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Hi everybody, welcome back. My name is Professor Paul Day. I'm a professor of geology and we're going to be talking about continental drift today. Continental drift is the prelude to plate tectonics, plate tectonics being the unifying theory of geology. When we left off with our previous lesson, we had a dispute between the Neptunists who believed that everything was carved through water and through catastrophes and that that was the primary way that the world was
0:28
shaped - the surface of the world was shaped - and people believed that for hundreds of years. That was overturned right around 1830 by people like Charles Lyell who began to publish and push this idea of a plutonist uniformitarian world which is to say, a theory that is dominated by time and by heat as opposed to water and catastrophes. And what this lesson gets into is how we go from the question of how much heat, how much time to this unifying theory
1:02
of plate tectonics, we need to bridge the gap by talking about what eventually leads to continental drift. I hope you enjoyed this lesson and uh let's jump right in. We got a lot to talk about. Now, if you remember where we left off in our previous lecture, we realized that the Plutonists have a dilemma. They've wrestled away the world's view of how the the surface of the earth was created. And the old Neptunists, for example, believed everything was flooding and that a series of
1:31
catastrophes and/or miracles occurred that created the mountains, created the river valleys, created the ocean basins, created the continents, and that it happened in a very short period of time. But by by the 1830s, the Plutonists had successfully shown that the Earth was governed by processes that had three major features. That they had been happening over very long periods of time. In other words, the Earth was not 6,000 years old, but was considerably older than that.
1:58
That 2), that these had been happening in the past much as they happen today. This is a concept called uniformitarianism. The idea that the Earth is much how it is now in terms of its processes, how it was then. In other words, rivers flowing across the surface behaved in much the same way that they behave today. The formation of river deltas much as they did today they would have happened in the past. And third the processes made the Earth dynamic. In other words,
2:30
it was evolving and changing and operable. In other words, it had a dynamo. and it had an energy source in it. By virtue of an internal heat source that caused the rock cycle. One of the great things that James Hutton identified was the existence of the rock cycle, the three different kinds of rocks and that the material within the rocks, the little bits, if you looked very carefully at them, you would realize that, you know, if you're looking at a sandstone, the
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the bits of the sandstone are derived from other pieces of rock that were taken from somewhere else. And so he envisioned this rock cycle that was constantly cycling Earth's materials around. This is an astounding observation. And of course, where did the observation that there was an internal heat source come from? Well, was pretty clear: there were volcanoes. These were clearly observed. The volcanoes showed that there's an internal heat. They were producing lava today,
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Right. The old neptunist idea was that lava was a primordial, which is to say an original material that precipitated out of seawater. Uh well, unfortunately for that viewpoint, it doesn't stack up when you have volcanoes that produce basalts today. So the question that the the Plutonists have now put onto their shoulders now that they've kind of taken over the discussion, is how much time
3:53
are they talking about? Remember James Hutton had brought in this concept of "no vestige of beginning." Right. "No vestige of beginning, no prospect for an end." No vestige of beginning means he doesn't see any start to the beginning of the Earth or geological time. How much heat is involved? So the earth is hot. How much heat? How did the heat move through the Earth? How did it flow through
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the Earth? You know, you have a volcano here, what's the heat source for this? How did the heat get into position? How did it melt the rock? These are very complicated questions. Some of these questions aren't even fully understood to this day. And of course, what did "dynamic" actually mean? Well, how did the Earth actually manifest that energy? It also leads to the question of why do mountains rise? How do rocks cycle? How long do these events actually
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take? Right? The Neptunists had answers for this. They said it was a series of miracles over a very short period of time. The Plutonists are coming in saying, "No, no, no, no. This is a very long, very long process." And so they have to answer these questions. Where did the ocean basins come from? Where are the mountains coming from? How old is everything? So the very first person to try to
5:16
tackle these issues in geology was an American geologist by the name of James Dwight Dana. Now James Dwight Dana is somebody who was steeped in academia. He was beloved by his colleagues. His students raved about him. He would proselytize people into the geological sciences and everybody who knew him, had nothing but great things to say about him. With that said, he had a fantastic reputation as a scientist and people respected him deeply. And so he
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came out with this idea and this is going to be an idea that's going to be widely accepted. So, American geologist James Dwight Dana proposed that the Earth's features including mountains were formed as the Earth cooled and contracted from a molten state. We call this the "Contraction Theory." Okay. So, the cooling was believed to cause wrinkling of the Earth's crust leading to surface deformation. So, remember that when things are cooling down,
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say a piece of metal or just about anything, as it cools down, it contracts. And the idea was, or the theory was that the Earth was originally this molten ball in space right when it formed, right at the beginning, and that it was cooling and as it was cooling it would contract in certain areas and it would bow down and that wrinkling would cause the surface deformation. Now the continents would have been the very first
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things to form. So in his theory, continents were thought to have formed first as lower temperature, lower density minerals - and we'll get into this in a future lecture - but this is your quartz and your feldspars, very famous minerals, which tend to be relatively low density, but they also melt at a very low temperature. It also means that as the Earth is cooling down, they would be the very
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first minerals to form. And so what he is saying is that as the Earth is in a molten state, you got the quartz and the feldspar and that is all congregating very quickly into what we now call the continents. And that the ocean basins would cool second and this would be higher temperature, higher density minerals. This is your olivine and pyroxenes. These are the rocks that are formed of olivines and puricines tend to be darker in color. um they
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tend to be higher in density and so they tend to sit low, right? And so his argument is you're going to create the continents first and then rapidly thereafter you're going to crystallize the the minerals that create the rocks that are in the ocean basins. Dana's theory suggested that the oceans and the continents were permanent. In other words, North America would form, Asia would form, Europe would form, that these were permanent and primordial features of the
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Earth and didn't exchange places. We call this the "Permanence Theory." So the idea is that North America is where North America is. It has never been an ocean. It has always been a continent. It has never traded places. It is never altered. The same thing is true of Asia. The same thing is true of Africa. That they are in the exact same places and that they were never ocean basins in the past. This permanence theory, or this permanence form of the contraction theory
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was endorsed famously by Charles Lyell. Charles Lyell of course remember is the famous friend of Charles Darwin and he is also the person that is out there pushing for the plutonist idea in the first place in his famous original textbooks that were moving all across the Earth and later on were a major part of what inspired Charles Darwin's observations on what would later become evolution. The biggest problem that contraction theory had was explaining the origin of mountains.
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Mountains are something that you wouldn't expect if the Earth is cooling and contracting. You could expect that the ocean basins which are already made of the denser materials are just going to sink down. But the continents are always just going to be kind of right alongside them, and there wasn't really a very good explanation brought out. However, James Dwight Dana and his fellow geologist James Hall, they proposed that you could in fact produce large mountains this
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way. Here's their proposal. What they were saying is is that the contraction induced stress would be concentrated at the boundaries between continents and oceans. In other words, they're saying, you know, as the earth is pulling in and contracting, the ocean basins and the continents were pushing up against each other. They were applying a lot of compression at those juncture points, and that you would notice that the existence of deep ocean
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trenches would form adjacent to coastlines and later form what we call "geocynclines." So you'd have land on one side, land on another side and as the earth contracted you would have the subsidance of the ocean floor and what would happen is is it would start to fill in with sediments and water. And as those sediments and water would pile in, it would actually add more weight and press it even further. So these geocsyncclines would then fill with sediments eroded from the adjacent
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continents. And then as contraction continued - so now it's being filled in, it's almost like you're folding things in that as it's loading itself with lots of sediments, let's say lots of silt and sand and clay, and it piles in, eventually those sides start to fold in and form compression from the land masses as that cooling continues with the earth. So as contraction continued these sediments
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within the geocyncline would be subjected to folding and uplift forming mountain ranges along the continental margins. The idea being that you're squeezing the mountains up and you would get these large folded ranges. Now is there any evidence to s to support this? And of course, the answer is yes. Let's have a look at a couple of images. So, it doesn't take very long to look around the Earth and notice that the Earth has large mountains. Here's a mountain in Namibia. And
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here's another one in the Swiss Alps. And you'll notice that it's made of these layers. These layers are marine in origin. In other words, they were formed in an ocean. Here we see another mountain - these are the Swiss Alps that are made of marine rocks again. And notice that they're folded. They're crumpled. Almost as if the Earth had compressed and squeezed these rocks straight up into the sky. Right. So these are what we call "folds." Very folded material here. Folded material
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here. And this is how in their view, this is how you get marine rocks onto the continent folded and pressed up high into these large mountain belts. And this was just kind of how they felt about it for a very long period of time. Now, permanentism has a serious drawback, which is the continents aren't moving. The oceans are always oceans. Right. The mountains are forming because of the compression through cooling of the earth as it all comes together. So, things are really
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effectively pretty stationary. You're just kind of squeezing certain things and crumpling them as the Earth cools over time. So the problem that they're going to have is that we have life on planet Earth and one of the things we realize is that there's fossils in different parts of the earth that match other parts of the earth where they shouldn't. Right. So it was further proposed that
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continents and oceans were occasionally connected and disconnected via land bridges. And this would explain similar fossils across the continents. For example, South America and Africa. There's very similar fossils. We're going to talk and actually show you some of the images of these here in a few slides. The land bridges were thought to rise and sink connecting land masses. So just like the crumpling that we're seeing, the idea is that here between Antarctica,
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they would have South American flora, this would be the plants. We actually find fossils of them in Antarctica. We also find the same fossils that are here not only here in Antarctica but also in Australia. There's Australian plants that are found in New Zealand. Australian plants found in Antarctica. So the idea was - the the question was was there these land bridges that would heave up from the seafloor rise up and allow the animals or the plants to move across to the
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additional continents because they weren't moving. Right. Panama was always cited as an example. The Bearing Sea was another one that was always cited. There were other ones that were projected to have existed between South America, Antarctica, Africa, and the Australian continents here as well as New Zealand. And so a series of land bridges would allow the animals to move around in theory - supported by biogeographical evidence. This is identical species distributions.
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These are animals, say land animals that have no business living in Antarctica. Right. However, there's no evidence for sunken land bridges that has ever been found in the deep oceans. And they looked for a very long time. They assumed that they were there. And when they went to go and look in these areas, in fact, they discovered that the ocean is actually fairly deep here. It's quite deep here as well and here. So, there's no land bridges in any of these locations.
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It's just not bearing out. Right. However, it was an important part of the theory for a very long time. Somehow the animals have to get around in order to leave their fossilized remains behind. Now, not everybody bought into Dana's idea of contraction with the geosyncline and the compression happening at the boundaries between the oceans and the continents and of course the land bridges. In 1885, Austrian geologist Edward Seuss began publishing his version of contraction theory,
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and he posits some new things. As the earth contracted, the crust wrinkled and folded like a dried apple leading to the formation of mountains. And so this is the "Dried Apple Theory" as I like to call it. And here we have a picture of a dried apple. And one of the things you'll recognize about dried apple is no part of the apple is safe. Everything is wrinkling and crinkling and pulling in. There's not areas that are left high and dry and other
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parts that are that are subsiding more quickly. The entire thing is wrinkled over time. The areas that contracted the most formed ocean basins. In other words, as the contraction happened, the areas that contracted the most would lower the most, and therefore would be covered with water while the remaining elevated areas became continents. Those would stay high. Ocean basins could rise and continents could sink in this theory,
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putting it at odds with Dana's permanence theory. So, we're going to show you a map here in a couple slides that demonstrates this clearly. Dana's group is not going to like this proposal. So, this is a major alternative to this and this is referred to as "contractionalism." So this is the contractionalism theory and here's a image of Edward Seuss in 1890. He's very famous. He was very influential for a lot of geologists for you know 20 or 30 years,
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maybe even longer, his ideas played a major role but his observations it turns out are pretty key for where we're going with this discussion. Under contractionalism Edward Seuss assumed that the Earth's initial crust was continuous. Right. Remember Dana said you had continents and then later on you would crystallize out the ocean basins which were made out of different minerals. Edward Seuss is saying "no the entire thing is continuous but it probably
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broke apart as the earth's interior shrunk." So as the earth's interior shrinking the outer layers had to catch up. They got crumpled underneath here. And so there's a diagram here from the time and here we see the layers of the Earth which they believed that there were good layers of the earth that had been even posited by Benjamin Franklin who had said so to his French colleagues while he was ambassador for the United States. We see these layers over here and of
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course as the Earth contracts, these layers would crumple underneath and cause these ridges these uplifts to happen and that caused a lot of the surface deformation. That was one of the things that he saw that internal buckling was a big deal. With further cooling the continents became unstable and collapsed to form the next generation of ocean floor. So what had been been formerly ocean now became dry land. So what he was saying is this - that you have this initial surface,
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this starts to cool and as that cools it becomes the ocean basin. This part that hasn't cooled yet is still the continent. Now as this subsides this will start to cool down also and this may even surpass the existing continent. So now this becomes the ocean basin and as the Earth contracts this is the process. Right. And so, by doing this, by making this type of argument, you can explain some pretty interesting things. So the interchangeability of continents and oceans
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explain the presence of marine deposits on land. So here we see the Grand Canyon. The Grand Canyon, the northern rim of the Grand Canyon is over 2500 meters high, which is about 8,000 feet, a little over, it's roughly 8,000 ft. These are marine rocks located right here, well above sea level. So, you could explain this by arguing that this used to be an ocean basin that was collecting a large amount of sediment. And then what has happened since then?
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Well, the rest of the area, the Pacific Ocean in particular, would lower itself down and all of a sudden now the what used to be the old ocean basin is now high and dry. And so now you're looking at these sediments up here at the Grand Canyon. So it's an explanation for this. Another major thing that Seuss sought to explain was the similarities in fossils. We just described that Dana was trying to explain it through land bridges. Seuss's theory also explained the widely
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known similarities of fossils in India, Africa, and South America by attributing them to an early geological period when these continents were still contiguous. In other words - he's got an image right here - and what he was saying is here you've got South America, Africa, we have India, Australia. He doesn't have Antarctica on here, but he has these land masses on here. And what he was saying is is this area here was all still high and the animals would run
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across this whole area and it's since subsided and the Atlantic Ocean has cooled down over here, the Indian Ocean has cooled down sufficiently where those land masses have subsided leaving South America and Africa and Australia still high leaving those fossils exposed at the surface. This continuous land mass right here he called Gondwanaland. He actually named it after Gondwana which is a
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region located in India located about right here. But he called it Gondwanaland to separate from Gondwana because of his understanding of the fossils of the area. He was also the first person to argue for the existence of something called the Tethys ocean. The Tethys ocean would have been the ocean located north of Gondwanaland where all of these continents would have been risen up in the past, and conceptually introduced what would later be called Laurasia. He conceptually
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argued that there was a continuous continent located north of Gondwanaland across the Tethys sea that would have connected the Asian continent with North America, what he called Laurasia. So...or what would become known as Laurasia. So these are very forward-thinking ideas that would be picked up by pioneers in geology in the coming decades. This is very fascinating, but it's also very speculative. Right. Because all the evidence is at the bottom
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of the ocean. He could be completely wrong. Now, one of the most important people to get involved in this discussion of the evolution of Earth from its origins to what it looks like today was William Thompson, better known to us today as Lord Kelvin. He lived from 1824 to 1907. He was born in Belfast, Ireland, and he was a mathematical and physics prodigy. He was doing incredible things
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by the age of 16. His publications are still read from his early years. He entered the University of Glasgow at the age of 10, studied at Cambridge from 1841 to 1845. And here's a photo of him at age 22. He was already a distinguished scholar by that point in his life. He was knighted in 1866, became Lord Kelvin in 1892 in recognition for his physics contributions - specifically he developed the Kelvin temperature scale and established key thermodynamic principles that
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inform us to this very day. So thermodynamics is effectively the way that heat and energy move through systems. Heat in particular. And so he was an expert in this and he wanted to use conductive heat flow to model Earth's cooling and assign a numerical age to the Earth. Nobody had ever been able to put a date on this and he had a deep interest in seeing if this could be done. Now, Lord Kelvin was pulled in by some objections he had to James Hutton's
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ageless earth. He strongly objected to this. He argued for a finite Earth age. In other words, that it had a beginning. Remember, James Hutton said "no vestige of a beginning" and Lord Kelvin wasn't going to let that fly. He said "no that there clearly has to be a beginning here." So he argued for a finite Earth age while aligning with the contraction theory's cooling model to explain Earth's dynamics. So the cooling model, remember in contraction is that everything
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is just that heat is being conducted out of the Earth and as that conduction is happening, the Earth is crinkling down. In 1867, Kelvin had a telling exchange with the Scottish geologist Andrew Ramsay after a lecture on the geological history of Scotland. Kelvin relates the following and this is a very important point and what drew him into the question to begin with. "I asked Ramsay how long a time he allowed for that history. He answered
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that he could suggest no time limit to it. I said, 'You don't suppose geological history has run through a billion years?' 'Certainly I do.' '10 billion years?' 'Yes.' 'The sun is a finite body. You could tell how many tons it is. Do you think it has been shining for a million million years?' 'I am as incapable of estimating and understanding the reasons which you physicists have for limiting geological time as you are incapable of understanding the geological reasons
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for our unlimited estimates.' 'I answered'" - and of course this is Lord Kelvin speaking - "'I answered you can understand the physicist's reasoning perfectly if you give your mind to it." Lord Kelvin was very frustrated by this and it was very easy for him to actually come up with certain dates by plugging them into the thermodynamic models into the mathematical models that existed even at that time that we still use to this day. And he would in fact carry out that
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very process. So Lord Kelvin endorsed contraction theory in 1863 using thermodynamic principles and in 1863 he estimated earth's age at 20 to 400 million years in a book called - er, an article called "On the Secular Cooling of the Earth." Later on he would refine this to 20 to 40 million years based on improved knowledge of the cooling rates of rocks and Earth's geothermal gradient. So the
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geothermal gradient is how fast the earth heats up as you move through it. So if you go down a kilometer, it's going to be possibly 20 degrees Celsius hotter than it was the kilometer higher. And so the idea is is that as you go into the Earth, it would get hotter. And that was actually known to him based upon evidence that was collected from miners. He used heat flow and thermal conductivity assuming a molten origin. And so here's his graph. This is where he
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did his plots. This stuff is still publicly available. You can review it. And many people have read and reviewed and criticized deeply his results because people aren't going to like the fact that it's only 20 to 40 million years. So Kelvin's age estimates were challenged post 1904, especially by somebody named Ernest Rutherford. Now, Rutherford is a big deal in chemistry and physics. And if you're somebody that studies isotopes, you you immediately come across
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Rutherford. Radioactivity in particular due to discovery of the ubiquity of radioactivity. So, radioactivity, it turns out, is a heat source. You put a radioactive material in an insulator, it'll heat that insulator up. That radioactivity is a heat source in rocks. Rutherford is a big deal. He would go on to get the Nobel Prize in chemistry in 1908. This suggested to geologists at the time that new heat was being generated in the Earth's interior and therefore
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earth could be far older than Kelvin's estimate. Now the one thing that is kind of interesting here is Rutherford gets his Nobel Prize in 1908. Lord Kelvin dies in 1907. So the discovery of radioactivity occurs in 1904. Sometimes it is said that Kelvin didn't know about radioactivity and therefore didn't incorporate it into his calculations. That's actually incorrect. Kelvin did know about radioactivity but he was skeptical of it. Deeply skeptical of it. So Kelvin
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remained skeptical of how radioactivity worked. He didn't like the transmutation of elements, how that worked. In other words, how uranium became lead over time. he didn't really fully like what was going on there. But then again, it was still kind of a very early part of the science and unfortunately he didn't live long enough to see it all worked out - as also did the quantity of heat
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energy provided by the new source. So he wasn't persuaded that radioactivity was nearly as big a deal as people were making it out to be. Ernest Rutherford does talk about this, because a lot of people again will say that Kelvin didn't know about radioactivity. But this is the truth here from Ernest Rutherford. "I came into the room which was half dark and presently spotted Lord Kelvin
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in the audience and realized that I was in for trouble at the last part of my speech dealing with the age of the Earth where my views conflicted with his." So Rutherford was one of those people that believed very strongly that the Earth was had to be much older. Right. The geologists wanted the Earth to be older because they felt there wasn't enough time. The church, of course, wanted it to be younger because they wanted it to fit with the 6,000-year narrative. Kelvin wasn't
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making anybody happy with his number. Okay. So his age of the Earth conflicted. "To my relief, Kelvin fell fast asleep. But as I came to the important point, I saw the old bird sit up, open an eye, and tilted a baleful glance at me. Then a sudden inspiration came and I said, 'Lord Kelvin had limited the age of the Earth, provided no new source of energy was discovered. That prophetic utterance refers to what we are now considering tonight. Radium!' Behold, the old boy beamed upon
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me." That's from Ernest Rutherford. So it is entirely true that Kelvin knew about radioactivity. He had taken an analysis of it, was not persuaded that it was as big a deal as people were making it out to be. It certainly not sufficient for him to change his numbers which he maintained until his death in 1907. But radioactivity it is now known to contribute about 50% of the Earth's internal heat that we are now seeing manifested at the surface. So that
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heat that is getting out of the Earth about 50% of it from radioactivity. An extremely interesting conjecture was made in the 1800s about the way that the Earth's crust was behaving based upon some observations that were being made largely in Scandinavia. It's called "isostasy." It was proposed in the late 1800s. Isostasy posits that Earth's crust floats. So floating almost like kind of boat floating on a denser semifluid mantle adjusting to balance weight. So just as
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you put weight into a boat, the boat presses itself a little bit further into the water, but as you take the weight off, it it pulls itself back out. It was proposed by two different people, uh, two different models. The first one was George Airy and the other one was John Pratt. These two models came out both in 1855. Airy's model is the concept that thicker crust under mountains, thinner under oceans. Remember the crust is this big tall mount, or tall elevation,
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and that to compensate for that the same way that you're putting something into a boat you're going to have something, the entire root of it is going to go extend further into the mantle. So Airy's model is that thicker crust under the mountains, but thinner under the oceans. The oceans were lower, they were thinner and therefore they weren't being bouyed up as high. So if we look over here, if you have a mountain range, you would have a big root located underneath this
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that extended deep into the mantle, some type of obstruction. Pratt's model was a little different that density variations explain the crustal balance. So if you have a mountain, well, that's just simply because the material there is less dense, not because there's more of it. Right. And so it's less dense here, so as a consequence the mountain range rises up. But there's this thing
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called the compensation depth. This is where the pressure at that location is equal. Right. Something called the lithostatic pressure at that point would be equal. This explained why Earth's crust maintained equilibrium which is to say the Earth's surface seems to be always trying to maintain some type of uniform level. So if glaciers were melting in a certain area, what one of the things they recognized was that the area would start to bouy up and start to move up
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and of course this implied that the mantle could flow to take into that space underneath it. This will become a major observation that was going to be built off of by several other workers in the 1900's and this concept is still integrated into modern Plate Tectonic theory to this very day. So it's an important thing to understand. The Pratt and Airy models are extremely important. Now there are some important shortfalls of contraction theory and a lot of people noticed them.
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The models of Earth's cooling and contraction suggested a limited amount of contraction, not enough to explain the observed deformation in the mountain belts. So, yeah, sure, maybe the Earth is contracting as it's cooling, but some of these mountains are really big. They're really tall, and it's really hard to explain using contraction alone how, for example, Mount Everest got to the
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extreme height that it is. The theory struggled to explain the existence of landlocked mountain ranges and areas experiencing stretching and extension rather than just compression. So for example, the American Southwest where Arizona is and the area to the north into Nevada is extending but those mountains - or in other words the area is pulling apart - but there's mountains there! Contraction theory says "no, everything should be coming together. They should be pressing together"
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and that's just not the case in the American Southwest, as well as numerous other places. Certain polices in Asia in particular. It's really bad at explaining why there's mountains in the middle of the continents. For example the Ural mountains which is what we're seeing photographed right here. How did these mountains come into being under geosyncline theory? The discovery of radioactivity challenged Kelvin's cooling model, a key assumption of the contraction theory. Okay.
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So all of a sudden contraction theory seems to be pushed by the fact that we've got this radioactive heat or this radioactivity that's generating heat within the earth. Maybe that's counterbalancing some of this contraction. So why are we getting these actions? It was subsequently argued that radioactive radioactivity merely slowed the rate of cooling but did not counteract it. So this is one of those things where the amount of radioactivity was a big deal for the heat flow
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issue for Kelvin, but very quietly it was also said "but it might not be so much heat that it's stopping the contraction. It's just merely slowing it down." This is a pretty sloppy approach to the problem. The model does seem to work. A lot of people like it. In fact, when I was a young student at the University of California at Los Angeles, several of my instructors were trained in contraction theory, that was the model that existed at their time and they thought
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it was a great theory, but they believed the new theories were much better, but that they saw that this did explain a lot of things, but they also admitted that it was sloppy, that it had mistakes, and it and it created these conundrums that they couldn't easily and readily resolve. Now, not everybody thought the earth was contracting. Believe it or not, there was an opposing view that the earth was expanding and it's called the "Expanding Earth theory." The
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expanding earth theory was first proposed in 1899 by Roberto Montovani and it posits that earth's diameter has increased over geological time. In other words, it was small and now it's large. Potentially explaining what appeared to be continental drift and other geological phenomena. So the key observation - what drove this, why did people have an interest in this? Because this is a kind of a bizarre idea that the Earth is sitting there getting bigger and bigger and
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bigger - is the key observation is the fit of the continents. Right. South America and Africa and their similar fossils would suggest they were once closer on a smaller earth. So remember we were saying that there's - in fact there's a great picture right here. This would be Gondanaland right here. This would be South America. This would be Africa and Australia and Antarctica. They're all located right here. And if you have it all together and you simply expand the
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Earth's diameter out, as you do so the continents float away from each other. And as they float away from each other into their modern positions, they would take the fossils with them. So this is the expanding Earth theory. There was some proposed mechanisms. You know, there's always the question, how in the world would the Earth ever do this? At least under the contraction theory is pretty obvious..Right? The Earth was hot and it's cooling down. That's the
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mechanism. Under the expanding theory, you have to get a little creative on this. And so they had internal processes like mantal degassing or core expansion. Maybe the core is you know getting bigger for some reason. There was also the idea of an external accretion of cosmic material. This was much less widely accepted. This is the idea that the Earth is still accumulating and getting bigger over time and that as it got bigger and it assimilated this material into the Earth it was
38:50
rolled into the mantle - as opposed to staying up on the crust - would sink in and in doing so it floats the crust out over time. So here we have a picture of Roberto right here. He was a young man at the time. But expanding earth was always considered a fringe idea by most scientists even at the time. But there were a few people that kept looking at the puzzle piece fit of the continents and saying there's seems to be more to it than this. And certainly in the case
39:20
of Gondwanaland where all the fossils seem to match together, it seemed to demand that somehow we got to get these continents in close proximity to one another. Now it's very important to bring in a important point of vocabulary, a term that was being used especially in the early 1900s uh the term "fixist." So a fixist was somebody who believed that the Earth's continents and major geological features remain stationary and do not move significantly over time. This describes the
39:51
geosyncline theory. This is the ideas of Dana. This is the ideas of Seuss. Like you know Dana and Seuss both agreed that the continents weren't sliding around and moving around. They agreed that they were staying stationary. They were staying exactly where they were. Now they might change their height a little bit, especially in the case of Seuss, in fact, Dana didn't necessarily accept that. Dana would accept that the sea levels could rise and fall, but he wouldn't necessarily
40:16
accept that the continents were rising and falling significantly. So, most geologists could only envision up-down movements on the Earth's surface related to cooling or expansion. So most of the time when Dana was talking about things, remember the geocyncline theory, there were these down-warps as the Earth was cooling and there would be a huge amount of sediment that would go in there and it
40:36
would get squished up as the earth contracted down and took up that space. In the case of Seuss, things were constantly leveling down and then there would be little crinkles forming mountains along the edges. All of these were widely accepted hypothesis until the 1960's which is to say the fixist hypothesis, with the exception I should say of the expanding earth. The expanding earth hypothesis was considered a fringe idea for people even at that time
41:10
but it was again based upon some important observations that needed to be addressed. The mobilists however are a different group of people. These were the mavericks of their time. They looked at things a little differently. The mobilists were a group of people - they originated right around 1900. They really got their legs underneath them right around 1910, 1920. And they were around a little bit before then, but they're an interesting group. The
41:39
mobilists argued that the Earth's continents were not fixed but could drift across the globe over time causing geological phenomena like mountain formation and earthquakes. Until the 1960s, most mobilist ideas were deemed as pseudocience. It was almost something like you would read in a fictional novel. So the idea of how the world might have changed over time according to the mobilists is that right now it might look like this, we're seeing up over
42:08
here the globe over here to the right, North America, South America, Eurasia, and Africa with Antarctica at the bottom. But if you went back in time, you could link these things together almost like puzzle pieces. And that is something that caught their attention and they felt that this needed to be addressed. So, it's about time that we introduce continental drift, in particular the Taylor-Wegener hypothesis, the most famous version of it. Continental
42:38
drift was an important mobilist hypothesis that challenged the belief that continents and ocean basins were fixed. This hypothesis, of course, would have been strongly opposed by the academics of the time, you know the followers of Seuss and Dana - the fixists. Right. It had been long noted that the shapes of continents on opposite sides of the Atlantic Ocean seem to match. The people that had noticed this in 1596 was a mapmaker by the name of Abraham Ortellius.
43:11
Theodor Lilienthal in 1756 had noticed this. Alexander von Humbolt noticed in 1801 and again in a publication in 1845, he noted this. And Antonio Pellegrini mentioned this in 1858. In fact, he published an image which is quite famous showing this is what Africa and South America, North America and Europe look like today with a speculative map of putting South America with Africa at the same time showing that they could seem to fit together like a puzzle piece.
43:46
It's kind of interesting. So, it seems like it's just a matter of time before somebody's going to take the plunge and actually make the proposal and say maybe the continents are moving around on the surface of the Earth. And that did finally happen in 1908. The first person to propose this was a person by the name of Frank Bersley Taylor. He's an American. He was influenced strongly by the ideas and evidence of Edward Seuss, so he was not, despite the fact that he is from North
44:13
America, he was not strongly influenced by Dana's view like most other North American scientists were. He was influenced very strongly by Seuss's evidence. And he was the first geologist to propose that continents could drift without expansion or contraction. So imagine, he's saying "it doesn't matter if the Earth is contracting or expanding, that's not relevant to me. As far as I'm concerned the Earth is staying the exact same size over time. I think the continents are
44:40
just moving across the surface." And he called this Continental Creep. And he posited some pretty obvious things from where it looked like things were going. That the continents moved on the Earth's surface, that Africa and South America were once joined together, and that the continents, or the collisions of continents could uplift mountains like the Andes, the Alps, and the Himalayas. So when you took two continents, you ran them together like India running into Asia,
45:12
you'd create very large mountains. And one of the things he proposed was that there was this nice puzzle piece relationship between South America and Africa, and of course this explains why you have fossils in close proximity to one another. This was published in - I'm sorry - he gave the presentation in early 1908. However, his proposal was not taken seriously and it kind of went nowhere. It was not a really big deal for a lot of people. He was kind of the
45:45
only person saying it for a long time. Well, for a for a while, I should say. Maybe not a long time. Which leads us to a very important person named Alfred Wegener. Alfred Wegener was born in Berlin, Germany in 1880. He did not know Frank Taylor's idea, his hypothesis. Apparently, he came up with this all on his own and independently. He earned a doctorate in astronomy from the University of Berlin in 1904. So, he's an astronomer. But shifted
46:15
focus to meteorology and geophysics influenced by his studies and expeditions in Greenland. This is a very big part of his life. He's studying old meteorological phenomena, but he's also looking at paleocclimates, which is to say ancient climates on the Earth as well. So he's very interested in a lot of different things. And he first proposed his version of continental drift or "continental displacement" is one of the ways you can translate his thing because he
46:45
published it in German - his hypothesis in 1912 independent of Frank Taylor's hypothesis. He published his book "the Origin of Continents and Oceans" in 1915 in German. Now, this is a very important thing that I'm going to bring up here is his publications. So, he originally published it in 1915 in German. The second edition came out in 1920, also in German. The third edition in German was then translated into English in 1924. And then the fourth edition in
47:20
1929 was translated into English in 1966. I'm going to go ahead and just let you know: he passes away in 1930. So the only thing that ever came into the English language from his writings was translated in English in 1924 was his third edition. His fourth edition, which is easily his best edition, he never actually saw the English translation of it. It wasn't published till 36 years after his passing. So this is Alfred Wegener. Very interesting person in mind and we're going
47:54
to go into his hypothesis that he has here. It's called the Taylor-Wegener hypothesis, though they both independently came up with it. And there are subtle differences between the two of them, especially with respect to mechanism. And turns out that Taylor wasn't real thrilled about having being kind of lumped with Wegener and Wegener seems to have been kind of silent on the issue, but it's unusual to take two people with hypotheses and immediately lump them together.
48:20
Usually, it's when two scientists publish or work together that you lump them together. But anyways, we'll just go ahead and put this out. So Wegener's continental drift hypothesis posits the following: that once there was a supercontinent which he called "Urkontinent" - "Ur" in German apparently means "primordial" or "original" - that there was an original continent consisting of all Earth's
48:44
land masses - something we would call a supercontinent today - once existed but that during the Mesazoic...this we now know is to be 200 million years ago, he did not know it was 200 million years ago, we now know it today...this supercontinent began fragmenting. It began to break up 200 million years ago. He proposed that there was at the same time a panthalastic ocean. So if you have an "Urkontinent," then there's also a singular "all seas" global-wide ocean, all together
49:17
at one time. So he posits that there's this panthalastic ocean. In his 1920 edition he first used the term "Pangaea." So he didn't say it in his 1915 edition, he brings it up in his second edition and that means "all lands." It's a name which stuck, so people no longer refer to it as Urkontinent - in fact in the English translations later on it was "Pangaea" and so they referred to it as Pangaea and this would be instead of Urkontinent and panthalassia. Wegener's theory was
49:52
more comprehensive, evidence-based and systematic than his mobilist predecessors. So other people had said, "Hey, I think the continents are moving and here's some, you know, here's some ideas for why this or you know why I think this based upon shapes and fossils," but Wegener decided he was going to go ahead and approach this in a very investigative approach and he was going to convince the world that he was on to something and that this needed to be examined a lot more
50:18
closely. He was clearly a true believer in it but he was one of the few people that thought about it. Over Taylor's objections, it was called the Taylor-Wegener hypothesis at the time, though there were important differences between the ideas, and we we talked about that already. So this is an actual image from his book that came out in 1915 showing the breakup of Pangaea. And of course what also happens is the panthalastic ocean also starts to fragment
50:49
as we start to create the Atlantic Ocean and the Pacific Ocean and they all start to separate out and the oceans wind up getting divided up as well. This is a very revolutionary idea. So let's talk about some of the evidence that he's proposes in his book. And if you get the opportunity to get his book - and every geologist should have a copy of it on their desk...I have a copy of it in my office and I consult it regularly - Wegener's
51:18
evidence is pretty interesting. One, he notes what everybody else notes, the jigsaw puzzle fit of continents. So here we have a jigsaw puzzle. And what he noticed was the same thing that everybody else was noticing: that South America and Africa seemed to be a really nice fit. But he also said that North America and Europe were also a really nice fit. Um, but we're going to concentrate on this down here. And so he's saying these things were once joined. And if you
51:43
can show that South America and Africa were once joined, then it's entirely possible that North America and Europe were also joined.. Right. If you could just prove that the idea is valid, then suddenly things become pretty interesting globally, not just in Africa and South America. Alfred Wegener also took a very close examination of the fossil evidence. What he realizes is that the fossil evidence was extensive. It wasn't just there was a couple of fossils that seemed
52:11
to match. There were fossils in places where they shouldn't be. In particular, what one of the things that he noticed was glossopterus...see if see if I can say it again...glossopterus. It's a little difficult to say. It's a plant and it's a type of plant that doesn't grow in cold areas. However, that plant is found in the Transantarctic mountains going through the center of Antarctica. It's found in Australia. It's found in South
52:41
Africa. It's found in southern South America. And what he realizes is if you put it together there's this nice belt of distribution and you can follow it straight across. The same thing was happening with lystrosaurus, that we find bones of lystrosaurus, fossils of lystrosaurus in the Transantarctic mountains. We find it in India. We also find it in Africa. And when you put them together there's this nice belt of distribution.
53:14
The same thing is going to happen with this dinosaur from the triacic here. And here's a mosasaurus. Mosasaurus connects between Africa and South America. And these are fascinating. When they all come together, it wasn't just one species or two species, but it was all of these species that came together and all seem to have these overlaps that only make sense when you put them all together. And this seems to suggest that they were all once not only connected,
53:41
but that they were also probably located a place where the climate was more favorable for these types of plants than exist today. So in other words, they're not just drifting side to side, but also possibly north and south as well. Another important thing that he noticed was that rock and mountain similarities existed across the continents as well. So when he went to look at these things in detail, one of the things he realized is that the rocks,
54:10
they'd be the same age across. So when you went to, you know, this area here between Argentina, this is South Africa over here, and one of the things he realized was these folds. So there's a series of folds, the Cape foldings. And here we can see the rocks are nice and folded. These are rocks that are also nice and folded in Argentina. These folded rocks are basically the same type of marine rocks. They're the same age. They have the same structure. It's the same structure,
54:39
same rocks. It even has the same fossils in it. And they're located here in South Africa. They're also located here in Argentina. And when you go to the Falkland Islands and you put the whole thing together, you realize that these folds continue right through the Falkland Islands, And they all seem to come together nicely when you fit the continents together: same rocks, same structures, and the mountain ranges themselves also seem to match as well. Speaking of mountain ranges,
55:06
he also recognized that there was this really nice correlation between North America, North Africa, and Europe, that when you put all of the mountains together, when you put the continents together, the mountain chains all seem to zip together with the British Isles located right here in the center. Caledonian mountains over here. The Atlas Mountains are located over here and unrelated, but we actually have coastal mountains on northwest Africa, the Appalachia or the
55:35
Appalachian Mountains moving through the United States. And one of the things we recognize is that there is a a very common red sandstone. It's the old red sandstone. It's Devonian in age. And if you look at this image right here, it's located right here across the top. And just out of curiosity, just as a little quiz to yourself, do you recognize this photograph? Because I have shown it in a previous lecture. Yeah, if you don't recognize it, this is Hutton's unconformity. So,
56:06
if you didn't recognize that, there it is. Here are the Devonian rocks of this red sandstone located here across the top, and here's the unconformity with the layers going in the other direction here. Now these Devonian rocks are the same age as these Devonian sandstones over here. Now the old thinking at the time was that it was all the same unit; that the red sandstones located over here in the British Isles where James Hutton was looking at the time, is the
56:34
same thing as the Devonian sandstones that we're finding over here. We now realize that what was probably in place at the center of what we now see as the opening of the Atlantic Ocean, was a very large mountain range. And we see sand being shed and forming along the coastal areas on both sides of the mountains. And so here we see the nice sands over here. And we find sands located across the top of Hutton's unconformity, the old red sandstone of the Devonian. Very famous outcrop.
57:09
Now I did mention that Alfred Wegener was a expert on paleoclimates and he was very interested in paleoclimates and one of the things he recognized that there was evidence of past climates in the form of glacial deposits in tropical regions. You know you don't normally get glaciers leaving rocks in the middle of the tropics. That's very unusual. Coal in Antarctica. So coal forms when you have peat bogs and you have flowing water and that's not the case
57:41
in Antarctica at all. Everything's frozen. All this suggests that the continents were once in different climatic zones. And so when you put it all together, you put together Pangaea, we'll notice that we have this nice tropical belt of deposition right here. We have glaciated areas down here at the southern portion, Antarctica, India, Australia. Believe it or not, there's glacier marks - there's evidence of glaciers moving north into India - and nobody would
58:12
ever mistake India as being a glaciated area in the southern portion. It's climactically, it's very different than what we would expect for that. But yet it's there. When you put it back with Africa, suddenly it makes sense. South America, we find these glaciers. The coal swamps all seem to line up suddenly, Right. And then we get deserts. The deserts seem to form these nice belts to the north and to the south of those tropical areas. And then of course we also have
58:40
the formation of reefs in areas where what we now call the Tethys sea where...we talked about the Tethys sea before. He agrees that the Tethys sea was a real thing and that we have these reefs located along this belt and they all line up very well when you put together Pangaea. This is fantastic evidence.
59:04
Now in proposing these ideas, Alfred Wegener is going to be also discounting some other ideas. So he he has some contrasts with the prevailing ideas of the time. One, he's rejecting land bridges. Land bridges were the accepted way of the world at that time. Wegener dismissed the idea of sunken land bridges connecting the continents. He challenged static continents. He opposed the widely accepted view that continents were fixed, arguing that they
59:33
drifted over time...that there was no major vertical movement. That basically the continents weren't rising and falling through some type of cooling process or expansion process, that the Earth was effectively maintaining its size, its diameter, and that the continents were sliding across the top. Okay. Let's see here...he disregarded theories of continents sinking or rising vertically, emphasized lateral movement across the Earth's crust...that there
1:00:05
was once a single continent, what we now call Pangaea. Remember what Pangaea means? "Pan" meaning "all"; "gaea" meaning "lands." He proposed all continents were once joined into a single supercontinent. And so again if we follow this out pangaea starts to break up. Here's Laurasia and Gondwanaland. These are two terms that you should be very familiar with at this point. He accepts that these two
1:00:27
things are here. That Pangaea actually breaks north and south first and then it starts to separate out through the Jurassic. It fractures out through the Cretaceous and then eventually North America and South America would in fact form a narrow land bridge, but for reasons beyond what the old fixists were saying, that this would have been just a part of the process and that the present day is explainable in this way.. This is again very revolutionary. Now as I've
1:01:00
already said in the past Alfred Wegener's ideas of continental drift are going to be rejected just like what happened to Frank Taylor. Alfred Wegener's theory of continental drift was rejected for three main reasons. One, it contradicted prevailing beliefs. Right. His ideas challenged the established scientific consensus that continents and ocean basins were stationary and primordial. Right. The idea that the Earth was very ancient and that North America
1:01:29
and South America were fixed in their locations, Asia was fixed in its location and the idea that the continents are somehow sliding around on the surface just had no credibility with the fixist idea which was prevalent at the time. Wegener lacked credibility. So, as a meteorologist, he was seen as an outsider to the geology community, leading some to dismiss his hypothesis as "geopoetry!" Right. "Geopoetry.". This was a put-down for what he's doing, or as "Germanic
1:01:59
pseudoscience", especially by North American geologists in the 1920s. The North Americans were especially hostile to it, largely because they were kind of in the camp of Dana, Not that Dana was hostile to any of these ideas. He wasn't alive at the time, but his ideas had really taken root and people strongly believed that the geocyncline model had most of the answers they were seeking and they just needed to dig a little further and they would
1:02:27
get it all worked out. Alfred Wegener is saying not only is Dana not right, but he's saying you guys are all wrong. This is a totally different world view. Again it's a complete revolution. But the primary reason it was rejected was the apparent lack of an acceptable mechanism. Even if the Earth's continents could move, which they didn't accept that it could, how could it how would that happen? What is the mechanism by which the Earth is moving things around? Are the continents
1:02:59
moving around? Does that mean that the moorings - the root of the continents is also moving around? And what's dragging it around? What would be the reason for it? So, Wegener suggested that continents move through the denser oceanic crust like ships through water. In other words, it's kind of plowing through. I wouldn't say that that was his primary proposal. I mean, he's coming up with ideas. And he's not one of
1:03:28
those people that comes out and says "this is what happened." What he's saying is "if you guys need a mechanism, this might be a way that this could happen." Right. But one of the things that he's not giving up on the ideas is that the continents are drifting. He might not know how it's happening, but it's happening in his worldview. He proposed that tidal forces from the gravitational pole of
1:03:50
the sun and the moon combined with the Earth's rotational forces provided sufficient force to push continents around the globe. Unfortunately, geoysicists came in and they calculated this and they immediately realized that this was not sufficient. So that could not happen. So he didn't really have a very tight mechanism. That's not to say he had to have a mechanism to be right. You could be right and not know the mechanism. The continents could
1:04:15
drift and you could be wrong about it. But the one thing that the geocyncline people had was they had a mechanism. Right? The Earth's cooling was a big part of this, and he wasn't incorporating that dynamo into his theory. He was saying that most of the forces are coming from a totally different source. not necessarily from the inside of the Earth, but from something external to it. With that said, there's some caveats... that he did have conversations with people about the Earth,
1:04:42
that the Earth's mantle was doing something called convection. And he did that very early on, but he didn't put that into his book in 1915, unfortunately for him. Now, there's no doubt about it. The father of continental drift is Alfred Wegener. While Frank Taylor's idea is certainly baked into this, the person who will always be the person that championed it, wrote the books, tried to get people to understand it, worked hard to get people
1:05:11
to understand it was Alfred Wegener. He died in 1930 during an expedition in Greenland when he was about 50 years old while attempting to supply a remote weather station. In fact, he passed away on his...sometime either on or shortly after his birthday, November 1st of 1930. And this right here, this photograph, is the very last known photograph taken of Alfred Wegener. His body would be discovered sometime later. Um, and it's a very sad way for for him to
1:05:42
to lose his life. I want to include this little quote: "Alfred was not just a scientist, but a dreamer who saw the Earth as a living, moving thing. His kindness made you believe in his ideas as much as his evidence did." And this is written by his father-in-law, Wladimir Köppen, I think is how you say that. And so again, very sad, but this is his very last photograph. And unfortunately, he perished shortly after his birthday, if not even on his birthday. Really,
1:06:10
they don't know exactly the day that he died. He went into the snowstorm and didn't return. Now, I don't want to leave you with the impression that continental drift had no support. It didn't have much and those that did were mavericks. Right. This is a group of maverick scientists that are looking at Alfred Wegener and kind of following up on some of his ideas. And one of the supporters of this was this idea of continental drift was a very important geologist named Arthur
1:06:38
Holmes. And here's his photograph down here in 1912. A young man. He made a huge impact on the geosciences in his lifetime. He lived all the way until 1965. Arthur Holmes was a pioneer in radiometric dating of minerals and he was also a well-respected academic. So he was very familiar with radioactivity. He was very familiar with all of these controversies with Lord Kelvin,. and he was convinced that the Earth was also quite old. And so he was also very familiar with
1:07:12
how heat flow worked and all these other things. So he's kind of this person that's coming in and re-evaluating Earth's mechanisms and how it's transmitting heat from within to the outside and this concept of radiometric dating. And he made two major contributions to our understanding of the Earth. In the 1910s, Holmes showed that earth was billions of years old. So he was actually put able to put a date on it that Kelvin wasn't. And he did it using radiometric dating. This,
1:07:40
of course, billions of years old being enough time for continents to drift significant distances. So suddenly continental drift was a viable thing. There was plenty of time to make that happen. Whereas under the old regime, under Kelvin you get 20 to 40 million years, that's not enough time to make continents move large distances. The interesting thing about Arthur Holmes is that in the 1940s he calculated the earth to be 4.5 billion years old plus or minus 100 million
1:08:07
years. This is, by the way, this is the accepted age of the Earth to this day. So this is a number that has held at this point for almost a century. This is an incredible finding that he's added. In 1928 he issued a paper that was entitled "Radioactivity and Earth's movements." So this is while Alfred Wegener is still alive. Remember he dies in 1930. Holmes proposed that convection currents in the Earth's mantle could drive continental movement. And for this to happen
1:08:42
the mantle had to behave like or to had to be a rheid which is a solid that flows like a fluid over very long periods of time. So it's solid, but over long periods of time it acts like a fluid. it can flow. Think of it as something like tree sap. Right. Tree sap coming out of the wood hanging on the edge of the tree. It will just sit there as a big bead, but over time it'll slowly start to drip and drop down. And his idea, this is actually figure number one from his 1928 paper,
1:09:19
this black area here is the Earth's core. And what he is suggesting is that the mantle which is this unit between the crust which is the outer edge right here down to the core was circulating. Right? Circulating. That's what we mean by convection. A convecting current is where we have things that, as they cool down...as things cool down near the surface, they contract. They become more dense and dense things tend to sink. Once it gets down near the core-mantle
1:09:55
boundary, which is...again, here's the core, here's the mantle...it begins to heat back up again and it starts to expand. And as it expands out, it reduces its density and it begins to rise back up into the mantle. And so it's creating this chain. Like boiling water circulating in these large convection cells within the Earth. And so this was one of his proposals. And he said if the Earth was doing this then suddenly you
1:10:25
have a mechanism for continental drift which was something that he was interested in deeply. Now in the 1929 book issued by...his fourth edition by Alfred Wegener...he makes a reference to people being interested in this convection idea right, in 1929. The problem is that he dies in 1930 and the book wasn't translated until 1966 into English. So for a very long time the most of the English
1:10:54
speaking world did not know that Alfred Wegener was aware that there might have been this driving mechanism within the mantle. But it was definitely there in his fourth edition in 1929. So for mantle convection to happen, we now know that the mantle has to have the dual nature of acting like a fluid over long periods of time but acting like a solid over very short periods of time. And the answer to that question actually came through an interesting
1:11:24
study that occurred over a period of several centuries. In the 18th century, Anders Celsius,... yeah, the same guy that the Celsius scale is named after. It's interesting. Lord Kelvin has the Kelvin scale named after him. Celsius has the Celsius scale name named after him. And both of these people have deep interests in what's happening with the Earth. I don't think that's a coincidence. I think that's just what scientists do. But in the 18th century, Anders Celsius noted
1:11:51
apparent sea level drops along Sweden's coast and started marking tidal benchmarks to track the changes. And one of the classic places where this happened was this rock right here. There's a tape measure down here. Here's current sea level. Notice right up here where that arrow is. There's a little line. You can barely see it. Let's move over to here. Says 1731. That is one of the title lines in fact that Celsius himself directed to have put onto that rock. It's still
1:12:20
there. It's a very famous rock. It's one of the "seal rocks" that exist in that area. They call them "seal rocks" because seals used to actually sit on top of the rocks apparently. The original interpretation of this phenomenon, which he had been told about by people, which was that the sea seemed to be moving out and away and that the land was becoming dry. So he interpreted this as a decrease in sea level. This was interpreted widely as a decrease
1:12:49
in sea level in Sweden. The problem was that when you went to other places in the world to see how much sea level change had happened, you didn't observe it. There was very little sea level change, in some places there was none in and in fact if you were to go to southern California you would see a very different rate of sea level change there. So in 1865, Thomas Jameson who is a Scottish geologist identified rising land in Scandinavia as post-glacial rebound. What
1:13:23
he said was that it's not that the sea is dropping, it's that the land is rising. And he had detected it was becoming common knowledge at that time in the geological community that there had been an ice age that had left large ice sheets across much of the northern hemisphere, specifically in Scandinavia, and that the areas where the ice sheets had retreated was where they were seeing the most uplift in that area. And so the idea of postglacial rebound that the isostasy,
1:13:58
remember that concept of isostasy we brought up earlier - that if you remove the weight of the ice from off the land that the land would rebound or push itself up to fill in that gap. So, post-glacial rebound raised shorelines due to crustal uplift after the melting of ice sheets that had recently existed. So, we now have a...here's a more modern map of it. Don't worry about the fact that
1:14:29
it's not in English. All you need to understand is that the amount of rebound that we're seeing here is determined by these circles. So, the maximum rebound is right here and it moves out. So there was a big glacier that was here that was pressing down on the crust and since it's receded that whole area is now starting to move up in some areas over 200 m. So that's really high. That's
1:14:51
a tremendous amount and it's still rising to this very day about 1 centimeter per year. 1 centimeter per year. That's about four-tenths of an inch for those of you that live in the United States. Now what does this imply about the mantle? Okay, so this rebound indicated that the mantle which was pushing the land up or rebounding after the ice sheet was removed was a ductile rheid. So ductile means it's moving and it's flowing and doing...it's acting like something that could
1:15:27
bend and and deform: a rheid. So in other words, over a long period of time it's ductal. over a short period of time. It's effectively not. And therefore had properties like a slow-moving fluid and it had the principle of viscosity. So viscosity is a measurement of a fluid's resistance to flow. So for example, honey is more resistant to flow. It does flow but honey will flow very slowly compared to say water. So the viscosity, things that are higher viscosity,
1:16:03
tend to flow very slowly. They resist flow. So we we have a slow-moving fluid, which is say it's viscosity...that it has viscosity...but was rigid enough to transmit seismic waves. Earthquakes can move through it and this was actually very well known at that time. In 1935 Norman Haskell...and this guy is another genius....he's like Arthur Holmes...really makes a difference in the geocciences...but Norman Haskell comes in and he calculates the Earth's mantle viscosity by
1:16:36
modeling the post-glacial rebound in Scandinavia. He says, "Hey, we now have a rate, and from that we can figure out the viscosity!" By using the uplift rates from the raised shorelines and the tidal gauges, he figured out the viscosity. And for those of you that need to know, it's ten to the negative twenty-one pascal-seconds is the answer that he got. I don't have it here in the presentation, but if you want
1:16:59
to do the calculation yourself, you you can do that. And based on these findings, in 1935, a geologist by the name of Pekeris was able to demonstrate that heat from radioactive decay in the core could drive slow, large-scale mantle convection. In fact, he would go to the point of saying that it did...eventually. I mean, he eventually got to the point where he actually says it DOES drive convection. In fact, it has to happen. So this is an image from his 1935
1:17:31
paper where he is showing convection currents circulating around. These are...the convecting mantle as we're seeing here with the cells coming around the core. So, they're arguing that yes, it is a rheid. It has viscosity and the mantle also acts like a solid over short periods of time. So it's kind of both. And this is very confirmational of the isostasy information, the isosity model that we were talking about earlier as well as the concept of convection
1:18:06
within the mantle. And so we now have the theory of continental drift. It's a theory, right? The theory was intriguing to some geologists, especially in the southern hemisphere. In fact, it was much more popular in the southern hemisphere than it was in the northern hemisphere. But it was also purely theoretical. Nobody had made a clear observation of mechanism anywhere on the Earth, but it was slowly gaining in popularity, but it was also gaining in notoriety.
1:18:37
People, especially in North America, really did not like it. The South Americans and South Africans actually found it pretty interesting and were working with it. In particular, a geologist by the name of Alexander du Toit. So Alexander du Toit, that's how the Americans pronounce his name, by the way... Alexander du Toit... even though he has a French surname, a South African geologist who provided strong support for continental drift through his work on geological
1:19:05
similarities between South America and Africa and his book, "Our Wandering Continents." So he wrote that in 1937. There was...Arthur Holmes wrote a textbook and he incorporated continental drift as his primary mechanism in 1931. So this is starting to become a thorn in people's sides, especially those that believed very strongly in the contraction theory. Continental drift remained unfashionable in most academic circles until the 1960s, especially in North America.
1:19:39
And, so I just wanted to include this. This is from Alexander du Toit 1937 book. This is an image where he is showing a geosyncline. So he's saying, "Hey, we we got geiosyncs here"...these fold belts that I was talking about earlier. And he connects it all the way from Africa or through Africa, South America, into Antarctica, and into Australia all the way across here. And he shows his map of Laurasia and Gondwanaland, and what it all looked like at that time.
1:20:10
Fascinating people, absolutely fascinating people. But again, now we have a theoretical framework, but the question is where's the observation? How do we know who's really right? And the problem is is that all of that information is at the bottom of the ocean. And the technology to be able to look at the bottom of the ocean really wasn't developed until 1944, 1945, maybe a slightly before when the outbreak of World War II had happened and the technology started to be brought
1:20:42
into naval battles to be able to look for submarines. And finally, they were able to get imagery of the bottom of the ocean and actually confirm what it was that was really going on down there. Were there geosynclines forming down there....exactly as Dana had suggested there were? Was there kind of these sunken continents, almost like Atlantis, as Seuss had kind of implied might be down there? Were there these land-bridges down there? Or was there something that might
1:21:12
indicate that the continents were drifting? That was a major discovery in the 1940s and 50s when they finally started to put together all of that data and that's where we're going to pick up for the next lecture on plate tectonics and the origins of that thinking. I'm going to take a couple more slides. I want to talk about Lord Kelvin. If you are just interested in the plate tectonics thing, we're done with that a little bit. But I want to come back
1:21:44
to Lord Kelvin again because there's a chance that we might want to vindicate him on some of the stuff that has happened. He gets a lot of ridicule for having this very poor age of 20 to 40 million years, but there's actually something scientifically very interesting baked into that number that they should have picked up on at that time. And if you're interested, stay with me for a couple more minutes while I go through it. So, Lord Kelvin....vindicated? Lord
1:22:14
Kelvin's 20 to 40 million year old Earth estimate was controversial right from the start. Remember, the geologists didn't like it because it didn't give him enough time for all the processes, the erosion of continents, the rebuilding of mountains and things like this to occur, and geologists just simply said it just doesn't give enough time and the church felt it that it allowed too much. So he wasn't making anybody very happy with his dates.
1:22:41
He had two key assumptions for his 20 to 40 million year old Earth and they were this: that the cooling was conductive not convective, and 2) that no undiscovered energy source existed within the Earth. Right? So later on people would come in and say there was radiation and radioactivity and that's a newly discovered energy source for the Earth. And he was not persuaded by that. Let's examine those two points. So his rejection of radioactivity as a
1:23:13
heat source was cited for a century as the reason for his error. And...you know, like I said, when I was a young scientist, a young geologist learning at the University of California, one of the things we would hear about is the poor treatment of Kelvin's age. All of the young students would hear about it, and this was kind of common knowledge that he had been ridiculed for this. However, incorporating radioactivity with conduction yields a negligible age change
1:23:46
as Kelvin had assumed. So when you actually do the numbers and you model it the way that Kelvin would have modeled it, the amount of heat from radioactivity is only 50% of the total heat coming from the Earth. which to him would say if it's only 50% of the heat coming out of the Earth and I have a 100 million year age or a 20 million year age and it's 50%, What are you really telling me? It's still going to be cooling down and we still have a pretty
1:24:19
valid number right between 20 to 100 million years. In fact, there's a wonderful paper that's written about this published in 2007 where they actually concluded the same thing. However, when you run the numbers and you assume convection, the Earth's age suddenly becomes two to three billion years old. In fact, if you add radioactivity at that point, it seems to run it up to about four and a half billion years old, which is the currently
1:24:48
accepted age under Arthur Holmes. So the problem doesn't seem to be necessarily with the fact that he doesn't know how to do math or that he used a incorrect model and that this was a problem. Kelvin's error wasn't rejecting radioactivity. More so his problem was accepting the geological model of his time. He was somebody that believed in the contraction theory. The contraction theory believed that the earth was cooling through conduction. So Kelvin's incorrect age estimate
1:25:21
revealed flaws in the tectonic model more than the ignorance of radioactivity. Had people said, "Listen, if the Earth has to be 20 to 40 million years old to cool down to the levels that it has right now, the problem isn't with your heat calculation, the problem is with the model that we're using." And this is something that they should have held up in the mirror and said, "We need to evaluate this." But they did not. In fact, what they did was they ridiculed the result
1:25:46
of the age and said it has nothing to do with the geological reality and you can't understand what it is we're doing. We have infinite amount of time. The other thing that he showed was that the Earth wasn't ageless. This was a very big deal. He showed that the Earth has a a very clear beginning. It has to have a beginning and that is something that he needs to get credit for. Is he vindicated for this? Well, he accepted the cooling model of the geologists at the time, but
1:26:15
what he should have been able to put together was either the model is wrong or the date is wrong. And if the date is wrong, then the model must be wrong. So, very fascinating situation. Is he completely vindicated? I I think we could partially vindicate him. He's like I said, I think he received a lot of bad press from the geologists. You see it in a lot of the textbooks to this day that he got it wrong because he didn't know about radioactivity. One, he knew
1:26:44
about radioactivity and he rejected it. Two, if you incorporate radioactivity into the model, it doesn't make a big difference in the age of the Earth that he would have gotten. And third, had he used convection instead of conduction, he would have gotten an age estimate much closer to the accepted age that Arthur Holmes would generate about a hundred years later. So fascinating, right? Had he done the things that Arthur Holmes was pursuing,
1:27:10
he might have gotten that age calculated correctly and we would have actually been on the right track at that point. All right. So, I just wanted to say that few notes about Lord Kelvin, whether or not he's vindicated or not. Again, 2007 paper. In fact I'll put it into the the subject description for this video...a link to that paper if you want to read it yourself. I actually drew upon it very heavily in the preparation of
1:27:40
this lecture. So thank you again for sitting through this and thank you for your interest in earth sciences and we'll see you in the next video when we talk about plate tectonics. Thank you.
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