The Science of Everything Podcast - Episode 128: Wind, Deserts, and Coasts

Episode Date: June 6, 2022

A discussion of the effect of wind on landscapes, focusing on deserts and coastlines. I outline the processes of eolian erosion, transportation, and deposition occurring in deserts, with particular focus on the formation and types of sand dunes. I also discuss other desert landforms, such as the desert pavement, mesas, and alluvial fans. I conclude with a look at coastal processes, including cliff erosion, emergent and submergent coastlines, and beach erosion processes. If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

you're listening to The Science of Everything podcast episode 128, Wind, Deserts and Coasts. I'm your host, James Fodor. Now, this is our second in our series of episodes looking at geomorphology and landscape features, and in this episode we're going to focus on what's called aeolian erosion or eolian processes. So these are processes that are dominated by wind activity,

and because of their dry climate, deserts are one of the major aeolian dominated environments. We're also going to talk about coastlines, because although you might think of those as dominated by the action of water, in fact, wave action is preeminent in shaping many coastlines, and waves are driven by the wind as well. So that's why we're talking about coastlines here too. There's no real recommended pre-listing for this episode, although it makes sense in the context of the previous episode on weathering erosion and rivers, so episode 127, for a little bit of background as to where we're going, but this episode should mostly stand on, stand by itself. And what we're going to look at

is the processes of erosion, transportation, and deposition in the context of deserts, and talk a bit about different desert landscape features and morphology. And then we're going to do basically the same thing for coasts, focusing on coastal landforms, cliff erosion and processes that shape beaches. So just to set the scene before we jump right into that, remember that in the previous episode we talked about the weathering and erosion processes that shaped environments in which rainfall was dominant and particularly rivers and streams. And the key processes that are important to discover when we're talking about how landscapes develop over time are erosion, transportation, and deposition. So we talked about those last time and we're going to cover those

again. Remember that erosion refers to the breaking down of material, particularly rocks, into smaller pieces. Transportation is the movement of those smaller pieces or sediments from one place to another, and deposition is the depositing of those smaller pieces into a new environment. And so the overall process, a set of processes that's taking place as our planet is sort of, at the surface of our planet, is shaped over time, is one of eroding some parts, transporting the materials elsewhere and depositing them in other parts. So basically everything's trying to be sort of worn down and homogenized by the processes of, you know, wind and rain and the energy from the sun and so forth. Of course, at the same time, the continents are moving around and there's new, there's new

land being thrusted up from geological processes and so forth. So that's constantly providing a new source of, well, energy and material for these processes to act upon. So the surface of the earth at any one time is given by the dynamical interaction of these processes. The geological processes are pushing up and moving around the tectonic plates and volcanoes and other materials. And then the processes on the surface of the planet dominated by heat from, ultimately heat from the sun, which leads to erosion, transportation, and deposition of materials. So in the last episode, we looked at how those are manifested in the case of streams and rivers in particular. And in this episode, we're going to look at how those processes are manifested

in mostly dry environments or at least environments where wind is dominant instead of the effects of rain and water. And so as I mentioned, an eolian process is one in which wind activity is dominant, and so an oelian environment is one in which those wind processes are dominant. Now typically this is, these occurs mostly in desert environments. And the reason for this is sort of unsurprising because deserts don't have much rainfall. They do have some, right, but not very much. And so they're quite dry. So there's not a lot of moisture. There's not a lot of water available for those processes to be dominant, at least in most cases. Obviously, you do have flash floods in deserts, but generally it's wind processes that are dominant. As I mentioned,

we're also going to be talking about coastal processes because coastal processes are often dominated by wave action, which in turn is also caused by wind. So the first thing that we're going to talk about is erosion in aeolian environments. And so this is about how material is broken down into smaller pieces by the force of wind or by the action of wind. And there are three main ways that this happens. Deflation, abrasion and attrition. So I'm going to look at each of those in turn.

So first of all, deflation. So deflation refers to the lifting and removal of loose material from the surface by wind turbulence. So generally this is sort of similar to how material is material is eroded by water in that it occurs by sort of pushing the particles along. So you may recall in the last episode we talked about saltation. That's where a particle like a grain of sand or something is lifted up and sort of hops a short distance and then lands downstream in the case of a stream or downwind in the case of wind.

Even smaller particles can be suspended in the wind for a longer periods of time where they remain aloft through turbulence and updrafts and so forth. or they can be sort of moved gradually along, sort of pushed along the surface. That's like pebbles or larger grains that are sort of pushed along the surface. So any of those mechanisms are similar. It's just in terms of how large are the particles that are moved by that process. And so deflation is one of the main mechanisms by which the looser material, like not necessarily completely loose, but not sedimented materials.

So these sort of looser materials like partially compacted sand, for example, or silts, the process by which these are moved and are broken down into smaller parts and then moved over time is called deflation. And deflation can occur very rapidly in some context, for example, if there is a loss of vegetation cover, which then removes the effective roots, possibly overlying branches or other vegetation that keeps the sediment down. Removal of that can lead to very rapid deflation and just effectively the remaining loose material is just sort of blown away. So in addition to deflation, there's also abrasion.

Abration is a process by which wind-driven grains knock or wear at material. Essentially, it's a scouring action of the wind. So you think of going onto a sandy beach or something when there's a high wind, and you can feel the grains of sand in the wind sort of hitting against you and abrasion against your skin. That's what abrasion is. So it produces a polishing and a pitting and grooving shapes. It tends to smooth out and even polish exposed surfaces in the desert.

So it tends not to move large quantities of sediment, but it does have a strong effect on the surfaces exposed rocks or columns or anything in the desert or like metal as well. The third form of erosion that's dominant in aeolian environments is called attrition. Attrition is the wearing down of particles caused by collisions when those particles are stuck in a moving fluid or entrained in a moving fluid. So basically these are, this is the wearing down of particles that are suspended in the wind itself or, you know, in the air itself. So what attrition tends to do is round down and smooth out the sand grains and they give them a very distinctive sort of frosted kind of polished appearance, which you don't get from like, or don't typically get from sand grains

in the beach or elsewhere. It happens only really in environments in which there's a lot of wind and not a of moisture. So the difference between abrasion and attrition is that abrasion is when basically wind-driven grains knock against like stationary materials, whereas attrition is the grounding down and smoothing of particles in the air itself or held by the wind itself. So that's like collisions between the particles in the wind, as opposed to the wind-borne particles hitting other things. But, you know, they're similar in various ways. And so all of these processes, deflation, abrasion and attrition result in the breakdown and the removal and smoothing surfaces in desert environments. Now, the combination of these processes, but particularly deflation, tends to lead to the production

of something called a desert pavement. So essentially what happens is over time, the smaller and looser grains tend to be removed, so particularly silts and clay, so the finest particles. they tend to be removed because they're the easiest for the wind to sort of take hold of and push away, right? And so over time, you tend to get a concentration of larger and larger particles like pebbles, rocks, and grains of sand. And over time, these sort of, so the finer particles are increasingly removed, and you get a concentration of these larger, like, pebbles or sand-sized grains. And they kind of fuse together, or they'll fit very finely together over time and fuse slightly, maybe through a bit of, crystallization of any minerals for the small amounts of water that do exist there, the water is evaporated and small amounts of minerals crystallized,

which helps to hold the pebbles or the sand grains together. And you get this flat surface, which is tightly fused together, mostly consisting of, as I said, like different sized rocks fit very finely together, called desert pavement. So it almost looks like it's a pavement, like it's been artificially manufactured, but it's purely through natural processes. And it's very hard and typically quite a very hard, and typically quite a flat surface, almost a form of sedimentary rock, although I don't know that

it's technically a rock, because I don't know that it's been lithified, but it's sort of close to that. It's a very densely packed collection of sediments, really. And this desert pavement is what forms the, well, the bottom or the base of many desert regions. So you might have dunes of sand, which we'll talk about later, that blow around on top of that desert pavement, or other loose sediments that sit on top of that periodically, even then are sort of blown around. But the desert pavement sort of forms a hard base on which, other things sit in the desert. Desert pavement is a very interesting phenomenon and from what I've read it's still not fully understand how it is formed although I presented one of the theories about the

sort of deflation and removal of finer grains which then leads to a concentration of the larger ones left which then kind of fused together or partly sort of fit very closely and then fused together but but there there appear to be other aspects to it as well so it's very very interesting how that works So that's a little bit about the erosion processes, and typically, as I said, in deserts, you get a removal of the finer grains, so particularly clays and silts, so the smallest particles. And what you have left are the larger particles that are moved around, but not entirely removed by the wind. The next thing we want to talk about, though, is the process of Iolaean transportation, so this is the movement over long distances of the particles after they've been eroded. So I mentioned earlier that particles can be transported by similar methods that we talked about in the case of streams. So suspension in the wind, saltation like skipping and bouncing over the surface,

or gradual creeping, like rolling and grinding along the ground for the larger pebbles and another particular matter. I also mentioned that vegetation is very effective at suppressing aeolian transport. Vegetation cover as little as maybe 15% is enough to eliminate most types of sand transport by the wind. So this is why in environments where there is more moisture, where there's even a modest amount of rainfall, sufficient to bring about even a small amount of vegetation cover, you tend not to have large amounts of aeolian transportation because there's enough vegetation to prevent it. You still have similar transportation like in streams, right, as we talked about in the previous episode, but not by the wind. However, in deserts, obviously, you typically don't have that vegetation cover or no vegetation cover or even less than 15%. And therefore, there is large scale. movement of or transportation of sediment by the wind. One of the most important and interesting, I guess, processes or examples of Eolian transportation is Loess. So Loess is a silt-sized sediment. So these are the smaller size of particles that are typically removed in deserts by the wind over time that's formed by an accumulation of wind-blown dust. So there's basically dust that's blown off from

deserts. About 10% of earth land area is covered by Loess or similar deposits. And it forms, when it's sort of, again, removed from desert environments and then transported by very large, like thousands of kilometers it can be transported in the atmosphere, and then eventually it's deposited at destination locations, and forms a sort of homogeneous pale yellow type of sediment, and it's mostly made up of quartz, feldspar, and mica. It is a very rich farm soil for a number of reasons. One of the reasons is that it resists like slumping, so it has a very steep angle of repose, so you can use it in like vertical farming, like it's done in China, where They have like terraces sort of at different heights next to each other.

So you can use it, for example, for farming on steep hills. It also has quite good drainage properties. So there are a whole bunch of reasons as to why it's particularly good for agricultural productivity. And many of the most productive regions of the world, including Yellow River Valley and China, many parts of the Ukraine, parts of the Midwest of the United States and southern parts of the United States and other areas that are particularly agriculturally productive have lowest deposits or similar deposits. and some of that is blown by desert regions like contemporarily. Other deposits of Loess have been left behind by debris from glaciers. But anyway, so that's an example of large-scale transportation of silts by Eolian processes. But again, the basic idea is simply that the smallest types of particles, so silts and clays, mostly,

are transported and removed from desert environments, leaving only the larger particle, so sands and upwards. And this is effectively the reason why many desert environments are dominated by sand. Of course, we tend to think of deserts as consisting of sand dunes, but actually only a small fraction of deserts are sandy deserts. Most deserts consist, if you sort of look at Google Images, you'll see what deserts typically look like. Most of them consist of just like a rocky desert pavement, as I mentioned before, with rocky terrain and very sparse vegetation, maybe some shrubs. some grasses and so forth. So most deserts are not, they're not consists of just rolling sand dunes. But sand is ubiquitous in deserts, and one of the reasons for that is because effectively it's the smallest particle, like the smallest grain size that's left behind after all of the aeolian erosion

and transportation. So the smaller particles are removed by the wind and the sand grains are the smallest size that's left. A related question one might have is, well, how does the sand get there in the first place? Like, why is there so much sand in desert? even if most deserts are not sandy deserts, but still, where does it all come from? And I think the best way to put this is simply that sand is everywhere. It's just mostly we don't call it sand. In a geological context, sand refers to a particular grain size, which is larger than silts, but smaller than like pebbles.

And so sand, you can see the grain size with the naked eye, but you have to sort of look very closer to see that, whereas I don't think you can see the grain size of silt with the naked eye. But the point is sand is everywhere in this. sort of geological sense, it's just that most of the time we don't think of it as sand, because it's mixed with moisture and organic material, and so we call it soil. Basically, the main difference between soil and, like, sand or silt or other, other materials, is just the organic content and the moisture content. I mean, there are some other differences as well, like the content of trace content

of minerals and other things, but predominantly it's organic content plus moisture. So in most environments that are not deserts, we talk about soils, right, because there's a significant organic component as well as enough moisture to sort of bind it together and it forms what we normally call us a soil or mud or something. Whereas the moisture and the organic content is mostly absent in desert environments. In addition, most of the smaller particles, because of the lack of moisture and vegetation to keep them there, are blown away. So what's left as sand. So that's sort of where the sand comes from. It's not that it is particularly prevalent in desert so much as it's what's left over after the smaller particles are removed and when there's no, or very little

moisture and vegetation and organic matter to keep it together. So as I said, even though only a small fraction of deserts are sandy deserts, sand dunes are still quite prevalent in many deserts, and it's useful to think about how they work and some of the different types of sand dunes as well. Also, sand dunes are one of the probably best known examples of deposition. So remember you have the erosion and transportation of material, or where does it end up? We talked about what happens with Loess, right, that it's blown very large distances and often is deposited. in environments where it contributes to the rich agricultural soil. Well, that's the case for certain silt-sized sediments,

but what about sand grains, which are larger? Well, those tend to be consolidated or moved around in sand dunes. And so this forms part of the deposition side of things. So a dune is a land form composed of sand that's driven by either wind or water. And typically here we're focused on wind-driven sand. So it typically takes a form of a ridge or hill or a mound, and an area that's dominated by such sand dunes is often called a dune system or a dune complex. Depending on a number of factors, there are a wide range of different shapes or forms that dunes can take.

And the main factors that contribute to this are how much sand is available, how strong the winds are, whether the wind typically blows in one direction or varies between a number of different directions, and also how much vegetation is present. So basically the more sand there is, the stronger the winds are, the more unidirectional the winds are, and the less vegetation there is. All of those things contribute to bigger sand dunes. But different combinations can lead to different results. I'll go through the different types of sand dunes in a moment, but I just want to talk about the sort of process by which sand dunes sort of form and move over time, because sand dunes are not static. Sand dunes are actually constantly moving, at least in normal conditions.

what happens is that the sand sort of clumps together maybe it gets caught on a small piece of vegetation or idiosyncratic dip or some feature of the ground that that leads some sand grains to clump there and then more clump on that and you get the accumulation of a mound which we call a sand dune once that happens sand dunes typically form an asymmetric shape where the upwind side so that's the side from which the wind is coming it has a shallower slope and then the downwind side has a steeper slope. What happens is that as the wind blows, you know, from the upwind to the downwind side, sand grains are picked up from the shallow sloped upwind side and they are moved gradually, you know, in the jumps, the saltation. They move gradually pushed by the wind up the sand dun and then eventually down the other side. So they are

eroded from the upwind side and deposited in the relatively calmer air of the downwind side. over time that results in the whole sand dune moving because they're eroding the sand grains are eroded systematically from one side and then moved transported to the other side so the whole sand dune over time gradually moves downwind as it's pushed by the wind effectively not as a whole unit but like small grains at a time the whole thing is moved down wind and sand dunes can also combine with each other if they sort of clump together or they or they can be split depending on the local geography and the wind, prevailing wind forces. So that's effectively how sand dunes work. Let's talk a little bit more about some of the different types of sand dunes. There's a very

characteristic type of sand dune called a bachan dune. This is a crescent-shaped sand dune where the points of the crescents point down wind, which kind of makes sense, right? Because effectively the dune is anchored in the center by the sort of thickest area and then the ends are sort of pushed on either side downwind by the prevailing winds and it sort of curves backwards from the wind on each side. So they tend to form in areas where there's a hard ground surface and a moderate supply of sand and a relatively constant wind. If you have a larger supply of sand, effectively that leads to is more and more of these Barkan dunes forming and colliding with each other. And if you sort of imagine adding a bunch of them together and kind of averaging over that, what you'll get is a series of

transverse dunes. So you lose the crescent shape and instead you just form long dunes that are perpendicular to the direction of the wind. Again, they form in sort of similar conditions to Barkan dunes except when there's more sand available because basically you get more and more of the Barkarns sort of sitting on top of each other and you lose the crescent shape because there's sort of more sand all over the place now. Now, in some sense, the opposite of a transverse dune is a longitudinal dune. So instead of a straight line perpendicular to the wind, this is a straight line that is parallel with the wind. You might wonder how that can form. Longitudinal dunes typically form when there is a relatively small supply of sand and also wind that changes direction or that comes

from different directions. And so what you'll have is the longitudinal dune being kind of parallel to the sort of average prevailing direction of the wind, but it's kind of buffeted on both sides. to some extent by winds that come from like one side or the other side and kind of keep it in the longitudinal shape. So yeah, when you when you have winds that is more variable, you tend to get the longitudinal dunes. So again, that's the dune is along the direction of the average wind instead of perpendicular to it in the case of the transverse dunes. Now there's another main type of sand dune. These are called parabolic dunes. These are very similar to barkhounds. The difference is that they're effectively in the opposite direction. So remember that a Barkan dune is a crescent-shaped

dune where the points of the crescent face down wind. Well, a parabolic dune is similar, except now the points of the crescent face upwind. So it's sort of facing the wind. These are more common in coastal areas, and the main reason is because they tend to be stabilized by vegetation, particularly at the horns or the pointy edges of the sand dune, and in areas where you have an abundance of sand and also relative abundance of vegetation, you tend to see these being formed, effectively, because the pointy edges are stabilized by the vegetation, and there's enough sand for them to form, whereas, again, Barkhans occur typically in environments where there is relatively little sand and little vegetation. So it's a bit unpredictable, as you might have sort of gathered, as to

exactly which type of sand dune can be formed, because you can have effectively many different combinations. you can have the crescent barcans where the points face away from the wind or in the parabolic case the points face towards the wind. You can have transverse dunes which are perpendicular to the wind or the longitudinal dunes where the dunes are parallel with the wind, like running along the direction of the prevailing wind. And it all depends on how constant the wind is, the relative amount of sand available and the amount of vegetation. I mentioned that all of these types of dunes tend to move over time as sand accumulates on the upper edge of the downwind side. So it's removed from the upwind side and deposited on the downwind side. And then over time, that leads to an increase in the angle of repose until the sand starts

slipping down the other side and the whole dune moves. Now, there's another related phenomena which I wanted to discuss briefly called sand ripples. And you would have seen these. They occur in deserts as well as on beaches and really anywhere where you have sand and a fluid so that fluid can be water or the air. And these are regular wave-like patterns that you find in the sand. And they're still under current study as to precisely how they form. It's quite complicated. But basically, they arise due to, I mentioned before, the saltation of sand grains. So that's the gradual sort of hopping as they're pushed by the wind. Now, the speed of the wind determines typical hop distances. So the stronger the wind, then the further it pushes the sand grains, which makes sense, right? But the ridges themselves,

the ripples themselves are actually formed by a complicated interaction between the precise environment of like the initial environment of the sand as well as the speed of the wind. So basically what happens is that locally like over a small local region, let's take the example of the wind, a bit of occurs in water as well, you tend to have a sort of a standing wave pattern of hops where you tend to have certain regions where the grains sort of clump up a little bit for whatever reason there was maybe slightly more initial grains in one particular region than a few centimeters downwind there are slightly fewer or something like that so the grains tend to clump in one region and there is slightly fewer in another region and so that tends to be accentuated by by the wind so

more and more of the grains clump up where they were already slightly more and then more are pushed away from regions where they were slightly fewer and and that tends to happen in a regular way or at least a relatively regular pattern it's not a perfect pattern but it's a relatively regular sort of wave pattern, right? Tiny initial differences in height get magnified and then homogenized into these series of regular ridges that form the ripples. And basically, this is generated by the energy that's imparted to the sand grains by the wind. That's why you have relatively regular patterns, right? Because there's a relatively similar average distance that particles are going to be moved. So if there's a region where the particles of sand tend to get concentrated and then they tend to get

hopped a relatively constant distance away from that, then you're going to get a new area that they're concentrated and a new area. So you can imagine the sand grains hopping from one sort of small ridge, we're talking a few millimeters above the surface, so they're not a steep ridge, but stand grades hopping from one relatively small ridge to the next one to the next one. Obviously, this isn't so perfect, right, because there's a lot of variation. But as long as this is what's occurring on average, then you will tend to get these fairly consistent sand ripples, instead of the particles just landing all over the place in it, washing out as a big homogenous constant,

which is not what you typically see. You tend to see these ripples. And so that occurs because of these typical hop distances combined with small initial differences in grain density becoming amplified and then sort of evened out because of the typical hop distances. So it's a very interesting phenomena, which I'd recommend actually looking into if you're interested,

because it's sort of fascinating how something so apparently, designed or planned out as these nice even ripple patterns emerges from just the wind blowing over a bunch of sand. Okay, so that's enough on deposition and sand dunes for the moment. Now I'm going to talk about just some other examples of desert landscapes or desert geomorphology that I wanted to mention. As you presumably know, a desert is just really any barren area of landscape where there's very little precipitation. In this episode, I'm really only focusing on hot deserts. You may know that the Poles like North and South Pole are also deserts because they receive very little precipitation, but I'll talk about those in a different podcast because they're quite different because of how cold

they are, right? And it's dominated by snow instead of sand and so forth. So here I'm really just talking about hot deserts. Because of their lack of precipitation, the conditions for life are relatively hostile in deserts. All life needs water and water is lacking in deserts, really by definition. Also, the fact that there's relatively little water available means that there's little plant life and little plant life means limited availability for animal life as well. There are still plant life and animal life in deserts, but it's much more limited, and not of it lives, well, in the case of animal life, underground. The fact that there is relatively little vegetation also means that the surface is exposed

to the processes of erosion and transportation much more than other environments, as I discussed previously. About one third of the whole land surface of the earth is covered by deserts or semi-deserts. I mentioned earlier that many people think of deserts as covered by billowing sand dunes, but sandy deserts are only about 20% of all the deserts in the world. Most deserts consist of barren rocky terrain with scattered shrubs and grasses. So at the base you have the desert pavement, and then on top of that, to some extent, depending on how much sand is present, you'll have a number of sand dunes, maybe lots of sand dunes, maybe only smaller scattered ones,

and then around about you'll have rocks here and there, and then there's some shrubs and grasses, depending on a number of sand dunes, depending on exactly how much rainfall it gets. That's what a sort of typical desert looks like. Another thing that I should mention, actually, is the association of deserts with cacti. So cacti is a member of a family of plants, consisting of about 1,700 different species.

And all of these are succulent plants, meaning that they store a large amount of water, fluid, in the tissue, sort of in their thickened leaves. And that's obviously an adaptation to living in very dry. environments. However, many people don't realize that cacti specifically is a specific family, and the family of cactus plants is only found in the Americas, except for one species that's found in Africa. So one specific species. I don't know why that one species is found in Africa when all of the other ones are only found in the Americas. But the point is that, apart from this one species,

deserts in Africa and especially the Middle East and Central Asia and Australia don't have cactuses in them. Now there are other succulent plants that live in these environments, but they're not cactuses. So the association with deserts and cacti is a bit misleading. Maybe it's driven by American television, I'm not sure, but actually, yeah, deserts in the rest of the world don't have cacti in them, except for that one species, again, that lives in some African deserts. So the Sahara, for example, no cacti in the Sahara. Anyway, that's just an interesting side note. But coming back to the desert landscapes, so again, picture the desert pavement with some number of sand dunes being pushed around by the wind. There are other features that are

fairly common in desert environments as well, but remember that in all of cases, the landforms are shaped predominantly by the wind. And so what we want to do is think about how the wind over long periods of time affects the local morphology of these different features. So I already talked about how the, how the process of deflation leads to the production of the desert pavement and also transportation of sand dunes over time. A few other features that are fairly prevalent in deserts are cliffs and messes. So a messer is an isolated, flat, topped, like ridge or a hill that's often found in deserts. And then it's bounded on all sides by fairly steep cliffs. Messas typically form when there is overlying relatively resistant layers of hard rock on the top of them

that sits on top of relatively softer, more easily eroded, often sedimentary rock. So there's like a top that sits on the uppermost flat region of the messer, which sort of protects it from further erosion, but it still erodes at the sides, because that's where the relatively softer or more easily eroded sedimentary rock is located. You can have messes in other locations as well. I believe the reason why they're particularly associated with deserts is just because you don't typically see the exposed sides of the cliffs in other environments where typically you'll have soil that is then covered by vegetation and that keeps the rocks from eroding as rapidly or as easily as they do in desert environments. Another feature of deserts is a blowout. A blowout is a sandy

depression which is caused by the removal of sediments by the wind. So this is erosion and then transportation away by the wind. Often they form, I think I mentioned this earlier, when you have patches of bare sand that relied on vegetation to remain stabilized and then for whatever reason the vegetation is removed or disrupted, that can lead to the whole region, and by region, this can be anywhere from like a few meters to many, many kilometers, the whole region being disrupted and massive amounts of sediment rapidly being eroded

and transported away by the wind. This process of erosion of the bare sand that's no longer being stabilized can continue until a non-erodable substrate is reached. So perhaps a desert pavement that's further down or maybe some sort of rocky substrate or igneous rock or something like that that can't be eroded as easily by the wind. And this can happen extremely rapidly if the overarching, if the overlying terrain is destabilized, for example, by construction work or human activity or by natural processes. and massive areas can be completely denuded of sand. So one example of this is the Qatarra Depression in northern Egypt, which lies up to 150 metres below sea level and covers about 20,000 square kilometers. And much of the sediment that once lay in this, what is now a depression,

was removed by wind erosion over the past several million years, and now consists of low-lying sand dunes and salt marshes. But it's interesting just to see how intense these processes can be and how large an area they can operate over. Another interesting phenomena found in desert landscapes is a ventifact. So this is a rock that's been abraded, pitted, etched or grooved or otherwise polished down by sand or ice crystals in the wind. And this includes phenomena called mushroom rocks, where you've probably seen these where effectively it looks like a mushroom stalk with a kind of cap on the top, right, where there's a sort of a thin pole thing in the middle. and then there's a swelling up located above that.

This occurs when you have in desert environments relatively greater erosion at a level that's closer to ground level. This is effectively because it is difficult for the wind to keep so many sand grains aloft at the higher you get off the ground. So the force of the erosion is highest nearer to the ground. I think the winds can also be stronger nearer to the ground, depending on the local environmental factors as well.

But for a combination of reasons, you have much greater erosional force near the ground, whereas higher above the ground it's not as great. So you tend to have more erosion of the rock nearer to the ground. There may also be, in combination with that, a differential resistance to erosion. So you may have different strata of rocks that are, say, more susceptible to erosion nearer to the ground, and then above that is some sort of more resistant rock that's above that.

So a combination of these factors you have, these rock structures where it's sort of eroded, down the bottom, but a region at the top is preserved, so you get these sort of mushroom rocks. But many facts go far beyond that, and really can include any type of rock, be it large or small, that is shaped and pitted and so forth, or polished by wind activity. And these are quite common in deserts. Another desert feature is an alluvial fan. So this is a fan shaped, kind of like a delta, but it consists of typically sand and other sediment particles that stretches out in some sort of depression or a valley after often like a dry riverbed or a gully enters into a wider depression.

Basically that can happen as a result of a stream carrying a certain particle load and then when when the stream enters into a broader gully or a much wider stream bed then the water loses a lot of its velocity because it spreads out loses velocity and then a large number of the sediments fall out of solution, and so it deposited right where the stream has entered that wider region, a wider stream bed or whatever it be. And so the grains are just sort of deposited straight there and form this sort of fan shape.

Now, this could have occurred either before the region became a desert or the particles may be moved in water in the intermittent periods when deserts flood, and so the alluvial fan builds up over time in these during those instances of flooding. But you can see lots of images of these in desert environments where you have these like a little gully or streambed typically dry that enters or it that meets into a broader area or wider stream bed or a larger gully area. And then you just see this huge deposit of sand grades. It's just like piled up there in this sort of fan shape. So that's an alluvial fan. So I think that concludes what I wanted to say about winds and

deserts. The summary there is that deserts because of their dryness and because of the lack of vegetation, they're dominated by aeolian processes or wind processes. And we talked about the sand dunes. We talked about the methods of erosion, including like deflation and abrasion and attrition. And we talked about how different desert landscapes, including messes, blowouts, Vendifax and Alluvial fans are all shaped by these processes of wind. So now we're going to move on to talk about coastal landforms and how particularly focusing on how the wind is dominant in these environments as well, although in a somewhat different way. So another piece of commonality between coastal landforms and desert landforms is that both

typically, not always, but typically have large deposits of sand. Obviously, when you have deposits of sand along the coastland, we call that a beach. Beach sand is actually quite different to desert sand. I think I mentioned this before, that desert sand grains are typically significantly rounded and kind of polished by the forces of the wind over long periods of time, whereas that doesn't happen to the same extent in a beach environment. And so beach sand and desert sand are actually different. So desert sand is typically not very useful for construction or for making cement, for example, because of the fact that the grains are much more rounded. Beach sand is much more sought after for that sort of thing. But anyway, let's get into talking about coastal landforms. So the coast is really just where the land meets the ocean. And I'm not really going to talk much about the ocean today. That will be for another episode. I'm just going to be talking about the coastal landform processes that dominate there.

According to the United Nations, about 45% of the world's population live within 150 kilometers of the sea. So about half the world's population lives fairly close to the coastline. And there's obviously a lot of reasons that one of which is being historically, I mean, and still today really, that ocean transport is much easier and quicker than land-based transport. And so if you live by the coast, it's easier to engage in trade and move goods around and also transport to other environments. Also, many people live by rivers being a good source of water and rivers, well, most rivers inevitably end up in the ocean. Now, there are two major different types of coastlines. I mean, there's different ways of classifying them, but this is one major. dichotomous classification between emergent coastlines and submergent coastlines.

So an emergent coastline is one that has experienced a fall in the sea level. So it's emerged from the sea is sort of a simple way to think about this. Either because of a global sea level change, so you can think of that as a fall in sea level, say during an ice age as more water is frozen at the poles, then there could be a fall in the sea level. Or local uplift of the surrounding continental plate. In either case, or whatever the reason is, an immersion coastline is one where there has been a landforms that are originally under the water and are now protruding above the sea level.

Emergent coastlines are identifiable by coastal landforms which exist above the high tide, so like raised beaches, for example. So whenever you have any kind of coastal landforms that are identifiable as coastal landforms that exist above high tide, that's an immersion coastline. And the reason is effectively because there's no way. for the ocean to have directly formed those if it's above high tide, right? Because the ocean, the water never gets there. So raised beaches that exist above the level of high tide, those sorts of things are going to be formed as a result of an emergent coastline that at one point in time, maybe millions of years in the past, was submerged. Now, the opposite of an emergent

coastline is a submersion coastline. A submerged coastline is one of which the sea level has risen. And that could either be due to global sea level rising, due to melting of the glaciers and ice caps and so forth. Or it could be due to local subsistence, so basically land subsiding and falling closer to the sea level, or other processes that change the local level of the land. So submergent coastlines started higher up and then sort of pushed down closer to the sea level. And submersion coastlines are identical by featuring submerged or drowned landforms. So an example would be a fjord or a Rhea, which is a drowned valley. So emergent and submersion coastlines have quite different features. Another couple of features that I wanted to mention that are characteristic of coastlines are estuaries and lagoons.

An estuary is a partly enclosed coastal body of water that is brackish water, so salty water, that has one or more rivers or streams flowing into it. But it also has a connection to the open sea. So an estuary is kind of like a drowned river, if you like. It's sort of like a river that connects to the sea and therefore has some intrusion of the saltwater. into it so that it has become salty to some extent at least. So an esturoy can be one indication of a submerged coastline where initially there was a river that consisted, like a river that connected to the sea, but then the sea level rose. And so part of that riverbed and the region of the river that was close to the ocean, part of that became sort of invaded by the ocean and has become brackish,

and it's partly oceanified if you want to think of it that way. That's not the only way that an estuary can form. Now a lagoon is a bit different. Lagoon is a shallow body of water that's separated from a larger body of water by a narrow neck of land, like a barrier island or a peninsula or something. And often, although not necessarily, lagoons consist of salt water. I'll talk a bit more about that later when we discuss a barrier island. But a lagoon is effectively a small shallow lake that's separated from the ocean, typically the ocean, by a small neck of land of some form. or an island sometimes. Mangroves are also an idea that you might hear associated with the coastline.

So a mangrove is any type of shrub or tree that grows in relatively saline or brackish water close to a coastline. So it's not a specific type of plant, but it's really any plant that grows well in sort of salty, brackish environments near the coast. So those are some sort of major coastal landforms. Let's talk about a few important processes that occur at the coastline. And the two main ones that I want to focus on here is cliff erosion, and then we'll talk about processes at beaches, you know, beach processes, and more about sand there. So first of all, cliff erosion.

So at beaches where you have cliffs that meet the ocean, obviously that's not all coastlines, but many coastlines do feature cliffs. What tends to happen is that the cliffs are gradually eroded away. They're gradually forced backwards by the eroding action of the waves. And remember that the waves are caused by the pushing of the water, or the surface of the water, by the wind. So ultimately this is wind-driven effects through the interaction with the ocean. Most of the sediment that is deposited along the coastline, including much of the sand that you find it like sandy beaches, is the result of erosion of surrounding cliffs by the effect of the waves from the sea. Because of this process, sea cliffs gradually retreat landward, because of the constant undercutting,

of slopes by the waves. So when we talk about undercutting, this is because the cliffs are obviously raised above sea level, because that's what makes it a cliff. And so the wave action actually hits at the base of the cliffs first. So what tends to happen is first you'll have cracks that develop, which is caused by the action of the waves, and it might find a particular weak path or weak section of the rock, which erodes away first. That forms a crack. Then the crack grows as there's further hydraulic action and abrasion. That forms a cave, which then is widened and increased in size and eventually becomes larger and larger until you have a headland arch, which is, you've probably seen these before, that looks kind of like a bridge, right? There's a

big region that's excised, taken out, or eroded away from the cliff, and then there'll be a part that is still in place. It looks kind of like an arch, and that's called a headland arch, headland forming a natural arch. And eventually that is continually eroded away until the connection between the arch and the rest of the cliff is broken. And so what you have is a tall rock stack that's left standing by itself. So it's now separated from the main part of the cliff. And eventually then the top of that is eroded away, leaving a stump. And then that's eventually eroded away as well until, well, there's nothing left.

So it's a gradual process of first undercutting cracks growing into caves, which grow into arches, which are then cut, leaving behind stacks, which are then eroded down, leaving stumps, and then those eroded away as well. So gradually than that process pushes inwards and arose away more and more of the cliffs. This obviously takes long periods of time. Another aspect of this is the way in which wave energy is directed into headlands. So these are the bits that stick out of the cliff. Or there may be cliffs that are interspersed by relatively quiet beaches,

and then there's a cliff on the other side of that and so forth. Whenever you have headlands like this that kind of stick out, what happens is that there is there's a diffraction of waves as they pass through whenever you have a wave that passes into a relatively smaller aperture so the waves come from open ocean and then they pass between two headlands they're now in a confined space and so they tend to diffract they bend they bend inwards and what this does is it changes the direction of or the parts of the waves that are closest to the headland causing them to bend inwards it's a little bit hard to show this but if you like see a diagram it's sort of instantly clear how it is that when the waves move into the space between the headlands, they bend around and kind of are concentrated, the energy is concentrated and pushed inwards onto the headland. So the energy is directed inward towards the headland, and that tends to erode it backwards, faster than it would otherwise be the case. Whereas in between, the space in between the headlands, it's actually relatively quiet because the energy has diverged away. So the wave energy

diverges away from these relatively quiet beaches, and it's converged on the headlands. So this is a process that amplifies the erosion, erosional processes that I mentioned before. And there's also how on like the same line of coast, you can get places that are quite quiet beaches with relatively modest wave action. And then very energetic environments where there's a lot of wave action very close to that because of this diffraction effect that the waves are bent around. And so they're diverged away from some areas and concentrated on the headlands surrounding those. So it's very interesting how that all works. So that's cliff erosion. let's talk a bit about the beach processes, the processes that operate on those relatively quiet beaches.

So the beach is a landform that exists alongside a body of water, so here we're focused on the coastline, and it consists of lots of loose particles. We talk about sand, but it's not all sand. Many beaches comprise of rock or gravel, pebbles, or bits of sand combined with particles of biological origins, such as pieces of shells or coral or algae. The exact composition obviously depends on the beach and the environment. So as I mentioned, beach sand is primarily formed through erosion of surrounding cliffs and headlands that's concentrated in these regions over thousands of years. Sand and other sediments can also be deposited at beaches by rivers, if there's a surrounding river, but a lot of it comes from the surrounding erosion of cliffs.

When there is a sufficient accumulation of sand, the beach tends to act as a barrier that prevents any further erosion. If there's an accumulation of sand at a place, then you tend not to have erosion of the underlying rock because it's buffered by the sand. So sand on beaches kind of protects further erosion, at least of that local area. Obviously, we know that waves exist at beaches, so the waves come in and then they go out again.

That process is actually a little bit more complicated than you might have thought, so let's go through that a little bit. When waves approach shore, they gradually slow down. That occurs because a wave is actually not, when you have a wave coming in, it's actually not a series of water particles that move.

It's not like the water particles are moving from the open ocean onto the beach, onto the beach. Really what happens is the water particles move in fairly small circles. And the wave itself is really the energy that's being transported. Obviously, there's some water particles that move, especially when it actually breaks on the beach itself. But for the most part, water particles don't move very much. It's mostly the water particles moving in place, like circling around, moving in a circle, and then the energy is passed on from one to the other. That's what a wave is, really. It's the transfer of predominantly a transfer of energy, not a transfer

of matter. As that wave gets closer to shore, eventually as the water gets shallower, the bottom of the wave becomes compressed against the bottom of the ocean, the bottom of the shore, as it approaches shoreline, it becomes shallower and shallower. When that happens, there's effectively a drag force on the whole wave slows down. However, in order to ensure that there's still a transfer of energy at the same rate, which is sort of needed in order for the complicated maths to work, which we might go into. What happens is that as the wave is slowed down, the amplitude tends to increase.

So the wave slows, but the amplitude increases. And what that means is that the wave gets taller. And as it gets taller, eventually, the top part of the wave is traveling faster than the bottom part of the wave. Here I'm talking specifically about the visible part of the wave on the top of the ocean. The top part of that travels faster than the bottom part

because the top part, it doesn't have the same drag with the rest of the ocean. Like the top part of the wave is separated, from the rest of the ocean, right? So it can kind of move faster without being pulled back by the rest of the water. And so it kind of goes too fast and tips over. And that's why you have wave breaking or the breakers, right, where the wave kind of tips over and then collapses in on itself. And that occurs in what's called the surf, right, where you have the breaking of the waves. So all of that process that the gradual increase in the amplitude of the waves and then the

the breaking and the crashing down of the waves is caused by the fact that the bay gets shallower as you move closer to shore, that slows down the waves, which then causes their amplitude to increase, and the height goes up, but then they tip over and break in the surf region. So after breaking in the surf zone, the waves continue to move in, and now they run up the sloping front of the beach. And this is called the swash, right? So it's after the waves are broken, and this is where the, instead of sort of being like vertical, if you like, the waves are now moving kind of horizontally just up the beach. And so that occurs after breaking. The water then runs back out again to the ocean in the backwash. So there's the surf where the waves break, then they rush up the beach

in the swash, and then back again, the water goes back again in the backwash. Again, only a small amount of water is actually moved in each breaking of the waves. Mostly it's a transfer of energy, whereas only a relatively small amount of water is actually moved. Now, one interesting thing about waves is that they rarely approach the shore exactly parallel with the shoreline. In fact, the waves approach it kind of whatever direction is determined by the prevailing wind. because, you know, waves are produced by the wind. So often that's at a kind of an oblique angle, the wave press coming in an oblique angle to the beach.

And that means that the swash also pushes up, you know, the waves break at an oblique angle, the swash pushes up kind of diagonally at an oblique angle along the beach. But the backwash always goes straight back out to sea because by the time you get to the backwash, that's not determined by the wind anymore. That's just while the water's going back out to sea,

it takes kind of the direct route. So this combination of diagonal in but straight out results in a gradual pushing of the sand along the shore. This is called longshore drift. And over time it results in, well, over short periods of time, it actually result in you being pushed down along the beach. If you've played on a beach before you realize that you, if you're not careful, you're gradually pushed down the beach by the longshore current. Over longer periods of time, it actually moves the whole beach or moves large regions of sand along the beach. And this tends to result in landforms called spits. So a spitz.

bit is a deposition bar. It's basically a big hunk of sand that's been moved off of a coast by the effect of longshore current. It's a little bit hard to describe what they look like. They can be very long and thin and they're kind of curved, right? Because basically they're formed by the prevailing wind pushing the surf in one direction and then the, again, the backwash goes straight back out to sea. So gradually the sand is pushed. So if it's always pushed in one direction, it tends to be eroded like from the left side of the beach and then deposited on the right side of the beach and form these long spits where the sand is being deposited over time. You can actually have situations where spits are being deposited on both sides. If the sand is subject to longshore

currents in opposite directions, from whatever reasons, depending on the local geography, you can actually get two spits meeting each other from opposite directions, and that forms a cuspate foreland, which is basically a long straight peninsula that sticks out into the ocean instead of a spit, which is often more at an angle to the shore. A similar phenomenon, a similar phenomenon is a barrier island. Barrier Island is a long coastal island that consists of sand and it's often subject to change during storms and other disruptions. So often they're not entirely static. They're like easily changed or subject to disruption during storms. And typically what these do is they create protected areas just behind the coast. So a barrier island often separates a lagoon

from the main part of the ocean, as I mentioned earlier. And they can often protect wetlands. have plants and animals that can flourish in this sort of relatively mild environment that's protected from the waves and the rest of the coastline. So it's not entirely clear how barrier islands form. They occur in many different coastal environments. They may form from spits that become detached or from coastal ridges that then become partially submerged. There are also other hypotheses as well as to exactly where barrier islands come from. But however they form, they're a very important part of many coastal environments. So one final aspect of this is, as I mentioned, a beach is a very dynamic environment. The sand is constantly being moved around by the effect of longshore current

as well as the erosional forces that maybe produce extra sand or carry it off because of longshore current and other factors like that. And so when humans build, well, anything on beaches or near beaches, what they often find is that the beach moves around them. The beaches eroded away inland. The sand is carried off in spits. There's other changes like to barrier islands and things. And so it's difficult to maintain a beach in a form that humans want it to be, like that that they originally built it. So there's a lot of work that goes into maintaining and engineering beach environments by building piers or jetties or other barriers that prevent the movement of sand, for example, by longshore current, by building floodgates and other things to protect the

inland from flooding. So there's a lot of complex engineering that actually goes into this, because beaches are highly dynamic and we as humans, you know, we wanted to stay the way that it was when we originally built stuff there or when we originally settled there. So it becomes a bit of a a bit of a problem. That's all I wanted to cover today. Hopefully you found the show interesting. What we did was focus on the effects of mostly wind action in both deserts and coastal landforms. In the next episode in this series, we will look at the cryosphere, so that is the poles and glaciers and the effects of frozen water on the earth. And we may also look at groundwater as well if we have time. So thanks for listening. Hopefully you enjoyed the show. If you did, you may

consider supporting the show by making a donation. You can go to the Patreon page for the show or make one-off donation through my email address. That's Fods12 at gmail.com. That's FODDS12 at gmail.com. Feel free to also get in touch with me there with questions, suggestions or feedback. I love to hear from my listeners. Also, if you want to promote the show, you can leave a favorable review on iTunes or Spotify or whichever other podcast aggregator you use. I greatly appreciate every such review. Thanks again for listening and I'll talk to you next time.