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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Perhaps the most common way that sedimentary rocks are created is through transport and deposition by water. In a recent podcast with Paul Hoffman on the Snowball Earth, we talked about sedimentary rocks created by the transport and deposition of sediments by ice. In this podcast, we'll talk about wind-driven, or aeolian, processes and the rocks that they produce. Mackenzie Day is an assistant professor in the Department of Earth, Planetary and Space Sciences at the University of California, Los Angeles. She's an expert in aeolian processes, both on Earth and elsewhere in the solar system. Mackenzie Day, welcome to Geology Bites.

Mackenzie Day: Thank you. Thank you for having me here.

Oliver Strimpel: How can we tell whether a sedimentary rock is aeolian?

Mackenzie Day: One of the things that we look for when we're looking at dune deposits, or ancient dune strata, is the angle of the strata. If you see a rock that's mostly sandstone, and you see some thin layers that are dipping to one side, and they're all kind of stacked up next to each other, and they usually tangentially approach a surface below, and sometimes they're cut off at the top, and you see that pattern stacked up, that's a very characteristic dune pattern. And if it's aeolian, then you'll have these really, really fine, we call them pinstripe laminae, very, very thin layers stacked up next to each other. And context is a really big indicator in trying to figure out, OK, is this a water-driven dune system, or is this a wind-driven dune system? Wind on Earth can really only move things that are about a millimeter in size. So if you're seeing indicators of wave activity or very large things moving, you know that's definitely not a wind system. But if you're seeing those nice pinstripe laminae, that's very characteristic of aeolian processes.

Oliver Strimpel: What about things like the grain spectrum of size, or the maturity of the grain, how rounded it is, or any specific composition? Is any of that distinctive?

Mackenzie Day: Yes, those are good indicators too. Wind is a kind of slow and steady process. So wind will take sand grains and it will transport them really long distances. The sand will bounce around, and as it's bouncing around, what happens to those individual sand grains is any protrusions on the sand grain will get rounded off. So windblown sand tends to be among the most spherical things you can find in nature. Oftentimes the surfaces of the sand grains will also get kind of frosted, from those impacts and from a little bit of chemical alteration at the surface. And then you mentioned grain size distribution. Because a particular wind can move sand of a particular size, oftentimes dunes have a very, very tightly unimodal distribution of sand, meaning the sand is all pretty much the same size, with just a little bit of variability.

Oliver Strimpel: Can you give us some examples of where we see aeolian rocks in the world today?

Mackenzie Day: Yes. The ones that I'm most familiar with are in the southwestern United States. I did a lot of work for my PhD looking at Jurassic aeolian sandstones, the Navajo Sandstone, the Entrada Sandstone, so in the Arizona, Utah, Colorado area. Parts of the Grand Canyon have really beautiful aeolian sandstone exposed. There are lots of national parks in the US, Arches National Park, Canyonlands National Park, that have these massive, massive accumulations of aeolian strata, and they've been exposed in these cliffs, and you can see them really nicely. And there's some interesting alteration that gives you these really pretty colors. The Wave in Arizona is a classic example of an aeolian sandstone, with some really beautiful iron-rich post-depositional alteration that gives it these yellow, brown, orange colors in this nice, beautiful pattern.

Oliver Strimpel: But presumably that was then a very large desert environment at one point.

Mackenzie Day: Exactly. The Jurassic Entrada Sandstone is one of the largest deserts in the history of Earth, ever.

Oliver Strimpel: Can we infer anything else about the environment in which an aeolian sedimentary rock was deposited?

Mackenzie Day: Yes, there are certainly lots of things that you can infer. So with one set of cross-strata, or one kind of layer where the individual laminae are all dipping and then downlapping, or approaching that underlying surface, you can get information about the ancient transport direction: which way were the dunes migrating? If you look a little bit deeper, and maybe you get a little bit lucky, you might be able to see where there are superimposed smaller bedforms. So do you have a large dune that had baby dunes migrating across it, and what was their transport direction? And then if you have both of those things, that tells you more complicated information about the wind: the wind was coming at an angle. So we can get lots of really, really precise information about the ancient wind and the ancient behavior of the dunes.

In any one layer you can get that information, but as these dunes are passing, they're leaving behind just those small basal strata from the dune, and that's what we see as cross-strata. So each layer, or each set of cross-strata, represents the passage of one dune. And as you look up section, or look to younger and younger rocks, you can see what's happening with the dune field over time. How is it developing? Is it growing? Are you getting larger and larger dunes? Is it dying out? Are you getting smaller and smaller dunes? Are they changing shape, or are they changing direction? There are lots of really interesting and complicated behaviors that you can put together when you start to look at the whole system.

Oliver Strimpel: When we try and reconstruct that environment in the geological past, then, can we do that by using analytical formulae for how dunes might develop as a function of wind speed or sediment supply? Or is that done by looking at rocks today or doing experiments today, and then trying to extrapolate back to much longer timescales?

Mackenzie Day: It's a little bit of everything. There are scientists working primarily on the rock record; there are people working on numerical simulations of dunes. So at this point we've got several very high-fidelity numerical models that will start from a random bed of sand and then develop into dune fields, and look at how those dune fields evolve and behave and change over time. And as inputs to those models, we use sediment supply and wind direction. A lot of this has come from experiments, wind tunnel experiments and experiments out in the field. There are some examples where people went and bulldozed a small area and then watched how it developed into dunes over time, or experiments in wind tunnels at smaller scales, where you can control the sediment supply and control the wind direction. And if you've got a rotating table, you can change the wind direction even in an experimental system. So yes, the short answer is that we're coming at this from all sorts of different directions, trying to understand how the wind and the sediment are interacting to make these patterns and bedforms.

Oliver Strimpel: Can you give some examples of the kinds of shapes that the dunes assumed, based upon the fact, as you just mentioned?

Mackenzie Day: Yeah. So we talked about dunes as having a stoss and a lee side. Most dune types have a very steep, roughly 30-degree slope on their downwind side, and that's where sand is accumulating, and then it avalanches down that steep downwind side. And then on the upwind side, they have a much shallower stoss, or upwind surface, of about 10 to 12 degrees. And that asymmetry is from the aerodynamic shaping of the dune, as sand is piling up and getting transported over the surface.

If we're looking at it in plan view, or sort of a bird's-eye, top-down view, dunes come in all sorts of different shapes. Barchan dunes, or half-moon-shaped dunes, are very common when you have low sediment supply. So when your dunes are fresh and new and developing, oftentimes you'll start with barchans. Those can interact with one another. They can combine into transverse or sinuous dunes that have curvy plan-view exposures, and then migrate downwind to become bigger systems. You can also have linear dunes, very, very straight, long dunes that fork and have these, we call them tuning fork or Y junctions. They're really, really straight, and then occasionally they'll bifurcate. And those are almost always found in systems where you have two wind directions. So if you've got the wind coming from different directions, it will move sediment in one direction, and then it'll move it another way. And so it creates this extensional dune that's migrating a little bit differently from your kind of classic barchan, which is migrating downwind as sand piles up the stoss and then avalanches down the lee side.

Oliver Strimpel: And is that shape, whether it's straight or whether it's moon-shaped or sinuous, is that what gets actually cemented into the geological record when it forms a rock?

Mackenzie Day: Usually what's actually in the geologic record is just the very, very bottom of the dune field. Dunes on Earth can be multiple hundreds of meters tall, but in the rock record you're only ever preserving just the bottom few percent of the height of the dune. So from that few percent, if you look at the orientation, if you're able to see multiple different cuts through the same set, you can often tell whether it was a barchan dune or a sinuous transverse or linear dune, because the dynamics of how these different dune types are forming means that they look slightly different in the rock record. It's very, very rare to preserve an entire dune. There are a couple of unusual examples, instances where a dune field was covered in lava, or where it was flooded very rapidly and then limestone was deposited on top, so you do actually see the whole dune shape. But for the most part, what's left behind is just the very, very bottom couple of meters of sediment.

Oliver Strimpel: So I guess your best tools then are really the microstructure versus the macrostructure of a dune.

Mackenzie Day: Yes. Looking at that 10-centimeter scale: what's the orientation? What's going on at the set boundaries, et cetera? Those are very powerful tools for understanding the system.

Oliver Strimpel: So you have your own wind tunnel. Can you tell us a bit about that and experiments you do there?

Mackenzie Day: We do. We do. We built a brand-new sediment transport wind tunnel at UCLA, and it's designed specifically to look at dust and sand and blow things around. There are plenty of other wind tunnels around in the area, but they're mostly run by engineers, and the engineers get very upset when you put sand and dust in their wind tunnel. It's got a big fan on one side, and it's a large Plexiglas box that pulls air through. And we use that to study how sand is bouncing across the surface; sand when it impacts other things, whether that's rock or equipment or vegetation, and how the wind-transported sand damages different materials; how it is flowing around those different materials; how bedforms start from a random bed of sand and then grow over time.

Oliver Strimpel: Do aeolian rocks also preserve fossils, just like subaqueous rocks do?

Mackenzie Day: They do. It's not as common, right? If you think about deserts on Earth now, there's a lot less macrofauna. There are a lot fewer big animals in the desert than there are in forests and swamps and things like that. So some of the best places to preserve fossils are in mud, where you can have depressions, and then those depressions can hold their shape and then be filled in. Sand, aeolian sand in particular, is usually loose and unconsolidated and doesn't hold its shape very well. So there are examples of dinosaur bones and trackways in desert systems, for example, and burrows, and beetles, and various fossils in ancient wind deposits, but they're not very common.

Oliver Strimpel: When we form a rock out of an aeolian deposit, it has to be cemented somehow. And I'm wondering if the cementation process is distinctive, or different from the subaqueous type of cementation you get when you consolidate a deposit that was formed underwater.

Mackenzie Day: Oftentimes it can be a very similar process. Cementation happens when some sort of fluid comes in and fills the pore space, and then dissolved ions crystallize new minerals out to fill up that pore space. Most of the time in aeolian systems, we're talking about sands made of mostly quartz and feldspar, and then you've got some fluid that comes through and has maybe a lot of calcium carbonate dissolved in it, and so that calcium carbonate will crystallize out as calcite and form calcite minerals that cement all the sands together. Because you're right, a loose pile of sand doesn't make a rock. You have to bury it, oftentimes; that's how you get it to a place where you can have fluid flow, and compact it over time, heat it up, potentially, and cement everything together in order to make an actual rock. That's lithification, the process of turning it into a rock.

Oliver Strimpel: In the introduction, I mentioned that you also study aeolian processes elsewhere in the solar system. Where else have we seen aeolian deposits?

Mackenzie Day: Many places, actually, at this point. We've seen dunes, or, we're using the term dunes fairly loosely here, but we've seen wind-formed bedforms on Earth, on Mars, on Venus; we see them on Titan. We've also seen them on Pluto, and on Comet 67P, Comet Churyumov-Gerasimenko, which was visited by the Rosetta mission. It took pictures of the comet's surface, and nobody was expecting there to be dunes on a comet. There's no atmosphere, really, not a constant atmosphere anyway. But if you look at those pictures of the surface, sure enough, there are very regularly spaced, beautifully patterned bedforms on the comet. So we're seeing them in many more places than people would have expected 10 or 20 years ago. And I think that really speaks to how fundamental dunes are to that surface-atmosphere interaction.

Oliver Strimpel: Let's talk about each of these a little bit. So on Mars, the gravity is only, what, about a third of that on the Earth, and the air is like 100 times thinner. So what can we tell about the processes there, and how can we study them?

Mackenzie Day: You're right. On Mars, the gravity is different. The atmosphere is much thinner. It's a different composition as well; it's mostly CO2. But we see dunes on Mars that are shockingly similar to the dunes on Earth. They have the same shapes; they're similar sizes. The grains are quite similar; they're sand-sized grains, so kind of fractions of a millimeter, 100 microns or a tenth of a millimeter, in that range. But one of the great things about dunes on Mars is that we can use the dunes, and our knowledge of dunes on Earth, to interpret what the wind is doing at the surface. If you think about Earth, we've got weather stations all over the place, and people are constantly predicting the weather. But on Mars, we don't have those. You know, we don't have weather stations on Mars. There are a couple with the rovers, and a few landers have been able to measure the winds, but Mars is a big place, and there's a whole planet of interesting dynamics going on. And so what my group has done a lot of is use the aeolian geology at the surface, dunes, wind streaks, other wind-formed features, to be able to interpret the wind on Mars, even without actually being there on the surface.

Oliver Strimpel: Can your wind tunnel help at all, or are conditions on Earth so different?

Mackenzie Day: The wind tunnel can help. My group and many others have worked on experiments that are trying to understand these processes on Mars. The gravity is different, obviously; the timescales can be a little bit different. We use analog models, or kind of scaled models, to scale everything down so that it fits in the wind tunnel. And I've done some experiments with scale models that answer questions like how the pattern of wind erosion in a crater evolves over time. And one of the things we're particularly interested in is how much wind it takes to actually get sand moving on Mars. One of the interesting results that's come out of the Curiosity mission is that we are actually seeing sand move in the wind on Mars at lower wind speeds than people had thought theoretically possible.

Oliver Strimpel: Is there anything you could do in your wind tunnel to simulate the lower gravity conditions?

Mackenzie Day: Yes. Because we can't change the gravity, and we can't change the atmospheric density, we are using low-density particles. So instead of studying sand with the density of rock, quartz and basalt and the regular sand that we would find on Earth, we're using materials that have a low density. For example, one of the experiments we've done recently is look at ping-pong balls and how ping-pong balls are transported in the wind in our wind tunnel: how much wind does it take to get a low-density particle off the ground? We're moving towards actually 3D printing our own, so that we can get very specific scaling to be appropriate for not only Mars, but Titan, Pluto, et cetera.

Oliver Strimpel: So Titan is fascinating. I guess you don't have water or water ice there, but you have methane. So what is forming the dunes on Titan?

Mackenzie Day: We don't have water on Titan. Titan's sand is almost certainly some sort of organic hydrocarbon ice. There are lakes and oceans and rivers on Titan, but they're methane, methane and ethane. And we don't know right now what the dunes are made of. And the Dragonfly mission is slated to get there in, I think, 2035. So it's in the works right now, and it's going to be this large multi-rotor helicopter that flies around Titan and can sample the sand and sample all the different materials. That mission will get us a lot more data. I'm very excited about that, as the dunes person. But at the moment we only kind of know that it's hydrocarbon ice, and that's where it stops.

Oliver Strimpel: Hydrocarbon dune, that sounds very exotic. And even more so, perhaps, you mentioned the comet. What is forming the dunes on a comet?

Mackenzie Day: So comets are mixtures of rock and ice, volatiles, so CO2, water ice, things like that. And as a comet gets close to the Sun, those volatile components sublimate, so they go from being solid ice to gas. And the gas leaves the comet, because the gravity is so low, but it flows across the surface. And apparently just that little bit of sublimation, or gas exposure across the surface, is enough to transport the dust and the smaller grains on the surface of the comet and organize them into these bedforms.

Oliver Strimpel: When you have a giant volcanic explosion, that dust goes all the way up into the stratosphere and then circles the Earth a few times, and eventually, after a few days or even weeks or months, even years if you have something as big as the end of the Cretaceous impact to deal with, can we see those deposits? And do those exhibit aeolian structures, or is that just a very different context?

Mackenzie Day: We do see those. We see those on Earth, and we think we see them on Mars: ash deposits on Mars. Mars has these massive volcanoes, and so you would expect some ash deposits on the planet. Dust is also a big issue on Mars, because everything on Mars is covered in a thin layer of dust. But with sand, gravity is really the dominant concern, physics-wise: gravity is pulling the sand back to the Earth, or back to the planet. With dust particles, because they're so small, the surface charges on dust and ash can have a significant influence on how the small particles are getting transported. So for example, it's very hard to suspend sand in the atmosphere for long periods of time, whereas ash, you're right, you're lofting it into the stratosphere, and it can transport its way across the planet. When Mount St. Helens erupted, it threw this big ash cloud up into the atmosphere, and a lot of it rained down and was deposited very close to the volcano, but a lot of it was transported all the way from Washington State out to Montana, hundreds of kilometers away. So yes, certainly that's an aeolian process, right? It's being transported in the wind. And it is possible to get bedforms in ash deposits, but they're usually very close to the source, and come from collapse and transport with not just ash but other things mixed in there.

Oliver Strimpel: So I heard that strong winds can actually transport dust from the Sahara Desert all the way across the Atlantic to Central America, even if it's not in sufficient quantity to form a natural dune. Does wind-transported dust around the world have an impact on the natural environment?

Mackenzie Day: Certainly it does. We've talked mostly about dunes so far, and dunes are composed of sand, but smaller particles are very important for the geology of the Earth and the way we live our lives as well. Dust, because it's small, can get up into the atmosphere and can actually stay there for a significant amount of time, and so dust from the Sahara makes its way all across the planet. Dust is also an important component of agricultural soil. There are a lot of nutrients that plants need in dust. And so one problem that's happened in the past, with the Dust Bowl, and what's happening a little bit in parts of the world, agriculturally speaking, at the moment, is that dust is being removed from the top layer of soil in places that are very dry and places where we have drought. The wind is coming through and stripping those fine grains away, and that's a problem for agriculture.

Oliver Strimpel: And what about the effect on the climate?

Mackenzie Day: Dust, because it's so small and because it can stay in the atmosphere for a long time, can have significant impacts on weather and on the temperature of the Earth. Small particles like dust lofted into the atmosphere reflect the Sun, so they'll actually lower the temperature of the Earth. In other places, you can have dust settle on ice at the surface, settle on glaciers or ice sheets, Greenland, Antarctica, et cetera. And when you have a layer of dust on top of that ice, because it's dark compared to your very bright, very reflective ice, that dust will actually heat up in the sun, and it will melt the ice.

Oliver Strimpel: With climate change, we're seeing shifting rainfall patterns and the spread of desert to areas that formerly had vegetation, especially in Africa. Desert sands are encroaching on cities bordering the Sahara. Do your studies help us understand this process better, and perhaps inform mitigation strategies?

Mackenzie Day: Absolutely, yes. The more we can understand about dunes, and how they move and how they evolve, and how they can be disrupted, the more we can use that from an engineering standpoint to make sure that the dunes aren't encroaching on places where we don't want them. There's a great example: the capital city of Mauritania is slowly being encroached on by this large, large dune field. And if you look at the outskirts of the city, you can see places where agricultural land is half covered by dunes, and old structures are slowly being covered by dunes. And coastal cities have this problem as well. The coasts are great places for sand sources, right? The ocean makes a lot of sand, and so dunes form along the coast and sometimes can migrate to places that we don't want them to. So as we understand their interactions with vegetation, and how they migrate, and how they change over time, that will be very important for mitigating dune problems.

Oliver Strimpel: Mackenzie Day, thank you very much.

Mackenzie Day: Thank you for having me.

Oliver Strimpel: For more about Geology Bites, as well as illustrations that support this podcast, you can go to geologybites.com.