White Paper Blog

Just to get the use of this blog started, I have gone over the NAS document on grand challenges in Earth Evolution, the IRIS Planning document on seismological grand challenges, and the outline of the COMPRES document, and put together the following straw man list of ten outstanding geodynamics problems for our consideration. They are little more than titles at this stage, but perhaps they can serve to get us going.
Feel free to say why we should promote or demote any of these, and offer substitutes or additions.

Peter's "Top Ten" Geodynamics Problems:

Dynamical origins of plate boundaries and plate tectonics

Nature of mantle convection in space and time

Causes and consequences of Earth's changing magnetic field

Formation of the ocean, atmosphere, crust, and core

Magmatism, massive volcanism, and mantle plumes

Earthquake dynamics in the context of the Earth system

Evolution of mountain ranges, plateaus, and basins

Water and carbon cycling within the Earth

Geothermal heat: sources and resources

Variable rotation, surface adjustments, and global change

Geodynamic drivers of biosphere evolution

I think Peter hit the major issues in geodynamics quite well. There are probably some we can add or link, but that's not what I want to do here. I want to start a thread on where the future of some major issues are possibly going. To me there are 2 big motivations, one is pressing need, and the other is future discovery. In the first we'd probably agree that there's at least climate (and energy) and it would be important to explore what we can offer that's useful. In the second, to me the most breath-taking discoveries have to do with extra-solar planets because we are now getting more than the 1-3 data points we've been working with (i.e., Earth, Venus, Mars). And the discoveries are starting to blow our perceptions of what's normal out of the water (e.g., where giant and terrestrial planets are in their orbits around solar systems). But also the issues of "climate" and "exo-planets" are related b/c astronomers are clearly looking for the sweet-spot of terrestrials (so far just "Super Earths") in the habitable-zone, i.e. where there's a temperate climate at least as far as our assumptions of what life needs (given the preponderance and versatility of carbon and water, probably not a terrible assumption).

So following these motivations are some thoughts.

a. The present coupling of tectonics and the hydrological cycles through weathering and formation of carbonates is thought to buffer CO2 and provide a long-term stable climate. It's often presumed that this is a condition for a stable climate when looking at the habitable zone (HZ) in other solar systems, or in considering what is "natural" in our climate and what would we need to do to restore it to (by things like sequestration in mafic rocks). BUT is that the only dynamical or coupled system that stabilizes climate? Presuming b/c it's what happens on Earth is therefore the only way it can work is possibly valid but possibly also myopic. So what other mechanisms like this are possible? Of course it's an open ended question but it has 2 important points: (1) it influences what other modes in which our own climate might work and thus could influence decisions on mitigation strategies, and (2) it influences what we expect to find with regard to habitable zones as we find more and more terrestrial planets in other systems. But a very simple example is that, while on Earth mountain building and weathering are sinks of CO2, perhaps on a non-plate planet, basaltic volcanism might leave enough mafic material to weather and pull out CO2; however, with low viscosity of basalt, these are perhaps not tall enough features on Earth to induce enough weathering so would need something else (smaller planet and less gravity, but then this causes faster thermal evolution). That's an obvious one and perhaps the example of Mars and Venus shows it doesn't work.... but why? It's also barely different than present mode of operation. Are there others?

b. Influence of climate on tectonics. Is it just water that permits plate tectonics? Or does temperature influence the lithospheric stress state and/or healing? (Full disclosure: This is more than a bit self-serving I admit, since both Adrian Lenardic and Mark Jellinek, and my student Billy Landuyt and I have independent models about this sort of climatic influence on plate generation.) But if there is 2-way coupling, then is there an unstable feedback, i.e., temperate climate needs tectonics which needs temperate climate, and if you screw up one the whole system goes to another state (Venus or Snowball Earth).

c. If climate influences mantle dynamics, tectonics, and thermal evolution, what's the influence of the rise of life? Peter's last item concerns geodynamic drivers of the biosphere, but maybe there is 2-way coupling here also. E.g., the rise of oxygen with photosynthesis is a carbon sink and possibly perturbed the planet into Snowball state, which shut off weathering also. Interestingly, the text-book story about how Earth moved out of the Snowball state is by volcanic return of CO2.... but apparently climate models can't get out of Snowball states easily and have to invoke other things (like dust to reduce albedo). Just an aside.

d. As we get more data points for planets in other parts of the galaxy, we need to consider to what extent different solar system compositions (e.g., from different star types, sizes, ages, and regions in the galaxy with different "metallicities", which remember to an astronomer means anything heavier than He) influence how planets convect and work. Even on a more detailed level, "local" planetary evolution and composition has some effect; e.g., to what extent has the presumably (maybe) higher Fe content of Earth after the putative impact with Mars-sized body (Theia) influence our convective state.

With regard to exo-planets, Diana Valencia and Rick O'Connell have been ahead of the curve; while I don't entirely buy the "inevitability of plate tectonics on Super Earths" argument, they're pushing the envelope and trying heroically to bring in lots of ideas from different fields. I would be happy to get Diana's input on this if we want to pursue this line.

I didn't really dig into the "useful approach to pressing needs" (i.e. climate, energy) issue other than very vaguely in item "a". Mitigation in mafic rocks (Peter Kelemen is one of the big champions of that) and geothermal (Peter O mentioned this above) are things to explore also. But this is long enough so I'll stop there.

Dave

I like Peter's general topics on the list. I would like to add something that is related to items 5 (magmatism etc) and 7 (evolution of mountains etc), and is in my view a goal for modeling plate tectonics on the million-year timescale: the role of melting and melt segregation on multiple scales (from the compaction length to orogenic scale) in the evolution of mountain belts, basins, etc. This means moving beyond deformation models that are based on effective rheologies for a single component and instead thinking about multi-component models (e.g., two phases at a minimum). It also means coupling time-dependent chemistry into plate-deformation models.

I also really like Dave's suggestion of adding the perspective from extra-solar planets. Could we distill Dave's suggestions into a bullet in the top ten list, for example: Water, plate tectonics, climate, and the biosphere. Under this heading, one could have a section discussing problems for Earth evolution and a second section on what we hope to learn from extra-solar planets.

A subset of the writing committee met at the GRC last week -- me, Dave, Bruce, Rick, Jason, and Mike. We had a good discussion of grand challenge topics, although the composition of this subset ensured that deep Earth topics would be over-represented. Here is the list of titles that we came up with, not necessarily in order of their importance. You will see that we have intentionally used more vivid language than in previous lists.

1. Causes and consequences of intraplate volcanism

2. Dynamics of continents and their margins

3. Generation of plate tectonics

4. Origin of Earth's changing magnetic field

5. Early days of the Earth

6. Fluxes between the solid Earth, atmosphere, hydrosphere, and biosphere

7. Geology of the core-mantle boundary region

8. Thermo-chemical evolution of the mantle and core

9. Alternate Earths: evolution of planetary interiors and their surfaces

10. Geodesy, surface adjustments and climate change


What I am going to do now is try and merge these with the suggestions from Lucy and Mousumi, and hopefully some from Mark,
into a fully updated list, and will post that new list here.

After collecting everyone's suggestions at and following the GRC, here is my second cut at the top ten unresolved geodynamics problems:

1. Causes and consequences of intraplate volcanism

2. Dynamics of continents: interiors, roots, and margins

3. Generation of plate tectonics

4. Origin of Earth's changing magnetic field

5. Early days of the Earth

6. Water, tectonics, climate, and the biosphere

7. Geology of the core-mantle boundary region

8. Thermo-chemical evolution of the mantle and core

9. Alternate Earths: evolution of planetary interiors and their surfaces

10. Earthquake dynamics in the Earth system

I will now be getting in touch with everyone, to find out who is willing to contribute to each.