I spent the last week touring Meteor Crater in Arizona. It was a unique opportunity sponsored by LPI that gave us unprecedented access to meteor crater to aid in our understanding of impact cratering processes. It began with 3 days of touring the crater followed by 4 days of investigating a particular problem.
This was lead by the lead scientist at Meteor Crater and the head scientist at LPI (the Lunar and Planetary Institute).
Introduction to Meteor Crater
We toured along the east rim then back again along the west rim viewing the crater from two different perspectives. On the second day we explored the interior of the crater. The beauty of Meteor Crater lies in its location and how well preserved it is.
Think about the Grand Canyon. It’s only ~150 km away from Meteor Crater, and in fact they both lie on the Colorado Plateau which means the geologic layers are the same (at least before impact).
You may have already learned that the Grand Canyon is like a picture through time. Cosmos, a Space Time Odyssey illustrated this with an nice animation that separates each layer to highlight how each different layer has a different history for how it formed.
Meteor Crater penetrated through to the top 3 or four layers, Moenkopi, Kaibab, and Coconino. You can learn more about these in D. Kring’s guidebook (2017), but they each speak to a different environment by which they form. Coconino likely formed by sand dunes. Kaibab formed when Arizona was covered by a sea of varying depths (with fossils). Moenkopi formed during a time of a flood plain, similar to the Florida Everglades.
What happens when impacted is that the impactor (~30m in size) penetrates ~30m below the surface releasing large amounts shock pressures into the ground that rebound into the meteorite.
It acts like poking a pen through paper. The paper breaks and the ring bends back. In fact, they bend back over one another, folding. The result is that the lowest layer is folded on top of the other layers inverse to how they are normally layered. The diagram below shows the “fold hinge” having been mostly blown out in ejecta while the Moenkopi remains. However, overtime, this hinge is eroded (as well as the upper layer) and it becomes more difficult to see.
This overturned rim consists of a part of the blanket of ejecta that extends outside the crater.
This overturn is idealized. Sections eroded. Some shear; sections of Kaibab are missing between Coconino and Moenkopi because it slid outward further into the ejecta blanket. Furthermore, Kring suggests there were likely two other types of ejecta. A base-surge unit of the finest particles thrown up by as ejecta that is the last to fall and coat the rim and ejecta. This unit is known to exist from atomic bomb testing that formed craters. The other unit, which actually was the target of our research project, was known as Fall-out or Fall back ejecta.
Research Project: Fallback ejecta at the west rim
Fallback material is a layer of debris that is deposited over the overturned ejecta/rim. Kring describes its radial extent as unknown in the field book, but in person he made it seem as though its radial extent would be minimal. He described it as highly shocked material that is shot nearly straight up into the air before falling back down. This suggests that the layer it produces should be a mix of material, rather than distinctly inverted layers of the original target rock (see figure below). The idea that it goes straight up suggests it can’t extend too far out (without further modification).
Our research objective of this trip was to investigate region of possible fallback material by mapping its extent and identifying key characteristics within it. This means looking for key things like moankopi above the two layers (when it should be below) or evidence of high impact pressures with things like shocked Coconino.
Note, a previous version of this blog went into more detail about the project. I regret that decision. The blog wasn’t meant to expose too many details about the project, especially with the potential for publication. I have a habit of discussing my research openly, and I realize this is a unique situation that deserved more discretion.
General Thoughts and Take Away
I wish I had a clearer idea of what to expect going into this. The first 3 days or so are introduction. The next day is an applied field project to expose us to the area and investigating a problem. We measured ejected boulders to calculate their velocities. Then we moved on to this project, but the transition went from being guided to self ran. There wasn’t a clear distinction that, hey, from here on out you’re guiding yourself. That lead to a bit of confusion at the start, and even Kring blowing up at us, even going so far as to call us “by far the worst group of students I’ve ever had,” after telling us we were picked for this very competitive school because were believed to be the best of the best. That bothered me and several others. That isn’t how you teach. That isn’t how you motivate. What’s worse, I have to ask myself do I really want to talk about this here? Because, I could risk throwing away my chance to get another field school sponsored by Kring or LPI. At the same time, the thought that I’d be willing to work with someone who would treat me this way makes me feel…pathetic.
Note, images of Meteor crater are not to be reproduced for professional use. Images must legally be obtained by contacting Meteor Crater enterprises.