This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites. I'm Oliver Strimpel, a former astrophysicist and museum director with an endless curiosity about the geology of our extraordinary planet. Most rocks have a history stretching back tens, hundreds or even thousands of millions of years. During that time, some rocks have just sat there and been largely ignored, while others have been subjected to enormous pressure and/or temperature changes. Knowing what has happened to a rock during its lifetime gives us crucial insight into how geological processes work, such as how mountains form or how economically valuable mineral resources originated. But how can we find out what sort of history a rock has been subjected to?

Clare Warren is a senior lecturer in the School of Environment, Earth and Ecosystem Sciences at the Open University. Her work has helped us understand how and when deeply buried rocks preserve a record of that burial and subsequent journey up to the surface. Armed with this record, she determines the processes and mechanisms that bury, transform and bring rocks up to the surface. Clare Warren, welcome to Geology Bites.

Clare Warren: Thanks very much for inviting me, Oliver. It's great to be here today.

Oliver Strimpel: The planet is covered with rocks. So how do you decide which rocks might tell you something interesting about the geological questions you're addressing?

Clare Warren: There are three broad rock types that we find on Earth: igneous ones that have crystallized from molten rock, sedimentary rocks, and metamorphic rocks. And it's these latter ones, which have been subjected to high pressures and high temperatures, that tell us about how mountain belts formed in the past. And it's that process of mountain building that I am particularly interested in, and particularly ones that form where two continents collide.

Oliver Strimpel: So I presume, to get the rocks you're interested in, you have to go to mountains.

Clare Warren: Yes.

Oliver Strimpel: But they can be vast.

Clare Warren: Yes, they are huge, and it's difficult sometimes to figure out which parts of the mountain belt to go to to find rocks of interest. And different parts of mountain belts can tell us different parts of the history. So some parts tell us about the early history of formation; some parts tell us about the later parts of formation. So we have a rough idea of where to go to, but then once we're there, sometimes you've just got to start exploring and see what you find.

Oliver Strimpel: And when you're up there, are there rocks just lying around and say, "That looks interesting, I'll grab that one"? Or how does it go?

Clare Warren: It really depends on where you're going. Some places now are protected in law; they're geological heritage sites. Of course, you can't sample from there, and we always operate on a leave-no-trace kind of basis. So when you take the geological hammer, you don't want to leave evidence of where you've collected samples from. But otherwise, pretty much, apart from local laws about collecting things, you can go and take samples where you want to. I mean, obviously there are practical difficulties when you're working in modern mountain belts, because sometimes the altitude's really high, so there's not much oxygen in the air. Sometimes there isn't much exposure, so you can't see many rocks for the taking. And sometimes, where there's been glacial action, for example, it's actually really hard to sample at all, because there are no nice sharp edges that you can hit off with your hammer. But generally you can go and take what you want.

Oliver Strimpel: So when you find a rock you'd like to focus on, do you start by just having a good look at it where you actually find it in the field?

Clare Warren: Yeah, we start from a really broad scale. So sometimes you might take a pair of binoculars out and have a scan of what's in the distance, and you look for different colors and different textures, and you might look for the way that rocks have folded or sit next to each other. And then once you've identified an area of interest, that's where you start getting up closer and having a look at how the rocks fit together on the outcrop scale, so on the sort of couple-of-meters scale.

Oliver Strimpel: And then once you've done that, do you find that it's pretty clear which rocks you really want to hone in on? Are they ones that flash in sparkling colors, or does it take a bit of hit and miss to get rocks that are really going to tell you the story you're after?

Clare Warren: Well, that's a really interesting question, because obviously our eyes are nearly always drawn to the most beautiful rocks, and those are the ones which will end up on our mantelpiece, or on the office equivalent of a mantelpiece. But actually, quite often it's the dull-looking rocks next to the pretty rocks that yield the best scientific information. So there's one particular place in western Norway where everybody gets taken on these geotourism holidays, and a lot of people have got a lump of this beautiful green and red rock sitting on their desks. But it's actually the sort of rather dull, finer-grained, black-looking rock next to it that yields the minerals that tell us about the pressure, temperature and time history, and give us much more information about how that mountain belt formed. So often you don't actually know which are the best rocks until you get back and start analyzing them in the lab.

Oliver Strimpel: But I presume you whip out your hand lens. All the geologists I've seen have hand lenses hanging around their necks.

Clare Warren: Yeah, so we can tell quite a lot by just looking at a hand sample. We can, for example, identify the different minerals that are in the rock and how they fit together. So it might tell us something about roughly whether this rock formed at 500 degrees, which is fairly cool for a metamorphic rock, or 800 degrees, which is quite hot, and whether it's been faulted or whether it's been sheared out like toffee. But until you actually take the rock back to the lab and start looking at the chemistry of the different minerals, there's only so much that you can tell with a hand lens.

Oliver Strimpel: OK. So you've whacked off your samples with a hammer, and you've put them in your bag, you've got home. And now you really want to try and uncover this history that tells us what the rock has been through. So what's the next step?

Clare Warren: So I guess the next step then is to make a thin section. These are thin slices of rock that are about 30 microns thick, about as thick as your hair, and when you cut rocks that thinly, you can actually start to see through them. So the first thing we do is look at the rocks under a microscope, which often confirms what we've seen with the hand lens, but it's much clearer to see which different types of minerals there are. We might identify some that are too fine-grained to be seen with a hand lens. And then once we've identified what types of minerals are in the rock, we take those thin sections to higher-powered equipment. So there's, for example, an electron microprobe, where we can measure the chemistry of the minerals, specifically the major elements, things like silicon and iron and aluminum, things that make up quite a lot of the composition of the rock. And it also helps us to identify which elements are in the different minerals.

We can then also take that specific section of the rock to a mass spectrometer, and that'll tell us much finer detail about the elements that are in really tiny concentrations in the rock. Or, for example, if we want to know the age of the rock, we need to start looking at isotopes, so forms of the same element that behave alike but have different masses, and you need really high resolution in the mass spectrometer to be able to tell those apart. And once we've got all that information, we should be able, with a following wind, to tell the pressure of the rock, the temperature of the rock, and hopefully the age of the rock as well.

Oliver Strimpel: So the complete history of a rock, if only you can get it, then would be the history of the temperature and the pressure, the pressure telling us the rock's depth, over all the time since the rock was first conceived, was a twinkle in the eye of the magma it came out of, something like that.

Clare Warren: Yeah. So if you think about a rock a bit like a tree, the tree rings on a tree can tell you time from when the tree was just a sapling all the way through until it was a stately, huge tree 400 years later. And a rock behaves a little bit like that as well. So if we imagine our nice beach sand, for example, on the edge of a nice coast, and then the ocean disappears and that bit of coast collides with the next bit of continent, that bit of sand on our beach will eventually get buried, and will turn into a new, different rock type and grow different minerals. But all of that takes millions of years. And so we need to look to see when the tape recorder starts, to see when that rock starts being buried, how deep it gets, at what time it gets to its greatest depth and its highest temperature, and then its journey back towards the surface. So the best rocks in my eyes are the ones that record that longest period of time and tell us what the pressure and temperature and time were doing over millions of years.

Oliver Strimpel: Well, let's start with how you can figure out pressures and temperatures that the rock has been subjected to.

Clare Warren: So these are recorded mainly by the minerals that make up the bulk of the rock, and there are different ways that you can record pressure and temperature. Sometimes it's by the way that magnesium and iron, for example, swap between two different minerals. So if you know the magnesium and iron concentration of one mineral and another mineral, and you know that they grew together at the same time, you can calculate the pressure and temperature that those grew at. There are other, newer techniques, and more high-powered techniques, where you need to look at the trace element concentration of titanium in a particular mineral; that might also tell you pressure and temperature. So it's basically by chemistry, and by assuming the magical status of equilibrium, and that's of course always the difficult bit. But if we make a bunch of assumptions and do some calculations, and we know the chemistry, then pressures and temperatures that appear to make sense usually fall out of that.

Oliver Strimpel: Are there any particular minerals that are particularly useful for determining pressure and temperature?

Clare Warren: So the best ones for the rocks that I work on are minerals called garnet, which are nice, big, bright red ones, and minerals like biotite, that's the shiny brown mica that you quite often see twinkling in rocks. So those are two really good ones, because they record pressure and temperature really well. Other minerals: there's a mineral called rutile, which is most commonly used for making the white color in your toothpaste. That's a really, really good pressure-temperature indicator mineral as well.

Oliver Strimpel: So the detailed compositions of telltale minerals such as garnet and biotite occurring in a particular rock depend, in ways unique to each mineral, on what the temperature and pressure was while they were crystallizing from the melt. For example, it turns out that one especially sensitive indicator of the temperature at which these minerals crystallized is the relationship between the iron-to-magnesium ratio in the garnet and the iron-to-magnesium ratio in the biotite. So if you use these very sensitive instruments, you have to measure the concentrations of iron and magnesium in these two minerals, you can get at the temperature.

Clare Warren: That's right. And garnet grows over a long period of time as well. So it's a particularly good mineral for recording pressures and temperatures, because it will record different values in its core compared to its rim. So, coming back to tree rings, for example, garnet is a particularly good recorder of pressures and temperatures, because it does grow over such a long period of time.

Oliver Strimpel: Let's talk about the time dimension, then. How do you figure out the time at which various things happen to a rock?

Clare Warren: So we record or measure geological time using radioactive decay. There are some elements which decay, for example uranium, which decays to lead. And there are some minerals in a rock which will concentrate uranium and exclude lead when they start crystallizing. So there are a couple of good minerals: zircon is one; another's monazite. They all have these fantastic names, minerals. And these are particularly good geological clocks, because again, they grow over long periods of time, and they form these different shells of different composition at different times. We can actually start using those as the tape recorder. The problem with those minerals is that they don't record pressure and temperature. So we then try to link our pressure-temperature minerals, like garnet, with the age information that our chronometers, our clock minerals, record.

Oliver Strimpel: How do you get this radiometric information out of a rock?

Clare Warren: So that's where we have to go to our mass spectrometers. What we do is we have these thin sections of these rocks, and we find the minerals, the clock minerals, in our rock using, for example, the electron microprobe. And then we use a laser with a spot size of about 10 microns, so that's about a third of the size of your hair, just to laser tiny little spots all the way across these minerals. And then we collect the chemistry information from those minerals: how much uranium it's got in it, how much lead it's got in it, and the isotopes of those two elements, as well as some of the other elements, like thorium, or the rare earth elements. They've also got some weird and wonderful names. And we collect all that information spot by spot. These grains are probably about 100 to 200 microns big, so tiny, tiny. We put a whole row of spots on these grains, and then we collect all that age information. So it is chemistry magic, really. Much better than I ever did at school.

Oliver Strimpel: Cool. So if I can spell this out a little more, you start off with a mineral such as zircon, which has a certain amount of uranium trapped inside it, and it decays to a particular isotope of lead. And then when you put the zircon into the mass spectrometer, you measure how much of the lead decay product there is compared to the uranium, and since you know the half-life of the decay process, you can tell how long ago the zircon crystallized and trapped the uranium inside it, starting the radiometric clock.

Clare Warren: Yeah, that's right. And the first people to carry out age dating of rocks like this would measure all the uranium and all the lead in the whole rock, and of course they didn't have mass spectrometers then. That's about 150 years ago. So it was very complicated and very time-consuming to get one age. And now we can measure these tiny portions of these tiny minerals, and measure hundreds of these minerals in a day, and get much more accurate and precise information out. So the idea that we can use radioactive decay has been around for a long time, but the techniques that we now use give an amount of information that would have blown the heads of the scientists of a hundred years ago.

Oliver Strimpel: OK. So now, using these radioactive methods, you can get the crystallization age of a particular crystal, when it forms. So does that then tell us the age of the rock that it's sitting in?

Clare Warren: It can do, but metamorphic rocks are a little bit complicated, because they have changed their form from one type of rock to another type of rock, and it's the change that I'm particularly interested in. But some of the minerals in the rock will still record the age of the original rock, for example if it came out of a volcano originally, or crystallized as a large granite body underground, and then it got metamorphosed later. So what we then need to do is to try to unravel which of the ages relate to different parts of the process that we're interested in, and that's where all these tools of linking pressure and temperature and age, using these geochemical fingerprinting tools, come into play. So it really is like a detective story, like a forensic science story, trying to link little tiny bits of evidence from one mineral and trying to patch that in to when that other mineral was growing.

Oliver Strimpel: So nature doesn't make it easy for us. There isn't any one mineral whose tree rings will record both the age, from the amount of uranium and lead, and the composition, say, like garnet, to tell us the temperature and pressure at the time. We've got to cross-correlate different kinds of minerals.

Clare Warren: That's right. And we can also use the way that different minerals sit alongside each other. So for example, when garnet grows, it will engulf other minerals that are in the rock at the time that it's growing, because if it comes across something that it doesn't want to make part of its crystal structure, it will just grow around it. So garnet might contain different inclusions of monazite in the core and different inclusions of monazite in the rim. And so those different inclusions can then tell us how long that garnet took to grow. So there are these different ways and mechanisms of linking the garnet and the monazite, or the garnet and the zircon, together, just by the way that they're growing.

Oliver Strimpel: So let's say, by applying this whole battery of techniques, that you've managed to glean enough information to tell you what the rock has been through since it was formed. Can you give us an example of a rock you've done this for?

Clare Warren: So I think some of the most interesting examples I've worked on recently come from the Himalayas. I actually went to Bhutan, on the eastern side of the Himalayas, and went all the way up to the northwest corner of the country. And so we were camping at altitudes of about 4,000 meters or so. So this is proper hardcore mountainous field geology. But we're trying to unravel how the Himalayas formed: which rocks were colliding between India and Asia, and when, and what then happened to them as they got buried and brought back up to the surface, because obviously the rocks that are at the surface today were buried at depth 10, 20, 30 million years ago.

So we found this one place with two specific samples that I was looking at. One was a beautiful green and red rock, a high-pressure metamorphic rock, that told us something about the very depths of the Himalayas. And then there were other rocks nearby, which were metamorphosed at much shallower levels, and we were trying to figure out how these two different rock types fit together. So were they just recording different parts of the story, but had had the same history? Or did they have a different history and then were placed together somehow in the vast washing machine that was underground?

So one of the rocks told us a lot about the early parts of the Himalayan history. India and Asia collided about 50 million years ago, and the metamorphic processes that heat up rocks and squash rocks start getting recorded in the Himalayas about 35 million years ago. And these older, these 35-million-year rocks, are quite rare in the Himalayas, because most of the time those haven't come up to the surface yet. Actually, it's the younger rocks, which were metamorphosed at a lower level, which are now at the surface. So we found this one rock that told us a lot about this early history, and then this deep, high-pressure rock was much younger than that. And so that was also then a bit of a puzzle: how do you get a young, deep rock suddenly a kilometer away from a shallower, colder rock? But pulling all these different pieces of the geochemistry and the field relationships and the mineral relationships together tells us a lot about that washing machine process of how India and Asia finally collided. So yeah, it's a fascinating story, which we only managed to unravel back in the lab, from two rocks that we happened to manage to get to at 4,000 meters altitude, on a day when we actually had enough strength and oxygen to wield a hammer to take these two bits home with us.

Oliver Strimpel: So this washing machine process you're uncovering is the somewhat chaotic burial, partial melting and uplift of the Indian rocks caught up in the collision with Asia. Is this process specific to the Himalayas?

Clare Warren: The details of how the rocks are now sitting together in the Himalayas are quite specific to the Himalayas, but what we're finding is that the processes that operate as two continents collide, and which bits of rock tend to ride over each other and start melting and start getting mangled around, have actually been happening for billions of years. Some of the oldest mountain belts that we have evidence for on Earth today, so about 2 billion years old, actually formed in much the same way as the Himalayas, the youngest mountain belt of these continental collision types, forming today. But what's unusual is that the Himalayas tend to only expose the rocks at the very surface today, so the higher levels of the mountain belt. And over time, those will erode, and we'll start then looking at the deeper levels. And so the older the mountain belt is, the deeper you look down into the structure. So what's great about these rocks that we've been finding in the northwest of the Himalayas is that they're showing us evidence from some of the deep roots of the Himalayas at the surface today, and that gives us a tie point, so that we can link the really old mountain belts with this young mountain belt and say, hey, look, the processes in the roots of these mountain belts were actually very, very similar in the past to the present day.

Oliver Strimpel: So there's been no major differences in the way that plate tectonics operates for 2 billion years, in this particular circumstance, and that's quite interesting. Is there a particular ancient mountain range that you compare the Himalayas with to come to this conclusion?

Clare Warren: So I haven't particularly been to this mountain range. It's called the Trans-Hudson Orogen, and it's in Canada, and that's about 1,800 million years old. But other workers have worked there, and the kinds of metamorphic maps that they are producing are very, very similar to the Himalayan ones when you look at them on a broad scale. So the types of rocks that we see, the rates and timescales of the processes that operate in those mountain belts, and this general pattern are very, very similar.

Oliver Strimpel: Plate tectonics has been around for most of the history of the Earth, it would seem.

Clare Warren: Yeah. Some of the bits of plate tectonics appear to be much younger, but the way that continents collide appears to go back at least half of the age of the Earth.

Oliver Strimpel: Are there any new tools emerging now that might help us elucidate even more about the rock's history than we can today?

Clare Warren: Well, what we're finding is that the more we look, and the finer and finer the detail that we look at, the more and more we find. So there's a particularly interesting instrument called the atom probe, which allows you to take tiny, tiny bits of minerals, so 50 nanometers (and we were talking about microns before, so much, much smaller), and peel off, atom by atom, the structure of that mineral. And all those atoms then fly off into a detector, and how they hit the detector you can then use to unravel where that atom was in the structure originally. So it's a very, very high-powered mass spectrometer. And so you can actually see, for example, where the lead is sitting in a zircon, and where probably the original uranium was sitting as well, and whether that lead has moved. So you can actually match the original element that decayed with the new element that it decayed into, atom by atom by atom. And that is just phenomenal.

Oliver Strimpel: Clare Warren, thank you very much.

Clare Warren: It's been an absolute pleasure. Thank you.

Oliver Strimpel: For more about Geology Bites, as well as pictures and diagrams that illustrate this podcast, you can go to geologybites.com.