2023-12-12 18:39:22
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In 1933, during an expedition to the Libyan desert, scientists discovered numerous strange small pieces of yellow glass in the sand. Results from the fusion of rocks containing silicasilica, natural glasses are not specifically rare. They are indeed found in many places around the world. Their formation, however, requires high temperatures, which explains why the glasses are generally found in volcanic contexts or meteorite impacts.
A debated origin
However, the fragments discovered in the Libyan desert are in no way linked to a volcanic eruptionvolcanic eruption, and seem very different from the usual tektites produced during the fall of meteorites. Libyan glasses are in fact much richer in silica than the latter. They also come in large fragments and are found in large quantities in the desert. Some studies had concluded that they could be fulguritesfulgurites, glasses produced by the fusion of sand during lightning impacts in the desert. A theory which, however, did not achieve unanimous support.
A meteorite impact, but no crater
The mystery could finally have been partly solved thanks to modern analysis techniques. A team of scientists has just determined that these glasses were produced under very high temperatures (around 2,500°C) and very high pressures (130,000 atmospheres). The results were published in the journal American Mineralogist.
Extreme conditions that are only encountered during a meteorite impact… or the explosion of an atomic bomb. But considering the age of the glasses, which is 29 million years, this last hypothesis is definitively invalidated! Although the origin of the fragments is now established, the story still contains several gray areas. In fact, no crater was found nearby. Considering the quantity of glasses, it should nevertheless be of a substantial size. Further research will therefore still be necessary to fully lift the veil on the origin of Libyan desert glass.
A meteorite fall is behind the mystery of yellow glass in the desert
Article by Laurent SaccoLaurent Sacco, published on May 20, 2019
Libyan glass is a natural rock found in the desert, mainly in Egypt. It is most likely the product of the heat released by the arrival of a celestial body. It is now believed that this heat was indeed produced by the formation of an impact crater and not by an event similar to that of the Chelyabinsk meteorite.
As its name does not indicate, the “Great Sea of Sand” of the Libyan Desert is located mainly in Egypt, and only overflows into Libya, Sudan and Chad. There is an oval region of approximately 130 km of north-south extension (latitudeslatitudes N 25°02′ – N 26°13′) and 50 km of west to east (longitudeslongitudes E 25°24′ – E 25 °55′). It is famous in the world of meteorite enthusiasts because you can discover samples of a strange yellow to light green rock, with a more or less transparent appearance, called “Libyque glass”.
For almost a century, this glass has intrigued naturalists, such as Théodore MonodThéodore Monod, and there are good reasons to believe that, just like the iron of Tutankhamun’s dagger, it has a meteoritic origin although it is indirect. Moreover, Libyan glass was known in the time of the pharaohs in particular because it was used to make the scarab which adorns the pectoral of Tutankhamun (born around -1345, died around -1327). In fact, it had obviously been used by humans for longer than that, as many Libyan glass cutting sites are associated with the presence of prehistoric tools from the Neolithic (9,000 BCE), when there was then a “Green” Sahara with rivers and lakes full of fish, and men lived there surrounded by elephants and hippos.
Libyan glass is composed of 98% silica and 2% aluminalumina, and contains some traces of iron oxide, titanetitanium and zirconiumzirconium. It was the potassium-argon method that made it possible to date it, but above all the fission trace method. We then obtain ages ranging from approximately 29.5 to 28.5 ± 0.4 million years. Several theories explaining its origin have been proposed which can be grouped into two classes: that involving very terrestrial phenomena and that involving the fall of a small extraterrestrial body, meteorite or comet.
Excerpt from the show Impending disaster, meteorite impact. The Chelyabinsk meteor could have caused significant damage as evidenced by the craters left by the impacts of other meteors. But the forces exerted on the celestial object caused it to implode in the atmosphere.© Nat Geo France
An explosion of 100 megatons of TNT?
It was this last hypothesis that ultimately prevailed but the discussion still continues to know whether Libyan glass is the product of the fusion of terrestrial rocks during the formation of an impact crater or simply the product of radiation. of a celestial body having exploded in the atmosphere. In the latter case, researchers report the equivalent of the famous Tunguska explosion in 1908, and more recently that of the Chelyabinsk meteorite. This would have resulted in a ball of fire similar to that of a nuclear explosion whose thermal radiation would have heated the sand of the Sahara to form this glass comparable to trinitite, a rock created by the exposure of sand to radiation from an atomic explosion.
But the power of the explosion at the origin of the Libyan glass, 100 megatons, must have been much greater than that of June 1908 – the energy of which was estimated, for its part, at 5 megatons of TNT, i.e. the equivalent of 600 Hiroshima type bombs, — and from a stronger to that of February 15, 2013, above the Russian city of Chelyabinsk, which was around 30 bombs, again, of the Hiroshima type.
No impact crater from the age of Libyan glass has been found which could be associated with its formation but the debate has undoubtedly just been relaunched with an article published in the famous journal Geology and from two researchers, Aaron Cavosie, from Curtin University in the USA, and Christian Koeberl, from the Natural History Museum in Vienna.
They examined zirconzircon grains in samples of Libyan glass to deduce that they had temporarily changed phase to become a type of mineral called reidite. It is named after the scientist who first created it at high pressure in the laboratory in 1969, Alan F. Reid. It is actually a very rare mineral on the Earth’s surface that is only found associated with celestial body impacts.
This passage of zircons in their transitional phase corresponding to reidite, before returning to their initial state, would not be possible with the pressures predicted by the hypothesis of an explosion in the atmosphere. We would therefore have finally decided between the two hypotheses but we can bet that the researchers will only be convinced when we have unambiguously identified a 29 million year old astrobleme astrobleme in the region of the “Great Sea of Sand” of the Libyan Desert.
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