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What caused the 'Eye of the Sahara'?


Still Waters

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For millennia, the Eye of the Sahara was hiding in plain sight. That's because this huge and mysterious geologic formation is hard to spot from ground level, walking around on Earth.

It turns out we only really discovered this incredible bullseye in the sand when we began sending humans into space, but even now we've found it, scientists still don't fully understand it.

The Eye of the Sahara, which is more formally known as the Richat structure, is located in the western Sahara Desert in Mauritania. On the ground, it's about 25 miles (40 km) across.

http://www.sciencealert.com/scientists-still-don-t-know-what-caused-the-eye-of-the-sahara

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I think a smaller tinier one on the upper left corner of the picture ... anyone else see it ?

~

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Erosion feature of a dome related to Pangean era tectonism? I'd assume?

*reads story

Yeah, that's what they're saying. 

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It's an eerie, creepy feature, ancient and anomalous. Nothing like it in the rest of the world. If  I was to let my imagination run riot I would think something enormous and heavy rest there for a very long time and then left. Something old and almost Lovecraftian in nature and certainly nothing related to human activity. Whatever might have dwelt in what once rested there would have witnessed our evolution. Total fantasy, of course, but such an evocative place!

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It should be said that eroded domes/fold features aren't very uncommon.

This one is more symmetrical than most (more magmatic-derived rather than tectonism-derived) and when it's scaled 6:1 vertically it really "pops" out of the landscape.

Also, third_eye, good eyes, you're seeing a kimberlitic plug associated with later activity in the igneous complex.

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I have a knack for picking out details like that , something I picked up during my days in the advertising and promo-media days ... :tu:

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This amazing thing is one of the true unexplained mysteries out there.  I find it fascinating and quite beautiful from above..  But why does it exist - if it's a natural consequence of some type of erosion, why aren't there any other similar structures?  Ironically, the most likely explanation seems to be that a dome of volcanic rock came up from under the layers of sedimentary rock that you can see to create a 'dented upward' reguion, and that perhaps, after a small eruption that may have blown away some of the rock towards the centre, the dome then settled back down, and the weakened sedimentary areas collapsed and eroded much more quickly than the surrounds.. Or something like that..

 

From the Wiki:

 File:Richat Structure - SRTM.jpg

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I would say its unexplained insofar as no geologic explanation is ever perfectly 'right'. But I would argue that it is fairly well understood, for all that.

From a thesis paper (Matton, 2008) on the topic, see picture.

RS.gif

So in our first stage we see a basaltic magma chamber at lower crustal levels "doming up" the surrounding area, and expressed via dykes and some surficial magmatic activity - very steep faulting occurs, fracturing (opening perpendicular to the greatest stress - magma coming up from below). We can see the various layer of ss, ls, and seds at the surface. The basaltic dykes, although we're only seeing a slice here, are likely ring dykes, roughly circular in map view, and subvertical.

Second stage, the magma chamber is differentiating (via fractional crystallization, etc.) and we develop a rhyolitic magma on top of the basaltic magma. A lot of hydrothermal activity is taking place, courtesy of the fractures and magma chamber. Extremely extensive throughout the entire middle of the complex. We see that this stage (roughly in the late Cretaceous, to place us) is when the kimberlite erupted, along with carbonatite on the other side.

Then, as more dissolution and hydrothermal activity take place, eventually the felsic, high viscosity, eruption-prone center decides to erupt. The volatiles and mush gone, space is made. We see how the center slides down the faults/ring dyke conduits, in a very caldera like fashion. The chamber presumably dies out, solidifies.

Finally, erosion (like that of cuestas) takes place, levelling out the entire area, preferentially leaving more resistant rocks as "rings" around the structure.

Is any theory perfect? No, this one probably isn't either. It does explain the rocks that are there, in the arrangement that they are in. Could there have been more of a collapse, rather than an eruption at stage 3? Possibly. We see a similar process with salt anticlines... It is interesting that there are so many interacting magma types here, we've got basalt and rhyolite (so essentially a bimodal system, we see that at Yellowstone, even a wee bit at Hawaii) and then the undersaturated alkaline magmas shooting through the system.

So it's a fascinating area, to be honest. Not inexplicable, however. All features seen here are seen in other places - although this is a fairly isolated, well-preserved, and highly symmetrical example. I'm definitely oversimplifying the geology here, but the general info I gave is solid - in line with current research on the area.

 

 

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