The Eye of Africa

Deep in the western Sahara Desert, in the country of Mauritania, is a spectacular circular structure that can be seen from space called the "Eye of Africa" or the Richat Structure (Guelb er Richat).

This structure is about 50 km (30 mi) across and was only discovered in the mid-1960s when Project Gemini spacecraft observed it from orbit.

The Richat Structure in Google Earth (21°07'30" N, 011°24'00" W)

Oblique view from Google Earth

It's pretty impressive.  Check it out in Google Maps and you'll see it's visible as you zoom out to orbital distances.

So what the hell is this structure? "Theories" on the Internet abound. Here are a couple of comments I saw on one web site:

I have a suggestion. It looks like a hole that could have been opened up when God flooded the Earth. “In the six hundredth year of Noah’s life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.” -Genesis 7:11 Later God plugged the holes: “The fountains also of the deep and the windows of heaven were stopped, and the rain from heaven was restrained” – Genesis 8:2

There can be no question that it is a standing shock wave. It is well known that it is not a result of direct impact. I have a theory that I have not seen posted elsewhere. I think it is a shockwave as a result of a large object impact at the antipode of this location. The current antipode to the Richat Structure is East of the Coral Sea. Tectonic plate movements may have shifted the actual impact. This makes the most sense to me. The structure is simply too symmetrical and undulating to be anything BUT a shockwave. If this is correct, it would have formed very rapidly and would have been incredible to witness. 

What about it being a wortex generated by dominating winds in the area?

Looks like a crash landing site. From what exactly? That’s an interesting thought. 

Its Earth asshole!

(OK, maybe the last comment wasn't serious!)

This is why you have to be careful about what you read on the Internet, boys and girls. Lots of people have crazy ideas based on nothing but ignorant, wild-ass speculation.

If you have a lot of money you don't know what to do with, it is possible to do a Land Rover 4x4 tour of this area. The problem, however, is that the Richat Structure is so big that you can't really see it from the ground.   That's why it wasn't even recognized until the Space Age.  Here are
a couple of images from the ground showing not much more than rocks and hills.

Geological circular features like  the Richat Structure are generally caused by one of three things - a meteorite impact, volcanic activity, or a domical uplift from an igneous intrusion.

When this structure was first noticed, it was thought to be a meteor crater. Once geologists got out there, however, it was determined that this structure had none of the characteristics that would have been expected from an astrobleme (impact crater).

The sedimentary strata surrounding this structure is upwarped, but not heavily broken up or overturned as is common in an impact crater. The multi-ringed structure of sedimentary rock without a depressed center is also not typical of craters. Large impacts also alter the crystalline structure of quartz grains in the surrounding rocks (called shock metamorphism) and there's no sign of that here.

[Dietz, R.S., et al. 1969. Richat and Semsiyat Domes (Mauritania): Not Astroblemes. Geological Society of America Bulletin 80: 1367–1372]

Geological fieldwork also showed that this structure is not a simple volcanic caldera (a large volcanic crater).  There is, however, evidence of past igneous and volcanic activity here (movement of molten rock - magma - in the subsurface).

Geologically, the structure is composed of three nested rings dipping outwards from the center.  They are composed of a sequence of  sedimentary rocks dating from the Upper Proterozoic Period (about 650 million years ago) to the Ordovician Period (about 450 million years ago).  The center of the structure is a limestone shelf with a lens of breccia (rock created from angular fragments of rock cemented together).  The whole structure is intruded by various types of volcanic rocks.

So what caused this?  A proposed mechanism (G. Matton, 2008) for the formation of the Richat Structure is as follows:

In Stage 1, prior to 100 million years ago, there was an intrusion of magma from the lithospheric mantle below the continental crust.  This intrusion domed up the overlying sedimentary rocks and formed radial and ring dike fractures above the intrusion.

In Stage 2, around 100 million years ago, there's reactivation of the magma chamber and the intrusion of different composition magma.  Groundwater circulating through the fractured, overlying rocks gets heated by the hot igneous intrusion and acidic leading to the dissolution of the overlying limestone.  The limestone collapses into sinkholes and gets cemented into a breccia.

In Stage 3, more igneous activity as magmas rise and interact with either shallow groundwater or surface water lakes in the center of the structure and explosive activity leads to collapse of the central part of the structure, much like a piston in a cylinder.

In Stage 4, erosion creates the circular, tilted layers (cuestas) of resistant quartz sandstone and exposes the central part of the structure where the limestone breccia and volcanic rocks occur.  Earlier radial and ring dikes are exposed in places.

All resulting in this.

NASA Landsat 7 image of the Richat Structure

Really cool!