About science; About the Earth's mantle

This is about science produced by the California Institute of Technology and originally broadcast by station KPP C. Pasadena California. The programs are made available to the station by national educational radio. This program is about the Earth's mantle with host Dr. Albert Hibbs and his guest Dr. Don Anderson associate professor of geophysics Here now is Dr. hit according to rumor in ancient time as a man thought the earth was flat. And when they finally decided it was round after all that picture of it was fairly simple a ball of dirt and rock covered here and there with oceans of unknown depth. The modern Geo physics the physics of the earth has been developing a much more complicated description of the interior of the earth and our guest today Dr. Don Anderson has been working out some of the newly discovered and most detailed portions of that picture. The structure of the Earth's mantle. Dr. Anderson is a graduate of Cal Tech and has been on the staff of the Seismological Laboratory there since 1961.

And Don I'd like to start by asking what really is the Earth's mantle What does that word mean and what other parts are there of the inside of the earth and how do you tell one from the other. Ok fine the Earth's mantle of that region of the earth which extends from the base of the crust which is seven to 50 converse that depending on whether you're under oceans or whether you're under mountains and extends down to the core which is approximately 3000 kilometers it is by far the most the largest part of the earth by volume and next to the crust is the most important part of the earth as far as our day to day activities are concerned. But what's what's the difference between the crust and the mantle is a different kind of rock. Well really the one word definition of the the crust and mantle in the definition of the discontinuity separating them is based on size and logical data and what we measure in seismology is the travel time where the velocity of the various kinds of elastic waves move how they are darting from earthquakes. That's right mostly starting from earthquakes but recently we've been using very large

explosions also but the the crust itself was discovered using earthquake waves back about one thousand ten or so on the the mantle is undoubtedly made of a different kind of rock than the the crustal rock and it is not clear whether we have ever seen a piece of the mantle rock or some very dense rocks that are observed at the earth's surface that some geologists think have indeed come from the mantle to the mantle. It is suspected that rock from the mantle is a considerably more dense than a rock in the average of piece of rock from the crust. Right and this is the way we detected from seismology the fact that it's denser and also has a higher velocity of propagation of elastic way. So those two are related by the way the first yes they seem to be very closely related when the velocity goes up the density also goes up and we are able to build fairly detailed models of the density variation in the Earth's interior by just measuring the elastic properties of the rock. Functional and by timing the waves by timing away I mean from a known position of an

earthquake could ride at a speed at which they travel and then this you can figure out where things are. That's when I take a tremendous number of earthquakes and computation of waves to build up any picture. Yes indeed it does it takes many thousands of earthquakes too. To sample all portions of the Earth's interior and our present knowledge has grown over the last 50 years and by now we of course have many hundreds of thousands of earthquakes that have been analyzed. So there is a crust of 10 20 30 miles thick over the surface of the earth. That's right. Then underneath that a mantle of one or two thousand miles thick Hornets three thousand kilometers down to the core itself and the earth itself is about sixty three thousand or sixty three hundred kilometers in radius. So it's about half my radius which makes it a tremendous amount of volume underneath that and there's a core of what's the difference between the core and the mantle. All the core of the boundary separating the the mantle from the core is where the most profound

discontinuity is in the Earth's interior. The velocity of seismic waves takes a sudden decrease at this boundary and the density takes a sudden increase. And from. What we think we know about the relationship between velocity and density and composition. The core seems to be mainly iron although it has some 20 percent lighter alloying element in it to reduce the density below what the density of pure iron would be at that depth. The core is liquid. It's probably molten iron. I say this you tell from the type of the ways it will go through the core. Right there's two kinds of waves that travel in a solid the compressional wave and shear wave and a liquid there is just the compressional wave since a liquid does not have a rigidity and therefore does not transmit your waves not side to side waves that would issue a right sure wave as he's initially called an S-wave. It meant secondary wave or shake way. Right I see so that these don't come through the car. These dumplings come through the core That's right. But for this of course you need it do get an earthquake all the way on the right on the

other side of the earth. To see a wave it's coming through the car very nearly or out and that's right in fact that's the way one builds up a picture of the Earth's interior is to look at earthquakes or explosions at successively larger and larger distances and one sees successively deeper and deeper into the mantle and then at a certain distance approximately one quarter of the way around the circumference of the earth. We. The way you start getting into the core and the velocity in a corps is less than the last you know metal so the waves that enter into the core are retracted downward and we enter into what is known as a shadow zone. No energy comes directly up into this zone from the core and we have to go out much further in order to see these waves that have indeed gone through the core. And when we finally get to the point where we see these waves they are just compressional and there are no shear waves that we can identify as having passed through the core. So there's quite a distinct change of properties that there's a very distinct change in problems here not only in the composition of the material which we believe goes from something like

a silicate rocks not the ordinary kind we see at the surface but the way they would be at great pressure and. Almost pure iron and the iron is not solid it is fluid so we had with a compositional change and a change of face from a solid to a fluid at this time. At this core mantle boundary. When I was up within the mantle itself was by far the major hunk of the earth. I suspect from what I have heard recently that this is not a single type of rock with a single uniform characteristic throughout but may be differentiated or at least separated into more complicated picture than just one uniform slab. Right the Earth's mantle gives every indication of having been very strongly differentiated. The current belief both by many seismologists also by chemists and geologists is that the earth than initially was a fairly

homogeneous collection of particles. It then heated up and in the process of heating up the iron in the interior melted and drained downward from the core and the lighter material the lighter silicates came up were to form the contents in the mountains. And in addition to this differentiation the primary differentiation which separated the earth into the three major parts of the crust the mantle and the core the mantle itself. I became zoned. It became locally depleted of certain elements in order to form the continents. It's laterally and homogeneous and in addition to the differentiation which is just the physical separation of different material by by density there are also major phase changes which happen in the earth in addition to the melting at the core My eyes are as you say lighter material sort of floated up in this process of the heavy going to bottom of the earth heated from the inside. What about the idea of the earth was hot to begin with and has been cooling off with this.

Is this in conflict with that notion or is that motion no longer popping I was one of the the early popular held views to current ideas are now that the planets including the Earth are created from fairly cold. Your attic material. And a silicate phase of these meteorites contain uranium potassium and for him and these when they're trapped inside of a body such as the earth or heat the body up and eventually required to differentiate. So the picture we have there is here as it has been something has been heating up from from a colder saying rather than cooling down from a hot cinder. And that's exactly the picture we have right now although the the end result for the earth is not too much different. It turns out that the earth will reach the melting point of of the common meteoritic material such as iron and such as silicates fairly early in the history of the of the earth so that we did indeed go through a point where the earth was at least partially moment and this was

early in the history but earlier in history it was called. I see those cold I'm heated up than I was a beginner in the in a cooling off phase again. Well assuming you have the second heating cycle as soon as you exceed the melting point of any of the constituents then of course you can allow the the light material to flow upward the most material fill up or in this is a very efficient way to remove heat the only reason the earth heated up an initially was because the heat was generated by the decay of the radioactive elements much quicker than it can be conducted outward but it was us and they were dry well insulated from that is exactly right. But as soon as melting can occur then the material can and flow outward and the removal of heat is very efficient very effective in a very rapid and then things slow down again. When they solidify and heat up again so it is sort of a sickly thing. However if partial melting is as extreme as there's a large amount of melting then a large amount of the radioactivity is concentrated toward the surface of the earth and the interior will then probably be cooling off there after I

say so of course radioactive material be the hottest it would tend to rise in favor of. Minerals of wealth equally density. It's the reason for the melting in the first place then tends to get trapped in the liquid phase and is brought up to the surface I see it's not that it itself as it is hotter lighter but it's just associated carried along with the molten metal that's right it's sticks with the first face to melt so in all this process the iron melted in center down to the metal in the lighter rock rose up to the top and that's where we live and in between now is this is this massive heavier rock and that's exactly right and that's the the object of my attention. Well how have you been paying attention to it how do you look at this buried mass of rock. Well there are several ways there is the classic way which uses earthquakes which generate the compression on the shear waves as I mentioned before and just the compression wave in the core. One determines the velocity of these waves simply by measuring the rate at which they cross various seismometers on the surface of the earth.

And after one has many hundreds of earthquakes at all possible distances it's possible to reconstruct the velocity in the interior of the earth that must have led to this distribution of velocity that's observed on the surface of the earth. However there are complications in the standard method. If the velocity in the earth decreases anywhere this means the rays are bent downward instead of upward and they have to penetrate to a much deeper depth where the velocity starts to increase again before they are retracted upward where we can actually measure them. Because after all we have to have the energy to reach the surface of the earth otherwise it doesn't do us any good. Could you have some other complicated paths. There are great obligated following underneath and the low velocity zones are the most complicated features to study a low velocity zone is simply an area where the velocity decreases with depth instead of increasing with depth. These are very complicated regions to study and there is quite a controversy about whether the mantle does indeed have a low velocity zone. This would have great implications. The

velocity is lowered when the temperature. Rises or when the rock is partially molten So the presence of walking isn't the possible presence of magma chambers in the upper mantle. The possible presence of convection cells in the upper mantle are very intimately connected with the presence or absence of a low velocity zone in the upper mantle. It now seems fairly certain that the major part of the earth's surface is immediately underlaying by a low velocity flare which is right under the scroll with this whole layer be somewhat liquid or would it be used the word chambers would be the low velocity or perhaps even liquefied material be distributed in blobs here and there. It would be more likely to be distributed in blobs and some shear waves do indeed traverse this material so it can't be completely molten. On the other hand there are regions of the earth where shear waves seem to be very rapidly attenuated such as would be the case if there was a

large molten areas and these are also the areas where we expect a partial melting on other grounds or as we see it in volcanoes and some some of these regions. There isn't a connection between the regions that you can identify with the slow wave character and the lack of sure wages a connection between that and obvious volcanism on the surface right. In many cases there is an obvious connection in other places there is a pronounced low velocity zone a no. Obvious president volcanism this is true for the major part of the ocean floor. The most pronounced low velocity zone seems to be under the ocean floor. The. Tectonic regions of the earth and regions where there are indeed volcanoes also have a pronounced loss the zone. So it's not clear at this point whether the whole crust of the earth is underlined by partially in the endzone. This is somewhat material only being allowed to escape in certain preferred areas or if indeed the magma only exists in these areas where there are surface volcanoes. Well this is essentially something that then.

Extends also perhaps not completely but extensively as a band throughout the mantle all around the earth breaking the mantle down now into a layered substructure right the low velocity zone seems to be essentially worldwide. There are areas where it apparently is missing there are areas where it is. It is less pronounced these areas are the shield areas the older areas of the earth which have presumably differentiated their crust out of the mantle and have removed the heat producing elements upward and this part of the upper mantle being depleted of the low velocity low density material to form the continent and also depleting it of the heat producing reactive elements will lose the essence of what it takes to make a little bit last night and I see it was this slow Last Days on them and other places where it does exist near the top of the mantle. It's the low velocity zone is very laterally and homogeneous it rises near to the surface under under oceans it possibly comes through the surface and volcanic

regions and it extends to greater depth under continents. What kind of depths are we talking about. You know we're talking about 50 kilometers deep to about 150 kilometers deep is this zone we're talking about still a very upper portion of the mass. It's the very upper almost across to the mantle writing called the crust the mantle and the knot that is there more structure as a possible to get any further down and get any picture of the globe last season. Has been one of those controversial regions and it's also one of the most complicated regions to study and because you have to know what a seismic wave is doing on the way down and on the way up you have to understand in detail the top of the mantle before you can study further down so the complications introduced by the low velocity zone have slowed us up here but now we have finally seen through this zone and we are seeing some structure in the upper mantle with some great detail now below the low velocity zone there is a a region of the earth that is some 600 kilometers thick in which the

velocity and the density seem to increase extremely rapidly much more rapidly than you would expect just by compressing material just by putting it under more pressure. There are two relatively abrupt discontinuity in the upper mantle. Each one is about oh 50 to 100 kilometers thick. And if you were to draw density versus depth you'd get something that looks something like a staircase with these two discontinuity. Now these discounted these are probably caused by solid solid phase changes. That is to say the common rocks that we see at the surface of the earth the silicates cannot keep their same structure when they are subjected to great pressure. They will collapse into a more dense form in the same way that graphite collapses to diamond when it's put under extreme as it also wanted to observe that under a laboratory test can you exert these pressures and see it happen. Right the solid solid phase changes have been known to physicists and to do physicists since high pressure experiments were first started in the laboratory. However most of these these

phase changes were materials that were not particularly pertinent to the composition of the earth. However as techniques have improved higher and higher pressures have been achieved in the laboratory. And in fact just last year it was possible to. Observe a solid solid face change from a silicate to a form that was some 10 percent denser in the laboratory. What kind of pressure to take to do this required pressures that went up to 150 killer bars. This would correspond to something like 400 kilometers depth in the earth. I see so we are just now reaching the point where we can reproduce in the laboratory conditions that exist in the upper part of the mantle and then so with with this you can then check out the idea that not only that changes occur but get some measurement of how much of a density change goes along with it he's exactly right and these experiments that have just been performed show that the discontinuity will occur at about four hundred

kilometers just where we see a discontinuity and the density jump there will be of the order of 10 percent just what we had inferred previously from. So even some of these early laboratory measurements are beginning to check the data that you can observe with seismology. That's right. We have used the Earth as a laboratory in the past. This is the only way to get these high pressures and we propose various theories to explain what we see and. Then the people with the high pressure labs go to check these theories and everything looks like it's falling into place fairly well right now. Is there any conceivable way of getting pressures which or equivalent to several thousand kilometers depth. Finding out what would happen only way this is possible now is with the use of shock waves. This is a very expensive and dangerous experiment whereby any explosive is used to fire a metal plate at a rock the rock. And in fact the whole laboratory disintegrate in the course of this experiment but using very rapid photographer you it's possible to calculate both the pressure and the density

that is involved in the course of the shock wave experiment and hence to construct any equation of state for the rock in question. But you don't have anything left of the rock to look at after the experiments on which made is true. And not only that but many tens of experiments must be run in order to reconstruct the whole equation of state because each experiment really just gives you one point on E and the pressure density diagram and you need many of these points in order to draw a curve that relates density pressure in a way that you want to study the earth. Well speaking of getting points and putting them on diagrams. Is it possible to use any sort of active seismology to carry on your experiments by using explosives. And. Producing your own waves to probe down into the earth or do you always have to wait for nature to do it for you with an earthquake. Well nature is the cheap way. But of course you have to have instruments monitoring all the time since it is not yet possible to predict when earthquakes will actually occur.

D. The technique of using artificial explosives is the technique that's used by the expiration industry of course the oil company people use on small charges and deep holes and many geophones to actually map out the layers in the upper part of the earth. We use similar techniques to study the crust. You can use relatively small explosives perhaps 100 pounds in a lake to study the structure of the earth's crust how deep can you probe with a hundred phone chargers. I wonder how do you know. It really really is just telling you about the very uppermost part the crust unless you're in a very efficient area to generate seismic waves from an explosive. And this would be and say deep lakes Lake Superior for instance is a wonderful place to shoot. Last year many shots of explosives were dropped in this lake and profiles are run all the way across the country. But these experiments were primarily for crustal structure. If you want to study the

deeper mantle then you have to wait until there's a big nuclear test somewhere in order to get the kinds of energy that will travel many thousands of kilometers. And of course when you do the nuclear tests are rather restricted as to exactly where they happen. So you're somewhat limited in that it was presume you can you know in your own spot it's not where you want the next bomb. That's very true unfortunately for four seismologists Fortunately for other people I suppose. But I suppose there's one advantage you know exactly where and when the explosion happened so you. Yes I mean that's the only thing that's a source of error and standard sized logical methods. Since you don't know exactly where the earthquake is and exactly what time it happened it is a little bit difficult to get with any great precision the structure of the earth. However if you have a very accurately timed and located seismic event such as a nuclear event then it is possible to. Study the earth in somewhat more detail and with somewhat greater accuracy. And not only

that the BOM records are very much more simple than earthquake records. So if there are complications of the size of a gram we attribute them to the earth rather than to the. To the earthquake itself. So the bomber was a classical point source of energy that uses very simple equations at least to start out with. That's right there's been a lot of things on seismographs that have been unexplained in the past we haven't begun to tap all the information that is contained in a seismic gram. Many people do not attempt this because we never really knew what an earthquake looked like and it could be complicated. So many of these bumps and wheels that we see on seismographs we. Tended to attribute to just complications of the earthquake itself. However with the bombs we see the same complications and we goals and now we are encouraged to interpret them in terms of features of the earth from the stuff the wave passed through rather than me and our GTS that right there is many discontinuity. Now it turns out in the earth in the mantle in the core in the crust and these discontinuity both reflect and refract seismic

energy and lead to many possible different past paths through the earth. Now there are so many possibilities out there. There are Wiggles that interfere with wiggles on a seismograph. But when we have a simple classic point source such as a bomb we are inclined to interpret these wiggles and try to explain the whole seismic Graham this was very difficult. Before we had a large explosion you have sound actually like sound echoing off of a mass of buildings in a city that's right exactly where you see and where the sound came from and where the buildings are while you're all blindfolded and have nothing but he isn't exactly how I would think this would really be quite a complication. For example even the getting the depth of an earthquake at the same time you're trying to use that same earthquake to figure out the characteristics of material at depth. Yes this has led to many difficulties it's a bootstrap operation before you know where the earthquake is you have to know the structure of the earth very accurately in order to to take the arrival times of the waves and then

take them back to the source. On the other hand you can't study the structure of the earth. You can't determine its velocity unless you know when and where the earthquake happened. So it's a round robin type affair or just I don't really have time for the last of computations just to check to see if you finally get a picture that fits all the data. That's right. It's helped a LOT of course as a number of seismometers around the world has increased the more you have. And the more closely you are to the the closer you are to the actual event itself and the less likelihood there is of large misidentification know where it is and how deep it is and when it actually happened. So we now have many hundreds of seismic stations around the world so the accuracy is improving all the time even for the earthquakes. We are building up a picture of a mantle which is a complicated layered structure certainly not a simple single model that.

I learned about when I took my sophomore course in geology. But apart from the pockets of magma or zones directly associated with volcanism. It seems to be a fairly static mantle from the least as I understand the picture you've given me. And yet I have heard of some ideas that the mantle is undergoing some sort of convection and is actually at a very slow rate turning over. Perhaps carrying He energy in the process accounting for the drifts of continents and perhaps even for the building of mountains at the borderline between two of these turbulent convection cells. Does anything you have done indicate any of this type of dynamic motion in the mantle. Large scale motion of this huge turbulent time of course seismology cannot measure philosophies of convection seismology is basically a science

of studying the earth the way it is today. Going beyond that is somewhat speculative. However the picture that has evolved of the structure of the interior of the earth is consistent with the continents having differentiated from the material which is immediately below them in the process of the early. Nothing in differentiation that I mentioned. So this wouldn't require that there is a new material constantly being brought up by by some sort of convection it just rose straight up and that's why of course that is convection. Mass movement however it's quite a different story than having continuous convection cells which are throughout the whole mantle. And the president volcanism that we do see the earth can this now be accounted for by the pockets of that you have identified in the in this low velocity zone. There is no doubt that the volcanoes are occurring around the surface of the earth and they must be underlined by magma chambers. As I said

previously it's not clear whether the whole world is underlined by magma chambers and they just rise where there's cracks or weak zones where there are just these magma chambers under volcanoes we know there's magma chambers under volcanoes. Well Don thank you very much for giving us a. Closer picture of the Earth's mantle it seems that the Earth is getting steadily more complicated the more we look at it. And I didn't we have a few more complications to go yet. I think we do. Thank you. This was about science with host Dr. Albert Hibbs and his guest Dr. Don Anderson. Join us again for our next program when Dr. Peter less a man will lead a discussion about the early universe about science is produced by the California Institute of Technology and is originally broadcast by station KPCC in Pasadena California. The programs are made available to the station by national educational radio. This is the national educational radio network. It's a one k PPC and the California Institute of Technology

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