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GEOSCIENCES

Scars of a cataclysm in Central Brazil

Rocks found in the states of Mato Grosso, Goiás, and São Paulo reveal earthquake and tsunami caused by a meteorite some 250 million years ago

Between 2012 and 2015, American geologist Eric Tohver drove thousands of miles through São Paulo, Goiás, and Mato Grosso. Then a researcher at the University of Western Australia, he visited quarries and cliffs near Brazilian roadsides in search of 250-million-year-old rock formations, hoping to find evidence of the destructive impact of the meteorite that created the largest crater in South America.

Just over 250 million years ago, a meteorite with an estimated diameter of 4 kilometers (km), traveling at up to 17 kilometers per second, collided with the Earth in what is now the heart of Brazil, leaving a 40 km crater that is today home to the municipalities of Araguainha and Ponte Branca, in the state of Mato Grosso. Some years ago, Tohver and researchers from Brazil, the United Kingdom, and Australia estimated that this highly destructive impact—which released millions of times more energy than the atomic bombs dropped on Japan at the end of World War II—would have instantly annihilated everything within a 250 km radius. The force of the shock would also have sent a huge amount of dust and water vapor into the atmosphere, as well as 1,600 gigatons of methane, a greenhouse gas that may have contributed to higher global temperatures at the time and the worst mass extinction event in the Earth’s history (see Pesquisa FAPESP issue no. 211).

In an article published in the journal Geological Society of America Bulletin in January this year, the American geologist and his colleagues described having found scars of this destruction almost 1,000 km from the crater. “We had already speculated through previous research that the damage could have reached such a distance,” says Tohver, a visiting professor at the Institute of Astronomy, Geophysics, and Atmospheric Sciences of the University of São Paulo (IAG-USP). “Now we have evidence that it actually did.”

At the point of impact, the meteorite carved a hole almost 2 km deep and displaced huge blocks of granite, resulting in the formation of the Arnica mountain range in the center of the crater (see Pesquisa FAPESP issue no. 140). In the moments following the collision, a mega-earthquake with a magnitude of up to 10.5 on the Richter scale, tens of times more powerful than the most devastating earthquakes on record, would have caused the ground to tremor thousands of miles away.

At the time, the continents were grouped in a single mass of solid earth called Pangeia, which extended from the very north of the planet all the way to the south pole. The rocky blocks that would later become Brazil were located in the southwest of this supercontinent, and the region between the states of Mato Grosso and Goiás and the north of Argentina were covered by water. The earthquake shook and broke apart the layers of mud and sand at the bottom of this immense body of water, generating structures known as clastic dikes that now appear as vertical shafts of rock cutting through a larger mass of a different rock type (see infographic).

Crystals found in rocks formed after the impact are close to the age of a major extinction event

Clastic dikes and tsunamiites
Tohver explored the Brazilian countryside with German geologist Martin Schmieder and Brazilian geologists Claudio Riccomini, from USP, Lucas Warren, from São Paulo State University (UNESP) in Rio Claro, and Cristiano Lana, from the Federal University of Ouro Preto (UFOP), where they discovered clastic dikes from the time over a vast region of what would have been the bottom of a shallow sea or a lagoon, now buried by the Paraná sedimentary basin. They identified such formations in the municipality of Alto Araguaia in Mato Grosso, 50 km from the center of the crater, and as far as four times farther away in Mineiros and Montividiu, in Goiás. They also found a larger number of clastic dikes from the same period in the municipalities of Rio Claro, Piracicaba, and Limeira, almost 1,000 km from the point of collision, and there are indications that there are more of these dikes in the states of Paraná and Santa Catarina. “If the effects observed at such a distance from Araguainha really were caused by the impact of the meteorite, the earthquake it caused may have had a magnitude of 10 or more,” says seismologist Marcelo Assumpção, who coordinates the USP Seismology Center and did not participate in the research.

In the rock layer immediately above the dikes, Tohver and his colleagues found evidence of another effect of the impact: a powerful tsunami. Aptly named tsunamiites, these formations contain debris and rock fragments, some of which are not native to the area where they are deposited. “In the minutes that followed the earthquake, a huge tsunami would have swept across an area of thousands of kilometers,” Tohver says.

The researchers are interested in tsunamiites because as well offering evidence of specific phenomena, they can contain zircon crystals, an extremely hard mineral (the oldest of which are almost the same age as the planet) that acts as a kind of geological clock. Microscopic zircon crystals extracted from tsunamiites found near Porangaba in São Paulo and Santa Rita do Araguaia in Goiás were formed around 253 million years ago, according to Tohver. This figure is very close to the estimated age of the impact (254.7 million years) calculated by dating rocks from inside the crater.

“It is difficult to estimate the exact age of the crater because the dating methods for this age range are still imprecise, varying by up to 3 million years,” explains geologist Alvaro Crósta, from the University of Campinas (UNICAMP), one of Brazil’s leading experts on craters. “I believe the current estimate is very close to the actual age of the impact.” In the early 1980s, Crósta demonstrated that Araguainha was formed by the impact of a meteorite and estimated its age at 285 million years. “At the time, I knew there was a high chance of error because the technique used was not the most accurate.”

Tohver is pleased that the most likely age of impact is 253 million years because it would mean the meteorite hit the Earth around the same time as a mass extinction event that eliminated 90 percent of life on the planet 252 million years ago, marking the end of the Permian geological period and the beginning of the Triassic period. In 2013, Tohver, Riccomini, Lana, and other scientists proposed that the meteorite that created the Araguainha crater was not the immediate cause of the extinction event, but that it contributed to climate change on a global scale that eventually led to the loss of almost all life. Its indirect effects would have been as devastating as those of another famous meteorite that later created the 180-kilometer Chicxulub crater in the Gulf of Mexico and contributed to the extinction of the dinosaurs 65 million years ago. “It is plausible that the Araguainha impact and the subsequent earthquake, tsunami, and expulsion of methane were the trigger for a major extinction event,” says Warren, who also works in paleontology.

It is a controversial proposal. The prevailing theory among geologists and paleontologists is that the Permian extinction was caused by climate change after massive volcanic activity in Siberia. It has also been suggested that other impact craters—around the same age as Araguainha but larger, such as the Wilkes Land crater in Antarctica and the Bedout crater off the coast of Australia—contributed to the catastrophe. But their origins have never been proven. “The Araguainha impact certainly would have caused a major change locally, including the extinction of some fauna and flora species on a regional scale, but I would not say that it triggered a mass extinction,” says geologist Elder Yokoyama, a professor at the University of Brasilia (UNB) who has already studied the crater.

For Max Langer, a professor of paleontology at USP in Ribeirão Preto, the new Araguainha dating could lead to a review of the Permian extinction hypotheses. After all, “until the Chicxulub crater was discovered,” he says, “there were several other theories explaining the extinction of dinosaurs.”

Scientific article
TOHVER, E. et al. End-Permian impactogenic earthquake and tsunami deposits in the intracratonic Paraná Basin of Brazil. Geological Society of America Bulletin. Jan. 2, 2018.