Contraction-based tectonic models were largely obsolete by 1910 under the double attack of isostacy and radioactivity. Continental Drift was one idea that attempted to fill the need for a high-level tectonic theory. The readings place drift in the context of contemporary geology and examine why it was enthusiastically received in some areas but rejected in others.

Much of the evidence and logic of continental drift will be familiar to you from your geology classes. However, as you read Greene’s and Stewart’s presentations of Wegener’s ideas, try to consider how this theory would have appeared to geologists in 1912-1926. How does his idea relate to the two main tectonic frameworks (contraction or at least lateral shortening; isostacy and vertical equilibrium)? If you read carefully you will encounter not just the geological evidence that was correct (the bulk of it) but also some of the evidence that was flawed (i.e., glacial moraines split by drift, the rapid movement of Greenland). To quote Greene, continental drift in 1912 “was a legitimate but very tentative deduction from a great body of geological and geophysical evidence assembled in the last quarter of the nineteenth century, one of many different hypotheses created from the same materials. It had no particular claim to predominance of the theories of Willis, Chamberlin, and Joly, and it faced similar objections.”

Harris, section 28: The usual overview.

Greene, chapter 12: Radioactivity, Continental Drift, and the Fourth Global Tectonics, 1908-1912 (available online)

This is the final chapter of Greene’s study of 19th century tectonics and it places continental drift in context as another in series of attempts to construct a global tectonic synthesis. This selection also covers the ideas of John Joly that advocated a theory based on radioactivity (you can skim through this part). At the bottom of p. 279, Greene turns to one of the critical issues for all theories: were oceans and continents permanent features or could one be changed into the other? Next come the early drift ideas of Pickering and Taylor (p. 280-284) – this can be skimmed this over (although it is nice to see these two given their due!). In turn, these lead into the ideas of Wegener. The summary (p. 284-290) is brief but nicely discusses the tensions with the advocates of isostacy (mostly American) and contraction (mostly European), some of the geological evidence, and the geophysical arguments. One important note is that this summary is based on Wegener’s 1912 presentation which was more speculative and included less geological information that the 1924 English-language version we usually encounter.

Stewart, chapter 2: The Rise and Fall of Continental Drift Theory

Stewart’s book (and a similar one by Le Grand) look at plate tectonics and opens with a chapter on continental drift. The section on geological thought in the early 1900s (p. 22-28) covers familiar topics – which you can skip. The second section (p. 28-34) covers drift theory. The discussion of Wegener is valuable because it looks at the actual three-part presentation used in Wegener’s 1924 English version so you can appreciate his argument. The section on responses to continental drift (p. 34-42) focus on the attacks of Jeffreys, the varied presentations at the 1926 AAPG symposium, and the support provided by Du Toit and Holmes. The last few pages (p. 42-44) can be skipped except for the thoughts on why continental drift was not widely accepted (middle part of p. 43).

To do

• Early 20th century tectonics
  • Joly’s theory was rather unique – what was the basic idea?
    • Could this really save geosynclines?
    • Where did the actual deforming forces actually originate?
  • Broadly speaking, what geological or geophysical data/features could contraction-based theories and isostasy accommodate?
    • What could they not explain?
• Wegener’s Continental Drift
  • How did he argue for and against the adequacy of  isostacy and contraction?
  • What geological evidence is most striking?
  • What kinds of geological phenomena could his idea explain that were previously unexplained?
  • What kind of geophysical evidence or ideas did he incorporate?
  • How did he deal with traditional “fixist” idea about continents?
  • What made continents move?
• What were the arguments against continental drift?
  • Include “factual” geological objections, geophysical arguments, tectonic assumptions, and arguments based on ideas about how geologists should work.
• Subsequent developments
  • What were the major contributions of Du Toit and Holmes?
  • Can you make sense of the geographical variation in the level of acceptance of drift?
  • In light of the general state of tectonics, why was drift not more widely accepted?

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