A Sixth Tektite Field Changes What We Know About Earth's Impact History
Brazil has just joined an exclusive planetary club. For the first time, scientists have confirmed a major tektite field—natural glass formed by the extreme heat and pressure of a cosmic collision—on South American soil. The discovery, published in the journal Geology, identifies a strewn field of impact glass stretching across more than 900 kilometers, filling a critical gap in humanity's incomplete map of where asteroids and comets have struck Earth.
The glasses, named geraisites after the Brazilian state of Minas Gerais where they were first found, represent a collision event that occurred approximately 6.3 million years ago, during the final stages of the Miocene epoch. Researchers led by Álvaro Penteado Crósta at the State University of Campinas have now cataloged over 600 specimens, ranging from fragments smaller than a gram to pieces weighing up to 85 grams. This discovery matters because tektite fields are rare—only five major ones had been recognized globally before this find.
Why This Matters for Impact Science
Until now, the known tektite fields were concentrated in Australasia, Central Europe, the Ivory Coast region, North America, and Belize. South America's geological record of large impacts remains sparse and heavily skewed toward much older events. The Brazilian field plugs a significant gap, suggesting that major cosmic collisions may have been more frequent than the terrestrial record indicates—or, alternatively, that smaller impact events go unrecognized because their remnants are mistaken for ordinary volcanic glass.
The geraisites themselves are distinctive. At first glance, they appear black and opaque, marked by tiny cavities where gas bubbles escaped during rapid atmospheric cooling. Under intense light, they reveal a translucent, grayish-green hue—distinctly different from the vivid green moldavites found in European tektite fields, which have been prized for jewelry since medieval times. These color and composition differences serve as geochemical fingerprints, confirming the geraisites' impact origin rather than volcanic or terrestrial sources.
The Forensic Evidence
Geochemical analysis clinches the case. The glasses contain 70.3% to 73.7% silica and show remarkably low water content—between 71 and 107 parts per million. By contrast, volcanic glasses like obsidian typically contain 700 ppm to 2% water. This extreme desiccation is a signature trait of tektites, created when molten material is ejected into the upper atmosphere and cools too rapidly for water to be retained. The presence of lechatelierite, a glassy silica form that requires extreme temperatures to form, further confirms an impact mechanism.
Argon isotope dating from three independent analyses all converge on an age of roughly 6.3 million years, with minimal variance. This consistency points to a single, discrete impact event rather than multiple strikes over time.
The Missing Crater and What Comes Next
Here's the mystery: no crater has been found. This is less unusual than it sounds—only three of six classical tektite fields worldwide have associated craters. The Australasia field's impact crater is believed to be submerged in the ocean; the Brazilian impact likely struck continental crust of the São Francisco craton, one of Earth's oldest and most stable geological features. Researchers are now planning aerogeophysical surveys—magnetic and gravimetric mapping—to hunt for buried or eroded crater structures.
The next phase involves sophisticated impact modeling. By analyzing the spatial distribution of geraisites and their physical properties, scientists aim to calculate the impacting body's velocity, angle of entry, and the energy released. Early indications suggest a substantial collision, though smaller than the Australasia event.
This discovery underscores a broader truth: Earth's impact catalog remains incomplete, and the planet's cosmic collision history may be more eventful than we've realized.
