South America’s geological structure is an enormous kaleidoscope of blocks of rock that broke apart, came together and moved about impressively. In Pirapora do Bom Jesus, a municipality 60 kilometers from the city of São Paulo, the geologist Colombo Tassinari, a professor at the University of São Paulo’s (USP’s) Institute of Geosciences (IGc), shows evidence of these transformations, which dozens of geologists have been studying in depth for at least 50 years, and which his colleague from USP, Benjamim Bley Brito Neves, synthesized in an article recently published in the Journal of South American Earth Sciences. “Everything here used to be at the bottom of the sea, more than 600 million years ago”, states Tassinari, as we arrive at the top of a hill in one of the neighborhoods in the municipality of Pirapora do Bom Jesus. Then he stops at a plot of land on the corner, lined by blackberry bushes bearing fruit ? in front there is a municipal school with white walls and a local grocery store that sells buckets, plastic balls and flip-flops. In the gully that runs along the side of a tarmac road, Tassinari points out an example of this evidence: the pillow lava, bodies of basaltic magma in the shape of bubbles or, as the name suggests, of pillows.
“The outermost layer of the pillow lavas was formed when the hot lava that flowed from the ocean crust cooled down as it came into contact with the sea water”, explains Tassinari, who is working with Bley and other geologists to reconstruct the turbulent – and unfinished – geological history of South America. There are more rocks of the same type on the other side of the valley that is cut by the Tietê River, which in this area is very polluted, with slow-moving dark water, covered by white blocks of foam. As we climbed up the hill he indicated a natural limestone deposit and pointed toward an old magnetite mine – other vestiges from the bottom of a sea that disappeared as the result of a collision between tectonic plates that were moving in opposite directions. The force of the impact between the plates was so strong that fragments of the ocean crust, which were at an estimated depth of 4 thousand meters, were driven into the continent and today can be found at an altitude of roughly 600 meters above sea level (it is quite possible that they created hills that were even higher than this).
Researchers from USP, the University of Brasília (UnB), the Federal University of Mato Grosso (UFMT) as well as other Brazilian geological research centers normally examine the origin and the composition of parts of this immense jigsaw puzzle, sometimes offering more general visions such as those provided by Bley. At the same time, specialists from other countries – Argentina, the United States, Spain, Germany, England, Denmark and Australia – are working to understand the formation of their own continents. They meet on a frequent basis to help each other or to see how the continents used to fit together , given that blocks of rock now found in South America used to be side by side with others that are now located in North America or China.
Bley’s most recent article complements another that he wrote in 2008, together with Reinhardt Fuck, from UnB, and Carlos Schbbenhaus, of the Brazilian Geological Service, and which was published in the scientific journal Precambrian Research. The two papers offer an overview of the remarkable diversity of the ages, shapes, sizes, composition and origins of the blocks of rock from the earth’s crust that came from different directions, came together, pushed against each other, joined together or destroyed each other, consolidating the South American continent’s geological skeleton. Very old blocks of rock, more than 2.5 billion years old, found in the State of Bahia and along the banks of the Amazon River, are near other younger rocks, roughly 2 billion years old, in Brazil’s Northeastern Region and in Mar del Plata, in Argentina, and the youngest ones, which are about 500 million years old, such as the Mantiqueira mountain range, which starts in the south of Bahia and runs all the way down to Uruguay. These layers of rock may start just a few meters below the surface and reach a depth of 40 kilometers.
In South America, as everywhere else across the globe, a continuous process of destruction and reconstruction is taking place. Another easy to see example of these tectonic collisions is found at Itatiaia National Park. Its basic geological structure is the result of lava released by a volcano, but after this a major flow of lava flowed through the area. In their book Itatiaia – Sentinela das alturas (Editora Terra Virgem), Umberto Giuseppe Cordani and Wilson Teixeira, who are also from USP, show the sequence of tectonic movements that led to the formation of Agulhas Negras (Black Needles) Peak and of the cliffs whose sides bring to mind the petrified roots of an immense tree.
“Where the Brazilian Center-West region is now, there used to be an ocean the size of the Atlantic, between 900 million and 600 million years ago”, says Fuck. In 1969, after having worked for five years on the geological mapping of the State of Paraná, he joined UnB as a professor and got down to the task of studying the geology of Brazil’s central region. His analysis indicated that there used to be an arch-shaped chain of volcanic islands, as in the Philippines, as a result of the shock between the oceanic plates. The destruction of the islands produced a chain of mountains similar to the Himalayas, which stretched for 1,500 kilometers from the southern part of the State of Tocantins to the south of the State of Minas Gerais. And then that too disappeared.
Tassinari believes that the former oceanic basin of Pirapora do Bom Jesus, which he first began to study 30 years ago, should be more valued. And moreover, that it could become another reason for visiting the city, currently best known for its religious festivals and for a church that started being built in 1725. “We have already spoken with the mayor and are fighting to get protection for these relics from the earth’s history”, he explains. According to him, this is the only area in the State of São Paulo that has a relatively well-preserved oceanic crust.
Another indication of areas of extinct oceans are the deep sea sediments, like those found in Araxá, in the State of Minas Gerais, and in Afrânio and Dormentes, in the State of Pernambuco. “The life of an ocean is extremely short, rarely lasting much beyond 200 million years. The reason for this is that since the oceanic crust is thinner than the continental crust, it is constantly being recycled”, states Cordani. He, along with Bley and Tassinari, are currently the main researchers on a thematic project that is under way, coordinated by Miguel Basei, from the Geosciences Institute.
There are other syntheses being developed. Cordani and Victor Ramos, from the University of Buenos Aires, are coordinating the drawing up of the new tectonic map – or of the major geological structures – of South America, under the supervision of the Brazilian and Argentine geological services. This project brings together about 40 of the continent’s geologists, who are synthesizing information accrued over the last 30 years, since the last version of the map was produced. Cordani opens one of the versions of the new map on the table, which is on a 1:5 million scale: it’s a mosaic of markings in various tones of red, blue and yellow, representing South America’s different ages and geological structures. “No, we can’t publish it yet. It’s just a draft”. They aim to present the final version in August 2012 at the international geology congress in Australia.
The map is full of black lines of different lengths. These are fractures or flaws, which can separate the blocks of rock and leave room for other rocks. About 30 million years ago, volcanic rocks filled in the fractures that had been formed much earlier, in structures more than 600 million years old, forming the base of the land now occupied by Greater São Paulo, São José dos Campos, Taubaté and other cities in the Paraíba Valley. To the north, the city of Manaus was formed over the top of rocky sediments that are just a few million years old, but underneath there are rocks that joined together around 500 million years ago.
The oldest rocks in Brazil are found in the country’s Northeastern Region. In the 1960s, as one of the founders of USP’s geochronology laboratory and its coordinator, Cordani accompanied the teams from the State of Bahia’s Bureau of Mines, which was carrying out a geological survey of the state. In the central part of Bahia they found a rock that turned out to be the oldest one in Brazil, but the dating methods back then were still very inaccurate, with a margin of error of close to 100 million years.
Even so, Cordani presented his results at a congress in Beijing in 1983 and the rocks from Bahia, estimated to be 3.4 billion years old, were among the oldest ones in the world. “In 1991 I took them to be analyzed in Australia and it was confirmed”. Nowadays he could simply cross the lawn in front of his office and use the high-resolution ionic micro-probe, a sophisticated piece of rock-dating equipment that came into operation a few months ago in a building in front of the Geosciences Institute.
Two years ago, Bley, Fuck and Elton Dantas, from UnB, identified even older rocks on the South American continent: 3.6 billion years old and located in the western part of the State of Pernambuco. For Bley, this episode was especially sweet – and not just because he was born nearby in Campina Grande, in the State of Paraíba. He roamed across the western part of the State of Pernambuco 50 years ago, shortly after graduating in geology from a university in Recife. “I used to walk around there during the daytime and at night I would read Os sertões with light from a kerosene lamp, on the pavement in front of the hotel São Pedro in a village that was part of the municipality of Ouricuri”, he recalls. “I saw that there had been very little study done of the area and I promised myself that one day I would come back.”
The rocks in Pernambuco were almost as ancient as those found in Canada, which are 4 billion years old. They are among the little evidence left over from the early days of the earth, which was formed 4.7 billion years ago from a rotating cloud of gas and dust. For many millions of years, all that existed was melted rock, volcanic eruptions and a toxic atmosphere. The first bacteria, capable of surviving temperatures close to 100º Celsius, only appeared on earth about 3.5 billion years ago.
The Carajás mountain range in the State of Pará, and the region formerly known as the Quadrilátero Ferrífero [Ferriferous Quadrangle], in the State of Minas Gerais, also contain very ancient, 3 billion year-old rocks. “As to when they arrived here, or where they came from, we have no idea”, says Bley. At USP’s Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG), the physicist Franklin Bispo Santos and his PhD advisor Manoel D’Agrella Filho are working to determine the magnetic direction of the rocks of the State of Mato Grosso and Roraima at the time they were formed, between 1.9 and 1.4 billion years ago. Knowing this may help them to determine where the rocks came from and whether or not they made the journey together. This technique, known as paleomagnetism, might strengthen or weaken the geologists’ hypotheses. “The problem”, states Santos, “is that this is a very laborious process and sometimes it takes years to complete the interpretations”.
A river and an island
South America was formed from these older cores, which became bigger by incorporating others. According to Cordani, the Amazon craton doubled in size during the Proterozoic era, the longest of the geological periods, lasting for about 2 billion years. Cratons are immense blocks formed by various types of rock, normally more than 1 billion years old, which function as a relatively stable set within the crust for at least 100 million years. The Amazon craton is 4.4 million square kilometers in size, which is equal to 52% of Brazil’s territory. The oldest part of it, more than 2.6 billion years old, lies in the State of Roraima and in the west of the State of Pará, which was joined by other blocks of granite that make up the Guyanas and part of Venezuela, and then by other, more recent rocks. The highest rocks left a valley through which the Amazon River began to flow, and sediment from the river formed the island of Marajó.
Two and a half billion years ago there was a turnaround in the history of the earth, with sharp drops in temperature. This enabled the formation of the crust, the planet’s uppermost layer, previously occupied by a hot magma soup. A supercontinent known as Kenorland may well have taken shape at this time, when the atmosphere began to receive oxygen, which is essential for the survival of more sophisticated microorganisms, from which multi-cell organisms developed. “How sure are we that this supercontinent existed? Between 20% and 30%. There is still a great deal of controversy”, informs Bley.
Another supercontinent may have formed between 2.2 and 2 billion years ago. Much later it broke apart and its pieces joined up yet again to form Rodinia, which brought together virtually the entire continental mass of the earth between 1 billion and 850 million years ago. Rodinia began to break apart about 800 million years ago, forming eight continents, which moved around and then joined up once again, forming yet another single supercontinent called Pangea.
“Look at this”, says Bley, pointing to one of the maps on the wall behind his desk. “Pangea broke into pieces about 230 million years ago, forming the great oceans, the Atlantic, the Indian Ocean, the Arctic and the Antarctic. The Tethys Ocean, which was enormous, vanished. This block, India, shifted 200 kilometers, moving from south to north”. Initially joined together in a single block of Pangea, South America and Africa began to move apart about 220 million years ago. “The current States of Pernambuco and Paraíba were the last two areas to detach themselves from Africa”, recounts Bley.
Most of South America became relatively stable about 60 million years ago. The fragments of Rodinia formed a relatively stable area from Venezuela to Argentina, the South American platform, a vast group of blocks of rock completed by sedimentary basins such as the Paraná basin, with roughly five kilometers of sediments. “Depressions formed on top of this set of sedimentary and volcanic rocks and it is here that you have the Paraná River and its tributaries”, explains Bley.
However, to the west there is an area that is still geologically unstable, namely, the Andes mountain range, which resulted from the convergence of the Nazca plate and the South American continental plate. The Andes are still rising, incorporating the Nazca rocks, which sink into the earth’s mantle, melt and then come back to the surface. “The Nazca plate is moving at the rate of one centimeter a year”, observes Tassinari.
The oceans are also in a process of transformation. “The Atlantic is expanding while the Pacific is closing”, informs Bley. The result – 200 million years from now, the continents will come together again”. Although it is a long way off, the continent that is likely to result from this merger has already been given a number of names. One of these is Amasia, given that it is likely to bring America and Asia together once again.
1. South America in the context of the supercontinents (nº 05/58688-1); Modality Thematic Projects; Coordinator Miguel Ângelo Stipp Basei – IGC/USP; Investment R$ 3,611,085.27 (Fapesp).
2. Paleogeography of the Amazon craton during the Proterozoic in the formation of supercontinents (nº 2007/59531-4); Modality Regular Line of Funding for Research Project; Coordinator Manoel Souza D’Agrella Filho – IAG/USP; Investment R$ 317,316.92 (Fapesp).
NEVES, B.B.B. The Paleoproterozoic in the South-American continent: Diversity in the geologic time. Journal of South American Earth Sciences (in press).
FUCK, R.A.; Neves, B.B.B. and SCHOBBENHAUS, C. Rodinia descendants in South America. Precambrian Research. v. 160, p. 108-26. 2008.