When the Sertão was sea – basic science

By Fabrício Caxito

The separation from Pangea formed an ocean. Could it be that there weren’t any others?

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The theory of plate tectonics, which became established in the 1950s, revolutionized the way the world map was viewed with its watertight separations of water and land. Pioneers such as the German meteorologist Alfred Wegener and the South African geologist Alexander du Toit used evidence such as the adaptation of the coastlines of South America and Africa and the presence of similar rocks and fossils on the two continents and hypothesized that these two masses existed until about 130 years ago Unified in a single continent, Pangea, millions of years ago. The discovery of the split of Pangea from the Jurassic period, which gave birth to a new ocean, the Atlantic, spawned the idea that the continental masses move over time in an eternal dance of the continents, the oceans opening and closing the creation caused new mountain ranges in which two continental masses collided.

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 successive cycles of opening and closing in geological time? ? We now know that the earth is about 4.5 billion years old, long enough that several oceans have opened and closed, and several fragments of continents have collided and have formed supercontinent positions in very different positions than they are today.

One clue to solving this puzzle is to look for traces of ancient oceans in today’s continents that squeezed and closed as continental masses collided. The difficulty in conducting this research, however, is, among other things, that it is difficult to find parts of ancient oceans within the continents.

The seabed is made up of very dense rocks, mainly basalts, a dark rock very rich in iron and magnesium minerals, very different from the granites that characterize the continents, rich in silica and aluminum, lighter elements. Fortunately, pieces of basalt that were once part of the ocean floor got stuck in the middle of the continent’s rocks in some very special situations and so were preserved for future research.

This happened, for example, in subduction zones, in which one tectonic plate penetrated beneath the other and sank into the earth’s mantle. During this movement, pieces of basalt from the sea floor could have broken off the submerged plate and climbed onto the plate on which the continent was previously located. The small pieces of basalt were ultimately 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 color specific to basalts and other rocks associated with them, and for this reason they were called ophiolites, from the Greek “ophios” (snake) and “lithos” (rock).

Finding ophiolites in the field and getting your hands on a preserved piece of the ancient ocean is a great 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 to which I belong found and described a piece of ocean of around 820 million years ago and he named it “Mount Oreb Ophiolite “. The study was published in the journal Geology and helped to elucidate the configuration of ancient tectonic plates, continents and oceans in northeastern Brazil.

Interestingly, the Monte Orebe region lies north of the Sobradinho Dam in Bahia. And it was the construction of the dam that inspired the famous song of Sá and Guarabyra (again inspired by the prophecies of Antônio Conselheiro): “The hinterland will turn into seas, it is in your heart / the fear that one day too the sea will turn into hinterland ”.

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Fabrício Caxito is geology professor and philosophy student at UFMG.

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