Tennessee Crater Inches Toward Recognition 113
tetrahedrassface writes "Slashdot carried the story of an-as-yet unverified impact crater in Tennessee a couple of years ago. After a few weeks of fairly hardcore sample taking, digging, obtaining some good images and manipulating them, I'm proud to report the first batch of evidence in favor of it being an impact site. The primary smoking gun is the presentation of an astrobleme, obtained from High Resolution Ornithographic Images taken in 2008. Also of note are the melted/deformed rocks, magnetic crater dust, and the fitment of the crater rim to a circle. A rented plane and a bunch of photographs today and it's pretty obvious that it's a crater, folks. Cheers!"
typo? (Score:5, Informative)
ornithographic or orthographic?
Bird pics? (Score:5, Informative)
... High Resolution Ornithographic Images ...
As taken by birds? Perhaps you meant orthographic? [wiktionary.org]
Were you able to exclude karst? (Score:5, Informative)
Possible. But I'd sure want to exclude karst features (e.g., sinkholes) before accepting an impact origin for a topographic feature in a known karst area. Mapped karst features are awfully close to where the "crater" site is.
Also, your shattercones don't look like real shattercones [wikipedia.org], which have a nested, cone-shaped geometry. What you illustrate looks more like an ordinary concoidal fracture [wikipedia.org]. Break a rock by any method and you can get those.
If you want to identify impact melt convincingly, then ordinary macroscopic pictures won't do. You need a thin section [wikipedia.org] and some petrographic microscope work. Sometimes even forest fires can melt rocks on the surface, or a camp fire if it is big enough.
Some terrestrial minerals are highly magnetic, but also quite resistant to weathering, and will get left behind (and even concentrated) while other minerals are altered. For example, magnetite [wikipedia.org]. If it were demonstrated to be metal (i.e. unoxidized), especially if a combination of iron and nickel, that would be suggestive, but you'd still have to exclude man-made contamination.
Missing from your sampling is context. Disconnected rocks removed from their geological context are not as useful as understanding how they were arranged in the field. This is especially true for features like shattercones, which should have a clear geometrical relationship to the crater (i.e. basically a radial arrangement). If you are sampling from rubble on the surface, rather than bedrock, you really don't know what you've got. It could be transported by river, gravity (mass wasting), or (not sure if possible at this location) glaciers. It might not even be local. A lot of your samples have lichens and weathering rinds suggesting that these aren't particularly fresh samples (this is why geologists bring geological hammers and suitable eye protection to use them).
In short, you've got an interesting feature, but you are still far from demonstrating it is a crater.
Impact crater? Evidence? (Score:5, Informative)
Sorry, I'm not seeing it. Everything I see posted here can be adequately explained without the presence of an impact. Sinkholes are also often round. Magnetic material can accumulate in and weather out of most rock types, and can be introduced through secondary contamination. Vugs are, and other holes/porosity often are, the result of dissolution and weathering (coincidentally, the same processes form sinkholes under the right conditions).
To convince me, do several things:
1. Cut open the weird looking metal pieces and acid-etch them to reveal any Widmanstätten patterns. (Note: the metal in meteors will not react quickly to water or other weak acids, but limestone will. Iron minerals commonly occur in most rock types, not least of all sedimentary rocks, including limestone, that are likely to form sinkholes. Magnetic minerals are actually pretty common. I'd be a little surprised not to find small grains of magnetic material pretty much anywhere on Earth.)
2. Take the rocks to an expert and get an opinion. This often annoys most geologists a little bit since, as one recounted to me, he's had hundreds of people bring him "meteorites" over the years and precisely zero of them were actual meteorites. But if you can get an appropriately trained geologist to glance at them it should be moderately easy to see that they are or are not meteorites, or represent a rock that has melted. If that person can't say definitively, he/she may be sufficiently intrigued to investigate further.
3. If the rocks look interesting to the geologist, running through an electron microprobe and electron microscope will reveal many more interesting things about their precise chemical composition and microscopic structure, as will thin sections under an optical microscope. For the metallic parts reflected light microscopy will tell a great deal. The presence of certain high pressure SiO2 polymorphs is diagnostic in rocks from the impact zone, and iron/nickel composition is a very good indicator in a suspected meteorite.
4. Careful mapping will help, as will a geologic map of the area. Geologic mapping is not difficult, but defensible results require practice and a thorough understanding of geologic principles. Look for the character of the ground, the distribution and size of different material both vertically and laterally, its composition, texture, the nature of contacts between areas with different materials, and the orientation of any different layers, amongst any other notable characteristics. Relate your findings to those on existing geologic maps. Create your map on top of a high resolution topo map.
5. Consider multiple working hypotheses, and keep an open mind. For example, the two obvious hypotheses are that this feature represents a sinkhole or that it instead represents an impact crater. Find as much evidence as you can that contradicts or informs both of those ideas; consider all evidence in light of them. For example, you might observe that the area is characterized by shallow crystalline silicate metamorphic bedrock that is not subject to dissolution, thus pointing away from a sinkhole origin. On the other hand, you might note there are caves in the area, topographic maps show creeks and stream ending abruptly, that there was cement production there in the late 1800s, limestone clasts show traces of pyrite, and there's little evidence of breccia.
It's a logical fallacy to conclude an unusual process must be responsible for an observed feature when your evidence can be adequately explained by more pedestrian processes. Make sure you have solid evidence that can't be explained by more mundane processes before jumping to a novel conclusion -- ad hoc conclusions are inimical to real understanding and the process of science. If an impact is still a reasonable explanation after carefully considering your evidence in light of other hypotheses, systematically write up your findings. Start by giving an overview of the general area, then the feature itself, then the details of specific observations you've made a
Re:Were you able to exclude karst? (Score:5, Informative)
Re:Can someone go check my impact crater candidate (Score:2, Informative)
Number 1 is Hicks Dome, a structural dome related to igneous processes.
Number 2 looks like erosion at the interface between different geologic layers, probable a ridge of Monongahela Group sandstone and softer Conemaugh Group shale and siltstone. There's nothing about it besides a little bit of roundness that's very crater-like. If this structure has a name I don't know what it is. The fact that it appears to be at the intersection of topography that's very steeply eroded to the west and more subtly incised to the east suggests it's a chance erosion feature between two different rock types of different toughness. If you look around that area you'll see there are lots of other eroded bowls. These are common shapes that peaks and ridges take as they erode.
Number 3 is I think called Burke's Garden, representing an eroded structural dome in an area of complex folding and thrust faulting in the ancient roots of the Appalachian Mountains. The outer, younger, sandstone layer was eroded way providing access for the elements to go to work on the less resistant shale and Knox Group limestone and interior. If you scroll around a bit you can see many ridges running running along the mountains, some of which are folded, faulted, and eroded in to similar structures -- just generally with a finer aspect ratio. This section of strata just happened to get abused by the just right combination of folding and thrust faulting in the right places to make the structure look a little rounder than most. Simple geologic mapping (which has already been done -- you just need to find a copy of the maps) should very conclusively demonstrate the structural features you see here are exclusively accounted for by the conventional compressive orogenic processes that formed the core of the ancient Appalachian Mountains.