After "what is this rock", one of the most common queries we receive at Ask-a-Geologist is this: "I've found a circular structure on my property or on Google Earth. I think I've found an impact structure."
Q: Hello AAG,
I've been sitting in my room recovering from an accident so I've been "traveling the world" using Google Earth. But I believe I may have found an impact crater in New Mexico.
The coordinates are 35 53 52 N 104 53 22 W
If this is, in fact, confirmed to be an impact crater, I would love to name it 'Noble Crater' as this is becoming an increasingly rare trait in modern society and hope this discovery will IMPACT young, budding geologists in a positive way...pun intended.
- Dustin K
A: Hi, Dustin - hope you're getting better.
You probably wouldn't be surprised to know you're not the first person to try doing this... or even the millionth. Gene Shoemaker, the "Father of Astrogeology", started an impact site search using air photos in the late 1950's, and started using LANDSAT when it became available in the early 1970's. When he died in 1996, he and his wife Carolyn had already spent many years trying to map all the impact sites in Australia, the best-exposed ancient continental craton on the planet - just to get a sense of how frequently the Earth gets whacked as a function of time. If you're being shot at, it helps your planning a bit if you know how often.
To the case at hand:
If you go to the geologic map of New Mexico (http://geology.about.com/library/bl/maps/n_statemap_NM.htm)
...you will see that your target area is in a large sedimentary province. A quick scan at the surrounding terrain shows that there has been significant tectonic activity (folding and uplifts, etc.) in this region.
If you look at the Earth Impact database (http://www.passc.net/EarthImpactDatabase/NorthAmerica.html), you will see that there is just one confirmed impact site in New Mexico - the Santa Fe impact structure (6 - 13 km in size, over a billion years old, located at N 35° 45' W 105° 56').
The level of geologic mapping in New Mexico (and pretty much all of North America for that matter) is such that it's highly unlikely that an impact in New Mexico hasn't been previously discovered by the literally hundreds if not thousands of geologists who have been roaming it for the past century or so. That doesn't mean every impact has been found, nor does it mean that you haven't found something round. The problem is really two-fold:
1. There are a LOT more, simpler reasons for a round-ish geologic structure than an asteroid impact. These include tectonics, halokinesis (salt diapirs or domes), dissolution, magmatic intrusions, and even human involvement. Any circular structure in a sedimentary province is far more likely to be caused by any of these.
2. For this reason, it's really, really difficult to prove that a circular feature is an impact site to the satisfaction of other geologists. For starts, most of the known impact sites are ancient, and have been highly modified if not mostly erased by tectonics and erosion over time. We should look like the Moon, but we don't - and tectonics and our atmosphere are why. Only about 15 of the ~180 known impact features are "meteorite impact structures" - that means that parts of the original bolide are still present - the rest are just "impact structures". The ginormous Tunguska event in Siberia in 1908 flattened a forest the size of Rhode Island - but left no crater nor any measurable fragments. It was an atmospheric detonation. Thus it's typical for impacts to be lacking any obvious "smoking gun" evidence.
Here is some helpful content for recognizing the subtle evidence of an impact feature: http://www.passc.net/EarthImpactDatabase/Criteria.html
There is more information, including some helpful images, in an article I wrote for Scientific American: http://volcanoes.usgs.gov/jwynn/1998SciAm-Wabar.pdf
From what I can see of your feature, it is an intersection of folds in sedimentary rock - but subsurface dissolution could also be involved. I have insufficient information about the local stratigraphy to say if this could be caused by halokinesis.
My suggestion for YOU, however, is not to give up, but KEEP EXPLORING. You really deserve praise for not feeling sorry for yourself - but for instead trying to DO something. For this effort you get five stars ***** from a US Geological Survey scientist. Keep up that attitude.