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Geology

Land in Transit

Super continent that existed a billion years ago gains new contours

Latest map of Rodinia includes land located in Brazil’s northeastern backlands Izan Petterle/SambaPhoto

Approximately 1 billion year ago, a significant part of the planet’s land was part of the super continent Rodinia, a name derived from the Russian term for Mother Earth. Even experts seem to agree on this. But the re-constitution of these vast lands is a difficult puzzle to solve, despite all tools of modern geology. To come closer to this distant past, an international group set up in 1999 by Unesco joined efforts and collected new evidence on the respective continents. Though the group did not reach a consensus, the February issue of Precambrian Research published the latest version of the evidence. The planet’s land mass was joined together in a single continent, most of it entirely south of the Equator. The timing of the evidence was very appropriate, as Unesco had designated the year 2008 as the Year of Planet Earth.

According to this multinational study, most of Rodinia was located south of the Equator. “We know this because of the vestiges of a glacial era that indicate which regions were closer to the poles at that time,” explains Benjamim Bley Brito Neves, from the Geosciences Institute at the University of São Paulo (IG-USP). Territories that are now India and China, which nowadays lie in the Northern Hemisphere, used to lie on the Equator. The blocks that nowadays integrate Australia and Siberia were in the north. The Amazon Region was to the south of Laurentia, which nowadays is North America, with a fragment to the east, now part of Mexico. The western part of Africa lay to the south of the Amazon Region, with part of its territory in the South Pole. The region that is now part of Brazil’s northeast backlands, where the São Francisco River lies, was part of the western portion of the Amazon Region and the western region of Africa. What is now the State of Bahia lay further to the northwest, in the block that is Congo today.

Compared to how the continents today, this map might seem insane to the eyes of non-geologists. It looks as if the parts that form the world were randomly mixed together. The experts see some logic in the movements of the terrestrial land masses, but the available information they have about those distant times does not allow for any certainty.

The information that enabled geologists to pinpoint the continents of that era more accurately is skimpy because it is preserved in rocks and rare rocky formations. Reinhardt Fuck, a geologist from the University of Brasília (UnB), who was a member of the Unesco team, explains that volcanic rocks are a precious material because they are formed by means of a very swift cooling process that crystallizes records of the terrestrial magnetic field at that given moment inside the rocks. Millions and millions of years later, an expert can analyze this paleontological data and establish at what distance from the pole that given rock was formed and which direction it was pointing to, at that moment. Based on this data, the researcher can reconstruct the path which that part of the continent has covered since its creation. This time feature – the age of the rocks – is determined by dating techniques that use radiogenic isotopes, whereby chemical elements are transformed through radioactive decay. “Uranium and thorium isotopes are transformed into lead isotopes,” says Fuck, “at a rate that we are reasonably familiar with.” This rate allows geologists to estimate the age of the rocks on the basis of the proportions of these rocks’ elements.

Paleo-magnetism solves a part of the puzzle: it allows scientists to arrange the pieces in the original direction, the upper part facing upwards and so on, and at the correct distance from the poles. But which part was on the right and which was on the left? How do the parts fit together? Finding this information – paleo-longitude – requires very meticulous work: it entails analyzing the chemical composition and other properties of the rocks and rocky compounds of each area being analyzed and the search for similar rocky formations in other parts of the world. “Comparing fragments of the earth’s crust on the basis of geology is what geologists do every day,” says Fuck. When they find rock formations with similar composition and age on different continents, they assume that those regions were joined together at some point in geological time. Thus, the pieces of the puzzle are slowly being put together, but fitting in the parts depends a lot on interpretation. “Each person has his own opinion,” says the geologist from UnB, “and hypotheses obviously abound.”

This is why the map of Rodinia has been constantly changed ever since its first inception in 1991 (see Pesquisa FAPESP nº 75). At that time, Canada’s Paul Hoffman, currently at Harvard University in the United States, committed what Bley jokingly describes as “an act of scientific irresponsibility” and, at the same time, “a stroke of genius.” Consulted on why plant fossils indicated the absence of barriers to the free movement and reproduction of live beings, Hoffman collected the (insufficient) information about this that had already been published by him and by other researchers, and proposed a super continent. This first version was necessarily inaccurate – and this is why it can be described as irresponsible – but it had an important effect in that it motivated experts from all over the world to search for a better way to fit the pieces together.

So far, Brazilian geologists have found rocks approximately one billion years old – the era of Rodinia – in very few areas. The latest map of the South American descendents of this super continent was also published in the February issue of Precambrian Research. Prepared by Fuck, Bley and Carlos Schobbenhaus, from the Brazilian Geological Service, the map shows that representatives of Rodinia are concentrated in the southern part of the Amazon Region, in the State of Mato Grosso, and in the Northeast Region, especially in the States of Bahia and Pernambuco. “South America is a mosaic of fragments of Rodinia,” states Bley.

Manoel d'agrella/usp Dikes that are now in the region of Ilhéus, in the State of Bahia, already existed at the time of RodiniaManoel d'agrella/usp

Nothing is certain
Parts of the rocks required to reconstruct Rodinia are no longer accessible – they lie under mountain ranges, on the bottom of sedimentary basins or on the bottom of the sea. The cost of collecting samples in these areas is prohibitive for researchers, who ultimately depend on huge ventures such as oil drilling. Samples of this kind allowed the region of the Paranapanema River in southeast Brazil to be included in the Unesco map; the rocks are hidden under the basin of the Paraná River and this is why the region had been ignored in reconstructions of Rodinia until now.

Brazilian researchers are quite convinced that the Amazon block was part of Rodinia, and was possibly located near the continent of Laurentia, which is now North America and Greenland. Maybe the Amazon block was next to Laurentia. However, discussions abound on the relative position of the two land masses. Manoel D’Agrella Filho, from the Institute of Astronomy, Geophysics and Atmosphere Sciences of the University of São Paulo (IAG-USP), is one of the scientists unconvinced by the version published by the Unesco group. In his opinion, the Amazon block collided with the southern part of Laurentia, and then went around the southern part of Laurentia clockwise. This model, proposed by the Eric Tohver, an American who is also from IAG, explains the scars on the North American continent caused by the collision – geological deformations known as the Greenville belt – and positions the Amazon block during the Rodinia era to the southeast of the current position of North America.

The chief difference between the international proposal and that of the researchers from IAG is not the relative position of the current Amazon Region and North America. The IAG researchers disagree on the characteristics of the collision between the two land masses. In D’Agrella’s opinion, the map of Rodinia was more dynamic than it seems to be in the current proposals. A study he coordinated was published this year in Earth and Planetary Science Letters, one of the world’s most prestigious earth sciences publications. This study reinforces the notion that the Amazon block slid around Laurentia and emphasizes the idea of a continent in which the relative position of the land masses changed constantly. Concerning the work of the Unesco group, the IAG researcher maintains his opinion, but admits that as yet nobody has been a winner in this debate. “The paleo-magnetic data can be interpreted in several different ways,” he says. Furthermore, the available information does not allow any of the hypotheses to be refuted.

Another bone of contention concerns the São Francisco River basin. The map prepared by the international group shows that the block where the São Francisco River basin is located lies in Rodinia. In D’Agrella’s opinion, however, this interpretation does not take into account vestiges of the huge Brasiliana ocean – which at that time allegedly separated most of the African and South American blocks, including the São Francisco River region, from the block formed by the Amazon Region, Laurentia, and western Africa. In D’Agrella’s opinion the said ocean actually separated Rodinia from the area that comprised the current São Francisco River basin and Africa’s Congo and Kalahari regions.

Roving continents
No matter what the size or shape of it is, a huge continent cannot persist. “We are sitting atop a thermal bomb,” explains Bley. Below our feet lies a solid lithosphere, or the earth’s crust, measuring 150 to 300 kilometers. This is an extremely thin membrane when compared to the rest of the planet – some six thousand kilometers to the center of the Earth. The extremely high temperatures of the earth’s mantle, the layer below the crust, confers viscous characteristics to the minerals that comprise it, and which, in the course of millions of years, produce movements whose effect is to release heat. When a super continent is formed, the heat accumulated under the lithosphere can crack it, much like a hot tea kettle does when it is placed on a glass table. This is what happened to Rodinia: the continent broke up into four huge masses – Laurentia, Gondwana, Baltic and Siberia – which about 230 million years ago came together again to form the continent called Pangea. This more familiar-looking super continent is the origin of today’s world map.

While geologists discus hypotheses and dig up rocks in their pursuit of answers, the continents continue their tireless migration. The oceanic plates are heavier than the continental ones and that is why they tend to move under the continents. In this process, the Pacific Ocean creased South America, created the Andes Mountains and causes frequent earthquakes along the seacoast. Very slowly, the Pacific Ocean is closing in, while the Atlantic and Indian Oceans, the Tasmanian and the Red Seas, and the Persian Gulf are expanding by a few centimeters every year. If the current routes remain unchanged, geologists predict that within roughly 50 million years Asia and America will once again meet and turn into a huge new continent. Although this continent does not exist yet, it already has a name: Amasia.

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