This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. One of the most appealing aspects of geology is the way it can reveal the common origin of rocks that today are separated by thousands of miles. In a recent episode with Rob Strachan, we saw how a single, albeit extended, orogeny, the Caledonian orogeny, formed rocks now in the northern British Isles, eastern Greenland and western Norway. But the Appalachian Mountains are also part of this same story, even though they now lie across the Atlantic Ocean. These mountains stretch for over 2,000 miles, all the way from Newfoundland in Canada to central Alabama in the United States.

Cees van Staal has been studying the Appalachians for over 35 years, focusing especially on the large-scale tectonics of their formation. He is an emeritus scientist at the Geological Survey of Canada and an adjunct research professor in the Department of Earth and Environmental Sciences at the University of Waterloo in Ontario. Cees van Staal, welcome to Geology Bites.

Cees van Staal: Thanks for inviting me.

Oliver Strimpel: Since I said that the formation of the Appalachian Mountain belt is related to the Caledonian orogeny, let me quickly recap the very high-level tectonic context of that orogeny. It took place between the final breakup of the supercontinent Rodinia, about 600 million years ago, and the formation of Pangea. It could in some ways be viewed as a first stage in the assembly of the Pangean supercontinent. So around 430 to 400 million years ago, three of the large continental masses that had rifted apart from Rodinia collided and formed Laurussia. Those colliding continental masses were Laurentia, which corresponds to present-day North America, Greenland and Scotland; Baltica, which corresponds to Scandinavia; and Avalonia and Ganderia, which correspond to the various terranes preserved in Ireland, Wales and England and parts of mainland northwest Europe. These collisions comprise the main phase of the Caledonian orogeny. So, getting back to the Appalachians, were these three continental masses also responsible for forming the Appalachians to the west?

Cees van Staal: In part, because Baltica was not involved in the Appalachians, but certainly Avalonia and Ganderia were. But you forgot one thing, which is actually quite important. You forgot the Taconic-Grampian event, which is an Ordovician event that took place between 480 and 450 million years ago. And actually, that is one of the arguments that the processes that were taking place in the Caledonian orogeny and the Appalachian orogeny are related, because this arrival of a terrane outboard of Laurentia, with Laurentia on the lower plate, so Laurentia was the downgoing plate, is very similar all the way from Norway into the southern Appalachians. This event involved the obduction of ophiolites and the arrival of island arcs at the Laurentian margin. And it is after that event that the subduction changed its polarity from south-dipping (Laurentia was in an east-west orientation at that time, lying very close to the equator), and subduction started to go underneath Laurentia, from south to north. So the realization that this event was very similar was one of the linkages between the Caledonian and Appalachian orogens.

Oliver Strimpel: OK. That was the Taconic-Grampian event. When we spoke earlier, you said that you have probably seen more rocks in Ireland and Newfoundland than anyone. Was it the close relationship between the rocks on either side of the North Atlantic that initially led us to this tectonic picture?

Cees van Staal: Well, this is not a new idea. People have been commenting on the similarities between the geology of the British Isles and the Appalachians in North America for many years. Some of the people you can mention are Hank Williams in Newfoundland, at Memorial University, and John Dewey, who was a professor at Oxford. Early on, after the acceptance of plate tectonics in the 1960s, they started to realize that it was important to correlate these features, because they saw that there was probably a similar tectonic history.

Oliver Strimpel: Can you tell us just how the rocks on either side of the North Atlantic relate to each other? And do those relationships extend all the way down the entire length of the Appalachians?

Cees van Staal: The rocks are similar even on a lithological level. But yes, certainly the rocks in the northern Appalachians, so the rocks which stretch from Connecticut all the way to Newfoundland, and especially Newfoundland, because it's the closest to the British Isles, have a geology that is very similar to what you see in the British Isles, especially Ireland. The correlations are based on the tectonic setting of the rocks: whether a rock is an arc, or a rift, or a margin. So we're correlating rocks of similar tectonic setting, looking at the provenance, looking at the tectonic histories of the rocks. We're using all kinds of tools for that: paleontology, the fossils in the rocks, the ages of the rocks.

Oliver Strimpel: Let's talk a bit more about each of these types of relationship. How is the tectonic setting related across the Atlantic?

Cees van Staal: Well, for instance, we determine that a rock, let's say, is an island arc, and we use igneous geochemistry for that, and we see that these rocks are very similar to modern-day island arcs like we see in the Pacific Ocean, like in the Solomons or in Fiji, whatever. So if we see an arc association, let's say, in Ireland, and we see the same arc association in Newfoundland, and they have very similar ages, and the faunas in the associated sedimentary rocks and everything else are very similar, then the correlation is a valid way to go.

Oliver Strimpel: I understand that we see evidence of extension that took place in association with this collision. Can you talk a bit about that?

Cees van Staal: Yes. During the opening of the Iapetus Ocean, we notice that there was hyperextension, which means that the continents were rifting apart without producing a lot of magmatism and going into spreading very rapidly. During this hyperextension, eventually you bring the lithospheric mantle to the surface, onto the ocean floor. We can see that today off the coast of Spain and Portugal, and also maybe off the coast of Ireland. These rocks are preserved, and they are very similar both in the British Isles and in Newfoundland and in the rest of New England. So we see an association of ultramafic rocks and sediments that were deposited very close to the Laurentian margin. We call these ocean-continent transition rocks, and we have dated them, so we know that they formed very early on in the history of the opening of the Iapetus. We know for sure that they were not an intraoceanic terrane or whatever. They must have formed during the rifting period, and the fact that these rocks are very similar is another link between the processes that took place during the opening of the Iapetus Ocean, the ocean that existed in the early Paleozoic and was then closed. And then eventually another ocean was closed after the arrival of Avalonia; it was called the Rheic Ocean. These linkages are important for us, to correlate and see whether the processes are very similar. So this is one of the arguments we have used.

Oliver Strimpel: I also understand that we see a particular kind of mafic igneous rock called a boninite. Is that something that is correlated on either side as well?

Cees van Staal: Yes. Boninites are very rare, especially in the Appalachian-Caledonian orogen, but they are relatively common in this first terrane that arrived at the Laurentian margin. You can trace these boninites from New England all the way into Norway, so that suggests a common origin. Boninites are high in magnesium and silica and highly depleted in high-field-strength elements such as titanium, and they form under specific tectonic conditions. So the fact that these rocks are very common in this particular terrane, which we can trace all the way along the length of the orogen, is another argument that these rocks are related.

Oliver Strimpel: You mentioned provenance. So how is the provenance on either side of the Atlantic related?

Cees van Staal: We study the sediments which are associated with the rocks involved in these collisional events. We're looking at fossils, and we've known for a long time that these fossils are very similar along the length of the orogen. Rocks that formed close to Laurentia have very different faunas from the rocks that come from other continents, like Baltica or Gondwana. And we also look at things like detrital zircons. We date the detrital zircons with uranium-lead methods, and they tell us what the hinterland is, where these rocks are coming from: the rocks that were eroding, whose detritus was then transported by rivers to the continental margin and deposited in sediments. These rocks are now locked in the orogen. They have been through the mangle; they have been deformed and metamorphosed. But they still allow us to determine what the hinterland, the source rock, was. So we can separate rocks that had a Laurentian source from rocks that had a Gondwanan source, because the hinterlands are very different. They have very different basements and very different histories prior to the Appalachian-Caledonian orogeny.

Oliver Strimpel: Is it the trace element signatures that are different?

Cees van Staal: It's the ages. For instance, Gondwana is characterized by a lot of Neoproterozoic activity. The Neoproterozoic is the time period between 1 billion and about 550 million years ago. This activity is very rare or absent in North America, but it's very common in Gondwana. So if you see a lot of these Neoproterozoic zircons showing up in your sediments, that will be an argument to say, well, these rocks didn't form close to Laurentia; they come from Gondwana. So these are the kinds of things we're looking at. Another thing is, for instance, 2-billion-year-old rocks, which we call Eburnean. They are very common in Gondwana, but they are not common in Laurentia. So it's the different age groups that show up in these sediments that are typical of where they come from.

Oliver Strimpel: You also mentioned that ophiolites were obducted in the Appalachians. Can you tell us about those?

Cees van Staal: Yes. Some of the best ophiolites preserved in the world occur in the Canadian Appalachians, especially in Newfoundland, and to a lesser extent in Quebec. These ophiolites were very important in the acceptance and development of plate tectonic models for the origin of the Appalachian-Caledonian orogen, which is one of the longest-studied orogens in the world.

Oliver Strimpel: Are these ophiolites in good condition, then? Can we actually see the structures associated with them and say something about the processes that took place?

Cees van Staal: Yes, some of the ophiolites preserved in the western part of Newfoundland are extremely well preserved. One of them is the Bay of Islands Ophiolite, very famous. It's very big, and it lies on top of the Paleozoic Laurentian margin. So the whole process associated with the formation of the ophiolite and its emplacement onto the margin is relatively well preserved, and all kinds of things that are very important for understanding how these ophiolites formed and how they were emplaced are in part preserved in these ophiolites. For instance, the metamorphic soles: a sole of rocks that was glued to the base of the ophiolite is well preserved. These soles basically demonstrate that the ophiolites were emplaced while they were still very hot, so the rocks that were accreted to them at the base were metamorphosed. Hence the term thermal sole, or thermal aureole.

Oliver Strimpel: And you can see the thermal aureole and metamorphic sole in Newfoundland ophiolites?

Cees van Staal: Yes, very much so, on several of them. These soles are very well preserved and were studied early on, and they were very, very important in developing the ideas people had on how these ophiolites were emplaced.

Oliver Strimpel: Were the Appalachians originally formed as a Himalayas-scale mountain range, and then are the rocks on the surface today the exhumed mid-crustal roots of those mountains?

Cees van Staal: No, the Appalachians weren't. Only parts of the Caledonides were, in the Scandinavian Caledonides, where Baltica was underthrusting Laurentia for a very long time. In the Appalachians, only a few segments may have been like the Himalayas, like southern New England, where we have evidence that there was a long-lived, high-standing tectonic plateau, like Tibet. But in the rest of the Appalachians, in northern New England and in Newfoundland and the rest of the Canadian Appalachians, the orogen was not like the Himalayas; it was more like what you would see today in Iran, along strike from the Himalayas. These rocks were much lower grade. We see volcanic and sedimentary rocks of relatively low metamorphic grade preserved, and the reason for this variation may be the obliquity of the convergence. It's more oblique to the north, and there was underthrusting in southern New England, first by Avalonia and then by Africa during the closure of the Rheic Ocean. That underthrusting was very long-lived in southern New England, so it kept pushing up the overlying crust to high levels and kept it in place for quite a long time, about 70 million years.

Oliver Strimpel: So this was Avalonia and Gondwana underthrusting, similar to the way the Indian plate is now underthrusting Tibet.

Cees van Staal: Exactly. The Indian plate has been underthrusting Tibet and Eurasia, because Tibet forms part of Eurasia, for about 50 million years. India is a thick continental plate, very buoyant, and it has been pushed underneath Eurasia, so you get double crust, relatively light crust. It's the same process as if you have a plank of wood and you put another plank of wood on it: the amount of wood that comes out of the water will be higher. That is basically the principle of isostasy, and that happened in Tibet because you stuffed something of low density underneath Eurasia.

Oliver Strimpel: The highest point along the Appalachians today is just over 2,000 meters high. Is the present-day topography what's left of the topography that was generated by the orogenies that formed the Appalachians?

Cees van Staal: Generally no, because after the Appalachian orogeny was finished, there was about 200 million years of denudation until the opening of the Atlantic Ocean, so the orogen must have been eroded back to sea level. So the present-day topography is largely due to processes first associated with the rifting and opening of the Atlantic Ocean. You get rift shoulders, and magmatism, the Central Atlantic Magmatic Province, basically pushing up part of the overlying crust, so forming topography. And then, subsequently, in recent history, in the last 60,000 or 70,000 years, until about 10,000 years ago, we had ice ages, and the ice was basically loading the underlying crust. When the ice disappeared, the crust started to rebound, producing topography, and erosion hasn't had enough time to completely erode this orogen back to sea level. That's why we see all the varied topography in the Appalachian orogen.

Oliver Strimpel: So do the hills of the present-day Appalachian Mountains actually correspond to the location of the original accretion and orogenies that took place? And if so, is that to do with structural inheritance, in which later stresses are accommodated by movement along former faults in orogens, where the rocks are actually weaker?

Cees van Staal: Reactivation of faults during subsequent rifting? Most likely, yes. But the hills have no relationship to where those faults are, so the topography is to a large extent not related to where these reactivated faults are. But yeah, reactivation of the faults during the opening of the Atlantic most likely occurred.

Oliver Strimpel: But not exactly where the Appalachians are today? Like along the seaboard, for example, but not in the hinterland?

Cees van Staal: Of course, the main rifting event took place along the eastern seaboard, but also, to some extent, further back into the hinterland of the orogen. The amount of motion on these faults is not very well defined, and the ones on the eastern seaboard are largely buried beneath sea level, so we don't really know exactly what happened to these faults. But it's not a coincidence that new oceans form along existing zones of weakness. It's a well-known relationship; we've seen it in many other orogens as well. Because these rocks are relatively weak, they have major faults that may continue all the way through the lithosphere, and so if you start rifting, those zones of weakness are going to be reused. Nature is like that. It always takes the easiest way out.

Oliver Strimpel: But that's not directly responsible for the Appalachian Mountains that we have today, then?

Cees van Staal: No. But yeah, I mean, we can see the rifting. For instance, if you go to the westernmost boundary of the Appalachians, you can see a huge topographic difference between the North Shore of Quebec and Labrador and the Appalachians, and that may have something to do with the rifting episode that opened up the Atlantic Ocean. In parts of Arctic Canada, for instance on Baffin Island, they have relatively high-standing mountains, which are probably related to magmatism associated with the opening of the Atlantic Ocean.

Oliver Strimpel: So the structures we see today, then, are consequent on much later events, and some of those later events took place and had an effect right there because of the adjacent rifting that took place, which was indeed affected by inheritance. As I mentioned, the Appalachians are over 2,000 miles long. Are there any analogs of such a long collisional mountain belt anywhere else on Earth?

Cees van Staal: Well, the one that comes to my mind is the Central Asian Orogenic Belt in eastern Asia, which runs from Siberia and Kazakhstan through Mongolia and northern China all the way to the Pacific Ocean. I mean, it's an extremely long mountain belt, and also extremely wide, and probably the longest accretionary orogen, in the sense that it's an orogen where many pieces coming from the south were accreted to the Siberian Craton, forming a long-lived orogen. That mountain belt is much bigger than the Appalachians and the Caledonides together.

Oliver Strimpel: You're still working on the Appalachians. In fact, you're setting off for a Newfoundland field trip later today. What questions are you tackling now?

Cees van Staal: We know, or at least we think we know, the general tectonic framework, what happened, but we want to know the details of the processes: how these rocks were exhumed and then started eroding, because as soon as they come to the surface, they will start eroding. We want to check that by looking at the syntectonic sedimentary basins that formed during those processes. We want to see the unroofing history, and that will tell us about the processes that were responsible for the burial and then the subsequent exhumation of these rocks. So we're looking at the sedimentary basins, we're sampling rocks through the stratigraphic record, and we're dating the minerals that can be dated by uranium-lead methods, minerals such as rutile, titanite, monazite and apatite. Each of these minerals has a closure temperature and formed at specific metamorphic conditions, and so we can try to reconstruct the processes that were associated with that exhumation. That's actually a very exciting and very promising new line of research. Hopefully we will be able to present these data in the near future.

Oliver Strimpel: So this is generating what people refer to as the time-pressure-temperature kind of loops in that temperature-pressure space.

Cees van Staal: Yes, exactly. We can determine approximately when the rocks were buried to their maximum depth, if we are lucky, and their maximum temperature. But then what we want to know is how these rocks got back. That's basically part of the PTt loops, as we say: pressure, temperature, time. And that tells us something about the processes that were associated with that, whether the rocks were heated up during this process or remained refrigerated. The results that we have so far are very interesting. I'm very excited about them.

Oliver Strimpel: You've spent almost all of your long research career on the Appalachians. What made you interested in them initially?

Cees van Staal: That's serendipity. I came to Canada in 1980 to do my PhD, and I did my PhD on Appalachian rocks. I got very interested in those rocks, and then I was hired by the Geological Survey of Canada soon after finishing my PhD. In their wisdom, the Geological Survey gave me the task of basically figuring out the Appalachian orogen. They gave me a lot of freedom, and I used that. I was basically driven by curiosity about how these various rocks correlate. It's like in any terrain: the relationships between rocks are the most important thing, and they may only be preserved in some places. So if rocks in Quebec give you very good relationships, then they may explain relationships in Newfoundland as well; those kinds of arguments. And that's why I kept working on it. It's a fascinating orogen. It's one of the longest-studied orogens, but there are still lots of questions to be answered.

Oliver Strimpel: Such as?

Cees van Staal: The most important outstanding tectonic questions concern the origin of the various terranes that became isolated in the Iapetus and Rheic oceans, terranes like Ganderia, Avalonia and Meguma, which were sequentially accreted to Laurentia. The evidence for these accretions is preserved in the Salinic, Acadian and Neoacadian orogenies, which occurred between 430 and 360 million years ago. These questions concern where these terranes formed, why they departed and became isolated from their source continent, how they traveled across the oceans towards Laurentia, and exactly what happened during their odysseys.

Oliver Strimpel: And what kind of evidence might help us answer these questions?

Cees van Staal: Provenance studies, paleomagnetism and tectonic reconstructions. This may help answer other outstanding questions, such as why subduction started, or was ongoing coevally, at several places in the Iapetus and Rheic oceans. And then, of course, all these lines of evidence will help us to understand exactly how these oceans closed, and eventually how the supercontinent Pangea formed.

Oliver Strimpel: Cees van Staal, thank you very much.

Cees van Staal: You're welcome, Oliver. It was a pleasure. Thanks.

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.