An international contingent of geologists has identified the inaugural field of tektites in Brazil—natural glass typically created when an asteroid impacts the Earth with immense force. Previously, only five such tektite fields were known across the globe. This finding not only expands the registry of these impact zones but also draws the scientific community’s attention to a prehistoric cataclysm in South America that went unrecognized until now.
Our planet is continuously bombarded from outer space. Daily, tons of meteoritic material enter the atmosphere, most of which incinerates long before reaching the ground. Occasionally, larger objects strike. These events result in impact craters, such as the renowned meteor crater in Arizona, USA, which spans over a kilometer in diameter. Nevertheless, over time, these kinds of features can be obliterated by tectonic plate movement and erosion.
A meteorite impact releases massive amounts of energy, generating intense heat and pressure; some rocks melt, while others vaporize. The resulting incandescent liquid material is ejected outwards. During its flight, it rapidly cools and solidifies, transforming into glassy bodies. Scientists term the smaller fragments spherules and the larger ones tektites.
Five extensive tektite fields are currently recognized worldwide. Each is linked to an ancient impact event and aids in reconstructing the history of Earth’s collisions with cosmic bodies. This roster has now been augmented by the field discovered in Brazil. The breakthrough was made by an international geological team led by Alvaro Crosta from the Institute of Geosciences at the State University of Campinas.
Crosta and his team located the tektites in the state of Minas Gerais across a 90-kilometer stretch. Approximately 600 specimens were collected. The smallest weigh less than a gram, while the largest reach 85.4 grams. These samples exhibit the typical “aerodynamic” shapes characteristic of tektites: spheres, droplets, discs, and dumbbells.
The principal challenge was definitively proving they were tektites and not volcanic glass, as they are visually almost indistinguishable. Spectroscopic analysis proved crucial, revealing an exceptionally low water content: only 0.0071–0.0107 percent. For contrast, volcanic glass contains up to two percent water—meaning volcanic glass harbors roughly 200 to 300 times more water.
Tektites are considered among the driest substances on Earth. Their formation involves intense degassing during extreme heating, causing the water to virtually all “boil off.”
Furthermore, the tektites contained lechatelierite—an almost pure silica form that arises when quartz melts under exceptionally high temperatures, such as those generated by meteor impacts.
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Following the publication of Crosta’s team’s paper in the journal Geology, reports of tektite finds began arriving from the neighboring states of Bahia and Piauí. Consequently, the debris dispersal area expanded to over 900 kilometers. Such an expansive scale points toward a tremendously powerful event.
The scientists dated the “glass” using the ratio of argon isotopes ($\text{}^{40}\text{Ar}/\text{}^{39}\text{Ar}$), determining its age to be 6.3 million years. The team suggested that the precise age might be refined following further analyses, but the reality of an ancient impact is now undisputed.
As for the impact crater itself, it remains unfound. No features in Brazil or neighboring countries match the required age profile. However, geologists do not see this as a contradiction. Impact craters have only been discovered for three of the six known tektite fields globally. The others have either not yet been located, have been erased by erosion, or are concealed beneath younger sedimentary layers.
Isotopic analysis yielded another clue. It revealed that the asteroid struck the ancient continental crust, which is over three billion years old. Most likely, the impact occurred over the so-called São Francisco Craton, one of South America’s oldest geological blocks of crust. If this hypothesis holds true, residual evidence of the crater should be sought precisely in that region.
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