The Science of Everything Podcast - Episode 129: Glaciers, Ice, and Groundwater

Episode Date: June 30, 2022

Concluding our series on geographic landforms, here I discuss glaciers, including their global distribution, formation, movement, and various glacial formations such as moraines, drumlins, and fjords. I also examine the role of ice in shaping periglacial landscapes, including the effects of permafrost and other frost action processes. I conclude with a brief discussion of groundwater, aquifers, and the hydrological cycle. 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 129, glaciers, ice, and groundwater. I'm your host, James Fodor. Now, this is our third and final in our recent little series of episodes looking at geography and geomorphology and landscapes. And in this one, we're going to be looking at glaciers, ice, and also groundwater. And so we're going to start off by talking about glaciers and where they are found and how they form. and then we'll talk about as we have been doing the different types of effects that glaciers have on the landscape, so erosional transportation and depositional processes. We'll then talk a little bit about periglacial landscapes, so permafrost and the actions of ice

and other effects that that has on the landscape, and conclude by talking a bit about the hydrological cycle and groundwater, which is sort of a continuation on from talking about the effects of permafrost. No very specific recommended pre-listing episodes today, although if you've listened to the past couple of episodes in the series, you'll have kind of a background as to the style of discussion and the general direction that we're taking. So, that being said, let's make a start and talk a bit about glaciers. So I'm sure you've all heard of a glacier before, but maybe don't know so much about them unless you live in an environment where they are fairly common. I live in Australia, where there are, I don't think there are any glaciers in continental Australia, actually. certainly no very large ones. Mostly glaciers are found in

Greenland, Alaska, parts of Canada and Russia. A few other places as well, but those are some of the where the biggest ones are. So whether or not you're familiar with them will depend on what part of the world you're from. But a glacier is a persistent body of ice that is constantly moving under its own weight.

So the easiest way to think about what a glacial is is essentially that it is a frozen river. It's not like a frozen lake, It's not a static body of water. It's constantly flowing downhill, albeit fairly slowly, under its own weight. So glaciers move over time, and we'll talk a bit more about that later. About 11% of Earth's land area is covered by glaciers or ice sheets. And that's down from about 30% during the past the most recent glacial maximum period. But even so, 11% is still quite substantial. And glacial ice is the largest reservoir of, fresh water on earth. It holds about 70% of all the world's fresh water is stored in glaciers and

ice sheets. Just for a bit of context, when we talk about glaciers and ice sheets and the past global maximum, I think I've talked about this in the past, but it bears repeating because this is widely misunderstood. The earth is presently experiencing what's called the quaternary glaciation, in other words, an ice age. This began about, I think, one and a half-ish million years ago. Ice age is typically last for one or two million years. I think this is about the fifth one in Earth's history-ish. An ice age is defined as a period when basically there's a substantial quantity of ice at the Earth's poles, because those are the culter parts of the planet.

So we've been in one of those for over a million years. However, during a glacial period, or an ice age, the variations in the Earth's orbit about the Sun, called Molankovic cycles, as well as some other factors, but particularly the Malankovic cycles, result in periods of relative warming and relative cooling over a number of different cycles. I think there's like a 10,000 year cycle and a 100,000 year cycle. Usually what you'll see in Ice Ages is that there will be a roughly 100,000 year cycle of glaciation and interglacial periods. Currently, we are in what is called an interglacial period of the quaternary glaciation. And this started about 10,000 years ago.

So what this means is it's a relatively warm period where the glaciers retreat and the, world is a bit hotter, but there still is substantial amounts of ice at the poles, and therefore we're still in technically an ice age or a glaciation period. So an interglacial period of an ice age. So technically we are still in an ice age, even though often we talk about the ice age, with reference to pre-roughly 10,000 years ago, when we were, before the interglacial period began, before the relative warming period. Back during the glacial maximum or around that period of time, around like 20, 30,000 years ago, large portions of the, the Northern Hemisphere, particularly in continental Europe and in North America, were covered

by huge glaciers, huge continental glaciers, which we'll talk about a bit more later. Currently, we still have glaciers, but they're much smaller than they used to be, and they're shrinking every year because of global warming. So glaciers don't form everywhere where it's cold. There are some regions which are called polar deserts where there's insufficient precipitation, meaning that there's not enough snowfall feed the glaciers, because glaciers are ice, and so you need the snowfall to feed them. Cold air isn't sufficient. You also, you also. also need the moisture there. And so they don't happen everywhere where it's very cold, only when there's sufficient snowfall. Now there are two major types of glaciers, alpine glaciers and continental

glaciers, also called ice sheets. Alpine glaciers, named after the Alps in Europe, so they're found in mountain ranges, often in the valleys between peaks or ridges around, like surrounding different sections of the mountain or different. Most alpine glaciers originate from a high mountain snowfield, so that's up high near the peaks of the mountains. So they originate in a snow field there where a lot of snow builds up and then it gradually flows downhill. Over time and as the snow moves, it becomes compacted and increases in density.

And so it forms this mostly solid mass, which is the glacier. And that slowly moves downhill. We'll talk a little bit in a moment about how it moves. But for the moment, just think of it slowly sinking downhill or sliding downhill if you like. Eventually, the glaciers reach a sufficiently low altitude where the temperature is higher, because obviously its temperature is lower up in the mountain peaks and it increases as you as you get to lower altitudes. And at that point, the snow begins to melt faster than it's accumulating.

And so that will be the point where the glacier terminates. That's called the terminal part of the glacier. So glaciers are particularly alpine glaciers, are in a constant state of trying to reach equilibrium, like many systems. So you have the upper parts of the glacier, which is in a higher altitude, where it's colder, and it's receiving more snow to accumulate. So it's called the accumulation zone, where basically the mass of the glacier is being built up, because the rate of accumulation is faster than the rate of evaporation and melting. And then at lower altitudes, what's called the ablation zone, the rate of melting and evaporation is faster than the rate of accumulation in those zones. Obviously, it's seasonal as well, because typically glaciers gain mass in the wind. winter and then they lose it during the summer, so it varies year to year. But if you sort of average

over that, you can think of parts of the glacial, the upper part, which is the accumulation zone, and the lower part, which is the ablation zone. And there's a line in the middle, which is called the equilibrium line. And that's the line where it's just exactly balanced. Basically, you can think of points of ice or small regions of ice at the equilibrium line have an exactly equal rate of accumulation of new snow and melting and evaporation of existing, ice and snow over say the course of a year. And so that part of the glacier is exactly in equilibrium. Now, that particular, like if you imagine putting a flag on the equilibrium line, that particular point is going to move down to lower altages because the entire glacier is

moving downhill. But the equilibrium line, you can imagine it as it's not associated with the specific flag that you've put in place, right? But you can imagine if you're sort of hovering above and you had a laser marker marking a point on the glacier, right? The specific ice will move down, but that part of the glacier will stay around the same position over long periods of time, assuming there's no overall changes in the mass of the glacier. So if the glacier is expanding or contracting over time with, say, climate change, then the equilibrium line will move. But if it's in equilibrium over long periods of time, the equilibrium line will stay in the same position.

So it's like cars on a road, right? the marking you can think of as the equilibrium line which is kind of on the road that the specific cars will keep moving past it which is analogous to the particular regions of of ice which continually move down the glacier but the equilibrium line will stay in the same place assuming that the glacier is not receding or expanding over time so that's an alpine glacier we'll talk a bit more about some of the features of alpine glaciers shortly but i want to introduce continental glaciers as i said also known as ice sheets these are much much larger than alpine glaciers we'll talk a bit more Alpine glaciers. So Alpine glaciers are found in particular regions of mountains, right? And they can be

fairly large. But continental glaciers are enormous. So these cover not just mountain terrain, but an entire region over an area greater than 50,000 square kilometres. Currently, the only continental glaciers on earth are found in Antarctica, which cover much of the continent, and Greenland, which cover much of, well, technically it's an island. In the past, there were, as I said before, large continental glaciers covering much of North America and Northern Europe as well, as well as one in the southern part of South America, the Patagonian ice sheet. But those have subsequently melted and retreated to maybe a few alpine glaciers in those regions. But because of the warming that's occurred in the last a few tens of thousands of years, those continental

glaciers or ice sheets don't exist anymore. So currently we've just got the two, the Antarctic ice sheet and the Greenland ice sheet. The Antarctic ice sheet is the largest. single mass of ice on Earth. It covers an area of about 14 million square kilometers. About 90% of all the mass of ice ice sheet is in Antarctica, in the Antarctic ice sheet. If melted, it would raise sea levels by about 60 meters, which is pretty substantial. The Greenland ice sheet is the other continental glacier that exists today. It occupies about 80% of the surface of Greenland, and if melted, it would raise sea levels by about 7 meters. So it's still quite a lot, but it's really puny compared to the Antarctic ice sheet. Remember that I said that glacial ice contains about 70% of all the world's fresh water

and 90% of all the earth's ice masses in Antarctica, which means that, you know, about two-thirds, give or take, of all of the freshwater on Earth is in the ice in Antarctica, which is pretty interesting, I think. So those are the two different types of glaciers, alpine and continental. Now, there's one other phenomenon that I wanted to talk about, which is technically not a glacier, but it's related, so I'll mention it here, and this is sea ice. Sea ice is essentially just frozen seawater. Because ice is less dense than water, or than liquid water,

frozen seawater floats on the ocean surface. That occurs in freshwater as well. And sea ice covers about 7% of the earth's surface, or about 12% of the surface of the ocean. Obviously, it only occurs over the ocean. So most of the world's sea ice is enclosed within the polar ice packs, either of the North Pole in the Arctic or the South Pole around the Antarctic. The north and south poles are different because

well, the South Pole is located in Antarctica, which is a continent that has a continental glacier or ice sheet over the top of it. And then there is sea ice that surrounds the continent of Antarctica. Sea ice freezes during winter and then much of it melts during summer, although there's still always some sea ice around, depending on season and so forth. But the Arctic is different because there's no continent in the Arctic. It's sort of surrounded by the continents of, you know, like Greenland and Northern Canada, and Russia in a way that the South Pole isn't, right? But the actual region of the Earth, where the pole is, is actually over ocean. So the Arctic is covered by sea ice. The actual

North Pole itself is covered by sea ice, and the extent of the sea ice expands and contracts with the season. So it's extremely extensive during winter, and it's much reduced during summer. But there's always some sea ice, even at summer at the North Pole. So sea ice typically isn't very thick, and this is the main difference between sea ice and a continental glacier. If you just sort of look at it from a satellite photo, you may not really appreciate the difference. Continental glaciers can be kilometers thick. They are very, very substantial, huge blocks of ice. So that's Antarctica Greenland. Whereas sea ice is generally quite thin. It's often only a few meters thick. It can be more than that. I don't know exactly what the limit of sea ice is,

but it's typically not very thick. So you can have ships called ice breakers that are able to move through sea ice. They need to be specially designed to do so, but they can do that because the the layer of ice on the surface of the sea is not that thick, whereas you can't take a ship through Antarctica, right? It's a continent, like that's not going to work. So it's really quite different in terms of what's happening there. Now, another thing to know about is an ice shelf. So an ice shelf is basically a bit of sea ice that's connected to a glacier or ice sheet. As the glacier flows down the coastline and onto the ocean surface, it kind of forms an ice shelf. So ice shelves are only found off the coasts of Antarctica, Greenland, Canada and the Russian Arctic.

So they're not found in too many different places. But again, there is a difference between an ice shelf and sea ice. So an ice shelf is basically the bit or the section of a glacier, typically continental. I'm not sure if they're alpine. I think some alpine glaciers run into the sea as well. but an ice shelf is where you have a bit of a glacier that's basically overflowing onto the sea, and so that forms the ice shelf, whereas sea ice is just frozen ocean water at the top layer of the ocean. So again, ice shafts tend to be much thicker than sea ice. The boundary between the ice shelf, which is floating on the surface of the water, and what's called the anchor ice, which is the land-based glacier that it's come from,

is called the grounding line. So there's essentially a line where it goes from being a glacier to being an ice shelf. And ice shafts can be quite thick. They can be like 100 metres to a kilometre thick. Again, because glaciers can be extremely thick, especially continental glaciers. And what's happening is because there's a continual accumulation of new snow falling at the source regions of the glacier. That pushes down the rest of the glacier. So it's moving down the slope over time and pushing the lower parts of it into the ocean.

So it's like a conveyor belt process. It's constantly pushing out the ice below it. And so eventually what happens is that the seawood front of the ice shafts carves off due to melting by the seawater or currents or other factors or just sort of tensional forces. Eventually it will cleave off and that forms an iceberg. So again, an iceberg is not sea ice. It's not formed from sea ice. I mean, I don't know if it's ever possible for icebergs to form from sea ice. But for the most part, icebergs are formed from bits of ice shelves that cleave off and then start floating around the ocean effectively. with currents. Sea ice is typically not that dangerous because it's not that thick, and if you're prepared for it, you know, one can deal with that. There are ships that are designed to move through it, whereas icebergs can be extremely large. And as you probably know, only about what, I think it's one seventh or one eighth of the volume of an iceberg is

actually above the surface of the water. Most of it is below the surface of the water because of the buoyancy of ice. So you won't see much of the iceberg. So icebergs come from glaciers that spill out over the ocean and then eventually forming an ice shelf and then eventually carve off to form icebergs, which then float around and eventually melt away, whereas sea ice is just frozen ocean that mostly occurs around the coast of Antarctica and the North Pole. All right, hopefully that's helpful for sort of distinguishing some of these different phenomena here. Let's now move on to talk about glacial processes or a bit more about how glaciers work and how they move and how they form over time and so forth. So how do glaciers form and sort of

survive. Well, to understand this, we need to introduce a concept called glacial mass balance. So this is just the difference between the accumulation and ablation. So ablation is the rate of loss of the mass of the glacier due to sublimation and melting. So melting, you know what that is. Sublimation is directly going from a solid to a gas state. So in this case, it's the ice directly going to a gas. That takes more energy than melting. So I'll often just say melting because the distinction is not too important for us here, but just bear in mind that sometimes the ice sublimes directly to a gas. So what we have is a situation where the glacier, as I've said, at the lower altitudes is ablating faster than it's accumulating, and the higher altitudes

is accumulating faster than it's ablating. There's an equilibrium line in between those. I also mention that this is seasonal. So during the summer, the glacier as a whole typically has positive ablation, so it has a net mass loss during the summer and a net mass gain during the winter. Overall, over the course of the year, you might expect that to be roughly balanced if the glacier is sort of neither expanding nor retreating. But these days, and since the 1850s, actually, most glaciers in the world have been retreating. And that's been accelerated dramatically in the past 30 or 40 years. In fact, you can see interesting compilations of images online of the rate of glacier retreat. It's quite, I was going to say, compelling, but not in a positive way, like a disconcerting

and compelling at the same time, just the rate at which these huge, masses of ice are being lost. But that's how we understand the change of glaciers over time is through the mass balance. Now, when we talk about movement of glaciers, there's several mechanisms by which glaciers move, because they're basically a huge, solid mass of ice. So movement is not exactly easy, but because they are so massive, there's a huge force that's pulling them downhill, and that's the ultimate source of why they move, right? So they flow downhill under the force of gravity. However, because glaciers are so thick, there's actually different properties of the ice at different depths through the glacier itself. So there's a few different phenomena that we can

investigate to sort of understand how exactly glaciers move. And that, again, it kind of depends on how deep you're looking through the glacier, like whether it's the top, kind of in the middle or down at the base right near the sediment that it's moving along. So let's sort of start from the bottom. At the bottom, there's a phenomenon called basal sliding. So basal just referring to the base, the bottom of the glacier, where it's contacting the sediment underneath. Basel sliding is the act of a glacier sliding over the underlying bed due to melt water under the ice acting as a lubricant. So basically when the ice contacts the bed underneath, there's obviously huge pressure from all the ice above it. That pressure pushing down plus the fact that the sediment is not part of the ice layers, a separate layer.

That melts a small amount of ice forming a lubricant, essentially, that allows the mass of the glacier to slide. slowly downhill. So that's called basal sliding. It's facilitated by the lubrication of meltwater. Then there's bed deformation. So again, the bed is just like the sediment or rock that's underneath the glacier. And because of the huge forces that are applying to the bed by the glacier, that's basically it's pulling the bed underneath it along with it as it slides downhill.

You can think of this as if you place a, this is not a perfect example, but if you place a heavy piece of furniture or object on a carpet, which is in turn on placed on like the floorboards or something, if you push the piece of furniture, the carpet will move with it, like it will slide, I suppose it depends on the exact carpet, but it may slide along the floorboards with the heavy object. That's kind of what's happening in this case, although it's not so separate as like the floorboards from the piece of carpet. What's rather happening is that the bed deforms, right? So the uppermost layers of the underlying bed, especially if they're like sedimentary layers. they're kind of stratified, will shear off and it will kind of like bend forwards along the

direction of flow of the glacier. So the bed itself gradually deforms, which helps to move the overlying glacier along with it. And again, that's coming about essentially because of kind of frictional forces, plus just the sheer force and mass of the overlying glacier. Thirdly, we have internal deformation. So we just talk about bed deformation. This is the change in the sort of shape and shearing of the bed. But that also happens to the glacier itself. itself. So this, for the same reason, because of the weight of the overlying ice, causes deformation of the ice crystals. Again, that kind of deforming and movement and bending helps the glacier to kind of move downhill. It also causes cracking in the uppermost levels of the glacier,

basically because the glacier tends to be brittle around the top and more plastic down the bottom because the difference is in pressure. Like if you're at the top, the pressure is not very high because there's not much ice on the top of you, whereas if you're right down near the base, the pressure is very high. And so the ice, becomes more plastic. Plastic, remember, meaning that it can sort of bend in shape, whereas brittle, it kind of cracks. So what tends to happen is that near the top of the glacier, the ice cracks and forms crevices, which are these basically big vertical holes in the ice as it's kind of cracking. Just like, you know, if you cook bread or a cake or something in the oven,

cake's probably a better example. You may see that it forms cracks, depending on how well you follow the recipe, I suppose. And that's basically just because it's, it's sort of, sort of becomes, as it dries out, it becomes a bit more brittle and parts of it like crack and separate from each other. Again, that's a loose analogy for what's happening here. The basal, the lower parts of the glacier, become plastic and they are kind of deforming and also slipping along with basal sliding, whereas the uppermost parts, because they are not as plastic, they are being kind of bent and then pulled apart because they're kind of breaking as the lower parts are sliding and bending around. So the glacier is actually structurally quite complicated because you've got these different

layers that behave differently. And internal deformation near the base helps the glacier to move downhill as it's kind of bending and warping. And the final phenomena is glacial quakes. And these are sudden shifts in the glacier of up to a meter in a very short period of time causing a local earthquake. So these are non-s seismic quakes because they are not caused by plague technotics. They're just caused by this huge mass of ice that's sliding downhill. glaciers typically move quite slowly around one meter per day but as I said during these glacial quakes they can move like a meter in very short periods of time so they're moving much faster

also remember that when we say that glaciers move at around one meter per day that doesn't mean the whole glacier moves one meter downhill it means that the regions of ice move one meter downhill but at the very lowest parts of the glacier the terminus of the glacier at the end of the ablation zone, that part is evaporating faster than it's being replenished. Whereas at the accumulation zone, we're adding to it and extending the glacier, if you like. So we're sort of adding to one end and chopping off the other end and the whole thing's then moving down. So bear in mind that when we say it's moving a meter a day, that doesn't mean that after a year,

the whole glaciers move 300 meters downhill because we're also taking off one from one end and adding to the top end at the same time. And that's the equilibrium that we talked about, mass balance. There are also times when the glaciers can move very substantially in short periods of time. And these are called periods of glacial surge. Glacias can move up to 100 times faster than normal, maybe at 10 kilometers a year, which is extremely rapid for a glacier. From what I understand, this is not fully understood as to why or how this happens,

but it's thought to be due to factors such as the availability of meltwater as a lubricant, deformation of the bed, and other factors like that that maybe can reach a critical threshold for a period of time, where it's able to move much faster, and then that sort of lapses and returns to slower rates of movement. Okay, so we've talked a bit about glacial mass balance and how glaciers move, particularly basal sliding, bed deformation and internal deformation are the ones to think about there. So it's important to bear in mind the nature of this sort of plastic bending and sliding along the bottom and the brittle cracking on the top of the glacier.

As we talk about, its effect on the different landforms, because that's the sort of process that brings about these landforms. effect. So as in the previous couple of episodes, we're going to focus on the effect that glaciers have on the landscapes that they exist in. And again, just like in the previous episodes, we sort of break this up into three different categories. There's erosion, transportation, and deposition. So again, erosion is where pieces of rock or sediment are broken off, and where sediment is, where rock is like broken down into smaller pieces. So that's erosion. Then transportation is moving those from the location where they were eroded to somewhere else, generally downhill.

And then deposition is when they're deposited and a new landscape feature or formation occurs as a result of deposition. So we talked about how rivers engage in these erosional transportation and depositional processes. We talked about how wind engages in these processes in the context of deserts. Now we're going to talk about how glaciers engage in these processes in the context of glacial landforms. And again, these glacial landforms are most prominent in areas of mostly northern latitudes where there are larger numbers of glaciers or where there were glaciers in relatively recent geological times.

Because remember, like 20,000 years ago, is very recent in geological time. So the fact that the Ice Age, you know, the last glacial maximum was a while ago in terms of like historical time, that in geological time is still extremely recent. So we're still seeing many of these landforms in like. parts of like Canada, the United States, Russia, and parts of continental Europe today, even though there are no glaciers there now. Okay, so let's start with erosional landforms or erosional processes. The most basic phenomenon is glacial striations. So this is basically lots of marks and scratches

on rocks that were once located underneath a glacier. So as the glaciers grow, the huge weight of snow and ice crushes and abrades and scours the surfaces beneath them. and that produces characteristic patterns of this sort of scouring. And you can look up images of these. It's quite interesting. This general process, though, apart from producing these sort of striations, also leads to other more specific phenomena, which I'm going to talk about, well, now. Also, I will mention that I may get the pronunciations of some of these a little wrong, because many of them are derived from non-English words, so please bear with me there.

So the first erosion land feature that we're going to talk about is called a CERC. It's spelled C-I-R-Q-U-Y-E. so I think it might be a French word. And this is a kind of a scooped out small valley. Well, I mean, they can be relatively large, but like a relatively shallow valley, typically, that's located at the head of the valley where glaciers originate. So it looks like kind of a scooped out region, kind of like a little shallow bowl or plate or something in the landscape.

Basically, this was a region where the glaciers originated, where there was in the accumulation zone. In the past, when there was a glacier in the region. as the glaciers have retreated, then these cirques are left over. The actual valley where the main body of the glacier was moving downhill, because of its shape and huge mass, it tends to scour out the valley and form what's called a U-shaped valley. So they have a characteristic U-shaped cross-section.

So they have very steep, straight sides and a relatively flat bottom. By contrast, valleys formed by rivers or carved out by rivers tend to have a V shape, as most of the erosion occurs just like at the bottom, where the water is passing through. But in the case of a glacier, it's spread out. The ice spreads out over a wider region. Obviously, it flows much more slowly, and so it has a different mechanical action to a stream.

So it tends to scrape out and gouge out these very sort of deep and wide, U-shaped cross-sectional valleys, forming a U-shaped valley, carves it out as it sort of scours down and roads away the rock and sediment. Now, when the ice recedes, the valley remains, and we call it a U-shaped valley, and it will be littered with small boulders and other pieces of sediment. that were transported with the ice. When a valley like this fills with ocean water, so creating an inlet from the ocean,

these valleys are called fjords. So that's what a fjord is. It's basically the leftover scraped out, gouged out, U-shaped valley from a past glacier. Norway has a lot of fjords because back during around the period of the last glacial maximum, it was essentially covered with a continental ice sheet. Next, erosional land feature we're going to talk about is called an errate. It's spelled A-R. E with a diacritic on it and a T.E. I'm not sure which language that's from. Now, an array is a narrow ridge of rock that separates two valleys. So you can think of it as formed when, I mean, this is how they are typically form. Two glaciers erode parallel U-shaped valleys. And so there'll be kind of a peak, a very narrow ridge that separates them, right? The two bits of a U that are kind of next to each other. So that's called an array. Again, when the glaciers recede, you don't really see the glaciers aren't there anymore, but you still see these ridges.

The next landform that we're going to look at is called a Roche Mutine. This is a rock formation created by the passing of a glacier. It's a little bit hard to explain what it looks like without sort of showing you an image. But basically, it's a characteristically shaped rock formation, which has its shape because of the way that the glacier interacts with it. So these are formed at the base of a glacier. The glacier moves over them and erodes over the top. And basically what happens is that there's a, there's kind of an ups and a ups and a glacier. stream and a downstream side, so they're asymmetrical. They look a little bit like the shape of the

wings of an aircraft, right, where you have a sort of slow tapering towards the back, but towards the front, it kind of rounds and is more sort of blunted. And so essentially what the glacier does is it is that it grinds and forms striations over, as it abrades over the tapered part, and then around the front, it kind of plonks down. I don't exactly know how to describe the action, but it kind of pushes down. And so it kind of rounds out that part. And that's where you observe a process, called plucking, which I think we'll talk about in a moment. So anyway, it's a little bit hard to explain, but mutinase, these are special rock formations that are caused by the passing over of glacier over an underlying region of bedrock. So the point is that it causes asymmetric erosion

resulting in the abrasion on one side and then the plucking on the other side. Finally, we've got drumlins. Drumlins are also asymmetrical, but they're shaped differently. So they, again, also found kind of at the base of valleys where previously glaciers passed through. But they are shaped more like a canoe. They have heights of maybe like 10 to 50 meters, but they can be very long, like a kilometer in length. And they're formed by continental glaciers as they move across the landscape. And basically, if there's any kind of underlying bit of rock or sediment that forms a lump,

that's basically then dragged across and streamed out by the glacier. And so you have this sort of tapering canoe shape. All right. So that describes some of the erosional process. It's probably the most important one there to keep in mind is the U-shaped valley, because that really encapsulates how glaciers move down. and really carve out the landscape. Now let's talk about some of the mechanisms by which glaciers transport sediment

because they are very effective at transporting large quantities of sediment. And probably the most important aspect of this is that unlike liquid water, so unlike streams, glaciers do not sort sediment. So they don't deposit sediment that's sorted by its size. Now that tends to happen with most streams essentially because as the streams lose velocity, as they lose energy, the heaviest, biggest, sediments are dropped out first and then the slightly smaller ones are slightly smaller ones than that. And so that's why you have this sorting, as it's called. We talked about that a couple of

episodes ago. But glaciers don't do that because they're solid, right? There's no sense in which the larger pieces of sediment drop out first compared to the smaller ones. They're all carried at the same time. This what's called undifferentiated material, this mixture of sediments of all sorts of different sizes from clay right up to boulders, is called till. And so this glacial till is the material that's deposited from glacial ice when it recedes. This is distinct from fluvial sediments which are typically stratified by size. So glacial till is undifferentiated or not stratified by size. And so that's one characteristic of glacial landscapes is if you have a lot of this undifferentiated mixed sedimentary material lying around. And I'll talk about more of that in a

moment when we get to deposition. But in terms of the transportation mechanisms itself, how it is it transport this material, we've just looked at the erosional processes, that grind down and break apart the sediment from the surrounding rocks, the glaciers then transport that material in two main different ways. Plucking and abrasion. And I mentioned these just before with reference to the Roche Mutane. So let's go over that again. Plucking occurs when glaciers flow of a bedrock and then they are able to soften and then lift up pieces of rock into the ice. Probably the easiest way to think about this is if the glacier is sort of flowing over a piece of underlying bedrock, which is sort of slightly elevated relative to surrounding reach.

imagine if the glacier kind of moves across it and then because it's then moving down off the elevated piece of rock it kind of lands with a thud if you like it falls down over the other side at a relatively small distance that then at the edge that kind of plucks off bits of rock as the glacier kind of bumps over that it plucks off bits at the end and those pieces of rock gradually loosened and then broken off and are lifted up become embedded in the glacier so those are carried on at the lower part, around the base of the glacier, and they form part of its sedimentary load. These plucked off bits of rock can be of all sizes. They can be like large boulders or, you know, small sand grains or even even clay. So that's plucking. Abration is different. That occurs more

on the upstream side of the mutinase, and that results from where the glacier is grinding. The base of the glacier grinds against the rock at the base and forms, you know, striations. But it also, it sort of smooths, polishes, and also forms these striations in the underlying bedrock, and that the material that's removed as it's doing that, as it's kind of forming these striations, smoothing, polishing, and sand-propering the rock beneath it, it pulverizes the rock and produces something called rock powder, which is made up a very tiny rock grains. These then add more material, which is carried by the glacier, it becomes embedded in the base and carried along. These processes don't always occur just at the base of the glacier.

They can also occur around the sides of the valley. They can also occur at a base but that might be at a higher altitude that then, you know, because different glaciers then can converge and merge with each other, right? So it may have been the base of a glacier higher up, which then kind of becomes the top of the larger glacier as it merges down. So the point is, even if sediments and rock powder was originally created at the glacial base, it can end up on the top of the glacier. Or it can be kind of, it can end up on the sides of the glacier as these processes also occur at the sides of the valley as the glacier moves down and, you know, gouges out this U-shaped valley.

So this moves us into then deposition, although, again, this distinction is a little bit fuzzy in the middle. But now, well, what happens to all of this sediment, this glacial till that is formed as the glacier gouges out and plucks off and abrades the rock and the sediment surrounding it? Well, there's different ways that these sediments can be deposited and different landforms that can result, unsurprisingly. I'll start by talking about a moraine, which you may have heard of before, probably one of the

better known landforms of glaciers. So a moraine is any accumulation of unconsolidated debris, or glacial till, we talked about that before. That's the unsorted sediment. That occurs kind of in lines, right? Either along the sides or at the edge of a current glacier or of a past glacier. So moraines can exist on both present day and also in the sites of past glaciers. So there's different, as I said, there's different places they can occur. moraines are typically always kind of like long straight lines or relatively straight lines but they can occur either down along the edges of a glacier as a result of the material built up with the interaction of the glacier with the sides of its valley or they can occur along the middle of a glacier this is called a medial moraine and this happens when you have the merger of two different glaciers or multiple contributory like tributary glaciers merging together as the valleys connect and it gradually flows downhill. Just like you have different tributaries that then come into and combine into a river, you have the same thing with glaciers, right? As different valleys merge together

as you sort of move downhill or down the mountain. So the lateral or edge, like side moraines, again, like lines of rock essentially and sediment, then kind of merged together and you can get a moraine that's then in the middle of the larger glacier. So that's called a medial moraine. In addition to these moraines along the edges and along the middle of a glacier, you also typically have one at the end of a glacier. That's called the terminal moraine. And that basically represents the rock and sediment that's being deposited at the very end of the glacier when it's sort of reached as far as it gets downhill.

The terminal moraine may move over time as the glacier expands. Then that rock and sediment may be entrained again in the glacier, taken up again as it gradually moves downhill. Or if the glacier is a retreating, which is much more common these days, you may uncover more terminal moraines as gradually the glacier. retreats uphill. There's also something called ground moraine, which is basically just a moraine that's spread over a large area, not confined to like a particular line along the edges, along the center, or along the end of the glacier. So moraines are important because they kind of tell you about

the shape and structure and trajectory, if you like, of the glacier. And they can show you in the past where the terminus of the glacier was based on looking at its terminal moraine. Okay, so, but basically though moraines occur when the glacier reaches a certain point and then drops the rocks there, or the rocks in sediment is still like sitting on the glacier, as is the case for like the medial moraine, for example, along a glacier, it may be sitting on top of it, or it may be in a position where the glacier used to be, but it's since retreated. There are, however, many other important depositional features of glaciers. Eskers are long winding ridges of stratified sand and gravel.

Frequently, they're several kilometres long and they are. often used as like walking tracks today because they kind of suit that purpose. They're just like elevated but relatively narrow and they kind of wind through the landscape. They look a bit like railway embankments because of their relatively uniform shape. Eskers are formed mostly from ice wall tunnels of streams that used to flow either within or underneath the glacier. Basically, for whatever reason, slight differences in temperature pressure or composition or whatever else, there will be some parts of the glacier either embedded inside it or often at the base just under the glacier, like due to some melt water at the base there, where there were streams

that ran through the glacier, so liquid water, and it was surrounded by ice wall, right? As that stream passed under the glacier, it would, as all streams do, deposit certain amounts of sediment. Eventually, either if that stream froze or dried up or whatever, or if the glacier retreated, which is, again, the case today with many of these landscapes that have been uncovered by the retreat of the continental glaciers since the end of the last global maximum. With the retreat of these glaciers, what we see is the leftover deposits from those streams that once flowed underneath the glaciers. And so these form these long winding ridges because they were following the glacier as it went downhill.

So that's pretty cool, right? So Eskers, they're kind of laid down mostly like parallel to the glacier or along its path down hill. Okay, so we talked about Eskers. Kame's. Kame is an irregularly shaped hill or mound that's composed of sand. gravel and till, so again a mixture of different sediment sizes, that accumulates in a depression on a retreating glacier and is then deposited on a land surface as the glacier retreats.

So basically, if you have a glacier, like anything else, there's going to be some regions on the top where it's, you know, a little bit higher and some reasons where it's a little bit lower. I mean, they're usually relatively flat, but they're not going to be perfectly flat, right? In some relatively small depressions on the surface of the glacier, you may have an accumulation of sediment that kind of fills up that little, little, little hot. hole, right, little depression. While the glaciers around, then it's just sort of a bit of till that's located in a little hole on the top of the glacier. But the thing is, if the glacier retreats, or if it melts, then that accumulation of sediments may be deposited at a particular

location, like as the glacier retreats eventually, it's just sort of dumped out, and it will be dumped on the surface of the land in a particular location, so it forms this little hill or mound. In some sense, the opposite of a cane is called a kettle lake, or sometimes just kettles. And this is a depression or a hole that's been left by a retreating glacier. So often if they're filled with water, they're called a kettle lake, although sometimes they dry out and then, well, it's not really a lake anymore. But so just as a cane is basically a bunch of sediment that's been left behind by a retreating glacier, a kettle lake originally is a result of a bit of ice, blocks of ice that are left behind by retreating glaciers, which then become big. buried in sediment and kind of form a hole in the ground or a partial hole in the ground.

And then there's some sediment maybe falls on top of them or surrounding them. Eventually, of course, the ice melts if the region is warming. And what's left behind is a little depression. And that's called a kettle hole, which if it's filled water, it's called a kettle lake. So this gives the landscape a dimpled appearance. It's very rough. Like there's all these little holes or depressions that are due to the kettle lakes or kettle holes. And then all of these little mounds, which are the cames.

and then these winding through it are these eskers, these long winding ridges that are, remember, formed by the sediment left over by streams, which originally ran under the glacier. In addition to these sort of moraines and other piles of sediments that have been left over, so post-glacial landscapes are highly pocked and disrupted as a result of the action of the glacier. That's why there have been sort of subject to a lot of study and a very characteristic of the landscape. And again, all of these processes can be understood in terms of the more fundamental processes of erosion, transportation, and deposition, which we've been talking about for the last few episodes. All ultimately driven by the energy of the sun, right, as the ice falls on the uppermost regions of the glacier, which, you know, the ice originally was evaporated because of the power of the sun, and then it was moved to that part of the earth by wind, which is also powered by a differential heating of the earth by the sun, and then it falls and then gradually moves. under the force of gravity downhill, and that drives all of the forces of erosion and transportation that leads to these landforms that we talked about. So again, all of this is driven by the power of the sun. All right, that concludes a discussion of glaciers per se, but I want to also talk a bit

about what are called periglacial landscapes, and these are basically land regions that are not necessarily directly affected by glaciers, although they might be, but are kind of nearby, or in other In other words, they're just very cold regions that are affected by, particularly frozen, like permafrost is one of the main characteristic features of periglacial processes and other effects of ice. So let's talk a bit about this. Permafrost is ground that is continually frozen, so below zero degrees, for two or more years in succession, so continuously frozen. It can be located on land or under the ocean, although usually we focus on permafrost on the land.

about 15% of the northern hemisphere, or 11% of the surface of the earth as a whole, is underlain by permafrost, including substantial regions of Alaska, Greenland, Canada, and Siberia, and those will be familiar now because those are also the main regions where we see large glaciers. So permafrost has quite a lot of structure to it, actually. It's not, well, I mean, it is frozen ground, but there's a bit more to it than that, right? So at the very top of the soil, or sediment, if there's not a lot of organic matter, but I'll just say soil. So at the very top layer of the soil, you have what's called an active layer. And it's called active because this layer here freezes and thaws annually.

And the reason for that is because it's so close to the surface that it's subject to the temperature variations that occur because of weather and climate patterns right above it. So it's active because it's close enough to that to be substantially affected by those changes of temperature in the atmosphere above it. So it's active, right? It's constantly freezing and thawing. I should say the active layer doesn't exist if the area was covered by ice. So like in Antarctica or regions where there's a continental glacier, you don't have this active layer, right? But I'm talking about regions that are not covered in ice, then you have this active layer of the soil or sediment that is just below the surface that freezes and thaws annually. So beneath the active layer, you have a region which is always frozen year round.

And so this is the start of the permafrost, right? However, the parts of the permafrost that are just below the active layer still have a fair amount of seasonal variation in temperature because they're closer to the surface, right? So enough for them to be affected by the temperature. It's just that they never quite get warm enough to thaw. So that's what differentiates the boundary to the active layer, right? Does it ever get warm enough over the course of the year to thaw? If the answer is yes, then it's part of the active layer, right? Which is not actually permafrost.

Below that, if it never thaws, then it's part of the permafrost. But it still kind of varies in terms of temperature. Eventually, if you go down far enough, you reach what's called the isothermal permafrost. That's a region where the temperature does not vary from season to season, because it's far enough below the surface of the ground that it's not really affected very much by what's happening on the surface, really at all. And so at that point, there's no fluctuation in temperature based on the seasons. I mean, over long periods of time, with climate change, it may change, but it doesn't fluctuate from season to season. If you keep going down further and further, going down through the isothermal permafrost, you will eventually reach a point where the temperature has increased to a point where the soil is frost-free.

And so that's the end of the permafrost or the base of the permafrost. Remember that because of the effects of geothermal warming, as you go down further into below the surface, the temperature increases. When you're near the surface of the earth, this effect is swamped by seasonal and also daily, variations in the temperature, right? So you don't really observe that for for relatively shallow depths. And this is the case for permafrost regions as well, where the active layer, the daily and also seasonal variations in the temperature of the active layer, swamp any effects of increasing warmth with depth, right? But when you get to the isothermal permafrost region, which, remember,

doesn't change seasonally, because it's too far down to be affected by that, then you get a gradual warming effect as you go down further and further, because it's closer to the center of the effectively and it starts to heat up gradually. It's pretty slow, but it does have an effect. And eventually you get to a point where it's warm enough that it never freezes. And that's the base of the permafrost. So there's sort of two competing effects here. There's the fluctuating effects of season and day in the active layer, and then to a lesser extent just below that. And then there's this isothermal permafrost, which is below that, which gradually warms up very slowly as you go further down, until a point is reached where it's too warm now for

the ground to freeze and then it's frost free of soil from there down. So why do we care about permafrost? Well, one of the interesting things about permafrost is that it has a strong effect or major effect really on how humans can interact or how they can build in those environments. Building on permafrost is very difficult because the heat of the building or pipeline or whatever it is that you built on top of it will typically warm up the permafrost and then partially melt the ground underneath it thereby destabilizing the structure. So what you have to do is build foundations on wooden piles, basically building on stilts. The transit atlaska pipeline, for example, uses heat pipes that are built on vertical supports over much of its length,

which prevent the pipeline from sinking down into the soil, because if you just put the pipes on the ground, then the heat from that would melt the permafrost over time, and then the pipeline would sink into the soil, and that would be disrupted. I think eventually it would rupture. which obviously is not a good thing. And you can see some Google images if you're interested of houses that were improperly built on permafrost soil, which then melt the ground and then sort of sink into the ground. Remember because ice freezes when it expands. And so if you melt it, the surface of the soil effectively forms a depression.

It goes down. That then causes whatever is built on top of it to kind of sink and break up. Another way around this problem, if you don't want to use stilts or supports, is to build on a very thick gravel insulated pad. which is another way to avoid this problem. Now, another aspect of the interaction of permafrost with humans is the fact that permafrost contains huge quantities of frozen organic material and frozen methane and other such compounds.

And the amount of carbon dioxide that's sequestered thus in permafrost is many times, I think about four times, the amount of carbon dioxide that's been released into the atmosphere by humans, like ever, like in modern times. So release of greenhouse gases by thawing permafrost is a major problem, problem. And it's one of the examples of a positive feedback to global warming, right? Because as the globe warms, permafrost melts, which releases greenhouse gases because of all of the organic material and methane and other things that were frozen there, which then contributes to global

warming, which then melts more permafrost, which then releases more greenhouse gases and so forth. So there are some people who are worried about tipping points where there may be such a rapid positive feedback as a result of melting permafrost that there's not necessarily very much we could do about it if we got to that point. I don't know exactly what the current state of the research is on the sort of tipping points and exactly how big a problem this is. In the future, I'll do a series of episodes on global warming where we'll talk about this. But for the moment, I just wanted to note that that's another interesting aspect of how permafrost affects humans or how it interacts with humans. All right, one final aspect of periglacial landscapes that I wanted to talk

about is the action of frost. So this is distinct from permafrost, which is frozen soil. frost also has other effects on periglacial landscapes. So one is called frost heaving, and this is basically where the frozen soil kind of thrusts and pushes upwards, because again, freezing means it increases in volume. And this can force layers of soil up by as much as 30 centimetres or something, which could cause significant problems, depending on what use the lands being put to. Ice wedges are also quite pronounced in periglacial regions. So this is where there's a small crack in the ground.

You have water that fills. that, it freezes, the freezing water expands, which pushes apart, cleaves apart the surrounding rock or sediment, widening the crack, then in the summer or during the day or whenever it is, that ice melts. Later on, it will be filled with more water, which then freezes and widens it further, right? So it's a gradual process of kind of squeezing and levering apart the land in that particular region. And this could result in really huge ice wedges, which freeze, as a result of freezing and thawing over periods of many years. exerting large forces and pressure on the surrounding soil.

Another interesting phenomena is called patterned ground. I mean, it's literally patterns in the ground, often geometric structures like squares or triangles or things like that. I mean, not perfect triangles. But just look it up on Google Images. It's easy than try to explain it. But there's geometric patterns that form in the ground with relatively symmetrical and relatively regular shapes.

Not perfect, but relatively so. And they look man-made, or some of them do at least. But they're not. They're natural phenomena. And they occur in certain types of soils and with certain sedimentary grain sizes. Basically what happens is if there's some regions where there are relatively finer sediments surrounded by some areas where there are slightly larger sediments, the water-saturated areas where there are fine sediments have a greater ability to expand

and contract as they freeze and thaw, because there's more water there, essentially. So they expand and contract more. So what that does is it kind of pushes apart the regions surrounding it. So basically think of it this way. There's certain regions that have larger sediments. They don't expand and contract as much, whereas the regions with the smaller sediments, they expand and contract more.

So they push into and push up against the regions with larger sediments and repeated through enough time that smooths out irregularities and so forth. And what this leads to is patterns of like the larger sediments in these geometric shapes surrounding the smaller sediments. It's more to it. There's more to it than that because there's also the effect of ice wedges, which can also be involved in these patterns as well. But it's a very, very interesting phenomenon.

I recommend looking it up. It's fascinating. Okay, so that's all I wanted to say about periglacial landscapes. We talked a bit about ice wedges, patterned ground, permafrost, and effect that has on humans. Now, before finishing up, I wanted to say a little bit about the hydrological cycle and the importance of groundwater. I think I may have mentioned this a little bit in one of the past episodes.

I actually can't remember, but because we talked a bit about permafrost, I wanted to then link that back to what happens to groundwater during the rest of the time, right, in regions where the soil is not frozen. So before we get to that, let's talk a bit about the water cycle or the hydrological cycle. So this is just the process that describes the continuous movement of water on above and below the surface of the earth, the continual recycling of the water content of Earth, which overall doesn't change very much. It just kind of moves around. So there are a number of major reservoirs of water on Earth, and there are flows that connect,

well, the water content of those different reservoirs. The largest reservoir of water on Earth is, of course, the ocean, unsurprisingly, which contains, I think, about 97% of all the water on Earth. Of course, that's salt water, so that's distinct from, remember I said earlier, that glaciers contain about 70% of Earth's freshwater, but that doesn't mean they contain 70% of Earth's water, because freshwater is, of course, different from saltwater. So the oceans contain nearly all of Earth's water, but other major areas, reservoirs include, of course, ice sheets or glaciers. We talked about those. That's where most of the

freshwater is. In addition, there is water in the atmosphere. So the actual size of the atmosphere, in terms of the amount of water that it stores, is quite small, but it's very important because of its effect on weather patterns. The next major reservoir of water is groundwater, which comprises, I think, like one or two percent of all of the water on Earth. And we'll talk a bit more about that in a moment. So behind the ocean and groundwater and glaciers, which we've all talked about, all of the other reservoirs are extremely small, right? So basically, all of the water is either in the ocean or underground, like in groundwater, or it's frozen in glaciers. The tiny amounts that are left are divided between the water and the atmosphere, water in fresh lakes, water in saline lakes, water in rivers,

and moisture that's actually in the soil. That's distinct from groundwater, which is like a lower depth. So those are the different reservoirs. Then we can also talk about the flows that connect those different reservoirs with each other. The most obvious one being precipitation. So that's water vapor in the atmosphere, which condenses and falls on the surface of the earth. Mostly as rain, but also it can fall as like snow and hail and so forth. About 80% of global precipitation occurs over the ocean, which is unsurprising because oceans occupy the majority of the surface of the earth. But some proportion of the precipitation, I guess about 20%, falls over land. Now, what happens to water once it falls on the land? Well, there are a number of possibilities. One major thing that happens

is that the water is just evaporated back and returns to the atmosphere. So this is called evapotranspiration. Technically, there's actually two processes there. There's evaporation, which is, well, you know what that is, right? The water turns back into a gas. Transpiration is actually a biological process in which water vapor is released from the pores of plants into the air. But they're often combined together because I think they're typically measured together and they're kind of similar processes. I mean, transpiration is technically like a way that evaporation occurs. It's just biological in nature. So we sort of separate it out. So there's evaporation from water surfaces, transpiration from vegetation. But they both add together to form evapotranspiration.

That's what happens to the majority of the water that falls on the earth's surface, actually, the land surface. It's just evaporated back. But there are some other fates as well. There is what's called surface runoff. This is when water moves over the surface of the land, not underneath, but on the surface of the land. This includes in streams, but also it can be just like over the surface of in like grass or wherever else. It doesn't have to be in a stream. Eventually, runoff will either end up in lakes or reservoirs, or it may infiltrate and into groundwater, or it returns to the ocean. Or, of course, it can evaporate as it's running off.

I just mentioned infiltration. So this is the flow of water from the ground surface into the ground. And basically that's where groundwater comes from. So it's infiltration from the surface, either kind of directly from wherever the rain fell on the surface, or after surface runoff moves the water somewhere else, and then it infiltrates as it's moving or when it reaches its destination, it infiltrates and becomes part of groundwater. So infiltration is distinct from percolation. They're kind of similar, right?

But basically, infiltration is when water goes from being on the surface of the earth to being under the surface of the earth. And percolation is water that's flowing vertically down through the soil and rocks under the influence of gravity. So it's basically being sucked down, if you like. That's not quite the right word, but it's being pulled down by gravity through rocks and soil. Not from the surface of the earth, but pulled down deeper. And that's how you get water reaching greater depths is through percolation.

What happens to all this groundwater? Well, some of it, of course, is used by here. humans, some of it is extracted by plants, you know, the roots of plants. The rest of it will be eventually, will eventually outflow to the ocean. So groundwater can flow out, sort of through the ground out to the ocean at places, obviously, along the shoreline. You can also have the reverse movement, so you can have what's called saltwater intrusion, where saltwater actually infiltrates from the ocean into the groundwater, or into the ground, I suppose, that is under the surface of the ocean, or like near the shoreline.

So there are a lot of these different processes. It's obviously easiest if you can sort of visualize them in a diagram, but hopefully I've sort of generally painted the picture of water comes from the ocean as it's evaporated. Much of it just falls back as precipitation onto the ocean. Some of it falls as rain on the land. Of that, much of it is evaporated straight back from the water surface or via transpiration in plants. Some of it runs off as surface outflow to the ocean. other parts of the water runoff on the surface to lakes or reservoirs, which then evaporate.

Still other runoff will eventually infiltrate into the ground and become part of groundwater, possibly percolating down to greater depths. And that groundwater will then be extracted by humans or plants or eventually outflow to the ocean. So that's why they say all water eventually goes to the ocean, because that's pretty much the... Remember, this is all pretty much acting under the influence of gravity, so you can only go downhill so far until you end up at sea level, and then you're at the ocean. So gravity pulls water down and eventually pulls it out to the ocean. The energy of the sun evaporates it and returns it to the clouds where it can then move around and precipitate on the land.

Just like these other processes that we've talked about, this is all driven by the energy of the sun as well as the force of gravity. Okay, now just a few more things to say about groundwater before we finish up. Groundwater is water that's present beneath the earth's surface in either rock or soil pore spaces. We often think about rock or soil as being fairly compact, right? But actually there's always pores there, spaces of air where that's available for water to fill up if there's water present. About 30% of all readily available freshwater on earth is in the form of groundwater. Pretty much all of the rest, as I mentioned before, is in the form of glaciers. Now, in groundwater you have what's called a saturated zone and an unsaturated zone.

In the saturated zone, which is further down, all of the pore spaces between grains of sediment or grains of soil, is saturated, all of that is taken up by water. So that's where the water pressure is higher. In the unsaturated zone, obviously not all of the pore space is taken up by water, some proportion of it is, but some of it's not. So the unsaturated zone is higher up in the soil, and it extends from the top of the ground down to what's called the water table. So the water table is the start of the saturated zone, where most of the pore spaces and fractures of rocks and so forth are saturated with water. The pressure that water is under, in increases as you go further down, obviously, because there's more of the overlying rock, and that

pressure is felt on whatever water is there. So water in the saturated zone is typically going to be at greater than atmospheric pressure, whereas in the unsaturated zone is at less than atmospheric pressure. So the boundary between those, where the water table is, is typically going to be around atmospheric pressure, unless there are overlying rocks which prevent the water from escaping, which we'll talk about in a moment. So an aquifer is an underground layer of water-bearing permeable rock or rock fractures or unconstlated sediment. So it's not the water itself, although I think it's used to refer to that as well, but it's actually the rock or the sediment or whatever that contains that water. Groundwater from aquifers can be extracted using a well.

So that's what wells are for, right? Therefore, extracting water contained in aquifers that exists in the saturated zone below the water table. If you are too greedy, if you dig too greedily and too deep, you can pump out too much water from the aquifer, which can result in the water table lowering because you've removed more of that water out. That causes the pause between sediments to shrink a bit because you've removed all that water from between them, which causes subsistence. So the surface of the earth actually moves down because you've removed all that water from it effectively. That causes ground collapse. So this can look like craters on plots of land. It can also, in extreme cases, lead to sinkholes where you just have these huge holes appearing

in the surface of the earth. Another problem that can occur with groundwater is that it can become polluted, which is quite difficult to clean up. So this typically occurs when you have improper disposal of waste on the surfaces of the land, which then is dissolved into the water seeps down and enters the water table, and then is pumped up by wells or other uses of aquifer water. Another issue that can occur with groundwater is seawater intrusion, and that's what I mentioned before. Basically, if you pump out too much water out of the aquifers, that reduces the pressure, and so that's going to increase the amount of seawater that flows in, whereas previously it was kind of kept out by the pressure of the freshwater there, now it's flowing in, because

remove all that fresh water. And so infiltration of seawater is not going to be good because then your aquifer might salt up, which is obviously not very helpful. It's also made worse by sea level rise, which again increases the amount of seawater that's kind of available to infiltrate there. When you dig a well down and reach an aquifer and start removing water from that, that tends to create what's called a cone of depression around the aquifer, which is a lowering of the water table immediately surrounding that well. And again, if that gets too extreme, the water table will lower and lower and eventually the well could dry up, right? Where effectively the bottom of the well is now above the water table because you pumped out

so much of that water. Groundwater is replenished over time, or aquifers are replenished over time as a result of infiltration from either surface runoff or from direct rain. I mean, all of it comes from rain eventually, right? But if you withdraw water too rapidly, then you're going to deplenish the aquifer faster than it's being replenished. And so over time, it will become depleted, which obviously is a major problem. Typically, what happens is that there'll be areas that are called recharge areas, which receive large areas of rainfall. And then there are areas where there's relatively less rainfall and a higher amount of withdrawals, either due to human activity or to vegetation.

So it's sort of similar to a glacier where you have these accumulation and ablation zones, but now it's obviously an aquifer. The water table typically more or less follows the topology of the overlying land. So it's sort of like roughly following the surface. but there can be exceptions if you have some sort of confining bed. So typically this will be rock that's less porous, that doesn't allow percolation of water down to a lower level. So there's sort of different levels of this.

When there's a completely impermeable layer of rock or sediment, that's called an aquaclude. A relatively impermable layer is just called an aquatard. So what you can have sometimes is a situation where there is a aquaclude or an aquatard, which separates an aquifer which is above with a, and that's called an unconfined one, basically an aquifer which has sort of free access to the surface. And below that, below the occluding layer, there can be what's called a confined aquifer, which over most of its area doesn't have direct access to the surface. If it's to be recharged at all, then it does have to have some access to the surface,

but that may only be like in a small region in its recharge area, right? and over most of its area, the aquifer is separated off from the surface by this confining layer. You can still tap into a confined aquifer, which will result in you being able to access that water. That's called an artesian well, where you get below this sort of confining barrier and reach the confined aquifer below. The danger to that is that because it's confined, then there's going to be relatively limited recharge, recharge like replenishment of the water in that aquifer, and so it can be very easy to deplete those. the rate of exchange of water in confined aquifers is much, much slower, obviously, because they don't have ready access. I think that there's a confined aquifer somewhere in Western Australia, I think it was,

where they're extracting water that they think is about a million years old. That is that the water was deposited there about a million years ago, which I think is really cool. Anyway, so that concludes what I wanted to say about groundwater and also the episode. Just a review, we talked about glaciers, the different types of glaciers, Alpine continental and also the distinction with sea ice. We talked about glacial processes, glacial mass balance, and how glaciers move through basal sliding and bed deformation and internal deformation. We then talked about some of the glacial landforms, including erosional transportation and depositional landforms, and all of the mechanisms that contribute to that. I then talked a bit about periglacial landscapes, including permafrost, frost action, and the effect of permafrost on humans.

We then concluded by talking a bit about the hydrological cycle and the different flows and reservoirs of water on the surface of earth. and concluded by taking a bit of look at groundwater and aquifers. So hopefully you found this episode interesting. If so, consider giving the podcast a favorable review on the aggregator of your choice, such as iTunes or Spotify or whatever it is that you use. All reviews are appreciated. If you would like to get in touch with questions, suggestions, or any other feedback, you can email me.

My address is FOD12 at gmail.com. That's FODDS1.2 at gmail.com. We've also got a Facebook page, which you can like and follow to receive occasional updates about the podcast and as well as just sort of express your support and let people know about the show. So once again, thank you very much for listening and I'll talk to you next time.