GD
Geodynamics
The Sassy Scientist

The Sassy Scientist

I am currently employed at a first tier research institute where I am continuously working with the greatest minds to further our understanding of the solid Earth system. Whether it is mantle or lithosphere structure and dynamics, solid Earth rheology parameters, earthquake processes, integrating observations with model predictions or inversions: you have read a paper of mine. Even if you are working on a topic I haven’t mentioned here, I still know everything about it. Do you have any problems in your research career? I have already experienced them. Do you struggle with your work-life balance? Been there, done that. Nowadays, I have only one hobby: helping you out by answering the most poignant questions in geodynamics, research and life. I am waiting for you right here. Get inspired.

The Sassy Scientist – Jekyll and Hyde

The Sassy Scientist – Jekyll and Hyde

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.

Senna asks:


I’m torn between mantle dynamics and lithosphere dynamics as a research topic. Which shall I choose?


Dear Senna,

Could you follow my little Jekyll and Hyde routine over the past two posts, railing about lithosphere dynamics and mantle convection? Some people did not, as my inbox will testify. This is basically what it has been over the past few decades. Let’s be clear: we don’t live in the 70’s anymore. We’re capable of more subtlety and have to live in the in-between: the world is full of grey tones (yes, also shades of grey).

Numerical modelling of mantle convection systems that includes oceanic and continental lithosphere has greatly improved. Producing tessellation of the Earth’s surface and long-term supercontinent cycles in combination with realistic surface kinematics is a big step forward. Does this mean we completely understand the solid Earth system? Definitely not. Do mantle convection modellers have to realise that the relative importance of the mantle on surface kinematics may only be a transient feature on a regional scale, and that surface motions may be unrelated to mantle flow? Maybe. Do lithosphere dynamics modellers have to realise that the relative importance of the mantle on surface kinematics may only be a transient feature on a regional scale, and that this doesn’t mean that the mantle definitely always or never influences surface motion? Maybe. Does everybody involved need to realise that their idea of the truth may not be the whole truth? Definitely. Is there a place for both lines of inquiry into understanding the solid Earth system? Undoubtedly.

I’ll come back to your question. There is no mutual exclusivity between mantle convection and lithosphere dynamics. It’s all part of the same system. So whatever you choose, just realise this: do not blindly step into the mantle convection gateway or lithosphere dynamics portal to the maze of solid Earth dynamics and kinematics. Both alleyways will probably end up in the same central square of enlightenment.

Yours truly,

The Sassy Scientist

PS: This post was written after getting hammered repeatedly by the Editor-in-Chief over my previous two posts.

The Sassy Scientist – Hyde: Lithosphere Dynamics

The Sassy Scientist – Hyde: Lithosphere Dynamics

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.

Senna asks:


I’m torn between mantle dynamics and lithosphere dynamics as a research topic. Which shall I choose?


Dear Senna,

I don’t know what came over me when writing last week’s post. Must have been something I ate. Sometimes I just get some crazy idea stuck in my head. For example, I completely misconstrued ideas on the relative importance of mantle convection over lithosphere dynamics.

Last week I was babbling on about plate tectonics, and how the focus on the lithosphere shifted attention away from mantle convection. To be clear: this was for good reason. Granted, McKenzie and Parker’s (1969) concept of rigid plates on a shell was simplified. That was also the point: it explains some first-order observations like relative plate motions and seismicity. No, plate boundaries are not necessarily narrow zones of deformation (Kreemer et al. 2014). Yes, plates can also be flexed through loads on top, or radial mantle tractions from below (Watts 2001). We know that the Earth’s lithosphere can generally be considered to be a visco-elastic plate that is primarily moved by forces acting at its boundaries. Does this preclude seismicity or deviations from principal stress directions far away from such plate boundaries? Of course not.

It is obviously a great idea to base model predictions on non-unique gravity and geoid observations, seismic tomography, and Earth’s normal mode undulations for structures 2500 km away from the surface. The surface. You know, that place where we actually have direct observations. Sure, just extrapolate some rheology measured on a rock sample down to the lower mantle using some equations of state. Should be similar, right? Do you have a kinematic plate tectonic reconstruction based on loads of geological and paleomagnetic data? Just pop it into a mantle convection model to see whether it is feasible. You measured some SKS splitting data? That’s definitely a measure of present-day mantle flow. You infer instantaneous dynamic topography at the surface in the order of a couple 100 meters? Mantle convection models predict kilometers of instantaneous dynamic topography, so you must have made a mistake in your observations.

Do you want to understand what’s driving deformation of the lithosphere? Choose lithosphere as a research topic. Do you believe in fairy tales? Look into mantle convection: it’s full of magic.

Yours truly,

The Sassy Scientist

PS: This post was written after recovering from a major headache from reading last week’s post.

PS2: I won’t be surprised if there is going to another post on this topic next week. How well do you know your Scottish literature?

References:
Kreemer, C., G. Blewitt, E.C. Klein (2014). A geodetic plate motion and Global Strain Rate Model, Geochemistry, Geophysics, Geosystems, 15, 3849-3889, doi:10.1002/2014GC005407
McKenzie, D., and R. L. Parker (1967), The North Pacific: An example of tectonics on a sphere, Nature, 216, 1276
Watts, A. B. (2001), Isostasy and flexure of the lithosphere, Cambridge University Press, 458 pp.

The Sassy Scientist – Jekyll: Mantle dynamics

The Sassy Scientist – Jekyll: Mantle dynamics

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.

Senna asks:


I’m torn between mantle dynamics and lithosphere dynamics as a research topic. Which shall I choose?


Dear Senna,

This is an easy one: mantle dynamics. Don’t you want to be part of cutting edge research, utilising state-of-the-art numerical modelling codes and high performance computing facilities? You can employ every solid rock rheology known to man and generate new lithosphere through volcanism. Nowadays the lithosphere and crustal layers are implemented so convincingly that we can easily match seismic tomography slices, passive margin architectures and even subduction zones to the scale of accretionary wedges. Inherited weak zones can lead to supercontinent cycles. The sky is the limit!

I am the first to admit that mantle convection has been overshadowed for quite a while by plate tectonic theory ever since the early days of Holmes (1931). Wilson’s (1965) tessellation of the Earth’s surface and the straightforward connection to relative plate motions (e.g., McKenzie and Parker 1967) and mid-ocean magnetic anomalies (Vine and Matthews 1963) resulted in a longstanding main focus on lithosphere dynamics (Forsyth and Uyeda 1975). Even though early work, by for example Morgan (1971), showed that simplified systematic mantle convection explained first-order observations as well, it took quite a while before mantle convection became more popular. To be clear, researchers from several fields studying the mantle didn’t help very much: lingering discussions on whole mantle convection vs. separate flow cells, whether slabs or plumes can penetrate the 660 km boundary, and problems with incorporating a realistic lithosphere with proper rheologies that produced surface deformation predictions similar to the actual Earth’s surface held back widespread acceptance that mantle convection can explain everything. Nowadays, computational prowess and numerical model sophistication is at a point where we have overcome these issues: once again, the lithosphere is simply the thermal boundary layer of the mantle convection system.

Yes, the lithosphere is tessellated and undergoes continuous reorganisation — we can model this with mantle convection. Yes, there are probably thermo-chemical piles in the lower mantle — we can incorporate these. Yes, we infer that India has moved faster than any continent does at present — just add a mantle convection cell. Yes, the surface has topography that we can’t explain with isostasy and flexure — it is induced by radial mantle flow. I’m forgetting other issues here, obviously. The point is: studying lithosphere dynamics is going to be an obsolete exercise in a couple of years. Join the mantle convection party and be prepared for the future!

Yours truly,

The Sassy Scientist

PS: This post was written after reading quite a bunch of lithosphere dynamics papers trying to explain surface observations without using the mantle!

References:
Forsyth, D.W. and Uyeda, S. (1975), On the relative importance of the driving forces of plate motion, Geophys. J. R. astr. Soc., 43, 163–200
Holmes, A. (1931), Radioactivity and Earth movements, Transactions of the Geological Society of Glasgow, Geological Society of Glasgow: 559–606
McKenzie, D., and R. L. Parker (1967), The North Pacific: An example of tectonics on a sphere, Nature, 216, 1276
Morgan, W. J. (1971). Convection Plumes in the Lower Mantle. Nature, 230, 42-43, http://dx.doi.org/10.1038/230042a0
Vine, F. J., and P.M. Matthews (1963), Magnetic anomalies over ocean ridges, Nature, 199, 947
Wilson, J. T. (1965), A new class of faults and their bearing on continental drift, Nature, 207, 343

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.