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Geophysucs

Journey to the Center of the Earth

Some 245 million years ago a meteorite caused a crater 40 km across on the current border between Mato Grosso and Goiás

ELDER YOKOYAMA/IAG-USPAfter impact: a rocky nucleus that was 2 km down surfaced and formed the Arnica hillsELDER YOKOYAMA/IAG-USP

Since he exchanged the hectic life of the big city for the peace and quiet of the countryside six years ago, Ruy Ojeda, an advertising executive from Paraná, has never tired of talking about the delights of the land he has adopted as his own: tiny Ponte Branca, in the Southeast of the State of Mato Grosso, on the border with Goiás. What attracted him was not the tranquility of this town of fewer than 2,000 inhabitants, or the region’s natural beauty, where trees with the twisted trunks and thick bark of the Cerrado [savannah]  gradually give way to pasture. The reason for his enchantment is a phenomenon that occurred a long time ago and that to this day Ojeda does not really understand: the appearance of a huge crater formed by the impact of a meteorite that fell 245 million years ago close to where Ponte Branca and the neighboring town of Araguainha are located.

Ojeda first heard about the crater, whose formation is only now beginning to be better known about thanks to recent geological and geophysical studies from São Paulo and Campinas,  in July 2002, when he accompanied the field work of Claudinei Gouveia de Oliveira’s team from the University of Brasília. Astonished at the possibility of being able to see this scar from very far away, Ojeda wasted no time. He climbed the Arnica hills – the highest point in the region, 16 km away from Ponte Branca – and looked in all directions in the hope of finding a huge hole. He saw nothing that reminded him of a crater, but he did not give up looking and visited farms in the region asking about this hole. However, he only managed to find the crater, the biggest one in South America ever caused by a falling celestial object, when he learned to decipher the information of scientific documents. “I hadn’t realized that we all lived inside it”, he says.

Like Ojeda, most of Ponte Branca’s 2,000 residents and the 1,300 inhabitants of Araguainha do not know that both towns were born from the belly of a crater made by a meteorite; in fact many people do not even believe it exists. It is easy to understand why. The crater is so large – 40 km across – that from the Arnica hills, its central point, it is impossible to see the hills that form its rim. To get an idea of its size, a crater such as this one would swallow the entire Metropolitan Region of São Paulo, South America’s largest metropolis, which includes the capital itself and 39 neighboring municipalities.

It is not just those who live there that find it difficult to see that the towns are at the bottom of a crater: the first is close to the center, the region directly affected by the meteorite, and the second is closer to the rim, where extensive chains of semicircular hills arose from the impact. Scientists also took a long time to notice the crater. Its structure, in the form of a ring that is 40 km across, was first identified in the 60’s in geological studies carried out by Petrobras. However, the strongest indications that this was really a crater appeared later. In 1973, when analyzing the first images of Brazil taken by the North American Landsat satellite, geophysicist Robert Dietz and geologist Bevan French suggested in an article in Nature that the Araguainha region was in a depression that might be an impact crater formed by a rock coming from space, a structure that geophysicists call an astrobleme.

Still, the circular shape seen from space might also represent the remains of an extinct volcano, covered by sediment, which was a question that intrigued researchers for years until the images studied by Dietz and French drew the attention of the recently graduated Brazilian geologist Alvaro Crósta, who was beginning his Master’s degree in the remote sense at the National Space Research Institute. In 1978, after days traveling on dirt roads full of potholes, Crósta arrived in Araguainha and Ponte Branca and traveled around the region, analyzing the different types of rock that dotted the landscape. On this expedition he found the characteristic signs of a crater formed by the impact of a meteorite, such as fragments of sedimentary rocks that look like the tip of a Christmas tree. They’re called shatter cones, which Crósta described in an article published in 1981 in Revista Brasileira de Geociência [the Brazilian Geoscience Journal], at the same time the work of German geologist Barbara Theilen Willige was published; she had arrived at the same result independently and had estimated the age of the crater at 285 million years.

Crósta analyzed rocks formed by the pressure and heat of the impact and calculated the age of the shock at approximately 300 million years. “But at that time there were no suitable dating techniques and I was already guessing that it might have been more recent”, comments the geologist, who is currently a professor at the Geosciences Institute of the State University of Campinas (Unicamp). Subsequent dating using more accurate techniques established that the meteorite fell in the region 245 million years ago.

At that time the Earth was very different from the one we know today. The climate was hotter and dry and the tectonic plates, immense blocks of rock that form the current continents, were still joined to one another, fused into a single continent called Pangaea. This super-continent, which extended in a north-south direction, divided the globe in the middle and was bathed to the east by the Thetys Sea and to the west by a vast ocean called Panthalassa that covered almost all the Earth. What is most noteworthy is that it is precisely at this period that the greatest of the five mass extinctions that devastated the planet occurred. From the fossils found in different parts of the world one can estimate that 96% of the species that inhabited the oceans and 70% of those that inhabited the land were eliminated 250 million years ago, a date that marks the transition between the Permian and Triassic geological periods. There are even those who believe that this extinction favored the supremacy established by the dinosaurs that were to later arrive on the scene.

It is highly unlikely that the Araguainha meteorite was solely responsible for the biggest extinction of life ever experienced on the planet. Still, it may have contributed to it to some extent, since the shock released such a large amount of energy that it caused more damage in the whole region than was imagined, as is revealed in the extensive work carried out by the team of Yára Marangoni, from the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP).

In 2005 Yára brought together specialists from different areas of USP and Unicamp and in journeys by that took two days from São Paulo the researchers decided to visit the region to investigate how the meteorite had affected the deepest layers of the earth’s crust, which are now exposed in the center of the crater. By combining different techniques, this work has been enabling scientists to recalculate the intensity of the shock and the deformation caused below the ground. After almost two years of studies and three expeditions to Araguainha, Yára’s team now has a more precise idea of what the region was like before the meteorite hit it and the depth of the wound that was opened in the planet’s outer layer. It is also managing to get a better estimate of how much it has “healed” and how much has been wiped out by the wind and rain.

When a block of rock 4 to 6 km in diameter fell from the skies at thousands of kilometers per second close to where Araguainha is today, the region was a vast marsh, covered in salt water several meters deep. The violence of the impact had an immediate effect on the region for up to 15 km in all directions: the energy of the shock vaporized the water there was and carved out a hole almost 2 km deep, says Yára’s group in an article published in October in the Geological Society of America Bulletin. The point that suffered a direct hit from the meteorite was submitted to extreme pressure, but for a short time. So, just as someone who falls onto a trampoline is thrown back into the air, when the pressure at the center of the impact ceased,  a huge block of granite, a very hard and ancient rock with a depth of two km, was pushed to the surface – a long way from the center of the Earth that was visited by the explorers of Jules Verne’s book. This nucleus, which is almost 5 km in diameter, is part of the raised zone in the center of the crater and includes the present Arnica hills, the very same that Ojeda visited years ago looking for the crater; it is also known as the dome of Araguainha.

How has all this been discovered? Simple: geologists Cristiano Lana, Ricardo Trindade and Elder Yokoyama analyzed the rocks that form the region’s relief and saw that layers that should be hundreds of meters down appeared on the surface, as if the Earth’s entrails had been exposed. “The pressure of the impact melted part of the sediment and after it cooled in the center of the crater there appeared a 100 m thick layer of a rock that contains microscopic fragments of glass”, says Trindade.

With equipment that measures variations in the acceleration of gravity and allows the density of the rocks in the region to be estimated, Yára and Marcos Alberto Vasconcelos evaluated 300 points inside the crater. They noted that the energy released in the shock caused damage far below the present surface. “Almost 2 km down, it’s possible to detect the effects of this impact in the granite, which cracked and became a lot less dense than it generally is”, reports the geophysicist.

The effects of the shock spread far beyond the nucleus and caused wrinkles in the earth’s crust. Three dimensional maps produced from satellite images by Lana and Carlos Roberto Souza Filho, from Unicamp, show that concentric circles were formed around the site of the impact, just as when a stone is thrown into a basin of water. A first circular chain of hills, up to 500 m high and kilometers long, rose up 12 km from the impact site and a second chain a little further away, between 14 and 18 km from the nucleus.

Of course, not everything remains the same as it was then. “In these millions of years the hills have lost between 250 and 350 meters of their height due to wind and rain”, Yara explains. Despite this natural wear the researchers say that the crater remains very similar to the one formed right after the impact. “It’s difficult to have access to a crater that has well conserved structures, like the one in Araguainha”, says Trindade. Many geologists believe that craters excavated by meteorites are much more common than is imagined. When the Solar System was being formed the planets closest to the Sun, including the Earth, were heavily bombarded by meteorites. The difference here is that climatic conditions and the movement of the tectonic plates have obliterated part of this history, which is still recorded in the craters of Mars, or even the Moon.

As a first step to protecting Araguainha, years ago Crósta registered the crater in the list of the Brazilian Committee of Geological and Paleobiological Sites along with the main national geological sites that are candidates for classification by Unesco as a World Heritage Site. At the start of this year the town administrations of Ponte Branca and Araguainha and Ibama signed a document proposing the creation of an environmental protection area within the crater. “This is a way for the region to obtain funds for preserving the crater’s structures and adopting initiatives such as setting up panels explaining what happened there and arranging education and disclosure programs in local schools”, says Crósta. “If we don’t pass on this type of information there’s a risk that the rocky outcrops will be destroyed.”

Ruy Ojeda, who until March was Secretary of the Environment and Tourism for Ponte Branca, sees conservation of the crater as an opportunity for an economic rebirth in the region , after having become poor once the mining of precious stones ceased in the middle of the last century.  When he discovered the crater five years ago he became enthralled with it and has since read everything he can find about it. As a result he decided to share his knowledge about this distant past with residents of the region and other towns. “I lose all notion of time when I’m talking about this subject”, says Ojeda, who is known as the ambassador of the Araguainha dome.

The Project
Geophysical and petrophysical characterization of the structure of the Araguainha impact (nº 05/51530-3); Modality: Regular Line of Aid to Research; Coordinator: Yára Regina Marangoni – IAG/USP; Investment: R$ 257,847.75 (FAPESP)

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