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The Richat Structure – Mauritania


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The Richat Structure

It is one of the nature`s wonders that scientists cannot identify the main reason for its shape, it is a giant forest reaches 30 miles diameter that you can only see it from the outer space till the time being they cannot solve the riddles of this place



The Richat Structure, also known as the Eye of the Sahara and Guelb er Richat, is a prominent circular feature in the Sahara desert of west – central Mauritania near Ouadane. This structure is a deeply eroded, slightly elliptical, 40-km in diameter, dome. The sedimentary rock exposed in this dome range in age from Late Proterozoic within the center of the dome to Ordovician sandstone around its edges. The sedimentary rocks comprising this structure dip outward at 10°-20°. Differential erosion of resistant layers of quartzite has created high-relief circular cuestas. Its center consists of a siliceous breccia covering an area that is at least 3 km in diameter.
Exposed within the interior of the Richat structure are a variety of intrusive and extrusive igneous rocks. They include rhyolitic volcanic rocks, gabbros, carbonatites and kimberlites. The rhyolitic rocks consist of lava flows and hydrothermally altered tuffaceous rocks that are part of two distinct two eruptive centers, which are interpreted to be the eroded remains of two maars. According to field mapping and aeromagnetic data, the gabbroic rocks form two concentric ring dikes. The inner ring dike is about 20 m in width and lies about 3 km from the center of Richat Structure. The outer ring dike in about 50 m in width and lies about 7 to 8 km from the center of this structure. Thirty-two carbonatite dikes and sills have been mapped within the Richat structure. The dikes are generally about 300 m long and typically 1 to 4 m wide. They consist of massive carbonatites that are mostly devoid of vesicles. The carbonatite rocks have been dated as having cooled between 94 to 104 million years ago. A kimberlitic plug and several sills have been found within northern part of the Richat structure. The kimberiite plug is estimated to be about 99 million years old. These intrusive igneous rocks are interpreted as indicating the presence of a large alkaline igneous intrusion that currently underlies the Richat structure and created it by uplifting the overlying rock.



Spectacular hydrothermal features are a part of the Richat structure. They include the extensive hydrothermal alteration of rhyolites and gabbros and a central megabreccia created by hydrothermal dissolution and collapse. The siliceous megabreccia is at least 40 m thick in its center to only a few meters thick along its edges. The breccia consists of fragments of white to dark gray cherty material, quartz-rich sandstone, diagenetic cherty nodules, and stromatolitic limestone and is intensively silicified. The hydrothermal alteration, which created this breccia has been dated at using the 40Ar/39Ar method as having occurred at about 98.2 ± 2.6 million years ago.
Initially interpreted as a asteroid impact structure because of its high degree of circularity, it is now argued to be a highly symmetrical and deeply eroded geologic dome. Despite extensive field and laboratory studies, geologists have found a lack of any credible evidence for shock metamorphism or any type of deformation indicative of a hypervelocity extraterrestrial impact. Coesite, an indicator of shock metamorphism, had been reported as being present in rocks samples collected from the Richat structure. As the result of the further analysis of rock samples from this structure, it was concluded that barite had been misidentified as coesite. In addition, the Richat structure lacks the annular depression that characterize large extraterrestrial impact structures of this size. Also, it is quite different from large extraterrestrial impact structures in that the sedimentary strata comprising this structure is remarkably intact and "orderly" and lacking in overturned, steeply-dipping strata or disoriented blocks. A more recent multianalytical study on the Richat megabreccias, concluded that carbonates within the silica-rich megabreccias were created by low-temperature hydrothermal waters, and that the structure requires special protection and further investigation about its origin..



Imagine if you were an alien species intent on conquering the earth by force.  Now, you might just appear over the various capital cities of the world and wait for your countdown to get to zero or you might, being a little timid of the explosive force that you are about to unleash, wish to do it from a safe distance.  What you would need to look for, then, is a handy bull's-eye – on the bull's-eye that is the Earth itself.

Look no further, alien invader.  The Richat Structure in Mauritania provides the perfect target towards which you can aim your death ray, annihilation laser or whatever you call your extraterrestrial weapon of mass destruction.  It’s almost as if another species, in a previous visit, had chalked in a target already and then become bored and wandered back to Betelgeuse.

However, although to all intents and purposes it looks as if it could have been lasered in to the landscape the Richat Structure – (when not described as a bulls-eye it is called the outsized ammonite) is a completely natural phenomenon.  Still unknown to many it gained the attention of the first astronauts because, unlike certain great walls, it is so very visible from space.

The Sahara Desert, in which the structure is situated, does look rather featureless from above.  So when astronauts glimpsed this prominent almost circular mark – with a diameter of almost 50 kilometers – it piqued their interest.  It has, since the early days of space exploration become something of a landmark for our planet’s space farers.  It is also occasionally called the Eye of Africa – and you can see why.

It was for a number of years thought to be the impact crater of an ancient meteor strike against the Earth – as much because it has a high degree of circularity than any real scientific hypothesis.  However, it is now thought to be what is known as a symmetrical uplift which, over the millennia has become visible (from space at least) due to the erosive effects of desert winds.

Geologically speaking the Richat Structure is known as a dome.  A dome consists of symmetrical but dipping anticlines.  Put simply an anticline is a fold in the earth consisting of various beds with the very oldest at its core.  Think of the top part of a babushka doll from Russia, with all the heads inside the largest one, get x-ray vision and you have a general visual picture of an anticline.

As the millennia progress so various layers of the anticline are exposed by erosion – at least in the case of the Richat Structure.  The rings you can see forming the ‘beds’ are made of Paleozoic quartzite which is between 200 and 500 million years old.  The dome explanation is thought the best – the lack of what is known as shock metamorphism (rock altered by the impact) at the structure puts a further dint in the theory that it was cause by a meteor.

Legend has it that there is a hotel smack bang in the middle of the structure – yet that is a myth easy to disprove by simply looking at images of the Richat Structure.  The ‘hotel’ is more a collection of wooden huts, there for the few hardy tourists who make it there and they probably sing We Don't Need Another Hero around the camp fire when they do. Why the Richat Structure is almost circular, however, remains a mystery yet to be solved.

aster

 

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