Feature article in October Earth Magazine - fossilized shockwaves

For Earth Magazine's October 2011 edition, I wrote a feature story about the discovery of the Santa Fe impact structure in New Mexico. To see the field evidence of this discovery first hand, I followed retired geologist Tim McElvain out into the Precambrian rocks of the Sangre de Cristo mountains. The crux of his geologic field evidence was finding shatter cones -- rocks that are deformed in a very special way during large meteorite impacts -- and preserved in only a few places on Earth.

Here's a link to the online version of the article:
http://www.earthmagazine.org/article/santa-fe-impact-crater-discovery-series-fortunate-events


Below is an annotated photo I took of a Santa Fe shatter cone outcrop that formed in Precambrian schist. For reference, the field notebook I used for scale is about 4 inches tall. You can see the yellow outlines I made around the most prominent shatter cones -- there are millions at this location -- ranging from meter-sized to millimeter-sized.

Shatter cones are fractal, which means their telltale shape repeats all the way down to the microscopic scale. The cones "point" to the center of the impact structure or crater, which is where the meteorite hit the Earth. A shockwave was created by this massive meteorite impact, and caused permanent deformation (shatter cones) to the Precambrian schists as it passed through the bedrock.

So when you see a shatter cone you're basically looking at a fossilized shockwave!

Unfortunately, no one has discovered an impact crater here in Washington State, though it would be nice to have a local one to investigate. More people to get  out and take a second look at road cuts, upturned beds, and gravity anomalies around the state. If you think you've found something, shoot me an email and I'd be happy to take a look.

You can check out all the confirmed meteor impact sites worldwide at this website: http://www.passc.net/EarthImpactDatabase/index.html

Earth Magazine (http://www.earthmagazine.org/) is published by the American Geosciences Institute.