A coincidental cosmic collision
The two near-perfect circles of Quebec's Lac à l'Eau Claire (Clearwater Lakes) look like the result of a binary asteroid, a pair of space rocks striking the Earth simultaneously. For decades, this was the accepted theory. Modern radiometric dating of the impact melt rocks tells a different story. The two impacts were not simultaneous at all; they are separated by approximately 180 million years. Their close proximity is now considered a remarkable coincidence.
The eastern crater, measuring 26 km (16.2 miles) in diameter, is the older of the two. It was formed by a 2.5-km-wide asteroid that struck a shallow sea during the Ordovician period, about 460-470 million years ago. The western crater is larger, at 36 km (22.5 miles) wide, and more recent. Its impactor, a slightly larger asteroid about 2.6 km wide, hit the continental landmass of Pangaea during the Permian period, approximately 286 million years ago.
Evidence etched in stone
The definitive proof of the lakes' cosmic origin lies in the rocks of the Canadian Shield they occupy. Geological surveys and drilling expeditions, some conducted on the frozen lakes in the 1960s, revealed unmistakable signs of shock metamorphism. These signs include shatter cones, which are conical fractures in rocks caused by high-pressure shock waves, and planar deformation features (PDFs) in quartz crystals. These microscopic structures can only be formed by the extreme pressures of a hypervelocity impact.
The western crater, being younger and less eroded, presents a more complex structure. A prominent ring of islands, about 10 km across, rises from the water. This is the central peak-ring of the crater, an uplifted structure of fractured bedrock and solidified impact melt. The eastern crater also has a central peak, but it is less pronounced and is submerged beneath the lake's surface. Analysis of melt rocks from the eastern crater suggests the impacting asteroid was a type of ordinary chondrite.