Archives / 2018 / March

Help us fight patriarchy, one comic strip at a time!

Help us fight patriarchy, one comic strip at a time!

Women in science/geodynamics: a topic we have discussed before and should continue to discuss, because we’re not there yet. In this new Wit & Wisdom post, Marie Bocher, postdoc at the Seismology and Wave Physics group of ETH Zürich, discusses a range of all-too-common encounters women face and a possible solution to awareness: comics (drawn by Alice Adenis, PhD student at ENS Lyon).

Credit: Alice Adenis

You know it is so much easier for women in science these days

“Oh I don’t hire female PhD students anymore: they get pregnant and then they’re lost for science”

“Yes, I remember you, you were wearing that red dress last time”

“Now that you have responsibilities, you can’t get pregnant again”

Oh, yes, they needed a woman for this committee, that’s why they asked you

And my two personal favourites:

“You should be happy that someone called you an angel [author’s note: in a professional setting], that means that you are beautiful, what are you complaining about?”

“I do not understand why women need to work, I mean, my wife did a marvelous job raising our children while I was working, I don’t get why this way of life has to change.”

These statements have been heard in real life, in the professional setting of the research lab (or at a conference), and were directed towards real humans, who share the particularity of being both women and Earth scientists (I know! Crazy, right?). If you have said similar things or think that some (or all) of these sentences are not that big of a deal, please go to the end of this article: I have a small text just for you! Anyway, I am pretty sure I’m not the only one to find this type of comments disturbing, to say the least. And I want to do something about it.

Credit: Alice Adenis

One of the first steps in the fight against sexism is to identify and describe the various ways it is expressed in our community. Research in geodynamics is definitely international, and patriarchy comes in different flavours all around the world. Each lab has its own blend of cultures and individuals that leads to different climates. That is also true for conferences and other events. As a result, the experience of working as a female in academia and developing as a scientist varies.

Credit: Alice Adenis

However, the patriarchal power structures and strategies are similar, even if the degree to which those are expressed in a specific setting varies. Here is a diagram that, I think, sums up the variety of barriers to gender equality we face in academia pretty well:

Diagram summing up the different barriers to gender equality in academia, taken from Holmes (2015).

Credit: Alice Adenis

The sentences I quoted in the beginning of the article, and illustrated by Alice Adenis throughout this post, are examples of sexist microaggressions (look up the interactional circle in the diagram!). Generally speaking, microaggressions are, according to Derald Wing Sue (2010), “brief, everyday exchanges that send denigrating messages to certain individuals because of their group membership”. In the context of sexism, they remind women of the stereotypical roles society has assigned to them: we should be pleasant to the eye; our most important achievement should be to become a mother; we are not as competent as men in science, and therefore any attempt to reach parity in committees means that women are helped or preferred over more competent men… Taken individually, and depending on the person they are addressed to, they go unnoticed, they are annoying, or they are deeply hurtful. Put together and accumulated over time, they create a chilly climate for women in academia and contribute to discourage young female researchers to pursue an academic career.

Credit: Alice Adenis

These sexist microaggressions are the subject of an initiative to which I contribute, together with Alice Adenis, Claire Mallard, Maëlis Arnould, Martina Ulvrova, Mélanie Gérault and Nicolas Coltice: the project “did this really happen?!“. We gather testimonies of everyday sexism in academia, translate them into comics, and publish them on this blog. The aim is to show the nature of current everyday sexism in academia, to make it visible to people who do not see it, and to start a conversation in our community on how we can do better, be more inclusive and more respectful of each other. To achieve this goal, we need you, dear reader. You want to contribute? Here is what you can do:
• Enjoy reading the comics, and think about how you would have reacted in such situations
• Share the contents of the website on your favorite social media
• Print some comics and put them in the common room of your lab to start discussions
• Share one of your personal stories with us, anonymously or not, through this form

Finally, you can join the course on unconscious bias and the session on ‘promoting and supporting equality of opportunities in geosciences’ of the next EGU general assembly in Vienna!

Hope to see you there!

Credit: Alice Adenis









A text to those who do not see why I ‘make such a fuss’ about some people sometimes saying stuff which are ‘maybe a bit sexist’.

Marie Bocher

You might feel like I’m attacking you. I am not. I’m against sexist behaviour – not against people. I am not fighting against men, I am fighting against patriarchy. I have very rarely encountered profoundly sexist people, and I am convinced that the people who did say the sentences I gave as an example meant no harm. Moreover, I have also said sexist (and racist) stuff and will probably say more in the future, because – like the majority of researchers right now – I grew up and live in a white-supremacist and patriarchal society, and this affects my behaviour even if I don’t want to, even if I am a convinced feminist, fighting for a world with more equality.

That being said, here is how I interpret the example sentences and why I think they are not acceptable:

“You know it is so much easier for women in science these days”
This sentence is a classical variation on the concept that women are now favoured over more competent men because of parity issues. While the discrimination against women during the recruitment process has been documented (see for example this article on CV selection, this article on the letter of recommendations, and this article on the same topic), I am still trying to find a study on all these incompetent women who steal the jobs of competent men…

“Oh I don’t hire female PhD students anymore: they get pregnant and then they’re lost for science”
By saying that, you suppose that every woman will systematically want children and renounce her career plans as soon as she becomes a mother. This results in restricting women to only one of the many lives they could choose for themselves. This is also gender discrimination and illegal in a lot of countries.

“Yes, I remember you, you were wearing that red dress last time”
When you say that, you send the message that you are paying more attention to my appearance than to what I have to say. This is objectifying and out of place in a professional setting.

“Now that you have responsibilities, you can’t get pregnant again”
Choosing to have a child or not IS A PERSONAL DECISION. Please do not give your opinion on these matters unless your colleague actually asks for it.

Alice Adenis: Cartoonist, Data Scientist, and PhD in geophysics

“Oh, yes, they needed a woman for this committee, that’s why they asked you.”
You are implying that I am not competent for this committee, but only selected for my gender. That is insulting.

“You should be happy that someone called you an angel [author’s note: in a professional setting], that means that you are beautiful, what are you complaining about?”
See the red dress remark.

“I do not understand why women need to work, I mean, my wife did a marvelous job raising our children while I was working, I don’t get why this way of life has to change.”
Again, this restricts women into one role, that does not necessarily fit everybody.

 Holmes, M. A. (2015) A Sociological Framework to Address Gender Parity, in Women in the Geosciences: Practical, Positive Practices Toward Parity (eds M. A. Holmes, S. OConnell and K. Dutt), John Wiley & Sons, Inc, Hoboken, NJ. doi: 10.1002/9781119067573.ch3
 Sue, Derald Wing. Microaggressions in everyday life: Race, gender, and sexual orientation. John Wiley & Sons, 2010.

Subduction through the mantle transition zone: sink or stall?

Subduction through the mantle transition zone: sink or stall?

The Geodynamics 101 series serves to showcase the diversity of research topics and methods in the geodynamics community in an understandable manner. We welcome all researchers – PhD students to professors – to introduce their area of expertise in a lighthearted, entertaining manner and touch upon some of the outstanding questions and problems related to their fields. For our latest ‘Geodynamics 101’ post, Saskia Goes, Reader at Imperial College London, UK, discusses the fate of subducting slabs at the mantle transition zone.

Saskia Goes

Subducting plates can follow quite different paths in their life times. While some sink straight through the upper into the lower mantle, others appear to stall in the mantle transition zone above 660 km depth. Geodynamicists have long puzzled about what controls these different styles of behaviour, especially because there appear to be correlations between sinking or stalling with faster or slower plate motions and mountain building or ocean basin formation, respectively. In the long run, how easily slabs sink through the transition zone controls how efficiently material and heat are circulated in the mantle.

The word subduction derives from the Latin verb subducere, which means pulled away from below, but metaphorically can mean to lose footing or remove secretly. Definitely, when Wegener first proposed continental drift, people were unaware that subduction is removing plates from the Earth’s surface. We now know this process is not quite so secret. The plates creak in earthquakes as they sink into the mantle, in some cases all the way through the mantle transition zone to about 700 km depth. Furthermore, where the subducting plate bends below the overriding plate, it creates deep-sea trenches with prominent gravity and geoid signals. This bending is a very important part of subduction dynamics, as I’ll explain below.

The seismic Wadati-Benioff zones and gravity expressions were sufficient clues of the location of the downwelling limbs of a mantle convection system to help acceptance of plate tectonics in the 1960s. However, it took another twenty odd years until seismology yielded images of cold plates sinking into the mantle, and it turned out that the plates extend beyond the seismic Wadati-Benioff zones [Van der Hilst et al., 1991; Zhou and Clayton, 1990]. These images showed that some subducting plates flatten in the mantle transition zone (e.g. below Japan and Izu-Bonin), while others continue with little to no deflection into the lower mantle (e.g., below the Northern Kuriles and Marianas) (Fig. 1). Soon after, it was realised that many of the places where the slabs are flat in the transition zone have a history of trench retreat [Van der Hilst and Seno, 1993]. Furthermore, mapping of seafloor ages revealed that flat slabs tend to form where plates older than about 50 Myr are subducted [Karato et al., 2001; King et al., 2015].

Figure 1: Variable modes of slab-transition zone interaction

Many mechanisms have been proposed for the variable slab transition-zone interaction. We recently reviewed the geodynamic and observational literature and combined these insights with those from our own set of mechanical and thermo-mechanical subduction models [Goes et al., 2017]. This effort shows that not one single mechanism, but an interplay of several mechanisms is the likely cause of the observed variable subduction behaviour.

It has long been realised that viscosity increases with depth into the mantle, quite possibly including jumps at the major phase transitions in the mantle transition zone. The ringwoodite-postspinel transition that is responsible for the global 660 km seismic discontinuity, usually taken as the base of the upper mantle, is an endothermic transition under most of the conditions prevailing in the mantle today. This means that the transition will take place at a higher pressure and thus depth in the subducting plate than the surrounding mantle, rendering the plate locally buoyant with respect to the mantle. Both these factors hamper the descent of the subducting plate through the transition zone. However, a viscosity increase within acceptable bounds (as derived from geoid and postglacial rebound modelling) can slow sinking, but does not lead to stalling material. By contrast, the phase transition can lead to stalling, as well as an alternation of periods of accumulation of material in the transition zone and periods where this material flushes rapidly into the lower mantle, at least in convection models without strong plates. But does this work with strong plates?

Making dynamic models of subduction with strong plates is challenging because the models need to capture strong strength gradients between the core of the plate and the underlying mantle, allow for some form of plate yielding, maintain a weak zone between the two plates and adequately represent the effect of plate bending (a free-surface effect). Most models prescribe at least part of the system by imposing velocities and/or plate geometries. This however needs to be done with great care and consideration for what forcing such imposed conditions imply.

“Pulled away from below” is a good description of the dynamics of subduction. Subduction is primarily driven by slab pull, the gravitational force on the dense subducting plate [Forsyth and Uyeda, 1975]. And to “lose footing” reminds us that gravity is the main driving force. Gravity tries to pull the plate straight down (Fig. 2), so the easiest way for a plate to subduct is to fall into the mantle, a process that leads to trench retreat [Garfunkel et al., 1986; Kincaid and Olson, 1987]. Besides letting the plate follow the path of gravity, subduction by trench retreat has the other advantage that the plate does not need to bend too much. Bending a high-strength plate takes significant energy. Some studies have shown that if plates are assigned laboratory-based rheologies, such bending can easily take up all of the gravitational potential energy of the subducting plate [Conrad and Hager, 1999], so if plates are to sink into the mantle, they have to do this by minimising the amount of energy used for bending into the trench. As a consequence, strong and dense plates prefer to subduct at smaller dip angles while weaker and lighter plates can be bent to subduct more vertically [Capitanio et al., 2007].

Figure 2: If subduction occurs freely, i.e., driven by the pull of gravity on the dense slab with sinking resisted by the viscous mantle, it is usually energetically most favourable to subduct by trench retreat.

The angle at which plates subduct strongly affects how they subsequently interact with viscosity or phase interfaces (Fig. 3). Steeply dipping plates will buckle and thicken when they encounter resistance to sinking. This deformation facilitates further sinking, as a bigger mass. But plates that reach the interface at a lower dip may be deflected. Such deflected plates have a harder time sinking onwards, both because the high viscosity resistance is now distributed over a wider section of the plate and due to the spread-out additional buoyancy from the depressed endothermic phase boundary.

Figure 3: The subduction angle largely determines how the slab interacts with viscosity and phase changes.

So, variable plate density and strength can lead to variable behaviour of subduction in the transition zone. And we know plates have variable density and strength. Older plates are denser and if strength is thermally controlled, as most lab experiments predict, also stronger than younger plates. This implies that older plates can drive trench retreat more easily than young plates. And indeed this matches observations that significant trench retreat has only taken places where old plates subduct. Furthermore, significant trench retreat will facilitate plate flattening in the transition zone, consistent with the observation that flat plates tends to underlie regions with a history of trench retreat (even if that does not always mean trench motions are high at the present day). This mechanism can also explain why flat slabs tend to be associated with old plate subduction.

So what about the role of other proposed mechanisms? Our models with strong slabs show that only when slabs encounter both an increase in viscosity (which forces the slabs to deform or flatten) and an endothermic phase transition (which can lead to stalling of material in the transition zone) do we find the different modes of slab dynamics. Neither a viscosity increase alone, nor an endothermic phase transition alone leads to mixed slab dynamics.

Other factors likely contribute to the regional variability. In the cold cores of the slabs, some phases may persist metastably, thus delaying the transformations to higher density phases to a larger depth. Metastability will be more pervasive in colder old plates thus making older plates more buoyant and hence resistant to sinking than young ones. In combination with trench retreat facilitated by a strong slab at the trench, this can further encourage slab flattening [Agrusta et al., 2014; King et al., 2015]. Phase transformations may also lead to slab weakening in the transition zone because they can cause grain size reduction. Such weakening can aid slab deflection [Čížková et al., 2002; Karato et al., 2001]. However, several studies have shown that transition zone slab strength is less important than slab strength at the trench, which governs how a slab starts sinking through the transition zone.

The Earth is clearly more complex than the models discussed. For example, present-day plate dip angles display various trends with plate age at the trench. Lateral variations in plate strength and buoyancy can complicate subduction behaviour. Furthermore, forces on the upper plate and large-scale mantle flow may also impede or assist trench motions and may thus affect or trigger changes in how slabs interact with the transition zone [Agrusta et al., 2017]. All these factors remain to be fully investigated. However, the first order trends of subduction-transition zone interaction can be understood as a consequence of plates of various ages interacting with a viscosity increase and endothermic phase change.

 Agrusta, R., J. van Hunen, and S. Goes (2014), The effect of metastable pyroxene on the slab dynamics, Geophys. Res. Lett., 41, 8800-8808.
 Agrusta, R., S. Goes, and J. van Hunen (2017), Subducting-slab transition-zone interaction: stagnation, penetration and mode switches, Earth Planet. Sci. Let., 464, 10-23.
 Capitanio, F. A., G. Morra, and S. Goes (2007), Dynamic models of downgoing plate buoyancy driven subduction: subduction motions and energy dissipation, Earth Planet. Sci. Lett., 262, 284-297.
 Čížková, H., J. van Hunen, A. P. van der Berg, and N. J. Vlaar (2002), The influence of rheological weakening and yield stress on the interaction of slabs with the 670 km discontinuity, Earth Plan. Sci. Let., 199(3-4), 447-457.
 Conrad, C. P., and B. H. Hager (1999), Effects of plate bending and fault strength at subduction zones on plate dynamics, J. Geophys. Res., 104(B8), 17551-17571.
 Forsyth, D. W., and S. Uyeda (1975), On the relative importance of driving forces of plate motion. , Geophys. J. R. Astron. Soc. , 43, 163-200.
 Garfunkel, Z., C. A. Anderson, and G. Schubert (1986), Mantle circulation and the lateral migration of subducted slab, J. Geophys. Res., 91(B7), 7205-7223.
 Goes, S., R. Agrusta, J. van Hunen, and F. Garel (2017), Subduction-transition zone interaction: a review, Geosphere, 13(3. Subduction Top to Bottom 2), 1-21.
 Karato, S. I., M. R. Riedel, and D. A. Yuen (2001), Rheological structure and deformation of subducted slabs in the mantle transition zone: implications for mantle circulation and deep earthquakes, Phys. Earth Plan. Int., 127, 83-108.
 Kincaid, C., and P. Olson (1987), An experimental study of subduction and slab migration, J. Geophys. Res., 92(B13), 13,832-813,840.
 King, S. D., D. J. Frost, and D. C. Rubie (2015), Why cold slabs stagnate in the transition zone, Geology, 43, 231-234.
 Van der Hilst, R. D., and T. Seno (1993), Effects of relative plate motion on the deep structure and penetration depth of slabs below the Izu-Bonin and Mariana island arcs, Earth Plan. Sci. Let., 120, 395-407.
 Van der Hilst, R. D., E. R. Engdahl, W. Spakman, and G. Nolet (1991), Tomographic imaging of subducted lithosphere below northwest Pacific island arcs, Nature, 353, 37-43.
 Zhou, H.-w., and R. W. Clayton (1990), P and S Wave Travel Time Inversions for Subducting Slab Under the Island Arcs of the Northwest Pacific, J. Geophys. Res., 95(B5), 6829-6851.

Postcard from Singapore: Global Young Scientists Summit 2018

Postcard from Singapore: Global Young Scientists Summit 2018

Excite, engage, enable. These three words were the driving mission behind the gathering of over 250 PhD and postdoctoral fellows at the Global Young Scientists Summit (GYSS) in Singapore. In January 2018, Thomas Schutzius, Michael Zumstein, Daniel Sutter, and I had the distinct pleasure of representing the Swiss Federal Institute of Technology (ETH Zürich) at this year’s summit.

The GYSS is a multi-disciplinary summit, covering topics ranging from chemistry, physics and medicine to mathematics, computer science and engineering. With its theme “Advancing Science, Creating Technologies for a Better World”, GYSS focuses on key areas in science, research, technology innovation, and society, as well as their solutions to tackle global challenges. The speakers invited to the GYSS are globally-recognised scientific leaders, who are recipients of prestigious awards: the Fields Medal, Millennium Technology Prize, Nobel Prize, and Turing Award. They gave lectures and talks, and held discussions both with summit participants and also the public. During the summit, anyone could come across these extraordinary scientists at various venues such as the National University of Singapore, the National Library and the Science Centre Singapore.

One of my main takeaways from the conference is that the boundaries between the fields are blurring. This encouraged participants to ask more questions, even in fields they were not experts in. Indeed, they were not shy about doing this. I have also observed how more and more researchers are studying and working in Asia—just another sign of the times.

“I love the creativity, enthusiasm, and optimism of young scientists. It gives me energy!”, says Frances Arnold who was awarded the 2016 Millennium Technology Prize (MTP) for her innovations of directed evolution and efficient methods for creating enzymes. Stuart Parkin, the 2014 MTP winner for multiplying information storage capacity and enabling Big Data, finds the fresh way of thinking of the young generation of scientists especially interesting. “Meeting young scientists is always very stimulating since they often think differently from scientists and researchers later in their career who perhaps become too aligned with the dogma of the research establishment”, professor Parkin says.

Stuart Parkin, Millennium Technology Prize (2014). Credit: National Research Foundation.

I am tremendously grateful for having been given the chance to represent the Swiss Federal Institute of Technology (ETH Zürich) at the Global Young Scientists Summit (GYSS) 2018. I would describe attending the GYSS as a once-in-a-PhD opportunity, if not a once-in-a-lifetime opportunity. The week-long event is highly interdisciplinary—covering a range of topics from medicine to engineering and biology to physics. Moreover, the poster session is designed to bring together ideas and researchers from different fields. It provided me with an exciting opportunity to share broader insights from my PhD with researchers outside of geophysics. Singapore needs all the talent it can get, especially as companies and universities invest in new technologies and spread their wings abroad.

I certainly had several scientists whom I much admired and respected—mostly those who really were interested in understanding the fundamental workings of nature and not those who rather treated science and research more as a career path.

ETH delegation at GYSS 2018, from left to right: Thomas Schutzius, Daniel Sutter, Michael Zumstein and Luca Dal Zilio. Credit: National Research Foundation.

Singapore is much more than the sum of its numerous attractions. It’s constantly evolving, reinventing, and reimagining itself, especially with people who are passionate about creating new possibilities. It’s where foodies, explorers, collectors, action seekers, culture shapers, and socialisers meet and new experiences are created every day. Although small in physical space (the country is about half the size of Los Angeles), Singapore offers large opportunities for high quality research set on a breathtaking tropical island with a bustling metropolitan area. Whether I was navigating the crowds in Chinatown, exploring Hindu temples in Little India, eating diverse cuisines in a local hawker center or relaxing in the Chinese Garden, this beautiful island provided exciting adventures every day.

Solar supertrees are vertical gardens in Gardens by the Bay in Singapore, which are designed to mimic the ecological functions of real trees. Each structure is outfitted with an array of photovoltaic cells that collect and store solar energy throughout the day – power that’s used to illuminate the garden when the sun goes down each night.

I have immersed myself in a society with very different cultural norms and rules from my own. By witnessing the functionality of many of Singapore’s well-known programs, such as the housing development board and water treatment plant, we were enabled to expand our views of what a successful society can be. Upon returning from GYSS 2018 in Singapore, I have gained a refreshing new perspective on what it means to be a part of the international scientific community. My time in Singapore reminds me of the beautiful unity we all share: to further the progress of humanity and better our global society. Through its fundamental research and implementations, Singapore is an excellent example of how scientific innovations can be integrated for the welfare of society.

Being both strong and weak

Being both strong and weak

The Geodynamics 101 series serves to showcase the diversity of research topics and methods in the geodynamics community in an understandable manner. We welcome all researchers – PhD students to Professors – to introduce their area of expertise in a lighthearted, entertaining manner and touch upon some of the outstanding questions and problems related to their fields. For our latest ‘Geodynamics 101’ post, Postdoc Anthony Osei Tutu of GFZ Potsdam shares the outcomes of his PhD work, showing us that, like the lithosphere, it is OK to be weak sometimes!

Strength is not everything in achieving one’s goal. The lithospheric plate acts both strong and weak at times. This dual characteristic of the outermost part of the Earth, the crustal-lithospheric shell, is thought to have sustained plate tectonics throughout Earth’s history, in the presence of other controlling mechanisms such as the weak asthenospheric layer (Bercovici et al. 2000; Karato 2012). In the world of the lithospheric plates there is the saying “I might be strong and unbreakable, but sometimes and somewhere, I am very weak, soft and brittle” and this allows the plates to accommodate each other in their relative movements.

We all sometimes need to bring out the soft part in us to accommodate others such as friends, family or colleagues. For example, my graduate school, the Helmholtz-Kolleg GEOSIM, an experiment by the Helmholtz Association, GFZ-Potsdam, University of Potsdam and Free University of Berlin, brought together two or more experts in mathematics and geosciences to collaborate on and serve as PhD supervisors for answering some of Earth Sciences’ pressing questions. The many, many benefits of this multidisciplinary PhD supervising approach also came with challenges. Sometimes, the different supervisors would make opposing/contrasting suggestions to investigate a particular problem according to the experience of some students and myself. Then it falls on you as the student to stand firm (i.e. be strong) on what you believe works for your experiments and at the same time to be receptive (i.e. flexible or soft) to the different suggestions, while keeping in mind the limited time you have as a PhD student.

Figure 1: Schematic plot of the conditions in a subduction system (left) aiding or (right) hindering global plate motions.

The both strong and weak behavior of the lithospheric plates was one of the conclusions of my PhD study. Besides the strong plate interiors (Zhong and Watts 2013), weak regions along the plate boundaries, aided by sediment and water (see Fig. 1), are required to give the low friction between the subducting and overriding plates (Moresi and Solomatov 1998; Sobolev and Babeyko 2005), combined with a less viscous sublithospheric mantle. This combination was key to match the magnitude and direction of present-day global plate motions in the numerical modeling study (Osei Tutu et al. 2018). I used the global 3D lithosphere-asthenosphere numerical code SLIM3D (Popov and Sobolev 2008) with visco-elasto-plastic rheology coupled to a mantle flow code (Hager and O’Connell 1981) for the investigation. To understand the influence of intra-plate friction (brittle/plastic yielding) and asthenospheric viscosity on present-day plate motions, I tested a range of strengths of the plate boundary. Past numerical modeling studies (Moresi and Solomatov 1998; Crameri and Tackley 2015) have suggested that small friction coefficients (μ < 0.1, yield stress ~100 MPa) can lead to plate tectonics in models of mantle convection. This study shows that in order to match present-day plate motions and net rotation, the static frictional parameter must be less than 0.05 (15 MPa yield stress). I am able to obtain a good fit with the magnitude and orientation of observed plate velocities (NUVEL-1A) in a no-net-rotation reference frame with μ < 0.04 and a minimum asthenosphere viscosity of 5•1019 Pas to 1•1020 Pas (Fig. 2). The estimates of net-rotation (NR) of the lithosphere suggest that amplitudes of ~0.1– 0.2 °/My, similar to most observation-based estimates, can be obtained with asthenosphere viscosity cutoff values of ~1•1019 Pas to 5•1019 Pas and a friction coefficient μ < 0.05.

Figure 2: Set of predicted global plate motions for varying asthenosphere viscosity and plate boundary frictions, modified after Osei Tutu et al. (2018). Rectangular boxes show calculations with RMS velocities comparable to the observed RMS velocity of NUVEL-1A (DeMets et al. 2010).

The second part of my PhD study focused on the responses of the strong plate interiors to the convecting mantle below by evaluating the influence of shallow and deep mantle heterogeneities on the lithospheric stress field and topography. I explored the sensitivity of the considered surface observables to model parameters providing insights into the influence of the asthenosphere and plate boundary rheology on plate motion by testing various thermal-density structures to predict stresses and topography. Lithospheric stresses and dynamic topography were computed using the model setup and rheological parameters that gave the best fit to the observed plate motions (see rectangular boxes in Fig. 2). The modeled lithosphere stress field was compared the World Stress Map 2016 (Heidbach et al. 2016) and the modeled dynamic topography to models of observed residual topography (Hoggard et al. 2016; Steinberger 2016). I tested a number of upper mantle thermal-density structures. The thermal structure used to calculate the plate motions before is considered the reference thermal-density structure, see also Osei Tutu et al. (2017). This reference thermal-density structure is derived from a heat flow model combined with a sea floor age model. In addition I used three different thermal-density structures derived from global S-wave velocity models to show the influence of lateral density heterogeneities in the upper 300 km on model predictions. These different structures showed that a large portion of the total dynamic force generating stresses in the crust/lithosphere has its origin in the deep mantle, while topography is largely influenced by shallow heterogeneities. For example, there is hardly any difference between the stress orientation patterns predicted with and without consideration of the heterogeneities in the upper mantle density structure across North America, Australia and North Africa. However, inclusion of crustal thickness variations in the stress field simulations (as shown in Fig. 3a) resulted in crustal dominance in areas of high altitude in terms of stress orientation, for example in the Andes and Tibet, compared to the only-deep mantle contributions (as shown in Fig. 3b).

Figure 3: Modeled lithosphere stress field in the Andes considering (a) crustal thickness variations from the CRUST 1.0 model as well as lithospheric variations and (b) uniform crustal and lithospheric thicknesses.

The outer shell of the solid Earth is complex, exhibiting different behaviors on different scales. In our quest to understand its dynamics, we can learn from the lithospheric plate’s life cycle how to live our lives and preserve our existence as scientist-humans by accommodating one another. After all, they have existed for billions of years.



Bercovici, David, Yanick Ricard, and Mark A. Richards. 2000. “The Relation Between Mantle Dynamics and Plate Tectonics: A Primer.” 5–46.

Crameri, Fabio and Paul J. Tackley. 2015. “Parameters Controlling Dynamically Self-Consistent Plate Tectonics and Single-Sided Subduction in Global Models of Mantle Convection.” Journal of Geophysical Research: Solid Earth 120(5):3680–3706, 10.1002/2014JB011664.

DeMets, Charles, Richard G. Gordon, and Donald F. Argus. 2010. “Geologically Current Plate Motions.” Geophys. J. Int 181:1–80.

Hager, BH and RJ O’Connell. 1981. “A Simple Global Model of Plate Dynamics and Mantle Convection.” Journal of Geophysical Research: Solid Earth, 86(B6):4843–4867, 10.1029/JB086iB06p04843.

Heidbach, Oliver, Mojtaba Rajabi, Moritz Ziegler, Karsten Reiter, and Wsm Team. 2016. “The World Stress Map Database Release 2016 -Global Crustal Stress Pattern vs. Absolute Plate Motion.” Geophysical Research Abstracts EGU General Assembly 18:2016–4861.

Hoggard, M. J., N. White, and D. Al-Attar. 2016. “Global Dynamic Topography Observations Reveal Limited Influence of Large-Scale Mantle Flow.” Nature Geoscience 9(6):456–63, 10.1038/ngeo2709.

Karato, Shun-Ichiro. 2012. “On the Origin of the Asthenosphere.” Earth and Planetary Science Letters 321–322:95–103.

Moresi, Louis and Viatcheslav Solomatov. 1998. “Mantle Convection with a Brittle Lithosphere: Thoughts on the Global Tectonic Styles of the Earth and Venus.” Geophysical Journal International 133(3):669–82, 10.1046/j.1365-246X.1998.00521.x.

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