The Way of Science

2. Wegener's Objections

In its time, the "shrinking apple" model was so well accepted that extensive testing of the model seems to have been done only rarely. But the "shrinking apple" model is easily testable (falsifiable), and that is what Wegener and some southern hemisphere geologists did early in the 20th century. For example, if the "shrinking apple" model is basically correct, then one can predict that sampling the composition of oceanic and continental crusts would yield the same results. It was known quite early that this was not true: oceanic crust is not only much thinner, but also has a very different composition (abundant basalt, a very dense material, compared to the thicker, lighter, granite-rich continental crust). Isostasy, accepted as part of the "shrinking apple" model, posed its own problems. If mountains were wrinkles, they should have essentially the same density as the rest of the crust around them. Even in Wegener's time, gravity measure-ments indicated that mountains were lighter.

It's worth noting at this point that these tests of the old model are not "experiments" in the sense that most people use that word. These are examples of the comparative, observational approach, a set of techniques that is the major "scientific method" in such synthetic, historical sciences as Evolution and cosmology. You may wish to take time to think on these different approaches, since there is a major bonus question on the final exam, hint, hint.

The major objection that were made by Wegener, and on which you should concentrate, concern predictions about the Earth's surface based on radial vs. lateral motion of the crust, and on random distribution of surface features.

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a. Objections based on prediction of radial movement

If the "shrinking apple" model were correct, then movement of the crust would have been limited primarily to changes in altitude, or - in proper terminology - radial movement (i.e., along a radius of the Earth). If you imagine the "shrinking apple", an insect glued to the surface might see another such insect, one centimeter away, dip down or rise upward, depending on which one was in the bottom of the wrinkle. It is possible that the wrinkle might become so exaggerated that the upper portion would overtop, like an ocean wave, and move one insect a bit sideways (lateral motion). Under no circumstances would the "shrinking apple" model allow that one centimeter distance between insects to change significantly; certainly, one insect's bit of skin could not go on an extensive tour of the apple. The old model thus says that radial motion is expected, but lateral motion is forbidden. Examination of the Earth's surface for unambiguous examples of lateral movement is a way of falsifying the "shrinking apple" model.

1. Matching coast lines
   The oldest observation, long before Wegener, was the apparent match of coastlines on either side of the Atlantic Ocean. Take a look at your text, or any atlas. We will have a large map available for class discussion, so be prepared. The near mirror-image coastlines prompted many observers, before Wegener, to suggest that the two continents were once united. Wegener, however, was the first to examine this matching in detail, and add a great deal of additional evidence to support the idea of an ancient "super continent." Note also that, in addition to considering coastlines in the light of lateral movement, the shape can easily be considered under "violation of randomness," coming up soon.

   If the continents were not connected in the past (that is, the Atlantic Ocean did not exist), then what explanation did the defenders of the "shrinking apple" model use? "Coincidence" was the favored response. But consider this: coincidence is certainly possible, but if other independent (unrelated) matches were to be found, then the likelihood of coincidence drops dramatically. Remember the probability problems from earlier in the semester?

   The probability of a series of independent events happening together, by chance alone, is the product of their individual probabilities. As Wegener pointed out, matching of features other than coastlines makes coincidence a very unlikely explanation, and the case for lateral motion becomes much stronger.

2. Matching fossils
   Specialists in other fields had already accumulated examples of these matches, and Wegener marshaled them as evidence for "continental drift." One of the earliest correspond-ences to be noted was the similarities of plant and animal fossils on both sides of the Atlantic Ocean. In 1858, the year before Origin of Species was published, similarities of European and north American fossils were noted; after that, extensive work by southern hemisphere geologists and paleontologists found many identical or nearly identical species near the Atlantic coasts of South America and Africa. In both examples cited, it is important to know that these fossils were not found elsewhere. In particular, a distinctive group of extinct plants, the Glossopteris flora, convinced some southern hemisphere scientists that Wegener was basically correct, and that South America and Africa were united, about 200 million years ago. Wegener proposed that a single supercontinent existed then, which he called "Pangaea;" geologists now are more inclined to accept two such masses, Gondwana in the southern hemisphere and Laurasia in the north.

   Calling the fossil matches a coincidence was not an acceptable explanation for either side, since Evolutionary Theory makes such a condition effectively impossible. You will understand this fully when we study Evolution. How, then, did the defenders of the "shrinking apple" model explain the fossil evidence without invoking lateral continental movement? Since Evolutionary Theory says that maintaining identity of populations requires continual contact for breeding, two possibilities were proposed as explanations. The first was long-distance dispersal. Biologists have long known that some species were adapted to surviving extensive trips on debris, and might have accomplished ocean crossings by this method. Alas, many of the fossil species were reptiles, like the extinct genus Mesosaurus from coastal Brazil and S. Africa, and nowhere else. Based on what we know of modern related reptiles, Mesosaurus was almost certainly incapable of such salt-water exposure. Many plant species today maintain contact between individuals on coasts throughout the world; the commercial coconut is one such. Fruits of the coconut palm can easily survive long ocean journeys. Alas again; the Glossopteris flora show no such adaptations for long-distance dispersal, either by water or by other vectors used by modern plants (e.g., birds' feet or guts).

   2a. Counterargument - Land bridges
   The most popular way of supporting the "shrinking apple" model, leaving the continents and the Atlantic Ocean where they are today, and still allow relatively free mixing of populations was the concept of land bridges. Such bridges have been well-documented for recent geologic times, so the hypothesis has a valid starting point. Think of the isthmus of Panama, which is now an interrupted bridge, courtesy of the Canal, between South America and Central/North America. The Panama link allowed extensive flow of organisms in both directions (we received our opossum that way); we also know that such flow was interrupted periodically as sea levels rose and fell hundreds of feet. Another firmly established bridge is the link between Siberia and Alaska at the Bering Strait. Toward the end of our most recent ice age, the Pleistocene, so much water was locked up in continental glaciers that the sea level dropped by about 120 meters, exposing much land. This bridge, across which the original human colonization of the New World probably occurred, is more like what was proposed for links across the ocean (the "shrinking apple" model), in that it is a series of island "stepping stones" rather than a continuous link. So: if such land bridges occur today, and have come and gone in the past, why not use them to explain distributions of fossil organisms from millions of years ago?

   One major problem with the land bridge hypothesis is obvious: where are the remains of the bridges? Even in Wegener's time, it was clear that there were few such protrusions in this very deep ocean, and the depth made sea-level changes difficult to accept as a reasonable mechanism for how those "stepping stones" vanished. The solution? If the sea didn't rise, the islands must have sunk. Subsidence (sinking) of the whole ocean basin was proposed, as well as subsidence of the islands into the crust beneath. You should immediately see a problem here; try to find the flaw with the "sinking island" idea by recalling the phenomenon of isostasy.

   All of these arguments, both pro- and contra-"shrinking apple" model, should be clear in your mind before class discussion. If you are still unsure, and/or you wish to add more detail, try reading the two supplementary articles on Library Reserve.