Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/646479
Bill Langer is a consulting research geologist who spent 41 years with the U.S. Geological Survey before starting his own business. He can be reached at Bill_Langer@hotmail.com CARVED IN STONE 60 AGGREGATES MANAGER / March 2016 A while back, I was involved in a mineral resource appraisal of the Islamic Republic of Mauritania, a country in north- western Africa. My role was to describe the geology and market potential for development of about 15 industrial minerals including two of my favorites — aggregate and dimension stone. Mauritania has about 4,760 miles of roadways, 540 miles of which are paved. The roads, particularly those in the countryside, are dilapidated, and the extreme desert climate makes maintenance difficult. Road construction is a top priority for the government. Unfortunately, Mauritania has a real prob- lem finding sources of quality aggregate. There is plenty of sand; not much gravel. And although there are a number of rocks that may be suitable for use as crushed stone, many are not near population centers where the aggregate is needed. Dimension stone is another story. There is a railroad line for carrying iron ore from the mines at Zouérat to the port at Nouadhibou. Some potential sources of dimension stone occur near the rail line. If space can be made available on the rail line, dimension stone might be marketable. I think some of the rocks are downright gorgeous. Of course, the attractiveness of any particular rock considered for use as dimension stone is subjective and can be best explained as being "in the eye of the beholder." And speaking of the eye of the beholder, let's get to the real topic of this article — the "Eye of the Sahara." While that mon- iker might sound like the sequel to the adventure movie Romancing the Stone, it's not. The Eye of the Sahara is a massive, 25-mile diameter circular rock formation that stands out in the desert (and on the geologic map I was using) like a target. It is easier seen from above than from the ground and has long been a landmark for space crews. The Eye was once thought to be the result of a meteorite impact. However, it lacks shattered glass or shocked quartz associ- ated with high-energy impact craters. A subsequent theory is that it is a volcanic caldera, but the area lacks any of the defin- ing characteristic of volcanoes. The reigning theory at the moment is that the Eye of the Sahara is the result of differential erosion of a geologic dome. About 100 million years ago, as the supercontinent Pangaea broke up and the Atlantic Ocean opened up, an intrusion of mag- ma slowly pushed up from the earth's mantle into the crust, creating a dome. Over millions of years, the dome slowly shed layers of rock under the erosive power of wind-blown sand. Resistant quartzite beds mark the rings as high ridges while less resistive beds eroded into valleys. Today, the geologic feature is exposed as a relatively flat cross-sectional bulls-eye with the oldest rocks peeking out of the center with younger rocks occurring as rings getting progressively younger as one moves outward from the center of the Eye. This NASA photo clearly shows the high, circular quartzite ridges and low soft-rock valleys. The yellow areas on the left are windblown sand. Vegetation in the drainage channels is green, and blue shows areas of temporarily ponded water. I suspect it is unlikely you will ever fly over the Eye in a spacecraft. Instead, try visiting it via Google Earth. In the search box enter the coordinates 21 08', -11 24'. Zoom out to an eye altitude of 50 miles. How cool is that! Long used as a landmark by space crews, this 5-mile rock formation is likely the remains of a geologic dome. The Eye of the Sahara A quarry in the lava near the Glasshouse Mountains with Mount Tibrogargan in the background.