Most people are familiar with the geologic stars of the Earth's surface. Uluru clings to the Outback like a languishing Elle Macpherson. The Grand Canyon gets its colossal groove on to continuous tourist applause. Even the indie-favorite Drygalski Ice Tongue has a geologic sexuality that begs for Space-shuttle paparazzi attention. But for pure eye-of-Terra impact, you need to meet the Richât structure.

Latitude: N 21° 04'
Longitude: W 11° 22'

From space the Richât structure looks like a gigantic, wrinkly, gray-blue chameleon's eye peering skyward from Mauritania's otherwise featureless, sandy Maur Adrar Desert. It is a 38-kilometer-diameter series of concentric rings. It's so big that people didn't really know it existed until we started flying overhead on space missions. Can you imagine what James McDivitt and Ed White thought as they first glimpsed it from Gemini 4 in the summer of 1965? Looking in awe at the peaceful curve and exquisite intricacy of the earths surface, something slowly slid into view that was looking right back.

The "white" of this eye is the barren Gres de Chinguetti Plateau, rising some 200 feet above the surrounding desert. The rings are sharp-peaked ranges. The structure gets its blue color from Paleozoic quartiztes in the layers of eroded rock. Sadly, sand from the abutting desert has been slowly creeping in from the south and is occluding the "white" of eye in geologic time. But it is looking like the winds won't be able to get the sands over the outermost ridges of the "iris", so at most over the next thousand years it may develop a sandy white outline, exaggerating its eye-like appearance.

When you first see it you might think that it's an impact crater like nearby Aouelloul to the southeast and Tenoumer to the north, but it's not. If it was it would have slightly different physical characteristics. (Stronger dips in the strata, shatter cones and other shock-metamorphic effects, etc.)

Two other possibilities might explain the geological oddity.

1. Volcanic dome
The US Geological Survey maintains that it must have been formed by a volcanic dome that eroded, exposing its layers. Kind of like a jawbreaker that you bite into.

2. Anticline
But NASA disagrees. It points out that there is no igneous dome or even volcanic rock. It says that it was formed by erosion of uplifted sedimentary rock (known in geology circles simply as an anticline). But if this is an anticline, what did the uplifting? And why is it so round? Erosion and geologic drift tend to produce long lines of effects, not geometric shapes.

Still awaiting completion of their time machine, geomorphologists will just have to pick their side of this debate and stick to it. In the meantime, we can marvel at this beautiful landmark from the many images that are freely available online.

  • Big image (2 MB) is at http://www.solarviews.com/huge/earth/richat.jpg
  • Giant image (9 MB) at http://www.lpi.usra.edu/publications/slidesets/geology/images/tifs/sgeo_s13.tif

    Other images
  • http://edcw2ks15.cr.usgs.gov:8090/imagegallery/silverstream/pages/pgImageDetail.html?ImageID=162
  • Landsat 7, care of NASA: http://antwrp.gsfc.nasa.gov/apod/ap021028.html

Sources
  • http://www.image-contrails.de/mauritania/mauritania-met-richat-txt.html, which, in turn cites...
  • Geomorphology from Space: 1986; NASA publication edited by Nicholas M. Short, Sr. and Robert W. Blair, Jr.
  • http://antwrp.gsfc.nasa.gov/apod/ap021028.html

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