The Greatest Show on Earth Page 28
Cartoon inspired by Wegener’s ‘continental drift’ theory
In the theory of plate tectonics the whole of the Earth’s surface, including the bottoms of the various oceans, consists of a series of overlapping rocky plates like a suit of armour. The continents that we see are thickenings of the plates that rise above sea level. The greater part of the area of each plate lies under the sea. Unlike Wegener’s continents, the plates do not sail through the sea, or plough through the surface of the Earth, they are the surface of the Earth. Don’t think, like Wegener, of the continents themselves as being jigsawed together or being pulled apart from each other; it isn’t like that. Think of a plate, instead, as being continuously manufactured at a growing edge, in a remarkable process called sea-floor spreading, which I shall explain in a moment. At other edges, a plate may be ‘subducted’ under a neighbouring plate. Or neighbouring plates may slide alongside one another. The picture on colour page 17 shows a portion of the San Andreas Fault in California, which is where the edges of the Pacific plate and the North American plate shear past each other. The combination of sea-floor spreading and subduction means that there are no gaps between plates. The entire surface of the planet is covered in plates, each one typically disappearing by subduction underneath a neighbouring plate on one side, or sliding past another plate, while it grows out from a sea-floor spreading zone elsewhere.
It is inspiring to think of the huge rift valley that must once have snaked its way down the continent of Gondwana between the future Africa and future South America. No doubt it was at first dotted with lakes like the present rift valley of East Africa. Later, it filled with sea water as South America sheared away with rending tectonic agony. Imagine the view that greeted some stout dinosaurian Cortez as he gazed across the long, narrow straits at the slowly departing ‘West Gondwana’. Wegener was right that the jigsaw complementarity of their shapes is no accident. But he was wrong to think of the continents as gigantic rafts, ploughing their way through the seafilled gaps between them. South America and Africa, and their continental shelves, are but the thickened regions of two plates, much of whose rocky surfaces lies under the sea. The plates constitute the hard lithosphere – literally, ‘sphere of rock’ – which floats atop the hot, semi-molten asthenosphere – ‘sphere of weakness’. The asthenosphere is weak in the sense that it is not rigid and brittle like the rocky plates of the lithosphere but behaves somewhat like a liquid: yielding, like putty or toffee, if not necessarily molten. A little confusingly, this distinction between two concentric spheres doesn’t wholly correspond to the more familiar distinction (based on chemical composition rather than physical strength) between the ‘crust’ and the ‘mantle’.
Most plates consist of two distinct kinds of lithospheric rock. The deep ocean bottoms are covered in a rather uniform layer of very dense igneous rock, about 10 kilometres thick. This igneous layer is overlain by a superficial layer of sedimentary rock and mud. A continent, to repeat, is the area of a plate visible above sea level, rising to this height where the plate is thickened by additional layers of less dense rock. The undersea parts of the plates are continuously being created at their margins – the eastern margin in the case of the South American plate, the western margin in the case of the African plate. These two margins comprise the mid-Atlantic ridge, which snakes its way down the middle of the Atlantic from Iceland (which is, indeed, the only substantial part of the ridge that reaches the surface) to the far south.
Similar undersea ridges are rolling out other plates in other parts of the world (see colour pages 18–19). These undersea ridges work like elongated fountains (on the slow timescale of geology), welling up molten rock in the process I have already mentioned called sea-floor spreading. The spreading sea-floor ridge in mid-Atlantic seems to push the African plate eastwards and the South American plate westwards. The image of a pair of rolltop desks spreading in divergent directions has been suggested, and it conveys the idea, provided we remember that it is all happening on a timescale too slow for humans to see. Indeed, the speed at which South America and Africa pull apart has been memorably likened – so memorably that it has become almost a cliché – to the speed at which fingernails grow. The fact that they are now thousands of miles apart is further testimony to the vast and unbiblical age of the Earth, comparable to the evidence from radioactivity which we met in Chapter 4.
I used the phrase ‘seems to push’ just now, and I must hasten to backtrack. It is tempting to think of those upwelling ‘rolltop desks’ as pushing their respective continental plates along from behind. This is unrealistic; the scale is all wrong. Tectonic plates are much too massive to be pushed from behind by upwelling volcanic forces along a mid-ocean ridge. A swimming tadpole might as well try to push a supertanker. But now, here’s the key point. The asthenosphere, in its capacity as a quasi-liquid, has convection currents that extend throughout its whole surface, under the entire area of the plates. In any one region, the asthenosphere is slowly moving in a consistent direction, and then circling back in the opposite direction down in its deeper layers. The upper layer of asthenosphere under the South American plate, for example, is moving inexorably westward. And, while it is inconceivable that an upwelling ‘rolltop desk’ could have the strength to push the whole South American plate before it, it is not at all inconceivable that a convection current inching its way steadily in a consistent direction under the entire lower surface of a plate could carry its ‘floating’ continental burden along with it. We aren’t talking tadpoles now. A supertanker in the Humboldt Current, with its engines switched off, will indeed go with the flow.
That is, in summary, the modern theory of plate tectonics. I must now turn to the evidence that it is true. Actually, as is normal with established scientific facts,* there are lots of different kinds of evidence, but I am only going to talk about the most strikingly elegant kind. This is the evidence from the ages of the rocks, and especially from the magnetic stripes in them. It’s almost too good to be true, a perfect illustration of my ‘detective coming late to the scene of the crime’ and being driven inexorably to only one conclusion. We even have something that looks very like fingerprints: giant magnetic fingerprints in the rocks.
We shall accompany our metaphorical detective on a voyage across the South Atlantic, in a custom-built submarine capable of withstanding the daunting pressures of the deep sea. The submarine is equipped to drill down for rock samples, through the superficial sediments of the sea bottom down to the volcanic rocks of the lithosphere itself, and it also has an onboard laboratory for dating rock samples radiometrically (see Chapter 4). The detective sets a course due east from the Brazilian port of Maceio, 10 degrees of latitude south of the equator. Having travelled 50 kilometres or so through the shallow waters of the continental shelf (which for present purposes counts as part of South America) we batten down the high-pressure hatches and dive (what understatement!), dive down into the depths where the only light normally seen is the occasional spark of greenish luminescence from the grotesques that inhabit this alien world.
When we hit bottom at nearly 20,000 feet (full fathom 3,000), we drill down to the volcanic lithosphere and take a core sample of the rock. The onboard radioactive dating lab goes to work, and reports a Lower Cretaceous age, about 140 million years. The submarine grinds on eastwards along the tenth parallel, taking rock samples at frequent intervals. The age of each sample is carefully measured and the detective pores over the datings, looking for a pattern. He doesn’t have to look far. Even Dr Watson couldn’t miss it. As we travel east along the great plains of the sea bottom, the rocks are plainly getting younger and younger, steadily younger. About 730 kilometres into our journey, the rock samples are of late Cretaceous age, about 65 million years old, which happens to be when the last of the dinosaurs went extinct. The trend towards younger and younger rocks continues as we approach the middle of the Atlantic and the submarine’s searchlights start to pick out the foothills of a gigantic underwater mountain range. Th
is is the mid-Atlantic ridge (see colour pages 18–19), which our submarine must now start to climb. Up and up we crawl, still taking rock samples, and still noticing that the rocks are getting younger and younger. By the time we reach the peaks of the ridge, the rocks are so young they might as well have only just welled up as fresh lava from volcanoes. Indeed, that is pretty much what has happened. Ascension Island is a part of the mid-Atlantic ridge which protruded above sea level as a result of a recent series of eruptions – well, recent: maybe 6 million years ago; that’s recent by the standards of the rocks we have been sampling along our submarine way.
We now push on towards Africa, over the other side of the ridge, down to the deep plains at the bottom of the eastern Atlantic. We continue to take rock samples and – you’ve guessed it – the rocks now become steadily older as we move towards Africa. It is the mirror image of the pattern we noticed before we reached the mid-Atlantic ridge. The detective is in no doubt of the explanation. The two plates are moving apart as the sea floor spreads away from the ridge. All the new rock that is being added to the two diverging plates comes from the volcanic activity of the ridge itself, and it is then carried away, in opposite directions, on one or other of the gigantic rolltop desks that we call the African plate and the South American plate. The false colours in the pictures on colour pages 18–19 illustrating this process denote the age of the rocks, red being the youngest. You can see how beautifully the age profiles on the two sides of the mid-Atlantic ridge mirror each other.
What an elegant story! But it gets better. The detective notices a more subtle pattern in the rock samples as they are processed in the onboard laboratory. The rock cores pulled up from the deep lithosphere are slightly magnetic, like compass needles. The phenomenon is well understood. When molten rock solidifies, the Earth’s magnetic field becomes imprinted into it, in the form of a polarization of the fine crystals of which igneous rock is made. The crystals behave like tiny frozen compass needles, locked into the direction they were pointing in at the moment when the molten lava solidified. Now, it has long been known that Earth’s magnetic pole is not fixed but wanders, probably because of slowly oozing currents in the mixture of molten iron and nickel in the planet’s core. At present, magnetic north lies near Ellesmere Island in northern Canada, but it won’t stay there. To determine true north using a magnetic compass, sailors need to look up a correction factor, and the correction factor changes from year to year as the planet’s magnetic field fluctuates.
So long as our detective meticulously records the exact angle at which his rocky cores sat when he drilled them out, the frozen magnetic field in each core tells him the position of the Earth’s magnetic field on the day that the rock solidified out of lava. And now for the clincher. It happens that, at irregular intervals of tens or hundreds of thousands of years, the Earth’s magnetic field completely reverses, presumably because of major shifts in the molten nickel/ iron core. What was magnetic north flips over to a position near the true South Pole, and what was magnetic south flips to the north. And of course the rocks pick up the current position of magnetic north on the day that they solidify from lava welling up from the deep sea bottom. Because the polarization reverses every few tens of thousands of years, a magnetometer can detect stripes running along the bedrock: stripes in which the rock samples’ magnetic fields all point in one direction, alternating with stripes in which the magnetic fields all point in the opposite direction. Our detective colours them black and white on the map. And when he looks at the stripes on the map like fingerprints, he notices an unmistakable pattern. As with the false colour stripes denoting the absolute age of the rocks, the magnetic fingerprint stripes on the west side of the mid-Atlantic ridge are an elegant mirror image of the stripes on the east side. Exactly what you’d expect if the magnetic polarity of the rock was laid down when the lava first solidified in the ridge and then slowly moved outwards from the ridge, in opposite directions, at a fixed and very slow rate. Elementary, my dear Watson.*
To revert to the terminology of Chapter 1, the morphing of Wegener’s hypothesis of continental drift into the modern theory of plate tectonics is a textbook example of the solidification of a tentative hypothesis into a universally accepted theorum or fact. Plate tectonic movements are important in this chapter, because without them we cannot fully understand the distribution of animals and plants over the continents and islands of the world. When I spoke of the initial geographical barrier that separated two incipient species, I proposed an earthquake diverting the course of a river. I could also have mentioned plate tectonic forces, splitting a continent in two and ferrying the two gigantic fragments in opposite directions, complete with animal and plant passengers – the ark of the continents.
Madagascar and Africa were once part of the great southern continent of Gondwana, together with South America, Antarctica, India and Australia. Gondwana began to break up – painfully slowly by the standards of our perception – about 165 million years ago. At this point Madagascar, while still joined to India, Australia and Antarctica as East Gondwana, pulled away from the eastern side of Africa. At about the same time, South America pulled away from West Africa in the other direction. East Gondwana itself broke up rather later, and Madagascar finally became separated from India about 90 million years ago. Each of the fragments of old Gondwana carried with it its cargo of animals and plants. Madagascar was a real ‘ark’, and India was another. It is probable, for example, that the ancestors of ostriches and elephant birds originated in Madagascar/India when they were still united. Later they split. Those that remained on the giant raft called Madagascar evolved to become the elephant birds, while the ancestors of ostriches sailed off on the good ship India and subsequently – when India collided with Asia and raised the Himalayas – were liberated on to the mainland of Asia, whence they eventually found their way to Africa, their main stamping ground today (yes, the males really do stamp their feet, to impress females). Elephant birds, alas, we no longer see (or hear, more’s the pity, for if they stamped the very ground must have shaken). Far more massive than the largest ostriches, these Madagascar giants are the probable origin of the legendary ‘roc’, which features in the Second Voyage of Sinbad the Sailor. Although large enough for a man to have ridden, they had no wings, so could never have carried Sinbad aloft as advertised.*
Not only does the now solidly established theory of plate tectonics account for numerous facts about the distribution of fossils and living creatures, it also provides yet more evidence of the extreme antiquity of the Earth. It ought, therefore, to be a major thorn in the side of creationists, at least creationists of the ‘young Earth’ persuasion. How do they cope with it? Very weirdly indeed. They don’t deny the shifting of the continents, but they think it all happened at high speed very recently, at the time of Noah’s flood.* You’d think that, since they are conspicuously happy to discount evidence that doesn’t suit them in the case of the massive quantity and range of evidence for the fact of evolution, they’d pull the same trick with the evidence for plate tectonics too. But no: oddly, they accept as a fact that South America once fitted snugly into Africa. They seem to regard the evidence for this as conclusive, even though the evidence for the fact of evolution is, if anything, even stronger, and they gaily deny that. Since evidence means so little to them, one wonders why they don’t go the whole hog and simply deny the whole of plate tectonics too.
Jerry Coyne’s Why Evolution is True offers a masterly treatment of the evidence from geographical distribution (as you’d expect from the senior author of the most authoritative recent book on speciation). He also hits the nail on the head with respect to the creationists’ penchant for ignoring evidence when it doesn’t support the position that they know, from Scripture, has got to be true: ‘The biogeographic evidence for evolution is now so powerful that I have never seen a creationist book, article, or lecture that has tried to refute it. Creationists simply pretend that the evidence doesn’t exist.’ Creationists act as though fos
sils provide the only evidence for evolution. The fossil evidence is indeed very strong. Truckloads of fossils have been uncovered since Darwin’s time, and all this evidence either actively supports, or is compatible with, evolution. More tellingly, as I have already emphasized, not a single fossil contradicts evolution. Nevertheless, strong as the fossil evidence is, I again want to emphasize that it is not the strongest we have. Even if not a single fossil had ever been found, the evidence from surviving animals would still overwhelmingly force the conclusion that Darwin was right. The detective coming on the scene of the crime after the event can amass surviving clues that are even more incontrovertible than fossils. In this chapter we have seen that the distribution of animals on islands and continents is exactly what we should expect if they are all cousins that have evolved from shared ancestors over very long periods. In the next chapter we shall compare modern animals with each other, looking at the distribution of characteristics in the animal kingdom, especially comparing their sequences of genetic code, and shall come to the same conclusion.
* The Voyage of the Beagle. Victorian naturalists were given to value judgements of this kind in their books. My grandparents possessed a bird book in which the entry on the cormorant frankly began, ‘There is nothing to be said for this deplorable bird.’
* The rule seems to be that, on islands, big animals get smaller (for example, there were dwarf elephants the height of a large dog on Mediterranean islands such as Sicily and Crete) while small animals get bigger (as in the Galapagos tortoises). There are several theories for this divergent tendency, but the details would take us too far afield.