This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. When plate tectonics was first discovered, some parts of the planet quickly fell into place as textbook examples of plate tectonics in action. The best examples were the oceanic plates of the Atlantic and the Pacific, with their rigid plates forming at mid-ocean spreading ridges and being consumed at subduction zones. But other parts of the world were not so easily understood, notably the Mediterranean.

Laurent Jolivet is a professor at the Institute of Earth Sciences at the Sorbonne University. He's interested in the processes that drive the motion of the plates, as well as the motion in the mantle below them, a field called geodynamics. He has unraveled the geological history of the Mediterranean, which, to put it mildly, is not quite as simple as that of the Atlantic and the Pacific. Let me suggest up front that for this podcast in particular, it would be helpful to look at the illustrations on the Geology Bites website. Laurent Jolivet, welcome to Geology Bites.

In the introduction, I said that the Mediterranean is not a normal ocean. But before we talk about the Mediterranean, what do we mean when we talk about normal ocean?

Laurent Jolivet: The best example would be the Atlantic Ocean, opened between Eurasia on the eastern side and North America on the western side, or Africa versus South America. So it opened starting with a rift within a continent. Continental crust is thinned, and then it's replaced by oceanic crust. And from there on, the two continents move away from each other, and a large ocean is progressively formed during 100 or 200 million years. That's the case of the Atlantic Ocean. So the bottom of the ocean is everywhere oceanic crust, made of mainly basic rocks like basalts or gabbros. The Mediterranean is very different from that. It used to be a very large ocean, and it had a compressional history that modified it quite significantly.

Oliver Strimpel: So overall, the crust of, say, the Atlantic is extending, taking up more space on the surface of the Earth. But as you alluded to, the overall context of the Mediterranean tells us right away that something quite different must be happening. OK. And that's because we know that Africa is moving northwards and colliding with Europe, which should in fact be reducing the space available between the two. So there's a kind of paradox here.

Laurent Jolivet: There is a paradox if you think of the Mediterranean as a normal ocean. But actually, since the Late Cretaceous, say around 84 million years ago, the Mediterranean has been shrinking, because Africa has been converging with respect to Eurasia, moving northward with respect to Europe. And this convergence is accommodated within several subduction zones, where the African lithosphere is diving into the mantle underneath Europe. And these subduction zones, they have a special behavior. Actually, not that special, because it's quite frequent at the surface of the Earth: the fact that the subduction zone itself, the trench, if you like, exactly the place where the subducting lithosphere is disappearing into the mantle, is retreating. What I mean by retreating is that the trench is moving away from the continent under which the lithosphere is subducting. And this causes extension within a very wide domain that is shrinking. So Africa gets closer to Europe, and in the middle, bits of Europe are moving away from Europe by this process that we call slab retreat. So the slab is the lithosphere, the rigid plate that dives into the mantle.

Oliver Strimpel: And by retreat, you mean that the place where the slab subducts, the hinge, is moving backwards. What makes a slab retreat?

Laurent Jolivet: So the idea is quite simple. Let's imagine that you have in your hand a piece of paper, and you hold it with your right hand, and then you fold the piece of paper with your left hand and you pull it downward. And you will see the hinge of that sheet of paper moving closer to your right hand. And this is exactly what happens there in the Mediterranean. The hinge is the trench. Most trenches are the deepest parts of the Earth underneath the ocean. This is the trench; this is the hinge of that big fold. And the subducting lithosphere sinks into the mantle under its own weight, just like the sheet of paper that you are pulling downward. So the hinge retreats with respect to the upper plate, and this leads to extension in the upper plate.

Oliver Strimpel: Oh, I see. So as the hinge moves back, it creates more space in front of the hinge, which creates an extensional environment just in front of the retreating slab.

Laurent Jolivet: Yes, exactly.

Oliver Strimpel: So as that slab sinks down into the mantle, does it have an effect on the mantle? Presumably the mantle has to make way for the slab.

Laurent Jolivet: Yes, of course. This is a very important part of the subduction dynamics, because the slab is diving into the mantle, sinking into the mantle under its own weight. And you can even imagine subduction without convergence of the big plates at the boundary of your system. So there is this component of subduction which is due only to the weight of the slab, and there is another component of subduction which is due to convergence. And the total amount of subduction is convergence plus slab retreat. So the mantle underneath, of course, will be strongly affected by the sinking of the slab and by slab retreat. Imagine that you have your hand in the water and you move it toward you. The water above your hand or below your hand will, of course, follow the displacement of your hand. And this is exactly what happens in the mantle. And we can see this flow in the mantle using the tools of seismology, and especially what is called seismic anisotropy, which shows the orientation of minerals in the mantle, and this orientation depends upon this flow. So you see that the flow is parallel to the direction of retreat that you reconstruct from geological evidence.

Oliver Strimpel: Just to explain that seismic anisotropy refers to seismic waves, such as those that are caused by earthquakes, whose speed varies depending on what direction they're traveling in. We expect this to happen when crystals in the mantle are lined up as a result of motion, such as the motion induced by a sinking slab. So we see the mantle flow moving with this slab, in front of and behind the slab. What happens at the edges of the slab?

Laurent Jolivet: That's a good question, exactly. As when you move your hand in the water, the water just above or below your hand will follow exactly the motion of your hand. But we are in an incompressible fluid, so as soon as you move a piece of the mantle somewhere, you will create a movement somewhere else. And in that case, the motion of your hand will push the water below toward you, and then on either side of your hand, the water will return, making some sort of cells, rotating cells with an axis of rotation that would be vertical. That's what we call the toroidal flow, at the edges of each piece of slab.

Oliver Strimpel: So we have mantle flow following the slab in the middle of the slab, and at the edges, a kind of eddy, as you might see next to a paddle moving through water, and that's called toroidal flow. Perhaps we should mention that it's moving much, much slower, of the order of centimeters a year, though that certainly adds up over millions of years.

Laurent Jolivet: Exactly.

Oliver Strimpel: Does this flow exert an influence on the overriding lithosphere?

Laurent Jolivet: In fact, we have shown recently that the overriding lithosphere and this mantle flow are strongly coupled. We know from the analysis of seismic anisotropy, which gives you the flow in the mantle, and the direction of extension that has been recorded by rocks in the crust, so the direction of extension that you can see when you walk in the field, for instance, that these directions are very often parallel. And numerical models also show that this coupling is efficient, and so this flow of the mantle underneath the crust is one very efficient driver of the deformation in the crust. So the crust is extending, and it is also very strongly affected by the mantle flowing underneath. It is sheared, actually, by the mantle flowing underneath, and this explains a lot of the features we see in the extending crust in the Aegean or the Tyrrhenian Sea, for instance, in the Mediterranean region.

Oliver Strimpel: Is there a specific example where we can see the effect on the lithosphere of the toroidal mantle flow going around the edge of a sinking slab?

Laurent Jolivet: There is one very good example. That is the eastern Mediterranean. If you look at the GPS velocity field, which is the active motion recorded by space geodesy, so the GPS satellite constellation, you see that the main body of Anatolia (Anatolia is the plate that covers most of Turkey) is rotating counterclockwise about a rotation pole that would be somewhere in the eastern Mediterranean, say north of Sinai. And this system is rotating. Further to the east, Arabia is moving toward the north and the northwest. And so there is rotating Anatolia, and if you go further to the west and to the southwest, you have then the motion of the Aegean Sea and of Crete toward the southwest. And this draws an overall very large toroidal flow, with a rotation about a pole in the eastern Mediterranean.

Oliver Strimpel: Hmm, geometry seems quite complicated. I was sort of naively visualizing a wide plate subducting along a trench running roughly parallel to the northern coast of Africa. Had there in fact been such a large plate, and did it then break up?

Laurent Jolivet: Yes. Actually, it all starts with the initial configuration of Africa when it collided with Eurasia. In the Jurassic, there was a sort of promontory sticking out of Africa toward the north, that we now call Apulia or Adria, sometimes presented as a microcontinent, and this block collided first with Europe, way before the main body of Africa. So we had at that time this promontory of Africa colliding with Europe, and this started to form the Alps and the Carpathians. And then on either side, we had the oceanic subduction of the Tethys Ocean, to the east and to the west. So already the subduction zones were divided into two parts, the eastern subduction zone and the western subduction zone. But it was not yet the Mediterranean; it was still open, open toward the east and open toward the west. And then some 35, 30 million years ago, we had this collision in the east, at the longitude of Iran, and then the Zagros range started to form, and later, further to the north, the Caucasus. So the ocean was somehow locked in the east, and the same thing happened in the west, at the longitude of Morocco and Spain. So we had these two oceanic spaces subducting underneath Europe, separated by this continental promontory that we call Apulia.

Oliver Strimpel: OK. To summarize, in the Jurassic, about 150 million years ago, a former promontory of Africa called Apulia collided with Europe. The collision of two continental plates creates mountains, and in this case, the collision started to form the Alps and the Carpathians. Meanwhile, on either side of this collision, the subduction of the Tethys Ocean under Europe continued toward the north. This carried on until about 30 million years ago, when two more collisions took place, one to the east, around present-day Iran, and one in the west, around Morocco and Spain.

Laurent Jolivet: Yes, exactly. And this had very dramatic consequences. Because of the collision in the east and the west, the northward motion of Africa was considerably slowed, and this made the subduction regime change completely, because the subduction could not swallow Africa at the same velocity, because Africa didn't move northward at the same velocity as before. Subduction started to retreat on the eastern side and on the western side as well. So if you return to my comparison with a sheet of paper, if you don't have this convergence, the only way for the oceanic lithosphere to sink into the mantle at the same velocity (because the engine is there; it's the weight of the sinking lithosphere) is this slab retreat, which increases the rate of subduction despite the slower convergence. And this, 30, 35 million years, it's really the birth date of the Mediterranean, when this peculiar way of subduction started to proceed. And during that process, the subducting slab was torn into several parts, and these slabs got very narrow, sometimes, like below the Alboran, below Gibraltar, toward the west. The narrower the slab, the faster the retreat. You can imagine that it's easier to move your hand in the water than to move a very large board of wood, for instance, because of the resistance of the fluid.

Oliver Strimpel: So there are several different processes that you mentioned: there is slab retreat, there's mantle flow, and slab breakup or tearing. How do we know these things are actually happening?

Laurent Jolivet: So we know they are happening because of, well, three main reasons. The first one is the techniques we have to unravel the geometry of the slabs at depth, and this comes from seismic tomography. Seismic tomography is somehow analogous to echography, when we want to see what's inside the human body, except instead of using acoustic waves, we use seismic waves. We have 3D models of what's going on at depth, so the geometry of slabs: where they are straight, where they are curved, where they are detached, torn and so on. And the geometry can be quite complex. Then we have the geological record, and the geological record shows very clearly that the arcs, like the Hellenic Arc or the Calabrian Arc and so on, are moving away from Eurasia, and we know the rates of this motion. They can be reconstructed quite precisely, independently of any other data set. And finally, we have numerical models, and especially now 3D numerical models with rather high resolution, where you can put in all the materials you need, the lithosphere, the mantle, the asthenosphere and so on, and you can calculate the displacement and the forces and the thermal evolution. And you see that this process is feasible, first, and inevitable, as it correlates very well with what you see in nature.

Oliver Strimpel: You mentioned arcs. An arc is a curved chain of volcanic activity that we usually see on the plate that overrides a subduction zone, typically a few hundred kilometers from the trench and parallel to it. And if a slab is retreating, the active parts of the arc will move in step with the retreating slab. So to take the Hellenic Arc, for example, which includes dormant volcanoes as well as active ones like Santorini, it's created by the Hellenic subduction zone to its south, which is just north of Libya, which is subducting under the Aegean Sea. And as it retreats southwards towards Africa, the volcanic activity of the Hellenic Arc moves south with it. And that is in fact what we can see in the geological record.

Laurent Jolivet: Yes, exactly.

Oliver Strimpel: OK. So I'd like to ask about some of the geological events that many of us know about. Let's start with the strong earthquakes in the Apennines, such as the lethal one in L'Aquila in 2009.

Laurent Jolivet: The earthquakes in the Mediterranean, they are related to several different processes. Some of the earthquakes, they are related directly to subduction. Below Crete, or below the Adriatic Sea, or along the northern coast of Africa, you have plenty of earthquakes that are caused by convergence and compression. But you also have extensional earthquakes in the back-arc region. In the Apennines, yes, we have destructive earthquakes every 10 years or so, and these earthquakes happen within the Apennines, which is the backbone range of peninsular Italy. And these earthquakes are simply the consequence of slab retreat: the mountain belt that was formed in earlier times is presently collapsing. And if you look at the map of the earthquakes in that region, you will see a concentration of earthquakes, shallow earthquakes (when I say shallow, it's shallower than 10, 15 kilometers), all along the backbone of Italy. And these earthquakes are all extensional, and they're related to normal faults, extensional faults that move rather continuously, and from time to time you have a magnitude 6.5, 7 earthquake. And as these earthquakes are quite shallow, they can be rather destructive, as was the case in L'Aquila, as you mentioned. Yes, they are really a consequence of slab retreat, those quakes.

Oliver Strimpel: So as you mentioned earlier, because the lithospheric slab is retreating as it subducts, the overriding plate in front of it has to fill the space being created by the retreat, which causes it to stretch, and hence the extensional nature of the earthquakes you mentioned. I want to ask about another geological event that many of us know about, which is the explosive eruption of Santorini in 1600 BC that may have destroyed the Minoan civilization.

Laurent Jolivet: It is possible that the Minoan civilization was already severely damaged by a series of earthquakes. This is not sure at all, but it seems that when the Minoan eruption, the big eruption of Santorini, happened, the Minoan civilization was already in bad shape, and there were not so many people left on the island at that time. Also, there was apparently a series of earthquakes that were signaling that something weird and dangerous was going to happen. In contrast with what we see in Pompeii in Italy, no bodies were discovered within the volcanic ash, so probably the people living on Santorini had left already. But this eruption had consequences all around the Mediterranean, in Crete, with probable tsunamis, and as far as Egypt.

Oliver Strimpel: OK, let's talk about the volcanism in the central Mediterranean, such as Mount Etna in Sicily, and Vesuvius near Naples, that destroyed Pompeii in 79 AD, and the Aeolian Islands just north of Sicily. Why do we have volcanism today just concentrated in those specific locations?

Laurent Jolivet: In general, first, the subduction-related volcanism is due to the injection of water into the mantle by the subduction zone. Let me explain. The subducting lithosphere carries sediments. They contain a lot of water, and also the upper part of the oceanic crust underneath the sediments, and the upper part of the mantle, contain a lot of water. When these rocks go down at depth in the subduction zone, reaching 100 or 130 kilometers, the pressure increases drastically, and this increase of the pressure will sort of squeeze the rocks like a sponge, and water will invade the mantle below the overriding lithosphere, and this will lower the melting temperature of the mantle rocks and produce magmas. These magmas will then accumulate below the lithosphere, and then they will go up inside the lithosphere and then quickly up to the surface. So that's the general cause of subduction-related volcanism. And because there is a lot of water in these volcanic rocks, there are a lot of fluids and a lot of pressure accumulated below the crust, and this will lead to very explosive eruptions.

Then there is the precise location of these volcanoes. So they can be just a line, like in the Andes, for instance, a line above the subduction zone, and this is quite simple. But when the slab is retreating, and when the slab is torn into several narrow pieces, the mantle flow at depth will be more complex. As we said, we have toroidal flow and so on, and we'll have sort of windows opened by these tears in the slab. The magma will come up more easily through those windows, and you will see, for instance, in the southeastern Tyrrhenian Sea, north of Sicily, narrow bands of volcanic material. Some of them are aligned, like the Aeolian Islands, like Stromboli and Vulcano. Others are not visible at the surface; they are only found on the seafloor, and they are found with geophysical exploration. And probably one of the most spectacular examples of this volcanism related to slab tearing is Mount Etna. It's probably located just above one of those tears in the slab. There is still a lot of work to do to really prove this, but yes, there is a strong impact of the tearing of the slab on the distribution and the evolution of volcanism in the area.

Oliver Strimpel: That's fascinating, that slab tears serve to produce the window that shapes the volcanism above the gap. What is the long-term future of the Mediterranean?

Laurent Jolivet: Inevitably, the Mediterranean will disappear, caught between the huge bodies of Africa and Eurasia. And so the small oceans that have been formed during the Cenozoic, these back-arc basins that we were mentioning, together with the old oceanic crust that is in the eastern Mediterranean, and even the oceanic crust that was formed in the Cretaceous in the Black Sea, all those bits of ocean will disappear and be subducted, and some of these rocks will be integrated into the future mountain belt that will be forming there. So the geologists of the future, within, I don't know, 20, 30 million years, they will have a very hard job telling the difference between these paleo-oceanic domains. They will have very different ages, from 200 million years to 30 million years only. And this makes our job at present very difficult when we interpret very old mountain belts, like the Variscan or the Caledonian, because we don't know exactly the geometry of the system before the complete collision was achieved.

Oliver Strimpel: So we hope that future geologists will be able to dig up the papers you and others are writing today in order to find out what actually happened. What are the main puzzles about the geology of the Mediterranean that remain?

Laurent Jolivet: Well, a few questions. Why did subduction start in the first place? That's a big question. We know that somehow the motion, or relative motion, of Africa and Eurasia changed some 84 million years ago, but it's a question of the chicken and the egg. Did the motion of Africa change because subduction initiated in the Tethys Ocean, or is it the other way around: was subduction initiated because the motion of Africa with respect to Eurasia changed? So we don't really know why subduction initiated, in general. This is a very important question, not only in the Mediterranean but all over the world.

And then there are several other questions, maybe smaller scale but quite as important in terms of our society, like the concentration of mineral resources. We know pretty well now that the concentrations of gold, of iron, of copper, and all these minerals that are very useful for our economic development, are intimately related to the large-scale geodynamic evolution. This has been known for quite a long time, right after the advent of plate tectonics. But with the tools we have now, we can reconstruct very precisely the evolution of continental margins and plate boundaries through time. We can model them numerically, and can play with all the parameters, the velocities, the temperature, et cetera, the nature of rocks, and can then study the processes that lead to the concentration of mineral resources, but also the concentration of heat. Geothermal energy is one very nice lead toward greener energy in the future. And if we understand better why heat is concentrated in that place or this place, it will be much easier to prospect for geothermal fields. And the same for mineral resources: the better we know the processes that lead to the concentration, the easier it is to find new resources.

Oliver Strimpel: For my last question, I'd like to ask you about your current research.

Laurent Jolivet: There is one very exciting question, which is the vertical motion of the floor of the Mediterranean Sea through time, and not only the Mediterranean Sea but all the countries that are around. Venice, for instance, and the periodic flooding of Piazza San Marco with seawater: this is due to external processes, climate and tides and so on, but it's also due to the fact that Venice is slowly subsiding, and it is subsiding because it is within a crust that is loaded by the Alps, actually, by the weight of the Alps. So one important question is to disentangle the effect of sea level rise from the part due to internal dynamics, the mantle dynamics I was talking about. So with this complex geodynamic context of the Mediterranean (but it's true for other places in the world, just concentrated in a small area here in the Mediterranean), geodynamics leads to subsidence, geodynamics leads to uplift, and you can have uplift in one place and subsidence 200 kilometers from there. And if you are not able to deconvolve the signal and say which part is due to internal dynamics and which part is due to externally driven processes like climate, you cannot understand the system, and you cannot predict anything.

It's a very different topic, but we are also working with archaeologists on the island of Delos, where we have this huge sanctuary, the largest archaeological site of Greece, and on this tiny island, hundreds of temples. And these temples are made of marble, and there are very few marbles on the island, which is made mostly of granite and gneiss. So the idea is to find where these marbles come from, in order to reconstruct the commercial system at that time and where the rocks were coming from. This is also quite exciting, at the interface between geology and archaeology.

Oliver Strimpel: Laurent Jolivet, thank you very much.

Laurent Jolivet: Well, thank you very much for inviting me.

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