Desert - Wind: 3 of 4

本站添加: 2026-05-15

[00:00:02]Well, why is wind so effective in the desert? What makes the desert different? And it's simply the absence of vegetation. When wind blows across a surface in a human environment, it doesn't touch the surface because it's covered with either grasses or other vegetation. When wind crosses the surface in a desert, it's able to move the loose material at the surface. So simply the absence of vegetation is why we see the more effective work of wind. It, of course, decides not to work again.

[00:00:36]So you can see sometimes on vegetation where if you've got a prevailing wind, you'll get some weird anomaly like this, but you have to have like severe winds throughout the year from one direction. We don't get this kind of stuff around here. You might get some vent effects, which are erosion or an abrasion from the wind blowing material across the surface. So you get these neat little stripes that go across here from the abrasion. This takes hundreds of years or thousands of years in some cases. This is a dry valley in Antarctica where the wind comes down off the higher Antarctic plain and it blows down through this valley, doesn't allow snow to collect because it's constantly blowing through. You can see how it's eroded the bottom foot or so of some of these rocks just blowing the sediments across it, and kind of creating these sharp edges on these rocks here. Vent effects are small and minor.

[00:01:26]The role of wind in shaping the desert for as far as erosion is minimal at best. You get one weird thing I'll show you in a second, but a single flash flood through this area will do more weathering than centuries of wind erosion. So I have to keep in mind that water is more powerful in the desert than the wind. It does some neat stuff I'll get to in a little bit, but as far as actually the wind blowing sand against something and getting it to erode isn't as important as water flowing through the desert. You may get in some areas what's referred to as a yard, and you get a slightly erosion resistant clump of rock, and then the wind kind of blows around it. I just like seeing Yarding because it's a funny word too.

[00:02:12]Um, but these are a few meters high at best, and they have kind of an odd streamlined shape to them.

[00:02:20]When you're really, really bored, you can drive out to near Edwards Air Force Base. There's a handful of yard things out there. Um, kind of on the other end of the spectrum for me from alluvial fans, as far as how excited I am about yard things, uh, but this is the best that wind erosion or sorry, wind abrasion can do as far as shaping the desert, it's better at transporting material than it is at actually shaping things. So of course, let's talk about sand transport. And then hopefully you're looking at this diagram going hey, hey that looks like the one we had for water transportation. They're very similar where you've got the finer grained stuff that can be held in suspension from the wind, and you've got the larger grain stuff, tiny pebbles and sand grains that can bounce along and be carried by. If there's a high enough wind, the sand can be carried to higher elevation, but for the most part, the sand just kind of rolls across and bounces across the surface. Look at this. We're talking inches where most of the transportation takes place, not feet. So even in a severe windstorm, a lot of that material that looks like a a sandstorm coming at you, a lot of the stuff in the higher elevations of that in above like ten 20ft is mostly dust. The stuff close to the surface will be sand, but higher up is fine grained dust. Now there is one exception to wind only being able to move fine grained material. And that's back to Racetrack Playa in, uh, Death Valley. The wind has pushed these rocks across the lake surface, and they couldn't figure out how it was working, because it just doesn't make any sense. Um, there was various theories, everything from UFOs to pranksters, but once it was a national park and they were protecting it, they were seeing that every now and then these things would actually move across the surface of the lake. But they knew it had to be after rainfall, because that's the only time the surface was slushy enough and soft enough for the stuff to move. So they tried investigate it and they couldn't figure out what was going on. They were putting stakes around some of the rocks to see if they could track how they moved after a rainstorm, and they weren't moving, and they were going to do it one time, but it was going to be too cold and windy. So they didn't do it. And lo and behold, the one time they didn't do it is when they moved, when it got cold ice formed on the surface of the lake. And what that ice did, is it ever so slightly when ice expanded, lifted this up? Not completely, but it didn't let it sink into the mud like it normally would. And then it was windy, and so the rock acted like a sail on a sheet of ice. And so ice on top of mud has a very low friction. And so it was able to slide across the surface from that. And by the way, it's a federal offense. When you could go to prison, actually you would go to prison. Um, if you move one of these rocks. Because they are protected in a national park.

[00:05:19]So the sand transport the. This is where we get dunes from. And it's also another way to get a couple of unusual desert landforms. So where it's sandy, of course, it's going to correspond in large part to where we get the, uh, climates.

[00:05:36]There's some exceptions out there. And we'll come back to that a little bit later because we'll start looking at glaciers as well.

[00:05:43]Central Australia. Oh my God, what a barren, barren environment. So if we look at a dune in cross-section, that's from the side, you'll notice that the bedding planes aren't horizontal, because what happens is the sand gets blown up the side of this, and it actually scrapes away the top of this, and then it falls down the other side. So as it migrates, you'll see a new layer being built on this side. So the layer is built at kind of this odd angle. And so these betting planes are at an angle. And this is one of the ways when you see sandstone you can tell if it was deposited by water or by wind. Because water deposited sand will have horizontal betting planes. Wind deposited sand has these diagonal betting planes.

[00:06:35]So here's what I was trying to demonstrate before.

[00:06:37]Um, God, I love animated GIFs. They come in handy sometimes. So it dune migrates but it it it's not the whole body of the doom migrating at once. It's like take off this layer on the puzzle on this side so they don't. Dunes migrate through an environment. Here. You can see in Zion National Park the bedding plains of subsequent subsequent dunes on top of old dunes on top of old dunes.

[00:07:01]So in. Great Sand Dunes National Park in Colorado.

[00:07:06]They've got it's the valley shape kind of funnels the wind into going through this little pass over here. So the whole valley, the wind keeps pushing all of the sand to the same central location. So you end up with this massive dune. Um, I, we were just randomly driving through southwestern Colorado. Let's go see the sand dune and you get to it. It's huge. And it was so much fun because it was so unexpected to see such a large sand dune in that area. That's about 850ft tall. They don't allow RVs or RVs, um, vehicles, to drive through it so it's safe. Um, people will hike all over the place. Although I never really enjoyed hiking up the side of a sand dune, but it was still fascinating to see. Also. This is by the UFO Information Center. So if you find yourself at the Sand Dunes National Park, you can go to the UFO Information Center. And apparently, this is sarcasm. There's two vortices in the universe that spiral here. One in a counterclockwise manner and the other in a clockwise manner. And where those two intersect is where the aliens come to visit.

[00:08:19]That's what the woman was trying to explain to me.

[00:08:21]Um, I didn't want to tell her about the Coriolis effect, how they should be both spinning the same way. Because I wasn't an expert on vortices in the universe. You can also leave things for the aliens.

[00:08:32]And so it was summer. I didn't need my parking permit for Saddleback College anymore. So I left it here, and I figured, you know, parking is always a barrier at Saddleback. So if the aliens came to Saddleback, they'd have a place to park.

[00:08:45]And that's my old element.

[00:08:50]Let's look at some sand dune shapes. And the dune shape is a function of two things. Where's the wind coming from? Is the wind shifting? Is it consistent? Do we have multiple directions?

[00:09:02]Singular direction and the sand supply? How much sand are you getting? Because if you're a beach environment, you're going to have a near constant supply of sand. But if you're only supply is a dry lakebed, that's something that it's going to have a limited supply of sand when you go through it.

[00:09:18]So we're just going to focus on a few of the dune types. Um, the bahkan the longitudinal also known as Sif. Um, and then the star dunes, the sandy giants of the desert. So the bar can dunes. These are crescent shape. So what's happening is that you're an environment that is sand starved. In other words, the what the sand dunes are flowing across does not have a sand supply. So if it seems weird, why would that happen? It's like, well, maybe it's Sandy over here and the wind is blowing the sand from a sandy area to a non sandy area. We have a bunch of these in Southern California out in the desert. And what happens is the wind blows across a dry lakebed, picks up some sand and transports it to other areas. Now these may accumulate somewhere else and get a big pile of sand somewhere else. But right now, they're moving through a sand starved environment in the bottom of the the the, the parabola points of the direction that the wind is blowing toward constant direction, constant supply of sand, but just not a lot. Now a see if you would get. When you have kind of the win. Coming from two dominant directions. So you need a large, big broad plane for this to occur. I'm doing hand gestures like you can see what I'm saying. Um, you need a big broad plane for this to occur, and it's often seasonal, or the wind kind of shifts from a little bit to the north part of the year, then a little bit to the south for part of the year. So as that wind direction shifts, it kind of makes these long, linear sand dunes. We don't have a big flat area where we have it dry to get these or those large sandy deserts like the Sahara has. They just we don't have big flat areas that cover, um, hundreds of square miles that allow this to happen. We, you know, our western United States is kind of broken up a little bit with topographic features that just doesn't allow it to happen. So bark and dune.

[00:11:17]So this is after this is off the coast near the coast in southern Africa. And so the beaches over here and the prevailing wind blows out of the west and it blows the dune across the, the non sandy area.

[00:11:35]Seats in Australia. Central Australia. Relatively flat. Great conditions for the shifting winds to make these long linear sand dunes.

[00:11:46]Here you can see this long linear dune in the Sahara. I mean, this is all one giant sea of sand.

[00:11:55]Oh, look. Some erosional remnants.

[00:11:59]Those, of course, are the erosional remnants Insel Bergs.

[00:12:08]Now the star dunes. Here we've got converging winds. We get these in the Basin Range Province, because the different valleys sometimes will funnel wind into one central area. And they create these massive sand dunes and, uh, called star dunes. And theoretically, if you look at them from above, they have a star like shape to them. That's where they get their name. But they're the sandy giants of the desert. And when we were going to Great Sand Dunes National Park in Colorado, I'm like, oh, it's a star dune. It's the sandy giant of the desert in my course. My wife rolls her eyes as I'm making dad jokes. Um, and I kept calling it the Sandy giant of the desert. So we get out of the car, we're looking at the little display that the National Park Service has about it, and it says that Great Sand Dunes National Park is a star dune comma. The sandy giants of the desert are the sandy giant of the desert comet. And it goes on with the sentence. And I was vindicated by the National Park Service. Thank you. So, Stovepipe Wells, we've got them out there. And I love that there's this all the sand here. And it's most of it has been kind of blown away from this area, areas like this into where they would pile up and accumulate.

[00:13:19]Now, if those sand dunes build up over time, we're talking in geologic time and are compressed and become lithia fine. Here you can see these are former sand dunes, but then now are solid rock and it says sandstone. Zion National Park, which we'll come back to, is the greatest collection of ancient sand dunes in the world. A layer of sandstones 2000ft thick. It's phenomenal the amount of sand in this area, this, and I'll explain later where it came from. But this is your best place to see. Sand dunes. Ancient sand dunes in Zion National Park.

[00:13:55]Wind can also move fine grain material like dust, and so it can't transport sand very far simply because of its weight. And so once you move out of that dry environment where you start running into vegetation, any grasses will will trap the transport of the sand. However, dust can be at a higher altitude and so it can be blown over the plants and for longer distances. So here we're blowing dust out into the Red sea. Sometimes after sand and and conditions locally, we'll see fine grained material covering our vehicles outside.

[00:14:30]And that's the dust that's been blown in from the desert. The accumulation of that fine grain dust, the, the the clays this is referred to as loss.

[00:14:41]And loss is like flour. I say it lacks stratification. So as it builds up, you can't see the bedding layers like you can in other sedimentary rocks. And so it's it kind of those pieces are rough shaped and they kind of interlocked. So if you took flour and you kind of compacted into a cup and then turned the cup upside down, you'll see how that can maintain its like stability. But you don't see any layers in it.

[00:15:08]So you don't see layers of, of flour in what you've done. But that's kind of the consistency of loss. Now here's the kicker. Now obviously I've been talking about deserts. This is one of the sources for loss. But you can also get it from the other extreme. And that's glaciers. And it's not when it's glaciated. It's afterwards when the ice all melts, you're left behind with these large areas that don't have plants on them. And so imagine a bunch of melting ice on dirt. What's it going to create? Mud. So when that mud dries out, that's where the source of the clays. And so the wind can carry that material. It's a really important soil component. And so areas that have a source of loss in their soils have really good soil. So like the black earth belt in Russia, Ukraine, through here in the plains, the United States are some of the most fertile soils on Earth.

[00:16:02]And it's a result of the loss that's mixed in with the soil.

[00:16:09]So for soil, it's a, you know, you want clay and silt and sand and so you want to have that perfect mix. And so that's why you have to have all three.

[00:16:21]And that this just happens to be one of the more important ones. It's often lacking in some of the soil. Plus also has like strange characteristics.

[00:16:30]I'd mentioned flour that if you put it upside down under, you know, a compressed cup of flour, turn it upside down, it'll still kind of maintain its own shape. And so these loose deposits, you can literally just carve into them and it'll maintain it. If you try to do this with shale or something, it would weaken the slope too much and nothing to hold the base of it. It would probably collapse.

[00:16:50]But plus has a lot of vertical stability. There's one area of loss that has really, really, um, weird characteristics because there's lime mixed in with it. So this is a humid portion of China.

[00:17:04]So it gets rain in this area. But the source of the fine grain material is out in the taco making.

[00:17:10]And there's limestone deposits out there. And so some of the dust has a high lime content in it. So the lime blows into kind of a sandy region with rainfall. And so the lush deposits here, you're mixing lime and water, which is your ingredients for cement. And so when it finally does form, you get these weird like layers of loss that are cement like, but they're still soft enough where you can carve into them, but they maintain their stability. So you can literally build homes into the sides of the valley walls and carve out what you need for your open space in front of your house for maybe some other small garden, or in some cases, even areas where you could plant.

[00:17:57]Simply by removing the top layers or carving it into whatever shape you want.

[00:18:06]Other landforms. Greg and Hamada. Hamada. We won't deal with too much. Um, but Ergon rig an erg is a sea of sand, so the Sahara is an erg. So obviously these are Arabic words and I apologize to any of my Arabic speaking students how I'm destroying its pronunciation. I'm doing the best. Um, so here you can have a large flat area with an ample sand supply. So when you talk about a sea of sand, this is a sea of sand.

[00:18:40]In these herbs, you can get some unusual formations. And so, um, I'm getting to what an oasis is. And so I'm kind of getting the terminology out of the way. Um, so the, the wind, when it flows through the desert will kind of pick up some areas. It'll gain speed and remove material redeposited somewhere else. That's why you see those undulating surfaces like this as the wind kind of blows through. So this would be an area where the sand has been blown away. It's referred to as a blowout. And the depression is the blow out. The process is called deflation. And so if a surface is deflated, it's not like a tire that's deflated. That's different. This is when deflation takes place. It removes the material. So the removal of fine grain material, sand and clays, that removal process is referred to as deflation.

[00:19:34]So a deflation hollow would be the same thing as a blowout. It's just it's a depression that has had the stuff removed. An oasis is that when that blowout or that deflation hollow gets so low, you've actually removed the sand all the way down to the groundwater table. And so that's where the source of water is. It's it's at the groundwater table. But I mean, you've got to go pretty low to get to it. The oasis, the water tends to stabilize the blowout so it can remain in place sometimes for centuries, sometimes for just years. But knowledge of where these are in the desert is how people are able to survive in the Sahara Desert.

[00:20:14]Um, Europeans were unable to cross the Sahara because the locals wouldn't cooperate with them and they wouldn't tell them where these places were. And so the Europeans never were able to, I think until like the late 19th century, early 20th century, crossed the Sahara desert without help from the locals. Um, so the term oases, though unfortunately in the United States, is used more frequently than it should be. So sometimes if there's a spring in the desert, somebody will call it an oasis. And like I, it's not the same thing.

[00:20:43]Um, it's not a deflation hollow down to the groundwater table. It's just a spring.

[00:20:50]Now a rig. This is a stony desert. It's still from deflation, though. Um, and you may have noticed that I've got two landforms with three letters.

[00:21:01]The exact same three letters, just in a different order. How do you remember which is which when it's an erg and when it's a rig? Rig think rocks rig is rocky. And so this is your stony desert. We get these in the southwestern United States and in California. I love these these are pretty cool looking. So what happens is the deflation. You started off with the soil profile that through whatever happened over time in the western United States, it got drier and the plants died off leaving behind soil. But you know what? It's got clays and dirt rocks in it. And so when the wind blows through, it can't remove the rocks. It can only remove the sands and clays. So it leaves behind the rocks. So all of these rocks here, if you remove the sand and clay matrix, they're slowly going to get lower and lower. But they're going to build up on the surface. And so all of these rocks here would have been at a slightly higher elevation, maybe two three four feet higher.

[00:21:58]But if you removed all that sand they don't get moved in. They're left behind with this rig. What we refer to in the West as desert pavement, this is really fragile stuff. You don't want to drive across it because if you tear up this layer of rocks, you'll expose some of the clays beneath it.

[00:22:15]I to me, this is the California desert, the high desert. This is what I always think of when I think of the California high desert, because there's these are so common. Um, and yet they're tempting for off road vehicles. And please, please don't drive on them. There's places where this has already been destroyed. There's areas where you can go to your heart's content. But these are very fragile and unusual landscapes that take centuries, if not thousands of years to be formed.

[00:22:42]So here we put all our pieces together. We've got our Bahati in the background. We've got our dry lakebed, our playa. We've got our rig in the foreground in Wiley right here. Kind of to add a little color to it. Sparse vegetation, all of our pieces for a desert landscape. This would be in the Basin and Range Province. So not the Colorado Plateau. This would be in the Basin Range because that's where you've got the down drop block of the basin.

[00:23:12]The horse. The up block. The range over there.

[00:23:21]Now. Hamada. What's up? Sorry, I almost got a dead joke thrown in there. The, uh. Hamada. This is the rocky surface. This is exposed bedrock that's kind of breaking down in place. So it looks similar to a rig where you've just got rocks on the surface.

[00:23:37]But this is just the rock breaking down in place.

[00:23:39]Uh, we don't really use this term much in the United States, and we'll use other names for it, like slick rock or force. Um. Oh, I forgot the name in northern Arizona that they use for it. So I'm not going to worry about this just because we don't use it that much. So we put all the pieces together. Here's your exposed bedrock. The Yamada, the wadi with the alluvium, the bergs hook, all of our pieces. Sparse vegetation. There's a tree, a tree, all of this. And our desert landscape.

[00:24:12]All right, well, we'll get into differential erosion and Zion National Park when we come back for the last segment.