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Tectonics and Structural Geology

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Beyond tectonics: The present-day tides are the biggest they have been since the formation of Pangea

Beyond tectonics: The present-day tides are the biggest they have been since the formation of Pangea
“Beyond tectonics” is a blog series which aims to highlight the connections between tectonics and other aspects of the Earth system. In this iteration of the “Beyond tectonics” series we talk about how plate tectonics have affected the tides on Earth over geological timescales. We will talk about tectonics on the Earth since the formation of Pangea to the present day, and into the future, ending with the formation of the next Supercontinent in around 200 – 250 Million years from now.

 

Tides of the Planet Earth

The Earth’s tides are predominantly caused by the gravitational pull of the Moon, and the centripetal force of the Earth due to its rotation. The force of the Moon and the spin of the Earth cause two tidal bulges to form, one that follows the Moon, and one on the opposite side of the planet. These two tidal bulges move around the Earth with a period of 12.5 hours. When the buldge moves over a coast, a high tide occurs, and when a bulge is not over a coast, a low tide occurs. This is why there are two low and high tides each day. These tides vary in strength around the world because ocean and coastal morphology plays a large role in how the tidal energy is distributed.

 

How do tectonic plates affect the tide?

The surface of the Earth is broken up into pieces like the shell of an egg. These pieces, or plates, can be divided into oceanic and continental plate. The main difference between the two types of plate is their buoyancy. Both types of plate are buoyant, however, after it is formed at a mid ocean ridge, ocean plate becomes less and less buoyant with age. Eventually, after around 30 million years, ocean plate is less buoyant than the underlying mantle meaning it could sink if it reached a subduction zone. In the present-day oceans, almost all plate older than 180 million years old has sank back into the mantle at a subduction zone. This recycling of ocean crust is what causes the oceans to change shape. The continents are pulled together by the closing oceans (sinking ocean plate), and pushed apart by the opening ones (creation of ocean plate). In the present day, the Pacific is closing and the Atlantic is opening, causing the plate drift map to look like this:

The major subduction zones on Earth. Black arrows illustrate trench migration vectors, while open arrows illustrate plate velocity (credit – Schellart et al., 2007)

As the oceans grow and shrink, their width changes. This means the tidal wave in those oceans has either too little, too much, or just the right amount of space to flow in the ocean. In the present-day the Pacific is too big, the Indian ocean too small, but the Atlantic ocean is just the right size to make the tide resonant.

 

What is resonance?

To explain resonance, imagine the tidal wave moving across the Atlantic ocean and back, like a child on a swing. If you apply the force on the swing when it is at the highest point, you are applying a force at one of the natural frequencies of the system, so the energy you input will make the swing go higher. If you push the swing before or after it reaches its peak, then you might input some energy, but it won’t be as efficient. The Atlantic ocean is just the right width to allow the wave to “swing” back and forth. The input of energy is applied at just the right point, one of the natural frequencies of the tide, to allow resonance.

It is possible for this to happen in any ocean basin. An ocean basin can house resonant tides when the width of the basin (L) is equal to a multiple of half wavelengths of the tide  (𝜆 = √gHT),  where (T) is the tidal period, (g) is gravity, and (H) is water depth. Essentially, when the ocean basin has a width that intersects with a natural frequency of the tide, it will become resonant.

 

The Super-tidal cycle

The reason why the present day tides are the biggest since the formation of Pangea is because the Atlantic is currently resonant with the tide, i.e. it is in a Super-tidal period. Looking at the energy of the tides from when Pangea existed to the present-day (Green et al., 2017), we can see that during Pangea’s life the tides were weak. That status quo continued from 180 to 1 million years ago, when the Atlantic suddenly developed large tides. The Atlantic had been growing all that time, and had finally reached a width where the tide became resonant.

M2 tidal amplitudes since the breakup of Pangea (credit – Green et al., 2017)

The large present-day Atlantic tide is unlikely to last. Green et al., (2018) predict that this super-tidal period will last around 20 million years, and Davies et al., (2019) predict another won’t occur for tens of millions of years, depending on how the future Earth develops.

Therefore, what are the implications of the present-day Atlantic having such large tides? Further testing of the implications of the Super-tidal cycle is needed before any conclusions can be made on how it may affect the Earth system. However, the larger energy input into the oceans during a Super-tidal period may enhance tidal mixing, which means the ocean will have a better distribution of nutrients and oxygen, i.e. it is less likely for it to become stratified. What we do know right now, is that Supercontinents generally have very weak tides (Green et al., 2018), and periods of continent divergence similar to the present day, have larger, or sometimes Super-tides (Balbus 2014; Davies et al., 2019).

 

References

  • Balbus, S. A. 2014, Dynamical, biological and anthropic consequences of equal lunar and solar angular radii. Proc. R. Soc. A, 470. Available at: http://doi.org/10.1098/rspa.2014.0263
  • Davies, H. S., Green, J. A. M., Duarte, J. C., 2018, Back to the future: testing different scenarios for the next supercontinent gathering. Global and planetary change, 169, 133 – 144. Available at: https://doi.org/10.1016/j.gloplacha.2018.07.015
  • Davies, H. S., Green, J. A. M., Duarte, J. C., 2019. Back to the future 2: Tidal modelling of four potential scenarios for the next Supercontinent gathering. Presented at EGU 2019. Available at: https://meetingorganizer.copernicus.org/EGU2019/EGU2019-1004-1.pdf
  • Green, J.A.M., Molloy, J.L., Davies, H.S., Duarte, J.C., 2018. Is there a tectonically driven super-tidal cycle? Geophys. Res. Lett. 45 (8). Available at: https://doi.org/10.1002/2017GL076695.
  • Schellart, W.P., Freeman, J., Stegman, D.R., Moresi, L., May, D. 2007. Evolution and diversity of subduction zones controlled by slab width. Letters to Nature, 446, 308 – 311. Available at: doi:10.1038/nature05615

Meeting Plate Tectonics – Nicolas Coltice

Meeting Plate Tectonics – Nicolas Coltice

These blogposts present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Get to know them, learn from their experience, discover the pieces of advice they share and find out where the newest challenges lie!


Meeting Nicolas Coltice


Nicolas Coltice graduated with a PhD from the École Normale Supérieure of Lyon, France. He then became assistant professor at the Université Claude Bernard in Lyon, and ultimately, full professor. As of last year, he also holds a professorship position at ENS Paris, France. He has received an ERC grant for the project AUGURY and he is one of the co-founders of the manifesto ’Did this really happen?’, which addresses sexual harassment and inequality issues within sciences.

 

Nicolas Coltice. Credit: Eric Le Roux / Université Claude Bernard Lyon 1.

I think it is extremely important that models are supported by evidence or data.

Hi Nicolas, could you tell us about your research interests and the methods you use to solve your problems?

Sure! My research interest is focussed on mantle convection and geochemistry. The research I do is strongly directed to combine models and observations to understand, for example, the geochemical cycle. I also combine observations and inverse models to build tectonic reconstructions and 3D spherical models. I work a bit with geologists and so I sometimes go into the field. I think it is extremely important that models are supported by evidence (or data) and so I try to combine this as much as I can in my research.

You have been active on different topics. What achievement in your carer are you most proud of?

The one thing I’m most proud of is setting up an ERC team for the project ‘AUGURY’, which happened to have more women than men, which is quite rare in our field. I feel we made quite some progress on undermining the patriarchy within sciences with this ERC project. I’m very proud to work with my team. One of the good things that came out of ‘AUGURY’ is our manifesto ’Did this really happen?’. It is a website where we tell the stories on sexual harassment and gender inequality that women in sciences using comics. Besides advocating gender equality science I also teach, which I find very fulfilling and my teaching is well-received.

Good research needs time.

Did this really happen?. Courtesy of www.didthisreallyhappen.net.

It’s fantastic that you are making the community aware of these more social issues. In terms of research, how does that benefit society?

The application of my research to society is first of all doing the job by itself. Every day that scientists invest in understanding parts of our planet is beneficial to society, just by the very act of it. Publishing my work might give a breach to society and offer perspectives that were not thought of before. I guess a more concrete way my research benefits society would be in the reserve or resources industry, where we always like to understand better where resources form and why they form under certain condition. This will eventually help to actually find them and exploit them and the better we understand that, the less impact it will eventually have on the environment.

Every day that scientists invest in understanding parts of our planet is beneficial to society, just by the very act of it.

 

How do you see the future in geoscience?

In my opinion, good research needs time. Currently we are given very little time to do good research. If we want to change the publishing-focussed mentality, we need to start at the bottom. We do not necessarily have to create a big revolution, but from the inside we can collaborate and slowly change the system. For example, if you publish, public money is used to pay for your publication. This public money then often goes to stakeholders, which is not good! We can change this by publishing in different journals with different ethics. This way, we can slowly lower the pressure we feel on publishing nowadays. So in terms of future, I think we need to get back to the core, do good research.

Selected 3-D view state of the model. Continental material is highlighted in yellow. Figure from Coltice & Shephard, 2018 “Tectonic predictions with mantle convection models”, Geophysical Journal International, 10.1093/gji/ggx531.

When you feel it gets rough, stick with your plan and keep your relationship with your colleagues positive.

One last question, what advice would you like to give to Early Career Scientists?

When I was hired 15 years ago, times were different. If recruiters had the choice, they would always go for the youngest person, not necessarily the best. Nowadays productivity is the factor that counts most and is imposed on people which makes it very difficult to maintain an interesting profile at an early stage in your career. I would advise to find time and space to feel good and let go of the pressure you might feel in your work. I believe there is room for everyone, just keep the spirit up. When you feel it gets rough, stick with your plan and keep your relationship with your colleagues positive.

Nicolas Coltice. Credit: Nicolas Coltice.

 

Interview conducted by Anouk Beniest

Minds over Methods: Numerical modelling

Minds over Methods: Numerical modelling

Minds over Methods is the second category of our T&S blog and is created to give you some more insights in the various research methods used in tectonics and structural geology. As a numerical modeller you might wonder sometimes how analogue modellers scale their models to nature, or maybe you would like to know more about how people use the Earth’s magnetic field to study tectonic processes. For each blog we invite an early career scientist to share the advantages and challenges of their method with us. In this way we are able to learn about methods we are not familiar with, which topics you can study using these various methods and maybe even get inspired to use a multi-disciplinary approach! This first edition of Minds over Methods deals with Numerical Modelling and is written by Anouk Beniest, PhD-student at IFP Energies Nouvelles (Paris).

 

Approaching the non-measurable

Anouk Beniest, PhD-student at IFP Energies Nouvelles, Paris

‘So, what is it that you’re investigating?’ It’s a question every scientist receives from time to time. In geosciences, the art of answering this question is to explain the rather abstract projects in normal words to the interested layman. Try this for example: “A long time ago, the South American and African Plate were stuck together, forming a massive continent, called Pangea, for many millions of years. Due to all sorts of forces, the two plates started to break apart and became separated. During this separation hot material from deep down in the earth rose to the surface increasing the temperature of the margins of the two continents. How exactly did this temperature change over time, since the separation until present-day? How did this change affect the basins along continental margins?”

These are legitimate questions and not easy to answer, since we cannot measure temperature at great depth or back in time. In this first post on numerical methods, we will be balancing between geology and geophysics, highlighting the possibilities and limits of numerical modelling.

The migration of ‘temperature’ through the lithosphere is a process that takes time and depends heavily on the scale you look at. Surface processes that affect the surface temperature can be measured and monitored, yielding interesting results on the present-day state and variations of the temperature. The influence of mantle convection cycles and radiogenic heat production are already more difficult to identify, take much more time to evolve and might not even affect the surface processes that much. Going back in time to identify a past thermal state of the earth seems almost impossible. This is where numerical models can be of use, to improve, for example, our understanding on the long-term behaviour of ‘temperature’.

Temperature is a parameter that affects and is affected by a variety of processes. When enough physical principles are combined in a numerical model, we can simulate how the temperature has evolved over time. All kinds of different parameters need to be identified and, most importantly, they need to make sense and apply to the observation or process you try to reproduce. Some of these parameters can be identified in the lab, like the density or conductivity of different rock types. Others need to be extracted from physical or geological observations or even estimated.

Once the parameters have been set, the model will calculate the thermal evolution. It is not an easy task to decide if a simulation approaches the ‘real’ history and if we can answer the questions posed above. We should always realise that thermal model results at best approach the real world. We can learn about the different ways temperature changes over time, but we should always be on the hunt to find measurements and observations that confirm what we have learned from the simulations.

temperature_quick