Reading Notes for Class 22: Geophysics

The main challenge to the late 19th century contraction/lateral compression tectonic model arose from the newly developing field of geophysics. There are three inter-related themes: how crustal studies led to the idea of isostacy; the inadequacy of contraction to explain compressive tectonic features (i.e., things like nappes); and the resulting development of different tectonic models built around isostacy or in combination with other ideas.

Harris sect. 26

Overview of this interesting topic – what happens when physics gets applied to the Earth. We will deal with Chamberlin in our next class, along with Kelvin.

Greene (1982) chapter 10 

Greene here summarizes how studies of the earth shape and density led into the development of contrasting models about the earth’s crust. These ideas had implications for crustal shortening (and the ultimate demise of contraction theories) and inspired the theory of isostacy. One result was a new radiation of tectonic theories that tried to make sense of all this. As a result, this chapter has quite a mixture of ideas (some of which are bit unusual).

  • Studies of crustal structure that led to the Pratt and Airy models (p. 238-243)
  • Fisher’s critique of contraction (p. 243-246)
  • Reade’s solid earth model that was similar (but more detailed) than Hershel’s model (p. 246-248)
  • Dutton’s suggestion of isostacic dynamic equilibrium (p. 248-251)
  • Willis’ combination of contraction and isostacy (p. 251-254)
  • Reyer’s thermal theory with gravity sliding (p. 254-256)

Fisher (1881) Chapter XXII – Summary

Osmond Fisher was a British mathematician who began to publish on geophysics in his fifties. He wrote the first major geophysical text – your selection is the summary chapter of that work. Although in places, the logic is a bit tortuous (since we are only reading the summary) you will find that he argues for a thin rigid crust, fluid interior, and an Airy model. This is really a remarkable book!

In reading this summary, you will find some of the numbered sections are not too clear (presumably the full text is better). So, I want to give you a road map to reading this – what to focus upon and what to skip. Note that the chapter sections’ topics are listed at the start of the chapter, but not in the running text (sigh). You are welcome to read the entire chapter, but – if time is short – just read the sections underlined below and use the notes for the rest.

  • Section I-III: This presents his argument about the structure of the earth’s interior based on temperature and density considerations. It is rather remarkable in delineating the case for a solid crust, an intermediate liquid layer and solid core. I think that this is really the first definition of such features beyond the Pratt-Airy work in the 1850s.
  • Section IV is a statement of the basic cooling-based contraction argument
  • Sections V and VI are a bit unclear in detail (at least to me), so you can skip. What you need to know is that the essential argument is that the amount of terrestrial contraction would produce “inequalities” (think deformed belts, uplifts, etc.) that would average about 200 ft in thickness over the entire globe, but the actual observed amount is about 10,000 ft.
  • In Section VII, the main point is in the first paragraph. Skip the rest.
  • Section VII and IX can be skipped – these are side points or ideas that are rejected.
  • The key points of Section X can be summarized (sidestepping some of the details). Assuming that the crust is essentially granite and the underlying “fluid” basalt, Fisher estimated that the “undisturbed” crust (i.e., not in mountain belts) is about 25 miles thick. Using the analog of an iceberg, he points out that elevated areas will have a considerably deeper root.
  • Section XI is worth reading because it links this insight to Airy’s model.
  • Sections XII makes a key point. Fisher revisits the question of crustal thickness, arguing that the density and elevation of ocean crust (now viewed as basaltic in nature – recall Dana) lead to the conclusion that oceanic crust is thinner (20 miles) than undisturbed granitic continental crust.
  • Section XIII argues that the amount of shortening that cooling-driven contraction would cause across continents is 42 miles. He presents another estimate of 200 miles but rejects this because it would require ocean basins to be granitic which prior sections disprove.
  • Sections XIV-XVI can be skipped – these are largely rejections of other ideas for building up stress in the crust because they are inadequate.
  • Section XVII returns to the dynamic nature of the Airy model (which Dutton termed isostacy in 1889)
  • Section XVIII rejects Mallet’s model for causing crustal stress (usually attributed to contraction) by intrusive dikes (causing expansion!) – more on him when we get to seismology.
  • Sections XIX-XXI: This work closes with some thoughts on volcanism and links into some of the ideas on magma generation. What is worth noting is that Fisher uses the thin crust Airy model to explain why magmas differ (XX) and suggests some differences between coastal and oceanic volcanic regions.

Stepping back, the points to get from this first book on geophysics are: (1) the attack on contraction based on geophysical grounds; (2) the idea of a fluid layer below a relatively thin (25 km) crust; and (3) the support for isostacy – essentially vertical tectonics.

Fisher came to geology rather late in his career. He wrote from the perspective of applying physics to the Earth. His work, Dutton’s idea of isostacy, and the studies of the “figure of the earth” (Harris notes) described a very different reality than that envisioned by Suess and those who studied nappes. Most of the unusual tectonic models proposed between the late 1880s and World War I reflect attempts to bridge this gap. A similar tension arose between geophysics and geology in the 1920s over continental drift, and again in the 1950s in the run up to plate tectonics.

To dos

  • We are familiar from introductory geology with the Pratt and Airy models for crustal structure. What is often not discussed are the original implications for crustal thickness, strength and mobility – how does all this fit together?
  • What was Fisher’s argument against the adequacy of contraction? How did his work bring in geological information?
  • Reade’s model was unusual and relied more temperature variations. How was it supposed to work? Can you “see” the similarities to Herchel’s model?
  • What was Dutton’s isostacy model? How did it relate back to the Pratt-Airy debate? Fisher’s argument?
  • What do you make of Bailley Willis’ combination model? This was a significant contribution that was favored by many American geologists in the early 20th century.
  • Reyer’s models was an interesting attempt to bring together vertical crustal movements and nappes (!). So, how did this work? How well would this fit the Alps or (for that matter) the Appalachians (push from the Atlantic side)? What were the similarities to (or elements from) other models?
  • More general questions:
    • How believable are the models that rely upon interactions of vertically moving crustal blocks to create mountain chains (as revealed in the Alps or Appalachians)?
    • If we think from the perspective of geology circa 1900, what are we to think? What are our options? How do we bring isostacy (vertical tectonics) into line with nappes (horizontal tectonics)?

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