The Way of Science

3. Distribution of minerals

Wegener expanded the number of "coincidental" matches with many other Atlantic coast geologic features. Most of these cannot be discussed here, simply because of the background in geology necessary to appreciate the similarities. However, a few are easily understood at this level. Deposits of glacial conglomerates called "tillites" constitute one such match, and we will return to tillites in the material on ancient climates.

1. Mineral deposit
   Well before Wegener emphasized them in support of continental drift, matching deposits of commercially important ores had been discovered in these areas. See Hurley p. 59.
2. Ancient rocks - Cratons

   Figure 1. Distribution of ancient (cratons, stippled), and somewhat younger rocks (diagonal lines). Atlantic Ocean is much reduced to show matches.

   
   Another powerful match drawn from previously published literature concerned the age of geologic features called cratons. Most continental areas, in contrast to ocean basins, consist of blocks of similar material jumbled together. These blocks, averaging 3000-4000 km in diameter, are usually arranged in an irregular mosaic as a consequence of plate motion, and most of them are geologically rather young, a billion years old. Some blocks, which are a great deal less common, are now known to be very much older, about two billion years old. These ancient matching blocks are the cratons (see Figure 1.). As you accumulate knowledge of Plate Tectonics, you should be able to explain in class discussion and/or in an exam, why old continental crust is so uncommon. And - while you are thinking about the rarity of cratons - consider this information. Although ancient continental crust is very rare (and such old crustal material with fossils is even rarer), nevertheless the oldest such crust is about 4 billion years old. The oldest known oceanic crust is vastly younger: less than 200 million years old. You should be able to explain why that is so when you understand the new model, Plate Tectonics.

4. Distribution of ancient geological feature

1. Glaciers and ancient climates
   Let us return now to those tillites mentioned earlier, and to the phenomenon of glaciation. First, how and where glaciers form must be completely clear to you. Most students immediately answer the "where" question with "near the poles, and not near the equator except on mountain tops where it's cold." But why must polar regions be colder than equatorial regions? Think about that! Here's another background question to contemplate: Where does all that snow in polar regions originate? What is the source of the water, and what must the relationship be quantitatively, between yearly polar snowfall and yearly summer melt? Answering these questions should enable you to describe where and how glaciers form.

   Now consider glacial movement. It is easy to see that the glaciers can grow in height (radial movement). Henniker was under at least a mile of ice in the Pleistocene. But how about lateral movement? How could that occur? That it does occur is without doubt; it can be seen today in the receding glaciers, and until a few years ago, in glacial advance. Here's some help: If you were to press down hard enough on a chunk of lead, what happens to the metal? What might happen to a wooden surface under the lead, particularly if there were debris trapped between metal and table? What might happen to that debris as the "lead glacier" advances at its edges? Now try to put it all together. The glacial scars ("striae") tell you about direction of movement; the consolidated rubble (tillites) tell you about the extent of the ice. Take a look around Henniker and the campus; you should be able to find striae without difficulty. You might also contemplate all those round boulders, some quite enormous, scattered about or gathered into old walls. Why are they round? How did they get where they are now? (Merlin didn't move them, the way he deposited StonehengeĽ.)

   Now let us see how the evidence from ancient glaciations, and from other examples of Paleoclimatology, were used by Wegener to attack the "shrinking apple" model and bolster continental drift

   About 240 million years ago, according to work done by many geologists before Wegener, glaciers existed near the Atlantic coasts of South America and South Africa (that match again!). In addition to these, there were icecaps in India. At the same time (240 million years ago), there was no evidence for polar conditions in places like Siberia. In fact, regions which are now near-polar show evidence that 240 million years ago they were tropical or subtropical. You have been through the glacial evidence, and how it is used to demonstrate that ancient conditions were very cold; now let's see how paleoclimatologists determined that Siberia, etc. were very warm.

2. Coal and ancient climates
   You probably know that coal is mostly the element carbon, and it is produced from enormous quantities of dead plants. Although coal can be formed under cooler conditions, (that's another story), it is possible for geologists to determine, with considerable confidence, that the coal beds we are interested in (eastern United States, Europe, northern China, etc.) were formed under conditions of tropical heat and abundant rainfall.

   Given these data (that regions which are now polar were tropical, and simultaneously areas that are now subtropical were glaciated), we would have an absurd situation if the "shrinking apple" model were correct.

   It gets worse for the "shrinking apple" model. The glacial striae in eastern South America indicate that the ice advance would have started in the Atlantic Ocean (if it had always been where it is now), and then moved onto land. As you undoubtedly know, water freezes long after land; lots more calories have to be removed before this abundant liquid changes phase. The same bizarre situation would have applied to India, where glaciers would have had to form first in the warm waters of the Indian Ocean.

   At the time, these arguments were rejected by the majority defenders of the "shrinking apple" model in ways that now seem weak. Never forget, however, that "hindsight is 20-20," and what was at stake was a major upheaval in fundamental geologic understanding.

   One line of counter-argument proposed that true polar wandering had shifted the position of equator and poles far from their present locations. Note that "true polar wandering" refers to a change in the rotational axis of the Earth. There is another type of wander, that of the north and south magnetic poles, produced by changes in the dynamo deep in the Earth's metallic center. We will examine the second phenomenon later, so keep in mind that the two kinds of wandering are distinct and different. Although we know that both polar systems do wander, and that true (rotational) wander will produce climate effects, the amount of shift required was staggeringly large and very unlikely.

   In the case of the coal deposits, some geologists claimed that it was too difficult to distinguish temperate from tropical coals with any certainty. The freezing of the Atlantic coastal waters must have been caused by massive land bridges, blocking flow of warm southern water currents. Indian inland glaciers must have formed at very high altitudes, now gone. If these counter-arguments sound strained and ad hoc today, they didn't to the majority of specialists at the time, even when other independent paleoclimatologic evidence supported Wegener's position. Certainly, one characteristic of good scientific evidence is when independent lines of enquiry converge and reinforce each other, and Wegener was unquestionably presenting his position well.