A scar on the arctic
The Tunnunik Impact Structure is a massive, roughly 28-kilometer wide scar located on Victoria Island in the Canadian Arctic. For much of history, its true origin was a mystery. From the air, it appears as a roughly circular feature in an otherwise flat landscape of Paleozoic sedimentary rocks. Geologists had noted the anomaly, but without definitive proof, its formation remained unexplained. The entire region was scraped and flattened by glaciers, which removed the crater's rim and any melted rock, leaving only the eroded "roots" of the structure visible today.
The breakthrough came in 2010. Geologist Brian Pratt from the University of Saskatchewan and Keith Dewing of the Geological Survey of Canada were conducting an aerial survey. They identified shatter cones—cone-shaped fractures in rocks with distinctive radiating lines—in the center of the circular feature. These features are considered definitive proof of a meteorite impact because they can only be formed by the immense shockwave, traveling faster than the speed of sound through rock, that such an event produces. Their discovery confirmed Tunnunik as Canada's 30th known meteorite impact structure.
Deep time history
The impactor that created the Tunnunik structure is estimated to have been several kilometers in diameter. It slammed into what was then a shallow sea, striking layers of Cambrian to Silurian-aged sedimentary rocks, including dolomite and shale. The force of the impact was so great it brought much older rock from deep below to the surface; the oldest rocks now exposed in the center belong to the Neoproterozoic Shaler Supergroup.
Pinpointing the exact age of the impact is difficult. Paleomagnetic analysis suggests it may have occurred between 430 and 450 million years ago, during the Late Ordovician or Silurian period. Other analyses of the affected rock layers constrain the event to a broad window between 130 and 350 million years ago. The structure has a central uplift, typical for a complex crater of this size, which has been deeply eroded but is still identifiable. A two-kilometer-long canyon cuts through the edge of this uplift, providing an excellent cross-section for geological study.