This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. When the Earth formed, it was covered by a hot magma ocean. At some point it cooled enough to form a solid surface, thought to be basaltic in composition, like today’s oceanic crust. Then at some point the surface began to differentiate into different types of lithosphere: basaltic oceanic lithosphere and more silica-rich continental lithosphere. How did this process occur? Were the first ancient continents similar to the present-day continents? And did the continents form in a burst of activity at a certain point when differentiation occurred, or was it a gradual buildup over Earth history?

Renée Tamblyn studies the early Earth, and in particular the role of water in the formation of continents. She’s also exploring how the production of hydrogen from geological processes could have powered very early forms of life. She’s a postdoctoral researcher at the University of Bern. Renée Tamblyn, welcome to Geology Bites.

Renée Tamblyn: Thank you very much, Oliver. It’s an honor to be here.

Oliver Strimpel: Before we talk about how continents form, how confident are we that the basic picture I laid out just now is correct, namely that the Earth was born without any continents and that these had to form at some later point in Earth history?

Renée Tamblyn: So although we don’t have any rocks preserved from the earliest part of the Earth, particularly from about 4.56 billion years ago, when the Earth was first accreted, until about 4 billion years ago, which is the oldest rock we have, we don’t really have any picture of what the Earth may have looked like then, because it’s not preserved. But I think we are quite confident that the Earth certainly started as a magma ocean. And we know that from this magma ocean we could not make the continents, as you mentioned, these silica-rich continents. They need much more complex geological processes to form, rather than just forming from a simple magma ocean. So we know that at some point (and when is the big question) these continents first emerged, and that there must have been more complex geological processes at play that allowed these continents to differentiate. And we know this because the chemical components which make up the continental crust are very different from the oceanic crust, or what would have been quite similar to this magma ocean at the beginning.

The continents today are quite different from the continents in the past. The continents today are very rich in elements such as silica, as you mentioned, but also potassium, whereas the earliest continents, the ones from the Archean (this is the time frame from 4 billion years ago to 2.5 billion years ago), tend to be richer in elements such as sodium and calcium. So this leads us to think that the processes we see today making our continents, our potassium-rich continents, must have been quite different to the processes we saw in the Archean.

Finally, the question of when the continental crust actually formed is a huge question in the Earth sciences, because, as I mentioned, we don’t have a record from the first 500 million years or so, but also it’s a question of formation versus preservation. So we know we can form the continents, but we also know they can be destroyed. They can either be eroded, or they can be buried back into the mantle and lost to our geological record. So it’s a balance between creation and preservation.

Oliver Strimpel: Okay, so let’s actually talk about how continents form. Do any of the first continents survive to the present day to give us some clues as to how they formed?

Renée Tamblyn: So we do have evidence of some very, very ancient continents on the Earth, luckily for us, and as I mentioned, they’re quite different to the younger continents we see today. And this difference has spurred a massive amount of research into these earliest continents. We don’t know if they were the first ones, but we certainly hope they were some of the first ones. And these give us a lot of clues. But it’s kind of like you have an end product and you have to go back and work out how it was formed: what were the ingredients, what was the recipe to make it? And there are a few important things that we need to know. We need to know the source rock, because you can’t create a rock from nothing. There had to be some rock there before that was affected to create the continent, in this case probably melted or remelted. And we also need to know what exact process did that, which process modified it, remelted it, to make the continents that we see preserved now. And we think that either the starting composition or the process, or both, were different in the Archean to now.

Oliver Strimpel: Can we date the oldest continental crust fairly confidently?

Renée Tamblyn: Yes, we can. So as I mentioned, the oldest rock we have is 4 billion years old, but that’s a small amount of rock. The first continents we can really find on Earth seem to be from around 3.8 billion years ago. And then we have them all throughout the Archean. So we also have some nice ones from around 3.4 billion years ago, 3.2 billion years ago. So we seem to have a good picture of the continents throughout the Archean.

Oliver Strimpel: What can we learn about the processes that took place at the beginning and formed the first continents?

Renée Tamblyn: Yes. Interestingly, these Archean bits of rock only cover about 8% of the continents we have now. We call these cratons, and they’re all over the world, amazingly. But some of the oldest ones, and the ones which have had the most study, the most well preserved, are in North America, the Superior Craton. We have the very famous Kaapvaal Craton, which is in South Africa. And there are also several cratons in Australia which are particularly old, and many in India, Central Asia, China and also in South America. And they are 80% dominated by one series of rocks, one magmatic series of rocks. And we call these TTGs, which stands for tonalite–trondhjemite–granodiorites.

Oliver Strimpel: Say that a bit more slowly for me. Way too fast.

Renée Tamblyn: Tonalite, trondhjemite, granodiorite. So these tonalite–trondhjemite–granodiorites, they’re magmatic rocks. They’re igneous: they’re melted and they crystallize in the crust. And these are very high-silica rocks, very high in sodium, very high in calcium, and importantly, they’re not very dense. They’re very buoyant. This is what actually makes the continent: the fact that they’re buoyant and thick and therefore are above the ocean. So these TTGs, as we call them (we like to shorten it), seem to require sort of three things to form. First of all, we need a starting rock, as I said, something that they were created from, and in this case we know it’s a basalt. They come from the melting or remelting of basalts, or mafic rocks, as we also say. So these are rocks that are high in magnesium and iron. They also require temperature. To melt a basalt, you need to take it to fairly high temperatures, to more than 700 or 800 degrees Celsius, but intriguingly, not that high a temperature, so maybe less than 900°C. For a geologist, that’s not too hot. And lastly, they seem to require water, which is an interesting thing: free water in these rocks.

Oliver Strimpel: These cratons are mostly TTGs, but they’re not entirely TTGs, are they?

Renée Tamblyn: No, they’re not. Around 80% seem to be TTGs, and the other 20% is a series of rocks which we refer to as greenstone belts. And “greenstone” comes from the fact that these rocks tend to be green (very inventive), but they’re actually super interesting. They’re very diverse, unlike our TTGs, and they seem to be primarily made of volcanic rocks, so rocks which erupted. And these, unlike the TTGs, are not so rich in silica. They’re actually very, very mafic, so they have high amounts of magnesium and iron. And the rest of these greenstone belts, where it’s not volcanic, tend to be sedimentary. So we have a lot of sedimentary rocks also preserved in these greenstone belts.

Oliver Strimpel: So mostly these TTGs, but a certain number of these high-magnesium volcanic rocks. Those are referred to as komatiites, aren’t they?

Renée Tamblyn: Yes, true, including komatiites. That’s been my current area of work for the past four years, looking at these rocks called komatiites, which is a great name, named after a place called the Komati River in South Africa. And these are really strange rocks, because they are lavas and they’re incredibly magnesium-rich, so 20-plus weight percent magnesium, which is a lot. And interestingly, they seem to have erupted at extremely high temperatures, so maybe 1,600°C, which is incredibly toasty. And these incredibly high temperatures seem to come from the fact that komatiites were generated very deep in the mantle, actually probably very close to the core–mantle boundary. And they then ascend through the mantle as a mantle plume and erupt in the Archean to form these komatiites. And we don’t really find them post-Archean. There are some very rare examples, but most of them are in the Archean. So it seems to be something very special to the Archean. And that is because, simply, the Earth was a lot hotter back then.

Oliver Strimpel: Does the disappearance of komatiites and TTGs, I guess, does that happen at a particular time in Earth history, then?

Renée Tamblyn: Yes, absolutely. So it’s very intriguing that the TTGs seem to die out after the Archean. From 2 billion years ago are some of the last big swaths of them, and the komatiites become incredibly rare after 2.5 billion years. So they seem to have really dominated the geological record in the Archean, and then they suddenly start to really die out and become less well preserved in the geological record.

Oliver Strimpel: And do we have a theory as to why that is?

Renée Tamblyn: For the komatiites, the idea seems to be that the Earth cooled beyond some kind of tipping point. The Earth at the beginning was very warm, thanks to its accretion, the kinetic force of accretion, and also from the decay of radioactive elements, which slowly died out over time. So it seems that around 2.5 to 2 billion years ago, some sort of switch went off. Suddenly the Earth cooled to some point where we weren’t making these komatiites, these ultra-hot mantle plumes, as much anymore. Intriguingly, this also corresponds to some very significant atmospheric events, but maybe we can touch on that later.

Oliver Strimpel: So I’m trying to form a picture, then. So we have these TTGs, which were plutonic rocks. They’re part of that granitoid family, on the one hand. And then you have the volcanic rocks that, I guess, spewed out over them, the high-magnesium komatiites, and then sedimentary rocks as well, which I guess are secondary processes. And then after the end of the Archean, some new or slightly altered processes took place to continue to build the continents. Is that right?

Renée Tamblyn: I think that’s a very good way of phrasing it. Some people might say that that could actually be the emergence of plate tectonics. And when I say plate tectonics, I mean modern plate tectonics, where we have very stiff, rigid plates and very deep and very steep subduction, where the oceanic plate is thrust under the continental plate. And a lot of people align that with the end of the reign of TTGs and komatiites. I personally think the picture is a little bit more complicated. I think we have local examples of subduction and some sort of horizontal tectonics, probably in the Archean, but there does seem to be some major change.

Oliver Strimpel: As I mentioned, you’re especially interested in the role of water in the process of forming continental crust. Can you tell us about that?

Renée Tamblyn: Yes. So water is a vital ingredient in the continental crust, whether it be in the ancient Earth, in the Archean, or in the modern Earth. And the key thing is: how do we get the water to high temperatures? Because what we really need is water to lower the solidus of rocks like basalts, i.e., to make melting or remelting of these rocks easier, because that’s how we form either our TTGs in the Archean or our granites in the modern Earth. There’s been a lot of research looking into the sorts of rocks that might be able to carry this water, captured in the crystal structure, and also carry this water to higher temperatures. And by high temperatures I mean more than 700°, so the temperatures of melting of these basalts. In the modern Earth, we understand this very well in subduction zones, where we have oceanic crust which is being pushed down into the mantle, but in the Archean this is a bit more of an unknown.

Oliver Strimpel: You mentioned the solidus. So when rocks are wet, the solidus is lower, which means they melt at much lower temperatures. Is that right?

Renée Tamblyn: Precisely, yes, and also they tend to melt in larger volumes. It’s a very simple equation: the more water you put into a rock, the more it melts and the more you can extract. And of course what we’re interested in is this extraction, which is what forms these silica-rich, calcium-rich and sodium-rich rocks. So I would say the best way to make a continent is to add water.

Oliver Strimpel: So would it be fair to say that if you didn’t have the water, (a) you wouldn’t have the volume of rocks formed that we see, and (b) they would have a different composition because of the kind of partial melting that takes place?

Renée Tamblyn: Perfectly said. Yes. If we didn’t have water traveling into our crust and traveling into our mantle, we would have a very different world. We would probably have smaller continents, as you said, because there’d be less volume, and they would have a very different composition, because the melting reactions, the reactions of the different minerals, would be radically different.

Oliver Strimpel: Is the water part of the story as to why we don’t see TTGs forming later? There just wasn’t enough water to form that kind of felsic rock?

Renée Tamblyn: Oh, that’s a very interesting question. So we know that water is very important in making granites. Actually, this is only an idea that’s really come around in the past 25 years or so, or 30 years. We’re understanding how important water in the crust is, and most granites, especially the granites which we see being made today, say in volcanic arcs, actually do have a lot of water in their source region. The water comes from subduction zones, from subduction of oceanic crust. The switch between TTGs and granites is, I think, a little bit more messy than that. It could be a change in thermal regime, the Archean being hotter. It could be a source problem: the basalts forming the TTGs could have a very different composition to the basalts and ultramafic rocks which granites come from. And it could also be plate tectonics. Right now we have this, as I said, very steep, cold subduction, which seems to release water, which eventually creates a granite. Whereas in the Archean, perhaps we didn’t have that; perhaps we had more collision, or shallow, hot subduction. So there are really all these different things which come into play.

Oliver Strimpel: So if you melt the basalts and you make TTGs early on, it’s a very similar process that produces the granites of today, but they have less magnesium in them. Is that right?

Renée Tamblyn: Yeah, it’s kind of really hard to put a finger on this, because there are like three or four different reasons why that’s the case. So it’s all kind of tangled up. But yes, it’s a similar process to now. It just seems that these different ingredients, the starting composition and the processes, seem to be different. But water seems to be the key player all throughout the Earth’s history. It’s just a question of how you get the water to those temperatures.

Oliver Strimpel: What is the main difference then between TTGs and modern-day granites?

Renée Tamblyn: It simply seems to be this geochemical difference. It could be different starting compositions. You could explain this all by having different basalts in the Archean versus now, and therefore making different plutonic magmatic rocks. Or you could have different temperatures of melting, which make these different plutonic rocks. Or you could have different pressures of melting, which make different plutonic rocks. So this is also one of the reasons why our field has been tangled up for so long.

Oliver Strimpel: But the fundamental difference is that modern-day granite is very potassium-rich, so you have a lot of K-feldspars, but in TTGs it’s much more sodium and calcium feldspars instead.

Renée Tamblyn: Yes, exactly. That is the main difference.

Oliver Strimpel: Does that mean it’s got more plagioclase?

Renée Tamblyn: Precisely, yes: much more plagioclase versus K-feldspar. It’s a very simple switch. The other thing this could all be driven by is differences in fractional crystallization. So I’ve talked a lot about the partial melting processes, because that’s what I work on. But of course, once you make a magma, you can fractionally crystallize it, so you can form crystals which fall out, fall to the bottom of the magma chamber or magma system, and you can differentiate your melt that way. So that could also be one of the processes creating these differences between modern-day granites and Archean TTGs. There’s a reason it’s not solved yet.

Oliver Strimpel: Today, about 30% of the Earth’s surface is covered by continents. As I mentioned early on, I’m wondering, do we know if this kind of continental volume was formed in a burst early on in Earth history, for example when the TTGs started to form? Or did the amount of continental lithosphere accumulate steadily over geological time?

Renée Tamblyn: It is really one of the things that remains the most unknown. And as I mentioned, there is one big problem that hinders our understanding of this, and that’s the preservation bias. So does what we have preserved now on Earth, when we look back at our 8% of the rocks that are Archean, actually represent what was on the surface at that time? Or does it represent one type of rock which was able to be preserved more easily? Or is it chance? Is it luck? Because we’re always looking at this balance between creation of continental crust and destruction of continental crust. And those two things don’t have to be the same throughout time. They can change. And of course, because some of this crust has been destroyed, it makes it very hard to say something about what we’ve actually lost from the geological record. I would say that what we see in the geological record is impacted by a preservation bias. It does not represent the world at that time. So we have to really take this into account when we’re trying to understand our crustal growth models, as you say, which is the total volume of continental crust on the Earth.

Oliver Strimpel: I mentioned that you’re interested in the possible role of hydrogen in powering the earliest forms of life. First of all, can life really be fueled by hydrogen? Are we talking about molecular hydrogen? And presumably this was well before the Earth’s atmosphere became oxygenated.

Renée Tamblyn: That’s exactly right. So we’re talking about molecular hydrogen, or H2. And yes, this is well before the atmosphere was oxygenated. The Great Oxygenation Event was around 2.4 to 2.1 billion years ago, but we seem to have evidence of life well before that. So this is quite a fun thing, because these types of organisms are called Archaea, and some of them use hydrogen, some of them methane. They use all sorts of different compounds to get their energy. And this particular type, methanogens, as they’re called, use molecular hydrogen, H2, and CO2 to form methane. And this is their main energy source. And incredibly, they’re still around today. So we believe that they evolved in the Archean, but we can still find them today. They’re in our guts, they’re in our mouths, and they’re living in the Atlantic Ocean in hydrothermal vents, where they get hydrogen from rocks. So they seem to be quite prolific in the world.

But the key problem was that in the Archean, before the Great Oxygenation Event, the atmosphere was really primarily composed of nitrogen, carbon dioxide and perhaps methane or ammonia. So this meant that the first life forms had to be anaerobic. They couldn’t use oxygen to create energy. So this group of Archaea most likely evolved in the oceans or in shallow lakes in the Archean, where they were using this molecular hydrogen.

Oliver Strimpel: And how would the hydrogen be produced in these Archean oceans or lakes?

Renée Tamblyn: Yeah, it’s a fantastic question, because we come back to these wonderful komatiites I mentioned earlier, these high-magnesium lavas. So quite incredibly, the exact same reaction which traps water in these rocks, this hydration reaction, is also actually an oxidizing reaction. So during this hydration, the iron 2+ in the rock is converted to iron 3+. And because of the unique chemistry of these rocks, this in turn dissociates the water, so it traps oxygen in the rocks and releases hydrogen. And it’s quite amazing. It’s all kind of the same chemical reaction doing all of these things: hydrating the rock, oxidizing the rock and releasing the hydrogen. And because we think the oceans then also had quite a lot of CO2 in them (we see evidence for that in the sedimentary record), these Archaea would have had the CO2 and the hydrogen to combine to make methane and survive. And the intriguing thing is that this could also be a possible way for the evolution of life on other planets where there’s not any oxygen, but there might be water available, and also these oxidizing reactions taking place.

We can also go one step further, which is why I mentioned it before. You brought up, interestingly, this kind of dying off of the komatiites about 2.5 to 2 billion years ago, which actually directly correlates with the Great Oxygenation Event, this big switch in how the solid Earth and also the atmosphere seem to work. And there have been a few studies that have actually suggested that these komatiites produced so much hydrogen in the oceans that it acted as a drawdown for oxygen from the atmosphere, recombining to make H2O, water. And that actually the Earth cooling, and the komatiites starting to be erupted less and less, therefore producing less and less hydrogen, is what allowed the oxygen levels in the atmosphere to finally take off, leading to the Great Oxygenation Event.

Oliver Strimpel: What are you working on at the moment?

Renée Tamblyn: So at the moment I’m finishing up my work looking at the interactions between komatiites and the ocean and the atmosphere, to understand some of the chemical cycling that may have been taking place at that time. And I’m starting to look more towards TTG production, so creating models to try to understand exactly how these basalts may have melted, and to constrain what we were discussing before. Really, what were the greatest effects: temperature, pressure, water availability, starting composition? How did they all play a role in creating the TTGs?

Oliver Strimpel: Is the existence of the TTGs viewed as a puzzle, or do we understand enough not to be surprised if they’re there, but you’re trying to flesh out the details?

Renée Tamblyn: I would like to flesh out the details. The details at this point matter. It seems like there is a non-unique solution to creating TTGs, as I think I mentioned before. And there’s also a non-unique solution for going from these TTGs to the more potassium-rich rocks we have in the more recent geological record. So the focus of my next study is to work out what actually are the most important factors. And also, a lot of the studies have been very broad-scale studies, but I’m hoping to go more into depth, looking at very particular case studies in some of these Archean terrains, to try to wiggle out what are the most important parts of this recipe.

Oliver Strimpel: Renée Tamblyn, thank you very much.

Renée Tamblyn: Thank you, Oliver. It’s been very fun. I enjoyed it a lot.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you’ll also find a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.