Click for a transcript of the Unit 7 lecture.
Good morning. Welcome to Geosc 10, Geology Of The National Parks. Today we're going to be talking about my very favorite subject in geology, glaciers. My name is Sridhar Anandakrishnan, and I'll be your guide through some of the most amazing scenery on the planet. And some of the most important geology that we have today, in my opinion. We're going to be talking about glaciers, and ice sheets, and sea level.
And sea level impacts you. You might never see a glacier in your life, but I guarantee that you know about how sea level affects beaches and coastline communities, and your life is going to be affected by that.
So, let's go and first have a quick look at some of these lovely places, and then we'll come back to the presentation.
Today's tearing down mountains. Glaciers are wonderful at destroying mountains. They're one of the best ways we have to both destroy mountains and to make them beautiful. If you didn't have glaciers, these mountains would look very, very different than what they do today.
This is Yosemite National Park in California. It's just up from San Francisco. You can fly into San Francisco, and drive, and you'll be up there in Yosemite in three hours, four hours, something like that. It's one of the most famous places because of people like John Muir, and the amazing photography that has come out there. Some of the history of the place. So, it's something that is almost legendary in conservation circles and in mountain climbing.
This is Bridalveil Falls over on the right over there. And the classic shape of that valley, going up from it, that U-shape for it tells us that this was glaciated. And we'll find out why that U-shape tells us it was glaciated.
And you just look at it way in the back, there. You have Half Dome. And there's probably people that are crawling up the side of it. It's one of the classic climbs of the world.
Here's a close up shot of that Bridalveil and showing that sort of rounded valley heading up from the falls.
Here's Lower Yosemite Falls. There's all these waterfalls all through there. We're on the lee side, the Pacific side of the Sierra Nevada mountains.
Remember last time we talked about, in the Redwood National Parks section, how we talked about when the winds, these wet, cool winds come along, and they start to rise up the mountain, and they get colder and colder, the air gets colder and colder, and then all the water gets squeezed out. Well, here's where it gets squeezed out. It gets squeezed out in Yosemite National Park. And so you've got tons of water pounding down through there. And then, you have these beautiful valleys, and then all the water comes cascading down the sides.
Here's another picture of Half Dome. The glacier was coming right down that valley. You can just imagine 20,000 years ago, if you were standing where this photographer was standing, you'd just see that whole valley filled up with ice all the way to the back, and that ice would be flowing towards you. And as it flowed along, it would be carving out that glacier, making it a deeper and deeper, year after year. And that's why you get these vertical walls.
This is a photograph not in Yosemite, but we've gone off to Greenland at this point. This is Scoresbysund. This is perhaps what Yosemite looked like. You can see there's a similarity. You have this big mass of ice coming down. You've got these huge walls going straight up. 20,000 years ago, instead of this minuscule amount of ice that we have in here— it's an enormous amount of ice, but 20,000 years ago, that ice would have been half a mile or a mile higher up. It would filled up that whole valley.
This is near the edge of the ice sheet. There's a blue pond at the surface. The ice here is perhaps half a mile thick. So, if you were to sit there, and take your shovel, and start digging, digging, digging, you'd have to dig for half a mile before you got to rock. And you can see those big folds where the ice is flowing down. A bunch of cracks and crevasses. If you're trying to travel across that, you could fall very easily into those. Absolutely dramatic, gorgeous.
This is [UNINTELLIGIBLE] glacier in Greenland. And you can see it flowing. Those big stripes coming down towards you, those are individual glaciers that all have come down and merged together. And you can still see those bands where they came together, and then as a valley curves, it comes down.
Here we have a bunch of tributary glaciers where the green arrow and the red arrow are that have come together, and these amazing, huge crevasse. I wouldn't want to be walking across that glacier. You'd just be falling into those holes all the time.
Another photograph of the glacier. Here we have some evidence, just the first beginnings of a clue that glaciers can change their size. The blue arrows are pointing at where the glacier used to sit just a little while ago. About 150 or 200 years ago, that glacier used to be at a different place. And we can tell that because of the color of the rock, and the marks on the rock. And then, the glacier has retreated back from there in the last few hundred years.
This is looking at the side of a glacier, and you have all of these debris bands in the bottom, and that tells us that glaciers aren't just these clean, beautiful, white, ice and snow masses. They also have all of this rock and mud and debris along the bottom. And so, here's a second clue that glaciers can modify the landscape. They can rip out rocks and carry them away, and here's evidence that they do that. And they're very good at doing that.
All the places in this country that have lakes— Minnesota, the land of 10,000 Lakes. The finger lakes in New York. The Great Lakes themselves, Lake Michigan, Erie, Superior. All of them are there because there was an enormous glacier—an ice sheet, really— that sat on top of North America, that filled all of Canada and flowed down into the northern tier of the US, and as it flowed down, it just churned out these big lakes and ripped them up.
Here are some of the critters that live in Yosemite National Park. And here are some more. These are not in Yosemite. This is in Greenland. Then, you can see these striations were the glacier came by. But now, the glacier's gone, and there are ptarmigan wondering across that landscape.
Here's another example of what glaciers can do. Glaciers flow across this, and they just polish it. It's like taking sandpaper and rubbing it for hundreds and thousands of years across this rock. You smooth it and striate it.
Just beautiful. Snow avalanches coming down. Just some of the most dramatic scenery. I've got a confession to make. I'm not really a geologist. I'm an engineer. I started out my life as an engineer, and I worked as an engineer. And then, I got a job to go to Antarctica and work on an engineering project. And I said, man, I got to keep doing this, and I became a geologist, just so I could go and work on glaciers, and understand them, and continue to go to these absolutely beautiful places.
Look at this. Look at that ridge running up there. If there hadn't been a glacier there, you wouldn't have that ridge. The glacier came along and literally chewed away at the side that mountain. And there was another one on the other side of the ridge that chewed away at it, and the two of them formed this sharp ridge that runs up the side of them. And then, where there's two or three of them that come together, you get these sharp horns that go up into these very, very steep peaks. You wouldn't have it unless you had glaciers doing that. This is some of the most beautiful scenery that I've ever seen.
This is in Alaska. Another one of those nice, U-shaped valleys where a glacier had come pounding down that towards us. It's all gone now. Where did it go? Why did it go? That's what we're going to be talking about today.
Here's another one. I can just sit here all day and just show you pictures, and I'd be perfectly happy. Unfortunately, we're going to have to go and talk about why this is important, and we'll do that next. Let me just run real briefly to Antarctica, and we'll see some pictures there. And then, we'll go to the real stuff.
So, this is Antarctica. The reason we want to go here is that Antarctica is the best analog to what Yosemite might have looked like 20,000 years ago. There's still huge glaciers in Antarctica. This is a mountain range that's 10,000 feet high. And that glacier almost fills it right to the top. Yosemite National Park, the mountains there are 6,000 feet high, and the glacier once did almost fill that valley. Same way here, and these glaciers do fill their valley. And so, we can study these glaciers and learn a lot about what Yosemite might have looked like 10,000 years ago or 20,000 years ago.
Here's [? Ketlet's ?] glacier that's coming down towards us. You can see those flow stripes. And as I said, those are enormous mountains, but there's only a little bit of them, maybe 2,000 or 3,000 feet, that are sticking up out of the snow. All the rest of that valley— and you can imagine if you ripped off that glacier, you'd have this two mile deep valley that would be sitting there, and you'd be standing there looking up these two mile high walls.
Here's a valley. This is a dry valley. It's in Antarctica. For fairly complex reasons, the glaciers can't get into this valley. They did at one time. They carved it out, and now they've retreated, and they have these frozen lakes in the bottom of them. But here, you have one of those two mile high valleys. We're flying over it in an airplane and looking down at it. And way in the back, way, way in the back, you have the main Antarctic ice sheet.
Here's the Transantarctic Mountains. You have these beautiful, layered sedimentary structures a long time ago. These were much lower, and then they were raised up over the last 100 million years or 150 million years. And now they stand 10,000, 12,000 14,000 feet high. And then, right behind them, you can see that white area there is the East Antarctic ice sheet flowing down towards us.
Here's one of those glaciers trying to get through the Transantarctic Mountains, coming down and flowing right around through all the rocks, and trying to erode it, and trying to deepen those valleys.
This is Mount Erebus. This is a volcano in Antarctica. You can see the steam rising up out of the top of it. There's a big lava pool up at the top. There's lots of people that go up there and study it. It's 13,000 feet high. And all the sides of it are ice covered. It's so cold in Antarctica that even though it's this huge mountain that is a volcano, and there's this bubbling lava pool at the top of it, you still have this mass of ice that's covering the whole thing.
This is Mcmurdo Station in Antarctica. This is where we do our research. We fly in here. You can see it looks kind of like a mining town. It's a very gritty place. There are no trees there. There's no vegetation there. The soil is all frozen, so you can't dig underneath it to put all your utility lines, and sewer lines, and electric lines. All of those run above.
So, it's just a very gritty place. But you've got to have that to do your research, to have the airplanes fly around, and so on. This is not one of the most beautiful places in the world, but hey.
This is a view looking out from Mcmurdo, out to the ocean. The ocean is frozen there. It's so cold that the ocean simply freezes over. In some years, it never opens up. Some years, it just remains frozen all through the summer. And then the winter comes, and it freezes, and then the ice just thickens. And that might happen for two or three years in a row, but usually that area does open out.
This is one of those distressing photographs. You got to have fuel to fly airplanes. These are big fuel tanks. And then right in the background, you have this absolutely beautiful shot of Mount Erebus rising up above the town.
There's some critters that live in Antarctica. The most famous, of course, are penguins. I've been down there many times, and I still am delighted every time I see one, because they're just such charming little fellows. And here we are, enjoying watching one of them who is looking at us as well.
And here's a close up. This is an Adalie penguin. They're about a foot and a half or so tall, and just as curious and fearless as anything. They have no predators on land, so when they're walking around on the snow here, they are not bothered if anybody walks around.
And he's off. We're not quite sure where he's off to, he or she. And off into the distance.
This is a penguin rookery. The penguins are displaying. They're sitting there sticking their head up. And those little circles of rocks— I don't know if you can see them— those are their nests. There are no twigs here. There's no place to put their eggs, except the only way to protect them is in these little circles of rocks. And so those rocks become really valuable. And they'll steal them from each other, and they'll get into these titanic battles. When the one steals it, they'll go and fight each other, and they'll bounce against each other. And it's quite serious to them. It's quite amusing us.
This is a view of Erebus stretching up above us, and in the foreground, you have a hut from one of the early explorers from 1910. This is Shackleton's hut. This is what he lived in through the winter before he went on his epic journey to try and get to the South Pole. He didn't succeed, but it was quite a story. There's a close up of his hut and one of my colleagues waiting to go into it.
This is a glacier. We're looking at a glacier. It's coming down towards us. The front of it is breaking off and falling down, but you can imagine— you have to imagine, because we're down below— that it just heads up and up and up the side of Mount Erebus for almost 10 miles. It's a huge glacier.
And here's some people for scale. So, there's these people walking around at the base of it. And that's not even all of it. Just as much as there is above, there's probably three times as much as that below the ground, below what we're looking at, below where those people are standing. The scale of these things is so majestic, it's hard to imagine.
There's a close up of it. You can see all the cracks in the face, and where it's going to break off, and the next chunk is going to fall off towards us.
Here's another picture of a glacier flowing down around the curve and coming towards us.
And to give you a scale of that glacier, off in the middle of the screen, there are two huts. Those are actually fairly big houses. They'd be a house that you might see around town over here. A nice, single story Cape Cod or something like that. These aren't Cape Cods, but that's about the scale of it. So, there's a couple of huts over there, and they're just dwarfed by the edge of the glacier, which you can see to the right of them.
And here's one of these glaciers coming down, and all destroyed, and cracked, and broken, and crevassed. And you can't travel over it, which is why it's nice to have helicopters and airplanes.
And here's another shot of it from the air. And a close up of all the huge seracs and crevasses. And another shot.
So, we're going to end this slide show. Here's one helicopters that we use to fly around. And I think I might have a couple more pictures here. Mount Erebus again, and Castle Rock in the foreground.
Even I don't want to end the slide show here, but —and here's how we move around on the surface with these snowmobiles in the storm. This was a pretty bad storm. You can see the wind blowing across the surface. And this is what we live in. Some tents. That's the tent that we spend two or three months in Antarctica, sleeping in and doing our work from. Putting up a bigger tent. It's cold down there. You've got to wear your parka, pull up your hood.
So, we've had a little tour of some lovely places around the planet. And now, we're going to find out what it all means. Why are there glaciers? How do they flow? How do we know that they came and went? Why was Yosemite filled with two miles of snow a long time ago, and it isn't today? Those are the sorts of things that we're going to be talking about. What glaciers are. Erosion by glaciers. How a glacier makes a hole. How did it make Lake Superior? How did a glacier make the finger lakes? How did a glacier make the 10,000 Lakes of Minnesota? Those kinds of things.
And then ice ages. What evidence is there for them, and why do they happen? Why did it get cold, and why did it get warm?
A glacier is very simple. It's a mass of ice and snow that deforms and moves. By deformation, I mean it simply changes its shape. It goes from here to there. It flows across the surface of the landscape. That's all it is. And it does so under the force of gravity. Glaciers flow when snow falls on the surface of the land and it doesn't melt, so it accumulates. If it's cold enough that the snowfall doesn't all melt and disappear, as it does around here, year after year —every year it snows here, too, but we don't have any glaciers. And the reason we don't is all that snow disappears in the summertime.
But if you go to a place where it's cold year round, that snow remains. And then, there's more snow that falls on it the following year, and more snow that falls on it the following year. And eventually, if the ice gets thick enough, then you form a glacier.
Where is it cold? It's cold at the North Pole, and it's cold at the South Pole. Why? If you remember from last time, we talked about that. The sun is shining on the Earth. At the equator, all the sun's rays just slam straight into the equator. And so, all that energy goes right onto the equator. Up at the poles, that same amount of energy is smeared across a larger area because of the curvature of the Earth. And so, it's colder at the polls, warm at the equator. If it's cold at the poles, then that snowfall doesn't melt.
If you go up high mountains, we talked about that last time. As you go higher and higher up, it gets colder and colder. And you get to a point where it's so cold that the snow doesn't melt in the winter time. That's where we could form a glacier.
Or if it just snows so much and the summers are relatively short so we can't melt all that snow in the summer time, then you could form a glacier. For example, in the Olympic Mountains, they just get so much snowfall that even though it's a relatively warm place, these aren't huge mountains, you can still form a glacier.
You have glaciers at the equator. You have glaciers on Mount Kilimanjaro in Africa. And the reason for that is, they're high, and they get a fair amount of snowfall.
If it's cold but you don't get a lot of snowfall, then the ground just freezes and you get what's called permafrost.
Glaciers move and deform because of gravity. Gravity is one of the key forces in geology. I've talked to you about it over and over again, many, many times. We talked about it with mass movement last time. We talked about it with convection cells earlier on. And here it is. It's coming at us again. It's how a glacier moves and deforms.
When you make a pile of anything, the highest spot in that pile will put more weight at the bottom than the lowest spot, or a lower spot, on that pile. So, there's more force under the highest spot and less force under the lowest spot. And like everybody else, glaciers respond to forcing, and they want to move from where there's high force to where there is lower force. And so, the glacier simply moves off to the left.
I'm going to go to the drawing tablet now, and we're going to sort of summarize this part of it.
Here's the ocean. Everything always starts with the ocean. Here's land. And we're going to put some trees on land. And we're going to put some houses on here. And then, we're going to have it snow. We get water that evaporates from the ocean. It gets blown up onto land, and it starts to snow, and we'll show snowfall as these little X's, and down they come. It's snowing on the land.
And if it is cold enough, that snow will make a pile. And if it's cold enough, and the snow doesn't melt, it'll make a bigger pile. And a bigger pile. And a bigger pile, year after year. Until the weight of that pile gets large enough that the glacier wants to start— I'm going to remove these little snowfall marks, because it doesn't snow inside a glacier. It only snows on the outside. So, we'll get rid of some of these things.
So we've got a pile of snow and ice. And if we make that pile big enough, it will start to flow off to the sides. And the reason for it is that the force here is high. Why? Because it's got lots and lots of ice sitting on top of it. Lots of ice.
And the force here is low. Why? Not so much ice on top.
And so, the ice flows from where the force is high to where the force is low. It flows off to the side like this, and it flows off to the side like that.
This sketch has gotten a little messy. I'm going to start it all over again. We'll put the ocean in. We'll put the land in. And we'll put in our glacier. Now we've built this big pile of ice, and it's flowing off to the side. And when it gets to the edge, it will melt. And the water will simply run back into the ocean.
So why doesn't that pile disappear? It's flowing off the side. It's melting. It's going off. And the reason it doesn't disappear is it's still snowing on top. If you remove material off on the edge, if you add material up on top, and you flow from the one to the other, you can keep that glacier at its same size. If you get it just right, if you melt just the same amount as you add on top, and you flow that from the one spot to the other, and you do this continuously over and over again, that mass of ice isn't going to change its size. It's going to stay the same year after year.
This is known as the accumulation zone. That's where snow accumulates. Simple enough.
This is known as the oblation zone. Oblation is a fancy word for removal. To oblate is to remove. The oblation zone is where you remove ice and snow.
And then, the red line indicates flow from the one to the other.
We got to finish this picture out with one more thing, which is evaporation of ocean water followed by transport inland. And then that ocean water deposits as snowfall.
So do you see that cycle? You get evaporation in the ocean. It comes inland. It falls onto the accumulation zone of the glacier. It flows down to the oblation zone. It melts, and it goes back to the ocean. And if you get it just right, the glacier doesn't change its size and the ocean doesn't change its size.
And if the temperature is constant, then the system will come to equilibrium. It will come to a spot where you are removing just as much as you're taking in. And year after year, the glaciers will stay the same. If you change the temperature, if you start to warm things up or to cool them down, then that balance will shift. And that's what we'll talk about.
So this is how a glacier works. It flows from the accumulation zone to the oblation zone, and it does so under the force of gravity.
Glaciers always move from the high to low spot. The high on the surface of the glacier, what do I mean by that? Here we have that mass of ice and snow with the rock underneath it. The rock is relatively flat. The glacier will always flow from where it is high to where it is low.
Not the rock underneath. The rock underneath, you can see, is pretty flat. And the analogy is if you take a bowl or plate, and you start ladling molasses onto it, or you start to ladle pancake batter onto it. As you make a pile, and you keep ladling material into the middle of your plate, what happens? It isn't if your pile just goes straight up in the sky as you ladle onto it. Your molasses just doesn't build up and up and up. It flows off to the side. And which way does it flow? It flows away from where that pile of molasses is high to where that pile of molasses is low. That's all that glaciers do. They flow from where they are high to where they are low.
And in fact, they can even flow— I'm going to go to a new page. Now, we're going to have a glacier sitting in a bowl. The glacier will still flow in that direction even though the rock underneath slopes up. Doesn't matter. Even though that rock slopes up, the glacier will still flow in this direction because the top of the glacier is higher there.
Now, there's a limit to this. If you make the rock really, really steep, then at some point the glacier won't be able to climb that hill. But you have to get pretty steep. You have to get 10 times as steep as the slope at the top of the glacier for the glacier to change direction and head down.
So, this is flow of glaciers. And the balance, the hydrologic balance of evaporation and accumulation and oblation.
We're going to go back to the presentation, and then we'll come back to this drawing tablet in a minute.
Glaciers move from where the surface is high to where their surface is low. "Their" surface. Not the surface of the rock that they're riding over, but their surface themselves. They can even move uphill, like pancake batter flows up the sides of a bowl when you pour it into a bowl. The pancake doesn't all congeal right in the bottom of the bowl. It can actually lap up the sides of the bowl and eventually drip over the edges if you keep pouring more and more into it. The North American glaciers came up into Pennsylvania, and they flowed uphill into the mountains of Pennsylvania because of this process.
And here we have a cross section of a glacier. And you have flow of it from the left, in this case, where it says "Lake Ontario," to the right, where it says "Pennsylvania." Even though Pennsylvania's higher up than Lake Ontario— State College is higher than the bottom of Lake Ontario— when the ice filled it, the ice actually flowed, quote, "uphill." The rock heads uphill, but the glacier itself, the top of it, was sloped from Canada down towards the US. The glacier was higher in Canada than it was in the US, and so it flowed from the high spot of the glacier to the low spot.
When glaciers flow and deform, they don't just move as a mass. They don't just simply have this big fat thing, and the whole thing moves together. They actually deform and change their shape. And that's what's shown in the bottom graph, and I'm going to go back to the tablet and illustrate that.
We'll use that same picture that we had on the presentations. We have the glacier that looks like this. And the bottom of it looks something like that. This is Lake Ontario, and this is Pennsylvania.
If you were to drill a hole down through this glacier, a nice, straight hole— and people do that all the time. They do it to sample what's underneath. They do it to see how thick the glacier is. They do it to sample the ice itself. So, there's lots of reasons for doing it. You drill this hole in the ice, and you come back a year later or two years later, and you measure what that whole looks like. It won't be straight anymore.
If you come back in a year, that hole will look something like that. The top moved a lot. And by "lot," I mean 100 feet, 500 feet. Maybe as much as 1,000 feet after a year, maybe as much as 5,000 feet after a year. So glaciers don't move enormously fast. In a year, they flow anywhere from a few feet to a few thousand feet, but they don't move as fast as you or I could walk, for example.
So, the top has moved a lot. The bottom has moved a little. And there is this curve in between where different places inside of the glacier have moved different amounts.
So, the top is always moved the most. If I were to stand here on the top, I would always move forward, ahead of a marker at the bottom. But it isn't a straight line. It isn't a flat line. It's this sort of complicated curve. And this is a big area of research. People want to know what shape will that hole look like after a year or after 10 years, because it tells us a lot of a glacier flow. But this is a very simple thing, is you have deformation within the glacier that will change the shape of the glacier.
Now, in addition to that, you also have sliding. of the glacier over the base, over the rock. So, glaciers can slide as well. They might slide a little, they might slide a lot. It really depends on how much water there is down there.
And you can have water underneath glaciers. Even though glaciers are ice and snow, and they're very cold at the top, they can be very warm at the bottom because of the heat that's coming up from inside the Earth. And where is that heat coming from? You know, radioactive decay. Remember that. The inside of the Earth is hot. That heat is coming up. It's coming up all around us. If you look down, you'll see heat come up through your feet. You won't be able to measure it. There isn't a lot of it. But there's enough that the bottom of the glacier is warm.
So, you get sliding of the glacier over the base, and that's where the erosion goes on. As a glacier slides along, it rips up the rock, and it destroys the rock. It pulls it up and carries it away, and you have erosion of rock as the glacier slides along.
Let's go back to the PowerPoint.
Glaciers with lots of water at the bottom are good at eroding. If you get lots of water down there, then you get this sliding mechanism. Instead of simply having deformation within the ice, you get sliding at the bottom, and then the glacier can pull up bits of rock and carry them away. "Plucking" is when the glacier literally breaks loose small rocks. You have this big mass of ice. There'll be some small crack in the rock, and it will break off or pluck a piece of rock away and literally carry it off.
"Abrading" is when the glacier drags those small rocks. So, it's plucked one of these small rocks, and it's carrying it away. And as it's carrying it away, that rock acts like sandpaper. It actually scrapes away at the rock that's still underneath the glacier, and then will break off small bits of other rock that will also then be carried away and washed away because of all this water that's down there.
Any water flow under there really helps. If you've ever done any woodworking, you know that your sandpaper will get clogged after a while. You got to clean it off. This is the same thing. So, as you rub at this, if there's water underneath the glacier, that water will simply wash away all the loose material, and then you'll have this clean surface to rub some more. The water will wash away the loose material, rub some more, and the glacier will dig down and down and down, and deeper into the rock underneath.
Here's a picture illustrating that. Both you have plucking on the one side, abrasion on the other side. If you get one of these sort of sloped rocks underneath there, then you can have both of those processes going on. The ice is flowing over these, and it's plucking and abrading. And over time, it will carry that material away, and it will keep digging down deeper and deeper.
And here's an example from the Alps where the glacier would have flowed from right to left across there, and that face we're looking at is where all of the rocks were plucked away and gone. And the glacier, of course, is gone, but this is what the bottom of the glacier would have looked like when it was there. It's smooth on one side where the glacier came from. It's rough on the other side. It's known as a roche moutonnee or rock sheep, that looks a little like a sheep, sort of a rounded thing with this fuzzy edge on the side of it.
The abrasion can be seen. Anywhere there's been glaciers, you get these smooth, polished surfaces with these long, straight lines on them. And the long, straight lines always point in the direction that the glacier was heading. And those long, straight lines are simply from those rocks that it plucked. As they get dragged along the remaining rock, it just leaves these long striations and stripes on there.
The net result of doing all this work of abrading, and plucking, and going on and on, is to build mountains. When the glacier was there, it was just tearing away at the mountains, and it leaves these huge valleys, these deep valleys. And because glaciers are very wide and broad, the valleys that they leave behind are very wide and broad and rounded. And that's why a classic glaciated landscape, like the photographs we saw in the beginning of Yosemite, have this rounding to them.
As you go higher up the valley, and you get to the top, you get to where these glaciers have been chewing away at the side, and you get these very sharp ridges between them. And then if two of these glaciers come together or three of them come together, you get these very sharp-sided features that are diagnostic of glaciated landscapes. U-shaped valleys, hanging valleys, rounded bowls, and sharp ridges and sharp mountains. These are all the things that you'll see if you go up to Glacier National Park, for example.
Glaciers are really, really good at eroding. The finger lakes are there because the glaciers. The Great Lakes are there because of glaciers. The 10,000 Lakes of Minnesota are there because of glaciers.
Streams make a very different landscape. A stream can cut down really sharply where it is, but streams are generally not a mile wide. Streams are usually a few 10s of feet wide, or 100 feet wide, or something like that. You can go out, you can look at your stream that's out there. And they're really good at eroding, too. And they're really good at cutting down. But they're not good at cutting down across broad territories. They'll cut down right here, and then more rocks will fall in and they'll cut down, and more rocks will fall in. And so you get these V-shaped valleys when a stream has been there for a long time.
Any time you drive around here, and you look at the shape of these valleys, and they have these really sharp Vs to them, you know that a stream's been going through there. You go to Glacier National Park, you get these nice, rounded, broad U-shaped valleys.
And here's an example of one of those. Nice, beautiful, rounded valley, broad across the bottom. Now there's a stream running down the middle of it. And if you leave that situation alone, and you let that stream run in the middle of it for 50,000 years, that stream will cut down, and cut down, and it will turn it into a nice V. But because the glacier's only been gone for a few thousand years, it's still rounded and U-shaped.
Here's one of those lakes that's left behind from the glacier. And here's another fjord. This is an arm of the ocean that's come in. The glacier used to flow down through there. The glacier went away and the ocean came running back in.
More, just beautiful, alpine scenery and glaciers. Here's one of those really sharp features where the glacier's eaten away at the side of it and left these sharp ridges.
This is the Matterhorn. Very, very famous mountain in Europe on the French-Swiss border. And the reason it is three-sided like that— there's two that you can see and one more on the side, on the other side— is there were three glaciers on the three sides that were eating away at the side of it. And they kept eating away and making it steeper and steeper, and where those three came together, you get a horn. Beautiful place.
We're going to go back to the drawing tablet now and start to talk about ice ages.
We're back. We'll do this again. We'll make our glacier and land. Our ocean and land. And we'll put a big old pile of ice and snow. And if it gets big enough, we call it an ice sheet. It's no longer a glacier. It really acts like a glacier, but we call it something special, just because it's so big, if it covers a continent.
The size of these is enormous. 2,000 miles, 3,000 miles. All of Canada, all of Antarctica, all of Europe. Something like that. That's how big these ice sheets can be.
Let's make this ice sheet go away. Let's just melt it all. Let's just take a thermostat knob and just heat up the planet. Where is that water going to go? It's going to go to the ocean, right? And when it gets to the ocean, it's going to raise sea level. So, remove ice means you raise sea level. It's as simple as that.
And that has happened over and over again over the last million years. It's actually happened all through the history of this planet, but it has happened very regularly for the last million years that every 100,000 years, the ice grows, sea level drops, the ice disappears, sea level rises. It just happens again and again and again. And when the planet gets cold, there's enough ice that builds up that sea level can drop by more than 300 feet. So, there's a lot of ice that gets built up on Canada, Europe, Antarctica, Greenland, when it gets cold on this planet.
Why does it get cold? Why do we have these cycles? Why is it that every 100,000 years it gets cold, the ice grows, sea level drops, and then it gets warm, the ice shrinks, sea level rises? Back and forth, back and forth. So why ice ages? And really, what evidence for ice ages?
Let's look at the evidence first. We'll go back to the ocean. Ocean with big ice. So, we've got a big old ice sheet. All of North America's covered with ice. Europe is covered with ice. The Antarctic ice sheet is huge. Greenland is huge.
This is where the ocean would sit. It would be at some level. Sea level of the ocean. If I had built a house on the beach over there, I could look out and watch the ocean water lapping up against my house.
This is my ocean level with small ice. I make the ice sheets go away in North America. There's no ice in North America. There's no ice in Greenland. There's no ice in Europe. And sea level's going to rise.
What happens is that in this process of making big ice, you change the chemical composition of the ocean. So, let's take a look at that. I'm going to back up here a second, and we'll come back to this.
In the process of making big ice, you change the chemical composition of the ocean. And the critters that live in the ocean notice that. And when they die and their shells fall to the bottom of the ocean, their shells record that. So, all I have to do is go and find a shell from the time of big ice, and I'll be able to tell how much the ocean had dropped.
What happens is— I'm going to go to the next page over here— in the process of building big ice, we have to evaporate water. You remember that. You got to take water out of the ocean and dump it on these big ice sheets.
It turns out that there are two types of oxygen. Isotopes. Remember what an isotope was? Homer Simpson's favorite team, the Isotopes, the Springfield Topes. These are when you have the nucleus of an atom has a slightly different number of neutrons. It's still oxygen, because that's determined by the number protons. But if you have a different number neutrons, it acts the same way, it's still oxygen, but it just has a slightly different weight to it.
The lighter isotope evaporates more easily. So the ratio of light to heavy changes if we build, quote, "big ice."
So, we build this big old ice sheet. And the way we build it is by preferentially grabbing these light isotopes and dumping them on the ice sheet. So, we grab all of the light isotopes— not all of them, but more of them. Preferentially, grab the light isotopes and dump them onto Antarctica, and Greenland, and Canada. And so, you're left with more of the heavy ones in the ocean.
And the critters notice that. The critters notice that in their shells. Their shells are now built up with a different ratio of light to heavy isotopes, and we can measure that. We can find a shell from 30,000 years ago, pull it out, measure the ratio of light to heavy isotopes, and tell, oh, look. The only way this could have happened is if we pulled out lots and lots of the light isotopes from the ocean. We can take that same critter from today, pick up its shell, take it to the lab, measure its ratio, and know that, oh, there's lots and lots of light isotopes in the ocean today.
So, this is the evidence for it. But why?
Everything in this section of the class, tearing down mountains, is driven by the heat of the sun. Remember that. And glaciers very their size because of variations in sunlight. The amount of sun that hits the Earth changes. Not a lot, but enough over time that you end up with variations in sunlight. It gets cooler, it gets warmer.
I'm going to go back to the slides because there's a nicer picture of it, and I won't be able to sketch those variations as easily as the more professional illustration. We're just going to jump over these.
The heat of the sun drives all of this. The amount of sunlight varies according to three things. The shape of the orbit, and I'll show a picture in a second. You know how the Earth goes around the sun. And it does so in an elliptical orbit. And that ellipse gets more squashed, and more round, and more squashed, and more round every 100,000 years. It goes back and forth. Just wobbling back and forth, like that.
And as it does that wobble, as it does that going from more squashed to more rounded, the amount of sunlight that hits the Earth changes.
The amount of tilt to the Earth's axis. You all know the Earth's axis is tilted. That changes every 40,000 years. It goes back and forth, back and forth. And as it does that, the amount of sunlight that hits the Earth changes.
And finally, the direction of the Earth's axis slowly spins around like a top. And that changes about every 20,000 years. And as that changes, the amount of sunlight hitting the Earth changes.
And let's look at that picture. So on the left, diagram A is showing the orbit of the Earth spinning around the sun. And it's more squashed in blue and more rounded in the black, dashed line. And it goes between those two shapes every 100,000 years, slowly. It takes 100,000 years to do it, but it cycles back and forth, and back and forth
And we can measure this. Astronomers and physicists are really good at this, and they can measure the shape of the Earth's orbit, and they can tell us what it's doing. Similarly, on panel B there, on the right, we have the Earth's orbit tilted over by 23 and 1/2 degrees. And the amount of that tilt changes a little bit every 40,000 years. It goes from 23 and 1/2, down to 21, back to 23. And it takes about 40,000 years to do that.
And then finally, you have that precession, which is the direction of that axis moves around. Today, the Earth's access points at the North Star. You know that. You go out at night, and you can look up, and you can always tell which way is north because all you got to do is look for the Pole Star, right? You all remember how to find the Pole Star? Find the Big Dipper— that's that Big Dipper shaped set of stars— and then follow the end of the Dipper, and you'll go straight to the Pole Star.
And you always know that the Pole Star is where the axis is pointing at. Well, 10,000 years ago, if you had been standing around and you would try to see which way is north, you wouldn't have seen the Pole Star because the Earth's axis was going somewhere else.
So, those things happen over time. And as those three things happen, the amount of sunlight hitting the Earth changes. The sunlight in the Earth changes, the temperature changes, the glaciers grow, and the glaciers shrink.
Milutin Milankovitch predicted this in the 1920s. It's known as the Milankovitch hypothesis. He was a Serbian mathematician. Long before any of these isotopes data were available, he said this ought to happen. The amount of sunlight hitting the Earth is changing. He was an astronomer. He wasn't a geologist. But he calculated that the amount of sunlight hitting the Earth was changing, and he said, I'll bet you that that would have had effects on climate. And he was absolutely correct.
The amount of sunlight in the far northern hemisphere seems to control the ice ages. This is a huge topic of research right now. This is what everybody is really interested in in the climate community. Why? Because we as humans are changing the amount of CO2 in the atmosphere. We're changing the composition of the atmosphere. We're starting to warm the globe because of our activities.
And so it's no longer these solar orbital cycles that are controlling temperature. We as humans are starting to do that. And so we need to understand the natural cycles much, much better, so that we can pull out how much it is that humans are affecting it. And so, this is a huge area, topic of research. And we'll find out more about climate change as we go forward.
Changes in sunlight are relatively small, but have very big effects because of feedbacks. And you'll find out about feedbacks down the road a little bit.
So, I hope you've enjoyed your tour through some of these beautiful places on the planet. You've seen how glaciers are built. You've seen some of the evidence for glaciers changing their size and shape over time. And now, here is a good hypothesis for why those changes take place. It's still an active area of research, and maybe we'll change some of the details, but it seems to be a pretty tight one.
Thank you very much. We'll see you next time when we talk about coastlines, and sea shores, and how the ocean can help to tear down mountains.