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Calibrate your intuition

I am a geodynamicist. Geodynamics aims to understand the evolution of the earth’s interior and surface over time. The following simple exercise explores the subject of geodynamics in the context of the availability of data. There are many parallels with modelling systems in exploration geophysics.

Let’s imagine an ideally symmetrical earth with physical properties (density, viscosity, temperature, etc.) as functions of depth and time. A simple two- dimensional time–depth diagram covering the earth’s entire history and its interior will thus be a schematic representation for the subject of geodynamics. For the real earth such a diagram should be four-dimensional, but this rectangu- lar diagram will do for us. The entire diagram should be then covered by data points characterizing the physical state of the earth at different depths, ranging from 0 to 6000 km, and for different moments of geological time, ranging from 0 to around 4.5 billion years ago. However, the unfortunate fact for geodynamics is that observations for such systematic coverage are only available along the two axes of the diagram: geophysical data for the present-day earth structure and historical record in rocks formed close (typically within a few tens of kilometres) to the earth’s surface. The rest of the diagram is thus fundamentally devoid of observational data, so we have to rely on something else.

What else can we use? Scientific intuition based on geological experience and modelling based on fundamental laws of continuum mechanics! However, our intuition cannot always be suitable for geodynamical processes that are com- pletely out of human scales in time (a few years) and space (a few metres). We have to accept that some of these processes could look completely counterintuitive to us. For example, acceptance of plate tectonics was delayed by relying on commonsense logic: a solid mantle conducting shear seismic waves can only deform elastically not allowing for the movement of continents over thousands of kilometres. The crucial point that was finally understood by the scientific community is that both viscous (fluid-like) and elastic (solid-like) behaviour are characteristic of the earth, depending on the time scale of deformation. The mantle, which is elastic on a human time scale, is viscous on geological time scales (>10 000 years) and can be strongly internally deformed by very slow solid-state creep. 

The ways in which various geodynamic processes interact with each other can also be very difficult to conceive using only scientific intuition. This is why intuition in geodynamics should be — must be — assisted by modelling. In a way, modelling helps train our intuition for very deep and very slow geological processes that cannot be observed directly.

Another role of modelling is the quantification of geodynamic processes based on the sparse array of available observations. Consequently, the systematic use of both analog and numerical modelling is crucial to develop, test, and quantify geodynamic hypotheses — and perhaps most questions about the earth. 

Beware the interpretation-to-data trap

Don’t ignore seismic attenuation