Meet the Eye of the Sahara

Let’s dive down into one of the most interesting formations in the world!

A structure known as the Eye of the Sahara is an ancient circular geological formation in the Sahara Desert. This intriguing phenomenon is known as the Richat Structure or Guel er Richat in Arabic and its home is the Adrar Plateau in the Sahara. The Eye is located near Ouadane in the Adrar Region of Mauritania. In the local dialect, rīšāt means feathers and the structure is also known locally in Arabic as tagense, referring to the circular opening of the leather pouch that is used to draw water from wells, that it seems to resemble.

Initially, geologists believed it to be a gigantic impact crater but subsequent analysis of the sedimentary rocks inside the central dome have revealed it to be from the late Proterozoic, which has a time period of 1 billion to 542 million years ago. Impact craters on the other hand would have to be many more millennia older.

Image of the Richat Structure, Eye of the Sahara, from the International Space Station, 17 December 2011. Credit: NASA Earth Observatory

Taken by an astronaut onboard the International Space Station (ISS), this photograph places the Richat Structure—a geologic feature in Mauritania characterized by its concentric rings—in context with the extensive dune fields that surround it. Credit: NASA Earth Observatory

According to NASA: “In this type of geologic structure, rocks exposed in the center of the “bullseye” are older than rocks forming the outer rings. This structure measures 45 kilometers (28 miles) across and is made up of igneous and sedimentary rocks. There are several faults visible (lower left) where strata that were once continuous have been shifted apart.”

The Eye is an eroded geological dome, approximately 40 km (25 mi) in diametre. This dome exposes sedimentary rock in layers that appear as concentric rings, with additional igneous rocks exposed inside. Furthermore, there are spectacular rhyolites (the most silica-rich of volcanic rocks) and gabbros (igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma) that have undergone hydrothermal alteration. The rocks that the rings comprise of erode at different speeds and this creates the variety of coloured patterns seen across the surface. There are also pieces of other angled rocks known as megabreccia that add to the colours of the formation. Inside the dome is what is known as a limestone-dolomite shelf that comprises of a one kilometre-wide breccia, ring dikes, and alkaline volcanic rocks.

Satellite picture of the Richat Structure (false colour). CC BY

Richat Structure: A topographic reconstruction scaled 6 to1 on the vertical axis from satellite photos. False colouring as follows: Brown bedrock, Yellow or white sand, Green vegetation, Blue salty sediments. Image: NASA/JPL/NIMA

The Eye of the Sahara is likely to have formed through a process called “folding”, which happens when a body of rock, especially sedimentary rock, is squeezed from the sides by tectonic forces. This squeeze fractures it or creates faults if it is cold or brittle. But if it is warm enough it behaves with plasticity and becomes folded. Upward pointing folds are known as anticline and downward ones are called syncline.

Acheulean archaeological artifacts (stone-tool industry of the Lower Paleolithic Period characterised by bifacial stone tools with round cutting edges and typified especially by an almond-shaped flint hand ax measuring 8–10 in) were also found here and it was selected as as one of the first 100 geological heritage sites identified by the International Union of Geological Sciences (IUGS) to be of the highest scientific value.

Fun fact: The Eye of the Sahara was used by the Gemini spacecraft astronauts in the 1960s as a landmark.

A study from 2014 proposed that the separation of the supercontinent Pangaea may have played a part in the formation of the Eye of the Sahara. There is volcanic rock present in the structure suggesting that molten rock was pushed to the surface during this event, thus creating the dome shape, before being eroded into the ring formations that can be seen today.

Topographic map of Guelb el Richat. Elevation in meters. 10 m contour interval with major contour line every 50 m

Home to 3.7 million peope, most of Mauritania lies within the Sahara Desert, where dry climate conditions and periodic droughts create difficult living conditions. Most of the population lives along its Atlantic coastline, about 500 kilometers (300 miles) from the Richat Structure.

The amazing Eye of the Sahara structure has fascinated geologists ever since it was discovered and it is still considered one of the most important global geological sites. The best way to see it is from high up in the sky or from space, just like the Gemini astronauts did!

NASA Images 

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