Understanding the Deep Earth

Online Workshop

One of the things that came up in the Wednesday (2/24) session was the need for information about current debates or key questions across the geosciences.
Some of the other workshops have tackled this question and come up with resources about these questions in their own disciplines. See http://serc.carleton.edu/NAGTWorkshops/earlyearth/questions/index.html for an example from the Early Earth Workshop in 2007.
We had come up with two ideas along this line for the Deep Earth - the Plume Debate which Brennan's talk and resources give lots of material to work with, and what actually drives Plate Tectonics. If you have ideas for Key Questions or materials/information that should be on pages about these questions please submit them here so that we can start building this really interesting and important resource.

The existence and nature of geochemical reservoirs in the deep earth.

Length scales and time scales for mixing in the mantle.

Light elements in the core.

As we discussed briefly, it would be useful to go through the force balance arguments for PT driving forces, aka Fig. 4.20 from Moore and Twiss (1999). I know I'm working with old info a bit, but Chapter 10 of Cox and Hart (1986) has a series of graphs showing plate motions versus several of the possible plate 'forces' (Figs. 10-7 to 10-11).

Although that data may be outdated, the graphs might be useful starting points from which to move to 'better' and newer data sets, as they are relatively simple to explain and generally make intuitive sense. Using NUVEL derivatives to get better plate velocities and directions of motion, using GIS to more accurately estimate plate sizes and calculate plate volumes, using tomographic data to more accurately constrain how much of each plate is 'hidden' in the mantle but still attached, using newer mineral physics data to better estimate densities of minerals that are in parts of plates which are deep in the mantle could partly or all be used to better test hypotheses about which plate forces are most important.

I would also wonder for forces such as drag, how much its effects depend on depth of lithosphere/asthenosphere boundary and temperature at the boundary, which might have a pretty strong control on the viscosity contrast across the L/A boundary.

A lot of the specifics for data to answer these questions I suspect is well beyond me, but I'd be glad to help gather/collate data in whatever way I can...

One possible source of key questions is the Seismological Grand Challenges document created by the seismology community in 2009 http://www.iris.edu/hq/lrsps/. About half of the 10 challenges relate to the deep Earth, including
How do magmas ascend and erupt?
What is the lithosphere-asthenosphere boundary?
How do plate boundary systems evolve?
How do temperature and composition variations variations control mantle and core convection?
How are Earth's internal boundaries affected by dynamics?

Here is my favorite question from various conversations I had this fall:
What is the oxygen fugacity in the mantle? Since there is some evidence that suggests that Fe+2 may disproportionate into Fe0 and Fe+3 at very high pressure, is this even the right question to be asking?

This post was edited by Glenn Richard on Feb, 2010
Predictability is a fundamental issue concerning the Earth, the universe, and phenomena in general.

What are we currently able to predict about the future, and what can we expect to learn to predict at a later time, as our understanding advances?

What aspects of phenomena make them predictable, or impractical to predict?

The topic of predictability carries with it fundamental issues such as probability, chaos, the nature of models, and questions about what we can say about the time and place of future events.

Concerning the deep Earth, some issues are what we can predict about future changes in the magnetic field, the future configuration of plate boundaries, and seismic events caused by plate motions.

Kindred issues can be raised about the past. How much and what detail can we recover about past events, and what information is lost forever? Concerning the deep Earth, we can ask about the history of the differentiation of materials, the signature of the history of magnetic field, evidence of changes in plate boundaries, and many more questions.

... and while we are on the topic of what information about the Earth is accessible and what is not, we can consider what we can and cannot learn about phenomena or places that we can only study from a distance, such as the deep Earth. What are the surface manifestations of deep Earth processes? What experiments can we perform that can yield information about the deep Earth, in lieu of our going there?