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Geodynamics

Geodynamics 101

The rock whisperers…

The rock whisperers…

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. This month, Manar Alsaif, PhD student at Université Montpellier, discusses actual rocks and field work!

In a discipline increasingly shaped by models, what can the rocks still tell us?

Flicking through your typical geodynamics bodies of work, most of the papers are on some kind of modelling – be it analogue, numerical, or something using seismic data. This is hardly surprising considering that geodynamics is all about the depths of the Earth, where we cannot make direct observations. But at some point, we need to check the results of these models, and checking them means looking for the observables. This is ok for the global scale – we can measure gravity and magnetism and the like, but what about smaller scales? And more detail? And the kind of complexity we cannot yet model? Our observations are restricted to the Earth’s surface – but this is actually not a bad place start. There is still a lot that we can learn from good old fashioned field work. And in fact, a lot of the motivation for models comes from an observation of something not understood, or not previously thought of.
So if you need a little inspiration for your next research project, I implore you to literally take a hike!

Apart from a source of inspiration, where does field work fit into a geodynamics workflow? I’d say it fits on top (no pun intended), since all geodynamic processes have a surface expression (at least to some degree).
Take plate motion, for example. Whether a plate moves laterally or vertically, that motion is recorded in the rocks. Palaeomagnetism will trace lateral motion, while thermochronometry will give you a vertical history of the rocks. More often than not, it will reveal a complex history of the rocks and the plate in which they lie. This complexity includes a myriad of processes e.g. fluid action, metamorphism, deformation, diagnesis, etc. These are all processes that are still not fully understood but which we can address by picking up a rock and looking at its mineralogy, its texture, its veins, its contact with its surrounding rocks, its P-T history, its fractures, their strain patterns, etc.

This is by no means an exhaustive article on field methods, I merely mention some examples of how field methods can be useful. So if field geology can be so useful, why are there fewer and fewer scientists doing it? Well, there’s the popular misconception that field geology is only geological mapping, and that the world’s geological surveys have more or less taken care of that already. In reality, some geological surveys have done a marvellous job at mapping out the rock units, but half of a geological map is actually interpretation. This interpretation will constantly change with new understanding of processes and with new data, especially where rock exposures are few and/or flighty.

Apart from the misconception that all field geology has ‘been done’, there are some practical reasons why geodynamicists veer away from field studies. Firstly, there is a mismatch of scales. Generally, the smallest scale a geodynamicist will deal with is a plate – that is already a scale which is too large for field work in practice. But as we argued above, field studies can tell you so much, so what do you do? Go strategic! Pick a few practical locations on your plate, where you might find the products of the processes you’re looking at. For example, if you are looking at obduction, go look around the high pressure rocks, which have probably already been mapped – thank you, local geo survey. If you’re looking at active faulting, use topography and satellite data to help guide you, and then a little thermochronometry can go a long way. If you’re looking at processes behind magmatism, look around your magmatic rocks, and then let the powers of geochemistry come to your aid. There are so many other examples that field geologists do and new tricks that we could start to do with a little creative thinking.

Drone field geology, bridging geo-scales. Tectonic study of Northern Scandinavia by CEED U. Oslo researchers Hans Jørgen Kjøll and Torgeir Andersen. Picture provided by Hans Jørgen Kjøll.

This is all made much easier by using satellite data as a first line of attack. Never before have we had such fine satellite data to simply strategising as we do now.
So maybe it’s also time to move on from old fashioned geological mapping – especially where pretty good maps already exist- and move on to more comprehensive, strategic field campaigns. And remember, technology can be our friend, we need not shy away from it. The photo here is not merely a gorgeous landscape, it is a drone picture by Hans Jørgen Kjøll and Torgeir Andersen of CEED, U. Oslo (seen as the people-looking lines in the middle of the photo). They are seen here flying a drone to get high resolution field data in rugged, inaccessible northern Scandinavia, while simultaneously bridging the scale of typical field work to large scale tectonics.
Similar advantages can be had by using LIDAR, various GPS methods, shallow logging techniques, etc. Perhaps it’s time to stop thinking of geologists as the hammer-hand lens people, and of geophysicists as the gadget people, and of geodynamicists as the code people. Perhaps it’s time to blur the lines, work together and learn from each other.

All of this might eventually give us more real data to plug into our models, perhaps refine some of the parameterisation, or at the very least, give us something against which to compare our model predictions.

After all, George Michael said it best: “Let’s go outside”!

Magma oceans

Magma oceans

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 we will talk about magma oceans!

When you think about the Earth a long, long time ago, in its late stages of formation, which picture comes to your mind? A lot of volcanoes erupting? Lava running from cracks in rocks? Meteorites falling? Maybe dinosaurs roaming the land? Well, first of all, there was certainly no life, and the dinosaurs only showed up somewhat recently in the history of the Earth (~240 million years ago, while the Earth is ~4500 million years old). In fact, our planet was a much more extreme environment that one might think… Even if frequent, meteorites would only arrive every few thousands of years. Temperatures at the surface were extremely high, as high as 2000 K (1727 °C). Yeah, forget it, too hot even to go to the beach. That, and… there was no land, because a big part of the rocks that form the Earth were molten and our planet was covered in a deep magma ocean. An ocean you definitely don’t want to go in for a swim.

Weather prediction for the Earth, 4.5 billion years ago: really hot, the sort of hot that doesn’t allow you to work on your tan because… well, everything was molten. This photo was in fact taken in the Hawaii Volcanoes National Park, in May 1954 during the eruption of the Kilauea Volcano. Credit: photo by J. P. Eaton, May 31, 1954

What?! A magma ocean?

That’s right, a magma ocean. Think of it as a body of magma, consisting of a mixture of molten or partially-molten rock, volatiles (dissolved gas and gas bubbles) and solids (suspended crystals). The magma behaves as a liquid, which means that the quantity of solids floating in the system is not enough for the solid particles to connect with each other. When that happens, we are left with a crystal mush, where the magma flows inside a porous rock, instead of having solid particles floating in the liquid, as happens in a magma ocean.

A magma ocean is an extreme environment that is vigorously convecting. Several laboratory experiments suggest that the consistency of such systems is something like honey, with uncertainties depending on several factors such as pressure, temperature, the amount of water and the presence of crystals. Yeah, extremely hot molten rocks flowing like honey covering all the surface of the Earth and extending to thousands of kilometres depth. Very, very different from today’s Earth, hum? By definition a magma ocean needs to encompass a substantial fraction of the planet (something like more than 10%), otherwise we can call it a magma pond or a magma lake. No swimming in these as well. Also, no ducks there.

Sounds a bit like science fiction, right?

Yeah… However, the idea has been around for some centuries. In the late XVII century, Gottfried Wilhelm Leibniz proposed that Earth began as a uniform liquid, and differentiated compositionally as it cooled. Others, such as Comte de Buffon, or Lord Kelvin tried to calculate the age of Earth based on the assumption that the Earth was once completely molten and cooled to present-day (and is still cooling).

Ironically, humankind had to go to the Moon in order for the magma ocean hypothesis to evolve to its modern form. Samples returned from the Apollo missions showed that the whiter part of the crust you can see when you look up to the Moon is composed of rocks called anorthosites. In short, and without going into detail, the only mechanism for the formation of these rocks that has attained general acceptance is if they formed through flotation of crystals to the top of a magma ocean on the early Moon. Other rocks recovered from the Moon are also consistent with a fractional solidification of a magma ocean.

If the Moon went through a magma ocean stage it is likely that the Earth underwent it too. Just as the other rocky planets in the inner Solar System: Mercury, Venus and Mars. In fact, various observations point to the occurrence of magma oceans in the early evolution of rocky planets. We know this through the observation of, for example, iron meteorites, or on Earth because we know that a large iron core exists. A magma ocean facilitates the formation of the core early in the history of the planets. Iron is a heavy metal that is thought to have arrived on the early Earth mixed with silicon and silicates. A very hot magma ocean would have melted the rocks and metals and allowed the heavier liquid iron to sink down to the centre of the Earth to form the core.

Thus, it is widely accepted that magma oceans are significant events in the earliest stages of planetary evolution and set the initial conditions for their future evolution. Meaning that understanding how a magma ocean evolved on Earth is key to understand why life exists on our planet.

How did magma oceans form?

The late stage of formation of the Earth and other rocky planets was an energetic process, where violent impacts between protoplanets happened. The energy provided by impacts was the largest energy source to heat and melt the Earth. There were however other sources of heat I should mention such as the conversion of gravitational energy of formation into heat, heat losses from the core at the core-mantle boundary, radioactive decay (mainly short-lived isotopes such as 26Al and 60Fe, which are important in the early stages of planetary formation), electromagnetic induction heating and tidal heating (caused by the proximity to another large object). Basically, a large amount of energy was being provided to the juvenile rocky planets. Thus, again, it is very likely they underwent one or multiple large-scale melting events.

It is probable that in the last giant impact to hit the Earth, the impactor was the size of Mars. Such a big impact had enough energy to melt at least a substantial part of the mantle. Oh, and this impact is also believed to have formed the Moon. Pretty cool, right? The Moon-forming impact was most likely in the origin of the last major, deep and global magma ocean on Earth. But how did the magma ocean evolve in such a way that set up the initial conditions for the evolution of a tectonically active planet, able to sustain water at the surface, an atmosphere, and life?

BOOOMMM! And the Earth was molten! And the Moon was formed! Artist’s depiction of a collision between two planetary bodies. Such an impact between Earth and a Mars-sized object likely formed the Moon. Credit: NASA/JPL-Caltech

How did the magma ocean evolve on Earth?  

Fortunately for us, the magma ocean solidified, and Earth’s conditions are very different from back then. According to the conventional view, as a magma ocean loses heat from the surface, its temperature drops, and it starts crystalizing from the bottom-up. When there are enough crystals present, a solid matrix is formed, and because overall this structure flows slower, the heat loss is also much slower, and thus crystallization is prolonged. Estimates for the transition from a liquid- to a solid-dominated regime range from thousands of years to several hundred million years. However, fully crystallizing the mantle takes billions of years. It might sound a bit strange that the uncertainty in the lifetime of a magma ocean exhibits such large differences. This is due to the fact that we don’t know much about such a system (we never observed one…), and the data available is rare and hard to interpret. These are the sort of uncertainties that we have to deal with when studying the evolution of a magma ocean on Earth:

  • Was an atmosphere present? After the Moon-forming impact the surface temperatures were very high (~2000 K), so the heat transfer from the interior to the surface of the magma ocean is expected to be very rapid. But, these values can be buffered, and timescales of crystallization extended if an atmosphere is present. In the beginning an atmosphere composed of vaporized silicates (literally vaporized rock!) covered the Earth. Afterwards, as the magma ocean cools down, it degasses through bubbles that rise up and burst at the surface to form a thick and insulating steam-dominated atmosphere. Such atmosphere can substantially extend the cooling time by millions of years. As we will see next, this has important implications in the mode of crystallization of a magma ocean. Finally, once sufficient cooling is attained, the atmosphere collapses into a water ocean, and our planet starts to look more like the “Blue Planet” we recognize today.

How does the magma ocean crystallize? In the classic view, shown here, it crystallizes from the bottom-up. Credit: Diogo Lourenço

  • Could the surface be solid? The existence of a solid lid at the surface would slow down the cooling of a magma ocean, and therefore increase its solidification time. However, it is unlikely that such a lid developed in bigger planets because of: (1) the possible existence of an insulating atmosphere that keeps the temperature at the surface high, (2) the major possibility of any solidified material to sink, and (3) any small impactor during the cooling of the magma ocean would disrupt the formed lid. The only likely way to form a relatively complete conductive lid on a planetary-scale magma ocean is by flotation of buoyant minerals to the surface. Like in the Moon, remember? However, this mechanism is likely to occur only on small planets.
  • How do crystals in a magma ocean behave? One of the most important and complex questions regarding the crystallisation of a magma ocean is whether the crystals continue to be suspended in the liquid, or whether they settle, being removed from the liquid magma ocean. This is a challenging problem to address due to the lack of information about the conditions in a magma ocean, and the complexity of the physics of settling versus entrainment in a vigorous fluid flow. In general, longer timescales of magma ocean freezing imply relatively slow convection within the magma ocean and may allow for crystal settling. On the other hand, short timescales imply fast turbulent convection, and hence no crystal settling. On Earth, with all the uncertainties, both styles are acceptable to have operated, but most likely a combination of the two types of solidification existed in a magma ocean, where at first crystals remained entrained, and as the magma ocean cools down crystals started to settle.
  • Full-mantle overturns? Another complexity in the evolution of a magma ocean is that full-mantle overturns are predicted. Two types have been proposed: (1) Thermal overturns, where the (partially-molten) mantle resulting from the solidification of a magma ocean is gravitationally unstable. These instabilities can grow to a solid-state overturning of the cumulate mantle. (2) “Compositional” overturns. As the magma ocean crystallizes, a progressive enrichment of iron and incompatible elements in the magma is expected, and ultimately, this leads to an unstable compositional density stratification because magma ocean cumulates will be denser as the crystallisation front proceeds. A single overturn and a pronounced stratification of the compositionally stratified cumulate layers are expected, which could delay mantle solid convection for billions of years. However, it might be the case that stratification is (partially) erased through progressive mixing due to multiple incremental cumulate overturns, instead of a single one.

How does the magma ocean crystallize? In a more more recent view, shown here, it crystallizes from the middle-out. Iron (Fe) tends to concentrate in the magma. Eventually, when the BMO is almost solid, the last-forming solids are highly iron-enriched, and probably dense enough to become stable against entrainment in the solid mantle. Credit: Diogo Lourenço based on Labrosse et al. (2007)

  • Did the magma ocean really crystallize from the bottom to the top? Alright, here we go for another strange bit. The conventional view presented above is that the magma ocean crystallised from the bottom-up. Makes sense, yes? However, evolution from a completely molten state resulting from the Moon-forming impact might have been different from the conventional view. Recent measurements suggest that it is possible that liquids are denser than solids at lowermost mantle depths, opening the possibility for alternative scenarios of crystallisation of a magma ocean from mid-mantle depths. This would imply that as the magma ocean crystallizes, a shallow and a basal magma ocean (BMO) are formed. The solid layer grows upward and downward, however it grows faster towards the surface because of rapid heat loss to the atmosphere. The BMO’s heat loss is buffered by the mantle, so it crystallizes much slower. Moreover, the basal melt layer is also cooling slower because iron and incompatible trace elements (such as heat-producing elements) would tend to be concentrated in the magma. Eventually, the BMO would almost completely solidify, with the last-forming solids being highly iron-enriched, and probably dense enough to become stable against complete entrainment in the solid mantle. These dense and solid piles could account for some of the observed large-scale features with reduced seismic velocities around the core-mantle boundary we observe today. Understanding whether the Earth had a BMO, and if so, its evolution, is a first-order question that has important implications for planetary thermochemical evolution, for example some reservoirs isolated from the rest of the mantle can be the result from the crystallisation of a BMO. Also, it would have affected the thermal history of the Earth and affected the history of the geodynamo in our planet.

Answering these questions, which are all connected to the chemical and dynamical evolution of a magma ocean, is very important in order to understand the initial conditions for solid-state mantle convection, which on Earth led to plate tectonics and life (but did not in other terrestrial planets). We should keep in mind that magma oceans are common and are still being formed in other galaxies, so maybe planets with magma oceans will one day become targets for direct imaging. Who knows?

I will leave you with some links and further reading below if you really got excited with this text! If you really really got motivated then you can also read my PhD thesis, I’m still recovering from that time, so it will make me happy to know it taught something to other people.

Alright, enough! Hope you enjoyed the read and remember to not swim in magma oceans (you might get arrested)! Bye-bye!

References and further reading:

Abe, Y. (1997). Thermal and chemical evolution of the terrestrial magma ocean. Physics of the Earth and Planetary Interiors, 100:27-39.

Ballmer, M. D., Lourenço, D. L., Hirose, K., Caracas, R., and Nomura, R. (2017). Reconciling magma-ocean crystallization models with the present-day structure of the earth’s mantle. Geochemistry, Geophysics, Geosystems, 18(7):2785–2806.

Elkins-Tanton, L. T. (2012). Magma Oceans in the Inner Solar System. Annual Review of Earth and Planetary Sciences, 40(1):113–139.

Labrosse, S., Hernlund, J. W., and Coltice, N. (2007). A crystallizing dense magma ocean at the base of the Earth’s mantle. Nature, 450(7171):866–869. 

Labrosse, S., Hernlund, J. W., and Hirose, K. (2015). Fractional Melting and Freezing in the Deep Mantle and Implications for the Formation of a Basal Magma Ocean. In The Early Earth Accretion and Differentiation, edited by Badro, J. and Walter, M., Hoboken, NJ. 

Lourenço, D. L. (2017). The influence of melting on the thermo-chemical evolution of rocky planets’ interiors. PhD thesis, ETH Zurich. [link]

Rubie, D. C., Nimmo, F., and Melosh, H. J. (2007). Formation of Earth’s Core, In Treatise on Geophysics, edited by Gerald Schubert, Elsevier, Amsterdam, pages 51-90.

Solomatov, V. S. (2007). Magma Oceans and Primordial Mantle Differentiation, In Treatise on Geophysics, edited by Gerald Schubert, Elsevier, Amsterdam, pages 91-119.

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To serve Geoscientists

To serve Geoscientists

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, Fabio Crameri, postdoctoral researcher at the Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway, joins us again. Continuing from his earlier post on the harmful use of the rainbow colour map, Fabio shares his thoughts on some of the expressions and phrases used in the community that propagate confusion, and how the new “Ocean-Plate Tectonics” concept offers relief for at least some on them. 

Blog author Fabio Crameri – in a shirt that translates from Tamasheq as “deserts” or “empty spaces.” You can expect no empty spaces in your lunchtime conversations after reading this post.

Do you, after reading the title, still wonder what this blog post is all about?
I’ll give you a hint, it’s about the Earth. No, wait, it’s about Earth, or perhaps earth, or isn’t it? And maybe it is a little bit about Moon, I mean the Moon. But also, it is about the Venus, I mean Venus.

It is not confusing, it is just well mixing.

You’ve got it; it is about confusion in the Geosciences. Confusion caused by symbols, letters, words and phrases through misuse, ambiguity or over-interpretation. So, after all, this is a blog post about geo-semantics rather than about culinary excursions.

The Geodynamics community is a diverse group of people with different backgrounds, native languages and customs. This is an attractive breeding ground for semantic related problems, particularly when you throw in some inherent peculiarities of the English language in which we largely operate.

Some use the symbol “a” for years, for years and years.

In line with a widely used standard definition (Holden et al., 2011) – but against the common convention of the Geosciences – the author of this blog post was using the unit of time “a”, or arguably just its symbol, for “years” (and I mean calendar years, neither financial years nor dog years), for years and years. A distinction between discrete points in time and the duration of time is at the heart of this confusion, and indeed has plagued a sub-selection of discussions, working groups and interpretations of the International System of Units (SI; e.g., Christie-Blick, 2012).

Figure 1. An ambiguously phrased situation near the recent end of the Cretaceous.

The symbol “a” for “annus” [year] (“Ma” being the symbol for 106 years, or “mega-annus”) in the Geosciences is most commonly used for a specific time or date in the past as measured from now. For example, “At 65 Ma (which is 65 Myr ago), the dinosaur looked up the sky.” (see Figure 1). On the other hand, “yr” for “year(s)” is commonly used for a duration of time, as in “The Cretaceous period ran for 79 Myr (from approximately 145-66 Ma).”. Other mutations within the convention of time in the Geosciences include “My”, “Myrs”, “Mya” or “m.y.” for “Millions of years”. Thus, the time unit and symbols for multiples of a “year” are likely amongst the most ambiguous expressions in the Earth Sciences, likely because, in contrast to the “second”, a universally applied scientific definition for the “annus” still remains elusive (Thompson and Taylor, 2008).

Such quibbling over semantics may seem petty.

Amongst other examples to cause geodynamic misunderstandings (e.g., Figure 2) might be the misuse of the phrase “stagnant slabs”? Are slabs ever really stagnant? Or are they just being deflected, slowing down, interrupting their downward motion, not directly entering the lower mantle at the same speed and trajectory as before?

Figure 2. One ambiguously phrased geodynamic explanation.

From the literature, you might be forgiven for having the false impression that slabs either fully stagnate around the upper-mantle transition zone or directly and effortlessly penetrate it; they likely do neither of the two (as explained in e.g., in an earlier Geodynamics101 post here).

When these slabs sink, and not temporally stagnate, they induce flow in the surrounding mantle. “Slab suction” is the downward suction induced by the nearby mantle that is set in motion through its dynamic coupling with the slab [e.g., Conrad and Lithgow-Bertelloni 2002]. Or isn’t it? “Slab suction” is also contrarily used as an upward directed force on the slab itself that is induced by the upper plate and might foster low-dipping shallow-depth slab portions in the uppermost upper mantle (unambiguously speaking of which: see again Figure 2).

The downward directed version of “slab suction” can induce “dynamic topography”. Estimates of the maximum amplitude of “dynamic topography” on Earth range from only a few hundred meters up to a few kilometres (see e.g., Molnar et al., 2015 and references therein). Such unusually large ranges of estimates are, as a general rule, a quite solid indicator for an underlying ambiguous definition, or in this case, rather a mix-up of multiple different definitions for the term “dynamic topography”. 

If you’re not confused, you did not pay attention.

As I keep talking about geodynamics, I hope we are all on the same page about subduction, one of the key players: Let’s assume planet XY has one single active subduction zone. Another subduction zone initiates on the opposite side of the same planet. Did “subduction” start once or twice on that planet?

It started once on that planet. Because “subduction” describes a process and not a physical feature; it is nonetheless easily mistaken for a physical feature.

And what about “plate tectonics”, the 50 yr old overarching concept that fascinates us, and for so many of us has become the foundation of our professional lives. Let’s approach this by considering the big question: When did “plate tectonics” start? Serious opinions in the plate tectonics community range from around 850 Ma (Hamilton 2011) all the way back to 4.3 Ga (Hopkins et al., 2008). – Remember what unusually large estimate ranges often indicate? – It is not surprising that the only commonly accepted specific answer everyone seems to agree on currently is that it depends on the very definition of plate tectonics.

So, what is the definition of “plate tectonics”? According to its original formulation, “plate tectonics” is the horizontal relative movement of several discrete and mostly-rigid surface-plate segments (Hess, 1962; see the corresponding visual representation in Figure 3). A generous interpretation of the original formulation might additionally define the plate-interface nature, but that is all.

Figure 3. As long as it is not overinterpreted, there is nothing wrong with the original definition of plate tectonics that solely describes the horizontal motion of several discrete surface plates: It does not discriminate the oceanic from the continental plate, does not consider the important framework of mantle convection, and does not specify the underlying key driver of the surface motion.

Considering the knowledge we have gained about the moving surface plates and their underlying causes and consequences during the past 50 yr, this is an extremely broad definition: As of today, we know that (A) the surface plates with their relative motion are an integral part of whole mantle convection (Turcotte and Oxburgh, 1972), that (B) Earth’s surface has a characteristic bimodal nature due to the partitioning into long-lived continental plates and short-lived oceanic plates (e.g., Wilson, 1966), and that (C) the latter are mainly driven by their very own subducted portions (i.e., all or parts of their slabs; Forsyth and Uyeda, 1975; Conrad and Lithgow-Bertelloni, 2002).

A clear, unambiguous and up-to-date definition for such a crucially important, wide-reaching concept is imperative. It is therefore not surprising that less ambiguous re-definitions have been suggested recently. To avoid propagating confusion, the introduction of alternative phases of plate tectonics that describe the various different possible modes of mantle convection during Earth’s evolution have been cast into the arena (e.g., Sobolev 2016). These include “plate-tectonics phase 1”, in short “PT1”, describing regional, plume-induced plate tectonics (e.g., until 3.0 Ga), “PT2” describing episodic, global plate tectonics (e.g., between 2.5-1.0 Ga), and finally “PT3” describing stable, global plate tectonics (e.g., 1.0-0.0 Ga). Other efforts result in different naming conventions, such as “modern plate tectonics”. However, apart from the fact that “modern” is a time dependent term, “modern plate tectonics” might be a somewhat unfortunate expression, as other planets like Venus might have undergone different, modern styles of plate tectonics than present-day Earth.

Stern and Gerya (2017) then actually suggests an entire update to the definition of “plate tectonics”:

“A theory of global tectonics powered by subduction in which the lithosphere is divided into a mosaic of strong lithospheric plates, which move on and sink into weaker ductile asthenosphere. Three types of localised plate boundaries form the interconnected global network: new oceanic plate material is created by seafloor spreading at mid-ocean ridges, old oceanic lithosphere sinks at subduction zones, and two plates slide past each other along transform faults. The negative buoyancy of old dense oceanic lithosphere, which sinks in subduction zones, mostly powers plate movements.”

Unfortunately, such a re-definition of the same old phrase makes it impossible to know which version of the definition (i.e., the original or the updated one) an author of a subsequent study should be applying and referring to.

In an effort to prevent all of the above problems, we recently introduced an entirely new concept; one that can coexist in harmony with the original definition; one that fully captures the dynamics of the oceanic plate according to our current knowledge. The concept is called “Ocean-Plate Tectonics” or, if you really like the term, “OPT”.

“Ocean-Plate Tectonics is a mode of mantle convection characterised by the autonomous relative movement of multiple discrete, mostly rigid, portions of oceanic plates at the surface, driven and maintained principally by subducted parts of these same plates that are sinking gravitationally back into Earth’s interior and deforming the mantle interior in the process.” – Crameri et al. (2018).

“Ocean-Plate Tectonics” captures not only the relative horizontal surface motion of plates, but crucially also accounts for (A) the importance of the whole mantle framework, (B) the bimodal nature of Earth’s surface plates, and (C) the underlying engine of the surface-plate motion (see Figure 4).

Figure 4. “Ocean-Plate Tectonics”, the unambiguous up-to-date definition describing the dynamics of the oceanic plate that crucially incorporates the bimodal nature of Earth’s surface, the convecting-mantle framework, and the key driver of surface-plate motion (after Crameri et al., 2018).

“Ocean-Plate Tectonics” is here to serve Geoscientists.

The concept of “Ocean-Plate Tectonics” is intended to bring together the extremely diverse research communities, but also the general public, to meet on common, fruitful ground in order to discuss and further develop our understanding of the fascinating dynamics involved in Earth’s plate-mantle system; the unambiguous “Ocean-Plate Tectonics” is here to serve us.

 

Christie-Blick, N., (2011), Geological Time Conventions and Symbols, GSA Today, 22(2), 28-29, doi: 10.1130/G132GW.1

Conrad, C. P., and C. Lithgow-Bertelloni (2002), How mantle slabs drive plate tectonics, Science, 298 (5591), 207–209, doi:10.1126/science.1074161.

Crameri, F., C.P. Conrad, L. Montési, and C.R. Lithgow-Bertelloni (2018), The life of an oceanic plate, Tectonophysics, (in press), doi:10.1016/j.tecto.2018.03.016 .

Forsyth, D., and S. Uyeda (1975), On the relative importance of the driving forces of plate motion*, Geophysical Journal of the Royal Astronomical Society, 43(1), 163–200, doi:10.1111/j.1365-246X.1975.tb00631.x.

Hamilton, W.B. (2011), Plate tectonics began in Neoproterozoic time, and plumes from deep mantle have never operated, Lithos, 123, 1–20, doi:10.1016/j.lithos.2010.12.007.

Hess, H.H. (1962), History of ocean basins, Petrologic studies, 4, 599–620.

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EGU GD Whirlwind Wednesday: Geodynamics 101 & other events

EGU GD Whirlwind Wednesday: Geodynamics 101 & other events

Yesterday (Wednesday, April 12, 2018), the first ever Geodynamics 101 short course at EGU was held. It was inspired by our regular blog series of the same name. I can happily report that it was a success! With at least 60 people attending (admittedly, we didn’t count as we were trying to focus on explaining geodynamics) we had a nicely filled room. Surprisingly, quite some geodynamicists were in the audience. Hopefully, we inspired them with new, fun ways to communicate geodynamics to people from other disciplines.

The short course was organised by me (Iris van Zelst, ETH Zürich), Adina Pusok (ECS GD Representative; UCSD, Scripps Institution of Oceanography, IGPP), Antoine Rozel (ETH Zürich), Fabio Crameri (CEED, Oslo), Juliane Dannberg (UC Davis), and Anne Glerum (GFZ Potsdam). Unfortunately, Anne and Juliane were unable to attend EGU, so the presentation was given by Antoine, Adina, Fabio and me in the end.

The main goal of this short course was to provide an introduction into the basic concepts of numerical modelling of solid Earth processes in the Earth’s crust and mantle in a non-technical, fun manner. It was dedicated to everyone who is interested in, but not necessarily experienced with, understanding numerical models; in particular early career scientists (BSc, MSc, PhD students and postdocs) and people who are new to the field of geodynamic modelling. Emphasis was put on what numerical models are and how scientists can interpret, use, and work with them while taking into account the advantages and limitations of the different methods. We went through setting up a numerical model in a step-by-step process, with specific examples from key papers and problems in solid Earth geodynamics to showcase:

(1) The motivation behind using numerical methods,
(2) The basic equations used in geodynamic modelling studies, what they mean, and their assumptions,
(3) How to choose appropriate numerical methods,
(4) How to benchmark the resulting code,
(5) How to go from the geological problem to the model setup,
(6) How to set initial and boundary conditions,
(7) How to interpret the model results.

Armed with the knowledge of a typical modelling workflow, we hope that our participants will now be able to better assess geodynamical papers and maybe even start working with numerical methods themselves in the future.

Apart from the Geodynamics 101 course, the evening was packed with ECS events for geodynamicists. About 40 people attended the ECS GD dinner at Wieden Bräu that was organised by Adina and Nico (the ECS Co-representative for geodynamics; full introduction will follow soon). After the dinner, most people went onwards to Bermuda Bräu for drinks with the geodynamics, tectonics & structural geology, and seismology division. It featured lots of dancing and networking and should thus be also considered a great success. On to the last couple of days packed with science!