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Geology

Mother Earth

Team from USP puts South America on the map of Rodinia, a supercontinent of 1 billion years ago

Ten thousand 2-centimeter high cylindrical samples of rock unveil the South America of 1 billion years ago, a very different picture from today’s world map. In those days, what corresponded to Brazil’s present day territory was a series of large islands, all distant from each other. The block that corresponds to the Amazon basin was separated from Goiás and from the Northeast by seas, and at the same time closer to the southern portion of the country than it is today, and almost stuck to what was to be North America.

Collected from the north to the south of Brazil, in the rest of South America and in Africa, the rock samples tell stories that made it possible for a team from the University of São Paulo (USP) to put together the jigsaw puzzle of the composition of Rodinia – Motherland, in Russian. This was one of the supercontinents into which the crust was divided about 1 billion years ago, in the period prior to Gondwana – the best known supercontinent, formed 750 million years ago, from the breaking up of Rodinia.

To reach these results, the researchers from the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) used the technique of paleomagnetism, which is based on the fact that when a rock cools down it creates a mark that shows the direction of the Earth’s magnetic field at that moment, and hence makes it possible to locate their position in relation to the poles.

Laurentia
Carried out in a project that started in 1998, this work was presented in October 2001 at a congress in Perth, Australia, and at an international symposium on Rodinia and Gondwana that met in Osaka, Japan. The discoveries complete and correct the first map of Rodinia, shown in 1991 by the American geologist Paul Hoffman, who is today at Harvard University, United States. Based on studies carried out basically in the Northern Hemisphere, Hoffman’s design showed a supercontinent whose center was a large block called Laurentia – nowadays North America. Around it, there were groups that formed Antarctica, Australia, Siberia, India, the Kalahari (east of Africa), southern China, Congo-São Francisco (part of Africa and of the Brazilian North-East) and the Baltic Continent (north of Europe).

The Brazilian group that had already been active in the area since the beginning of the 70’s then decided to offer a South American vision of Rodinia, adding pieces to Hoffman’s jigsaw puzzle. Under the coordination of Igor Ivory Gil Pacca, the group from the IAG trained its focus on rocks from the cratons, which formed the south of Rodinia. Cratons are the most ancient rocky blocks of the lithosphere – the outer layer of the planet, which is made up of the crust and the upper mantle, and which is about 100 kilometers thick.

The group collected material from Mato Grosso, Rondônia, Ceará, Bahia and Paraná. It also gathered samples in Argentina, Uruguay, Paraguay and Bolivia, and, on the other side of the Atlantic, in Gabon, Nigeria and in the Cameroon. Rocks of three types were collected: sedimentary rocks (arenites, carbonates and siltites), magmatic rocks (basalts, granites, gabbros and andesites) and metamorphic rocks (amphibolites, granulites, migmatites and gneisses).

As the analyses of this material were concluded, the group would add pieces to the original proposal. To start with, it showed that the central region of Goiás, the Rio de la Plata (which covered the south of Brazil), West Africa and other smaller blocks also participated in the formation and breaking up of the supercontinent.

The result is that the image idealized by Hoffman changed significantly: the Amazon region, for example, left the west side and went to the east. According to Pacca, Hoffman included the Amazon in the 1991 map on the basis of structural evidence and geological similarities, but with very little of the paleomagnetic information that is indispensable for a reconstitution of the ancient Earth. “It was almost a supposition, a possibility”, he comments. One of the merits of the team from USP was precisely that it gathered abundant data on the position of the Amazonian block in Rodinia. “Today, no doubts remain”, he avers.

The story extracted from the rocks strengthens the hypotheses on the look the Earth had 1 billion years ago – one fifth of its age. It is believed that at the time of Rodinia the planet was an immense ball of ice, probably the result of astronomical interference and alterations in orbit: the ice increased the reflection of light, which lessened the absorption of energy. Then came the other side of the coin: the intense volcanic activity of those days that emitted an enormous quantity of gases, which ended up originating a gigantic greenhouse effect.

The ice started to melt and, in some 10,000 years – an extremely short period in geology – the Earth’s temperature went from minus 50ºC (degrees Celsius) to plus 50ºC. The consequences for the still incipient life were immediate. “These events of stress favored the appearance of pluricellular beings”, says Ricardo Trindade, a researcher for the IAG team. “Before, what there was on Earth were cyanobacteria, very simple beings that were capable of surviving in adverse conditions”.

Rodinia in pieces
It was precisely in this period of intense climatic changes, around some 750 million years ago, that Rodinia started to break up, and the dance of the blocks came to form another supercontinent: Gondwana. It encompassed today’s South America, Africa, Antarctica, Australia and India. The blocks that were to make up Brazil started to come closer to the design of today, when the last of the great continents arose: Pangea, some 300 million years ago. It was the breaking up of Pangaea that gave birth to the current oceans and continents, roughly 100 million years ago.

But the dance goes on: it is estimated that the blocks are nowadays moving 3 centimeters a year, on average. The group from the IAG points out some tendencies: in the next few millions of years, cracks should appear in North America, in Asia, and between Africa and the Arabian Peninsula. Brazil and Africa will meet again, this time on the other side – the west of Brazil with the east of Africa.

Both the discoveries and the conjectures are based on the Theory of Continental Drift, presented in 1912 by the German scientist Alfred Wegener (1880-1930), but only consolidated in the 60’s. By the theory, the lithosphere is made up of deformable parts called tectonic plates. The plates move along the surface, they break and they join up, all driven by the heat from within the Earth.

Marked rocks
As one goes back in time, however, uncertainly grows over the actual movement of the plates, so that it is not possible to say that the map of Rodinia is a definitive one, even after the Brazilian contributions have been added. “In addition to there persisting some doubt as to which blocks were really part of Rodinia, opinions diverge as to their correct positions and where exactly they fit”, explains Manoel Souza D’Agrella, from the IAG group. In an article published in March in Geology , one of the most important magazines in the area, Ebbe Hartz and Trond Torsvik, from the University of Oslo, Norway, show evidence that the Baltic continent was actually in an inverted position compared with the one its is in today – the north was in the south, and vice versa.

“This finding has implications for the Amazon region, which usually appears stuck onto the Baltic continent, in the reconstructions of Rodinia”, says Hoffman, the author of the first map. According to him, a study that is to come out in Earth and Planetary Science Letters shows that Australia may not have been to the north of Mexico, one of the parts making up the block called Laurentia, 1 billion years back.

There are disagreements, but the researchers agree on one point: it would be impossible to recover this so remote history without paleomagnetism, a tool that the IAG and the other groups adopt to give an account of continental drift. The technique is based on the fact that when a rock cools down, whether 1 billion years ago or 100 million, it always records the direction of the Earth’s magnetic field. “Paleomagnetism determines the latitude in which the block of rock was to be found and its position with regard to the Earth’s axis”, explains geophysicist Márcia Ernesto, who shares with Pacca the coordination of the team from USP.

At the end of the 90’s, now with the first samples in hand, the group from the IAG found that the position of the pole registered in the rocks does not correspond to the current position: in one, it would be vertical, in another, horizontal, others were in various diagonal positions. The hypothesis that the Earth’s magnetic field may have altered in the course of time was ruled out straight away: “The magnetic poles of the Earth have always been very close to the geographical poles”, Pacca emphasizes. “And the Earth’s magnetic axis works like a great bar magnet, close to the axis of its rotation”. The explanation for the discrepancy in the magnetic orientation of the rocks is that the continents have really been moving.

Continental routes
Afterwards, an analysis was carried out on rocks of different ages, collected in the same place. The historical series would make it possible to define the recorded direction of the pole. A hypothetical example: 800 million years ago, the direction was horizontal, 600 million years ago, it bent to the left, and 300 million years ago to the right, and so on. The memory of the rocks files the routes. Gathering these tracks together and ordering them into a sequence of directions, the apparent polar drift curve arises – which clearly shows the paths followed by the continents. This is what happened with the arenites gathered in two waterfalls in Mato Grosso – one in Salto do Céu, near to Cuiabá, the other in Vila Bela da Santíssima Trindade, the former capital of the State, close to Bolivia -, which showed conclusively where the blocks that made up South America were.

Having traced the routes of the movement of South America and Africa, the researchers placed the former over the latter, and vice versa, and saw exactly where and when these continents drew close or moved apart. The fieldwork calls for patience and a certain dose of adrenaline for adventure. The last time they were in Rondônia, in July 2001, the group had to invade areas controlled by logging companies, and the university car was mistaken for an inspection. “It was most unpleasant”, Pacca says: “We were almost shot at”.

In the field, the researchers perforate the rocks with a special drill and take the cylinders out. A compass immediately determines the direction of the magnetic field at that moment, and the researcher takes note on the sample itself. In the laboratory, the samples are put through ovens that raise and lower the temperature alternately, to eliminate recent magnetic interference. Only the old record has to be left, which is assessed in a steel-plated room – “the place with the least intense magnetic field that there is in São Paulo”, according to Márcia.

Partnerships
The group received reinforcement from the Geochronological Research Center of the Institute of Geosciences (IG), also at USP. It is there that they do the chemical analyses and those of the radioactive elements, which attest to the age of the rocks. “As the margin of error is minimal, we can establish hypotheses on the formation of Rodinia that are more and more coherent”, says Wilson Teixeira, a director of USP’s Institute of Geosciences (IG) and part of the group. It was the IG, incidentally, that hosted an international encounter on Rodinia, in August last year.

The IAG group, nowadays an international point of reference and probably the only one to work with paleomagnetism in Brazil, is also acting in collaboration with groups from the São Paulo State University (Unesp) in Rio Claro and the federal universities of Bahia, Pará and Rio Grande do Norte, which facilitates the collecting and interpretation of the results. There are also partnerships with specialists from the universities of Berkeley (United States), Trieste and Padova (Italy), Sweden, Toulouse (France) and Buenos Aires (Argentina). The usual spirit of collaboration between scientists is strengthened by the fact that nobody knows where the rock that is missing may be in the jigsaw puzzle that each group is trying to put together.

The project
Participation of the Craton Units of South America in the Evolution of Supercontinents, Since Mesoproterozoic Times [RJS1] (nº 98/03621-4); Modality Thematic project; Coordinator Igor Ivory Gil Pacca – IAG/USP; Investment R$ 292,409.52

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