Highly Allochthonous

I say no, not a crater. It looks more like a knot in a plank of wood, the way lines run vertical (relative to the picture through it. A sink hole from an old river? I'm a chemist, what do I know?

Posted by: Propter Doc | January 9, 2008 12:15 PM

I'll look at the regional context later, but my initial gut-reaction is that it's not a crater. The way the rocks go around the feature, as pointed out by Propter Doc, makes me think it's some sort of dome structure?

What on (Google)Earth could be a new series...instead of finding something, you provide the location, and ask what it is...as you've done here. Go for it.

Posted by: BrianR | January 9, 2008 12:26 PM

Before I say anything (or search for the regional context on Google Earth), would you mind it if I point my Structural Geology class here and ask them to brainstorm possible alternate working hypotheses, and ideas for how they would test them?

Posted by: Kim | January 9, 2008 12:45 PM

The coincidence of the circular form making a bullseye in the outer structure tells me the odds are strongly against this being an impact crater.

Posted by: 6EQUJ5 | January 9, 2008 12:57 PM

Looks like a crater to me. But that's just 'cause "eye of Sauron" looks like a big hole to me!

Posted by: Khalil A. | January 9, 2008 1:16 PM

I'm not a geologist either, but I'll stick my neck out and say: highly eroded volcanic feature. The central circle is the lava plug, and surrounding N/S linearity is sedimetary(?) rock that was domed by the emplacement, then eroded to expose the edges of the layers.

There are also some dead obvious old cones just to the NW:
http://maps.google.com/maps/mm?f=q&hl=en&geocode=&time=&date=&ttype=&q=22%C2%B048'50.77''n+9%C2%B029'08.92e&num=10&ie=UTF8&t=h&om=1&ll=24.203758,6.964645&spn=0.214815,0.572662&z=11

Posted by: Eamon Knight | January 9, 2008 1:23 PM

I'm inclined to say something volcanic. But the color intrigues me. It looks like salt. It could be some kind of dry lake collecting salt, but that doesn't explain the shape, which could still be some kind old volcanic vent.

I look forward to the answer.

Posted by: John McKay | January 9, 2008 1:53 PM

At the risk of sounding like an idiot: A volcanic flow around an older sedimentary structure such as a dome, which then eroded away?

Posted by: decrepitoldfool | January 9, 2008 2:31 PM

To me that looks like one of the carbonate mud mounds. Middle Devonian?

Posted by: Luna_the_cat | January 9, 2008 2:38 PM

Not a crater - the surrounding material seems to have flowed around the object, so it was there first or at least at the same time, the shape is not consistent with any other crater I've seen, and finally there doesn't appear to be a rim or debris thrown up by an impact.

Looks to me as if there has been some flow around it, so it's a positive feature, or at least was. The three most obvious materials to flow (to me anyway) would be sand, lava or water.

Posted by: PaulG | January 9, 2008 3:01 PM

My guess is that it was an dome-shaped outcrop of something that has been mined/excavated.

Posted by: erik | January 9, 2008 3:43 PM

Sigmund ... Flashearth is nice, but can you get perspective views? ... plus, at least for me, it doesn't move around as smoothly (but maybe that's my computer, I don't know). How do you add overlays? Can you export .kmz files? I wouldn't discount GoogleEarth just yet.

Posted by: BrianR | January 9, 2008 3:51 PM

Without having any idea of the geology of algeria but to me this looks like a dome structure. Here is my theory: a shallow intrusive body is pushing the apparently(?) sedimental rocks upward. This intrusion-caused updoming of sediments is then eroded and the eroded material fills up the now roughly circular central depression covering the view to whats below. What strikes me is that the "layers" move more or less around the circular form and following them N and S they seem like a layer of rock that has been broken open.

Posted by: Mathias | January 9, 2008 4:33 PM

BrianR, like I said, I'm a geneticist. I am more interested in satellite imagery sites that allow me to see my house rather than using it for geological purposes. It actually scrolls fine for me but I don't know how to get perspective or overlays and I haven't tried to export the files.

Posted by: Sigmund | January 9, 2008 4:56 PM

This is a meteor crater:

http://maps.google.com.au/maps?f=q&hl=en&geocode=&time=&date=&ttype=&q=Carranya+Station+&sll=-19.243736,127.721558&sspn=3.676544,7.338867&ie=UTF8&t=h&om=1&ll=-19.177761,127.801552&spn=0.057477,0.11467&z=14

Posted by: Kieran | January 9, 2008 6:04 PM

Looks like a crater to me.

Posted by: GrEy | January 9, 2008 7:12 PM

Weird. It reminds me of a s-c fabric. The cool guess is it is some sort of igneous emplacement that acted as a "hard nut" in the regional foliation.
The simpler explanation is that it is the remains of a doubly plunging antiform. This is probably a better explanation, as it looks like this place ends up being low spot/salt pan. If this is an excavated anticline, and the rock exposed in the core is soft(er), it would become a low spot.

Posted by: flounder | January 9, 2008 8:57 PM

Thank you Sigmund for the link. I have to agree with DSW, the markings would indicate a varied layered burn pattern, and the deeper center inset seems to indicate a definite object having impacted. A meteorite would have greatly cooled before impact, and not shown such burn marks, if that is indeed what they are. A nuclear, or powerful bomb is my guess.

Posted by: ShadowMyth | January 9, 2008 10:33 PM

Volcanic plug plus erosion from the prevailing wind is my guess.

Posted by: Monado | January 10, 2008 12:45 AM

Ok, I've given my students nearly 24 hours to check this out, and none of them are playing, so I'm going to jump in. :D

I think that Andrew and flounder are on the right track. But some reasoning first:

1) The terrain information isn't available, so I can't look at an oblique view, but the shadows give some sense of topography. The dark features on the right of the image, for instance, cast significant shadows, so I would say that they are pointy features. (I suspect that they are the volcanic features that Eamon Knight found references to.) The "eye of Sauron" (I love it :D), on the other hand, doesn't cast much of a shadow.

2) There's a good example of an ephemeral stream channel on the left side of the image. North of the "eye," you can see dark lines that are probably canyons that feed it. So the area is somewhat raised above the basin floor.

3) The north-south features are regional - if you zoom out, you can see them in other dark areas. So we're looking at bedrock poking out from beneath the sand.

4) The dark, north-south lineaments curve around the "eye." That kind of pattern is typical of gooey stuff squashed around something that's more rigid, as Andrew and flounder implied.

So I think we're looking at structures that formed in the mid crust. I think the "eye" is a pluton (for non-geologists: molten rock that cooled underground), and the dark lines are the pattern of foliation around it. I think the pluton is probably older than the deformation, but that kind of pattern has also been described from syn-deformational plutons. (I would need to go look at the rocks to know for sure. Too bad it's in Algeria. It looks like a spectacular place for a field project. Except for no water, and really hot.)

As for the age... my guess is that it's part of the pan-African orogeny, when a block of rock from west Africa collided with another block that's now in east Africa, ummm, somewhere around 900 to 600 million years ago. (The Hercynian orogeny, which is like the Appalachian orogeny on Chris's side of the Big Pond, affected rocks closer to the coast. Alpine orogeny structures should be oriented closer to east-west, plus I would expect more topography from something that young.)

Posted by: Kim | January 10, 2008 1:23 PM

I have to agree with most of Kim's detailed analysis. Even in Google Earth there's not enough elevation detail to tell much about the topography. The two shadow-casting features to the east of the "augen" appear to me to be volcanic necks - there are a number of resistant dike-type features in the region, very reminiscent of Shiprock or the Spanish Peaks. One thing that makes me question Kim's pluton assertion for the "augen" is the lack of evidence that this is a similarly resistant (igneous) rock. True, not all plutons hold up well to weathering, particularly in humid environments, but in this desert environment I would expect a pluton to be more resistant. If that observation is correct, I'm more inclined to favor a salt dome type of origin, though I agree that an "augen" origin would clearly imply a more resistant body surrounded by ductile materials - decidedly not the case if this is salt.

Posted by: Ron Schott | January 10, 2008 1:52 PM

Kim's suggestion is definitely the one that sounds most plausible to me.

jdinuc: It would be highly unusual to find striations caused by ice of this magnitude. Reliefs of those are on mm-scale in bedrock.

Posted by: saxifraga | January 10, 2008 3:32 PM

saxifraga: Well, I guess that blows a giant hole in my theory! I also note the distinct lack of any objects that appear to be giant boulders lying around the area!

Hope we get an answer soon!

Posted by: jdinuc | January 10, 2008 6:53 PM

I think it is a crater. Reason: it looks like a crater. LOL! It looks like maybe an impact crater, but it also looks like it could be a volcanic crater. Maybe even more likely volcanic because of the symmetry.
Dave Briggs :~)

Posted by: Dave Briggs | January 11, 2008 10:42 AM

I think the bulk of the stratigraphy is a sedimentary package, with the dark strata either being varying sedimentary packages or sills. Though you would expect at least some crossing of strata with sills and these appear extremely conformable. I don't think these linear features could represent metamorphic banding, especially at this scale.

I believe what we are seeing is a tightly folded and sub-vertical (west-dipping?) sedimentary sequence (though to the east and west the folding is less tight and more chaotic).

The darker beds persist through the two prominent features to the east. If these were volcanic 'plugs' within a much older sub vertical stratigraphy I believe they would not contain these layers. I think these are more likely to represent resistant areas possibly silicified or altered in some way. You can see this layer has other resistive parts to the south too.

As for the main feature, I agree the sedimentary sequence wraps about about it.

Any body intruding this sequence, magma or diapir, should show some north-south elongation and not have a circular cross section.

I agree with Kim in that we are seeing large scale deformation partitioning about a pre to syn-deformational igneous body. The body may have originally been a laccolith-like body that has been captured with in the core of an isoclinal antiform. There appears to be some asymmetry about the core with a possible west-side-north sense of movement.

some more of these would be great :)

Posted by: Clay | January 11, 2008 11:41 AM

This "puzzle" points out the perils of aerial photo interpretation. Without an opportunity to get on the ground and look at texture, mineralogy, and fabric, we're (usually) throwing darts blindfolded.

Remember Colin Powell's powerpoint presentation at the UN?

Posted by: John Freeland | January 11, 2008 4:22 PM

Clay:
Understood, it's fun and necessary to brainstorm. Last thing I'd do, however is put my equipment on the block in front of a bunch a folks swinging rock hammers.

Posted by: John Freeland | January 12, 2008 9:55 AM

Has anyone suggested a RF Big Garnet yet?

Posted by: Lab Lemming | January 13, 2008 5:36 AM

When do we get to find out what it really is?

(not that I'm impatient or anything...)

Posted by: Luna_the_cat | January 13, 2008 11:58 AM

My guess is it's the hole that Chris has disappeared into.

Posted by: JasonE | January 21, 2008 8:50 AM