Massive asteroid impact 6.3 million years ago left giant glass field in Brazil

The findings were detailed in the journal Geology by a research team led by Álvaro Penteado Crósta, a geologist and senior professor at the Institute of Geosciences at the State University of Campinas (IG-UNICAMP). The project involved collaborators from Brazil, Europe, the Middle East, and Australia.

Before this discovery, only five major tektite fields were known worldwide, located in Australasia, Central Europe, the Ivory Coast, North America, and Belize. The Brazilian field now joins this rare group.

A 900 Kilometer Strewn Field of Impact Glass

The geraisites were first documented in three municipalities in northern Minas Gerais — Taiobeiras, Curral de Dentro, and São João do Paraíso — across an area about 90 kilometers long. After the study was submitted, additional finds were reported in Bahia and later in Piauí. As a result, the total known distribution now stretches more than 900 kilometers.

“This growth in the area of occurrence is entirely consistent with what is observed in other tektite fields around the world. The size of the field depends directly on the energy of the impact, among other factors,” Crósta explains.

By July 2025, researchers had collected about 500 pieces. With more recent discoveries, that total now exceeds 600. The fragments vary widely in size, from less than 1 gram to 85.4 grams, and can measure up to 5 centimeters along their longest dimension. Their forms match the aerodynamic shapes typical of tektites, including spheres, ellipsoids, droplets, disks, dumbbells, and twisted shapes.

What the Geraisites Look Like

At first glance, the geraisites appear black and opaque. Under strong light, however, they become translucent with a grayish green hue. This shade differs from the brighter green moldavites of Europe, which have been used in jewelry since the Middle Ages. The surfaces of the Brazilian specimens are pitted with small cavities.

“These small cavities are traces of gas bubbles that escaped during the rapid cooling of the molten material as it traveled through the atmosphere, a process also observed in volcanic lava but especially characteristic of tektites,” says Crósta.

Chemical Clues Confirm Impact Origin

Laboratory analysis shows that the geraisites contain high levels of silica (SiO2), ranging from 70.3% to 73.7%. Sodium (Na2O) and potassium (K2O) oxides together account for 5.86% to 8.01%, slightly higher than what is seen in other tektite regions. Trace elements such as chromium (10-48 parts per million) and nickel (9-63 ppm) vary in small amounts, suggesting the original target rock was not uniform. Researchers also detected rare inclusions of lechatelierite, a high temperature glassy silica that forms during extreme heating, further confirming an impact origin.

“One of the decisive criteria for classifying the material as a tektite was its very low water content, as measured by infrared spectroscopy: between 71 and 107 ppm. For comparison, volcanic glasses, such as obsidian, usually contain from 700 ppm to 2% water, whereas tektites are notoriously much drier,” Crósta points out.

Dating the Ancient Asteroid Impact

Argon isotope dating (⁴⁰Ar/³⁹Ar) indicates the impact occurred around 6.3 million years ago, near the end of the Miocene epoch. Three closely grouped age results were obtained (6.78 ± 0.02 Ma, 6.40 ± 0.02 Ma, and 6.33 ± 0.02 Ma), supporting the conclusion that they came from a single event.

“The age of 6.3 million years should be interpreted as a maximum age since some of the argon may have been inherited from the ancient rocks targeted by the impact,” the researcher comments.

The Search for a Missing Crater

No crater linked to the impact has yet been identified. According to Crósta, this is not unusual. Only three of the six major classical tektite fields have confirmed craters. In the case of the vast Australasia field, the crater is thought to lie beneath the ocean.

Isotopic geochemistry suggests the molten material came from Archean continental crust dating between 3.0 and 3.3 billion years old. That evidence points to the São Francisco craton, one of the oldest and most stable regions of South America’s continental crust.

“The isotopic signature indicates a very ancient continental, granitic source rock. This greatly reduces the universe of candidate areas,” says Crósta.

Future surveys using magnetic and gravimetric techniques could detect circular underground structures that mark a buried or eroded crater.

Estimating the Size of the Impact

Researchers cannot yet determine the exact size of the object that struck Earth, but they believe it was not small. The volume of melted rock and the broad distribution of debris indicate a powerful event, though likely less intense than the impact that created the enormous Australasia field, which spans thousands of kilometers.

The team is developing mathematical models to estimate the impact’s energy, entry speed, trajectory angle, and total volume of melted material. These calculations will become more refined as additional data on the distribution of geraisites are gathered.

The discovery adds an important chapter to South America’s impact history. Only about nine large impact structures are currently known on the continent, most of them much older and located in Brazil. The findings also suggest that tektites may be more widespread than previously recognized, but are sometimes overlooked or mistaken for ordinary glass.

Separating Science From Speculation

To address exaggerated claims about asteroid threats, Crósta works with undergraduate students to manage the Instagram account @defesaplanetaria. The page focuses on science communication and aims to distinguish genuine risks from unfounded speculation about meteorites and asteroids.

Impacts were common in the early solar system, when debris was abundant and planetary orbits were unstable. Large bodies shifted positions, sending smaller objects in many directions. Today, the solar system is far more stable, and major impacts are much less frequent.

“Understanding these processes is essential to separating science from speculation,” the researcher concludes.

Crósta has studied meteorite impact structures since his master’s research project in 1978. Over the years, he has received several grants from FAPESP (08/53588-7, 12/50368-1, and 12/51318-8).