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0:00
The idea of continents drifting about the globe is as old as the idea of flights to the moon, and until recently, equally laughable. But Earth scientists have been getting very excited about some new discoveries in geology and geophysics. To begin with, what is the evidence for continental drift? Professor Sir Edward Bullard explains the geometry.
0:24
Most people must have noticed that the coast of Africa is a very similar shape to the coast of South America. It was pointed out, oh… ages ago by Francis Bacon in the time of James the First,
0:39
but no one took the thing very seriously until Alfred Wegener, a polar explorer,
0:47
just before the first war suggested that once the two had fitted together like the pieces of a jigsaw puzzle. And in fact, they do fit really rather well.
1:02
You get a little gaps, in fact, I think the gaps on this model are probably bigger than the gaps in nature. There is a trick in fitting them together, that is that you go to fit them at the real edge of the continent. The real edge of the continent is not the shoreline, it's the place where the shallow sea changes suddenly into the deep ocean. So there's a bit of sea here, but this is all shallow sea and the thing that's been fitted is the real edge of the continent. Now,
1:32
you can do this fitting in the rest of the Atlantic. You can put,
1:38
if you’re clever enough to fit it in, you can put Greenland in there, and you can bring North America across and fit it — you've got to be a little careful with Spain. Spain has to be turned around a bit. This is about the only cheat there is in the thing. And then these two together will come around like this, and really, the jigsaw puzzle pretty well goes together. But if you have a jigsaw puzzle, not only have the bits got to fit together, but the picture ought to fit together too.
2:10
And it really does fit together excellently. And up here, this lot of rocks in this area are 2000 million years old. And then there's quite a sharp line. And then here there are rocks which are 500 million years old. Well now if you take this line between the two and go across this very narrow gap into Brazil, then you come out in a little village called, I think Sao Luis, here. And it's a wonderful thing. But the rocks at the western end of the village of Sao Luis,
2:44
are 2000 million years old, like these ones, and the rocks at the other end of the village are 500 million years old like these. The pattern really does continue across.
2:55
The pattern in other parts of the world can be more tenuous, but if you place Australia fairly close to India, there are certain geological contours, minerals and so on, which would seem to link the two continents together.
3:07
But it's not only rock formations that leave traces for scientists to look at. In the study of fossils, paleontologists have always been keen enthusiasts for the drift theory, and certain dinosaurs do provide evidence. Animals such as this little dinosaur here, coelophysis, will do very well. You can see from the hind limbs that coelophysis was a little fast-running dinosaur.
3:31
We know coelophysis from some deposits in Arizona and also in Rhodesia. So one way or another, he could get around the world between those two continents, and so could a lot of the other big dinosaurs which lived in the world with him. Unfortunately, we don't know the route by which they could get from one place to the other. Today, of course, you can't get across the Bering Straits, but in those days there are many reasons for believing that the Bering Strait
3:55
region was much warmer than it is today and also that there was dry land there. So all of these dinosaurs, for example, those that were living in North America, could have spread across the Bering Strait into Asia and then on down into Africa. In fact, lystrosaurus is probably the best evidence from the vertebrates of continental drift. Now, we've known this little reptile from deposits in South Africa and China and in India for quite a long time.
4:21
This is a skull which I think shows its structure quite clearly as the lower jaw down here, rather a downturn, snout, the nostrils and the eyes. Now, the important fact was the discovery of this animal in Antarctica only last December (1969). Now, for lystrosaurus to have got to Antarctica and to have survived there, implies, I think, such different positions of the continents and climatic conditions in the continents that surely continental drift is the only reasonable explanation.
4:52
In fact, the study of weather in prehistoric times, paleoclimatology, has shown that certain parts of the world have undergone extreme changes in temperature and climate. England, for example, must have been nearer the equator. We have here a typical well-rounded desert sandstone, typical of the Sahara today, and yet we're now in this bleak part of Durham. And this sandstone, which is about 250 million years old, gives you an excellent method of determining the direction of east in the past.
5:26
You can see from the dunes here that the wind direction must have been blowing these grains up and down over the dunes. So from this you can work out the wind direction. At that time, the wind was blowing from the east across here. And yet today it is quite clearly in this direction. So quite clearly Britain must have rotated relative to the poles since that time.
5:51
In the southern hemisphere, glaciated rocks of the Permian Age were recently found as far north as India. If one reconstructs what might have been the south polar ice cap at that time, the implications are fairly startling.
6:13
Even so, all the evidence so far was only an indication and not actual proof of continental drift. But during the last decade, the science of paleomagnetism has transformed the picture.
6:27
This solid wall of rock began life in liquid state some 300 million years ago. As it welled up to the surface from the interior of the earth, it cooled, solidified, and became magnetized by the Earth's magnetic field.
6:41
The direction of this magnetization will remain frozen in the rock until it returns to a liquid state.
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These scientists are taking a sample of the rock so that they can measure the direction of the magnetism. Might imagine that the magnetism in samples from all over the world would all point north-south. The answer is: they don't.
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Rocks of different ages from different places points to different north poles, but there's only one North Pole, and as it stays more or less in the same place in relation to the sun, it's therefore the rock samples that have moved.
7:51
If the samples have moved, then so have the continents. So how do we find out exactly where they were?
7:59
This is a model of the Earth's magnetic field as we see it at the moment. It can be well-represented by that which would arise from a large magnet buried at the center of the earth and the lines of force come out from the region of the South Pole and go in again in the region of the North Pole, and the compass needles on the frame follow the direction of this magnetic field at the Earth's surface. A rock forming on the equator now would acquire a magnetization parallel
8:31
with the Earth's surface and at some intermediate latitudes, such as England, it would acquire a magnetization dipping about 66 degrees into the ground and one forming a pole would acquire a direction of magnetization vertically into the earth. Well, the directional magnetization in this rock is nearly horizontal, and this means that either England was here when it was formed and it acquired this horizontal magnetization, or if the magnetic pole was in the Pacific in those days,
9:04
then we see that the magnetic field in England was horizontal. Either way, this means that when this rock was formed,
9:12
England was in equatorial regions relative to the pole in those days, and this is what paleomagnetism is all about. We can estimate the ancient position of continents by measuring the direction magnetization from rocks within them. At Newcastle University, scientists found it simpler to work out where the poles appeared to be and to plot a polar wandering curve. The first man to do the fieldwork for this was the man who is now professor of Physics, Professor Runcorn.
9:43
The earth has a gyroscope and the gyroscopic effects are due to its rotation — not only keep the axis of the station fixed in space, but also control the generation of the Earth's magnetic field so that the magnetic pole on average coincides with the geographical pole. Some years ago, I examined the directions of magnetization of rocks from the top to the bottom of the Grand Canyon, which cover a thousand million years. The results form a rather simple pattern.
10:17
The pole appears to move as viewed from North America
10:25
in an arc around the North Pacific. The surprising result, which came out of this work was that the polar wandering path from America was about 25 degrees west of the polar wandering path derived from rocks from Europe.
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This was the fundamental paleomagnetic evidence for the drifting of the continents.
10:51
If the polar wandering curves for rocks from South America and South Africa are brought together so that they match, not only are the two continents brought together, but the curves show the moment at which the two continents started to drift apart. 100 million years ago.
11:11
If the continents of the world are reconstructed for that date on paleomagnetic data alone, then the South Pole is centered on about Johannesburg, while Britain was exactly on the equator.
11:26
This means that with one sweep, paleomagnetism confirms all the previous evidence for continental drift.
11:36
Inevitably the computer has played its part, and at the Atlas Computer Laboratory, the computer is able to produce on film a projection of the break up of the southern hemisphere.
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Notice how the other continents flee from Antarctica, which seems to get stuck in the same place. This is a process which continues to this day.
12:03
Watch it again and this time look at India, the small triangle on the right, which hurls itself towards Asia.
12:14
Going further back in time, Australia has performed a sort of elephantine waltz with the South Pole as a partner.
12:42
Eventually paleomagnetism will take us back to the age of the earth itself, but for the moment the best picture comes from only a short time ago when life began. This time, 300 million years ago, we had two supercontinents, something like this, the northern one called Laurasia and Gondwanaland
13:05
in the south separated by the ocean, which is known as the Tethys.
13:13
These continents, the northern continents, were made up of North America and Europe and Asia. The southern continents had Africa in approximately this position, with South America attached to it, India,
13:30
Australia and New Guinea, approximately here and Antarctica around here and New Zealand across here. But if this was the Earth 300 million years ago, what was around the other side? Well, one possible suggestion is that the world used to be much smaller (Note: this hypothesis of a smaller earth has now been firmly rejected). And when you look at things more closely, you find that the continental jigsaw puzzle can be done on this smaller earth rather nicely. Now,
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naturally, as the Earth slowly expanded after this, the skin had to crack, and it did so around the borders of the Pacific Ocean. That is,
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around here and here on this smaller globe.
14:21
But ideas naturally are rather speculative about this and I think this will keep us busy for at least the rest of my lifetime.
14:38
So the evidence for continental drift seems unassailable, but it still leaves wide open the question of how such massive journeys were possible. How do you move thousands of square miles of solid rock such long distances?
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And what about the ocean floor in between? Since the war, oceanographic surveys have revealed huge mountain chains, higher than anything on land, running continuously through the Atlantic, Indian, and Pacific Oceans.
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Did the continents slide across the sea floor and over the mountains? The skeptics quite rightly said this was impossible, and it wasn't until paleomagnetism went to sea that an answer began to emerge. Two unrelated discoveries were waiting for a catalyst to link them up. The first was the strange phenomenon of polarity reversals.
15:40
We already suspected when we started on this work that the polarity of the Earth's magnetic field had changed. Now that sounds like a very complicated thing, but what I mean by it is this that at the present time, the Earth's magnetic field is pointing north. We mean that in the past it was pointing to the south much of the time to the north, it switched back and forth. And the problem for us was to find out when these north-south switches had occurred.
16:03
What we had to find in order to do this with some sort of a natural tape recorder that could give us a record going back many millions of years. We found our tape recorders and this is what they looked like. Every time a volcano erupts, a new layer of molten lava is spread down the mountainside on top of the previous one. Each layer has the Earth's magnetic field locked in it from the moment it cools, and where the sea has eroded away the mountain, the layers are exposed.
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We made measurements working our way down the cliff, found the uppermost lava flow was magnetized to the north, the second one — and then we would find the next lower one would be magnetized to the south. And at this exact point, we knew that we had captured the reverse of the magnetic field. We did this in many parts of the world, and when we put it all together, we had a detailed time history of the field as I've sketeched over here, we found that there was a long time when the field
17:05
had been pointing to the north, and then 800,000 years ago, let's say it was pointing to the south. Back and forth again. And in this way, we were able to build up a very detailed history of north-south switches going back something like three and a half million years. The other discovery was even more curious. The US Navy allowed a British scientist, Professor Ronald Mason, to tow a magnetometer behind one of their survey ships in the Northwest Pacific in order to measure the pull
17:37
of gravity over the sea floor. The map they produced showed clear stripes hundreds and sometimes thousands of miles long, which appeared to mark alternate strengths and weaknesses in the magnetic properties of the seafloor. But geologists could make nothing of them, and they remained a mystery for about ten years. The mystery was finally solved when the polarity reversal timescale was applied to the magnetic stripes by Fred Vine, one of two Cambridge geologists who produced
18:08
a revolutionary new theory.
18:11
The concept was first formulated in some detail by the late Professor Harry Hess of this department, and he made this simple, rather elegant suggestion that was rather startling at that time, that the oceanic crust of the sea floor is continuously being created at Mid-Ocean ridge crests, such as the crest of the mid-Atlantic ridge here in the South Atlantic. It is generated above an upwelling in the mantle and spreads laterally away from the crust,
18:40
thereby separating drifting the continents apart as though they were on a conveyor belt of upper mantle material. So I might represent this, therefore, by simple “Mickey Mouse” models such as this, illustrating the way in which the conveyor belt of oceanic crust and upper mantle material flows out from this central crack in the Earth's crust. Well now, in 1963, Drummond Matthews and myself then at Cambridge, added the simple rider to this and in effect
19:11
converted his conveyer belt of oceanic crust into a tape recorder. We suggested that as new crust is formed, solidifies at a ridge crest, frozen into it would be the current direction of the Earth's magnetic field. This will be subsequently moved aside as new material comes in and in this way successive avenues of crust normally and reversely magnetized will be built up, paralleling the ridge crest and symmetrically disposed about it. And indeed, going back to the anomaly map,
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the very detailed contour map of [unintelligble] for this area of the North-East Pacific, and after peering at it for some time, we could see that indeed there is a remarkable symmetry of the magnetic anomalies about the crest of this so-called Juan De Fuca Ridge in this area and they do indeed reproduce all the details of the reversal timescale in remarkable detail. So to summarize this, then we drew up this new colored summary map which emphasizes this symmetry about the ridge
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crest in a truly remarkable way, as incredible to think that it had been in the literature for four or five years and people hadn't recognized the symmetry. An aerial survey of a section of the mid-Atlantic ridge produced the same magnetic stripes. But still, there weren't many scientists who could accept the idea of seafloor spreading. In 1963, it was very frustrating because I have this idea and I'd realized that if it was true, the implications were enormous, that the ocean basins were young,
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ephemeral features, that all the oceanic crust, all the deep ocean basins that we see today were generated, say, within the past 200 million years of geologic time. Less than 5% of the whole of geologic time.
21:05
Was it possible that the whole of the sea floor crust was so new? The question had to be answered. So, suddenly the oceans of the world were busy with any craft the oceanographers and geologists could lay their hands on. The biggest was a specially-built deep sea drilling ship, the Glomar Challenger. Commissioned by the JOIDES Group and based at the Scripps Institution of Oceanography in San Diego, this vessel must be one of the most expensive craft ever built for pure research.
21:35
Its six engines hold it accurately on station while it drills in water several miles deep.
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The object is to recover from the ocean bed, a core of sediment representing millions of years of history from the present day back down to the moment when the first layer of sediments settled on the newly-formed rock and to see how old that first layer is.
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The ship drilled two sets of holes across the ridge in the north and south Atlantic specifically to test the seafloor spreading theory. And this is tremendously exciting because to a geologist, there's really nothing quite like a sample. You cannot determine the age of sediment, for example, from geophysical investigation. You have to have some of it in your hands. And in fact, by drilling all the way through the sediment layer and recovering not only the sediment just above the igneous rock,
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but in a number of cases the actual igneous rock floor of the ocean. It was found, without exception, that the age of this oldest material did become progressively younger as one approached the crest of the rise system.
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At the Lamont Geological Observatory, Columbia University, are stored literally hundreds of cores from several different ships.
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These long strips of mud are priceless. One millimeter can represent a thousand years of history, and they contain enough information to dispel any doubts that the crust of the earth has been moving and growing for millions of years.
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Seafloor spreading and continental drift are no longer a myth — they're fact. There's so much information coming in that scientists can now give us accurate dates as to when the continents separated. We have been extrapolating both the anomaly pattern and the age outward from the ridge axis to the flanking basins and the continental margins. And from this, we've been able to suggest that South America and Africa separated about 150 million years ago,
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and this agrees reasonably well with the geologic information, which suggests that the first occurrence of marine sediments along the coast of South America was about 130 million years ago. In the North Atlantic, extrapolating again from the ridge axis, using the magnetic anomaly pattern, we would propose that North America and Europe separated about 90 million years ago, and this agrees reasonably well with the geologic information.
25:04
This illustration shows the relative position of the continents about 75 million years ago. The North Atlantic is closed, and a point of interest is that Iceland did not exist at this time.
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Iceland has always been something of a mystery with its geysers, hot springs, and boiling mountains.
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Armed with the new ideas about geology, a team from Imperial College London has been doing a survey.
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Iceland is one of the few places in the world where one can observe seafloor spreading in action. This is because it sits astride the Mid-Atlantic Ridge, which comes up here and passes through the island like this.
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Now, it enters Iceland on the Reykjanis Peninsula here, and it follows the line of post-glacial volcanic activity which crosses Iceland and is split into two sectors. Now we are working on the southwest corner of the island on the Reykjanis Peninsula. And what we're doing is we're measuring the distance across the rift with the instrument developed by the National Physical Laboratory. And what we do is to bounce pulses of light across the rift with an instrument on one side and a reflector
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mounted on the other. And then we measure our distance by measuring the transit time of the light pulse, from one side to the other and back again. It's a little early yet to make any very definite assessment of our results, but as far as we can tell, they're certainly not inconsistent with a spreading of the conventional one centimeter or so a year, so in 1 million years time, Iceland would be about ten kilometers wider than it is now.
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In the middle of the Atlantic, the new crust of the earth is being formed, and as the continents slide apart to accommodate it, the question remains: where is it going? At the other end of the world is an island which also owes its origin to tectonic activity under the sea.
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The island of Tonga in the South Pacific was the place chosen by earthquake scientists, seismologists, to investigate the deep earthquakes underneath the islands, which seemed to originate from the deep ocean trench just offshore. Let me draw you a section through the Tonga Arc. Here we have on the surface with volcanoes, active volcanoes on top and the deep trench and out in the Pacific Ocean… this would be the water. The earthquakes occur in a zone which dips down to great depths.
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Now, the old idea was, is this earthquake zone formed… it was actually a giant fault such that the material moved down on one side relative to the material moving up on the other side shown in this fashion. In fact, they found that tremors coming up from 60 or more miles down in the hot, molten mantle, reached their instruments in Tonga much earlier than their instruments in Fiji.
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They also found that the tremors were much weaker when they got to Fiji. The two facts suggested that the material in the earthquake zone was of a different composition, a different consistency to the material on either side — the same consistency, in fact, as the upper crust of the earth. And it would seem that the earthquake zone and the ocean floor were all part of the same conveyor belt. This then, is a very convenient way to get rid of material created at the mid-ocean ridge system.
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We have material moving along here, bending over and descending into the mantle as a coherent slab. The next major step was taken by a very young geophysicist from Cambridge, Dan McKenzie, who provided a theory which tied together all the loose ends and formed the basis of a revolution in geology. I was sitting in the library at Scripps in Southern California reading an article by Sir Edward Bullard on the fitting of the continents, and it seemed to me that it was impossible to do
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this unless the continents, like South America to Africa, have remained rigid during their motion and had come together like that without any internal deformation at all. And what's more, it also seemed equally obvious that the seafloor also had to remain rigid because it was covered with magnetic lineations which had been formed on the ridge, and these had not got broken up during the motion. So, we then thought, well, perhaps the whole world consists of just
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a series of rigid, interlocking plates, like a mosaic. So, on this view, the place to look for plate boundaries and where the motion is and the deformation was where there were earthquakes. And we went back and we got hold of maps of all the earthquakes of the world and looked to see where they were and they follow the most dramatic, narrow lines all over the world. And these, we decided, would be the plate boundaries. This is where there would be defamation and not elsewhere.
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So, if we make this into a series of plates,
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then you can see that the earthquakes lie along the margins of these colored plates. The big ones are the Pacific plate, which consists of the whole of that region, the Atlantic and American Plate like that,
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and various other ones, Africa and Australia. And these are in relative motion and we've got a ridge — they're pulling apart, like in the South Atlantic — and creating new seafloor with its magnetic lineations. Where one plate overrides the other, the plate doesn't immediately vanish, but breaks up as it goes down into the mantle and causes, produces intermediate and deep-focus earthquakes, which you only find where the plates are being destroyed,
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like up here in the Aleutians and down past Japan. But in parts of the eastern Pacific, like in California, the Pacific is just sliding past the American plate and there's not overriding or underriding, but just sort of sliding sideways. The difficulty with the whole of this subject is not easy to photograph because the plates are so enormous. And even if you do it from a satellite, all the plate margins are underwater, so you can't really see what's going on there.
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So the best analogy we could get was with ice. Aerial photographs of large expanses of the Arctic show how the ice forms itself into plates where these plates pull apart water forms, new ice in the gap and where they're pushed together, they override each other.
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And this is exactly like what goes on in the Pacific where the thing is pulling apart here and here is the Pacific plate is being overridden by the American plate here
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producing these earthquakes.
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In Anchorage, Alaska, in 1964, just such an earthquake struck. It was actually filmed by a sailor on board this ship. They, of course, had no notion of what was happening to the crust of the earth several miles under their feet.
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Suddenly the harbor emptied itself of water as the surrounding territory leapt in the air.
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Just as suddenly the water came back in the form of a tidal wave, smashing everything in its path.
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At the time, scientists had no idea why such earthquakes occurred in this region. Now they realize that this is simply a boundary where two giant plates are overriding each other. When the Alaskan earthquake hit seismometers thousands of miles away, it practically wrecked the instruments, but it did provide a great deal of information about the movement of the two plates concerned.
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These instruments are connected directly to the Lamont Observatory, where one of the world's leading seismologist, Dr. Lynn Sykes, has invented a method for confirming accurately the direction the plates are moving. The cause of an earthquake is simply a slip or fault in underground rock. Can we tell what way these rocks are moving? Yes. The basic principle in determining a fault plane solution for an earthquake is that if we imagine that there is a long fault and we're looking at a map view,
34:25
if this side moves to the north and the side moves to the south, suddenly during an earthquake, what we find is that the first motions of the p-waves are such that stations in this quadrant receive compressions or outward first motions, these over here, dislocations or inward motions, dilatation and compressions. In other words, the first motions are divided into four different quadrants. By mapping these quadrants, it's possible then to determine the orientation
35:02
of the fault and the direction of motion on the fault.
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These are some of the seismograph instruments that we operate at Lamont. In fact, you can see an earthquake that was recorded about 3 hours ago. There's a very clear upward motion of the p-wave on this record. We need more information than this, however,
35:24
if we're going to examine the complete details of an earthquake. In fact, a worldwide network for earthquake information was set up by the American government for the detection of nuclear explosions. It now provides scientists with even more information than they need. Seismic stations around the globe report the arrival times of every single earthquake, large or small…
35:52
and a computer sorts them all out into a comprehensive picture, the date, time, exact location, depth, and magnitude of each event are given together with the first motions.
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So earthquakes have given us not only the boundaries of the plates, but also the relative directions in which they move. So much information is now available that a young research student from Los Angeles has built a sort of globe to test whether the movement of the plates matches the original hypotheses all around the world. It shows much better than you can on a flat sheet of paper how various plates move. If you take the boundary between South America and Africa,
36:33
this is the way it looked 20 million years ago with this essentially closed up. And then if you go forward in time to today, it opens up a little bit. Now, on the other hand, if you go around over here to the Pacific, this is the way that the East Pacific Rise looked 20 million years ago and this is the way it looks today. It opened up a lot more than the Mid-Atlantic Ridge did, just the way the magnetic anomalies show. And here in the Tonga Trench region,
37:00
this is what it looked like 20 million years ago. And if you go forward in time, you can see how it closes up just as it closes up along
37:11
that side of the Pacific Ocean over there. But one interesting thing to look at also is here in the vicinity of India — here 20 million years ago, India was much farther south, and if you go forward in time, you can see how India crashes in the southern part of Asia.
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The Plate Tectonics theory, only published two years ago, has already had an enormous impact. Even the Gemini astronaut pictures of the Red Sea and Aden show clearly how two plates here once fitted closely together.
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But it's the Earth Sciences which have been knocked sideways, in particular the fields of earthquakes, volcanoes, and mountain building. What happens, for example, when continents collide? Let's imagine, for instance, this is India, perhaps 100 million years ago and the Asian continent when there was once an ocean between the two. This is the old Tethyian Ocean. And what I've done here is to move India northwards, progressively northwards, and gradually push all these sediments which lie
38:15
on the bed of the ocean and squeeze them in a vice-like fashion and produce the Himalayas. And we can take it one step further and have the present day situation in which the the Indian Shield has gone down underneath Tibetan Shield and has doubled up the thickness of the crust with the Himalayas sitting on top of the system.
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If this is the result of plate collision in the past, can scientists tell us what mountains will be built in the future? Well, what we think one of the interesting areas is the Mediterranean, here. Well, we think what's going to happen is what has happened in the past along northern parts of India. This is an enlargement of showing that where the earthquakes now are in the Mediterranean region. And what's now happening is that Africa is moving northwards towards Europe and that the Mediterranean
39:07
is being under thrust beneath the Cretan Arc all the way along here. And presumably what's going to happen later on is that mountains are going to get built out of the sediment that's now in the Mediterranean.
39:25
The new Himalaya mountain belt being formed in the Mediterranean is of course producing the volcanoes and earthquakes in that area. But in Northwest America, the volcanoes in the cascade range on the decline. Or so everyone thought. It's now believed that the remains of a plate which built the volcanoes may still be descending underneath them.
39:47
This has sent scientists scurrying up to the edge of the craters to find out what's going on because the volcanoes were thought to be extinct. Large towns like Seattle and Portland have been built close by. It's now known that these towns will be wiped out. It may not be until man has moved to another planet, but it will happen. This volcanologist from the University of Oregon has instruments set around the rim of Crater Lake. He's trying to find out what's happening far
40:28
below the volcanoes roots.
40:35
When you think of eruption, people used to think, and mostly think of the hazard as it relates to volcanoes like the Hawaiian volcanoes, but you must realize that these volcanoes are quite different type and if we have an eruption, it's much more apt to be the very explosive, the very violent type. We might even be able to see something like a the creation of ignimbrites where material is mixed with water and it comes pouring out of the volcano at a very high temperature. It's so hot, in fact,
41:05
that it is actually being in particles but after it settles, it re-glues together and re-welds. This forms a welded tuff or an ignimbrite and — although there's tremendous areas of this in the United States — there have never been any eruptions of this type. But we have volcanoes that are not of the Hawaiian type but are the more dangerous type. As a matter of fact, we get several earthquakes a day and this seems quite a bit higher than we would expect to find
41:34
from a region that's inactive. And as a matter of fact, this whole study is exciting because it's the geology of now. Lots of times we can look at the rocks and say, this happened in the past, but with geophysics and particularly with microearthquake study, it's become very popular because it tells us what kind of tectonics, what kind of adjustment is the earth making right at this minute.
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It's in the various earthquake-plagued areas of the world that it becomes really urgent to know what's going on. One of these is California, where the famous San Andreas Fault runs in a straight line for hundreds of miles.
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Sometimes it's clearly visible, sometimes it's lost in the hills and valleys —
42:19
but it also passes through areas of extremely dense population. From Los Angeles, it runs north, smack through the village of San Juan Bautista and into the San Francisco Bay Area. In terms of plate tectonics, this line is a boundary between two plates which are sliding past each other — one of the edges of the world. Well, here we can look across the San Andreas fault that occupies the trough under the lake, toward the hills in the distance, which are on — what in a grossly oversimplified way —
42:54
could be called the Pacific plate. Now, that extends from here on out for thousands of miles into the Pacific Ocean. Where we're standing, we're on what could be called the North American Plate that extends thousands of miles out into the Atlantic Ocean. Now, that side opposite these two plates, of course, are moving with respect to each other — and that has moved something like an inch a year for millions of years and here in the foreground, we can see a variety of rock called a Franciscan rock
43:23
that characterizes the Eastern or North American plate. Locally, this does not occur on the opposite side because of this tremendous movement that has occurred between the two plates.
43:38
The region is, of course, honeycombed with seismic instruments. The strain gauge is simply a wire stretched between two points on either side of the fault line.
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The readings are fed straight back to seismologists at the U.S. Geological Survey unit at Menlo Park. Here, every tiniest tremor, every microearthquake, is monitored…
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sometimes as many as 50 a day.
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The strain gauges tell them how much the two plates are moving, and the answer is an inexorable six centimeters a year.
44:32
As these two great plates move past each other, some areas tend to lock periodically and hang up and then break through. And these two red segments on the fault,
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one broke in 1857, the other 1906. But strangely now those are very quiet areas. No movement do we detect, nor do we see any small earthquakes. These other colored areas, however, are creeping and are characterized by very small earthquakes. If we take a look at this section right here near Hollister over on this more detailed map, we can see some mapping by… microtremor mapping. We put out seismographs on these red triangles, and this allows us to identify very small earthquakes.
45:18
And this is a set of earthquakes in a very few months time in the Hollister area. But then look at the part of the San Andreas Fault up here in the San Francisco Bay area. Almost no earthquakes. This is now locked and yet we are sure that this will break again some day and perhaps produce another great earthquake.
45:44
In 1906, half of San Francisco was destroyed and San Franciscans today are understandably worried that it will happen again. It will. Scientists know that as each year passes, it will be bigger. But unfortunately, they don't know when it will happen. In the same way that our view of the globe was changed by the astronauts looking back from the moon. So the new geology will alter our idea of where the parish boundaries of our globe are.
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Every textbook from primary school to university will have to be rewritten as scientists get together all over the world. Geochemists, geophysicists, paleontologists — many people who used to work sort of among themselves, not understanding really what each other were doing, are now able to focus their efforts on two common problems to see the significance of what they're doing in a larger picture. This, I think, is always good. In practical ways, it will have an impact, too,
46:47
and I think in the very near future. For example, with earthquakes. We now, knowing the motions of the plates, are able to understand in new ways the reasons why earthquakes are generated and the effects that they have.
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In more subtle ways and human ways which are really the most exciting ways, for example, young people in the earth sciences now — once at a disadvantage and still somewhat from lack of experience and so on, lack of having been around the world and seen it all — nevertheless, with this new theory, have mental tools that are powerful enough to allow them to plunge into the mainstream of thinking and to attack important problems very early in their careers and getting even wider and more grandiose
47:31
but I think still being true, men at large have a better way now to understand, with no background in geology at all, to understand how the world was put together and how it has gotten to be what it is. This means, in some subtle inside the mind way, I think the ultimate long range benefit of this kind of thinking may be to let people appreciate better what their environment really is. What kind of balances really do exist in the earth.
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