A scar in the suburbs
In the dense urban sprawl of southern São Paulo, less than 40 kilometers from the city center, lies a nearly perfect circle 3.6 kilometers wide. This is the Colônia Crater, a geological formation whose true origin was hidden in plain sight for decades. From the ground, it appears as a wide, flat basin surrounded by a ring of hills that rise up to 125 meters. The structure was so subtle that its extraterrestrial beginning went unnoticed until it was spotted in aerial photographs in 1961.
Definitive proof of an impact event arrived much later. Geologists drilled into the crater floor, recovering sediment samples from hundreds of meters below the surface. Microscopic analysis of these samples revealed quartz and zircon crystals with fractures and deformities known as shock metamorphism. These features can only be formed by the immense pressures and temperatures of a hypervelocity impact. The findings indicate a pressure of 40 kilobars—40,000 times standard atmospheric pressure—and temperatures around 5,000 degrees Celsius at the moment of collision. The impactor is estimated to have been an object about 200 meters in diameter. The age of the event is still broad, with estimates placing it between 5 and 36 million years ago.
More recent studies have uncovered glassy spherules between 180 and 224 meters deep. These tiny spheres are melted rock that was ejected into a superheated cloud by the impact's energy, which then solidified and fell back into the newly formed depression. The chemical composition of these spherules matches the local rock, suggesting the original object may have been a comet composed mainly of ice, which would have vaporized completely.
A library of ancient climates
After its violent formation, the crater became a closed basin, gradually turning into a lake that trapped sediment for millions of years. This created a unique geological archive. Today, the crater is filled with a layer of sediment and peat up to 275 meters deep. This layered deposit is a record, preserving a continuous and detailed record of the region's climate history.
Scientists analyze sediment cores drilled from the crater, examining preserved pollen, spores, and charcoal layer by layer. This data allows them to reconstruct a picture of past vegetation and climate, particularly the history of the Atlantic Rainforest. The upper eight meters of sediment alone provide a detailed environmental record of the last 130,000 years. The full sediment column potentially holds one of South America's most complete records of climate change, stretching back millions of years.
The crater also has a bustling community. The neighborhood of Vargem Grande is located inside the crater floor, with around 40,000 residents. This makes the Colônia Crater one of two known inhabited impact craters in the world. The area faces challenges from irregular urban development that began in the 1980s, threatening the preservation of this important geological site.