Fundamental Science: Why are there still mountains in Brazil?

The last time there was a mountain-forming process in Brazil was approximately 500 million years ago, in a remote geological moment when the continents were united in a supercontinent, Gondwana. However, taking into account our knowledge about the mechanics of erosion processes, once this formative process is over, erosion in riverbeds and slopes would be responsible for eliminating a mountain in a few million years, reducing it to a hill or hill. not very high. In other words: Brazilian mountains such as Serra do Espinhaço and the Iron Quadrangle in Minas Gerais should no longer exist.

Why, then, do they insist on lasting? Because this is still one of the biggest enigmas of Brazilian geoscience. As a geoscientist, I confess to feeling a little uncomfortable because I still don’t know the answer. In Brazil, we understand a lot about the rocks that support our mountains today, but in a way we ignore why the mountains are still active. Taking into account the current stage of research on Brazilian rocks, it is easier to infer what happened before 500 million years ago than in the last thousand years. Despite this apparent paradox, there are some plausible explanations, but still little explored.

A daring hypothesis is that some mountain ranges such as the Quadrilátero Ferrífero are, in fact, gaining topographical expression. According to this thesis, if we could observe the mountains for tens of millions of years, we would see that the difference in elevation between the foot of these formations and their peaks would be increasing. What could such a phenomenon be due to? Rocks of a certain type, located deep below the mountains, are more resistant to erosion. Once exhumed at the surface, they would remain in the relief and support the highest parts of the landscape, preventing or slowing down erosion of the topography, while the surroundings, made up of less resistant rocks, would continue to subside. Over millions of years, this difference in erosive resistance would cause the mountains to gain topographical expression.

Another hypothesis, also daring, is that mountains in stages of decay can be rebuilt by processes associated with the Earth’s mantle, hundreds of kilometers deep. Between 100 and 600 kilometers deep, superheated rocks move slowly and, when very hot, rise to the base of the crust and act as a “chock”, raising the surface of the Earth for a few hundred meters in the form of a dome of hundreds of meters. kilometers long. If this occurs at the base of a mountain range, even if it is decaying, its elevation can gain a few hundred meters.

Another bold hypothesis is that surface erosion, responsible for causing elevation loss, may actually lead to the uplift of peaks. This goes against our intuition, but there is a coherent physical explanation. The earth’s crust, although made of hard rock, has a certain elasticity (as it is relatively warm at depth) and floats on top of the denser mantle. Therefore, a mass of rock, or even ice, of sufficient weight could bend this crust. Consequently, if this mass were removed, the previously formed depression would return to its original position in a vertical movement.

Now, imagine that, along the entire length of a mountain range, rivers carve their valleys causing the typical landscape of alternating peaks and valleys, as in Serra do Mar. The mass removed from the valleys, however, removes a substantial weight from the crust that supports the mountain range! Knowing that the readjustment due to the loss of mass does not occur only in the valleys, but in the entire region, it is plausible that the vertical adjustment causes an increase in the elevation of the peaks. In this scenario, it would be possible for the Serra do Mar, one of the largest coastal ranges on the planet, to gain elevation.

A last hypothesis is the lack of erosive tools. Despite the saying “soft water on hard stone, it hits so hard that it breaks”, it is not the water that is responsible for excavating the rocky bed of rivers, but the sediments, mainly the high caliber ones, that the water is responsible for transporting . Removing the sediments from the scene, little erosion occurs, even with abundant water.

And who is in charge of delivering sediments to the rivers? The slopes. In tectonically active environments, landslides provide the necessary sediments for rivers to erode their beds. However, without tectonic activity, that is, without earthquakes, much less landslides occur and, with this, less and less sediment and, therefore, less and less erosion. In computational tests, mountains whose incising process depends on this mechanism would persist for hundreds of millions of years after the end of tectonic activity.

It is ironic that we understand a lot about the rocks of the Brazilian mountains, but very little about the mountains themselves. The geoscientific community in general, and not just the Brazilian one, needs to explain the existence and morphology of several mountains that occupy regions that no longer build them, such as the Espinhaço mountain range and the Appalachians, in North America, and the surrounding coastal ranges. of the planet, such as Serra do Mar and southwest Africa.

Such mechanisms could explain the Brazilian mountains and suggest that they would be growing over millions of years. This is counterintuitive, fascinating and one of the main scientific challenges for geosciences in Brazil.

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Pedro Val is a geologist and professor at Queens College, City University of New York.

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