Dozens of volcanoes spew out large amounts of ash and molten rock projectiles that crisscross the air while still incandescent. Rivers of lava gush out of the widespread craters and trickle down the slopes, spreading and molding a new landscape. This is what the geologist Caetano Juliani, from the Geosciences Institute at the University of São Paulo (USP), sees when he sails along the Tapajós and Xingu rivers or climbs up the hills in the middle of the Amazon Forest in Pará state. However, there is no reason for concern: most people see nothing there other than a thick forest or deforested areas in which cattle graze mindlessly and run no risk greater than catching diseases transmitted by the hordes of mosquitoes or the heat, which, no matter how suffocating it may seem, is nowhere near the heat that volcanoes produce.
The scene that the geologist sees existed almost 2 billion years ago and only its scars can be identified by those who know what to look for. “This was hell on Earth,” jokes Juliani. The oldest known volcano dates back to this time, though today it is merely a rounded hill some 200 meters high (see Pesquisa FAPESP nº 81). In the last few years, however, the USP group has found further vestiges of dozens of volcanoes – de-characterized by erosion, but still carrying an unmistakable signature on their rocks. “My American colleagues can’t believe these rocks have been conserved,” celebrates the researcher. Witnesses of the volcanic events of those times, now known as the Uatumã event, are extremely rare anywhere in the world.
Such rarity gives great importance to the findings in the southern part of the Amazonian Craton, one of the oldest surfaces on Earth. It is a region of about 1.2 million square kilometers, accounting for 15% of Brazil’s area. Juliani’s group has tried to characterize it on their annual excursions of some 40 days, during which they cover rivers and trails, collecting rock samples, identifying formations and documenting the past in their notebooks. Furthermore, this scene as discovered is not limited to just this area. “The major volcanic cycles are usually planet-wide,” explains Juliani. According to him, what remains recorded in the Amazon region tells the story of a substantial part of the Earth back then.
One of the characterized areas lies in São Félix do Xingu, a municipality in southern Pará state, next to the Xingu river, where a great geological diversity provides a rich picture of this period, the end of the Paleoproterozoic period. Part of the work was conducted jointly with Carlos Marcello Fernandes (who completed his doctorate last year under Juliani and is now a professor at the Marabá campus of the Federal University of Pará/UFPA) and was published in the Journal of Volcanology and Geothermal Research.
When examining these landscapes, one must resort to imagination, strongly supported by a knowledge of geology, in order to find the volcanoes. Pointed hilltops, such as the one in the picture at the beginning of this article, are formed by the material that once filled the crater of a volcano. The slopes eroded over time, occasionally leaving behind chains of smaller hills in a semi-circle, which outline the area where the volcano once rose. Ignimbrites, the rocks formed by the molten material that typically streamed out of the volcanoes and ran down its slopes, are among the witnesses that enable one to reconstruct these phantom formations. These ignimbrites often contain fragments of pumice stone, rocks typical of volcanic areas that are porous like sponges.
The silica content of the region’s volcanic rocks, which also contain potassium, sodium and very little magnesium, is characterized by a highly viscous magma that, instead of flowing down like chocolate sauce along the slopes of the volcano, is hurled out in explosive eruptions. Residues of more liquid magma, which ran down the slopes in a fluid state, are further away from the volcano that gave rise to it, helping to distinguish areas that were close to active craters from those that were far away from them – one more way of mapping a surface teeming with craters.
Valleys of fire
Records of the force of the eruptions can be found in the incrustrations of the rocks known as volcanic bombs. Juliani tells us that pieces of magma would be thrown out and would fly through the air, becoming spindle-shaped because of the air’s resistance during their flight. Upon landing, they would perforate the layer of volcanic deposits on the ground. All of this would be slowly petrified as the entire set cooled down. Today, depending on the direction in which the rock breaks, these projectiles appear as circles, when the break is transversal to their trajectory, or as an elongated shape that thins out toward its end, in profile. This material enables geologist to look around them and to point out, as if they were seeing an erupting volcano, where the bombs came from.
The formations that reveal the geological composition and processes of those times include rocks peppered with little colored dots known as crystal tufts. During an eruption, a large volume of fine ash spreads through the air and eventually falls to the ground, forming a layer of fine sediment that geologists call volcanic glass. When crystal fragments, such as quartz, are expelled by the volcano and fall, they sink into the layers of ash, altering their configuration. The path of these crystals is preserved when the set crystallizes, providing further evidence for scientist to find out and calculate the direction of these minerals and the strength with which they fell.
As if the succession of volcanoes were not enough, the strip of ground that is marked on the east by the curve of the Xingu river, shown on the map that illustrates these pages, was also the site of a crack on the earth’s crust. From there, magma poured out through fissures along a stretch hundreds of meters long, in which the magma, as it poured out and flowed away from the crack, formed a valley-like structure. “Nothing like this had ever been described in Brazil,” Juliani tells us. Along with his students, he explored the entire former crack, documenting and collecting material.
Whenever they go to this area, the geologists return home with bags full of rocks. Not like travelers that pick up mementoes or boys that fill up their pockets with ammunition for a possible battle, but with definite objectives. Back in their laboratory, the collected rocks undergo a series of analyses that show their composition and age in detail. The samples, with one rock surface polished, are examined with the naked eye or with a magnifying glass. Samples may also be cut down with a diamond blade and then mounted on a glass plate, sanded down and polished until they are as thin as 30 thousandths of a millimeter. This makes them transparent and suitable for examination under a microscope. There are also other techniques available, such as chemical analyses, dating and electronic microscopy.
Juliani has also been examining the Tapajós river closely, partly with the help of his student Carlos Misas. There, they discovered rich deposits of alunite, which indicate that there might be rich veins of gold on that site. Alunite is a mineral formed only when the water that comes out of the magma at some 450 degrees Celsius (°C) reaches the surface still very hot, at about 130°C, altering the surface rocks. It also characterizes deposits of gold and copper of the porphyry kind. This clue suggest the presence of mineral deposits that, despite a low gold content, might hold a great volume, with more than 1,200 tons of gold. The researchers are collaborating with mining companies, which finance their work. It is a profitable investment for the firms, as geological studies provide information on where it is worthwhile looking for valuable minerals.
Regardless of whether minerals of great value are found, Juliani’s aim is to tell the story of what happened by means of work that is covered by the National Institute of Science and Technology (INCT) of Geosciences in the Amazon Region, headquartered at UFPA. “What we do is detective work,” he tells us. Thanks to growing knowledge about the Amazonian Craton, researchers are beginning to provide details of the Uatumã event, dividing this period of volcanic explosions in geographical and temporal terms. Besides volcanology, this historical reconstitution also allows one to detect the movements of the tectonic plates that made up the Earth’s crust at that time. “In the region we have studied there are indications that a tectonic collision, like the one that formed the Andes, occurred. Except that there aren’t such high mountains, and we don’t know why.”
Year after year, according to the funding and the heavy Amazon region rainfall, the team pieces these ancient puzzles together. The pieces that do not fit, such as the absence of mountains where the plates appear to have collided, and the peculiar preservation of the rocks in the south of Pará state, provide further encouragement. Such fascination for the challenge becomes evident in the words of Albert Einstein, the German physicist, which Marcello Fernandes chose as the epigraph of this thesis: “If at first the idea is not absurd, then there is no hope for it.”
JULIANI, C. and FERNANDES, C. M. D. Well-preserved late paleoproterozoic volcanic centers in the São Félix do Xingu region, Amazonian Craton, Brazil. Journal of Volcanology and Geothermal Research. v. 191, n. 3-4, p. 167-79. Apr. 2010