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When the sertão was sea – Fundamental science

By Fabrício Caxito

The separation of Pangea formed an ocean. Could it be that there were no others?

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The theory of plate tectonics, which gained strength from the 1950s onwards, revolutionized the way the world map was looked at, with its watertight separations of water and land masses. Pioneers such as the German meteorologist Alfred Wegener and the South African geologist Alexander du Toit used evidence such as the fitting of the coastlines of South America and Africa, as well as the presence of similar rocks and fossils on the two continents, and raised the hypothesis that, until about 130 million years ago, these two masses were united in a single continent, Pangea. The discovery of the split of Pangea from the Jurassic period, generating a new ocean, the Atlantic, sowed the idea that, over time, the continental masses move, in an eternal dance of continents that caused the opening and closing of oceans and the creation of new mountain ranges where two continental masses collided with each other.

If South America and Africa separated, as Wegener and Du Toit suspected, why could there not have been configurations between continents and oceans in the past that we can no longer distinguish due to the cycles of opening and closing that followed each other in the geological time? We now know that the Earth is about 4.5 billion years old, long enough for several oceans to have opened and closed, and several fragments of continents have collided and formed supercontinents in positions quite different from today.

One clue to unraveling this puzzle is to look, in today’s continents, for traces of ancient oceans that squeezed and closed when continental masses collided. The difficulty in undertaking this research, however, is due, among other reasons, to the difficulty in finding pieces of ancient oceans within continents.

The ocean floor is made up of very dense rocks, mostly basalts, a dark rock very rich in iron and magnesium minerals, quite different from the granites that characterize the continents, rich in silica and aluminum, lighter elements. Thankfully, in some very specific situations, pieces of basalt that used to be part of the ocean floor ended up getting stuck in the middle of the continent’s rocks, thus being preserved for future investigations.

This occurred, for example, in subduction zones, which are those where one tectonic plate entered beneath the other and sank into the Earth’s mantle. In this movement, pieces of basalt from the bottom of the ocean may have broken off from the submerged plate and climbed up to the plate where the continent that had been on top was. The small pieces of basalt ended up being preserved as splinters on the continental granites and sedimentary rocks. In other words, these rocks are very different from their neighbors. They have a dark green coloration specific to basalts and other rocks associated with them, and for this reason they were called ophiolites, from the Greek “ophios” (serpent) and “lithos” (rock).

Finding ophioliths in the field and getting their hands on a preserved piece of ancient ocean is a huge joy for scientists. This is what happened in the Monte Orebe region, in the hinterland on the border between Pernambuco and Piauí, when, in 2014, a group of researchers, of which I am a part, found and described a piece of an ocean from around 820 million years ago and he named it “Mount Oreb Ophiolith”. The study was published in the journal “Geology” and helped to unravel the configuration of ancient tectonic plates, continents and oceans in northeastern Brazil.

Interestingly, the Monte Orebe region is just north of the Sobradinho dam in Bahia. And it was the construction of the dam that inspired the famous song by Sá and Guarabyra (inspired in turn by the prophecies of Antônio Conselheiro): “The backlands will turn into seas, it’s in your heart/the fear that someday the sea will also turn into backlands”.

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Fabrício Caxito is a geology professor and philosophy student at UFMG.

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