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Geodynamics

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The Sassy Scientist – Key Papers

The Sassy Scientist – Key Papers

Every week, The Sassy Scientist answers a question on geodynamics, related topics, academic life, the universe or anything in between with a healthy dose of sarcasm. Do you have a question for The Sassy Scientist? Submit your question here.

Luca asks:


What is (in your opinion) the key paper in geodynamics and why?


Dear Luca,

There is not one key paper. It is simply impossible to point at one paper and say: “This is the one that changed geodynamics”. Obviously, without the early work of Wilson, McKenzie, Froidevaux, Morgan, Turcotte, and Schubert (and many others, the list is enormous, don’t be offended if you’re not on it) the field of geodynamics wouldn’t even exist. However, unless you are a bachelor student, it should be fairly clear by now that (Earth) science is an evolving beast that can take many shapes and lead to many dead-ends. Well-considered concepts are continually overturned, contradicted or adjusted.

So, it seems to me that you are looking for a shortcut. Unfortunately this shortcut doesn’t exist and you’ll just have to go through the literature. Painstakingly and critically evaluating papers. And you’re right: most of the ones you’ll come across will not be qualified as ‘key papers’. But this is a good thing. It’s better to go through an abundance of small steps to move science forward than to take major leaps while overlooking small but significant details that take you across a decades-long detour…

Still interested in science?

Yours truly,

The Sassy Scientist

PS: This post was written after getting annoyed by stupid questions.

Happy blog birthday!

Happy blog birthday!

Can you believe it, people? We have been running this blog for 2 years! What a milestone! Time to celebrate and look back at a year of great blogging.

Who are the champions?

We are the champions, my friends!

That’s right! We actually won a prize this year: we won best blog post of 2018 by public vote for a post by one of our editors, Luca Dal Zilio, about a conference he attended in Singapore. So we are now an award-winning blog. Hell yeah!
Technically this post was written during our first blog year, but hey, we only got the prize in January, so I think we’re totally within our rights to mention it now. Since we won by public vote, we would like to thank all of you – our readers – for your support! It really means a lot to us and strengthens the idea that some people actually read this blog!

And talking about our readers…

Who are you?

Contrary to popular belief, there are actually people who read this blog! Although I don’t have any data to back this up, I think most of our readers are actually scientists from the geodynamics, tectonics and structural geology, and seismology divisions of EGU. So much for trying to do outreach. If I’m wrong, please let me know!
I do have some data on how many people visit our blog. On average, we have 37 unique visitors per day (and that’s quite something if you remember that we only post on Wednesdays and now Fridays). Our 100 most viewed pages (and this includes the homepage, author profiles, tags, etc. as well as individual posts) each have seen 150 unique visitors on average with some posts having over 2000 unique visitors in the past year! These popular posts are usually commissioned and promoted on social media by our editor Grace Shephard. Little gems of blog posts with many views from her hand include The Rainbow Colour Map (repeatedly) considered harmful, Thirteen planets and counting, and How good were the old forecasts of sea level rise?

We have a global readership! Most of our readers are accessing the website from the US, the United Kingdom, Germany, China, and Spain. Interesting list of countries, right? Apart from that, we also have some geodynamicists in Turkmenistan, Rwanda, Pakistan, Myanmar, and Mozambique. Or maybe that is just a result of geodynamicists being on holiday in exotic places who are dying to read geodynamics news to distract themselves from their amazing holiday destination. Who knows?

Who are we?

We have a lovely blog team and it’s quite a big team as well! Currently, there are 7 regular editors, one mysterious, anonymous editor under the name of ‘The Sassy Scientist’ and one Editor-in-Chief (yours truly). Who would’ve ever thought there would be 9 people in the blog team? Last year, we were 5, so we have grown a lot. If you have already forgotten who we are, you can check out our recent introduction post.

But, these 9 amazing editors don’t have the time (or the expertise) to write all these blog posts themselves. Therefore, we heavily rely on the most amazing guest authors. During the past year, we had 20 guest authors who contributed one or more posts. So here is a big shout out to all the guest authors of the past year:

• Manar Alsaif
• Marie Bocher
• Daniel Bowden
• Kiran Chotalia
• Robert Citron
• Lorenzo Colli
• João Duarte
• Rene Gassmöller
• Lars Gebraad
• Antoniette Greta Grima
• Charitra Jain
• Kirster Karlsen
• Maria Koroni
• Laurent Montesi
• Andrea Piccolo
• Adina Pusok
• Nico Schliffke
• Paul Tackley
• Katy Willis
• Jonny Wu

Thank you so much. We couldn’t do it without you!

Behind the scenes

During our first 1.5 years of blogging we had a system in place where we had regular types of posts, such as Geodynamics 101, Remarkable Regions, Peculiar Planets, and Wit & Wisdom posts. Additional content that did not fit in any of these categories, would go into our News & Views or Conferences. This meant that all the editors were encouraged to find posts that fitted into a certain topic and then we hoped for the best. In practice, most of the responsibility lay with yours truly: the Editor-in-Chief. I was in charge of keeping an eye on the schedule and asking the editors to contribute. In the end, I wrote and commissioned most of the posts myself.

That was clearly unsustainable.

Whenever I had a lapse of vigilance, holes were more likely to appear in the schedule, because no one else in the blog team felt responsible for posting (and rightfully so). This lead to the infamous gaps in content around February (which seems to be a recurring yearly theme).

Again: clearly unsustainable.

So. This year, I got inspired at the EGU Blog Editor meeting a few days before the General Assembly and I thought of a complete new blog strategy during EGU. I know: I spent my time at EGU wisely…

We recruited a bunch of new editors and we have now successfully implemented a new schedule: all 7 regular editors are responsible for a blog post for 1 week in a 7 week cycle. They can commission blog posts, write them, give their slot to another editor who might have more blog posts lined up, or whatever they want to do, but they are – in the end – responsible for uploading a blog post on Wednesday. I still keep an eye on the schedule and fill gaps where necessary, but at least now I have someone to address whenever there seems to be an empty slot. We also added some extra repercussions to increase the responsibility of our regular editors: if they fail to upload a Wednesday blog post on time in their scheduled weeks twice within one year, they will automatically stop being editors. Of course, we want to keep everyone in the team, so everyone is encouraged to help each other out, if a gap in the schedule threatens to appear.
The Sassy Scientist is responsible for weekly Friday Q&As, which is a lot of work actually: 52 blog posts in a year is a lot. So for these posts, we are working with a backlog of at least 5 blog posts at all times to ensure that our Sassy Scientist can sometimes take a holiday. Currently, I am editing all the Sassy Scientist blog posts, but I’m hoping they can fly solo soon! One of the most difficult things is getting questions for the Sassy Scientist to answer. So far most of the questions have been asked by editors, although some wished to remain anonymous, so their names were changed. Therefore, we ask everyone to just e-mail the Sassy Scientist a question, leave a comment to one of the blog posts or on social media. You will remain anonymous, if you so desire, and we will make sure that your question gets answered soon (i.e., we will adapt the schedule accordingly).
The new system works well so far. Let’s see if we can keep it up!

So now what?

Well, onwards and upwards, don’t you think? We will try to keep providing you with geodynamics news on Wednesdays and Sassy Scientist Q&As on Fridays for another year. That’s twice as much content as in our first year! Theoretically. We have lots of great posts lined up as well as some very impressive, new guest authors who are dying to pen down their thoughts. If you would like to contribute to the blog, don’t hesitate to contact us by sending us an e-mail. Until then: enjoy the read!

The Sassy Scientist – Time Trials

The Sassy Scientist – Time Trials

Every week, The Sassy Scientist answers a question on geodynamics, related topics, academic life, the universe or anything in between with a healthy dose of sarcasm. Do you have a question for The Sassy Scientist? Submit your question here.

Sylvie asks:


What would you say is the main problem you encountered during your research career?


Dear Sylvie,

Easy: time management and focus. Doing scientific research is hard enough without dealing with deadlines, waiting for collaborators to provide their contribution… —

*ping* *ping* *ping*
ahem… sorry, just needed to check these emails

—…and adjusting paper manuscripts due to nitpicky comments from reviewers that took their sweet time to reply. Meanwhile, students constantly ask numerous “clearly defined” and ambitious questions… —

*knock* *knock*
Just a minute! I’ll be right out to talk to you about that hand-in assignment…

— …and I still need to finish those abstracts for AGU. I should first check the rules about authorship. Oh, only one first-author abstract this year. How am I going to handle this? I hope my co-conveners can take up some of the abstract checking — the list of abstracts is getting quite long… Did I already book my flight and hotel? I am getting sick of those hostels. Can I use my presentation from this year’s EGU?

*ping* *knock*
Just one more minute
*ping*

What was I saying?

Right: time management and focus. Initially, you wouldn’t say this could become a problem. However, after a while you’ll find that decent planning is critical: issues will snowball and reach a tipping point if you are not careful. This also concerns planning your future: designate some time in your busy schedule to evaluate the current status of your projects and assess your priorities for the next months.

Now, where did that student go…

Yours truly,

The Sassy Scientist

Ps: This post was written in the last weeks before my extensive summer holiday (I promised a real one this year) and the final weeks of the academic year: my favourite time of the year for answering your questions… 

Production and recycling of Archean continental crust

Production and recycling of Archean continental crust

Continents are essential for the development and survival of life on Earth. However, as surprising as it may sound, there did not exist a planetary scale numerical model to show the formation of the oldest continents until the recent study ‘Growing primordial continental crust self-consistently in global mantle convection models‘ in Gondwana Research by Jain et al., 2019. Hot off the press, the first author of this study himself, Charitra Jain, Post Doctoral Research Associate in the Department of Earth Sciences at Durham University, shares the scoop in our News & Views!

Why do we care?

Uniquely positioned within the habitable zone [1], Earth is the sole planet within our solar system that sustains life. Understanding the factors that make a planet habitable [2,3] are becoming increasingly relevant with the rapidly expanding catalogue of extrasolar planets over the last decade [4,5]. The operation of plate tectonics and the formation and stability of continental landmasses have played a crucial role in the atmospheric evolution and development of life on Earth [6]. Plate tectonics is a theory where the outer surface of the Earth (lithosphere) is fragmented into a number of mobile plates that drift at a speed of few centimetres per year relative to each other, atop a convecting mantle. Mountains, volcanoes, and earthquakes are found at the boundaries of these plates. Continents cover about a third of the planet’s surface area and are made of thin crust overlying a thick and undeformable cratonic lithosphere [7,8]. Even after having assembled a reasonable model for its origins and internal workings, many fundamental questions pertaining to Earth sciences remain unresolved, for example:

  • How did the first continents form and what accounts for their stability over billions of years? Was their growth an episodic process [9] or a continuous process with a significant drop in crustal growth rate around 3 Ga (billion years ago) [10]? How much of the continental crust has recycled into the mantle [11]?
  • What was the global geodynamic regime exhibited by early Earth? When and how did the subduction-driven plate tectonics commence [12]? Did the presence of continents play a role in a regime transition from vertical tectonics to horizontal tectonics around 3Ga [13]?

Owing to the increasing paucity of natural observational data as we go back further in time [14], numerical modelling constrained by experimental and field data has thus become indispensable to uncover the secrets of Earth’s evolutionary history. Our recent study [15] is a step in the right direction where we have developed a new two-stage melting algorithm to create oceanic (basaltic/mafic) and continental (felsic) crust in self-consistent global mantle convection models for the first time.

What’s new in these models?

Generally, two stages of mantle differentiation are inferred to generate continental crust as shown in the schematic in Fig. 1A. First, basaltic magma is extracted from the mantle. Second, it is buried and partially melts to form felsic continental crust. During the much hotter Archean conditions [16,17], majority of continental crust was made of Tonalite-Trondhjemite-Granodiorite (TTG) [18,19] rocks. Experimental data suggests that TTGs are formed when hydrated basalt melts at garnet-amphibolite, granulite or eclogite facies conditions [20,21] and specific P-T conditions [22] have been employed as a criterion for generating TTGs by the authors in their models.

Interested in the long-term planetary evolution, we parametrised the processes of melt generation and melt extraction. If the melt is generated within the top 300 km of the mantle (Fig.1B1), it is instantaneously removed from the depth (Fig.1B2) [23,24] and transported both to the bottom of the crust (plutonism/intrusion) and to the top of the model domain (volcanism/eruption) (Fig. 1B3). The intruded melt stays molten while a temperature adjustment to account for adiabatic decompression is applied, and tends to result in a warm, weak lithosphere. The erupted melt is rapidly solidified by setting its temperature to the surface temperature (300K), resulting in a strong and cold lithosphere [25]. The mass ratio of erupted to intruded melt is controlled by a parameter called eruption efficiency, which is tested extensively in our simulations. Geological data suggests that the majority of mantle-derived melts intrude at a depth, corresponding to an eruption efficiency between 9% and 20% [26].

 

Figure 1: A, One-dimensional compositional variation with basalt-harzburgite continuum consisting of a mixture of olivine (ol) and pyroxene-garnet (px-gt) mineralogies in different proportions. Upon initialisa- tion, the whole mantle has a pyrolytic composition. B, Cartoon depicting a section of a mesh column (not to scale) in a stage: B1, where crustal production has already happened; B2, after melt removal but before compaction or opening gaps in lithosphere for magmatism; B3, with the eruption and intrusion of the melt with the white downgoing arrows representing compaction of tracers/markers.

How do these results stack up against data?

By varying initial core temperature, eruption efficiency, and limiting the mass of TTG that can be generated in our simulations to 10% and 50% of basalt mass, we present results from two sets of simulations. The crustal volumes have the same order of magnitude and the crustal composition follows similar trends as reported from geological and geochemical data [10,27,28]. Interestingly, we report two stages of TTG production without needing a significant change in convection regime: a period of continuous linear growth with time and intense recycling fuelled by strong plume activity and lasting for around 1 billion years, followed by a stage with reduced TTG growth and moderate recycling. The production of TTGs happen at the tip of deformation fronts driven by the lateral spreading of plumes (mantle upwellings) that rise to the surface (Fig. 2). These results indicate that the present-day slab- driven subduction was not required for the genesis of Archean TTGs [29,30] and early Earth exhibited a “plutonic squishy lid” or vertical-tectonics geodynamic regime [31–33].

 

Figure 2: Thermal (top) and compositional (bottom and zoom-in) evolution with time for a simulation with initial core temperature of 6000 K, mantle potential temperature of 1900 K, eruption efficiency of 40%, and TTG mass limited to 10% of basalt mass. The lighter shades of teal in the composition field represent progressive mantle depletion (higher harzburgite content) with time.

What’s coming next?

These results are parameter-dependent to some extent and they would change in a 3D domain as that would limit the impact of plumes on lithosphere dynamics. Future models will aim for forming the viscous cratonic roots and incorporating the effects of water [34,35] and grain-size [36–38] on mantle rheology. Going forward, modelling coupled core-mantle-atmosphere systems will shed more light on the role of different tectonic modes towards planetary habitability [39,40] and help solve these contentious aspects.

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