At 62 miles (100 km) across, the Manicouigan structure in Ontario is one of the largest known impact structures in Canada, and the fifth-largest known impact structure on Earth. This circular, partly exposed crater was formed by the impact of a meteorite with a three-mile (5-km) diameter with Earth 214 million years ago, hitting what is now the Pre-cambrian shield. The Manicouigan River flows south out of the south side of the crater and drains into the St. Lawrence River. For some time it was thought that the Manicouigan crater resulted from the impact that caused a mass extinction that killed 60 percent of all species on Earth at the Permian-Triassic boundary, but dating the impact melt showed that the crater was 12 million years too old to be associated with that event.
The Manicouigan crater has been deeply eroded by the Pleistocene glaciers that scraped the loose sediments off the Canadian shield, pushing them south into the United States. The crater, exposed in bedrock, is currently delineated by two semicircular lakes that are part of a hydroelectric dam project.
The Manicouigan structure has a large central dome, rising 1,640 feet (500 m) above the surrounding surface, culminating at Mount de Babel peak. Manicouigan is characterized by huge amounts of broken, brecciated rocks and shatter cones that point toward the center of the dome. Shatter cones are cone-shaped fractures that form during impacts from the shock wave passing through the adjacent rock, and typically have tips or apexes that point toward the point of impact, but may have more complex patterns that form by the shock waves bouncing off other surfaces in the bedrock.
The Manicouigan crater also has a thick layer of an igneous rock called an impact melt, generated when the force of the impact causes the meteorite and the rock it crashes into to vaporize and melt, and then the melt can fill the resulting crater. The impact melt layer at Manicouigan is more than 325 feet (100 m) thick. There have also been high-pressure mineral phases discovered in the rocks from the Manicouigan structure—phases that form at pressures that could occur only as a result of a meteorite impact and are impossible to reach by other mechanisms at shallow levels of Earth's crust.
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