From mountains to microscopes
Instructions
To use this interactive, move your mouse or finger over any of the labelled boxes and select to obtain more information.
Welcome
New Zealand scientists are taking part in a multinational project to drill deep into the Alpine Fault. This active fault stretches for 500 km along the west coast of the South Island. By looking at pieces of rock (core samples) from deep within the fault, scientists hope to learn more about how and why earthquakes happen.
Studying core samples with microscopes is an important part of the drilling project. In this interactive, you can trace the journey of core samples from the Alpine Fault to the University of Otago. You’ll see how they are prepared and studied under the microscope, and you’ll learn how something as small as a microscopic grain of rock can shed light on something as big as the Alpine Fault.
1. Why study the Alpine Fault?
PROFESSOR DAVE PRIOR The Alpine Fault is particularly special because it’s a type of plate boundary where one side is sliding past the other. It’s what we call a strike slip fault. But there’s also a component of movement in the other sense that, if this is the Earth’s surface, one side is moving up, so the combined movement is like this, and the reason why that is so cool is that it means that rocks which form say at 10 or 20 kilometres depth get carried up to the Earth’s surface. And we can look using microscopy and all sorts of other tools at rocks that were created at say 10 kilometres depth, 15 kilometres depth, 20 kilometres depth. So there’s nowhere else in the world where you can do this. One of the things we’ve moved a little further forward with is a programme to sample parts of the Alpine Fault by drilling. And one of the big reasons for doing is that if we drill to 1 and a half kilometres, then we can sample materials that in a few hundreds and thousands of years time will be at the surface, and similarly, stuff which is at the surface now would have been at those depths a few hundred thousand years ago. So we can get a full picture including what happens at depth on this particular fault.
Acknowledgement Image courtesy of The Deep Fault Drilling Project – a multinational collaboration lead by GNS Science, the University of Otago and Victoria University of Wellington with researchers from the University of Auckland, the University of Canterbury, Liverpool University and the University of Bremen in Germany. Scientists from the United States and Canada are also participating.
2. What the cores can tell us
PROFESSOR DAVE PRIOR What we hope to learn by our work on the Alpine Fault is what are the physics by which rocks deform. The process of deformation includes things like earthquakes but it also includes the longer-term kind of squidgy deformation of the rocks where they’re changing shape without actually breaking. Those two are closely related because, at depth in the Alpine Fault, say, 10 kilometres or deeper, the rocks don’t generate earthquakes, the deformation all occurs by the rocks simply squidging. So if you imagine if you have a chocolate bar on a summer’s day and you try and pull a bite off it, it tends to stretch in your mouth like this, whereas if you do it on a winter’s day, it snaps. At depth, things are hotter, so the rocks squidge. At a shallow level, they’re colder, so they tend to break. And the deformation at depth, this squidgy deformation, is ultimately what drives the deformation at a shallower level, so the Alpine Fault is in a sense a natural experiment.
Acknowledgements Professor David Prior and Dr Virginia Toy, Department of Geology, University of Otago. Strike slip fault animation, courtesy of US Geological Survey. Stills of drilling rig and the Deep Fault Drilling Project site, courtesy of The Deep Fault Drilling Project – a multinational collaboration lead by GNS Science, the University of Otago and Victoria University of Wellington with researchers from the University of Auckland, the University of Canterbury, Liverpool University and the University of Bremen in Germany. Scientists from the United States and Canada are also participating.
3. Processing the cores
PROFESSOR DAVE PRIOR There’s a certain amount of processing that has to be done on a particular bit of drill core before you move onto the next one. One has to process that core very carefully to make sure it’s scientifically useful. And the key things are what their relationship is to where we got them from, so we need to know how deep they’ve come from and we have to make sure that information is tagged correctly to the core. And when the core comes out of the ground, if you can imagine the core like that, it has a top end and a bottom end, and we have to make sure we know which is the top end and the bottom end, so which way was up. We would like to know which way was north in the core as well. That’s actually a lot harder because the process of drilling is a rotary drilling process, so the core gets spun. Quite often a core in reality is not a 3-metre tube of rock but it’s a small bit of rock and a pile of rubble. So someone has to reconstruct, well, where does the bit of rock come from and where does the rubble come from? And sometimes there isn’t an easy answer to that.
Acknowledgments Professor David Prior, Department of Geology, University of Otago Stills of drilling rig and the Deep Fault Drilling Project site, courtesy of The Deep Fault Drilling Project – a multinational collaboration lead by GNS Science, the University of Otago and Victoria University of Wellington with researchers from the University of Auckland, the University of Canterbury, Liverpool University and the University of Bremen in Germany. Scientists from the United States and Canada are also participating.
4. Micro-CT: how it works
ANDREW McNAUGHTON The key advantages of micro-CT is that you can reconstruct a very accurate three-dimensional model of a life-sized object. Micro-CT works by taking a series of X-rays around an axis of rotation. So the easiest way would be to imagine a rubber duck in a paper bag and a ring of cardboard around it. If you got a really strong torch and shone it through the bag, you’d get a shadow on the ring of cardboard and then if you move the torch around another few degrees, you get another shadow but the shadow would be slightly different because you’ve taken it from a different angle. So then if you went round in a series of rotations right around, you’d get a whole ring of shadows. So each of those shadows would represent the same object but from a different angle. So the micro-CT works the same way except you’re using X-rays instead of light, so you get a ring of X-rays, and then you recreate what the original object looked like, because you know the angle and you know the direction that it was taken. Then you can go in and see the internal detail, so that’s the biggest advantage you’ve got. Everything in its place where it should be, you can really burrow into the bits you’re interested in.
Acknowledgments Andrew McNaughton, Microscopy Otago, University of Otago 3D animation of micro CT sample, by Hannah Scott, University of Otago
5. CT and micro-CT of core samples
PROFESSOR DAVE PRIOR One of the limitations you are always up against in trying to look at rocks is that we always look at two-dimensional sections, and in reality, of course, rocks are three-dimensional things, and some of the properties of them are highly dependent upon how things fit together in three dimensions. So the flow of fluids like water through a rock is a really important control on how they behave, and you can’t really understand the ability of fluid to flow through a rock without thinking in three dimensions. One of the places you get water in the rock are in pore spaces, basically holes in a rock, and if those holes are connected up, it’s permeable. If they’re not connected up, the water can’t get through. And if you imagine you have your permeability as long thin tubes, and you cut it one way, and you’ll see sections, which are like long thin things, and you think, “Oh, that’s nice and permeable.” If you cut it at 90 degrees to that, you’ll see a whole load of circles, which aren’t connected to each other and you’ll think, “Well that doesn't look very permeable.” And you don’t really get the answer unless you can put those together and see the thing in three dimensions. Now there are all sorts of variants of three-dimensional scanning which has come from innovations in medicine, which enable us to map out in three dimensions the density distribution in a rock. So looking for the three-dimensional arrangement of pore spaces is something that CT-type methods are very good for.
Acknowledgments Professor David Prior, Dr Virginia Toy, Andrew McNaughton and Hannah Scott, University of Otago Rock pore diagrams, courtesy of CO2CRC. 3D animation of micro CT sample, by Hannah Scott, University of Otago
6. Preparing cores for microscopy
PROFESSOR DAVE PRIOR For us to be able to look at the samples we’ve collected from the drill core using microscopes, we need to prepare it so that we can look through it using light, and also we can look at the surface in great detail using scanning electron microscopy and electron back scatter diffraction. So to see through it using light it needs to be thin, so the process is to use diamond-coated saws to cut out a small chip of the rock about a centimetre thick or so. And we grind flat one side of that and stick it to a piece of glass, then cut off most of the chip so we have a piece of rock of say a millimetre or so thick, and then we grind that gradually to make it thinner and thinner, and we use a microscope to assess how thick it is, to check until we get exactly the right thickness. And then from there, we clean up and polish up the surface. And so then we have a sample which is in fact 30 microns thick – 30 microns is really good for transmitted light polarised microscopy so we can see lots of distinctive features in the minerals, and we can see the microstructures, and the polished surfaces enable us to get really good quality images in the scanning electron microscope.
Acknowledgements Professor David Prior, Dr Virginia Toy and Brent Pooley, Department of Geology, University of Otago
7. Optical microscop y
PROFESSOR DAVE PRIOR Light microscopy is actually a really beautiful tool for rocks and minerals because rocks and minerals have specific optical properties, which means that we can distinguish the different mineral types very easily using nothing more sophisticated than an optical microscope with polarising filters in it. We look at what we call microstructures as well – the arrangement of those minerals. So a rock which has been deformed in the earthquake process will have lots of angular bits and fractures in it whereas a rock which has been deformed at higher temperature and has not undergone fracturing, then the grains will have elongate shapes, for example, so it might have been an original spherical grain and it gets pulled out to make a long thin thing. So we look at that first on the optical microscope, and the microscope enables us to look at features on a scale where we could also see the features by looking with our eye and go up magnification to a scale where we can see features which we’ll then recognise when we get to the scanning electron microscope. We can do a huge amount with an optical microscope, and it’s fast, it’s cheap, it’s easy to apply. It generates scientific information in its own right and it’s also an essential screening process for the samples which we’re going to use for more detailed types of analysis.
Acknowledgments Professor David Prior and Dr Virginia Toy, Department of Geology, University of Otago Rock micrographs courtesy of Dr Virginia Toy, University of Otago
8. EBSD: how it works
PROFESSOR DAVE PRIOR Electron back scatter diffraction is a technique which we use in a scanning electron microscope, and it enables us to measure the crystal structure and orientation for a very small point on a sample surface. If you have an electron beam coming in an SEM and this electron beam comes down and it hits a sample at a point, the electrons go into a sample and there’s a process which scatters them into every possible direction. Because crystalline materials have preferred lattice planes within them, and you can see the facets on this crystal are defining where some of those lattice planes are, then the diffraction process in crude terms focuses the outcoming electrons in those lattice planes. So this particular lattice plane will have a focused band of electrons coming out in all directions parallel to that. When you deform this, the crystal is allowed to slide on those planes of weakness, and what happens is that certain crystallographic directions get lined up. So if we can measure the way they’re lined up, it can tell us something about what were the temperature conditions, for example, under which the rock deformed and what way was it being pulled or pushed, because the alignment will relate to whether it was pulled out this way or whether it was pulled out that way.
Acknowledgments Professor David Prior, Department of Geology, University of Otago
9. EBSD and the Alpine Fault
PROFESSOR DAVE PRIOR Electron back scatter diffraction, in the case of quartz in the Alpine Fault zone, it enables me to measure the orientation of every crystal in a sample. So a rock which is strongly deformed will tend to have all of the crystals lined up, and the way they’re aligned tells me which way it was pulled when it was deformed in the ground and what was the temperature conditions. So by measuring those kinds of things, I can forensically extract that information from a rock which finished deforming a million years ago. We haven’t as yet applied this to the drill sample but we have hand samples from the Alpine Fault zone, and one of the things we’ve been able to establish with the electron back scatter diffraction is what the movement direction was at depth in the Alpine Fault in a way that was difficult by other means. So one of the things we’ve been able to establish quite well is that, on this fault where we know at the surface the movement is like this – so that you could draw a line on the fault and the fault plane is moving like that – that the movement at depth for the most part is exactly the same, and that’s cool because that means that we can link the deformation at depth, the squidgy deformation at depth, with the earthquake-generating deformation higher up.
Acknowledgments Professor David Prior and Allan Mitchell, University of Otago Stills of drilling rig and the Deep Fault Drilling Project site, courtesy of The Deep Fault Drilling Project – a multinational collaboration lead by GNS Science, the University of Otago and Victoria University of Wellington with researchers from the University of Auckland, the University of Canterbury, Liverpool University and the University of Bremen in Germany. Scientists from the United States and Canada are also participating
10. Putting it all togethe r
PROFESSOR DAVE PRIOR The thing about rocks is they have features with a huge range of scales in them. And no microscopical kind of rock is really a lot of use unless you can relate it back to the bigger scale, so for us in our study of the Alpine Fault, the biggest scale feature is our fault zone which is a kilometre wide by 500 kilometres long by going down to 20–25 kilometres depth, which is a very big scale feature. And if I’ve got a field of view 10 micrometres in a microscope, I want to know how that 10 micrometre bit relates to the Alpine Fault zone. There’s too much scale difference to try and draw a picture of 10 microns on the scale of the whole Alpine Fault zone, so the continuity of scale is that I will know where that 10 micron field of view is in a picture of say 100 microns field of view and that I’ll know where it is with a picture of 1 millimetre field of view. And that could all be on a scanning electron microscope, and then that 1 millimetre field of view, I’ll know where it is in a 1 centimetre field of view on the polarising microscope, and the 1 centimetre field of view, I know where that is looking at a whole thin section which is several centimetres long. And then I know where that thin section comes from in the rock because I’ve got a square I’ve drawn on the rock, and I know where the rocks come from in the ground and how it links up to other units. I could get some information from the 10 micron field of view by itself but it’s incredibly devalued if I don’t have that continuity back to its biggest scale context.
Acknowledgments Professor David Prior, Department of Geology, University of Otago Satellite map of South Island, courtesy of NASA Aerial still of Alpine Fault, by Lloyd Homer, courtesy of GNS Science Limited Rock micrographs and still of rock, courtesy of Dr Virginia Toy, University of Otago Still of rock cutting and Southern Alps, courtesy of Professor David Prior
