Testability in Earth Science

Geology is to a large extent the study of unrepeatable events which happened in the very dim and very distant past. As Kim rather nicely puts it:

That 1.4 billion-year-old granite whose contact aureole I’m studying? It’s done intruding, it’s cold, it’s eroding. It’s not going back to the mid-crust any time soon.

Superficially, this conflicts with the idea that rigorous science is grounded in testing and replication of your results (and, taken to extremes, culminates in the ‘where you there?’ bleating of the more virulent creationists). After all, we can’t run the Earth again to see if we’ve got it right; so how can you ever truly test a geological hypothesis?

It is certainly true that most geological research does not fit easily into the standard picture of experimental science, where you perform laboratory experiments designed to allow you to play around with the controlling variables, and fully explore the behaviour of the system being studied. Geologists don’t study past events directly, but rather work backwards from the traces that these events leave in the rock record. Nature has already run the experiments that we are collecting the data for, but she is usually rather coy about the details: you don’t precisely know the initial conditions (in the case of Kim’s granite, things like the temperature of the melt when it was injected into the crust, and how long the intrusion took to cool), and you certainly don’t have the luxury of playing around with them to explore how different values would affect what you see.

This has led some people to single out ‘historical sciences’ like geology and archaeology from ‘observational sciences’ like particle physics and chemistry. Personally, I find the distinction rather misleading: although the intellectual toolkit required to puzzle out the stories told by billion year-old granites is obviously not exactly the same as the one needed when mixing chemicals in a test tube or firing high-energy particles into each other, when it comes to hypothesis testing, there is not as much difference as you might think. In all branches of science, testing is less about straight repetition and more about the exploration of consequences. Your hypothesis or model is usually based on only a small subset of the possible observations you could make; what predictions does it make about the behaviour of the things that you haven’t yet measured? Real testing involves moving beyond the original experiment into previously unconsidered realms; if physicists just spent their days exactly replicating old experiments, we’d still all be sitting under trees waiting for apples to fall on our heads, rather than flinging space probes at Pluto via Jupiter.

Geology fits quite comfortably into this picture of science: you can’t make a melt crystallise again, but you can look to see if all the minerals tell the same story about starting temperatures and cooling rates – not just the ones you looked at originally. If you’re sampling desert sandstones for palaeomagnetic analysis, you’d expect your results to show that they formed at low latitudes and not at the North Pole. If you’re excavating dinosaur skeletons in Montana, you wouldn’t expect to find signs that humans have chewed on the bones.

There is also another sense in which geology is testable. When studying a particular granite intrusion, or mountain range, or ancient river delta, as well as trying to understand that feature’s specific geological history, we are also attempting to construct a more general model of pluton, mountain belt, or delta formation, one which can be applied to all plutons, all mountain belts, all deltas. We can’t test the predictive power of an orogenic (mountain-building) model by resetting the Himalayas with a slightly different convergence rate or a different thickness of crust, but we can achieve the same thing by taking advantage of the fact that the Earth is a very big and very old place, and has seen many mountain building episodes which are all unique in their own way, thus providing new and different experimental data to test against. For geologists, the Earth is our laboratory, and a repository of data from over 4 billion years’ worth of experimentation in various aspects of tectonics, volcanism, geochemistry, and climatology (to name a few), all in long-term storage in the rock record. By seeking out the results of these ancient experiments, comparing different runs from different locations and times, and using them to predict what else we should find, we are undertaking science in just the same way as any physicist or chemist. Anyone claiming otherwise is just bitter that they don’t get to run around in the mountains and call it work.

That said, I certainly wouldn’t mind receiving a grant to employ the services of the Magratheans

Categories: basics, geology

Comments (9)

  1. BrianR says:

    “…we are also attempting to construct a more general model…”
    I have an ongoing internal struggle where, on one hand, I have a desire to construct simple and elegant conceptual models. Models that succinctly explain a lot of the patterns and characteristics of a particular system. But, on the other hand, I have a desire to highlight the complexity and uniqueness of particular examples and get a little peeved with oversimplification (i.e., conceptual models).
    Let’s call it universality vs. complexity*
    As time goes on, I’m starting to realize that one needs to do both. I picture a playground see-saw (or teeter-totter) with universality and complexity on either side going up and down. And it’s going up and down on purpose…the whole point is to go back and forth. If one holds on to the simple, elegant models too tight, they will blow off exceptions to it as one-off examples that aren’t common. And, if all one does is characterize every last little detail of systems and does not attempt to ‘see the forest for the trees’ by testing results against conceptual models, then what is the point.
    Very good post and topic. I have a draft post discussing this stuff that has been sitting there waiting for several months.
    *complexity as in complicatedness…not the more formal definition of a complex system

  2. Kim says:

    Anyone claiming otherwise is just bitter that they don’t get to run around in the mountains and call it work.

    Yep. 😀
    Thanks for elaborating on your comment about “consequences.” Perhaps that’s a better term than “predictions” (which is what I usually say to students) – it’s very confusing to talk about predictions of things that have happened in the past. But it does come down to the same thing: looking for something other kind of information than what you already have.
    (By the way… are there any philosophers of geology? I know of one historian of geology/environmental science – Naomi Oreskes – but I don’t know of many people who study how geologists think.)
    And to Brian: I think we can’t stop characterizing specific examples. Partly for practical reasons – you don’t find oil, or metals, or water in an idealized conceptual model; you find them in a specific place. (And an idealized subduction zone is not what causes a devastating tsunami.) But also for more philosophical reasons: if you don’t look carefully at the natural world, you can not find the places where the simple, elegant models fail, and you will not recognize an unexamined process that turns out to be very important. (Like, maybe plate tectonics, or the role of carbon dioxide in climate.)

  3. BrianR says:

    Kim…re geoscience philosophy types…check out a short book called “Geo-Logic: Breaking Ground Between Philosophy and the Earth Sciences” by Robert Frodeman
    (link to Amazon)
    I read it a few years ago, and loved it. There’s great stuff about the aspect of field work in earth sciences (i.e., going out into nature and attempting to figure things out).

  4. bob koepp says:

    Rachel Laudan wrote a nice book titled “From Mineralogy to Geology” about 20 years ago. It focuses more on the history than the philosophy of Geology, but since Rachel’s training is in History and Philosophy of Science, she provides a conceptually nuanced version of history.

  5. Lab Lemming says:

    “Geology fits quite comfortably into this picture of science: you can’t make a melt crystallise again”
    Dude, you have just ignored the entire field of experimental petrology.
    It is the melting of rocks under carefu;lly controlled laboratory conditions that allows people like Kim to interperet more complex natural systems.

  6. Bob O'H says:

    I think there’s a lot of similarity between geology and evolutionary biology and ecology (two fields I work in), because they are all historical, and large-scale. Hence, a lot of philosophy of biology might be relevant.
    Philosophically, there is one big problem with these outdoor sciences – because you can’t do controlled experiments to directly test your hypotheses about hat you see out there, it’s much more difficult to infer causality – there can always be other lurking variables, well, lurking. I think that makes the science more interesting – you have to take little bits of knowledge, such as Lab Lemming’s experiments (BTW, Lab Lemming, I hope your boss isn’t called Cliff), and models and studies of other systems etc. to understand what’s going on in your lump of rock.
    Overall, I think we should feel sorry for the poor chemists.

  7. Chris Rowan says:

    Brian – indeed, as I’ve alluded to before, I think our love of messy reality, rather than wanting to approximate everything as a sphere in a vacuum, is possibly one reason why physicists look down their noses at geologists…
    Lemming – no slur on experimental petrologists was intended – you can certainly play around in the lab, and generate useful information, but that’s not quite the same as seeing things happen again in their original setting.
    And thanks for the book suggestions – I think I might be buying myself some Christmas presents from Amazon.

  8. Andrew says:

    Seconding the recommendation for Richard Frodeman’s “Geo-Logic.” He got a master’s in geology before his Ph.D. in philosophy. I need to re-read it, but as I recall his idea of what makes geology a special science is its emphasis on devising stories (that is, its 4D approach) and its common-sense style of thinking (devising models and testing them with new data). He also has a good word for geopoetry.

  9. BrianR says:

    here is a quote from Frodeman:
    ìScience in the field proceeds at a different rhythm…while the lab narrows and controls the flow of information, the fieldís perceptual and conceptual kaleidoscope exceeds our capacities to sort, test, and categorize it. Field scientists develop intuitive skills for parsing knowledge in implicit, nonpropositional ways.î