In Our Time - The Earth's Origins

Episode Date: July 5, 2001

Melvyn Bragg discusses the origin of the Earth. Ideas used to be very clear about its origins. Bishop Ussher, in 1654 arrived at an exact figure and specified it in his work Annalis Veteris et Novi Testamenti: He deduced that work on Planet Earth began at exactly 9am, on Monday 23rd October 4004 BC. The date was then printed in the margin of The Bible and preached from the pulpit, and right up to the nineteenth century to the left of ‘In The Beginning…’ was specified ‘Before Christ 4004’.Christian believers thought the creation story was solid as a rock…until the geologists arrived. First Hutton, then Smith, and then Lyell smashing away at orthodox belief in a way that made poor Ruskin quail, but in doing so they created a science. With Simon Winchester, author of The Map That Changed the World: the Tale of William Smith and the Birth of A Science; Cherry Lewis, geologist and author of The Dating Game: One Man’s Search for the Age of the Earth; John Cosgrove, Structural Geologist from the Royal School of Mines at Imperial College, London.

This BBC podcast is supported by ads outside the UK. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio four. I hope you enjoy the programme. Hello, we used to be very clear about the origin of the earth. Bishop Usher in 1654 arrived at an exact figure and specified it in his work Analis Vetteris at Novi Testamete.

He deduced that the work on planet Earth began at exactly 9 a.m. on Monday the 23rd of October, 400 BC. The date was then printed in the margin of the Bible and preached from the pulpit right up to the 19th century, to the left of, in the beginning, was specified before Christ, 4004.

Christian believers thought that the creation story was solid as a rock until the geologists arrived and how solid are rocks these days. First Hutton, then Smith, and then Lyle, smashing away at orthodox belief in a way that made poor Ruskin quail, but in doing so, they created a science.

With me to discuss the development of geology and our changing understanding of the structure of the earth is Simon Winchester, author of The Map That Changed the World, The Tale of William Smith and the Birth of a Science. Also with us is Cherry Lewis, geologist, and author of The Dating Game, One Man's Search for the Age of the Earth,

and John Cosgrove, structural geologists from the Royal School of Mines at Imperial College London. Cherry Lewis, can you give us some idea of where geology was up to the 18th century? Yes. I think what you've just said about Archbishop Bussia is really important because it puts in context the religious climate that was prevailing through certainly the 17th and 18th centuries that really dominated the understanding of geology. I think perhaps if we start with somebody like James Hutton who was beginning to get an understanding of what the rocks really meant, he trained as a doctor. in fact, but went on to become a farmer.

And it was while he was doing his farming, that he started to look at landforms, and he started to get a feel for the fact that weathering processes were extremely important. And he noticed that the sediments produced by weathering were then being taken down by the rivers and deposited in the oceans. And he built up this picture of an evolving cycle of rocks.

and gradually the rocks would be taken down to deposited in the sea and then over long periods of time the sea would be uplifted to form the rocks that we're standing on today. And he developed this theory of the earth, which really the main, I think, premise of it was that it involved enormous time scales and he was probably the first person to really appreciate the enormity of geological time.

But of course this went very much against all the religious teaching of the time, which was very much dominated by Archbishops' understanding that the earth had been created 4,04 BC, and therefore it couldn't be much more than 6,000 years old. But he was adamant. In nature, he said there is no deficiency in time. And he looked at the rocks and saw that it was really required the most enormous amounts. of time. We're talking here about the 18th century, aren't we? And up in Edinburgh, Hutton. Simon Winchester, you've written about William Smith, most engagingly in his geological map of the British

Arts, which was published in 1815. Was there a sort of apostolic succession from Hutton, from James in Edinburgh to William Smith? Not at all. Smith, born in 1769, would have been, I think, largely unaware of Hutton's belief. And I think he was in the early days guided by churchly belief. So he grew up in Churchill in Oxfordshire, and I think the rhythms of his days were dictated by the church. And his teachings, his belief, I think, for instance, with the fossils that he found and played with as a young boy

in the fields of the Cotswolds, his belief was probably when he was a child, that these were, inserted into the earth by some magical plastic force, the via plastic eras it was called, as a demonstration of God's omnipotence and the wonderment of God's creativity. Not that they were relics of once living animals. So in his early days, no, there was no apostolic succession. He didn't know what Hutton was thinking.

But then... He did the same sort of thing as... Well, that's the extraordinary thing. He did the same sort of thing. He went and looked. Once he went and looked, and this was the... the big key to Smith's success, is that he was a practical man. I mean, Hutton had seen these what, famous unconformities. There's one on the Isle of Arran and one at Sicker Point in Berwickshire, where he could see that rocks had been laid down, the angle of them had been

changed due to some inexplicable and then at the time unexplained force. This suggested to Hutton that something had evidently happened. Now, Smith saw essentially the same thing when he took work surveying a canal south of Bath. He reasoned that not only were rocks dipping, so they were turned down at an angle pointing towards, if you like, London and then the continent, but also, and this was the key to his discovery, that there were fossils of different types

in the different layers of rocks and that it was always the same specific fossil could be found in the same specific rock, whether the outcrops of these rocks were one mile apart or 50 miles apart and then he twigged that possibly these were therefore the same rocks laid down at the same time the same conditions prevailing. So he then thought if he could go round Britain, this very practical man, and look for all the outcrops in all the canal excavations that were being dug

thanks to the Duke of Bridgewater's amazing canal building mania at the end of the 18th century, then maybe he could draw a map. He could join up the dots and say Rock A occurs here in Shropshire and there in Lancashire, and so we can draw a map. And his genius, which really came out, first of all, exactly 200 years ago in July 1801,

by great coincidence, he sketched the first map of England and Wales. And then he went on for the next 15 years to create the proper one that the book is all about. come back to William Smith in a moment. But John Cosgrave, you're at the edge of modern geology. Can you just give us your view on the contribution that Hutton and Smith made? Did they, let's just be straightforward about it, did they lay the foundation for what you're doing now

and what's their significance in your area of study? I think in retrospect we can almost see Smith's contribution as a paradigm change. because prior to Smith, we'd had a collection of disparate studies of geology, fossils, minerals, volcanoes, earthquakes. Nothing had brought it together. But suddenly we get the appearance of this map, which showed the spatial and temporal disposition of rocks over the whole of the British, not the British, England and Wales.

And suddenly we could see things in context. And it was the first time that we'd been able to test theories. For example, if you think of what Smith, had done. He predicted that the dipping strata that's just been spoken about extended across the UK. He could predict where outcrops

would occur simply by looking at the topography and looking at the particular strata he was considering. So the acid test, the hallmark of a theory that's accepted scientifically nowadays, must be testable and he showed this eminently. In fact, one of his games was to

guess or work out what the outcrops would be in the and demonstrate to his friends as exactly what was happening. So here was a sound theory that could be tested, and it really does, in my opinion, it's a stamp of a paradigm change in geology. It brought the subject together. These disparate aspects of geology were brought together in this way.

And then we come to Lyle, who gave Darwin, as it were, permission. To have his theory of evolution. Did Lyle was he a pupil of or was he part of this? Did he feel a tradition was building up by that stage? Probably did, but I insist on. drifting back to Smith because it's impossible not to acknowledge the contribution that Smith paid to

Darwin's theory. He made a wonderful distinction between stratigraphy and paleontology. And as many of you were aware, the layers of rock are simply sediments deposited in oceans. Bread and butter slices, indeed.

But the interesting thing is Smith realized that those slices would not be necessarily the same rock type. They could change laterally depending upon the environment. So a particular stratum which represented a time slice might be a limestone here grading into a shale, grading into a sandstone. And he thought this is really difficult because how can I then check whether my rock in Whitby is the same as my rock on the English Channel?

The key was recognising that although the lithology's change in this time slice, the fossils remain the same. So he identified the importance of fossils in dating and implicit in his work is that the fossils will change as you go through the successions. the fossils are evolving. And this is 50 years before Darwin came in with its theory of evolution. Can we talk about the battle for the age of the earth? Can you kick us off here, Cherry Lewis?

I mean, so we have these, we have Hutton Smith and Lyle, we have Darwin. Let's put them all together just for the sake of this discussion. When did they really lock horns? We've still got Usher saying 400. We've still got Usher saying 4004. I think perhaps it starts with Lyle in that he really takes, on Hutton's ideas of this sort of continue cycling of the rocks. But he takes it one step further, and he says that not only have rocks been cycle for millions.

Well, he doesn't use the words millions in those days, but he gives the impression of this sort of infinite amount of time. But he also says that over that period of time, the geological events in terms of their scale have never changed. So whatever we see today going on, the processes that we see, have always gone on in the past. And they've never been any different. Is this uniformitarianism? Uniformitarianism. I thought I'd get it in. It's quite a mouthful.

But there were a number of people who had a problem with this, particularly the physicists, who had a very clear idea. And William Thompson, who later went on to become Lord Kelvin, was the leading physicist of his day. and he had particular problems with this because his understanding of the way that the earth had formed was from a molten globe, that it had been condensed from the nebulae and it had become a molten globe

and that it was still cooling down from that time. And he said that wherever you looked in the world, if you went down a mine or if you went dug a borehole, as you went down, it got hotter inside the earth. And this was the remnant heat from the time when it had been a molten globe. And he said, well, you know, if this is the case, since we understand the laws of thermodynamics, we should be able to calculate given that we know that the rate, or we can assume a rate that rocks cool up from a molten state.

And he initially calculated this to be between 400 and 20 million years, which was quite a decent range. And most geologists at that time thought the age of the earth was about 100 million. And they were quite happy with this. However, people like Darwin, who wanted much greater periods of time and who, if you like, had been given permission to have great amounts of time. Biology takes its time from geology, as Thomas Huxley famously said. He wanted much great amounts of time for evolution to occur. And so there was this real split between the geologists

who wanted to have great amounts of time and the physicists. This battle really revolved for a while around Kelvin, didn't it? And then Rutherford came in with Rutherford's theories of radioactivity, which trounced Kelvin's physics, just as Kelvin's physics had trounced the geologists' attempts to make a larger span. How did this battle affect geology, do you think, had then having this fight about time? Did geologists become rather worried about that position?

Did it make them change the direction in which their signs was going? I'm not sure that they were worried about their own position. Getting away from the idea, I mean, now we've escaped from the bonds of Usher, the belief that the earth was a great deal older than mankind, was a revolutionary thought. The church, of course, founded abhorrent. But then that the fossil record was relatively young compared to the age of the earth, something that you bring out from, Cherry, your discussion of,

Lyle, when he was looking at the bivalves under Mount Etna, he was able to calculate from the number of eruptions of Mount Etna and the amount of lava that was spewed out at each eruption, how old Etna probably was, 100,000, 200,000 years. And at the base of it were found life forms that were more or less identical to life forms that were then swimming around in the Mediterranean. And so it was obvious to him that 100,000, 200,000 years had passed with very little evolution of a life form, some perhaps.

But the number of years that life had existed on Earth compared to the age of the Earth was relatively minimal. To this extent, in the 19th century we're now talking about the late 19th century, geologists were becoming more and more confident, I think, that their thoughts about the age of the Earth. I think they did feel extremely threatened by Kelvin because at that point it really wasn't a very numerous science. It was very much an observational science, and geologists went out and recorded what they'd seen in the rocks, and they had an intuitive feel as to the enormous amounts of time that were required to deposit the rocks. But Kelvin was telling them, and here are the numbers. And he was a formidable figure, a real giant of his time.

And it was extremely intimidating, I think, for the geologists to be able to stand up and say, no, you're wrong. You know, we know intuitively without putting any numbers on it, that the age of the earth must be much greater than 20 million years. Because eventually, Kelvin, reduced the number of, allow the number of time. Was it radioactivity that was the breakthrough that enabled geology? It was. And I think it really first started to, to put numbers on the age of anything.

And after that, time has become the framework, I think, into which geology fits. John Cosgrove, can you tell us a bit about the discovery of Pangea and the theory of continental drift and why this fits into the development of geology? Yes, continental drift was a theory that gradually evolved, and it was first thought of as soon as the maps between showing Africa and the Americas were printed. Immediately people commented on the jigsaw fit between the two. And this idea was floating around for a long time.

But of course the problem was, no one could suggest a mechanism by which this particular drifting could occur, the idea of the continents drifting about the earth's surface. And it wasn't until much later when Vagner got an enormous amount of circumstantial evidence together that he felt confident enough or someone felt confident enough to actually say, look, this is a theory. We don't know the mechanisms, but there's so much circumstantial evidence to suggest that these continents were at one time clipping together, fitting together like a jigsaw puzzle, it must have occurred. The problem then was, what was the mechanism? And that's the

thing that really sunk the theory of continental drift. No one could on the case. So the idea was the pangier was there was one landmass, about a third of the planet, and two thirds were water, and then this one landmass started to split up. Precisely. He argued that about 200 million years ago, this pangier, broke up into the continents we now see spread around the Earth's surface. And you said his arguments, though, weren't strong enough. The arguments were very good. The circumstantial evidence is great,

but we need a mechanism whereby the continents actually can drift around the Earth's surface. And no person in geology could provide that. Sorry, can I ask him once? What explanation did he provide that? He didn't. He didn't. He didn't. He had the courage of his observations.

He's pragmatic German, and he said, well, this is the observation. and we have measured many, many things, and the evidence suggests this, and just because I can't suggest a mechanism doesn't mean it's not true. He had an idea. Well, he did have an idea that the continents were floating on a sort of sea of basalt, which lay underneath the continents

and which are now between the continents. But, yes, there was tremendous discussion as to how you could plow continents through this sea, of basalt. Can we come back to your man then, who, before we move on to the tectonics and modern geology, who actually did plot the time of the earth and

get there, despite Calvin and despite all the difficulties. If we could, if you could briefly tell us, yes. Well, Arthur Holmes was lucky, really, I think. He was in the right place at the right time. And he started studying

at Imperial College, in fact, just at the time when Radio It was only 10 years old, and it was a tremendously exciting science. And it was Rutherford who'd first suggested that radioactivity could be used to date rocks, but it was Arthur Holmes who actually took it on as a real science and developed it, and he used the technique of uranium-led dating. And he was the first person to say, well, actually, we think the age of the earth is the oldest rocks that he dated were 1,600 million years old. I mean, at that time, only if you...

What time is it? What we're talking about? Well, this is 1910, 1912. And up until then, most geologists had just about become comfortable with the idea, well, maybe the age of the earth was only 100 million years old. And now they're suddenly, you know, this whole order of magnitude greater, the rocks are telling us, and of course they're all the usual, well, it can't be right, there must be an error, and it went on for years and years,

and it really wasn't until the 1920s that the age of the... the Earth became established at about 2,000 million years, which is about slightly less than half what it is today. So I can ask you, Simon, and then, you John Cosgrove, what effect did this ageing have on the development of geology? The ageing gets settled in the 20s and 30s, 20s and 30s. So does that give another kick to the science? I wonder if it does.

It certainly gives a, you have to, the geological timescale that one has in the back of all geological books, just got longer and longer and longer. the Pre-Cambrian, which was sort of the oldest one ever talked about, really, and when I was reading Arthur Holmes as my textbook as a geology student in 1963, there was pre-Cambrian, and before that a great void. There was certainly no life. But now, of course, they've found life in the Pre-Cambrian,

and the Pre-Cambrian seems extremely young. Now, I think it's Gradstien and Ogg, is the widely accepted geological timescale that's in most... books today is an entity which is at its lower end absolutely vast, with all sorts of new names that certainly, I mean, Hadean suggesting, that's one of the earliest periods now of geology, suggesting birth out of Hades, out of some hellish construct, going back now to what, 4,600 million years. Can you, John, can you just tell us what effect it had this arrival at the vastness of time

to just give out this idea of the impact that would have? Well, the impact was really that it gave a framework in which we could understand the evolution of the earth. And it gave us the time to enable mechanisms that took many, many millions of years to be totally acceptable. And so I think certainly when it was introduced, it was a paradigm shift. But having established that we've got this period of time, that enabled us to think then about the mechanisms that were frustrating geologists, the mechanisms relating to the way in which the continents might move about the earth's crust. And that's where we come to play tectonics, don't we?

It is indeed. Could you tell us about that? Well, the problem was, as I mentioned, there was no mechanism to drive it. To drive this pangier, to break pangier up and to cause the continents to drift around the earth's surface. And what happened was, like everything else, you get people interested in different aspects of geology. There was a tremendous interest in earthquakes because of the threat it caused of people. And so gradually, seismic stations were built all around the globe. And when earthquakes go off, you get seismic energy, that's any.

transmitted around and through the earth and collected by these various receivers. So you can actually get a sonic scan, if you like, of the earth. And they could work out what the interior of the earth was like. This is how geophysicist worked out the structure of the interior of the earth. They found there was a central core about the size of Mars, made up of iron and nickel. There was an outer core, which was the rest of the world, basically, made up of heavy silicates. And right on the top floating was this scum, which we call the crust. And the analogy was that it's exactly like a blast furnace.

You have iron collecting at the bottom and the light silicate slags floating to the top. It's this gravitational separation. And so they found these three units of the earth, which were the core, the mantle, and this very, very thin scum. And what was particularly important in terms of plate tectonics was they found another boundary in the upper part of the mantle.

It was a boundary called the low-velocity layer because it wouldn't transmit certain kinds of seismics waves very easily. and this low-velocity layer was a wonderful discovery because it really was as it's known as the asthenosphere and in Greek as you probably know it means a weak layer and it allowed the top part of the mantle and the crust to decouple from the lower part of the mantle and geophysicist, a psychic upon geochemist came to the rescue

and they tried to explain what this could be and it was suggested it might be a phase change from one minute to another which would occur at a particular depth and temperature Now, as you probably know, something like carbon can exist in many forms. At the earth surface, it's generally existing as graphite, but under high pressures and temperatures, it becomes a diamond. And as materials go from one phase to another, the properties they have at that interchange, that exchange, are extremely different from the properties of the two end members. So we get this very interesting wheat layer occurring as we get this phase transition.

And that allowed the decoupling of the lithosphere, that is the mantle and the crust are, above the asthenosphere from the rest of the earth and provided a slip horizon which would allow platonics to occur. That's the point. So we're on platforms as it were. We are indeed. Sometimes 150 kilometers deep, sometimes 75 kilometres and we are on platforms

and we are still moving. Greenland is still moving away from America, North America or both. You're absolutely right. The idea of continental drift is fine, except now instead of the continents drifting, there are plates that drift and the plates that are made up of both continental and oceanic material.

Sorry, Simon, I'm up to you. No, I was just going to say, we, going back quite away now, and geology has advanced hugely in the last 35 years, but we went on an expedition to East Greenland in 1964, I think it was, six of us, sledging way up onto the ice cap to collect from the nunatics, from the outcrops of tertiary basalts, laid down, what, 25, 30 million years ago, we would take core samples of these basalts,

and using a sun compass would show, exactly where the rocks were aligned in relation to the sun and to the current North Pole, then would bring them back to Oxford and frozen into the basalts are little crystals essentially of iron, which act as little compasses. And all you have to do is to determine where those compasses are pointing to today, where, in other words, the North Pole was when those... basalts were cooling and the crystals of iron were freezing into them. This is it called paleomagnetism. You're looking at the old magnetism of the rocks.

And we found, without a shadow of doubt, that in tertiary greenland, in 30 million years ago, the North Pole was 15 degrees to the east of where it is today. In other words, Greenland had drifted 15 degrees westwards over 30 million years. Maybe the figures are wrong. I'm not sure, John, you would know better than I, but it was one of those early and marvelous discoveries of continental drift, that a plate, we didn't know it was a plate in those days, but sea floor spreading had taken place,

that Britain and Iceland and Greenland had all been moving apart over the last 20, 30 million years ago. So were you aware of paleo reversals, of the reversals of the magnetism? Not of the reversals of the magnetic fields. That's another feature, but no, as just six young 19-year-old geologists, when we got back to Oxford and found that the North Pole of Greenland, 30 million years ago was 15 degrees to the east of where it is today. Eureka, there is continental drift happening.

It was a wonderful feeling. John Cosgrove. And that's remarkable. It is the prelude to the idea of seafloor spreading. Can you just explain seafloor spreading? With pleasure. If you look at the centre of most oceans, there's a great crack, a great rift,

known as the Mid-Atlantic Ridge in the case of the Atlantic Ocean. And if we look at what happens along the Mid-Atlantic Ridge, we get the upsurging of magmatic material all the time. It's currently occurring now. It's building up the islands of Iceland. The age of the magmas that come up, as you move away from the ridge on each side, gets progressively older and older.

And as you just pointed out, Simon, as these magmas cool below what temperature called the curry temperature, any magnetic mineral within it, such as magnetite, will take on the magnetism of the present Earth. So we'll have a strip down the middle of the Atlantic now, which has got a magnetism corresponding to the present North Pole. But for reasons there isn't time to go into now, the Earth's magnetic field switches every so often.

The North Pole becomes the South Pole, and the South Pole becomes the North Pole. So if we go further out from the Mid-Atlantic Ridge, we find strips of basalt parallel to the ridge, which tell us that, no, the pole was in the opposite direction. We go further out, and the poles are switched again. And so this shows us very clearly that the oceans are growing incrementally

from that Mid-Atlantic ridge, like a conveyor belt moving out. Thank you all three of you very much, Sherry, Louis, Simon Winchester and John Cosgrove. Next week we'll be talking about Dickens and Society. Was he, as George Bernard Shaw said, more revolutionary than does Capitol. There you go. Thanks for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programs about history, science and philosophy at BBC.com.com. UK forward slash radio 4.