Historical Geology/Continental drift
Continental drift is the theory that the continents have shifted their positions over time. Today, the subject of continental drift has been subsumed into plate tectonics, the science which deals with every aspect of the motion of lithospheric plates.
To write about "continental drift" rather than "plate tectonics" is therefore something of an anachronism. However, there is such a lot to say about plate tectonics that it seems necessary for us to break down the subject into smaller pieces that are easier to swallow. In this article, therefore, we shall examine the notion that the continents have shifted so far as we can do so without discussing the mechanisms of plate tectonics.
The drift theory[edit | edit source]
As first formulated, the drift theory was that the continents had once been joined together in a single large continent (Pangaea) which then split into the modern continents, which then drifted into their current positions. We now know that this was only one of a series of similar events by which continents have alternately been pushed together and pulled apart. The animation to the right shows the splitting apart of Pangaea.
When the drift theory was first proposed, the "drifters", as they were called, had no idea how the continents could have moved, and such proposals as they came up with were contrary to reason and evidence (as was pointed out by their opponents, the "fixists"). But, as we now know, the evidence they produced that the continents had moved was correct, and should have convinced more geologists than it did at the time.
How did they know? The initial evidence for drift[edit | edit source]
As explained in the introduction to this article, what we will present here is the most superficial evidence that continents have shifted in their relative positions; specifically, that the Americas were once attached to the Old World along the eastern edge of the former and the western edge of the latter. What is presented below is therefore by no means the whole or even the majority of the evidence for continental drift; it is merely all the evidence that we can present without looking at the mechanisms of plate tectonics.
When continental drift was first proposed, the arguments rested on four key pieces of evidence.
- A glance at a world map shows that it looks like the Americas and the Old World once fitted together. Today, by measuring the submerged borders of the continental shelf rather than the coastline, and by using computers and the result in spherical geometry known as Euler's theorem, we can see just how good this fit is. Yet even without these techniques, a glance at a world map shows that there is something to be explained.
- There are geological features that seem to be interrupted by the Atlantic: for example, rock formations and mountains which appear to start in North America and finish in Scotland. We could perhaps try to ascribe this to coincidence or to forces as yet unknown; but when we take this in conjunction with the rest of the evidence, it is more parsimonious to conclude that the Americas and the Old World were once contiguous, which entirely explains why the east coast of the Americas and the west coast of the Old World underwent the same geological processes.
- Similar remarks might be made about the ranges of certain flora and fauna as indicated by their distribution in the fossil record. For some examples, see the map to the right, which shows the distribution of various fossil species across the southern continents, which are represented in the relative positions inferred by the "drifters". Such a distribution would be very difficult to explain if the continents had been at their present positions when these species lived. "Fixists" used to postulate "land bridges" rising and falling between continents to explain these biogeographical features, but this was very much an ad hoc explanation, for which they could provide no mechanism; and it would not explain other features that are explained by drift.
- Another piece of evidence that is inexplicable without drift is provided by past glaciations. For example, studying the striations, moraines, and other evidence for glaciation in South America, we find evidence of glaciers traveling westwards from the east coast of South America — as though glaciers had somehow climbed out of the Atlantic to roll across South America! But this is impossible and absurd: glaciers can't climb out of the sea. However, when we observe that there is also evidence of glaciers moving across Africa towards the west coast of that continent, and that dating methods put the South American and African glaciers at the same date, we can see that the absurdity is avoided by the hypothesis that the two coastlines were once contiguous.
How do we know? More modern evidence for drift[edit | edit source]
To the observations mentioned in the previous subsection we may now add further evidence that seems to belong in this article, since like the evidence in the previous section it does not require us to know how the continents move.
First, there is the fact that the continents are moving right now, as can be measured by GPS (the Global Positioning System), by SLR (Satellite Laser Ranging), and by VLBI (Very Long Baseline Interferometry).
Moreover, the continents are moving at the right rate. That is, if we apply geological dating methods to see how long ago the Americas parted from the Old World, if we measure the distance across the Atlantic, and if we measure the rate of drift using GPS, SLR, and VLBI, we find the numbers to be consistent.
If this in itself is not conclusive evidence that the continents have been moving for millions of years, it is at least highly consistent with that proposition.
We might also mention in this subsection the phenomenon of apparent polar wander, something which was only noticed in the mid-1950s, just before the plate tectonics revolution really got underway. You will recall from the article on the Earth's magnetic field that the magnetic poles observably drift around somewhat; also that information about the Earth's magnetic field is preserved in the rocks.
It follows that we should be able to find out how the positions of the magnetic poles have changed in the past. However, when we try to do this, we find that the rocks on different continents give discrepant answers: all the rocks in South America (for example) will tell us that 60 million years ago (again, just as an example chosen at random) the north magnetic pole was in such-and-such a place, whereas the African rocks will all tell us that no, it wasn't, it was in another place altogether. It looks, in fact, as though there were two north poles, one controlling the deposition of rocks in Africa but having no effect on the rocks in South America, while the other affected South American paleomagnetic data while leaving Africa untouched — and the situation only gets worse when we start taking into account the data from Australia and India and so forth, and apparently discover that there were lots of north poles 60 million years ago, one for each continent.
But we can reconcile all these contradictory data with the hypothesis of continental drift. All we need to do is suppose that when the paleomagnetic data were laid down, they were all perfectly consistent and showed the north pole as being in exactly the same place, but that since then the continents have shifted their positions relative to one another and to the poles, leaving the paleomagnetism in the rocks pointing in different directions for each continent.
This is more than an ad hoc hypothesis to account for the paleomagnetic data. If we want to reconcile the data, we have to do more than wave our hands in the air and say: "what if the continents were in different positions once?" Instead, we have to propose specific positions that they were in: and we find that this exercise leads us to the same conclusions as we arrive at when studying the other data, such as biogeography, glacial striations, geological formations, and other evidence which we shall mention in subsequent articles.
The drift controversy: a historical footnote[edit | edit source]
When the drift hypothesis was first proposed, it met with stiff opposition; opposition which, with the 20/20 view of hindsight, seems unjustified. Drift could, after all, explain a number of anomalous features in geology; whereas the "fixists" had only an ad hoc explanation for the facts of biogeography and no explanation for anything else explained by drift.
The drifters were handicapped by having no good explanation for how drift could occur; indeed, they were handicapped by having a bad explanation: that continents plowed through oceanic crust like icebergs through water; an explanation which was demonstrably false.
Also, they did not then have the technology to prove that the continents are still in motion: they had no GPS, no SLR, and no VLBI to appeal to. If they could have shown that the continents were indeed moving, then the problem of explaining how could have been left to future geologists.
Nonetheless, in retrospect the drifters should have been granted more acceptance than they were. It is, after all, usually the case that scientists accept the existence of a phenomenon before finding a mechanism for it; science could hardly have progressed otherwise.
However, some popular authors have exaggerated the opposition to continental drift, portraying the leader of the "drifters", Wegener, as a modern-day Galileo persecuted by a scientific rather than a religious establishment. This is far from the truth. Wegener was well-respected as a scientist: his textbook on meteorology became a standard work; his book on continental drift was translated into several languages; he had a professorship specially created for him at an Austrian university; he won the support of some other geologists, mainly in Britain and continental Europe; and he was able to get funding for his research. In fact, he died of overexertion on an expedition to Greenland costing $1.5 million in today's money, and if no-one had taken him seriously as a scientist he might have lived beyond 50.
Whatever the rights and wrongs of the drift controversy, it is a fact that drift was not widely accepted until the revolution in geology attendant on the birth of plate tectonics. We shall discuss this newer and more conclusive evidence in the next few articles of this textbook.