The readings deal with three inter-related topics: Kelvin’s challenge to geologists assumed time scale based on thermodynamics and physics; the continuing problem of developing tectonic models that were compatable with the evolving geophysical interpretation of the Earth’s crust; and radioactivity that would change both debates.

Harris sect. 26 (section on Chamberlin) and 27

Please re-read the section on Chamberlin (p. 167-168 in section 26) before reading the Greene reading. Section 27 deals primarily with Kelvin’s decade long argument with geologists and how radioacivity changed the “rules of the game”. The final section (“So where was tectonics around 1910?”) is best read after the other readings.

Hallam (1989) chapter 5: The age of the Earth

Hallam provides a good summary of the running controversy between geologists and geophysicists (at least those of the thermodynamic bent) over the age of the Earth. Estimates prior to the work of William Thomson (Lord Kelvin) were basically quantified guesses. Kelvin took a more rigorous approach and over 35 years (1862-1897) published a series of articles that progressively shortened his estimates of the Earth’s age. Hallam summarizes the “dialog” between Kelvin and geologists over this time span, and then turns to the resolution via radioactivity. Essentially this removed the force from Kelvin’s argument by changing the understanding of the heat balance of the Earth. We will primarily be interested in the conflicting views of traditional geologies and geophysics, so we can focus our reading a bit.

• p, 105-107: You can skip this. It summarizes geologists attempts to estimate the age of the earth. Most geologists avoided this question, but Phillips estimated the time since crust stabilization at 96 million years based on observed sediment thicknesses. At the other extreme, Darwin suggested 300 million years for the Cenozoic (!) based on the time he though was needed for natural selection to operate. (If you think about this suggestions, it may reveal why natural selection feel out of favor by the 1880s-90s in light of what comes next.)
• p. 107-112: This presents the heart of Kelvin’s thermodynamic arguments – read carefully.
• p. 116-119: The initial reactions of the geological community were initially rather accommodating – at least while Kelvin still allowed up to about 100 million years. It did inspire a range of different attempts to estimate the Earth’s age based on geological criteria, as seen here.
• p. 119-123: As Kelvin’s estimates became more restrictive, he provoked more opposition from geologists and geophysicists (who noted his assumptions of a very simple structure to the earth’s interior). This basically wraps up with a strong objection from Chamberlin – as further developed in the Greene reading.
• p. 123-129: This can be skipped because I think that my notes provide an adequate summary for our purposes. The critical point for tectonics is fairly clear: the discovery of radioactivity added a heat source to counteract heating. In one blow, this removed the basis of Kelvin’s argument and a basic tenet of tectonic models based on contraction and cooling. If you are interested in the details about the emergence of the time scale, please feel free to read this section.
• P. 129-132: Skip the “epilogue”

Here are some things to consider:

• What were the three approaches that Kelvin used to estimate the age of the earth or the solar system?
• What were his assumptions in making his estimates?
• What were his progressive estimates of the duration of the earth?
• Why were Kelvin’s works so highly regarded?
• Why was his work a threat to Lyellian interpretations? How did it relate to the directional synthesis?
• How did geologists respond to his estimates? (This varied considerably over time.)
• What were Fisher’s objections?
• How did radioactivity sidestep Kelvin’s calculations?

Greene (1982) chapter 11 (Available online)

This chapter picks up the story (from the prior chapter on isostacy) of the state of tectonics around 1900 with an American twist – the theory of T. C. Chamberlin, the third major global synthesis (after Elie de Beaumont and Suess). Following the work of Dana and Dutton, American geologists were less concerned with tectonic theory – to quote Greene: “an attitude of benign neglect in the matter of geotectonic theory seemed judicious and feasible.” Chamberlin revived the effort to develop a comprehensive theory. His idea was ultimately based on contraction but included the twin ideas of periodic deformation (“correlation pulsations”) and the permanance of continents. Like Suess, he linked tectonics to transgressions and regressions albeit in a different way.

However at the turn of the century (1895-1908), isostasy measurements proved to be a major challenge to all theory relying upon periodic/episodic isostastic adjustments of an unbalanced system. The gravity survey results (1895-1908) showed that the crust was essentially in equilibrium, under-mining the idea that continents were foundering due to isostatic adjustment. In fact, Suess would reject isostasy altogether along with any vertical compensation.

At the same time, the newly discovered phenomena of radioactivity (with the associated release of heat) also make cooling-driven contraction models less viable. As you might guess, the end result of all this geophysical development was a general loss of coherence in efforts to develop a geotectonic synthesis.

Here are some notes the Greene reading:

• The American perspective (p. 158-260)
  • This is forecasts the main objections to continental drift: notions of continental permanence and periodic deformation.
  • How do these reflect the persistent influence of Dana?
  • How do they flow into Chamberlin’s theory?
• Chamberlin (bottom of p. 260-267)
  • How did his theory work? (The main idea starts on p. 162)
  • Could it really explain nappe and fold/thrust belts?
  • How did he incoporate some of the ideas of Suess (contraction), Dana (ocean-continent interactions), and Dutton (isostosy)?
  • How did this all relate to his idea of a solid interior?
• Challenges (p. 267-275)
  • As you read this section, do not worry about all the details on the varied theories – the reason will be clear by the end of the chapter. Pay more attention to the general issue of the overall state of tectonics.
  • How did isostasy (as established by the analysis of field measurements, p. 267-268) challenge tectonic theories? (The text is quite good on how disruptive this was!)
  • Note the resistance from the geological community in general, Suess, and Willis (p. 268-272) – this can be skimmed over. Willis was an excellent geologist and his attempts to reconcile different ideas nicely captures how difficult it was to make sense of all this.
  • Read p. 272 (bottom)-275. After all this work on tectonics, we emerge with a remarkable loss of coherence in tectonics as an integrating framework.
    • What was the state of geotectonics circa 1910-1912?
    • What was the underlying tension?
    • What were the alternatives?

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