We will be working on two main projects over the last couple weeks of the course:

1. Preparation of your posters for a “poster day” on the last day of class. Please come prepared to develop the outline of your poster – meaning read the references and have notes on critical information (including a copy for me).
2. Construction of a “development map” for plate tectonics to summarize the evidence that converged as the theory developed in the 1950s to early 1950s. This project will complement the earlier class project on tectonics from the mid-19th to early 20th centuries. The project will be spread over the classes on seismology, paleomagnetism, sea-floor spreading and plate tectonics.  Much of this will be familiar to you, so your challenge will be expressing how the parts fit together. Hint: put things in a time sequence based on publication dates.

These readings deal with paleomagnetism, one of the key components of the theory. Paleomagnetism really involves two parallel stories: (1) the study of terrestrial records that led to “polar wandering” curves and the magnetic reversal time scale; and (2) the development of marine magnetic recordings and profiles with “anomalies” that were rather mysterious. As you read over this material, please make notes on the basic observations and interpretive issues related to these data. (You will be doing this for the next couple of classes.)

Harris section 30

Allegre (1988) p. 41-58

This selection covers paleomagnetism based on terrestrial data. You probably know some of this but the history behind the astatic magnetometer and the difficulties with the inversion issue may be less familiar. Be sure to read over his comments (p. 56-58) about the specialization within geological disciplines had allowed the major breakthroughs of “polar wandering” to be ignored by geologists with a “fixist” mentality.

Wood (1985) Chapter 6: A naval engagment, p. 139-148

Wood gives an account of marine paleomagnetism that opens (curiously) with Hess’ famous 1960 “essay in geopoetry” (p. 139-142 – skim) before turning to the geophysical data that inspired it: earthquake locations in the oceans, heat-flow measurements, ocean magnetic field measurements, and gravity measurements (p. 142-148) – please note the growing database on marine geophysics and how unexpected the findings were from the perspective of continental geology. Finally, the following section (p. 148-154) on sea floor spreading is worth reading now, even if we are getting ahead of ourselves

To do

The most critical “to do” is to make a list of the basic knowledge that accumulated very quickly during the 1950s. Here are some specific features to pull out of the readings. It would be helpful to note the years of major publications.

• Terrestrial
  • Polar wandering curves
    • What were the options given one polar wandering curve?
    • What was the implication of having two discordent polar wandering curves?
  • Magnetic reversals and time scale
    • Given magnetic inversions in some rocks, what were the possible explanations?
    • How were they overcome (both in theory and from data)?
    • How was the magnetic reversal time scale assembled by the mid 1960s?
• Oceans
  • Earthquake locations
    • What was the distribution of earthquakes in marine settings?
  • Heat-flow
    • What was found out about oceanic settings?
    • What was surprising?
  • Ocean magnetic measurements
    • What was actually measured?
    • What is a magnetic anomaly?
    • What were the map patterns?
  • Gravity measurements
    • You may recall that gravity measurements hurt isostacy by showing uncompensated anomalies over features such as deep-sea trenches.
    • What pattern did Girdler find along the Red Sea rift? How was it interpreted?

One of the remarkable features of this expansion of information about the oceans is how quickly it happened.  The study of the oceans opened up geophysics to move beyond isostasy and vertical motions to a more dynamic perspective that began to align with geologic ideas – well, some of them at least.

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