Understanding Isostasy and Earth's History

Welcome back to 51ers. I threw you for a loop last time. I do have another Black Sabbath shirt. This is it. It's hard to see because it's black on gray, which is not a really good color scheme for resolving text and detail damage. but this is also a Black Sabbath shirt. And this is the last chapter, I'm sorry, this is the last video for this chapter. So I made it through the whole chapter wearing a different Black Sabbath or Black Sabbath related t-shirt. shirt every video. That's pretty exciting stuff. I'm happy I celebrated by coming up with a new exciting hairdo for you, my 251 students and audience, and I hope that you like it. So this also is going to be, I think, a shorter video. It definitely, this, the last video, talking about the different layers of the earth, and this one on ice hostility and a few other topics. I think that it's good to have them split so you can really be able to swallow them and not be overwhelmed. But this will also be a shorter video, and this will finish Chapter 1. So let's just jump right in there. I'm going to drink a little coffee. You'll learn throughout this course that one of my best friends in the world and greatest loves is coffee. It's so good for you. So good for you. Okay, so... When we last left off, we were talking about the different layers of the Earth and the connection between that and isosceles has to do with the fact of both density and things floating on the thinosphere. and gravity and buoyancy. So really, isostasy is probably a new word to most of you. It was a new word to me when I first started studying oceanography. But isostasy is the interplay between buoyancy... buoyancy and gravity, right? So buoyancy is what keeps things afloat. And, you know, relative amounts of buoyancy will keep you sitting higher, more afloat and less so. And then gravity is the force that's pulling you towards the center of the earth, right? And so it's basically the offsets of those two forces. Isostasy is a state of hydrostatic equilibrium where the weight of columns of rock at some depth called the depth of compensation is everywhere equal. That's a lot of fancy words that I don't really understand. understand what they mean. So the example in the book and probably the easiest example to help you understand this even though I want to be clear that this is not actually isostasy in terms of the way it works with the continental and the oceanic crust. However, a good way to think about this is if you have a ship, so in this case they're talking about a container ship, right? These are the ships that go all over the world bringing goods, usually, cars and things like that, things that are made by humans from one country to another country. And so sometimes they're empty and sometimes they have a lot of a lot of lot of cargo on them. So when there's more cargo on them, the ship rides lower, right? This is true if any ship or a boat, if you put more stuff on it, it's going to sit lower and reach some point of equilibrium, and that point of equilibrium basically dictates where it sits. Okay, so a container ship that's empty is going to ride higher. A container ship loaded with cargo is going to ride lower, right? And so that is isosceles at work in relation to this container ship. Now, I'm going to ask you on the exam something about isostasy. And if I ask for an example of isostasy, do not tell me about the container ship. That's not what I want to hear. I want to hear something related to the Earth, related to continents and ocean, seafloor, and things like that. However, this is a good example and introduction to the theory. It's something you could wrap your heads around. So the other thing to keep in mind. is that as this whatever it is that's floating on whatever type of liquid is floating higher or lower, when it's pushed down the liquid that's below it is going to be pushed out. It's going to be pushed down and out and have to go somewhere else. And then when it is sitting higher, floating higher, liquid is going to come in to basically fill that space as it's floating higher. So isostatic rebound, this is really where it becomes important for geology and oceanography, is isostatic rebound is the upward movement of the crust due to reduced loading. I say the crust. Remember, there's continental crust and there's oceanic crust. You there. That's not a geology major. That better know this. If you are a geology major and you don't know this, you're fired from geology. You there. Seafloor. What kind of crust is that? What is it made of? What is the rock? We talked about it in the last video. Basalt. That's right. Continental crust would be granite. That's right. So this just says the crust. So it can be, that is indicating that isostatic rebound can happen both. in the ocean with oceanic crust and on land with continental crust. And so we'll talk about two different examples. The example in the text, which is a good example, is ice melting, in particular glaciers, glaciers melting, right? So we are coming out of an ice age. Even if we didn't have human-induced climate change, which is really accelerating the rate of change of climate currently on the planet, we are still... we would be coming out of an ice age regardless. That's where the Earth is right now. It goes into and out of ice ages. We're coming out of an ice age. So there's all of these glaciers that are on land, but they're melting. And as they melt, in particular with glaciers, as they melt, that water is going to go into the ocean. So the water is basically transferred. The weight of the water is transferred from the continents into the ocean. So that's going to change the amount of weight on the continental crust. So if ice is melting, ah. not an ice cube, but a glacier, a really big piece of ice that weighs a lot, then the crust is going to have less weight on it, and it's going to rise up because there's less weight on top of it. Another cause, and I'll show you a picture of this in a minute, or a diagram. Another cause of isostatic rebound could be the erosion of an oceanic volcano. And so this happens around the islands of Hawaii or any chain of islands. As you make a volcano, that's what happens. Again, this is a hotspot. We haven't talked about hotspots in detail, but we will to some extent. And so hotspots are places on the seafloor that are really hot, and as the seafloor passes over them, volcanoes are created. And so you're basically building this big mountain that becomes an island eventually if it breaks the sea surface and is big enough. But over time, that island is going to be eroded and become smaller again. So there's now less weight just under the crust at that particular spot, right? Not under the entire oceanic crust, but under that spot. And so the reduction in weight on the crust at that particular spot will relieve the isostatic pressure, and you get isostatic rebound, where the crust in that particular area will start to come up, or rebound. Rebound means basically rise back up, right? Okay, I hope that makes sense. So here's a picture of that. And in this particular case, they're showing you what happens when you have a big piece of ice on the continent, probably a glacier. And so you have the ice here, you see this nice dotted line, which you can imagine as some sea level or some important particular height. And it says down here, the loaded crust. pushes down and it sinks. And so normally this piece of land right here, the brown, would be straight across, but because of the weight of the ice, it pushes down. And so the crest actually sinks and bends, bows to the weight of this ice. And the mantle below the crust is pushed outward. This also coincidentally leads to these bulges on the outside of the ice. It says peripheral forebulge flexes out. I'm not going to make you memorize that or learn that. It's not about memorization. It's about learning. Memorizing is just being able to vomit facts. Learning is actually being able to take all the concepts and the facts and put them together and apply them to other situations that aren't the exact ones you memorized. So we're learning here. We're not memorizing. Sorry, didn't mean to use that word. So after this ice... melts, we now might have an ocean here, or maybe this is a lake, but it's less water, right? And so you see that the crust will actually rebound, and the mantle is going to flow back in to fill the space as the crust is now moving up, and these peripheral bulges actually over time will go down. Now these are the These are forces and reactions that occur over long periods of geologic time. So we're talking millions of years here. But sometimes, in the case at least of ice melting, we can detect. So in northern Europe, in northern Canada, where there are more glaciers than there are in Texas, and the water is melting at a measurable rate, If you have really sensitive instruments, you can actually measure this rebound. And they've measured the Earth actually rising, or the continent, continental crust rising up a little bit as the pressure from the ice is relieved a little bit. And this is, you know, in this case would be over the course of years that they can measure this. But it is measurable. So, and this is the idea of asystasy. That's a complicated one, but... It has a lot to do with how the continents and the oceans, when they interact with each other and they're all floating on the asthenosphere, how they interact with each other and the results of those interactions are why it's important to understand isostasy on kind of a basic level. By the way, I mentioned taking 252. The lab can help you with some of these concepts. There's a whole lab on isostasy that really... I think helps explain that concept a lot in more depth than we have time to in this particular class. So also in thinking about just the Earth and kind of an introduction to it, the last few slides are really looking at origins of the planet and life on it and some really broad basic definitions. The book does go into... more detail about how the planets formed and it's not a bad thing to know. It goes into various different models for that. I actually don't want you to, if you think it's interesting, by all means, I encourage you to check it out, but it's not something that I'm going to ask you about. I think it's a little beyond really the scope of what we need to know in this course. But it is good to know where did the water in the ocean actually come from, right? That's a reasonable question. there's a few places that we think actually two major sources. One of them is volcanoes. So volcanoes, they outgas steam, which is water vapor. And all of the data that we have points to the fact that there were more and more active volcanoes on early Earth. And so all of these volcanoes were... outgassing a lot of steam and some of the steam as the Earth cooled down. Early Earth was really hot. When I say early, I mean like, you know, first few tens of millions of years to maybe hundreds of millions of years, but a very long time ago. You're about to find out how old the Earth is if you don't already know. But early Earth was very, very, very hot. And as the Earth cooled down, part of the cooling down was because we actually got an atmosphere. Some of that atmosphere would come from the water vapor. Water vapor can help make an atmosphere. So as the Earth cooled down, the steam that was evaporating and was steam at the time condensed and fell into the basins that eventually became the oceans. So some of the water in the oceans originated from volcanoes way back in the day on early Earth. And some of it came from comets that we believe bombarded the Earth early in its history. This was shortly after... We think the Big Bang, or however the universe came into play, the Big Bang is the current scientifically accepted theory. As far as I know, I will admit that I haven't really looked into this too much in recent years. So shortly after that event, it's believed that there were a lot of comets all over the place. And comets, if you didn't know, comets are actually just giant balls of ice. So if there are a lot of comets bombarding into the planet, especially because we didn't have an atmosphere, at the time, so they're easy entry to crash into the Earth. Comets have a lot of water, and so comets also would create water to help make oceans. So where did the water in the oceans come from? The water in the oceans, that's still in the oceans. The water that's there now most likely came from outcast steam from volcanoes and comets way, way, way back in the day. So when I started teaching this course, this article had just came out, and so I threw a slide up here. It's now a few years later, but this article came out in 2016. And it's one of these questions that people always have. What is the earliest life on Earth? And it's a really, really difficult question to answer, as you can imagine, because the earliest life on Earth pretty much had to have been microbial life. We know that animals didn't show up until way later. You're going to find out how much later in a few slides. So the earliest life on Earth, it had to have been microbial. Microbes famously don't leave a lot of fossils, and so it's very hard to... trace that. And then also sometimes you wonder, like, is that really a microbe? It's just a dot. How do we know that's a microbe? There are different ways that we can do this that are beyond the scope of this course, but looking for elements that are more indicative of microbial life than sedimentary features or other features in igneous rocks. Also high concentrations in things like carbon and phosphorus. Anyway, the current, there's some arguments about this paper for sure, but... The current paper that's kind of hot that points to the earliest things that the authors think they can definitively say are microbes is 3.8 billion years ago. And it turns out we think, well, it's much later, I guess a few more slides. We think that the Earth is about 4.6 billion years ago. I'm sorry, 4.6 billion years old, which, and this is only... 0.8 billion years later, 800 million years later. And actually, if you look at this, 3.8 billion year old rock with potential fossil stromatolites, which are microbial reefs. We will talk about stromatolites a little bit down the road. So these features that you see right here, there should be a scale bar, but there's not here. But these features are on the scale of a few millimeters. And so that would actually mean that this is probably millions of microbial cells in these features. I think that they were... that the thing that they were attached to was probably down here, whether this was likely the seafloor, and that they were kind of reaching up for various reasons. For these to have formed, that means that they've already evolved for several millions to possibly longer years. And so they think that the earliest life appeared somewhere around 3.8 to 4 billion years ago. That's really old. Pretty cool stuff. World's oldest fossil is found in Greenland. Greenland is continental crust. Remember I mentioned that oceanic crust gets continuously destroyed as it's subducting under the continents. Continental crust doesn't get destroyed unless it bumps into other continents and so the oldest rocks are on continents and in particular the oldest rocks on the planet tend to be found in northern Canada, in Greenland, in Australia, and in some parts of Africa. So the other paper, I'm sorry, the other place you see papers like this talking about oldest microbial fossils, they are usually in Australia. Can you imagine holding a rock that's 4 billion years old? It just blows my mind. It's so amazing. So amazing. So one concept I want you to be aware of is the difference between auto and heterotrophs. This is a little bit biology, but it has to do with It also has implications for geology and chemistry. So everybody uses organic carbon for respiration. You do. The trees do. My pet cats do. I wish I had dogs. I like them better than cats. If I did have dogs, they would also use organic carbon for respiration. Some organisms make it themselves. These organisms are called autotrophs. Some organisms get it through... feeding or basically eating other things that have organic carbon and these are called heterotrophs. So autotrophs basically make their own food. The classic example of an autotroph is a plant, right? But that uses light for energy and then carbon dioxide as a carbon source. Sorry about that. And uses the carbon dioxide to make sugar and then we eat that sugar through either If you are eating plants and if you happen to eat meat, that is also a source of sugar. Mixotrophs cheat the system and they do both. So there are some organisms, dinoflagellates in particular, dinoflagellates are a type of phytoplankton that can form harmful algal blooms. That's how they tend to get the most press. They don't all form harmful algal blooms, but some of them can be quite nasty. They are both autotrophic and heterotrophic. These things are called nixotrophs and it's like winner takes all because they make their own food which is already pretty badass and if they are having trouble doing that because there's not enough nutrients for them to do that. They can eat food. So they really win either way. Currently, there are six known ways to fix carbon. And there are many, many, many, many, many ways to use pre-made organic carbon. These are a few just broad concepts that we want you to know. But another big event that changed the course of the planet ever after was the Great Oxidation Event. So the evolution of photosynthesis by a group of organisms called cyanobacteria, so the original photosynthesizers were bacteria, not plants or eukaryotic phytoplankton, bacterial phytoplankton, and the end result of photosynthesis is you take carbon dioxide out of the air and you release oxygen. Oxygen is basically their waste product. They put oxygen into the atmosphere. You do this a bunch, oxygen is really pernicious, it gets everywhere and it's hard to get rid of oxygen. You do this a bunch and you have oxygen in the atmosphere. So we know for a fact that in the early Earth, before the great oxidation event, there wasn't oxygen in the atmosphere. So there was a time in Earth's history where there was no oxygen and everything that was alive and everything that had ever been alive was what we call anaerobic, where oxygen is poisoned to that type of organism. Cyanobacteria came around and they started making a little bit of oxygen. But again, oxygen is really, it's really nasty stuff and it gets everywhere. It's hard to get rid of. So they started doing that and they started building up and not going away. And they kept doing that. They said, this is a fun party. Let's keep it going. And so cyanobacteria basically ruined the party for everything that had ever been alive on the planet prior to their arrival. So you can see here, great oxidation event. We believe this happened somewhere. Around two and a half million years ago, there's all these indications of life before then. So this slide is actually from 2010, from a paper in 2010. And so this figure is from a paper in 2016. So they're calling the earliest stromatolites at 3.5 billion years old, but possibly they were older than that. However, we know there was microbial life at least that far ago. It was anaerobic. There's some other bacterial fossils, other types of fossils. Here it says the oldest record that we know of macroscopic multicellularity occurred about 2 billion years ago. Potential prokaryotic, I'm sorry, potential eukaryotic fossils, so fossils with organisms that have nucleus, 1.5 billion years ago. That's not that far ago. And then the... oldest unequivocal animal fossils are only a few hundred million years ago about 500 million years ago so the way I want to point point out is that before this event where oxygen started being built up in the atmosphere so this on the y-axis we have amount of oxygen in the atmosphere on a log scale that means that this 10 to the negative it doesn't matter there's the higher it is the more oxygen On the y-axis we have time in billions of years ago, and so now is basically here, and going back this way is older, older, older. And before about 2.5 billion years ago, everything on the planet that was alive and had ever been alive was anaerobic. And then cyanobacteria came along, they put oxygen in the atmosphere, ruined everybody else's life. However, it led to the evolution of things like us that breathe oxygen. Oxygen has a lot of energy. and so it really changed the face of the planet and the type of organisms that can live on it. Oxygen levels have varied over time. This previous plot went all the way back to the beginning-ish, not quite, it should go a little longer, the beginning of the Earth. This one only goes back 600 million years, but it shows you as far back, at least for this figure, as we have records of amount of oxygen in the atmosphere and really this shows you that it changes over time. This purple line here is what it is currently. So again atmospheric oxygen concentration in percent it's about 21 percent now about 78 percent nitrogen and a few percent other one one percent-ish other stuff like carbon dioxide and you can see that like carbon dioxide oxygen levels have changed throughout time and crashes in oxygen have coincided with mass extinction. So you see when oxygen crashes at the bottom of that crash are periods of time where something happened that we call mass extinction where basically a significant amount of life on the planet died off. So the few things to remember we know we have microbes We don't always have life on the planet, not surprisingly. We think it started somewhere between 4 and 3.8 billion years ago, maybe-ish. There's two major, major, major divisions in life, autotrophs and heterotrophs. Autotrophs, a certain type of autotrophs that came along, put oxygen in the atmosphere, ruined it for everybody about 2.5 million years ago, but made it possible for us to show up. And that amount of oxygen in the atmosphere, it actually varies over time. So, and the... The last thing that I want to talk about is just dating the Earth. Well, and other stuff too, but this is how we do do dating. So when you want to date how old things are, the most common method is to use radiometric dating or to use radioisotopes. using the decay rate of radioactive elements to daughter elements to determine the ages of an object. This is not specifically a chemistry or specifically a physics class, so you don't have to know super-duper ultra-mega details. What you should know is that if something is radioactive, like carbon-14, it, or let's say uranium, because that's what they use to date really old things, uranium-235 is a radioactive version. of uranium, over time it turns into, it decays and turns into lead 207. And so you can tell how old something is by the amount of uranium that it still has left, because we know how often it turns into lead, right? And so this is, we do a similar thing with carbon-14. Carbon-14 has a half-life of about 5,600 years. And so after 5,600 years, a half of... of the carbon-14 and something is gone. And so if you want to date how old something that's sort of recent-ish, like animals, that's a common way to do that. And this is showing you, so on the y-axis here we have remaining parent isotopes and the sample age, it's really what we're interested in. And over here we have years, the sample age in years, and it's a linear. line, but we see that it's a logarithmic decrease, or exponential decrease, I'm sorry, in number of remaining parent isotopes over time, and this is number of half-lives. So every half-life, after a half-life, trust me, read the book, but after a half-life, you have half of the amount of radioactivity left, right? And you could only do that so many times before you have no radioactivity left. You could actually, you could do it about six times. And so this is how we can tell how old stuff is. And so the example that they have here, or that I have here, oh, this is from the book, this is not, is in uranium. And so a billion years ago, I'm sorry, this is the number of uranium-235 atoms. I was reading the wrong number. 4.2 billion years ago, let's say we started off with a million uranium-235 atoms. After 500,000 years, or 70,000 years, hundred thousand years, which is the half-life of uranium, we have half that left. We have about 500,000. And now we also, in that same amount of whatever it happens to be, matter, we have 500,000 lead 207 atoms, right? And then after another 700-ish thousand years, We have 250,000 uranium left and 750,000 lead. You see that it's reversed. And we keep getting less and less and less and less uranium. And you can use this if you can calculate the amount of uranium. in something, then you can basically back calculate how old it must be. And so what you would get is with carbon-14 in this example, you might say, well, we measured 100 pounds. parent isotopes. And so if we started off with a thousand, which you have to figure that out, then this thing must be 300-ish years old. So suffice it to say, this is how we know the age of the Earth, which happens to be 4.6 billion years. Ooh, wow, I really rambled on. You're going to have to hold on for a sec because we're going to do this thing. Right? Sorry that I rambled on for so long right there. This is the last slide though, so it'll only be another minute. There were not a lot of slides in this lecture, but you got me all excited talking about this stuff. I think it's really interesting. So we have one more slide left. Sorry about that. Okay, so as I was saying before George told me that I had to wrap it up, the Earth is... about 4.6 billion years old. And for most people that don't think about this all the time, that's a difficult number to wrap your head around, right? And we know that by looking at uranium-235 atoms of the oldest materials that we can get our hands on, which, again, would be from continents, because continents stick around for a long time because they are less dense than the oceanic crusts, and so they, when the two meet. Otrena crest goes underneath and gets destroyed. Continent stays above. So the radiometric dating tells us that we have a 4.6 billion year old planet. It's hard to wrap your head around what that really means, since humans live on an average of like 75 to 80 years, depending on where on the Earth you live, right? But that's a lot different than 4.6 billion years old. If I was a good scientist, I would have told you where I found this particular plot. I don't think it's from the book, actually, but you can Google, you know, the Earth history compressed into one year, basically, and you'll find a similar plot. I really like this. I think it's a good example. So basically what people have done is taken the course of one year and said, what if we... said one year is like 4.6 billion years. Like, when would some of these major things that happened in Earth's history have happened? So on January 1st, right, at the stroke of midnight, boom, it's Earth. Right? On February 1st, so about 4 billion years old, 4 billion years ago, we have the oceans forming. So the oceans are about 4 billion years old. And the oldest dated rocks. about 4 billion years old, right? So the age beyond that is extrapolated slightly, but there are a lot of scientists that have studied that question very hard, and so that's how they came at the age of 4.6 billion years old. April 16th, we're a full four and a half months into the year. We have the first known life, according to this figure, which came out before that paper that I just mentioned. So this says 3.6 billion years old, or 3,600 million years ago. Actually, it would be probably more like here. Maybe the beginning of April would be 3.8 billion years ago. And we actually discussed that even though 3.8 billion years ago, I guess I would actually be here, halfway between February 1st and April 16th, so I guess that's like early March, that actually the earliest life may have been pushed. as far back as somewhere between 3.8 and 4 billion years ago. But this is, so this is, we have life on Earth somewhere between February 1st and April 16th. It's not, then a lot of time passes. It's not until October 1st, right? Ten months into this year. So we are, we are 10 twelfths, right? Or five sixths, is that right? I guess not actually 10, 10 are 9-12, something like that. Nine months are done, we're beginning the 10. 10 out of 12, start of the 10th month, it says the ocean and atmosphere reach steady state like today. So basically that it took 3.6 billion years before the ocean and the atmosphere kind of looked like they are today in terms of 21% oxygen, you know, 78% nitrogen in the atmosphere. November 1st, we have primitive higher animals. This is 700 million years ago. Right in the scheme of the entire year in the history of the Earth, that is relatively recent. We have the beginning of well-known geology about 600 million years ago. Part of this is because we lose the seafloor, oldest seafloor rocks, somewhere around 250, 300 million years old in the sea now. Some of that ends up on land. So the beginning of well-known geology that we've studied well, that we feel like we have a good understanding about 600 million years ago. First fish, 500 million years ago. First land plants were only 430 million years ago. That's not that long. We're already at November 28th, already into almost December. December 10th says supercontinent. They mean Pangea. We'll talk about Pangea. First mammals, December 12th. 222 million years ago. We're mammals, right? And we're fairly evolved mammals. The earliest mammals that we know about. Only Two weeks before the end of the year, a little more than two weeks before the end of the year. That's crazy. The dinosaurs were killed off 66 million years ago, right? We know this. You probably all know that, right? We all watch the Discovery Channel. Discovery Channel used to have a lot of science on it now. Now it's like people doing crazy stuff. The end of the dinosaurs 66 million years ago, that would chart to December 26th, right? There's only five days left in the year. Man first appears, this block right here represents December 31st, the very last day before the end of the year. Man first appears at 6 p.m. four million years ago. When we say man... Is that, I guess, I guess that's Homo sapiens, right? But this is still way prehistory. The Ice Age begins about 1.6 million years ago. This is 7 p.m. before it's dark at midnight. So an hour later, on December 31st. 80 seconds to midnight, the Ice Age ends. That's 10,000 years ago, and so the Earth starts to look a little bit like it looks today. 17 seconds to midnight, Herodotus draws the map of the known world, 459 BCE. Three seconds to midnight, Columbus discovers America. Three seconds to midnight out of the entire year. So the entire history of white people in America, at least, is really recent. And 0.9 seconds to midnight, the Challenger expedition. So we only showed up on this planet about six hours before midnight in the course of the entire year. That would represent 4.6 billion years. So you can see the whole world. You can kind of wrap your head around this, but I think this is a neat way of looking at the history of Earth. Again, animals, biggie biggies. Animals existed for about two months, and humans only for six hours. Science of oceanography, less than a second. It's a very recent. As far as sciences go, it's a pretty recent one. So that is, boom, that's the end of your chapter. That's the end of chapter one. We're doing good. Got good pace. We got a lot of Black Sabbath. It's not the beginning anymore. We're past the beginning, starting in the next video. We'll be into chapter two, and so I can't wear any more Black Sabbath shirts. So we'll be done with the Black Sabbath shirts. But you don't know what I'm going to wear. It's going to be something different and more awesome. Not more awesome than Black Sabbath. Not a lot, but more awesome than my Black Sabbath shirts. So, thank you. Thank you for putting up with this slightly longer video, and I will see you in chapter, chapter two. See you then.

Welcome back to 51ers. I threw you for a loop last time. I do have another Black Sabbath shirt. This is it.

It's hard to see because it's black on gray, which is not a really good color scheme for resolving text and detail damage. but this is also a Black Sabbath shirt. And this is the last chapter, I'm sorry, this is the last video for this chapter. So I made it through the whole chapter wearing a different Black Sabbath or Black Sabbath related t-shirt.

shirt every video. That's pretty exciting stuff. I'm happy I celebrated by coming up with a new exciting hairdo for you, my 251 students and audience, and I hope that you like it.

So this also is going to be, I think, a shorter video. It definitely, this, the last video, talking about the different layers of the earth, and this one on ice hostility and a few other topics. I think that it's good to have them split so you can really be able to swallow them and not be overwhelmed. But this will also be a shorter video, and this will finish Chapter 1. So let's just jump right in there.

I'm going to drink a little coffee. You'll learn throughout this course that one of my best friends in the world and greatest loves is coffee. It's so good for you. So good for you.

Okay, so... When we last left off, we were talking about the different layers of the Earth and the connection between that and isosceles has to do with the fact of both density and things floating on the thinosphere. and gravity and buoyancy. So really, isostasy is probably a new word to most of you. It was a new word to me when I first started studying oceanography.

But isostasy is the interplay between buoyancy... buoyancy and gravity, right? So buoyancy is what keeps things afloat.

And, you know, relative amounts of buoyancy will keep you sitting higher, more afloat and less so. And then gravity is the force that's pulling you towards the center of the earth, right? And so it's basically the offsets of those two forces. Isostasy is a state of hydrostatic equilibrium where the weight of columns of rock at some depth called the depth of compensation is everywhere equal. That's a lot of fancy words that I don't really understand.

understand what they mean. So the example in the book and probably the easiest example to help you understand this even though I want to be clear that this is not actually isostasy in terms of the way it works with the continental and the oceanic crust. However, a good way to think about this is if you have a ship, so in this case they're talking about a container ship, right?

These are the ships that go all over the world bringing goods, usually, cars and things like that, things that are made by humans from one country to another country. And so sometimes they're empty and sometimes they have a lot of a lot of lot of cargo on them. So when there's more cargo on them, the ship rides lower, right? This is true if any ship or a boat, if you put more stuff on it, it's going to sit lower and reach some point of equilibrium, and that point of equilibrium basically dictates where it sits. Okay, so a container ship that's empty is going to ride higher.

A container ship loaded with cargo is going to ride lower, right? And so that is isosceles at work in relation to this container ship. Now, I'm going to ask you on the exam something about isostasy. And if I ask for an example of isostasy, do not tell me about the container ship.

That's not what I want to hear. I want to hear something related to the Earth, related to continents and ocean, seafloor, and things like that. However, this is a good example and introduction to the theory.

It's something you could wrap your heads around. So the other thing to keep in mind. is that as this whatever it is that's floating on whatever type of liquid is floating higher or lower, when it's pushed down the liquid that's below it is going to be pushed out.

It's going to be pushed down and out and have to go somewhere else. And then when it is sitting higher, floating higher, liquid is going to come in to basically fill that space as it's floating higher. So isostatic rebound, this is really where it becomes important for geology and oceanography, is isostatic rebound is the upward movement of the crust due to reduced loading.

I say the crust. Remember, there's continental crust and there's oceanic crust. You there.

That's not a geology major. That better know this. If you are a geology major and you don't know this, you're fired from geology.

You there. Seafloor. What kind of crust is that?

What is it made of? What is the rock? We talked about it in the last video.

Basalt. That's right. Continental crust would be granite.

That's right. So this just says the crust. So it can be, that is indicating that isostatic rebound can happen both.

in the ocean with oceanic crust and on land with continental crust. And so we'll talk about two different examples. The example in the text, which is a good example, is ice melting, in particular glaciers, glaciers melting, right? So we are coming out of an ice age.

Even if we didn't have human-induced climate change, which is really accelerating the rate of change of climate currently on the planet, we are still... we would be coming out of an ice age regardless. That's where the Earth is right now.

It goes into and out of ice ages. We're coming out of an ice age. So there's all of these glaciers that are on land, but they're melting. And as they melt, in particular with glaciers, as they melt, that water is going to go into the ocean. So the water is basically transferred.

The weight of the water is transferred from the continents into the ocean. So that's going to change the amount of weight on the continental crust. So if ice is melting, ah.

not an ice cube, but a glacier, a really big piece of ice that weighs a lot, then the crust is going to have less weight on it, and it's going to rise up because there's less weight on top of it. Another cause, and I'll show you a picture of this in a minute, or a diagram. Another cause of isostatic rebound could be the erosion of an oceanic volcano. And so this happens around the islands of Hawaii or any chain of islands. As you make a volcano, that's what happens.

Again, this is a hotspot. We haven't talked about hotspots in detail, but we will to some extent. And so hotspots are places on the seafloor that are really hot, and as the seafloor passes over them, volcanoes are created.

And so you're basically building this big mountain that becomes an island eventually if it breaks the sea surface and is big enough. But over time, that island is going to be eroded and become smaller again. So there's now less weight just under the crust at that particular spot, right?

Not under the entire oceanic crust, but under that spot. And so the reduction in weight on the crust at that particular spot will relieve the isostatic pressure, and you get isostatic rebound, where the crust in that particular area will start to come up, or rebound. Rebound means basically rise back up, right? Okay, I hope that makes sense. So here's a picture of that.

And in this particular case, they're showing you what happens when you have a big piece of ice on the continent, probably a glacier. And so you have the ice here, you see this nice dotted line, which you can imagine as some sea level or some important particular height. And it says down here, the loaded crust.

pushes down and it sinks. And so normally this piece of land right here, the brown, would be straight across, but because of the weight of the ice, it pushes down. And so the crest actually sinks and bends, bows to the weight of this ice. And the mantle below the crust is pushed outward.

This also coincidentally leads to these bulges on the outside of the ice. It says peripheral forebulge flexes out. I'm not going to make you memorize that or learn that.

It's not about memorization. It's about learning. Memorizing is just being able to vomit facts. Learning is actually being able to take all the concepts and the facts and put them together and apply them to other situations that aren't the exact ones you memorized. So we're learning here.

We're not memorizing. Sorry, didn't mean to use that word. So after this ice...

melts, we now might have an ocean here, or maybe this is a lake, but it's less water, right? And so you see that the crust will actually rebound, and the mantle is going to flow back in to fill the space as the crust is now moving up, and these peripheral bulges actually over time will go down. Now these are the These are forces and reactions that occur over long periods of geologic time. So we're talking millions of years here. But sometimes, in the case at least of ice melting, we can detect.

So in northern Europe, in northern Canada, where there are more glaciers than there are in Texas, and the water is melting at a measurable rate, If you have really sensitive instruments, you can actually measure this rebound. And they've measured the Earth actually rising, or the continent, continental crust rising up a little bit as the pressure from the ice is relieved a little bit. And this is, you know, in this case would be over the course of years that they can measure this.

But it is measurable. So, and this is the idea of asystasy. That's a complicated one, but... It has a lot to do with how the continents and the oceans, when they interact with each other and they're all floating on the asthenosphere, how they interact with each other and the results of those interactions are why it's important to understand isostasy on kind of a basic level.

By the way, I mentioned taking 252. The lab can help you with some of these concepts. There's a whole lab on isostasy that really... I think helps explain that concept a lot in more depth than we have time to in this particular class.

So also in thinking about just the Earth and kind of an introduction to it, the last few slides are really looking at origins of the planet and life on it and some really broad basic definitions. The book does go into... more detail about how the planets formed and it's not a bad thing to know. It goes into various different models for that.

I actually don't want you to, if you think it's interesting, by all means, I encourage you to check it out, but it's not something that I'm going to ask you about. I think it's a little beyond really the scope of what we need to know in this course. But it is good to know where did the water in the ocean actually come from, right? That's a reasonable question.

there's a few places that we think actually two major sources. One of them is volcanoes. So volcanoes, they outgas steam, which is water vapor. And all of the data that we have points to the fact that there were more and more active volcanoes on early Earth. And so all of these volcanoes were...

outgassing a lot of steam and some of the steam as the Earth cooled down. Early Earth was really hot. When I say early, I mean like, you know, first few tens of millions of years to maybe hundreds of millions of years, but a very long time ago.

You're about to find out how old the Earth is if you don't already know. But early Earth was very, very, very hot. And as the Earth cooled down, part of the cooling down was because we actually got an atmosphere.

Some of that atmosphere would come from the water vapor. Water vapor can help make an atmosphere. So as the Earth cooled down, the steam that was evaporating and was steam at the time condensed and fell into the basins that eventually became the oceans. So some of the water in the oceans originated from volcanoes way back in the day on early Earth. And some of it came from comets that we believe bombarded the Earth early in its history.

This was shortly after... We think the Big Bang, or however the universe came into play, the Big Bang is the current scientifically accepted theory. As far as I know, I will admit that I haven't really looked into this too much in recent years. So shortly after that event, it's believed that there were a lot of comets all over the place.

And comets, if you didn't know, comets are actually just giant balls of ice. So if there are a lot of comets bombarding into the planet, especially because we didn't have an atmosphere, at the time, so they're easy entry to crash into the Earth. Comets have a lot of water, and so comets also would create water to help make oceans.

So where did the water in the oceans come from? The water in the oceans, that's still in the oceans. The water that's there now most likely came from outcast steam from volcanoes and comets way, way, way back in the day. So when I started teaching this course, this article had just came out, and so I threw a slide up here.

It's now a few years later, but this article came out in 2016. And it's one of these questions that people always have. What is the earliest life on Earth? And it's a really, really difficult question to answer, as you can imagine, because the earliest life on Earth pretty much had to have been microbial life.

We know that animals didn't show up until way later. You're going to find out how much later in a few slides. So the earliest life on Earth, it had to have been microbial.

Microbes famously don't leave a lot of fossils, and so it's very hard to... trace that. And then also sometimes you wonder, like, is that really a microbe?

It's just a dot. How do we know that's a microbe? There are different ways that we can do this that are beyond the scope of this course, but looking for elements that are more indicative of microbial life than sedimentary features or other features in igneous rocks.

Also high concentrations in things like carbon and phosphorus. Anyway, the current, there's some arguments about this paper for sure, but... The current paper that's kind of hot that points to the earliest things that the authors think they can definitively say are microbes is 3.8 billion years ago.

And it turns out we think, well, it's much later, I guess a few more slides. We think that the Earth is about 4.6 billion years ago. I'm sorry, 4.6 billion years old, which, and this is only... 0.8 billion years later, 800 million years later.

And actually, if you look at this, 3.8 billion year old rock with potential fossil stromatolites, which are microbial reefs. We will talk about stromatolites a little bit down the road. So these features that you see right here, there should be a scale bar, but there's not here.

But these features are on the scale of a few millimeters. And so that would actually mean that this is probably millions of microbial cells in these features. I think that they were... that the thing that they were attached to was probably down here, whether this was likely the seafloor, and that they were kind of reaching up for various reasons.

For these to have formed, that means that they've already evolved for several millions to possibly longer years. And so they think that the earliest life appeared somewhere around 3.8 to 4 billion years ago. That's really old.

Pretty cool stuff. World's oldest fossil is found in Greenland. Greenland is continental crust.

Remember I mentioned that oceanic crust gets continuously destroyed as it's subducting under the continents. Continental crust doesn't get destroyed unless it bumps into other continents and so the oldest rocks are on continents and in particular the oldest rocks on the planet tend to be found in northern Canada, in Greenland, in Australia, and in some parts of Africa. So the other paper, I'm sorry, the other place you see papers like this talking about oldest microbial fossils, they are usually in Australia.

Can you imagine holding a rock that's 4 billion years old? It just blows my mind. It's so amazing.

So amazing. So one concept I want you to be aware of is the difference between auto and heterotrophs. This is a little bit biology, but it has to do with It also has implications for geology and chemistry. So everybody uses organic carbon for respiration.

You do. The trees do. My pet cats do. I wish I had dogs. I like them better than cats.

If I did have dogs, they would also use organic carbon for respiration. Some organisms make it themselves. These organisms are called autotrophs.

Some organisms get it through... feeding or basically eating other things that have organic carbon and these are called heterotrophs. So autotrophs basically make their own food. The classic example of an autotroph is a plant, right? But that uses light for energy and then carbon dioxide as a carbon source.

Sorry about that. And uses the carbon dioxide to make sugar and then we eat that sugar through either If you are eating plants and if you happen to eat meat, that is also a source of sugar. Mixotrophs cheat the system and they do both. So there are some organisms, dinoflagellates in particular, dinoflagellates are a type of phytoplankton that can form harmful algal blooms. That's how they tend to get the most press.

They don't all form harmful algal blooms, but some of them can be quite nasty. They are both autotrophic and heterotrophic. These things are called nixotrophs and it's like winner takes all because they make their own food which is already pretty badass and if they are having trouble doing that because there's not enough nutrients for them to do that. They can eat food.

So they really win either way. Currently, there are six known ways to fix carbon. And there are many, many, many, many, many ways to use pre-made organic carbon.

These are a few just broad concepts that we want you to know. But another big event that changed the course of the planet ever after was the Great Oxidation Event. So the evolution of photosynthesis by a group of organisms called cyanobacteria, so the original photosynthesizers were bacteria, not plants or eukaryotic phytoplankton, bacterial phytoplankton, and the end result of photosynthesis is you take carbon dioxide out of the air and you release oxygen.

Oxygen is basically their waste product. They put oxygen into the atmosphere. You do this a bunch, oxygen is really pernicious, it gets everywhere and it's hard to get rid of oxygen. You do this a bunch and you have oxygen in the atmosphere.

So we know for a fact that in the early Earth, before the great oxidation event, there wasn't oxygen in the atmosphere. So there was a time in Earth's history where there was no oxygen and everything that was alive and everything that had ever been alive was what we call anaerobic, where oxygen is poisoned to that type of organism. Cyanobacteria came around and they started making a little bit of oxygen.

But again, oxygen is really, it's really nasty stuff and it gets everywhere. It's hard to get rid of. So they started doing that and they started building up and not going away.

And they kept doing that. They said, this is a fun party. Let's keep it going. And so cyanobacteria basically ruined the party for everything that had ever been alive on the planet prior to their arrival. So you can see here, great oxidation event.

We believe this happened somewhere. Around two and a half million years ago, there's all these indications of life before then. So this slide is actually from 2010, from a paper in 2010. And so this figure is from a paper in 2016. So they're calling the earliest stromatolites at 3.5 billion years old, but possibly they were older than that. However, we know there was microbial life at least that far ago. It was anaerobic.

There's some other bacterial fossils, other types of fossils. Here it says the oldest record that we know of macroscopic multicellularity occurred about 2 billion years ago. Potential prokaryotic, I'm sorry, potential eukaryotic fossils, so fossils with organisms that have nucleus, 1.5 billion years ago. That's not that far ago. And then the...

oldest unequivocal animal fossils are only a few hundred million years ago about 500 million years ago so the way I want to point point out is that before this event where oxygen started being built up in the atmosphere so this on the y-axis we have amount of oxygen in the atmosphere on a log scale that means that this 10 to the negative it doesn't matter there's the higher it is the more oxygen On the y-axis we have time in billions of years ago, and so now is basically here, and going back this way is older, older, older. And before about 2.5 billion years ago, everything on the planet that was alive and had ever been alive was anaerobic. And then cyanobacteria came along, they put oxygen in the atmosphere, ruined everybody else's life. However, it led to the evolution of things like us that breathe oxygen. Oxygen has a lot of energy.

and so it really changed the face of the planet and the type of organisms that can live on it. Oxygen levels have varied over time. This previous plot went all the way back to the beginning-ish, not quite, it should go a little longer, the beginning of the Earth.

This one only goes back 600 million years, but it shows you as far back, at least for this figure, as we have records of amount of oxygen in the atmosphere and really this shows you that it changes over time. This purple line here is what it is currently. So again atmospheric oxygen concentration in percent it's about 21 percent now about 78 percent nitrogen and a few percent other one one percent-ish other stuff like carbon dioxide and you can see that like carbon dioxide oxygen levels have changed throughout time and crashes in oxygen have coincided with mass extinction.

So you see when oxygen crashes at the bottom of that crash are periods of time where something happened that we call mass extinction where basically a significant amount of life on the planet died off. So the few things to remember we know we have microbes We don't always have life on the planet, not surprisingly. We think it started somewhere between 4 and 3.8 billion years ago, maybe-ish. There's two major, major, major divisions in life, autotrophs and heterotrophs. Autotrophs, a certain type of autotrophs that came along, put oxygen in the atmosphere, ruined it for everybody about 2.5 million years ago, but made it possible for us to show up.

And that amount of oxygen in the atmosphere, it actually varies over time. So, and the... The last thing that I want to talk about is just dating the Earth.

Well, and other stuff too, but this is how we do do dating. So when you want to date how old things are, the most common method is to use radiometric dating or to use radioisotopes. using the decay rate of radioactive elements to daughter elements to determine the ages of an object.

This is not specifically a chemistry or specifically a physics class, so you don't have to know super-duper ultra-mega details. What you should know is that if something is radioactive, like carbon-14, it, or let's say uranium, because that's what they use to date really old things, uranium-235 is a radioactive version. of uranium, over time it turns into, it decays and turns into lead 207. And so you can tell how old something is by the amount of uranium that it still has left, because we know how often it turns into lead, right? And so this is, we do a similar thing with carbon-14.

Carbon-14 has a half-life of about 5,600 years. And so after 5,600 years, a half of... of the carbon-14 and something is gone. And so if you want to date how old something that's sort of recent-ish, like animals, that's a common way to do that.

And this is showing you, so on the y-axis here we have remaining parent isotopes and the sample age, it's really what we're interested in. And over here we have years, the sample age in years, and it's a linear. line, but we see that it's a logarithmic decrease, or exponential decrease, I'm sorry, in number of remaining parent isotopes over time, and this is number of half-lives.

So every half-life, after a half-life, trust me, read the book, but after a half-life, you have half of the amount of radioactivity left, right? And you could only do that so many times before you have no radioactivity left. You could actually, you could do it about six times. And so this is how we can tell how old stuff is.

And so the example that they have here, or that I have here, oh, this is from the book, this is not, is in uranium. And so a billion years ago, I'm sorry, this is the number of uranium-235 atoms. I was reading the wrong number. 4.2 billion years ago, let's say we started off with a million uranium-235 atoms. After 500,000 years, or 70,000 years, hundred thousand years, which is the half-life of uranium, we have half that left.

We have about 500,000. And now we also, in that same amount of whatever it happens to be, matter, we have 500,000 lead 207 atoms, right? And then after another 700-ish thousand years, We have 250,000 uranium left and 750,000 lead.

You see that it's reversed. And we keep getting less and less and less and less uranium. And you can use this if you can calculate the amount of uranium. in something, then you can basically back calculate how old it must be. And so what you would get is with carbon-14 in this example, you might say, well, we measured 100 pounds.

parent isotopes. And so if we started off with a thousand, which you have to figure that out, then this thing must be 300-ish years old. So suffice it to say, this is how we know the age of the Earth, which happens to be 4.6 billion years. Ooh, wow, I really rambled on.

You're going to have to hold on for a sec because we're going to do this thing. Right? Sorry that I rambled on for so long right there.

This is the last slide though, so it'll only be another minute. There were not a lot of slides in this lecture, but you got me all excited talking about this stuff. I think it's really interesting.

So we have one more slide left. Sorry about that. Okay, so as I was saying before George told me that I had to wrap it up, the Earth is...

about 4.6 billion years old. And for most people that don't think about this all the time, that's a difficult number to wrap your head around, right? And we know that by looking at uranium-235 atoms of the oldest materials that we can get our hands on, which, again, would be from continents, because continents stick around for a long time because they are less dense than the oceanic crusts, and so they, when the two meet.

Otrena crest goes underneath and gets destroyed. Continent stays above. So the radiometric dating tells us that we have a 4.6 billion year old planet. It's hard to wrap your head around what that really means, since humans live on an average of like 75 to 80 years, depending on where on the Earth you live, right?

But that's a lot different than 4.6 billion years old. If I was a good scientist, I would have told you where I found this particular plot. I don't think it's from the book, actually, but you can Google, you know, the Earth history compressed into one year, basically, and you'll find a similar plot.

I really like this. I think it's a good example. So basically what people have done is taken the course of one year and said, what if we... said one year is like 4.6 billion years.

Like, when would some of these major things that happened in Earth's history have happened? So on January 1st, right, at the stroke of midnight, boom, it's Earth. Right? On February 1st, so about 4 billion years old, 4 billion years ago, we have the oceans forming. So the oceans are about 4 billion years old.

And the oldest dated rocks. about 4 billion years old, right? So the age beyond that is extrapolated slightly, but there are a lot of scientists that have studied that question very hard, and so that's how they came at the age of 4.6 billion years old.

April 16th, we're a full four and a half months into the year. We have the first known life, according to this figure, which came out before that paper that I just mentioned. So this says 3.6 billion years old, or 3,600 million years ago.

Actually, it would be probably more like here. Maybe the beginning of April would be 3.8 billion years ago. And we actually discussed that even though 3.8 billion years ago, I guess I would actually be here, halfway between February 1st and April 16th, so I guess that's like early March, that actually the earliest life may have been pushed.

as far back as somewhere between 3.8 and 4 billion years ago. But this is, so this is, we have life on Earth somewhere between February 1st and April 16th. It's not, then a lot of time passes. It's not until October 1st, right?

Ten months into this year. So we are, we are 10 twelfths, right? Or five sixths, is that right? I guess not actually 10, 10 are 9-12, something like that. Nine months are done, we're beginning the 10. 10 out of 12, start of the 10th month, it says the ocean and atmosphere reach steady state like today.

So basically that it took 3.6 billion years before the ocean and the atmosphere kind of looked like they are today in terms of 21% oxygen, you know, 78% nitrogen in the atmosphere. November 1st, we have primitive higher animals. This is 700 million years ago.

Right in the scheme of the entire year in the history of the Earth, that is relatively recent. We have the beginning of well-known geology about 600 million years ago. Part of this is because we lose the seafloor, oldest seafloor rocks, somewhere around 250, 300 million years old in the sea now.

Some of that ends up on land. So the beginning of well-known geology that we've studied well, that we feel like we have a good understanding about 600 million years ago. First fish, 500 million years ago. First land plants were only 430 million years ago. That's not that long.

We're already at November 28th, already into almost December. December 10th says supercontinent. They mean Pangea.

We'll talk about Pangea. First mammals, December 12th. 222 million years ago.

We're mammals, right? And we're fairly evolved mammals. The earliest mammals that we know about. Only Two weeks before the end of the year, a little more than two weeks before the end of the year. That's crazy.

The dinosaurs were killed off 66 million years ago, right? We know this. You probably all know that, right?

We all watch the Discovery Channel. Discovery Channel used to have a lot of science on it now. Now it's like people doing crazy stuff.

The end of the dinosaurs 66 million years ago, that would chart to December 26th, right? There's only five days left in the year. Man first appears, this block right here represents December 31st, the very last day before the end of the year.

Man first appears at 6 p.m. four million years ago. When we say man...

Is that, I guess, I guess that's Homo sapiens, right? But this is still way prehistory. The Ice Age begins about 1.6 million years ago. This is 7 p.m. before it's dark at midnight.

So an hour later, on December 31st. 80 seconds to midnight, the Ice Age ends. That's 10,000 years ago, and so the Earth starts to look a little bit like it looks today.

17 seconds to midnight, Herodotus draws the map of the known world, 459 BCE. Three seconds to midnight, Columbus discovers America. Three seconds to midnight out of the entire year. So the entire history of white people in America, at least, is really recent.

And 0.9 seconds to midnight, the Challenger expedition. So we only showed up on this planet about six hours before midnight in the course of the entire year. That would represent 4.6 billion years.

So you can see the whole world. You can kind of wrap your head around this, but I think this is a neat way of looking at the history of Earth. Again, animals, biggie biggies. Animals existed for about two months, and humans only for six hours.

Science of oceanography, less than a second. It's a very recent. As far as sciences go, it's a pretty recent one. So that is, boom, that's the end of your chapter. That's the end of chapter one.

We're doing good. Got good pace. We got a lot of Black Sabbath.

It's not the beginning anymore. We're past the beginning, starting in the next video. We'll be into chapter two, and so I can't wear any more Black Sabbath shirts. So we'll be done with the Black Sabbath shirts. But you don't know what I'm going to wear.

It's going to be something different and more awesome. Not more awesome than Black Sabbath. Not a lot, but more awesome than my Black Sabbath shirts. So, thank you. Thank you for putting up with this slightly longer video, and I will see you in chapter, chapter two.

See you then.

Transcript for:Understanding Isostasy and Earth's History

Transcript for:
Understanding Isostasy and Earth's History