EARTH 501
Controversies in the Earth Sciences

Correlating impacts and extinctions

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In this activity you will explore the relationship between known impact events and sudden extinctions on Earth. Want to see a hand sample from an impact crater? Here is a piece of rock from the Vredefort structure in South Africa. The Vredefort is so big that you can't even see across it. The extreme heat of impact created glassy flow bands in the rocks of the crater ring. That's the black part in the photo. It's a little easier to see in outcrop scale, but it's still cool!

handsample from the Vredefort impact structure, South Africa
Rock from the crater rim of the Vredefort, South Africa, impact site.
Photo by E. Richardson.
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The world map shown in this screencast comes from the impact crater database maintained by the University of New Brunswick's Planetary and Space Science Centre. It shows the locations of known impact crater sites (noted as white dots on the map). In this screencast I explain the map in more detail. You can also read a transcript of my discussion of the map.

world map with known impact craters designated
World map with known impact craters designated, current as of 2008.
Credit: University of New Brunswick PASSC database.

As discussed in the screencast, it is instructive to see where craters are and are not found on Earth. For example, craters are conspicuously absent from the oceans, Antarctica, Greenland, the Amazon river basin, central Africa, and Indonesia. Is this real (i.e., is there a scientific reason why asteroids or comets would fall to Earth in certain locations?) or is this an artifact? The reason why craters are not found uniformly over the globe is a result of preferential preservation in some places and also some places on the planet are not as easy to get to, thus potential impact sites have not been explored there.

In the case of the oceans, plate tectonic activity means that the oldest ocean crust dates from the Jurassic, so no impacts older than that could be found there. In addition, oceanic crust is quite thin and not conducive to preserving evidence of an impact.

Impact Craters problem set

xkcd comics about correlation and causation
Image source: XKCD comics.

Directions

For this problem set, create your own document instead of downloading a worksheet.

  1. On the geologic timescale you constructed earlier in the lesson (see "Antipodes and geologic time"), mark theknown impact structures in the PASSC database according to their ages. To avoid monumental clutter, remove the class member data from your timescale and don't try to write the name of each crater on your timescale. I do want you to create a visual representation of how often the Earth has been hit by objects of various sizes, so use five different symbols or colors to represent the following crater diameter ranges:
    • <0.09 km
    • 0.1–0.9 km
    • 1–9 km
    • 10–99 km
    • 100+ km
  2. Now mark the "big five" extinction events on the timescale. Use a different symbol or color than any of the five from #1 above. (Make sure to include a key on your timescale.) These events are:
    • end-Ordovician (444 Ma)
    • late Devonian (385 Ma)
    • end-Permian (251 Ma)
    • end-Triassic (200 Ma)
    • end-Cretaceous (65 Ma)
  3. Make a scatterplot of crater diameter vs. crater age. [On the PASSC database, they used to have a 3-column table with name, age, and diameter of craters and you could sort the table on any of those categories. I don't really know whey they abandoned this way of displaying their data.Right now it takes a lot of extra work to make a table from which to make a simple scatter plot. I have done the sorting and organizing for you on this one. Choose the file format that works best for you. Let me know if you'd like a different format and I'll see what I can do.These data are current as of February 2016.] Impact Crater data in .csv or Impact Crater data in .xlsx or Impact Crater data in .pdf
  4. Where is the biggest known impact crater on the planet and how old is it?
  5. Why are there more impacts in the database the closer we get to the present time?
  6. How old is the oldest crater with diameter less than 1 km? Why aren’t there any older craters of this size or smaller?
  7. According to this database, about how often does the Earth get struck by an extraterrestrial object that makes a crater as large as the one found at Chicxulub? What bearing does this have on the debate between Keller's group and the other groups? (Note: If you aren’t clear what debate I’m referring to, then reread the article from The Economist called “Making an End of It”)
  8. How well do the "big five" extinction events correlate with known impacts? Specifically, does this database support the Becker group's idea that the P/T extinction is impact-related?
  9. Let the following be my hypothesis: All impact craters with diameter greater than 100km must have caused an extinction event. Can this hypothesis be proven true, or proven false, or not even usefully tested with this database? Explain.
  10. We are going to read a paper that deals with how life on the Earth recovers from a mass-extinction event next. See Figure 1 from this paper. You will see that apart from the "big five" extinction events, the Earth has undergone several smaller-sized extinctions and originations of life over the past 500 million years. Do any of these smaller extinctions coincide with impacts?
  11. Find the maximum innocuous crater size from this database. What I mean is, what is the largest crater since the beginning of the Cambrian that has no extinction event associated with it in time?
  12. One of the main points of this problem set is to explore the relationship between correlation and causality. Explain briefly what that relationship is and use a real-world example (doesn’t have to be from this problem set) to illustrate your point.

Submitting your work

Your completed problem set should include your timescale, your scatter plot of crater diameter vs. age, and answers to the follow-up questions. Save an electronic version of your work in the following format:

Impactcraters_AccessAccountID_LastName.doc (or .yourFileExtension).

For example, Cardinal relief pitcher Tyler Lyons' file would be named "Impactcraters_twl70_lyons.doc"

Upload your document to the Impact craters problem set assignment in Canvas by the end of week 3 of this lesson.

Grading rubric

I'll use my general rubric for grading problem sets to grade this assignment.