By Adriana Alves
We still don't know why they are not associated with great extinctions like that of dinosaurs
Brazil is one of the most stable countries in the world in terms of natural disasters. Unlike our neighbors, we are not haunted by the possibility of volcanic eruptions or earthquakes of great magnitude. But it was not always so.
The geological evolution of the Brazilian territory records from uplifts of mountain ranges as high as the Himalayas to volcanic fields of such impressive size that scientists call them LIPs – Large Igneous Provinces, giant igneous provinces.
The continental configuration of the Earth is defined by the plate tectonics. The perfect fit between the borders of the South American and African continents is the most compelling evidence that the two have been together. The opening of the Atlantic Ocean took place about 134 million years ago and was marked by the formation of a large volcanic province that covered about 1/4 of the country's territory. The lavas could reach up to 1 km in thickness, reached 1000º C and ran up to 10 km / hour. A small part of this large province was on the African continent and that is why it is called Paraná-Etendeka Province (90% of the province occurs in what we call the Paraná basin, which covers the state of the same name and a good part of the South Center of the country, and Etendeka is the name of a mountain field in Namibia).
The event lasted from 1 to 4 million years (a short time, considering that the Earth is 4.5 billion years old) and involved the release of 1.2 million km3 of lava. Volcanic provinces such as Brazil are rare events, which arouse the interest of geologists and geophysicists because they are not associated with the limits of tectonic plates (where volcanic activity is usually concentrated) and because they are contemporary with three of the five great extinctions that affected life in the planet.
The most significant example of this volcano-extinction association is precisely the one that almost decimated life on Earth. 252 million years ago, about 70% of life disappeared from the fossil record (90% of marine species perished and the event became known as the Great Dying). The only clue we have is the temporal coincidence with the release of 2 million km3 of lava observed in what is now Siberia.
But how do volcanoes kill on a global scale? The answer is not yet well designed, but it is believed that catastrophic climate changes resulting from the inflow of volcanic gases into the atmosphere (sulfur, carbon dioxide, chlorine and fluorine) have triggered a succession of events – global warming, acid rain, acidification and withdrawal of oxygen from the oceans – which would have caused a collapse of the food chain, with consequent extinction.
At this point, the attentive reader must be anxious to know the extinction rate associated with Paraná-Etendeka province. I'm sorry, but our lavas did not cause mass extinction. And why was there no extinction? Again, I'm sorry, but the answer is: we don't know yet.
A World War II anecdote can shed light on the issue. The Allies hired engineers to reinforce areas of planes that were propelled by bullets – reinforcing the most affected areas (wings and horizontal stabilizers at the rear) would increase the rate of return of the planes. The statistician Abraham Wald observed that the returning planes were precisely the ones that continued to fly after the attacks: it was necessary to find out which areas were affected in the planes that did not return. Wald suggested strengthening the turbines and the engine, and the return rate of the planes increased significantly. This logical error was called survivorship bias, survival bias.
Geologists have done the same as anecdote engineers. All projects dedicated to investigating the volcano-extinction association are focused on provinces associated with extinction events. Apparently, the answers will begin to be better outlined when we look at the missing piece of the puzzle: the information brought by the big provinces that did not cause extinction, like the Brazilian one.
Adriana Alves is a geologist and professor at USP
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