Research reveals the Amazon’s volcanic past

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An area of ​​around 55 thousand square kilometers that comprises the Northeast of Mato Grosso and the South of Pará; characteristics of an ancient volcanic formation 

If today the Amazon region is dominated by lush tropical forest, billions of years ago the area had intense volcanic activity, something that is already widely known to geologists and researchers, but whose research, due to characteristics of the forest itself, is still a challenge.

Research by Unicamp's Geosciences Institute (IG) brought new contributions to the investigation of the Amazon's geological past. Through the analysis of images and the composition of rocks extracted from the region, the study discovered that the eastern portion of the so-called Alta Floresta Mineral Province — an area of ​​around 55 thousand square kilometers that comprises the Northeast of Mato Grosso and the South of Pará — presents characteristics of an ancient eroded volcanic caldera formation, as found today, for example, in Yellowstone National Park, in the United States.

The discovery is one of the results of André Kunifoshita's master's research and was published in the journal Geoscience Frontiers. The article is also authored by Maria José Mesquita, professor at IG and research advisor, and Felipe Holanda dos Santos, professor at the Federal University of Ceará (UFC).

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From left to right, supervising professors Felipe Holanda dos Santos (UFC), Maria José Mesquita (IG/Unicamp) and research author André Kunifoshita

Reconstruct the past

The first research into the Amazon's volcanic past dates back to the end of the 2th century. Since then, researchers have been working to understand aspects of this past through the evidence found today. The three periods of volcanic activity that formed what is now called the Amazon Craton initially occurred 1,88 billion years ago, followed by another period 1,78 billion years ago and a third, XNUMX billion years ago. The formations derived from this last period are part of the so-called Colíder Group, one of the types of rock that make up the Alta Floresta Mineral Province.

The Group brings together aspects that arouse the curiosity of geologists, such as a possible association with the copper and gold formations found in the region. According to Maria José Mesquita, it is not yet known whether there is a direct relationship between volcanism and the emergence of these metals, which justifies the research. The professor also highlights that, around 2 billion years ago, the planet began to oxygenate, giving rise to the first living beings. “There are several things at stake when studying this period”, she assesses.

The hypothesis of the formation of a volcanic caldera arises from the analysis of radar images of the relief near the city of União do Norte. This formation occurs when a chamber that concentrates the magma empties, causing the layers above them to collapse and descend, forming the circular structure with edges. Despite the evidence, analysis of the rocks that make it up is necessary to reinforce the hypothesis that the site was formed from volcanism. "As we are working with very old rocks, we do not have all the evidence of this formation. We collect clues to unravel what happened in the past", explains Kunifoshita.

Thus, samples of rocks from the region were collected for dating — using the uranium-lead method —, petrography and geochemical analysis. The researcher verified the existence of rocks characteristic of effusive volcanic events, marked by the greater fluidity of the magma, which facilitates the release of gases and a calmer eruption — as is the case with rhyolites —, and rocks from explosive, more violent eruptions, where the extravasated magma mixes with ash and fragments of other rocks — in this case, the lapilli tuffo and other pyroclastic rocks. "Our hypothesis is that they are rocks derived from the same period, but from different volcanic pulses. Compared to today, it is like what happens on Mount Etna, in Italy, which erupts from time to time", he explains.

In this way, the research mapped the stratigraphic stacking of the region, that is, how the sediments released by volcanic activity accumulated and formed layers. Through this, it was possible to find another piece of evidence that corroborates the hypothesis of the eroded caldera, which is the existence of dikes through which the magma flowed, forming rings and radial structures around the caldera.

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On the left samples of rocks with characteristics of volcanic events (rhyolites, lapilli tuffo, pyroclastic rocks); on the right maps of where the rocks were found

Pieces of a puzzle

Studies that seek to uncover the geological past of the Amazon are faced with challenges that range from the difficulty of reaching certain locations and the great distance between cities, to the weathering processes typical of tropical forests. “Several study areas are covered by vegetation. The forest itself makes it difficult to find rock outcrops, they are not continuous”, says André, who also highlights how constant rain contributes to the alteration of minerals and the formation of clayey soils.

However, the researchers emphasize that the volcanic rocks of the Colíder Group, even though they are very old, have not undergone metamorphism that has modified their main characteristics. “Everything is preserved. This makes it much easier, we see the structures in the way they were created, which is incredible”, highlights Mesquita.

Location maps where field research is being carried out
Figure A (on the left) identifies the Amazon Craton and B highlights the Alta Floresta Mineral Province

Now in his doctorate, André researches the entire extent of the Colíder Group in the Alta Floresta region. The information obtained contributes not only to understanding the geological past, but also what can be expected from the present, such as the minerals currently available in the region. A complex work, worthy of the grandeur of billions of years of history. “It’s like finding parts of a puzzle,” he summarizes.

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Mapping location in União do Norte, district of Peixoto de Azevedo, in Mato Grosso

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