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0:06
So actually, earlier in my career, I introduced Tuzo Wilson in Toronto for a talk. So I feel that by introducing Jason, my career is complete. Now, Jason, I can retire. But to say that this is an honor is truly an understatement. The 20th century was characterized by all sorts of Scientific Revolutions, and revolutions in the arts as well. All right, we had physics, I came from a physics department, general relativity, quantum mechanics, produced a evolution in physics, in biology
0:36
we had the elucidation of the double helix. And in earth science, we had plate tectonics. All earth scientists know that there is earth science before plate tectonics, and earth science after plate tectonics. And plate tectonics really marks the modern age of global geophysics and global geology. And we're now, I guess you could say, in the post-modern age right of earth science. Well, we're lucky here today, we've been lucky at Harvard for the last 20 years or so, 15 years, Jason,
1:07
to have Jason, one of the principal architects of the theory of plate tectonics, among us. Jason is, of course, very well known for a number of papers. He synthesized a huge number of observations to produce a paper, now 50 years ago, published on rises, trenches, and great faults around the Earth. And what Jason understood better than anyone at the time, was that these boundaries were connected into a global network. But earth scientists know that Jason was even better known,
1:39
in a sense, for elucidating the geometry of plate tectonics. For he used really simple, elegant demonstrations, that for example, the San Andres fault. Its orientation isn't random, it reflects a rotation between the North American plate and the Pacific plate. He brought us an understanding of how one sort of gets a deeper sense of these rotations, and how they build plate motions, and ultimately build geology. He was also I guess you might call Jason the geometer of earth science.
2:13
The sort of Euclid of plate tectonics, although maybe I should get somebody more modern than that, Jason. Mirzakhani of plate tectonics. That insight in to geometry was important also in a classic paper he wrote, looking at what the stability of zones around the Earth were. Like some of them here. You can see many of them here, for example, where three plates come together. Is that situation stable? Will that situation prevail for another million years,
2:43
or 10 million years, or will we evolve out of that situation? I haven't even really talked about work that probably 75% of the earth science upstairs associate with Jason, more than any other, is his understanding of this constellation of hotspots, region of localized heat in the mantle, and what the geological and geophysical implications are of those. Most notably for Americans or Hawaiian-Emperor seamount chain. These are all really fundamental first order discoveries of scholarship.
3:14
And he's been awarded a number of medals that reflect that contribution. National metal of science in the early 2000s, the Japan prize, Wollaston Medal from the Geological Society of London. And, of course the most important of all, I guess, Jason, is an Honorary PhD from Harvard.
3:32
But the story that I like to tell about Jason, I'm not sure he likes me to tell it, but I'm going to tell it anyway. It's about six or seven years ago, I was in my office, and Jason comes by as he does a few times a day, and we have this incredible conversation. I said, Jason, I'm thinking of nominating somebody from for a prize. This is an internationally known scientist, and I wanted your sort of insight into this, since you've won so many of them.
3:57
And I described, Jason knew this scientist very well, and I described the scientist in great detail again. I said, what do you think? Japan prize? We thought, Wollaston prize? I said, what about the Vetlesen Prize from Columbia University? This is a very prestigious award that at one time was considered the Nobel of earth science. Or a Nobel version of earth science. And so Jason said, no, no I don't know that one. But he knew a lot about the Japan prize, for example.
4:26
I said, well, let's get on online and see who won that. Let's see if there's a pattern to those recipients. And then I looked, and I said, Jason, you won it in 2003. And I realized, Jason has forgotten awards that would be the pinnacle of my career. Which tells you everything you need to know. But let me end on a personal note. When I came to Harvard from Canada 10 years ago, nine years ago, I knew what a wonderful department it was. I've had an incredible time interacting
5:00
with all the scientists here, students and faculty. But I hadn't realized how lucky I would be that Jason would join our group. Jason is a participant in all of our group meetings. He shows my graduate students that I am definitely fallible. He gives them far more geological insight. But I think above all, he shows them, and I think this is a rule, having worked with some of the best, that the best scientists are most humble scientists. Jason doesn't have a lot of bravado,
5:30
he balances that weakness in me. But he's taught scientists that speaking quietly, and speaking with a really encyclopedic knowledge of data, is the central aim of global geophysics. And it's really been a central privilege of mine to have him in a group that I call our group. Jason. [APPLAUSE]
6:09
Yeah OK. I like to start with this slide on a lot of talks, and remind everyone that science isn't the truth. That it's a process of convincing others the best answer to a question.
6:30
Does this work? Yeah OK.
6:35
So to convince specialists, which is what you're supposed to do, you have to have a lot of data. You don't make arguments without showing, it's written on exams, no credit for the correct answer unless you show your work. And too much of high school, middle school, is teaching students how the world is, and not showing that science is really just a process. And with this process, we-- but you just teach the facts that are useful in general life, and not the process of doing science.
7:25
Modern geology, I think, really began in 1669, when Steno, a Dane who was doing work in Italy, came up with what are called Steno's laws. It's things are laid down flat, if there is a gap because of a valley or something that used to not be a gap, it went across, and it has been eroded.
7:59
Things that are on top are younger than things that are below. But anyway, this was really trying to organize what you observed. That the people before described nature very well, but they didn't go into why does it do this? How does it do this? One example is Pliny the Younger wrote a description of the eruption of the Vesuvius that covered Pompeii and Herculaneum. And those type of eruptions, he described it perfectly. That type of explosive volcano that sent a column up miles into the air.
8:47
It's called a Plinian eruption. But he didn't go into why. And this illustrates Steno's law. What's on the bottom is marked one, and it was the first thing that was put down in this area, because it's underneath the rest. And on top of that is a layer of the type that I'll call two. And on top of that is three. And then there was something that happened that went through these others. And so it has to be younger than three, to have gone through them all. And then there was this fault, that's
9:37
marked five, that offset all those others. And then there's erosion and something new in the first. And this is really trying to go about how even today a field geologist goes into the field and tries to figure it out.
10:01
The second really important person that, there were others, but I think this is the next really important. Was James Hutton. And he discovered what are called uncomformities. He found a lot of examples of younger, more fluid rock, inter-fingering into rock that was already there. And he saw seashells up high on the mountains of Scotland. And so he came up with a mechanism to tie this all together. This is an example, you can see these vertical, earlier rocks. They are sort of cut off and flat.
10:53
Other rocks were laid on top of them.
10:59
The second is an example of fluid material coming in to preexisting rocks. And he tied this together with what he called plutonism, that the things are laid down, generally level and flat. They get buried, in he said, in the oceans. They get heated, they melt and that melting starts a process that tilts things, and rises to the surface, and he came up with a mechanism of--
11:49
One of his things was that this process goes on for an eternity. It started an eternity before and it's going to last for eternity afterwards. His quote is that there is no vestige of a beginning, and there's no prospect of an end. And that was the motto on the Harvard t-shirt when I came here. That was. And I gave such a shirt to a friend who has retired and is emeritus at Princeton. And I told him never to wear it to faculty meetings. Just keep- because when a professor,
12:33
when he really retires, they can appoint somebody new.
12:41
The other part is that things really happen slowly. The present-day rate of erosion or volcanoes erupting, it wasn't a few cataclysmic events, like a Newarkian flood, or something even larger than that. And it's this present things have to be used to explain everything you see. You don't invent totally remarkable events to explain something. OK. The sort of third really key thing in the discovery was the recognition of the fossils in the rocks, and that there's an age progression of the fossils.
13:33
In the rocks of westernmost British islands you had you know one type of fossil. And you could tell that there were things on top of that that were different. And those were younger. And then there are younger things on top of that. And then there's a steady progression. But you can use these fossils with Steno's law with such. You can walk and see the continuity. But you can go hundreds of miles away and find the same fossils. You know that those are the same age
14:21
as what sort of beyond standard walking kind of distance. And the early part was done mostly by the British.
14:33
The Triassic, Jurassic, Cretaceous are generally French. Eura and Cretacea are parts of France.
14:48
And this, they called it the law of faunal succession. And as you get nearer the top, things become more and more modern. They become more like things that you have today. And way down, you don't have trilobites anymore. This was done by William Smith. This quotes, this is not William Smith. This is an actor on BBC. But it looks like him from the pictures. And it shows some of this fossils and it shows the map that he made of England. Each color is a different age and different type
15:36
of fossil matter.
15:40
And he constructed canals by profession. Engineer making canals to bring coal from Wales to London. And so he has to follow level lines, and through valleys, and he noticed that the sequence of these fossils in one valley that was the same as it was in the other. And he realized that you could use the fossils to tell where you were within the layers. You didn't have to follow it continuously. It was the same if you found it elsewhere. And this was an enormous step forward in about 1850.
16:34
So I'll jump about 50 years from that. And there was a unique sequence of these fossils that were found in India, in the province called Gondwana. And you found the same things in Africa, and in South America, and in Australia, and we've even now found that some of those are on Antarctica in the Permian age rocks.
17:13
But nowhere else. You don't find them in North America, you don't find them in Europe.
17:20
And this is an artist's descriptions as to what these fossils may have looked like before they became fossils.
17:35
The best known of these is this Glossopteris, it was sort of an early tree, a seed fern that you find the leaves of throughout these South America, Africa, but nowhere else. And an Austrian geologist, who never went to these places, but he read everything. But he never liked to leave home.
18:08
And he depicted these different places and thought there was once a continent that he called Gondwana Land, that was all of the seven continents. And this was the land, and there were oceans, but there were other places where you found similar things. North America, and in Siberia. And I've got a little more recent than his sketch. Historically, this was in the same journal where, the same issue of the same journal where Alfred Wegener first published his first paper on continental drift.
18:56
And you see these different regions. And they really thought that what's now the Indian Ocean was dry land, up high in the air. What's now the south Atlantic was dry land, then all of the region of North Africa and such was below sea, level because you find Permian kind of sediments, marine kind of sediments at that age in these places. And this is a little bit later, but it's Wegener's interpretations is that from the shapes of these things, they all fit together.
19:36
You don't need to have these oceans in between that these were just collected as a unit. And it's because the South America and Africa coastlines fit so very nicely, and you can do the same thing, putting India.
20:07
This is an alternate way to look at it. Now Wegener was not the first to do this. There are several other people, I've chosen just one, that had the same idea that things used to be together, and they have since come apart. This is one from, I can't read the number, but maybe 1860, a Frenchman. And this is from a paper by Frank Taylor, who was the president of the American Geological Society. He gave a talk on this in Berlin, but Wegener wasn't in Berlin.
21:01
But the idea that the continents were fleeing, and their leading edges are where you have the present mountain belts. That the continents moving is why you have mountains where you have them.
21:18
This is a map of the depths of the oceans as was known in about 1912. And it's amazing how these depths were measured. You took a rope, you tied a 300 pound lead weight at the end, you lowered it over the side of the ship, and the sailors brought in the 300 pound, with five miles of rope.
21:51
And you got a sounding. And the Challenger expedition of the late 1800s made 300 of these soundings around the oceans. And you can see that even then they had the hint that the mid-Atlantic ridge existed, and was shallow. This was actually of commercial importance in the laying of the telegraph cables, that they would, I mean, they're there more along that North Atlantic part, because of the tele-- maybe it was even AT&T, made the measurements to put in the cables.
22:36
And those cables were amazing things. I've seen one that comes out from a house of a friend of mine in France. And it is copper about as thick as your arm, coated with about 12 inches of rubber. And it stretched all the way from Brittany, France, to the Azores In laying these cables they used the world's mining supply of copper for a year. But Wegener saw this, and he noticed, not only was the coastlines of South America and Africa so beautifully fit, but that even
23:24
the oceans gave a hint of this. And in 1912, he proposed Continental Drift, that these were once one, that Gondwana, it wasn't the ocean sank, it was that the two continents pulled apart. And that this was an easier way, and it didn't have huge amounts of water going on land, off land. I mean imagine, form that whole area are rising by four miles. Where does that water go?
24:11
Everyone quickly agreed, said you're right, you've really pointed out a bad thing. But we don't have to have the whole area. We can get by with just a few land bridges connecting. So we just want the fossils to walk across. And so very little, they weren't fossils. Very little. And this land bridge has the interesting name Lemuria. Because the lemurs are in Madagascar, and they were in India, not the others.
24:48
Wegener remembered, because he wrote so much about this. He really hammered away at the idea here, convinced people. Wrote four books. You didn't really publish much in journals, you wrote books, like Darwin did.
25:13
He had many, many kinds of facts that all supported this, and made it easy to explain if they were once all together, but they've since moved apart. And I want to show one of these types examples. And this is his sketch, the reconstruction, he has Africa in present place, but has brought everything in. And South Africa, in South America, in such places this E for ice, in German, that you find evidence of glaceous in those places in the Permian. If this was the pole of the Earth, then,
26:07
and are all of these continents were at the South Pole, the equator would have been sort of the red line that is 90 degrees away from that point that I've marked as the pole. And there you find a lot of present day current Carboniferous coal deposits of the age of the glaciation. That was the jungle, that was the heavy vegetation being laid down and buried. And you get the coal. In between that you get sandstones, and salt, and things that are in the deserts.
26:42
Just like the right distance from the pole. You have the Sahara, or you have the Kalahari.
26:52
This is a different version of that. You can, when you see the glaciers, they leave, they bring the erratic rocks from everywhere. I mean the rocks around here all come from much further north. If you go, I've seen a little bit further west in the Berkshires, or southern Vermont, you can see scratch marks like are shown in the lower left, that the moving glacier makes grooves and the rocks. That there's a rock trapped between the ice. And it just pushes it along and carves a groove.
27:32
And you can even tell the direction. And you can see from the map that if you assembled them in Wegener's idea of what it was, that there is just sort of one area that was covered with ice. Not some ice north of the equator, if you keep it in the present place. And he was quiet, he was basically a climatologist. Which is why he used climate so much as his argument. He, unfortunately, died in 1930. On an expedition to Greenland, he wanted to winter over on the highest point.
28:19
He was not only a climatologist, he was an arctic climatologist. And this is a, the party that was over wintering sort of ran into trouble with supplies or something. And he, one of the Inuits took off on their own for a trek back to the small town on the coast to get a rescue party. And he died in that trip. And after he died, they just ignored him. I mean, there were a lot of people that were very upset at this absolutely crazy, stupid idea.
29:17
Not in South Africa, or Australia, or Argentina, they still very much kept this idea alive and worked on it. But the rest forgot it. And one of the quotes coming out of America, they said, oh, it's all very nice, but how does it help you find oil in Kansas?
29:43
So and then this, this was how mountains were made. The leading edge of these continents plowing through made the mountains on the leading edge. The backside was quieter, more like the coast where we are here, instead of on the leading edge. OK. So now, this idea was sort of revived by the late 1950s, and 1960s it became, you weren't considered a total idiot if you, you read textbooks. And I read one by a Harvard professor, freshman textbook, that was published about 1960.
30:35
And there's about a paragraph on this. And that's it.
30:50
But there was, the glacial rebound was discovered. You had the loading of ice in Sweden and Finland. The land was rising very rapidly where the ice had melted.
31:12
You discovered a lot about earthquakes that were mostly in a rim around the Pacific Ocean. It looked like that's where things were happening. And there was a whole new field called paleomagnetism. And I want to go through those. This is, the figure on the top left shows the area that was covered by ice, only 15,000 years ago. And the one on the right shows the rate that land is rising right now. And it's rising about a meter per century. And you have Viking villages that are many,
32:04
many hundreds of years old, and those ports are now many, many meters above sea level. And you have that. You have the recent measurements by leveling, as you start to build cities like Stockholm. And you get uplift of those places. And it was calculated that the Earth is viscous, and flows. It wasn't a solid. Because one of the big arguments against this is, but the Earth is solid, how can these continents move around on a solid? Another was over several decades the location of earthquakes
32:58
got better and better. You started having radio time signals in about 1920. And before that, what was the difference between clocks in India compared to those that were in Europe. You had astronomy, but it was, you might know it a number of minutes, not seconds, the correlation of the time. And these earthquakes, as I say, had these dipping planes that were, this so-called solid earth had earthquakes that went quite deep, seven hundred kilometers down.
33:47
And the idea that the Earth might be a little mobile, and things moving, became more popular. But what really turned the tides was the study of magnetism. And you take a sample of rock that was formed at some age that you know, and you take it out in the lab and determine what's the director of the magnetic field in that rock? And if you do a recent one, it points north. And there's a dip, about this much from what the latitude is here. If I were closer, in Canada the dip would be steeper.
34:36
If I was down in Venezuela, it would be much flatter. And this dip tells you how far you are from the pole. And you know which direction. And these little blue dots are what you would get the pole to be if you look at sort of relatively young rocks, sediments, volcanic rocks, and they generally are at the North Pole. But if you look at Permian rocks, as a particular example, in Europe. So these red places are places that the Permian rocks are at. They point to a place very far away.
35:29
And, so something has happened here. Either the pole used to be here, 250 million years ago, or Europe has moved. And that was a question. But when you look at more than just Europe, you look at North America too, the red line is the different ages, and the little red circle is Permian, is for Europe, and blue is for North America. And if you move the two by closing the Atlantic, the paths pretty much coincide. And this was in about pardon, 1830 or 62. And then this was sort of the turning point
36:24
that continental drift was thought of as, don't laugh at a person because you suspect he might believe it.
36:36
Now what really happened were all the discoveries in the ocean. And on the left, I have the three US oceanographic ships in the '20s and '30s. One of them didn't fare, well as you see at the bottom. A wooden ship is great for measuring magnetic fields and all, but it has certain disadvantages. And this is a seaplane tender coming back from World War II. It became renamed the pioneer. A seaplane tender, the shops were, that were used to repair the engines of the sea planes,
37:26
near the overhauls, make patches to the wings, and whatever you have to do to a Navy sea plane were taken out, and lab space put aboard. It's a nice, small, compact ship. And it switched that suddenly they were about, and the Navy was wild about the ocean, because all of that had been learned that was used against the German submarines. Knowing the properties of the ocean water, knowing some of the properties of the depths turned out to be extremely, the research was well
38:14
funded by the Navy for that. And 20 different ships began to explore the world's oceans in the US. And many types of measurements were made.
38:30
One was bathymetry, and by the late '50s, the map that is just the North Atlantic was published by a group at Vermont, Bruce Heezen. A sketch of what's called a relief map used shadows and colors to sort of indicate where it's rough, and what the elevation is. More than just a flat contour map. This is the whole world, so you get to see what got added to that.
39:12
One of the things that Heezen showed is that the earthquakes, which by the '50s we're getting to be pretty good, you could locate an earthquake to within 100 kilometers pretty surely. That's a better degree.
39:33
They really followed this rift that was in the middle of the ocean. Just incredible matching. Another is a technique that had been developed to try to find submarines, submarines are quiet and underwater, and you're trying sonar, but you don't find it. But it's a big piece of steel. And it makes its own magnetic field, combined with the Earth. And something is called a magnetometer that measures the strength of the Earth's field, was used very successfully to look for submarines.
40:19
And this area off the West Coast of the US was mapped for the depth. Very fine grid, the depth is classified. That was not released, even the tracks are classified. But two of the people that were aboard the ship were told this magnetometer, just to see what they would get. And it was this remarkable pattern. A very, very regular pattern. And the Earth, magnets are used to explore for mineral deposits, and they're very knotty. In one part of a county you'll have a stronger feel,
41:04
and there might be an iron ore deposit there or something. But the oceans, just for many hundreds of miles you just had these straight line magnetic anomalies. That the ship would go and measure the strength, you'd take out sort of the average, what the Earth was. And to make it possible to plot this, just where it's above the average, you color it black, where it's below the average, you color it, you leave it white. And this was a very puzzling pattern.
41:39
And almost immediately, the same year, Vacquier noticed that the track that went across here looks a whole lot like the track that goes across there. And these are two of those tracks, on the lower right, and I've colored them, and these places are 600 miles apart, 1,000 kilometers. Offset by this Mendocino fracture zone. And this is a puzzle. Who expected that the anomalies are going to be nice straight lines and clean? But that they're offset, it looks like it's a huge fault that's moved it.
42:26
Now we know, now, it's not a fault, but it does, that was the first, OK.
42:37
This is a photograph of Harry Hess who was in the Navy.
42:48
He was in the Navy because he did geological research aboard a US submarine in the early '30s. And submarines were very secret things, and you had to be in the military to be on one. And they made Harry Hess and Maurice Ewing lieutenants. They had PhDs, well medical doctors all the lieutenants, so they were lieutenants. And they were on the submarine. Now, six years later, when the war broke out, here's this guy who has a commission in the Navy, and he was called up.
43:31
And actually he was involved in the anti-submarine warfare in the Atlantic until that quieted down. And he wanted to go to the Pacific, and they made him captain of the ship. The Cape Johnson.
43:48
And he kept his depth chart, his depth recorder working the whole time. He measured a lot of things in Pacific, discovered the flattop seamounts called guyots, and generally became very knowledgeable about, and had the interest in the oceans. And in 1960, he wrote this paper synthesizing everything that was known at the time about the ocean floor. It had all these mysteries, all these new techniques, new data. And he wrote this, it was published in a book that was honoring his professor.
44:38
He knew the book wouldn't be out for another two years, because other authors don't always contribute their papers on the day that they promised them to the editor.
44:50
But he distributed 200 copies, and I have one, proud to say, in which he just presented, he interpreted all the data.
45:08
There's no data, particularly in this paper, except talking about, we all know that the ocean is about to uniformly five or six kilometers thick, the oceanic crust, and there's heat flow, and it was just a general things. There's no presentation of real data in this paper. And it's, I only know of one other paper that's important that has no data in it. And that was the one by de Broglie that laid out wave mechanics. It says you can interpret Bohr's atom
45:46
by saying that the electron has a certain wavelength, and that these are, depending on its momentum.
45:59
So he came up with this, at the end, this prediction. And amazingly, almost everyone is correct. I'm a professor, I couldn't help but grading it.
46:16
It was a habit at the time I did this. He's really only wrong on two of these 19, quite very profound, and there's a reason he was wrong on that. He wasn't really very, very wrong.
46:31
And essentially the idea is that the upper part of the mantle is cooler, and stronger, and it moves away from these rising currents in the middle. And the ocean floor, and the continent, move together. There's no continents plowing through an ocean. They're both on what we now call the lithosphere, that is moving, and pulling apart. And this is the key. Plate tectonics is just a small wrinkle on top of this credible realization that it's this strong, moving together
47:26
that really describes what is happening on the Earth. There were things close to it before, but they stretched, and distorted. It wasn't that they moved as a rigid unit. OK now.
47:49
At about the same time Hess's paper came out, it had been discovered that when you look at these magnetized rocks, you go to some volcano like Etna, something, and measure the direction. And they point north. And magnetized. But at some point exactly the opposite way. And there was a bit of confusion as, well, did the field change? Flip from north to south in some way? Or did the mineralogy in the rock just sort of spontaneously reverse the field? And once you had accurate age dating, which
48:39
came about in the '60s the early 1960, the really young rocks were all, there's none of these reversals. You get a little bit older, you get, however you, wherever you date this, the same age, it's all the same. With a little bit of measurement error, and what the dates are. And this reversal time scale was developed. And this graduate student at Cambridge in England had it been on a cruise, a British research ship in the Indian Ocean.
49:41
And they had measured rocks that were one way and the other way. And he wrote the paper, his advisor happened to be on his honeymoon, at this particular time. Probably had less to do with it.
50:03
And he said this, the theory, is consistent with, in fact, virtually a corrolary of current ideas on ocean floor spreading that Hess had developed, and periodic reversals of the Earth's field. That these stripes were, well, right now you're magnetizing one way, but a little bit earlier, when the field was opposite, the polarity makes the field a little bit weaker, because what's frozen in is opposite. And you can take the time scale that's at the bottom,
50:48
and at that particular latitude, and the direction ship is going, compute what you ought to see, if that is, right with it, right opposite. And you can see that it is incredible correlation with what you observe. Since you know the dates of these anomalies, you can say how fast the ocean is spreading. And this was an absolutely remarkable profile in the Southern Pacific Ocean on a ship that was called the Eltanin. And the center is the profile that
51:34
was measured, the one above it is that same profile, but just flipped, to show you how remarkable one side of this is compared to the other side. That, it's hard, you see that there's something similar here to there. But when I've just flipped it and put it upside down, it's really striking. And this is a computed from the time scale, and it agrees as well. Now, this time scale comes from this profile.
52:15
You're only able to measure the radioactive ages accurately for about two million years. So they just took that rate and expanded it and said, well gee, this stuff out in the end must be 10 million years old. If this is, if it was continuous. And this is Vine's map of that pioneer area that was in black and white. And it really shows how remarkable the two sides are. OK. And so you can see it for yourself, this is an area further out in the Pacific.
53:02
And this is the time scale from about 19 million years to probably 40, at the other side. And these are some of the ship tracks, that the Pioneer here over here is much denser, and it's secret. But this is what you can see. And you can see, this little cluster, it is the same. I mean, this must be the same age. Or way over here, you can see little bumps, three little bumps. They look very similar. And this is done for the entire ocean.
53:53
At the same time, this computer model of fitting the coastlines was done also at Cambridge. And people argued about Wegener being a little off because of the crudeness of his plot. And no, this coastline matches incredibly good. It's within 50 or 100 kilometers the whole way. Not only South America, but North America goes to Africa.
54:26
OK. So, I'll make one quick jump to plate tectonics. And this figure was published by Bill Menard in a paper in January of '67 And he says these great fracture zones in the Pacific are not great circles. And he had said that in his book that was sort of the Bible of the Pacific, but he realized it was wrong. And he demonstrated it by making a gnomonic projection, a great circle projection. And if these great fracture zones were, if they were great circles, they'd be straight lines,
55:20
and they're not. And I looked at this, when it was published, and said, the ones in the north sort of end upwards. And the ones that you get down further, bend the other way. And it reminded me of a figure in the navigation textbook that I had when I was in the Navy. And this was an example, the gnomonic projection is you put a light at the center of the globe.
55:52
Any plane that goes through the center of the globe is a great circle on the sphere. And any plane intersecting a piece of paper is a straight line. And you can see that the Meridians are straight lines on this gnomonic projection, and only the equator is a straight line. There's a great circle. And these curve up and curve down. So I looked at Menard's and said, boy, that just looks like, all of these fracture zones must be about a pole. And this is, you didn't have Xerox,
56:34
but you had some kind of wet process that made copies. And I made calculations. In about three days I had this that, I could compute the small circles. I had a trial pole somewhere in Siberia.
56:51
And I could match that pretty well. This is a cleaner version that appeared in the paper.
57:01
And that makes no sense. I mean, who cares if these great circles are concentric about someplace in Siberia? But there is an important geometry theorem, called Euler's Theorem, is that on a sphere, you can, with just a single rotation, you can move something to somewhere else. And I tried that on this globe.
57:35
This is a very poor globe, because it's cheap. And but just rotating it about this point, I can bring them close together. I mean I can match them. It's just a simple rotation. Now I, what you saw out here was, whatever was happening in the Pacific at the time the Pacific was made. But the test, the interest would be in the Atlantic. This was the first map of plates.
58:16
And in this paper, we make the assumption that gives this model mathematical rigor. We assume each crustal block is perfectly rigid. Now, there's a reason for this. Is that you can never prove that something is true, in science. But you can show it's false. And so you can set something up and test whether this perfectly rigid is false. And it turns out, it worked very, very well. It wasn't false. It's like you can't say, this is the cure for cancer. This might be better than other things,
58:59
or it's definitely not a cure for cancer, but you can't say this is the--
59:10
Science really is negative. That you can show something is not true. And so in this paper, I took the different fracture zones, it's over here on the right. And constructed the perpendiculars to that. And they sort of intersect at a point, with a little bit of error. This is, this other figure is more a demonstration of what the Euler principle is supposed to be. That the fracture zones will all be small circles about a pole. And in the lower left is the magnetic anomalies,
59:59
because if you're 90 degrees away from that pole, you're going to be opening the fastest. And with the help of Fred Vine, we did a bunch of these anomalies in the Atlantic, and showed that the peak, the higher values are in the south Atlantic, which is 90 degrees from where we thought the pole was.
1:00:29
These magnetic anomalies are known throughout the ocean now. You can-- Another kind of evidence you have is there are these island chains, and the islands are dated. And if you say, well, the volcanic source deep in the mantle is fixed, you can, from the age progression along, you can say what the plate's speed is. And the black lines here are rigid rotations for a set of motions for the Pacific. And the red are where there are islands, or atolls. And a large number of these have dates.
1:01:17
And it is an incredible agreement that fixed, the plate is rigid, and that's the, it's not only in the Pacific. There's a chain called the New England seamounts that come in, and when they get on land, they become the volcanics in the White Mountains, and that volcanics near Montreal.
1:01:46
So I want to end with a couple of figures. First, I will show you a GPS satellite.
1:02:01
Because originally, you can only look at measure rates at the mid-ocean ridges with magnetic anomalies. With GPS, you can measure any place on the Earth. There's no need to only do it, assume that the plates are rigid, calculate the motions everywhere else. These are very expensive. But they're military.
1:02:42
They're are about 30 of these in orbit, any instant you see nine in the sky. They're used so that the nuclear submarines can, when they get the radio signal that war has started, they can find out where they are, and they program their missiles to go where they're supposed to go. Or your cruise missiles can get through these low level through mountain passes and things, to where they want to go. So it's a huge cost, but it's in the Pentagon budget.
1:03:41
This is GPS in Asia. In the interior of the continent. You can see the continent is not perfectly rigid. It is, China's fleeing in the rest of Asia. Tibet is sort of pushing it as India moves in from the south.
1:04:07
Until the GPS, these space kind of measurements were done with the radio telescopes. And this is Westford, Massachusetts, the Haystack Observatory, going to one that's near Munich, Germany. This is about a nine year period. Every five days or so they would make this measurement. You can see that the rate of change of distance between Munich and Boston is about 17 millimeters per year, plus or minus 2 millimeters per year. But now from the magnetic anomalies
1:04:53
model, that the prediction was that it would be 16 millimeters per year. The black is the data from a space technique. The red line is just what plate tectonics, rigid plates, would predict. And it's not the only. This is Hawaii to Fairbanks, Alaska. And the data is shown as the data with the error bars. The straight line is the prediction, from rigid plates. And this late '90s and all was really heyday for plate tectonics really works. And you knew the continent, you had no way
1:05:53
of knowing what continents were doing other than just assume they were perfectly rigid. And you could calculate. And you had amazing agreement with when you could compare it. But this is an example. You knew this basin and range was spreading. You didn't really know much. But with the GPS, and the GPS is important simply because the receivers, the antenna, everything is, it can fit in the back seat of your car. The tripod weighs more than the receiver does.
1:06:32
You can go anywhere, make a measurement, come back a year later.
1:06:39
These measurements, you didn't move. You kept it at, these pipes go down 30 feet into the ground. So they don't move when it rains.
1:06:52
And in the west, what is happening now, as you get further and further from the mountains in Utah that are the end of North America, as you get closer and closer to the Pacific, it spreads, spreads, spreads, just like the shapes of the valleys and things too. California doesn't just move along the San Andres fault, the whole Central Valley is going north about a centimeter a year, compared to everywhere else.
1:07:43
This is rigid North America. It's not deforming very much at all. There are measurements, but they just look like dots. There's no arrow to them.
1:08:03
You get into the basin and range, you see the whole thing is spreading. When you get to California, you find out the whole Central Valley is moving north. There's a major fault on of the sort of the backside of the Sierra Nevada. Then you have sort of a super major fault, and you get to the San Andreas. And you start going about six centimeters a year. And so this is the map as to what these velocities are. And this is where GPS is today in the US.
1:08:39
And mostly in the east, because the west has already been surveyed pretty hard. And they keep moving them eastward. And then these are the velocities. And you can see, this is, you this looks like almost a rigid rotation.
1:08:59
If you take out that rigid rotation, you find out it's not. You know from the vertical measurements that up in Canada, around Hudson Bay, the land is rising. As you get a little further south, these yellow errors, or land is sinking a little bit. Peripheral bulge. But not only is it rising from Canada, as it comes up, it moves out a little bit. And you can see that taking away the average of the bulk of North America, this sort of motion away from the rising land, because the ice is gone.
1:09:44
As you get further south, you get down into sort of just random noise.
1:09:50
So, my conclusion is, if you assume the plates are rigid, you can precisely predict. That's the advantage. You can predict it if there's no distortion.
1:10:12
We don't need to predict anymore, because we can measure what it is. And that's even better. And what has happened is that, 50 years ago, any problem you solved had to be a perfect sphere, a perfect cylinder, a uniform viscous material. And you had functions that you learned to work in these different geometries, vessel functions, and whatever. Nowadays you don't need that. Because you can take something as complicated as a supersonic airplane and calculate the air
1:10:57
flow over it by a numerical computer. And that's essentially what has happened. The basic idea that it's rigid, moving, is still true. But we no longer need the assumption that it's perfectly rigid, because we don't care about that. We don't need to predict how fast South America is heading towards the incoming Nazca plate. We can measure. And that's the after take plate tectonics part that I wanted to say. There was a heyday in the '80s where all the data was just
1:11:41
coming in great, and now the data is the only thing that matters. Thank you. [APPLAUSE]
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