The first thing I want to explain is the very broad theme of my PhD, what the final goal is and what it is not. I want to start by talking about my research group, 10 PhD students who are all united by a common theme of study. That theme is ‘Deformation of geological materials’. What is the most important thing that we all have in common? ‘Geological materials’ is a pretty broad category. Into that category falls almost everything on and in the earth that isn’t currently alive or man-made! What unites us all is ‘deformation’, changing something’s form. We can do that in a number of ways, squashing, pulling, twisting, or a combination of all three, but the process, the way a particular material manages to deform, stays pretty much the same. The most important thing to remember about my PhD is that I am studying a process. While I am in fact studying actual rocks to do so, the most important goal is NOT to understand the geology of a specific area. If that happens, that’s great, but it isn’t the only or the ultimate goal.
So I’m trying to understand more about a process. What kind of process? Perhaps the subtitle of our research group will give us a clue. It reads: ‘Mechanical-chemical feedback and the coupling across scales’. That second part is really only there to remind you of the fact that geologists can study things as small as atoms or as big as continents, for much less than a second to much more than a billion years. Sometimes stuff we find out at one scale might be useful for studying another! The much more important part is the first bit, ‘mechanical-chemical feedback’. That’s the process we want to know more about, but it’s not very clear what we mean. ‘Mechanical’ and ‘chemical’ on their own make sense, and it is possible to see that they apply to rocks. Rocks can get squashed (mechanical) and they can take part in chemical reactions (chemical). When rocks get squashed and heated they can change their shape and, and the amount and type of minerals that they consist of. Rocks which have been changed by pressure and heat are metamorphic rocks, a name that you might have heard.
But why should there be ‘feedback’, why should these two separate things affect each other? Here we have to use some imagination. Imagine I have a cube of rock, which is made of a bunch of different minerals. I start to squash the rock (one kind of deformation). Just like some chemical reactions only happen after you reach a certain temperature (burning!), there are also reactions that only happen once you reach a certain pressure (for example the melting of ice when you press on it with the blade of an ice skate, allowing you to slide easily along). Imagine I press hard enough on the rock that a reaction happens. I don’t add or take away anything from the rock, but the mineral that is the product of the reaction won’t take up exactly the same amount of space, it will either be denser or less dense (different volume, same mass).
Imagine I make a reaction happen by squashing the rock cube. Let’s say the minerals I form take up more space than the minerals used up. That means they need to make space for themselves, by pushing outwards. They are pushing against the pressure from outside. That means that pressure on the rock is reduced. If the pressure goes down, the reaction can’t carry on (recall it needs a certain pressure to happen). So we have a negative feedback loop. The reaction stops itself!
If the product of the reaction takes up less space in the rock, less stuff is pushing outwards to resist the squashing, so the pressure inside the rock increases. That encourages even more of the product to form, which causes the pressure to increase... In that case you have a runaway reaction that would carry on until there was nothing more to react, positive feedback!
That does sound a bit complicated. You’ll be glad to know, geologists agree! We know a lot about how rocks behave when they get deformed and there are no chemical reactions, and we know a lot about chemical reactions in rocks when there is no deformation. Normally people studying a rock spend a long time proving that only one of the two processes happened, so they can understand it. Perhaps you see then why we need some PhD students to look at both at once!
Hopefully it’s clear by now that:
1) My PhD is about understanding a process, not the geological history of an area.
And:
2) The process I’m trying to understand is the way physical deformation and chemical reactions affect each other.
Onwards to part 2...
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