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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 oldest mountain range with host Dr. Albert Hibbs and his guest Dr. Leon silver professor of geological science. Here now is Dr. hymns for many years geologist and geochemist have been studying the age of the earth samples of Earth rock of samples of meteorites have both been used in the search. And the best estimate now is about four and a half billion years. But though the earth as a whole is probably this all the details on its surface are considerably newer. In terms of geologic time the mountains and valleys that cover the Earth's continents are relative newcomers generally in the hundreds of millions of years rather than billions. And this is understandable. Earthquakes keep raising some areas of the surface and dropping others. Volcanoes cover all surfaces with fresh lava glaciers and
streams where away old mountains and build up New Meadows and prairie's and all of this means that what the geologists see when they investigate the surface of the earth is a small fraction of its total history. It's very difficult to trace back this history more than a few hundred million years since so much of the evidence has been destroyed. Nevertheless every now and then a small piece of the ancient record is discovered and an example of such evidence is our subject today. To tell us about the discovery and its implications we have Professor Lee Silver of Caltex division of Geological Sciences. Professor Silver started out his academic career as a civil engineer at the University of Colorado but then he moved into the field of geology and geochemistry and it was in this area that he received his doctor's degree at Caltech. Since 1955 he's been on the faculty at Cal Tech and devoted a considerable amount of his attention to the geologic history of the southwestern portion of the North American continent. And we start out with How does a geologist
set out to discover the remnants of some ancient mountain. The rocks themselves have a significant record which can be read by a variety of techniques and methods. The basic elements of reading the rocks lies in recognizing sequence which has been superimposed. On the rock strata or the various rock formations sequence which is reflected by a superposition of straight-A the oldest data being at the base of the section. The youngest data being at the at the surface. Or sequence being developed by the imposition of a series of fractures or faults on rocks so that we can recognize that the fracture systems are younger than the rocks in which they are sort of superimposed. The effects of heating and the effects of. Of all types
of physical and chemical disturbance which permit us to compare whether or not rocks which are immediately associated are to be rated as older or younger than the rocks with which they're associated have to have an intimate familiarity with three sequences with it with this trait or that you see. I suppose again from examination of expose mountains in various areas where you finally build up a record of what kind of rock along this way or in this time sequence and you recognize it when you go somewhere else with this. That's right. The synthesis of the story for a large region of the crust is dependent upon putting together the pieces obtained in many different types of local situations. Rock a rests on rock B. Therefore in general rock is younger than rock be at another site we find that rock be interned. Is to be found resting on Roxy or for
Iraq. C is older than both rock. A n rock B. And when we find in a still another place. And fracture wood which cuts rock a and is filled with lava which is chilled a form what we call a dyke and we can recognize that since Rock A is older than this dike therefore rock be in Roxy or also or in the dyke and so we continue to synthesize these pieces by pieces of like it was a general picture. Well in this it was this the general technique that you use when you look for the extremely rock by just tracing back down through these layers and looking for still a new route not a newer but an older layer one. Still not yet on covered. Well it. I would be very helpful if we always had the mindless types of exposures that one finds in a place like the Grand Canyon where one can go and succession down through older and older rocks until you
find the oldest rocks in the region exposed in the bottom or the inner gorge of the Grand Canyon as the case may be. Was that what you found in this case you've been investigating here so now it's a matter of fact the study is is quite different and the oldest rocks happen to be amongst the highest rocks not the deepest rocks of this house. And this is the result of the peculiar history of our part of Southern California in which the movement of blocks of the crust. Along great factories which we call faults amongst which I'm sure you know Senator asphalt the same Gabriel felt movements along these factors have brought up for us in certain places rocks which were once very deep in the within the earth. These rocks now occupy some of the highest elevations in Southern California to instead of having a river cut down on the Rockaway was in the Grand Canyon you had and I was going to bring it up for your inspection right near this one that's right and that's one of the
that's what you might call a piece of geological luck. It's been most important most valuable for I thought that the San Andreas fault in California moved things horizontally because it also brings up and I think that's right. And one has to scale of the magnitude of the movements both horizontal and vertical. Geologists have long suspected that there has been a significant lateral displacement of the elements of the crust found on opposite sides of the Senate greatest fault by significant It's been suggested that these displacements may be as large as three hundred fifty miles positive demonstration of this is lacking but much evidence suggests that the crust on opposite sides of the fault has moved relative have moved relative to each other. In something like this magnitude on the other hand we find that a number of points along the San Andreas Fault a mountains that stand quite high and it's
quite clear that some of the fault movements along the San Andreas have contributed to the upward complement of movement this compliment may be on the order of a mile to three perhaps even as much as 10 miles. It's still a small fraction of the horizontal movement that's running motion is horizontal within some small vertical we would assess this is primarily horizontal motion but allowing for still a significant complement of vertical movement. Well how was this example of material you identified. Well the. Oldest mountains which we see the products remnant now in southern California are about seventeen hundred million years old. See that's one point seven million or one billion seven hundred million years old. Over a third of the estimated age of the untried. This is the oldest remnant of a recognizable.
Geologic formation that's been if not the oldest in the continent by any means in the central or interior part of the continent. In the Minnesota River Valley and southern Minnesota we know of rocks that are almost exactly twice as old. Three point forty three point five billion years. But this is old for this portion of the continent. Yes and as a matter of fact our interest in these rocks stems from our interest in trying to understand whether our continent in its present dimensions sprang full sized from the early formation Lepus odes. And the history of the earth or whether or not these continents and this continent in particular has grown from a very old center to successively younger and younger rimming or ringing. Increments of crust. We have a hypothesis known as Continental accretion which it stated that there was just a little bit of the granitic material which makes continents
in the early history of the earth. But the deep seeded process season and the Earth continue to operate and if slowly brought more of this material to the surface and permitted us to. Or permitted the continents to grow over a long period of time. Now that is one hypothesis that is not universally accepted as a matter of fact. I would not accept it without considerable reservations what is our alternate hypothesis with an alternate hypothesis would be that the ancient material which is now well preserved in the interior part of the continent once existed much closer to the margins but that the intense activities which have continued in physically and chemically changing the crust closer to the margins have destroyed you know almost all places. All evidence of this original ancient crustal material. And so the apparent rainin of an ancient interior by successively younger exterior belts is really more a reflection of
the areas of activity slowly progressing away from the center and out towards the margins of the continent. So that the general shape and position of the continent is quite ancient and the growth of the continent has been almost like quite like the spread of bacteria on a culture plate or something like that. Well that way from songs that would be one hypothesis certainly I would have to say that at this time the definitive evidence the really important information which we need to decide what is to be favored amongst these hypotheses is not really come forward. On the other hand we do have great promise of useful results based upon analytical methods and geological approaches which have developed in the last 10 or 15 years with this region where you found this particular remnant is the San Gabriel mountains just north of Los Angeles is a rather young range geologically speaking and yes there might appear to be a little bit of an
in congruity there but it's not really so. The mountain range as a as a elevated region which stands high above the Los Angeles Basin is certainly an exceedingly young feature. By. In terms of geological time it probably developed in much less than the last 1 percent of geological time. On the other hand we recognize that presently exposed in this very young mountain range our great masses of rock what we call rock complexes which formed during earlier episodes of mountain formation and heat we can recognize in the San Gabriel Mountains at least five major rock forming episodes each of which was probably associated with the appearance of an elevated area on this site at the time that those rocks formed. Was there any evidence that they in any way coincided with the position of the current mountain ranges are they each other separate location and position and on the
surface. Well the. We have considerable evidence based on regional studies that some of these more ancient mountain chains were once much more widely extensive than our present San Gabriel Mountains. Indeed some of them may have extended from the Pacific coastal margin clear across the continent to northeastern North America. So it would appear that it was quite different and ignorance quite quite different. The present San Gabriel Mountains over their length their height and their general form to the existence of a set of intersecting faults very large faults. These faults are factors along which the crust is moved. One of these sets is the very well-known San Andreas Fault set. Another set which is quite important but less well known as a set which is more or less east west and trend and is responsible for producing the series of east west mountain
ranges which we call the transverse ranges without an active region. Oh it's got very much about it as well. The concern of the seismologists really well very recently quite a controversy developed at a site on the Malibu coast called Corral Canyon concerning the establishment of a proposed nuclear reactor site on one of these fault systems and there was a great deal of controversy amongst geologists seismologists and various other experts as to just how active this East-West fault was I believe that the. Final Resolution or recommendation on that site was largely based upon the judgment as to whether or not this was an active one was the final outcome by by the way. It was recognized that faulting existed in the region and the Atomic Energy Commission investigating panel recommended that any design which for a nuclear reactor which would be acceptable at that site must take into account
the possibility of halting at the site so far as it has political legal decision goes this is a some sort of a possible act evolved a little certainly doesn't seem to be as active as a San Andres. Well. I would not like to insert myself in controversy right now. You know that's a kind of controversy that transcends scientific investigation. Exactly. How could you first of all let me ask you what was it particularly that you discovered when you discovered this remnant What was it a piece of rock the largest deposit of rock you worked well and it was if I can first introduce the word again the word rock complex. This was a region where a number of different rock types. Related in space in a very complex sort of way existed which had characteristics which permitted us to suspect that they were quite agent. These characteristics were primarily
assemblage of minerals and or groups of mineral types and arrangements of the minerals and patterns in the rocks which indicated that these rocks formed at high temperatures and under conditions of intense movement and rock shearing at great depths in the earth. These led us to suspect that these particular rocks had endured a longer and more intense history than most of the other rocks with which we were figures may be good candidates for ancient. That's right. And how did you identify the fact that they really were indeed. Willis was by the application of. Studies chemical studies to the relative abundance of this of certain radioactive elements and isotopes of these elements and to the relative abundance as of the isotopes of lead which are
produced by the radioactive decay of these radioactive elements which once you start which of the radioactive ones at start of the stick aging well the systems that we used are the uranium and thorium systems and in particular the isotopes uranium 238 uranium 235 and thorium 232. These decay by rather long and complicated pattern from nuclei which have about whether the respective masses that I'd just given to you two isotopes of lead which have the masses lead to 0 6 to 0 7 to 8. When we compare for example the amount of uranium 238 with the amount of lead to a six which is found associated with it and certain minerals we can make a calculation based upon knowing just how fast you're running 238 is transformed to lead to a six. We can
calculate the apparent age of that system. Now since we have three different radioactive isotope systems and since these isotope systems each have different kinds of or different rates of decay if we get stories which are internally consistent from each of these we have a considerable degree of faith in that. Jim the significance of the data was this kind of a check that enables you to get the 1.7 billion year data that's rock that's right. What is the flavor by the way the one. Well we used to uranium isotopes and uranium 238 has a half life of four and a half billion years. That means that if you have a very large number of atoms of uranium 238 at the end of a four and a half million years from the time you initially examine them you'd find that half of them had disappeared. And if you kept them in a real tight container you'd find that instead of having. That half that was that was missing you would now have an equivalent number of lead
to a six. So really what you're measuring is the time. But. Between now and some point at which that piece of rock was molten or at least was mixed up in some way that's right and we had it was they were measuring the time back to the point when the clock formed Noah. A mental clock. It has to be considered as being composed of various elements it's a container. And it is a contained group of isotopes. The mineral is the container and minute quantities of the radioactive isotopes are the actual clock materials. If that clock or if that container is a sound one and is capable of resisting. Fracturing or breakage due to heating through the long history of the rock then simply by counting the number of lead atoms that are there now. The number of uranium atoms that are there now and calculating back to the time when there were no original lead
atoms. We get an assumed or inferred age for that because any particular mineral that you use that's better than others for this purpose. Well there are a number of minerals which are used by people who work in the and the science or art dating. I happen to spend a great deal of time working with a mineral called zircons. This is the rock which is also a semi precious gem stone and is comprised of the elements zirconium silicon and oxygen. It turns out that zirconium as a chemical element has properties are quite similar to uranium and thorium. So that even though uranium and thorium are very rare on the crust of the earth if they are present they prefer to go into the minerals work on more than most other metals they could imitate as a Conium and get into the crystal. That's it. They occupy very similar sites and the structure is a matter of fact. Both uranium and thorium fit into those a Conium site better than the zirconium does.
They're better received and I say if only they were a little more abundant they'd be the basis for the crystals. That's right. You mentioned that the rocks you found were made quite great depths and under great heat about it. Did you have any way of estimating how deep. Well we have some estimates there as yet not as precise as we'd like them to be. We arrive at these estimates from again regional studies of one kind or another. Columns of rock which are exposed above this type of group of minerals this type of texture We also gather a great deal of data from experimental studies in the laboratory where we attempt to determine under what conditions a particular group of minerals formed together. If we had to estimate the conditions under which. The These very ancient rocks in the San Gabriel mountains formed we would have to say that it was probably at a depth of somewhere from six to 12 miles down
in the earth and probably at temperatures from over 500 to 700 degrees centigrade. So this is what allows you then to identify this as a portion of an old mountain range. Yes this would be the type of material which we would refer to as being characteristic of the roots not ranges that is the deeper the supporting material on which is the familiar upper parts are found it in and the other had been just the basin of an ocean that would not have been there would have been no such depth for us there. Well I would not have been the environment based upon the loading up of the rock which would have created such you know rolls and such textures or structures in the rock. We mentioned also this was brought up to the surface by earthquake Foden and which side of the San Andreas fault that you find this particular example. Well the San Gabriel Mountains of course are on the southwest side of the San Andreas Fault which is the Pacific Ocean side. And it is
away from the main mass of continental material that's one of the reasons why we're particularly interested in this so this is a portion of the. Confident this section to the west of the fault it is gradually moving northward as the fall slips and that's our present interpretation of Genesis implied that the sample you found was when it was made somewhat south of its present location. That would be really an occurrence we would draw and in order to confirm or deny this inference it would be appropriate to search across the senator asphalt that is on the North East Side and somewhere to the southeast. For a similar group of rock types which we could match hopefully with the complex that we have found in the sand you will know we have had an opportunity. Well we are in the midst of the survey at the present time and we have a great many ideas about possibilities.
As you pointed out earlier I have spent a great deal of time studying the distribution of the older rocks of the southwestern part of North America and there are places where I would like to go hunting to make this match. And I plan to do this in your future. And this would include the regions in the. Southeastern California desert and in the Arizona desert and perhaps down even into northwestern Mexico. Yes as you mentioned earlier the motion of the San Andreas Fault is perhaps carried at some hundreds of miles. The other half of this piece might really be quite a bit further south than the Los Angeles basin. Well of course I have indicated to you that it has been suggested that this crystal displacement of several hundred miles has occurred when it will be one of our most important objectives to be able to confirm or deny that such large displacements have really taken place.
I see so what what this hundred of hundreds of miles figure represents is an extrapolation from current motion back assuming that it's been going on for some time in part from matter and in part from suggestive evidence but evidence which even those people who suggest it do not feel it is absolutely compelling. So this this discovery of this remnant then not only is an identification of an old mountain formation but a possible check on the total motion over almost two billion years of a major fault system. That's right. I might say that it is not at all apparent to the geologist as yet for how long the San Andreas Fault has been in existence. That is it may be a relatively recent feature or maybe a very ancient feature. Most of those. Reflections of its existence seen in the form of the mountains and in the valleys and the great patches of Southern California are all very young features and we are not at all certain that the Senate race fault extends a very
long time back into the geological record. But that happens to be one of our major interests and trying to develop a time. Dependent record or history if you will of the San Andreas fault. Is there any way of estimating from what you've learned so far from these examples that what the wood of the formation might be that this is a piece of it was perhaps a narrow mountain chain so that once you discover that if you can locate the other pieces you have quite a sharp line and you can draw east and west or might it have been a very broad feature. Well. Faced with the fact that there are perhaps four or five remnants of mountain chains in this one young mountain right now. Will depend more on the combination than on any unique linear feature because most mountain ranges are indeed very broad. Features are not simple lines on maps they represent rod bands sometimes as much as several hundred or even a thousand miles
with. However in the San Gabriel Mountains we have the fortunate condition of having the intersection of a number of patterns of different ages and of different rock types. The combination perhaps provides us with the uniqueness with the unique combination which will permit us if we're fortunate to make a direct comparative match with rocks across the San Andreas fault at some unknown distance to the south and east. So this is in a sense the extension of what you mentioned earlier about the noticing the vertical layering of rocks to identify their ages is now noticing a three dimensional pattern to identify not only age but history of faulting relative motions of relative upheavals and subsidence is of very large regions very American Very well said. That's right. And you think one as your studies in this progress you might be able to draw how the map would have looked in the past with some accuracy.
Yes that's again one of our objectives we will try to develop a two dimensional map pattern from which we can infer a third dimension of depth. As well as the additional dimension of the time dependence system we thank you very much for coming with us today and giving us a picture of the ancient history of the earth. It's been my pleasure. This was about science with host Dr Albert Hibbs and his guest Dr. Leon silver join us again for our next program when Dr. Hibbs will lead a discussion about radio telescopes about science is produced by the California Institute of Technology and is originally broadcast by station KPCC Pasadena California. The programs are made available to the station by national educational radio. This is the national educational radio network.
About science
About the oldest mountain range
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California Institute of Technology
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University of Maryland (College Park, Maryland)
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This program focuses on scientific study of the oldest mountain range. The guest for this program is Dr. Leon Silver.
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Interview series on variety of science-related subjects, produced by the California Institute of Technology. Features three Cal Tech faculty members: Dr. Peter Lissaman, Dr. Albert R. Hibbs, and Dr. Robert Meghreblian.
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Guest: Silver, Leon T. (Leon Theodore), 1925-
Host: Hibbs, Albert R.
Producing Organization: California Institute of Technology
Producing Organization: KPPC
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University of Maryland
Identifier: 66-40-42 (National Association of Educational Broadcasters)
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Duration: 00:29:20
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