Controversies in the Earth Sciences

The K/T extinction reading assignment


Next we'll read a set of scientific articles together with some summaries intended for a general audience. These all deal with the K/T extinction event. The four scientific articles I have selected are intended to walk you through the development of the impact theory. The Alvarez paper first proposes this hypothesis, the Swisher paper identifies the Chicxulub crater as the site of the impact, and the Kyte paper identifies evidence of the meteorite itself. Lastly, for an opposing point of view, the Keller paper raises some arguments against the Chicxulub crater being the dinosaur killer.

I've arranged the press releases and summary articles in chronological order. Some of them discuss aspects of the four scientific papers, and some of them present alternative hypotheses for the K/T mass extinction. My suggestion is to read through the press releases and summaries first and then read the scientific papers. There is nothing to turn in for this assignment, but I have posed some questions to you to help guide your thinking. You will want to have a good grasp of the current thinking about the K/T extinction so that you can compare it to the Permian/Triassic extinction event, which we'll discuss later on in this lesson.

We will discuss these papers together via a discussion in CANVAS. This discussion will take place over the week of 14 -21 Feb. Cross-out shows what we did last time this class ran. I left it there instead of deleting it just so you can see what the assignment was. This semester I am asking you to read about the K/T on your own. We will discuss the P/Tr instead. Just FYI, I alternate between these two discussions each time I teach this class.

Reading Assignment


  1. Read the following press releases and summaries, which are available through LIbrary e-Reserves or through the link provided. You will see that there is a set of questions, below, to guide your thinking as you read.
  2. Read the following papers, which are available through Library e-Reserves:
    • Alvarez, L. W., Alvarez, W., Asaro, F., & Michel, H. V. (1980). Extraterrestrial cause for the Cretaceous Tertiary extinction. Science, 208(4448), 1095–1108.
    • Swisher, C. C., Grajales-Nishimura, J. M., Montanari, A., Margolis, S. V., Claeys, P., Alvarez, W., et al. (1992). Coeval 40Ar/39Ar ages of 65.0 million years ago from Chicxulub Crater melt rock and Cretaceous-Tertiary boundary tektites. Science, 257(5072), 954–958.
    • Kyte, F. T. (1998). A meteorite from the Cretaceous/Tertiary boundary. Nature (London), 396(6708), 237–239.
    • Keller, G., Adatte, T., Stinnesbeck, W., Rebolledo-Vieyra, M., Fucugauchi, J. U., Kramar, U., et al. (2004). Chicxulub impact predates the K-T boundary mass extinction. Proceedings of the National Academy of Sciences of the United States of America, 101(11), 3753–3758
  3. As you read, keep in mind these questions:
  • What is the evidence supporting an extraterrestrial impact at the K-T boundary? What is the evidence against it? What are the other hypotheses?
  • How was the size of the impactor calculated?
  • How does argon-argon dating work?
  • What are the main contentions of Keller's group? What do you think is the next step in resolving the debate between Keller's group and the pro-impact groups?

4. Enter the L3 Reading Discussion in CANVAS. You will see the questions above already there. It might make the most sense to organize this discussion as separate threads, each dealing with one question at a time and see where that leads us. Please participate by answering my question, responding to a classmate, asking another relevant question or furthering the discussion in another meaningful way. There is no discussion assignment this semester. Please read these papers on your own.

Grading rubric: Please see the rubric for teaching/learning discussions.