The Cerro do Jarau range of hills, which is some 200 meters high, stands out among the small hillocks of the Pampas in the municipality of Quaraí, western Rio Grande do Sul state, where Brazil’s border with Uruguay lies. The origin of these hills, which form a semicircle when seen from the sky, has always intrigued the state’s inhabitants and even gave rise to folk tales regarding the formation of the people of this state. Now it seems the mystery is about to be cleared up. A study conducted by the geologists Alvaro Crósta and Fernanda Lourenço, from the Geosciences Institute of the State University of Campinas (Unicamp), found proof that these elevations were formed by the impact of a meteorite that fell in the region millions of years ago, opening a large crater.
Anyone without the eye of a trained expert would be unlikely to see a crater at this site. Indeed, what one sees is not a hole generated by violent impact, but its edges, which rose up like the waves formed by a stone falling into a swimming pool. Furthermore, these edges are not as well preserved as they were in the past. In the course of millions of year, the wind, the rain and the Earth’s surface movements corroded Jarau’s edges, leaving the hills at their current height of 200 meters. Rocks forming a 3.5 kilometer wide ring highlight the more central region of the crater, where the crash probably took place.
Two years ago, Crósta and Fernanda, who at the time was his student on Unicamp’s geology course, made an expedition to Jarau looking for signs of the meteorite’s impact. For 10 days they walked up and down the hills collecting rock samples that they took back to Campinas. The analysis of the rocks under a microscope confirmed that they could only have been formed at extremely high temperatures and pressure, such as those generated by the crash of a celestial body.
The Unicamp team’s results corroborate the hypothesis put forth some 20 years ago by two researchers from the Federal University of Rio Grande do Sul (UFRGS). In the late 1980’s, the geologists Nelson Amoretti Lisboa, from UFRGS, and Marisa Terezinha Garcia de Oliveira Schuck, also at UFRGS at that time, analyzed satellite images and the region’s geomorphology and proposed that the Jarau hills could have been formed as a result of a meteorite impact. “We came up with the idea,” says Lisboa. “But we lacked the technical resources to examine the minerals.”
When they evaluated the structure of the rocks in the range of hills and their mineral composition, Crósta and Fernanda found two indications of the crashing of a celestial body. The first was the presence in Jarau of so-called fissures of clastic rocks, i.e., rocks formed from the fragments of other rocks. “These fissures may have different origins; for instance, they can be volcanic,” explains Crósta. “But the ones we found at Jarau have the characteristics of those that are formed as a result of the impact of a meteorite,” says the geologist from Unicamp, who has already helped to identify other craters caused by impact in Brazil.
The second and more conclusive piece of evidence came from a laboratory examination of the rocks. Under a microscope, Crósta and Fernanda saw that the rocks’ grains of quartz had suffered a phenomenon called a planar fracture. These signs appear as parallel lines of vitrified material and they are different from the natural structure of quartz crystals. “This is irrefutable evidence, as it can only be generated by deformation due to impact,” states Crósta.
The Unicamp geologist explains that these grains are formed at pressure levels far higher than anything on the Earth’s crust. Only in the planet’s deeper regions, such as the mantle, which extends from 30 km to 2.9 thousand km under the surface, do temperatures of thousands of degrees Celsius and pressures hundreds of thousands of times greater than those of the atmosphere enable the formation of structures such as those found in impact craters. However, the Jarau rocks had the characteristics of surface rather than of mantle rocks. According to Crósta, only the energy released by the crash of a body such as a meteorite could produce the pressure and temperature required to cause this type of deformation of quartz on the Earth’s surface.
The results achieved by Crósta and Fernanda have turned the Jarau range of hills into the sixth impact crater (or astrobleme, a Greek expression that means “blemish made by a heavenly body”) in Brazil. The number is small but will tend to grow over time. Not that other meteorites are expected to fall in Brazil in forthcoming years. What should increase, however, is the understanding of those that hit the country in the distant past. Geologists believe that the number of astroblemes known in the Southern Hemisphere is small due to a lack of broad geological surveys.
The conclusions of the Unicamp group are to be published shortly in the book Large meteorite impacts IV, edited by the American Geological Society. However, this is not the only book that talks about the Jarau range of hills. In the early twentieth century, João Simões Lopes Neto, a writer from Rio Grande do Sul, immortalized the region in the book Lendas do Sul [Southern Legends], dated 1913. In the text “A Salamanca do Jarau,” Lopes Neto tells a very ancient tale about when the Arabs were expelled from Spain. Among those who took refuge in the South of Brazil was a beautiful Moorish princess who, after entering into a pact with Anhangá-pitã (the demon of the native Indians), became a salamander with a stone head, called Teiniaguá, which then hid in the Jarau range of hills. It is to this mythological princess, who resumed womanhood thanks to the love of a sexton, that the mixed ethnicity of the Rio Grande do Sul people is ascribed.
Legend and war
This legend became mixed up with national history in connection with the Farroupilha Revolution, when Rio Grande do Sul rebels confronted the imperial armed forces in the mid-nineteenth century. The farrapos [meaning tatters, due to the rebels’ tattered status] used the Jarau range, which was part of the property of the rebel general Bento Manuel Ribeiro, as a privileged observation post in the Pampas. The site, which became known as the Jarau Lookout, was never part of the theatre of war, but all of this added to the old legend of Teiniaguá. It is said in the area that Bento Ribeiro made a pact with the Jarau creature to protect himself from the dangers of the conflict.
The confirmation of the origin of the Jarau range represents merely the starting point of a lot of work. In forthcoming years, the geologists intend to determine, for example, the true dimensions of the crater and of the celestial body that causes it. “We estimate that the original crater was about 13 kilometers wide, but it’s hard to figure this out precisely because the edges have been totally eroded,” says Crósta.
This piece of information is fundamental for calculating precisely the size of the meteorite that crashed into the area. The researchers from Unicamp assume that it was a rock about 600 to 700 meters in diameter. However, Crósta himself warns us that “This is an estimate based on another estimate.”
Another issue among the priorities of the Unicamp geologist is to discover when this impact occurred. This question has not been easy to answer. To determine the crater’s age, it will be necessary to find rock samples that melted exactly at the moment of impact and to measure the proportion of isotopes of the chemical element argonium contained in them. The problem is that such rocks may be very similar to all the others in most of the Jarau terrain, which consists essentially of basalt, an igneous rock formed at high temperatures, such as those found inside volcanos. Discovering these rocks would be like finding a needle in a haystack. “The sample that we are looking for may be no larger than millimeters and we have to find it in an area with a 13-kilometer diameter [equal to a town such as Americana, in inner-state São Paulo],” states the researcher.
While these rocks are not found, one must make do with yet another estimate. “We have a maximum age which is that of the youngest rocks [basalt] affected by the impact; these are some 135 million years old,” says Crósta. As the edges of the crater have been quite worn down through erosion, the researchers imagine that the crater cannot be very recent and that it is some dozens to 100 million years old.
Such dating is important because it can reveal another history, hidden in geological registers. This scale of impact may have strongly affected life in the south of South America, causing considerable local extinctions. “It would not be comparable with the event that took place 65 million years ago [which produced the Chichxulub crater under the sea, in the Gulf of Mexico], when the large reptiles became extinct along with 60% of life on Earth, but it would have had a substantial regional impact,” states Crósta.
The large Jarau hills may yet reveal more than the Earth’s past. The crash of a meteorite into basalt rocks possibly caused specific transformations that might allow one to differentiate their evolution from that of other types of rocks – and even to understand details about how other rocky planets were formed, such as Mars and Venus, where there is a lot of basalt. It is likely that Jarau stories will come to life during the course of the next few years, though this time it will be thanks to the researchers.
CRÓSTA, A. P. et al. Cerro do Jarau, Rio Grande do Sul: a possible new impact structure in southern Brazil. In: Large meteorite impacts IV. The Geological Society of America.