Where Did Dietz and Hess Find Additional Evidence That Supported the Idea of Continental Drift?

GEOLOGY/GEOPHYSICS 101 Program 6

THE BIRTH OF A THEORY

Plate tectonics is a revolution in Earth Science, a revolution so great that it may turn out be be equal in magnitude to the realization that the Earth is spherical, and, also, the realization that the Earth is like another planet and goes around the sun in its orbit.

Plate tectonics is a unifying theory, which explains many features and processes that we find on the Earth. It explains the locations of earthquakes and volcanoes. It explains mountain building and rock deformation on the continents, and even, in fact, describes the shapes and locations of the continents. It also helps us understand the youthful age of the sea floor and the unusual distribution of fossil and living organisms on the continents.

In fact, plate tectonics actually combine two other theories, continental drift and seafloor spreading into a comprehensive global theory.

It's curious that the continents on opposite sides of the Atlantic Ocean fit together so well, almost like a jigsaw puzzle. In fact, if I move South America and Africa back together, I see that they fit together almost perfectly. The black line that runs through the center of the two continents represents the distribution of a particular type of fossil called "Mesosauras," which was a not particularly noteworthy reptile, which was found only on these two locations and nowhere else in the world. Is it just a coincidence that the continents fit together so well, and that the distribution of a particular animal happens to fall in that place?

Well, the shape of the continents was noted almost as soon as maps were made of the new world. It was noticed first by Francis Bacon back in early 1500s. Very soon after that, there were suggestions that there had been a huge continent, which broke apart by undescribed forces. There was even a theory in the late 1800s that the Atlantic Ocean might have been formed by some cataclysmic event. I should note here that at the same time there was a very popular theory that the moon had been formed by being ripped loose from the Pacific Ocean leaving the huge gap, and this theory of the cataclysmic origin of the Atlantic Ocean was paralleling that theory of the origin of the moon. Of course, now we think that the moon was formed along with the rest of the Solar System, and so we don't think of these cataclysmic theories.

Edward Seuss , a scientist in the late 1800s proposed that there had been a single continent, only he proposed that the continent didn't split apart, but rather that the center of the continent sank by isostatic adjustment. This was an interesting theory or an interesting suggestion, but Seuss gave no explanation for why the center of the continent should suddenly become heavy enough to sink, and you remember that isostatic readjustments require some sort of a change in density or added forces.

The idea of continental drift really is an idea before its time. The idea was first put down in a more or less modern form by Alfred Wegener. In 1915, Wegener published an expanded version of an earlier book, which he called "The Origins of Continents and Oceans". Wegener was a good scientist; in fact, he was a meteorologist, who was most noted for his work on the Greenland Ice Cap where he died in 1930, having never seen his theory accepted by scientists. What Wegener proposed was that 200 million years ago, all of our present continents had been joined together at one place on the Earth and a supercontinent that he called "Pangea," and has since split apart, and the continents have now moved into their present locations.

It's an interesting suggestion, but suggestions in Science are not easily accepted. A scientist must have evidence and must also present a mechanism by which the proposed changes could have happened that are consistent with current theories and other models. Wegener was missing information that had he known would have explained the workings of this continental drift theory, and he predicted that understanding the sea floor would hold the key. Wegener's theory died about the same time he did but reemerged 30 years later in the 1960s with new evidence and new understanding of the Earth's interior. The study of plate tectonics also provides us with an excellent example of how the process of Science works, and how scientists come to accept or reject various theories, so before I introduce you to the program, let me remind you of the reading assignment.

This is Lesson 5, "Birth of a Theory,"

• and you should have already read the text assignment. The assignment for this program is to read Chapter 4, pages 65 to 79; that's the first half of Chapter 4, and you'll read the last half of the chapter for the next lesson.
• Be sure to read the Introduction, and, also, be sure to read the summary even though you haven't read the entire chapter yet.
• The Summary gives lots of good information that will help you understand this, and, if you need to,
• review the previous lessons to understand the bases for these three theories, the three theories of continental drift, sea floor spreading and plate tectonics and be sure to study all the photographs and diagrams carefully, especially in this chapter because these models will help you understand these processes,
• and don't forget to follow the study plan in the study guide as well.

There are several objectives to this lesson, and I'll review these with you briefly.

• We want to explain the Theory of Continental Drift.
• We want to list the lines of evidence used by Alfred Wegener to support his Theory of Continental Drift and the existence of the supercontinent.
• I want to discuss some of the objections to the Theory of Continental Drift and cite the evidence that contributed to a revival of the idea of continental drift.
• We want to explain the methods used to determine the drift history of the continents and summarize Harry Hess's Theory of Sea Floor Spreading and,
• also, to describe the causes of magnetic patterns on the sea floor,
• and how they allow us to measure the rate of sea floor motion and predict the age of the sea floor, and,
• finally, we want to be able to describe the rates of plate motion.

So before we actually start on today's lesson, I want to remind you once again that we have an exam coming up. The exam will occur after Lesson 6 covering Chapters 1 through 4 and to remind you to review previous material, as needed, to make sure that you understand.

This lesson will help us to understand the origin of the ideas of continental drift and sea floor spreading, and how they contributed to the theory of plate tectonics, and it also illustrates the way science works.

A few notes on the video before we start. In this video there are several animations of the motion of lithospheric plates that are very excellent. Now, keep in mind that this is a computer model that helps us to visualize what we can't really see happening, and we'll see this model reappear in several different contexts as we get into more material as the course goes along. When you watch these animations, remember that these are time lapse animations. The time that actually passes in these animations represents hundreds of millions of years, and, obviously, things are not really happening that fast. You'll especially note at subduction zones bubbles of magma rising through the overlying upper crust and mantle. These things don't really happen this fast. There are also many animations, which the narrator doesn't call attention to, and you should by this time be learning to develop a way to make associations between the pictures and processes that you see without having your attention called to them.

One example of this is the model of convection that you'll see in the video. In reality, the process of convection is quite chaotic, and models often show the process simply as a bunch of arrows, and you'll see in the computer model the different colors, which represent the different temperatures of material actually become quite chaotic. Another thing I want to note about this video is that for the first time, we start to introduce the names of some of the geologists, who were important in contributing these theories. It's not that the names themselves are important to memorize, it's that this is one way in which we honor people who come up with good ideas. The name actually can be a model for the ideas.

When we talk about sea floor spreading, if you can get a sense of who Harry Hess was from the video, then you'll always connect the name, Harry Hess, with sea floor spreading, and the whole process starts to unfold in front of you, and really, there are only three different sets of names that we have to remember.

• The first is Alfred Wegener and the Theory of Continental Drift.
• The second is Harry Hess and the model of sea floor spreading.
• The third is that of Vine and Matthews, who provided the evidence for Hess's Theory of Sea Floor Spreading.

Now, there's also a countertheme running through this lesson that has to do with the Scientific Method. Well, some people call it the Scientific Method anyway. I'm not sure it's really a method. Maybe, it's more like a process.

The Scientific Method, in fact, is a lot like a cooking method, that no two people do it exactly the same way, and, in fact, a variety of methods produces very similar results in many cases, and, I think, Science operates in pretty much the same way. Some times we try to learn about a particular method in which scientists follow, and scientists like everybody else don't really follow a method, but there are certain things that we need to do, and I'll talk about this after the video.

I'd like to take a few minutes now to talk about Wegener and Continental Drift. Much of this material will be covered in the video, but I'd like to take a few minutes to review some of this, so that you get to see it more than once.

As I mentioned before, Wegener was not the first person to notice the shape of the continents and how well they fit together. So what's the difference between simply noticing the shape of continents and coming up with a theory of continental drift. Does the shape mean something? Or is is just a coincidence.

Well, Wegener was a good scientist, wasn't willing to accept the coincidence, so he looked for evidence, which might confirm or deny whether or not the continents have actually been together. He likened this to the process of trying to put back together the page of a newspaper. If someone's taken a newspaper and torn it into many shreds, and you're able to very meticulously fit together the pieces of the newspaper, when you get all the pieces put back together, if the lines still make sense and still read across, there's a pretty good chance that the pieces came from that page, and this is basically what Wegener did with the Theory of Continental Drift.

He uncovered many different forms of evidence and, in fact, presented it as one of the best documented theories in the history of Science. It was very thorough, very scholarly, very well presented. The problem was that on top of all the evidence, he made certain suggestions about the way the process worked that weren't feasible, and after the video,

I'll come back and review some of the objections to the Theory of Continental Drift, but let me turn the attention now to looking at the lines of evidence.

When we talk about lines of evidence, as I mentioned earlier, Wegener was very meticulous in his putting together of the evidence, and he actually separated the evidence into several different areas. One of these, as already mentioned, is the fit of the continents in the jigsaw puzzle. It's interesting to note that if you look at the present shorelines, they only approximately fit, and processes in 200 million years might have modified the edge of the continents, and Wegener was very well aware of this, and, in fact, he was only suggesting a crude fit, but we remember that the edge of the continent is now submerged, and, in fact, the continental shelves, which extend into the sea from the continents are actually parts of the continental block.

If you put the continents back together at the edge of the Continental Shelf, the fit is even better than Wegener would have expected, and had he known the existence of the continental shelves and their exact shape, it would have provided extra evidence for his theory. In addition to the fit of the continents, Wegener also noticed that there were rock and structure correlations.

What I mean by that is that rocks of a particular sequence or a particular type are found on both sides of the Atlantic Ocean in very small regions. The tip of South of America and the bend in Africa, when you put the continents back together show rock structures that are the same on both continents. Well, I shouldn't say totally the same on both continents because part of the key to this evidence is the fact that if you look at rocks that are older than 200 million years, the rocks are very similar on the two continents, but if you look at rocks less than 200 million years old, then the rocks become different.

Now, the processes by which rocks are deposited have so many variables that it's very unlikely that you could deposit exactly the same sequence of rocks on two separate continents separated by 2,000 miles. I should also note that there are similar correlations between rocks in North America and Europe, and also between Australia and Antarctica, and even into India, which also go on to support the supercontinent of Pangaea as Wegener had proposed. Wegener was a meteorologist, and as a meterologist he was very interested in climate. One of the things that attracted his attention to the idea of continental drifts in the first place has to do with what we now call paleoclimate .

The word "paleo" means old, and "climate" means climate. By looking at old climates early in the history of the Earth, we can get some sort of an idea about what kinds of processes were operating in the atmosphere at that time. How do we learn things about ancient climates? We can't really go back in time and measure temperatures and measure rainfall and that sort of thing, but we still have records left preserved in the rocks that can help us to understand this.

For one thing, when glaciers accumulate on continents during rare glacial periods, as the glaciers move, they scratch the rocks below them and leave very distinctive marks, grooves, or striations in the rocks that allow us to know, not only that the glacier has passed over the area, but also to note what direction the glacier had moved. If you look at marks of ancient glaciation. By ancient, I mean, 500 or 600 million years ago on some of the older rocks in the continents, in the southern part of South America and Africa, these glacial grooves point in all different directions, and, in many cases, they go contrary to what you would expect.

Now, we'll study glaciers in some detail on a later lesson near the end of the course, but for now we might note that when glaciers form, large glaciers of the continental variety, they generally form in cold places near the poles and move toward the Equator. When you examine the glacial striations in the Southern part of South America and Africa, many of them point in the direction and indicated that the glacier formed near the Equator and moved away from the Equator. Not only that, but much of the area that was glaciated now lies near the present Equator. In addition to all this, we find the evidence of tropical swamps in the form of coal and other types of deposits that are now in very widely scattered locations.

The continent of Antarctica, of course, which is now near the South Pole and entirely frozen, contains rocks, which were formed in a tropical swamp. So do the rocks in Eastern Pennsylvania in the United States. Now, what this means, you see, is that either there's been a radical change in global climate where the climate has been so reversed that what's now the Equator was the Poles and vice versa, and that life forms, which now only accumulate in tropical conditions could have lived under polar conditions.

You either have to accept radical climate changes, or you have to accept the fact that the continents have moved, and Wegener preferred to think that the continents had moved. Fossils,which are the preserved remains of animals and plants, also give us many clues to the locations of the continents. I mentioned in the opening about the distribution of mesosaurus. It turns out that fossils which are greater than about 150 million years old are very similar on two continents, South America and Africa or North America and Europe, but fossils that are younger than 150 million years start to get more and more different from each other, the younger they become.

There are several examples of these types of fossils, of the mesosaurus, the lystrosaurus, two reptiles, and the glosopterus fern. The glosopterus fern is an extinct variety of fern, but it's highly significant in this respect because it's a special type of a fern known as a "seed fern." Most ferns reproduce by spores, but this is a seed fern, and in modern situations, seed ferns are only found in polar climates; in other words, in cold rain forests like those now found along the coast of Alaska and Western North America. The glosoptaurus fossils that we now find are found in a variety of climates: Antarctica, South Africa, South America, and India.

The video will go over these same points again and will give you many animations of the various features that I've been discussing here. Before we actually see the video, though, I want to come back to the idea of seafloor spreading.

The idea of a Scientific Revolution, the idea of a radical understanding of, the new understanding of Earth processes, comes about, not through continental drift but through new evidence about Earth magnetism on the Earth's interior. Once the idea of magnetic reversals was established and we began to understand more about how to read the magnetic record in the rocks, the tide of opinion rapidly shifted, and many geologists began to understand that the Earth's surface is very mobile, and the mantle is plastic.

The things that's to me most interesting about the theory of sea floor spreading in terms of looking at it as a scientific process is the way in which it was put together in the first place. The model that we now call sea floor spreading was first proposed by Harry Hess in 1963. Hess had been a ship commander during World War II and had access to an echo sounder, which he kept turned on all the time in the hopes that he'd be able to use the data to understand about the Earth. Hess was a trained geologist.

After the War, as he started putting together information, he began to develop the idea that the Earth's crust, especially the oceanic crust, is continually recycled and recreated, and it became apparent about the same time that the oldest sediments on the ocean floor are only around 200 million years old, as we mentioned last time.

In 1963, Hess wrote a paper published in the Scientific Journal, which he called an "Essay in Geopoetry." It's an interesting title, "Essay in Geopoetry." See, in order for there to be Science, there has to be evidence, and Hess virtually had no evidence for his theory. The only evidence he really had was the age of the sea floor rocks, the youngest, 200 million years old, virtually no other evidence. He proposed that the Earth's crust moves along the surface, is created at the mid-ocean ridges, and is moved across the upper mantle to be destroyed at subduction zones in the trenches. The continents according this model are sort of unwary passengers, passive passengers, if you like, on top of the lithospheric plates. If the moving plate happens to contain a continent, okay. If it doesn't contain a continent, it doesn't really matter because the depth to which the motion occurs is deeper than the base of the continents, and in the cases where two plates carrying continents collide with each other, you have certain features formed.

Hess also--I said he had no other evidence, he had one other piece of evidence, which has to do with earthquakes. We learned in an earlier lesson that the speed of earthquakes depends upon the density of the rocks, and that at a depth of about 100 to 200 kilometers, there is a so-called low velocity zone. Hess recognized that this low- velocity zone corresponds to a region of partial melting, and that means that the rock is more plastic and is capable of flowing, so in the model, the rigid upper part of the Earth's crust called the " lithosphere " slides over the lower part of the mantle through this lubricated layer of partial melting that he called the "asthenosphere." Okay, the real key to understanding sea floor spreading came by studying paleomagnetism. Remember the word " paleo " means old. Well, "paleomagnetism" means studying old magnetism. Remember from our previous lesson that rocks often contain a mineral magnetite, which orients toward magnetic north. So that means that when lava solidifies, the magnetic magnetite grains retain the orientation of the Earth's magnetic field at the time that the rocks solidify.

It turns out that when you study the rocks on the continents, you find various orientations, various alignments of the rocks in different parts of the continents. Trying to sort out this mess is a very difficult task. It seems that when you try to put together the directions that all of these various grains are pointed in different locations, you're forced to come to one or two conclusions.

Let me give you a specific example. Rocks about 200 million years old in North America point to a location near the present north magnetic pole around Hudson Bay. However, rocks of the same age in Europe show that the north magnetic pole was someplace else. Now, here we have a problem, you see, because our understanding of Earth's magnetic field is that we have only one North and South Pole, not several, so either we have to accept the fact that the Earth's magnetic field was different in the past, or that the magnetic poles have moved and that they moved in a particular way, or that the continents had previously been in a different place, and, sure enough, if you move the continents of North America and Europe back to where Wegener suggested that they had been in the supercontinent, then the rocks older the 200 million years point to the same North and South poles, so we're faced here with a problem of deciding which is more likely, the moving poles or moving continents.

Even with this new paleomagnetic evidence, not everyone was convinced. The study of magnetism was a new and untested technique, and this so-called remnant magnetism weakens with time, and it's possible that the rocks may acquire a secondary magnetism by other processes. In the 1960s Vine and Matthews discovered magnetic stripes on the sea floor. Now, these strips aren't really stripes. It's not like they're painted on the ocean floor, but by the 1960s it was accepted that the Earth's magnetic field periodically reverses itself. The pattern was reconstructed over several million years, and we see that there are about 170 reversals in the last 75 million years. Now, these magnetic reversals don't follow a particular pattern. They average about 450 thousand years between reversals, but there's really no pattern, and the exact cause of the reversals is unknown, but it's probably related to convection currents in the liquid core. The magnetic stripes are really variations in the strength of the Earth's magnetic field. See, the magnetism of the magnetite either adds to or subtracts from the Earth's magnetic field. If the orientation of the magnetite grains is the same as today's magnetic field, then it makes the field a little stronger. If the orientation is reversed, then it subtracts from the field, and the field is a little weaker, so that if you measure the magnetic field in the region of the mid-ocean ridges, you find that the intensity of the Earth's magnetic field fluctuates; it moves up and down as you travel over the ridge.

Vine and Matthews made a connection with sea floor spreading. The connection is a very simple one that they predicted that these magnetic stripes ought to be symmetrical with the ridge. What that means if you find one stripe on one side of the ridge, you find the matching stripe on the other side. And they then took Hess's sea floor spreading model and put it together something like this: Magma rises at the ridge, solidifies retaining the magnetic orientation of the Earth at that time. The spreading continues as new magma is injected. Each time the magma cools, it maintains that orientation. When a reversal occurs, all the magma that's erupted at that point takes on the new orientation, and then that spreads away from the spreading center as well. It's as if there's a tape recorder, which is recording the Earth's history.

Once the theory of sea floor spreading began to make sense, other methods of studying the Earth, including gravity and earthquake studies, and heat flow, and radioactive dating, and various other aspects began to put together this model of plate tectonics, and today we're able to calculate fairly accurately using the magnetic reversals, the rate of sea floor spreading, and we find that in the Atlantic Ocean the rates are from one to four centimeters per year and in the Pacific Ocean from three to ten centimeters per year. Just a sense of how long or how slow this process really is, I mentioned in a previous lesson how fast your fingernails grow.

The rate of sea floor spreading is about the same as the rate of your finger nails. You might look at it this way that in your lifetime the continents will move about as far apart as your height, so by the time you get to be 50 years old, the continents will be about five feet further apart then they were when you were born. Well, with all this in mind, let's watch the video.

Major funding for "Earth Revealed" was provided by the Annenberg CPB Project.

The rich diversity of land forms and geologic activity on Earth make it unique in our Solar System. Three decades of space exploration have taught us that most of our neighboring planets are geologically stagnant when compared with the dynamic Earth.

Until very recently, geologic phenomena such as earthquakes, oceans, volcanoes, and glaciation were thought to be unrelated aspects of nature. But in the 1960s Earth scientists had a bold and revolutionary realization that completely changed our concept of the Earth. They surmised that these diverse geologic phenomena could be explained by a single unifying concept, a blueprint of global geologic activity called the "Theory of Plate tectonics."

Plate tectonics is built on the premise that the outer skin of the Earth is composed of individual slabs or plates. These rigid plates, which can be continental in size and in some places hundred of kilometers thick slide around on a partially molten mantle layer below. Where the plates collide or moves apart or slide past one another, a great variety of geologic activity results. Ocean basins open and close. Mountain belts emerge, and volcanoes erupt, all accompanied by countless earthquakes and the formation of new rocks and land form. Although we still have a great deal to learn about the mechanisms in the Earth's interior, which cause the plates to move, most scientists agree that the concept of plate tectonics is valid.

Today the Theory of Plate tectonics is widely embraced, but the road to its acceptance was long, and at times fraught with controversy. During the Fifteenth Century, Europe began its great age of exploration. As navigators charted coastlines the world over, scholars became intrigued by the similarity between the Atlantic Coasts of Africa and South America.

In 1912, Scientist Alfred Wegener introduced his Theory of Continental Drift, which sought to explain this similarity. Alfred Wegener's one of the individuals, who in the Twentieth Century has had a major influence on geological thought. He observed like several before him the geographic relationship along the Atlantic Ocean, and that the obvious match between, particularly, the African and South American continent. This led him to conclude that the surface of the Earth, that the continents were not stable but, in fact, mobile and led to his idea of drifting continents. At the time it was not accepted; it was not well accepted in either geological or other scientific thought but ultimately became the foundation for what was a real revolution in geology.

Wegener poured over geological maps, maps that showed identical rock types, fossils, and mineral deposits in many different lands. He was intrigued by what he discovered. He began to see that certain patterns matched up if you carried them across major ocean basins, particularly in the Southern Hemisphere continents or what we now call "Gondwana," which is split apart into Antarctica, Australia, South America, and Africa. What he found is that certain plant fossils matched up very nicely on these different continents. He found certain climatic zones, glaciation zones, or coal bed zones matched up on these various continents, and they didn't make sense with where the continents are today in terms of the current climatic zones. When he fit Africa against South America, Wegener discovered that the rocks of a South African mineral belt coincided with similar formations in Brazil. A South African mountain range ending at the coast lined up nicely with a similar range in Argentina. Furthermore, the areas of ancient rock in Africa formed a continuous belt with those in South America.

Wegener wrote it is just as if we were to refit the torn pieces of a newspaper by matching their edges and then check whether the lines of print run smoothly across. If they do, there is nothing left but to conclude that the pieces were, in fact, joined in this way. So Wegener really put together the story for all the continents and wrote it down and collected all the evidence that he could about it, and so that's partly why we hear so much about Wegener because he brought it all together. Wegener's suggestion that the continents had once been joined and drifted apart to their present positions ignited a major controversy in the scientific community. Acceptance of Wegener's Continental Drift Theory seemed to be divided roughly along hemispheric lines.

I think the geologists who were working in the southern continents never had any doubt because the evolution of species is so clearly parallel on them, and they have so many other geological events that are in common between the South Africans, and South America, and India, and Antarctica once we began to explore it. There is just no doubt that they were all parts of one piece. But most geologists weren't working in the southern continents. They were working in Europe and North America, and they just didn't catch on.

Many scientists were especially disturbed by Wegener's explanation for how the continents moved apart. Wegener explained his Theory of Continental Drift based on the fact that the continents were moving through the ocean basins, and the continental crust was basically following through this oceanic crust. Geologists at the time could not understand how the rigid, tough continental assemblies could plow through the weak oceanic basins, and still it was the continents that were deformed to make mountain ranges. That seemed rather peculiar.

According to Wegener, the mechanism behind continental drift was the centrifugal force of Earth's rotation and the tidal drag of the moon and sun. Geophysicists quickly and correctly exposed the flaws in this reasoning. In fact, the most difficult aspect of this theory was that he alluded to a centrifugal force as explaining why, in fact, the continents drifted. It was shown quite conclusively that the forces were too small; the rocks were far too strong for this to have been the case. So people were able to reject his hypothesis completely at the time based on a mechanism which, in fact, was shown to be incorrect.

Because Wegener's explanation for why continents drift did not hold up, he couldn't convince most other scientists that the continents had, in fact, ever drifted at all. Some people like to say that scientists are so fuddy duddy, they refuse to let any new idea come in, and I think that's a bad way to think about this. We really are careful, and we don't want to leap on some glamorous new idea unless our data really does support it in our understanding of how the world works really can incorporate that, so, I think, we really did not have any data that was so cutting and so clear that we had to accept this theory, and so I think it was, they were actually being responsible not-- They were being conservative, but they were also being responsible scientists.

In 1930 Alfred Wegener died of a heart attack while attempting to cross the Greenland Ice Cap. By the 1940s with a lack of any conclusive evidence, the heated arguments about continental drift cooled off. Wegener's Theory, however, would soon, once again, be propelled to the forefront of scientific debate but in a most unexpected way. In order to understand continental drift, we really needed to know how the oceans are made and destroyed, and we had very little understanding at all even of the shape of the sea floor, and so we really needed some new technology. Oceanography ventures are very driven by technology, and it turns out-- I hate to admit anything was good about World War II, but it was very good for developing new technologies.

This new technology developed for World War II included the fathometer, an echo sounding device that instantly measured depth to the sea floor enabling troop ships to move into shallow water. Once transport commander, Harry Hess, made a habit of running his ship's fathometer continually. A geologist and Princeton Professor in civilian life, Hess wanted to learn more about the topography of the sea floor. He soon discovered that the ocean floor was not level as expected but was instead dotted with flat topped mountains, which he named "Guyots" Hess was perplexed by how these mountains formed.

Their flat summits could seemingly be explained by wave erosion, but why, then, did the Guyots lie so deep underwater? Hess thought he saw a clue to this mystery when he observed that the deepest Guyots lay farthest from the mid-ocean ridge system. Hess and a colleague, Robert Dietz, suggested that "Guyots" are, in fact, wave eroded islands, which submerge as the crust beneath them subsides. To account for this subsidence, they proposed a radical new theory, which they called "sea floor spreading."

Harry Hess had a bold imaginative concept, which he enunciated in 1962. It stated that the newly found mid-Atlantic ridge was actually the site of up welling mantle beneath the ridge, and that is it solidified. The melts derived from it solidified at the ridge, they were carried laterally piggyback away from the ridge, much as the froth in a pot of convecting stew would be carried away from the central up welling current.

What Wegener had evoked was continents floating on and plowing or drifting through ocean basins. What Hess and others showed was that the ocean basins themselves were growing from mid-oceanic ridges, and that over the course of time, they enlarged from the ridge and spread laterally. These would then be capable of carrying the continents initially much closer together and actually joined at the onset at the ridge away from one another, directly away from one another, by the spreading process.

The combination of continental drift and sea floor spreading, then, gave them both the mechanism, and the manner, and timing in which the continents had drifted apart from one another. If mid-ocean ridges were continuously creating new sea floor, it would seem that the Earth must be growing, swelling along the ocean ridges like a ball slowly being inflated. But scientists saw no convincing evidence for an expanding Earth. How, then could the excess oceanic crust be explained?

Hess suggested that the ocean floor simply sinks back into the planet at deep marine trenches. He claimed that this process, now called subduction, destroys the surface formed by sea floor spreading. Just as Wegener's hypothesis was weakened by lack of data, so, too, was the model of sea floor spreading. Hess cautiously referred to his own theory as " Geopoetry ," but supporting evidence would come from the emerging field of paleomagnetism.

While the term "paleomagnetism" refers to ancient magnetic fields of the Earth, we can learn about, and we can say something about ancient magnetic fields of the Earth because of the record they leave in rocks, and if mini rocks become magnetized when they are formed or some time after they are formed, when they become magnetized, they become magnetized parallel to the first magnetic field at that time, so if you can collect a rock sample, measure which way it's magnetized, and determine when it was magnetized, then you can learn about the orientation of the magnetic field at that time in the past.

By the early 1960s, geologists were mapping the magnetization of rocks worldwide. One startling discovery came from the ocean bottom where oceanographers discovered an unusual pattern of strong and weak magnetic stripes in sea floor lava. Cambridge University student, Frederick Vine, and his advisor, Drummand Matthews, were especially intrigued.

Two questions fascinated them. What causes the strong and weak parts of the magnetic patterns, and why is the pattern striped?

They soon realized that the changing intensity of the pattern was caused by the reversal of Earth's magnetic field. The evidence we have that the Earth's magnetic field has been reversed in the past is simply that we find rocks, old rocks, that are reversely magnetized; that is to say, they are magnetized pointing south rather than pointing north. There was much scientific debate at first because it's known that some rocks can be placed in a magnetic field pointing one way and become magnetized exactly opposite to that direction, and the fact that such rocks existed meant that it was possible that reversely magnetized rocks were simply recording normal magnetic field.

The clinching evidence came when scientists from the U. S. Geological Survey and from other places went on expeditions around the world, collected rocks of different ages, and were able to show that rocks collected from around the world of the same age always had the same polarity. Rocks that were younger than thousand years old were normally magnetized. Rocks older than 700 thousand years were reversely magnetized. This global pattern of magnetic polarity recorded in the rocks is what convinced people that it was the magnetic field itself changing polarity. Vine and Matthews noted that the striped reversal pattern is parallel to mid-ocean ridges. By 1965 they concluded the best way to explain this relationship is by sea floor spreading.

To check their conclusion, Vine studied the magnetic stripe pattern across several mid-ocean ridges. He predicted if sea floor spreading did occur, the magnetic pattern on one side of a ridge should be the mirror image of the pattern seen on the other. With startling precision, Vine's prediction proved correct. Vine and Matthews figured out this idea and published it in kind of an obscure paper to start with, but then when Fred Vine collected enough profiles across different mid-ocean ridges and saw that it really was true, he went on the road, and he was talking about it to anyone who would listen, and in December of '66, he gave a lecture at Scripp's Oceanographic Institution that just changed the whole direction of the institution.

I arrived there as a graduate student in January of 1967. I thought I was coming to this venerable old oceanographic institute that everybody would be a little bit bored, and they were just crazy. There were records strewn out on the floor all over halls. Everybody was just all in a chaos over this new idea. It meant a totally different way to explain almost everything on the sea floor.

A further test of the sea floor spreading hypothesis was carried out by Geophysicists J. Tuzo Wilson and Lynn Sykes. They were interested in the faulting which offsets mid-ocean ridges. If a feature such as a road or a fence is split and moved by an earthquake, one can deduce how the crust shifted along the fault by measuring the direction of movement and the amount of offset. Many scientists expected the same to hold true with faults cutting across the mid-ocean ridges, but Wilson and Sykes pointed out that if new crust is forming at ridges, that growing sea floor should shift along the faults in the opposite direction.

Based on his study of seismic records from the mid-Atlantic ridge, Sykes concluded that this, in fact, was taking place. Wilson and Sykes named the new class of geologic structure "transform faults," which served as further proof of sea floor spreading. As transform faults were being discovered, oceanographers prepared for the most intensive sampling of the sea floor ever undertaken using the research vessel, Glomer Challenger.

ln 1968 drill cores showed that the sea floor grows older with distance from the mid-ocean ridges. Together with the magnetic data and transformed faults, the cores indicated sea floor spreading must take place. Suddenly, scientists could easily explain many aspects of Earth's Geology and History, which had long been unclear. Not only could the distribution of fossils and paleoclimates be explained, but the origins of mountain ranges, mineral resources, large faults and magnetic field patterns also make sense. Geologists realized that Earth's surface is divided by the system of mid-ocean ridges, subduction zones and faults into giant plates, which slowly separate, converge, or slide by one another.

In 1967 Donald MacKenzie and Robert Palmer introduced the term " plate tectonics " to describe this revolutionary view of our world. I guess you could say that the emergence of the concept of plate tectonics is an example of the Scientific Method in action. We use a great number of kinds of observations, all different, all seemingly unrelated, and bring it together in a model that explains everything. Prior to the advent of plate tectonics, you know, we knew that we had mountain ranges. We didn't know why we had mountain ranges. We didn't know why there was this incredible sharpening. We knew that we had volcanism near the margin of the continents. We didn't know why. These things are explicable based on plate tectonics, and it is the fact that this new model, this new paradigm resolved so many divergent, seemingly unrelated geologic facts, observations that we already knew that allow us to move forward to a new level of understanding.

Plate tectonics has become one of the major unifying theories of Earth Sciences. In many ways it's comparable to the Theory of Evolution in life sciences. It provides that unifying theme upon which we can understand and by which we can understand a very wide array of processes, which are taking place in the Earth's crust, and so, I think it's quite clear that it is probably the most important contribution, which has been made to geological thought in much of its development.

Plate tectonics Theory fundamentally changed the way Earth Scientists view our world. Acceptance of the theory has taken hold worldwide, although a few skeptics remain. There are skeptics to every theory, and I guess you would have to say whether we are talking about evolution or a spherical Earth. You would have to say there are skeptics. There are people I am told who don't believes that the Apollo Mission actually went to the moon. My belief is that it is a very healthy thing that there are such skeptics because they test our honesty basically. It's the emperor has no clothes sort of phenomenon. We all believe in a new hypothesis and accept it so completely that perhaps we don't test where we should, and it is very important that these skeptics exist. Although plate tectonics explains many geological phenomena, certain questions about how it actually operates remain.

Earth scientists are still trying to understand the mechanisms of tectonic activity. In terms of the next challenge of plate tectonics, it really goes back to the initial debates back to Wegener's time, which centered on the actual driving mechanism. We know that the Earth is convecting, and that the movement of the plates is related to this convection. We don't really understand the dynamics of the convection very well. Whether or not the updrafts are actually driving the plates apart or whether the colder parts of the plates that are being driven back into the mantle are actually pulling the plates apart or whether it's some combination of the two, so it's this question that a lot of debate is focused on right now.

In terms of the more surface level processes, I think the major advances or frontiers really deal with rates of processes. How fast do certain plates move past one another, and how rapidly does that cause mountains to form, basins to drop down, basins to turn back into mountains. The plate tectonics revolution has had a profound impact on our view of Planet Earth even extending beyond the field of geology.

The theory of plate tectonics has been, if not instrumental, certainly one of the major contributors to us increasingly looking at the Earth as a single system, and all the components of it as being an inter-related part of that system, and our increasing understanding of an environmental system is essentially a follow through of that basic theme and something that's been very, very necessary. By the late 1960s, sea floor spreading and the concept of continental drift had gained wide acceptance.

Eventually, they were combined into a global model of crustal dynamics that revolutionized geology. The Theory of Plate tectonics has had a pervasive impact on the earth sciences. It's demonstrated that earthquakes and volcanism, mountain ranges, and ocean basins are all inter-related; that they are the consequences of tectonic activity at plate boundaries. It's provided new perspectives and imposed new constraints on all geologic interpretations, and the Theory of Plate Tectonics has shown the geologic diversity and activity of Earth to be the consequence of dynamic tectonism that's been functioning for billions of years. Most current research in plate tectonics focuses on documenting the movements of plates in the earliest periods of Earth History and on understanding the mechanisms of plate formation and movement. These mechanisms operate in a most inaccessible place, deep within the Earth's mantle, unraveling the dynamics of the mantle and its link with the plates is one of the most formidable and exciting challenges facing geologists in the future. The funding for "Earth Revealed" was provided by the Annenberg CPB Project.

The plate tectonics takes two separate theories and puts them together. On one hand, continental drift is well supported by Wegener's data; on the other hand, sea floor spreading is supported by the paleomagnetic data. Plate tectonics is the logical combination of both theories. The easiest way to understand this model is to see how the two separate theories work together.

The evidence for plate tectonics is overwhelming, and any good model in science, not only explains the evidence, but also relates facts from other areas and predicts as yet undiscovered facts. Plate tectonics is a good model. The evidence comes from many different areas of study, and scientists from widely diverse areas are drawn to the same conclusions: Earthquakes, volcanoes, mountain building, and sea floor processes are seen to be part of the same large scale global process.

Before we end the lesson, I'll remind you of the lesson assignment for next time.

• We'll continue in Chapter 4 where you should read the last half of Chapter 4 beginning on page 79 with the section on diverging plate boundaries. Now, this is a long chapter, and there are many concepts to learn, so it's going to take you more than one reading, but this is a fascinating subject, and it's crucial to understanding Earth revealed. It sets the stage for the rest of the course,
• so if you need to, review the first part of Chapter 4 and Lesson 5, and you can go back to look at some of the earlier lessons,
• and be sure to read the box beginning on page 86 about back arc spreading.

Well, that's all for this program, so study this fascinating subject well, and it'll really help you in the rest of the course; it'll help the rest of the material make sense.

Well, I'll see you next time.

Where Did Dietz and Hess Find Additional Evidence That Supported the Idea of Continental Drift?

Source: https://www.honolulu.hawaii.edu/instruct/natsci/geology/brill/gg101/Programs/program6%20BirthTheory/program6.html

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