Two impact craters, one inside the other, from separate meteorite strikes millions of years apart. The larger is 28 km wide. Lake Mistassini fills one crater, creating a distinctively shaped lake that puzzled geologists for decades.
Public domain, via Wikimedia Commons
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.
💡Fun Facts
For decades, scientists believed the two craters were formed by a single binary asteroid striking the Earth.
The western crater’s 10-km-wide ring of islands is the eroded central peak of the original complex impact structure.
The Ordovician impact (East crater) struck a shallow ocean, while the Permian impact (West crater) hit the supercontinent of Pangaea.
The craters are located within Tursujuq National Park, the largest provincial park in Quebec, covering over 26,000 square kilometers.
The park is remote and accessible year-round, but travel is highly dependent on weather conditions. The primary visiting season is summer (July-August).
Admission
Park access fees and activity fees apply. Costs for multi-day packages can be several thousand dollars.
Accessibility
The site is extremely remote with no road access. Visitors must arrive by charter flight, typically a floatplane, from the communities of Umiujaq or Kuujjuarapik. The terrain is rugged and undeveloped.
