Crater holds memory of the impact of a celestial body in a suburb of São Paulo

Microscopic analysis has confirmed that the Colônia Crater, near the Billings reservoir on the outskirts of Brazil's largest city, was produced by an asteroid or comet.

By José Tadeu Arantes  |  Agência FAPESP – Research has proven that the 10.2 square-kilometer Colônia Crater on the outskirts of São Paulo City in Brazil was produced by the impact of a celestial object millions of years ago. This geological formation is located on the southwestern edge of the Billings hydrographic basin less than 40 kilometers from Praça da Sé, which marks the city center.

The interior of the crater is filled with sediments, and the rim of the crater is covered with vegetation. The area was not recognized as an impact crater until the early 1960s, when aerial photographs and satellite images showed its circular shape. The first mention of the crater in specialized literature dates from 1961 and was published in Boletim da Sociedade Brasileira de Geologia in the article “Estudos preliminares de uma depressão circular na região de Colônia: Santo Amaro, São Paulo” by Rudolph Kollert, Alfredo Björnberg and André Davino, professors at the University of São Paulo (USP).

Because circular structures can result from other factors, such as volcanic activity, it was previously impossible to determine whether the crater was produced by an extraterrestrial body. Sufficient evidence for proving the impact hypothesis was presented in 2013 by geologist Victor Velázquez Fernandez, a professor at the University of São Paulo’s School of Arts, Sciences & Humanities (EACH-USP). Much of his work was done as part of a project supported by FAPESP that lasted until 2015 and was entitled “Registration of the geological and geomorphological elements from the Colônia Crater to establish a strategy of management and environmental preservation.”

Velázquez and collaborators reported their evidence in the International Journal of Geosciences. Their paper was entitled “Evidence of Shock Metamorphism Effects in Allochthonous Breccia Deposits from the Colônia Crater, São Paulo, Brazil.”

Later papers by Velázquez highlight another aspect of his research, which is the characterization of this geological formation as a natural heritage site that should be protected and preserved, including “The Colônia Impact Crater: Geological Heritage and Natural Patrimony in the Southern Metropolitan Region of São Paulo, Brazil” and “The Current Situation of Protection and Conservation of the Colônia Impact Crater, São Paulo, Brazil”.

“The impact created a hole with a diameter of 3.6 kilometers and a depth of approximately 300 meters,” Velázquez told Agência FAPESP. “Today a difference in altitude of up to 120 meters exists between the outer rim and the crater floor. Over time, intense weathering and sedimentation filled the hole almost completely and it became covered with vegetation, resulting in a flat area surrounded by hills. This evolution contrasts the evolution of other impact craters, such as Meteor Crater in the northern Arizona desert of the US, where the geological structure has remained practically intact because of the desert environment.”

In the case of the Colônia Crater, he went on, “it’s perfectly circular shape shown by aerial photographs provides strong evidence of an impact structure due to the collision of a comet or asteroid with the Earth’s surface but is not sufficient to confirm this hypothesis. Thus, we conducted a petrographic study and performed microscopic analysis of the sediments.”

According to Velázquez, the opportunity for a petrographic study arose when groundwater exploration was conducted for extracting drinking water, which included drilling boreholes in the sediment to a depth of 300 meters, reaching the crystalline basement rock. Thanks to cooperation between USP and the São Paulo State Basic Sanitation Company (SABESP), the university received sediment samples collected at 1-meter intervals.

“We found various types of evidence when analyzing the samples,” Velásquez recalled. “For example, there was very strong evidence of the transformation of several minerals, especially quartz and zircon. The transformation of these minerals requires pressure in excess of 40 kilobars, which is 40,000 times standard atmospheric pressure, and a temperature of approximately 5,000 degrees Celsius. Pressure and temperature at these levels are characteristic of the powerful release of energy that results from the impact of an object from interplanetary space.”

These findings and other solid evidence confirmed the impact hypothesis. The Colônia Crater is now included in the Earth Impact Database (EID), a list of the 188 confirmed impact structures from around the world. The EID is maintained by the Planetary & Space Science Center (PASSC) at the University of New Brunswick, Canada.

Geological Monument

It is not yet known whether the crater was caused by a rocky or metallic body, such as an asteroid, or by an object mainly made up of ice, such as a comet. “However, 50 new samples are currently being analyzed in Canada and could provide the missing data needed to determine what type of object caused the crater,” Velázquez said.

The ongoing investigation is also expected to date the event somewhat more precisely than current estimates, which range from 5 million to 36 million years ago.

“The importance of substantially improved dating is that it will create the conditions for a paleoclimate study based on the sediments found,” Velázquez said. “By analyzing the composition of the sediment column centimeter by centimeter from a depth of 300 meters, corresponding to the date of the impact, to the surface, corresponding to the present, we’ll be able to create a fairly accurate picture of how the climate evolved in South America, and by extension, worldwide.”

The Colônia Crater’s inestimable scientific importance lies in this potential, which cannot be realized unless it is protected and properly managed as a heritage site. The crater was declared a São Paulo State Geological Monument by the official body responsible for such things (COMGEO) in 2009, but part of the area was already occupied by that time.

“A small area at the northern boundary has a relatively longstanding settlement that was originally created by German immigrants who arrived in the 1840s, giving rise to the name Colônia Alemã, which means German colony. The reference to Germany was dropped during World War Two,” Velázquez said. “The more recently occupied area is larger. Urbanization of the occupied area began in the 1980s and 1990s and has been messy. It gave rise to a neighborhood called Vargem Grande, which currently has some 47,000 inhabitants,” and is not to be confused with Vargem Grande Paulista, a town in the metropolitan area, or Vargem Grande do Sul, a town near the large city of Campinas in a different part of São Paulo State.

The inhabitants of Vargem Grande in the crater are poor, and the urban amenities are highly deficient, as they are in most suburbs of major cities in Brazil. For example, no basic sanitation system exists in the area. The water table is near the surface because of the site’s sedimentary structure, so residents cannot excavate septic tanks and raw sewage flows openly through the area.

“In contrast with the northern rim, the southern portion is occupied by small permaculture farms engaged in organic market gardening. This is very interesting because these people are already committed to environmental conservation. Part of the area has dense forest cover and a few good trails for ramblers,” Velázquez said.

The importance of protecting the site as a geological monument led Velázquez and his collaborators to include topics relating to environmental conservation and land-use management in their research project.

“Focusing on protection and conservation, we’re working on three fronts. First, USP’s portal now features a website that offers a virtual tour of the crater, differentiating between researchers, students and the general public in terms of access,” Velázquez said. “Second, a comic is being distributed to primary schools in Vargem Grande that can be colored by local children so that they feel responsible for the area and become interested in helping to conserve it. The third front is fieldwork in environmental education and ecotourism.”

Since he began studying the area in 2005, Velázquez has observed a slow but consistent improvement in local community awareness regarding the importance of conserving the geological formation. “We interact well with the residents’ associations, which are now engaging with initiatives such as the Clean Crater Movement,” he said.

Only two of the 188 impact craters catalogued by EID are inhabited: Ries in Germany and Colônia in Brazil. In contrast with the latter, the 24-kilometer diameter Ries Crater is carefully managed and is the site of the first geopark in Bavaria, with an area of 1,800 square kilometers. The earliest remains of settlements in the Ries Crater date from the Paleolithic Period. With 53 communities, Ries Crater is the world’s most populous geopark.