potsdam.

Plate tectonics is a key geological process on Earth, shaping its surface, and making it unique among the planets in the Solar System. Yet, how plate tectonics emerged and which factors controlled its evolution remain controversial. The recently published paper in Nature by Sobolev and Brown suggests new ideas to solve this problem….

What makes plate tectonics possible on contemporary Earth?

It is widely accepted that plate tectonics is driven by mantle convection, but is the presence of said convection sufficient for plate tectonics? The answer is no, otherwise plate tectonics would be present on Mars and Venus and not only on Earth. The geodynamic community recognized that another necessary condition for plate tectonics is low strength at plate boundaries and particularly along the plate interfaces in subduction zones (e.g. Zhong and Gurnis 1992, Tackley 1998, Moresi and Solomatov 1998, and Bercovici 2003). To quantify the required strength at subduction interfaces, we have used global models of plate tectonics (Fig. 1A) that combine a finite element numerical technique employing visco-elasto-plastic rheology to model deformation in the upper 300 km of the Earth (Popov and Sobolev 2008) with a spectral code to model convection in the deeper mantle (Steinberger and Calderwood 2006). The model reproduces well present-day plate velocities if the effective friction at convergent plate boundaries is about 0.03 (Fig.1B). Low strength corresponds to subduction interfaces that are well lubricated by continental sediments (low friction; Lamb and Davis 2003, Sobolev and Babeyko 2005, or low viscosity; Behr and Becker 2018). In case of sediment shortages in the trenches (corresponding to a friction coefficient of 0.07-0.1), plate velocities would first decrease about two times (Fig. 1C) and then even more because of less negatively buoyant material having subducted into the mantle, leading to less convection driving force.

Effects of sediments on contemporary subduction according to global numerical models.

Figure 1. Global numerical model showing the effect of sediments on contemporary subduction. (A) The global model combines two computational domains coupled through continuity of velocities and tractions at 300 km depth. (B) NUVEL 1A plate velocities in a no-net-rotation reference frame (black arrows) versus computed velocities (blue arrows) for the global model with a friction of 0.03 at convergent plate boundaries. (C) Root mean square of computed plate velocities in the global model versus friction coefficient at convergent plate boundaries.

Hypothesis and its testing

Based on the previous discussion, we infer that continental sediments in subduction channels act as a lubricant for subduction. In addition, the presence of these sediments in trenches is a necessary condition for the stable operation of plate tectonics, particularly earlier in Earth’s evolution when the mantle was warmer and slabs were relatively weak. With this hypothesis we challenge the popular view that secular cooling of the Earth was the only major control on the evolution of plate tectonics on Earth since about 3 Ga. The hypothesis predicts that periods of stable plate tectonics should follow widespread surface erosion events, whereas times of diminished surface erosion should be associated with reduced subduction and possibly intermittent plate tectonics.

We test this prediction using geological proxies believed to identify plate tectonics activity (supercontinental cycles) and geochemical proxies that trace the influence of the continental crust on the composition of seawater (Sr isotopes in ocean sediments; Shields 2007) and continental sediments in the source of subduction-related magmas (oxygen and Hf isotopes in zircons; Cawood et al. 2013, Spencer et al. 2017). All three geochemical markers indeed show that just before or in the beginning of supercontinental cycles the influence of sediments is increasing, while it decreases before periods of diminished plate tectonic activity, like the boring billion period between 1.7 and 0.7 Ga (Cawood and Hawkesworth 2014; Fig. 2). The largest surface erosion and subduction lubrication events were likely associated with the global glaciation evens identified in the beginning (2.5-2.2 Ga) and at the end (0.7-0.6 Ga) of the Proterozoic Era (Hoffman and Schrag 2002). The latter snowball Earth glaciation event terminated the boring billion period and kick-started the modern phase of active plate tectonics.

Another prediction of our hypothesis is that in order to start plate tectonics, continents had to rise above sea level and provide sediments to the oceans. This prediction is again consistent with observations: there are many arguments for the beginning of plate tectonics between 3 and 2.5 Ga (see the review of Condie 2018) and, at the same time, this period is most likely when the continents rose above sea level (Korenaga et al. 2017).

Cartoon summarizing the factors that control the emergence and evolution of plate tectonics on Earth.

Figure 2. Cartoon summarizing the factors that control the emergence and evolution of plate tectonics on Earth. Enhanced surface erosion due to the rise of the continents and major glaciations stabilized subduction and plate tectonics for some periods after 3 Ga and particularly after 0.7 Ga after the cooling of the mantle. Blue boxes mark major glaciations; transparent green rectangles show the time intervals when all three geochemical proxies consistently indicate increasing sediment influence (major lubrication events); and, a thick black dashed curve separates hypothetical domains of stable and unstable plate tectonics. The reddish domain shows the number of passive margins (Bradley 2008), here used as a proxy for plate tectonic intensity.

What was before plate tectonics?

The earlier geodynamic regime could have involved episodic lid overturn and resurfacing due to retreating large-scale subduction triggered by mantle plumes (Gerya et al. 2015) or meteoritic impacts (O’Neill et al. 2017). Retreating slabs would bring water into the upwelling hot asthenospheric mantle, generating a large volume of magma that formed protocontinents. Extension of the protocontinental crust could have produced nascent subduction channels (Rey et al. 2014) along the edges of the protocontinents lubricated by the sediments. In this way, a global plate tectonics regime could have evolved from a retreating subduction regime.

What is next?

Despite of the support from existing data, more geochemical information is required to conclusively test our hypothesis about the role of sediments in the evolution of plate tectonics. Additionally, this hypothesis must be fully quantified, which in turn will require coupled modeling of mantle convection and plate tectonics, surface processes and climate.

References
Behr, W. M. and Becker, T. W. Sediment control on subduction plate speeds. Earth Planet. Sci. Lett. 502, 166-173 (2018).

Bercovici, D. The generation of plate tectonics from mantle convection. Earth Planet. Sci. Lett. 205, 107–121 (2003).

Bradley, D. C. Passive margins through earth history. Earth Sci. Rev. 91, 1-26 (2008).

Cawood, P. A., Hawkesworth, C. J. and Dhuime, B. The continental record and the generation of continental crust. Geol. Soc. Amer. Bull. 125, 14-32 (2013).

Cawood, P. A. and Hawkesworth, C. J. Earth's middle age. Geology 42, 503-506 (2014).

Condie, K. C. A planet in transition: The onset of plate tectonics on Earth between 3 and 2 Ga? Geosci. Front. 9, 51-60 (2018).

Gerya, T.V. et al. Plate tectonics on the Earth triggered by plume-induced subduction initiation, Nature 527, 221-225 (2015).

Hoffman, P. F. and Schrag, D. P. The snowball Earth hypothesis: testing the limits of global change. Terra Nova 14, 129–155 (2002).

Korenaga, J., Planavsky, N. J. and Evans, D. A. D. Global water cycle and the coevolution of the Earth's interior and surface environment. Phil. Trans. R. Soc. Am. 375, 20150393 (2017).

Lamb, S. and Davis, P. Cenozoic climate change as a possible cause for the rise of the Andes. Nature 425, 792-797 (2003).

Moresi, L. and Solomatov, V. Mantle convection with a brittle lithosphere: Thoughts on the global tectonic style of the Earth and Venus. Geophys. J. Int. 133, 669-682 (1998).

O’Neill, C. et al. Impact-driven subduction on the Hadean Earth. Nature Geosci. 10, 793-797 (2017).

Popov, A.A. and Sobolev, S. V. SLIM3D: A tool for three-dimensional thermo mechanical modeling of lithospheric deformation with elasto-visco-plastic rheology, Phys. Earth Planet. Inter. 171, 55-75 (2008).

Rey, P. F., Coltice, N. and Flament, N. Spreading continents kick-started plate tectonics. Nature 513, 405–408 (2014).

Shields, G. A. A normalised seawater strontium isotope curve: possible implications for Neoproterozoic-Cambrian weathering rates and the further oxygenation of the Earth. eEarth 2, 35-42 (2007).

Sobolev, S. V. and Babeyko, A. Y. What drives orogeny in the Andes? Geology 33, 617-620 (2005).

Spencer, C. J., Roberts, N. M. W. and Santosh, M. Growth, destruction, and preservation of Earth's continental crust. Earth. Sci. Rev. 172, 87-106 (2017).

Steinberger, B. and Calderwood, A. Models of large-scale viscous flow in the Earth’s mantle with constraints from mineral physics and surface observations. Geophys. J. Intern., 167 1461–1481 (2006).

Tackley, P. J. Self-consistent generation of tectonic plates in three-dimensional mantle convection. Earth Planet. Sci. Lett. 157, 9-22, (1998).

Zhong, S. and Gurnis, M. Viscous flow model of a subduction zone with a faulted lithosphere: long and short wavelength topography, gravity and geoid. Geophys. Res. Lett. 19, 1891–1894 (1992).

 

Remarkable Regions – The Réunion Hotspot

Remarkable Regions – The Réunion Hotspot
Eva Bredow at Réunion caldera.

Eva Bredow in front of the caldera at Réunion Island. Credit: Simon Stähler.

This week we again turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. Postdoctoral researcher Eva Bredow of Kiel University shares with us her long history with Réunion Island.

At first glance, Réunion is a relatively small tropical island, located between Madagascar and Mauritius, and from my personal experience, most Germans have never even heard of it. To be fair, it is much better known in France, because Réunion is officially a French overseas department, meaning that the eleven-hour flight from Paris is technically a domestic flight and that you can pay there with Euros (and I bet you did not know that a millimetre-sized outline of the island appears on every Euro banknote!). Besides, Réunion hosts one of the most active volcanoes in the world with one eruption per year on average. However, it rarely hits the headlines because the inhabitants live far enough away not to be overly threatened. And yet, for people interested in geodynamics, the name Réunion might actually have a familiar sound, since it regularly appears in hotspot catalogues and hotspot reference frames – a sure indication that there is more to discover.

For me, Réunion has been a very special place ever since I was a high school student lucky enough to visit the island in order to learn French. And who would have thought back then that hiking in this surreal volcanic landscape would be one of the first steps towards my decision to study geophysics? And what were the odds to stumble upon a PhD project years later, centred around the Réunion hotspot? Well, that is exactly what happened and in this article, it is my pleasure to give you at least a brief overview of why Réunion deserves to be called a remarkable spot indeed and how numerical modelling can help us to explore its geodynamic history.

NW Indian Ocean crustal thickness map.

Crustal thickness map of the north-western Indian Ocean with the entire hotspot track from Réunion Island to the Deccan Traps in India. Figure from Torsvik et al. (2013).

A deep root

The hypothesis that Réunion is an intraplate hotspot possibly fed by a hot, buoyant upwelling rooted deep in the mantle was already put forward by Jason Morgan (1971, 1972) in his famous papers outlining the classical mantle plume hypothesis. And as it happens, the Réunion plume has left a number of traces that fit the plume hypothesis extremely well and make it a kind of prototype for a deep plume and its surface manifestations. A brief look at a topographic map of the north-western Indian Ocean reveals not only the currently active hotspot at Réunion and the slightly older island of Mauritius, but also a clearly continuous (and age-progressive) hotspot track on the African and Indian plates, only split due to subsequent seafloor-spreading.

According to numerous laboratory and numerical studies that describe the mushroom-like geometry of a plume, the hotspot track is considered to be caused by the long-lived plume tail, whereas the voluminous plume head is supposed to create a huge flood basalt province in a relatively short geological time (Richards et al., 1989). In the case of the Réunion plume, the hotspot track starts at the Deccan Traps, a gigantic continental Large Igneous Province (LIP) in India. The LIP was created around 65 million years ago and the environmental changes triggered by the volcanic activities might have led to the extinction of the dinosaurs (an alternative theory to the Chicxulub impact in Mexico; Courtillot and Renne, 2003).

Further indications for a deep plume beneath Réunion include the broad topographic hotspot swell around the island, a geochemical signature of the volcanic rocks that clearly deviates from mid-ocean ridge basalts, and the present-day hotspot location above the plume generation zone at the margin of the African Large Low Shear Velocity Province (LLSVP).

Plume-ridge interaction

A more puzzling observation is the geochemical anomaly at the closest segments of the Central Indian Ridge, about 1000 km away from Réunion that implies a long-distance plume-ridge interaction. Already Morgan (1978) suggested that a sublithospheric flow channel connecting the upwelling plume and the ridge is responsible for the creation of the Rodrigues Ridge, a rather eye-catching feature not at all parallel to the hotspot track or recent plate motions.

And there is one more noteworthy hypothesis associated with Réunion, based on extremely old zircons found at Mauritius; it postulates that the hotspot track has (coincidentally) been created on top of a Precambrian microcontinent (Ashwal et al., 2017).

The RHUM-RUM experiment (completely alcohol-free…)

Concerning the (present-day) state of the Réunion plume at greater depths, seismic tomography is the most promising tool to answer the question if it is indeed fed by a deep plume or not. But given that the island is rather remotely located and a classical plume tail is expected to be quite narrow, there are plenty of technical obstacles, and it was not until 2006 that Montelli published the first seismic image of a continuous plume conduit reaching into the deep mantle. More recent global tomography models also image the Réunion plume as a clearly resolved, vertically continuous conduit at depths between 1,000 and 2,800 km (French and Romanowicz, 2015).

In 2012-2013, the French-German RHUM-RUM project (Réunion Hotspot and Upper Mantle – Réunions Unterer Mantel) aimed at an even higher resolved image of the plume. Therefore, 57 German and French ocean-bottom seismometers were deployed at the seafloor around Réunion for about a year (Stähler et al., 2016) – still the largest seismological experiment to image a deep oceanic mantle plume so far.

 

RHUM-RUM seismic stations

All seismic stations related to the RHUM-RUM project, with the 57 ocean-bottom seismometer stations shown in red. More information on the project can be found here.

With all that in mind, and as part of the RHUM-RUM project, I set up a regional numerical model with some colleagues from the GFZ Potsdam in order to assemble Réunion’s entire dynamic history. We used time-dependent plate reconstructions and large-scale mantle flow as velocity boundary conditions as well as a laterally varying lithosphere thickness in order to specifically simulate the Réunion plume (for details, see Bredow et al., 2017). In short: altogether, we were able to reproduce a crustal thickness pattern that at first order fits the observed hotspot track (although the method is not suited to reproduce a continental LIP such as the Deccan Traps). Moreover, the interaction between the plume and the Central Indian Ridge explained both the genesis of the Rodrigues Ridge and the gap in crustal thickness between the Maldives and Chagos – both features that have not been dynamically modelled before.

After our models were published, the active long-distance plume-ridge interaction beneath the Rodrigues Ridge was additionally confirmed by seismological studies in the RHUM-RUM project: first in a three-dimensional anisotropic S-wave velocity model comprising the uppermost 300 km (Mazzullo et al., 2017), and second by SKS splitting measurements (Scholz et al., 2018). Overall, these interdisciplinary studies confirmed Morgan’s long-standing hypothesis – more than 30 years after its original publication.

 

Cross section geodynamic plume model of Bredow et al. 2017.

Cross section of the geodynamic plume model, showing the long-distance plume-ridge interaction as predicted by Morgan (1978). Figure after Bredow et al. (2017).

Surface wave tomography showing the Reunion plume.

Cross section of the surface wave tomography model, showing the low velocity signature of the plume rising toward the base of the lithosphere underneath Réunion and the sublithospheric flow toward the Central Indian Ridge (CIR). Figure after Mazzullo et al. (2017).

The whole-mantle P- and S-wave tomography models from the RHUM-RUM project have yet to be published, but the (almost final) results presented at this year’s EGU (Tsekhmistrenko et al., 2019) were quite intriguing: while the plume conduit can continuously be followed down to the LLSVP in the deep mantle, the conduit is not as narrow and not nearly as vertical as classically expected!

Therefore I think it is quite safe to say that we have not yet heard the last of the Réunion hotspot and I hope that the next time you hear this name, maybe you will remember it as a rather remarkable spot on our planet…

 

Ashwal et al. (2017), Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius, Nat. Commun., 8, 14,086, doi: 10.1038/ncomms14086.

Bredow, E. et al. (2017), How plume-ridge interaction shapes the crustal thickness pattern of the Réunion hotspot track, Geochem. Geophys. Geosyst., 18, doi:10.1002/2017GC006875.

Courtillot, V. E. and P. R. Renne (2003), On the ages of flood basalt events, C. R. Geosci., 335(1), 113–140, doi: 10.1016/S1631-0713(03)00006-3.

French, S. W. and B. Romanowicz (2015), Broad plumes rooted at the base of the Earth’s mantle beneath major hotspots, Nature, 525, 95–99, doi: 10.1038/nature14876.

Mazzullo, A. et al. (2017), Anisotropic tomography around Réunion Island from Rayleigh waves Journal of Geophysical Research: Solid Earth, 122, doi: 10.1002/2017JB014354.

Montelli, R. et al. (2006), A catalogue of deep mantle plumes: New results from finite-frequency tomography, Geochem. Geophys. Geosyst., 7, Q11007, doi: 10.1029/2006GC001248.

Morgan, W. J. (1971), Convection plumes in the lower mantle, Nature, 230, 42–43, doi: 10.1038/230042a0.

Morgan, W. J. (1972), Deep mantle convection plumes and plate motions, AAPG bulletin, 56(2), 203–213.

Morgan, W. J. (1978), Rodriguez, Darwin, Amsterdam, ..., A second type of Hotspot Island, J. Geophys. Res., 83(B11), 5355–5360, doi: 10.1029/JB083iB11p05355.

Richards, M. A. et al. (1989), Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails, Science, 246, 103–107, doi: 10.1126/science.246.4926.103.

Scholz, J.-R. et al. (2018), SKS splitting in the Western Indian Ocean from land and seafloor seismometers: Plume, plate and ridge signatures, Earth Planet. Sci. Lett., Volume 498, 169-184, doi: 10.1016/j.epsl.2018.06.033.

Stähler, S. C. et al. (2016), Performance report of the RHUM-RUM ocean bottom seismometer network around La Réunion, western Indian Ocean, Adv. Geosci., 41, 43-63, doi: 10.5194/adgeo-41-43-2016.

Torsvik, T. H. et al. (2013), A Precambrian microcontinent in the Indian Ocean, Nat. Geosci., 6(3), 223–227, doi: 10.1038/ngeo1736.

Tsekhmistrenko, M. et al. (2019), Deep mantle upwelling under Réunion hotspot and the western Indian Ocean from P- and S-wave tomography, Geophysical Research Abstracts, Vol. 21, EGU2019-9447, EGU GA 2019.