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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Most of Earth's salt is dissolved in the oceans, but there is also a significant amount of solid salt among continental rocks. And because of their mechanical properties, salt formations have an outsize effect on the structure and evolution of the rocks that surround them. This gives rise to what we call salt tectonics: at first sight, a rather surprising juxtaposition of a soft, powdery substance with a word that connotes the larger-scale structure of the crust. Michael Hudec has been working on salt tectonics since 1989, when he joined the Production Research Group at Exxon. He is currently investigating salt tectonics in regions in which rifting has created salt basins, such as in Brazil, West Africa and the Gulf of Mexico, and has co-written a definitive textbook on salt tectonics. He is a research professor at the Bureau of Economic Geology at the University of Texas at Austin. Michael Hudec, welcome to Geology Bites.

Michael Hudec: It's a pleasure to be here.

Oliver Strimpel: What exactly do we mean by salt tectonics?

Michael Hudec: Well, I think you have to start by understanding what we mean by salt, because it's a somewhat slippery term. What most people mean when they say the word salt is table salt, which is sodium chloride. The mineral name for it is halite. At the Earth's surface, or in the Earth generally, a lot of the world's salt is actually halite, but there are actually a whole bunch of other minerals that are typically deposited with salt, and they have complicated names: carnallite, tachyhydrite, sylvite, anhydrite, gypsum and so forth. So salt is very often used in geology as a kind of catch-all term meaning all of those minerals, but the most important one is the mineral halite. Now, salt tectonics refers to the structures that form when this salt starts to flow, because in geology, salt is a fairly unique mineral. We think of rock as being a word meaning something hard and impossible to break. Salt actually is a rock that can flow at geologic strain rates and under geologic conditions, and so it's actually one of the weakest rocks on planet Earth. And salt tectonics refers to the whole set of processes and structures that are formed when the salt starts to flow.

Oliver Strimpel: What sort of size structures are we talking about?

Michael Hudec: Well, single salt domes are often, say, 5 to 8 kilometers tall and can be 10 or 15 kilometers across. Then you start getting larger composite salt structures that are made of many salt structures that have sort of flowed together. The largest ones on Earth are the Angola Salt Nappe, which runs for several hundred kilometers in the deep water offshore Gabon and Angola, and then the Sigsbee Escarpment in the deep-water Gulf of Mexico, which likely covers several tens of thousands of square kilometers. So these are simply enormous structures, bigger than most U.S. states.

Oliver Strimpel: Wow, that's absolutely enormous. I had no idea that we had so much salt. And where does it all come from?

Michael Hudec: Most salt in the Earth comes from evaporation of seawater. If you think about how much seawater has to evaporate to make salt... I mean, everybody knows seawater is salty if you taste it. If you take 1 kilometer of seawater and evaporate it, you end up with something like 12 meters of salt. And so to get salt deposits one or two or three kilometers thick, which many of them were, you start having to refill the basin with water many times, and evaporate all the water, and then refill it again and evaporate all the water. So it takes really just enormous quantities of water. If you had it all in one place, it would be a column of water more than 100 kilometers tall to make a lot of the salt basins on Earth. So it takes a while to deposit it, and it reflects the evaporation of just massive amounts of seawater. There are some minor salt deposits that come from evaporation of saline lakes, but by far the most important source on Earth is seawater.

Oliver Strimpel: Was the Earth endowed with its salt when it was formed?

Michael Hudec: Well, the Earth wasn't even endowed with water when it was formed, so it took a while for the rock cycle to get going, and most of the salt was originally bound up in minerals inside the Earth. And there was a process that went on for a long time called outgassing, where the early Earth had volcanoes and things spewing various gases into the air, which then formed water and oceans, and the salt was one of the substances that came from both erosion and outgassing of the original planet. So it took a while for a stable water cycle and a stable salt cycle to get going.

Oliver Strimpel: Do we see evidence of such vast amounts of salt anywhere else in the solar system?

Michael Hudec: Well, there has been salt tectonics reported on a whole variety of different planets: on Mars, on Europa, which is a moon of Jupiter, on Triton, which is a moon of Neptune. And it's all done from satellite imagery, saying, oh, that looks like it could be a salt structure. So we don't really have direct evidence, as in a sample that's salt, but there are weak things flowing around on other planets, and they are sometimes inferred to be salt. But we actually don't know the mineralogy. They could be made of borates or sulfates; they don't necessarily have to be sodium chloride, the one we're most used to. And in some cases, people think that you have salt-like tectonics going on as a result of ice, frozen water, in the subsurface. What you really need is anything that's weaker than the rocks around it, and you can get salt-like tectonics all over the solar system.

Oliver Strimpel: In my introduction, I said that salt formations have an outsize effect on the local structures. Why is that? And what kind of structures form when salt is involved?

Michael Hudec: Well, think about salt flowing in the subsurface. If it moves from point A to point B, and if there are rocks or sediments over the top of the salt, those rocks above the salt have to deform and move and go up or down or sideways when the salt is flowing underneath them. And so a salt basin looks very different from just a regular basin that does not have salt, because the flow of the salt underground, and sometimes up at the surface, has a huge effect on the rocks both above the salt and to the sides of the salt. So you get styles of structures that you just don't see anywhere else.

Oliver Strimpel: So do we generally see significant salt structures in any particular plate tectonic setting, for example plate boundaries?

Michael Hudec: Well, to get salt deposited in the first place, you need to take vast amounts of seawater and you need to evaporate the seawater. So, for example, there are no salt basins forming today in the Pacific Ocean, because even though you're evaporating lots and lots of water, that water gets replaced by water flowing in from the Indian Ocean and from the Atlantic Ocean, and from rivers coming in. So what you need is a lot of water evaporation, coupled with isolation of the basin, so that you cannot replace the water that's getting evaporated. And there are some plate tectonic settings that favor that more than others, in particular rifting, when a continent is splitting apart for the first time. So you've got Europe and Africa separating from South America and North America at the birth of the Atlantic Ocean. Well, initially there's a really long, skinny basin separating the two megacontinents from each other as they pull apart. And so that's an ideal situation for salt to be deposited, because there can be a lot of evaporation in that long, skinny basin, and it's hard to replace the water, for the water to get in down the axis of that long basin. So in that case, you had salt basins forming both offshore Brazil and offshore West Africa, and you also had salt forming in the North Atlantic as well. Nova Scotia's got salt basins, and so does Morocco; they are conjugates of each other. They were originally attached to each other. So very early in rifting is where you get salt deposition. Now, salt structures, when the salt starts to flow, actually sort of have the reverse provenance, because it's when continents collide that this salt, which may have been perfectly happy to just sit there in the subsurface for long periods of time... all of a sudden you've got two continents ramming into each other, and any salt that's there wants to get out any way it can. And so it starts moving all over the place. So many mountain belts have large contributions from salt structures in them, because the salt just wasn't happy when the continents started slamming into each other, and it started moving.

Oliver Strimpel: Do those structures sometimes make it to the surface?

Michael Hudec: They do. And in fact, there are some today in Iran. Iran's undergoing a plate tectonic collision with Arabia, the Arabian plate, and you've got salt basically popping out onto the ground surface and flowing sideways as what are sometimes called salt glaciers. You actually get salt flowing down the sides of mountains. And if you go on Google Earth, you can find them; they're very spectacular. Now, Iran's a nice place to see those because it's a desert, or large parts of Iran are desert anyway, because, as you can imagine, the more it rains, the more the salt tends to get dissolved the minute it comes up to the surface. So to get those things preserved, you need a way not to have them all dissolved, and a desert's a great place to do that.

Oliver Strimpel: So where do we see evidence of salt tectonics today?

Michael Hudec: Well, salt is active in a whole bunch of basins, many of them offshore, so you won't see them unless you have some sort of echo sounding of the seafloor, or seismic data to look underneath. But the Gulf of Mexico, both the northern and southern parts of the Gulf of Mexico, offshore U.S. and offshore Mexico, are just riddled with active salt tectonics. Salt is flowing all over the place in those basins. The same is true offshore West Africa, in Gabon and Angola. The same is true offshore Brazil. You've got active salt structures happening in the North Sea. So most active salt provinces are actually marine, and so the main people that know about them are people who look in marine areas, which could be researchers doing hydrography, but mostly oil and gas companies, who are looking for the oil and gas that is very often associated with salt structures.

Oliver Strimpel: So you mentioned the compressive environment. What happens to the salt then, if it doesn't come to the surface? Do you get domes, diapirs, forming? And is there a collapse of the material above it?

Michael Hudec: Well, when you squeeze salt, it really wants to get to the surface any kind of way it can. But if it can't get up to the surface, normally it will accumulate into large bulges, which form anticlines and dome the rocks over the top, and between the domes there are what are called synclines, the low areas where the salt flowed out of. But if there are any pre-existing salt domes that were there before the shortening started, it's kind of like holding a banana in your hand and squeezing the banana, and liquid banana comes shooting out the top because you're squeezing it shut. And so that's what happens to any pre-existing domes: they typically pinch shut, or close to shut, and the salt goes squirting out the top. So it can form a variety of different structures. It's also a very good lubricant, because it's so weak, and so if there is a salt layer someplace, rocks like to slide along salt layers. So that's another big factor, in both compressional environments, actually, and extensional ones: salt as a lubricant.

Oliver Strimpel: And how does that manifest in the local structures, when you have that sliding movement?

Michael Hudec: Well, when you have that, rocks can move tens of kilometers sideways, and so there are extensional structures in the area where the rocks are pulling away. It's sort of like the top end of a landslide. And down at the bottom, the downdip end of the landslide, there are shortening structures: thrust faults, folds and so forth. So if you take two continental margins, one of which has salt and one of which doesn't, and you crunch them both in the same plate collision, the one without salt is certainly going to be deformed, but the style is not going to be nearly as complicated as what you see when there is salt present. Salt just makes everything pretty much go nuts.

Oliver Strimpel: Has the presence of large salt structures had a significant effect on the way Earth's global plate tectonics takes place?

Michael Hudec: The presence or absence of large salt structures really has no influence at all on the overall pattern of global plate tectonics. The plates that were going to collide collide anyway. The rates don't particularly change. However, what you do get is a very significant modification in the structural styles that you see. Now, the structural styles of a plate collision zone in the absence of salt structures have been extensively studied and are really very well known. Classic examples include the Canadian Rockies and many other fold-and-thrust belts around the world. Now, when you have salt structures that are present, the whole structural pattern of the collision zone changes, because the first thing that happens in a collision zone that involves pre-existing salt structures is that the salt structures are the first things to activate. And so the pattern that results depends on the pattern of the salt structures, and you get patterns that are completely unlike the normal fold-and-thrust belts that form in margins without salt. So in the 1950s, '60s, '70s, even into the '80s, you had geologists who were unknowingly mapping collision zones that involved salt and trying to interpret them with standard fold-and-thrust-belt ideas. And they would be tying themselves into all kinds of knots, because there actually were salt structures there that were confusing everything, and it just wasn't a normal fold-and-thrust belt. And the geologists would have all kinds of problems trying to interpret a salt-involved margin that was involved in a collision as if the salt was not present. So starting in the late 1980s and into the 1990s, geologists began to understand the huge influence that salt tectonics could have on continental collision zones. There's been a pattern ever since then, one after another, of looking at collisional belts, identifying where the salt had actually been present, and reinterpreting all of these margins in a new way, with the recognition that they involved salt tectonics. So is there an overall control of salt tectonics on global plate tectonics? No, there's not. But does it change the style of what happens in plate tectonics? Yes, it does, very much. And one of the big advances in structural geology in the last 30 or 40 years has been the recognition that you really have to know whether salt structures are present or not in order to correctly interpret the margin.

Oliver Strimpel: Some of the places that you mentioned are also centers of oil production.

Michael Hudec: That's true.

Oliver Strimpel: Is there a connection between salt tectonics and the formation of hydrocarbon reserves?

Michael Hudec: There is, for a couple of reasons. One is that to get hydrocarbons, oil and gas, you need what's called a source rock. And a source rock is generally a very organic-rich shale in most cases; it doesn't have to be a shale. So you need a very organic-rich shale. Well, how do you get an organic-rich shale? Most basins have little microscopic creatures, or bigger creatures, dying all the time, and they accumulate at the bottom. But there are all sorts of organisms that eat organic matter, little bugs, both at the surface and in the subsurface. And so you need an environment in which those bugs are not active for some reason, so you preserve the organic matter better. And the kind of hypersaline, or very salt-rich, environments in which salt is deposited are also very hostile to the organisms that eat organic matter. They can't survive in hypersaline environments. So if you've got an environment that is either making salt or has recently made salt and is still very hypersaline, the organic matter that dies is very often preserved. There's nothing at the bottom to eat it, because the water is too salty. So one factor is that salt basins tend to have very good source rocks associated with the salt. And then there's the factor that all this movement of salt that I've been talking about, the flow of salt in salt tectonics, tends to make a lot of big structures, big anticlines. And many of those structures trap hydrocarbons. You know, an anticline is a big dome of sediments coming up. Well, hydrocarbons tend to be lighter than water, so they migrate up to the top of the dome, and they're trapped there. And so oil companies come and drill the top of the anticline and produce the oil and gas. So it's an environment very rich in traps for oil and gas. And then the last thing is that salt is itself a very good seal for hydrocarbons. And by a seal, we mean that if the oil or gas is trapped under the salt, it's very difficult for it to leak through the salt. It makes a good sort of trap over the top, and if for some reason you do manage to fracture it, the salt itself heals, because it recrystallizes very easily. So if you somehow manage to disrupt the seal, it heals itself. So salt basins are rich in oil and gas because there are good source rocks, because salt makes lots of traps, and because it's a very good top seal. For example, most of the oil on the Arabian platform, or at least a large part of it, which is why Saudis drive Mercedes-Benzes around, is sealed underneath salt. And in this case, it's not the mineral halite; it's actually anhydrite that is the principal top seal over a lot of the Arabian platform. It's another salt, but it still illustrates the point that salt makes a very good top seal for oil and gas.

Oliver Strimpel: In Saudi Arabia in particular, and the Middle East perhaps more generally, the oil is very close to the surface, isn't it? Is that because you're looking at the top of one of these domes?

Michael Hudec: No. The oil reservoirs in Saudi Arabia are actually much more widely distributed than in any single salt structure. And in fact, in most cases, the oil is actually not in one of those anticlines; it's in another type of trap entirely. And the main function of the salt on the Arabian platform is actually as a top seal, not in making the structures themselves.

Oliver Strimpel: To stop it sort of leaking out into the surrounding rocks.

Michael Hudec: Yes, to stop it from leaking away. Exactly.

Oliver Strimpel: Salt has been a valuable commodity for much of human history, and it has many uses even today. What are those uses, and do we get our salt by mining the salt structures we've been discussing?

Michael Hudec: Well, for many years we did. We don't generally get our salt from mining anymore. Going back into the Neolithic, the principal uses of salt were for food preservation. It was used in tanning hides, and it also had some medicinal uses as well. And so for that reason, a lot of early Neolithic settlements were located around sources of salt, and there are big archaeological sites going back many thousands of years around some of these. Some of these were salt springs, where the groundwater just had salt dissolved in it, and people took the water and heated it up in a pan and got the salt out of that. In other cases, the salt was exposed at the surface, and you had Neolithic miners going in and mining it. You can go look up the Salt Men of Iran online if you want to. A series of miners got trapped in cave-ins in some of these salt mines, and because salt is very good at preserving things, they basically got pickled inside the salt. They died and were buried by salt and were more recently extracted. And there are some museums in Iran that have these, you know, heads of Neolithic miners preserved in them. So it's sort of ghoulish, but it does illustrate very well exactly why salt was so valuable, because things are preserved very well when they're packed in salt. Up until, I'd say, 50 years ago, the most common way to get salt was by mining it, and so there were many salt mines around the world. These days, at least in the United States and Europe, most salt is made in factories by chemical reactions. It's not naturally occurring salt, although there are still parts of the world in which salt is mined. But in most of the industrialized world, it's actually made in factories.

Oliver Strimpel: What about evaporation pans in shallow seas, or even in the Dead Sea?

Michael Hudec: Yes, those still exist. And in many places where there were no naturally occurring salt basins, again, Neolithic or Middle Ages miners evaporated seawater one way or another, even in climates where you wouldn't think that was possible. I mean, England, for example, isn't a tropical environment, and you wouldn't think you could do that. In England, a "wich" was a saltworking, usually from water evaporation. So any English town with the word "wich" in it, Northwich and so forth, was originally a saltworking.

Oliver Strimpel: Salt was also used as a form of currency in antiquity, wasn't it?

Michael Hudec: It was. The rumor that Roman soldiers were sometimes paid in salt is correct. And in fact, the Roman word sal, which meant salt, was the root of our word salary.

Oliver Strimpel: Hence somebody being worth their salt.

Michael Hudec: I mean, actually, they were on occasion paid directly in salt. It was that valuable. And at other times, wars were fought over access to salt, because it was so critical for civilization, especially for food preservation. In lots of Europe, there were wars over cod, and salt to pickle the cod, or preserve the cod, fought especially in northern Europe.

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

Michael Hudec: I've been working on a couple of things. One of them is some of the salt tectonics going on in the very deep-water part of the Nile Delta, offshore Egypt. It's actually far enough out in the Mediterranean that it's offshore Cyprus; you've passed from Egyptian waters to Cypriot waters. And in that area, the salt has been flowing downhill under the weight of the Nile Delta, and it's actually ramming up against a big basement high that's out in the middle of the Mediterranean called the Eratosthenes Seamount, and it's making all kinds of really interesting structures that are visible on seismic data. And so we're working on a big 3D seismic cube to try to understand what's going on. And there have been some recent oil and gas discoveries in that area, which is why oil companies are interested in the research and are helping to support us financially to do it.

Oliver Strimpel: So the salt structures might be indicative of the presence of oil.

Michael Hudec: Not directly, but the oil and gas is certainly associated with salt structures, and you need to be able to correctly interpret the salt structures in order to find the oil and gas. Salt structures are so complicated that when many interpreters look at them for the first time on seismic data, their first response is to go, "What's that?" You know, it looks terrible. And so we try to provide techniques for understanding what's going on and being able to interpret it, so you can say, "Oh, I get it, that's what this is," which means the oil and gas would be over here. And so it's not so much that the oil and gas is directly reservoired inside the salt, or caused by the salt, but you need to be able to interpret the salt structures to find it.

Oliver Strimpel: Michael Hudec, thank you very much.

Michael Hudec: It's been a pleasure, Oliver.

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.