Chris Hadfield tweets incredible photo of geographical oddity in the Sahara Desert

Canadian astronaut Chris Hadfield (@Cmdr_Hadfield) treated us to another photograph from space before signing off from Twitter last night, but this one looked more like an ancient fossilized shell than a massive geological formation.

Originally thought to be the site of an asteroid impact, The Eye of the Sahara — more formally known as the Richat Structure — is a 40km wide, eroded rock dome in the west African nation of Mauritania that has been firmly identified as having a terrestrial origin.

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It likely formed as layers of rock deformed under pressure, folded and were pushed upwards to the surface to form what geologists call an 'anticline'. Since the layers of rock lower down were pushed up furthest at the centre of the fold, as the rock was eroded away over the eons, it exposed a tree-ring like structure, with the youngest rocks near the edges of the formation and the oldest rocks in the centre.