Giant Shatter Cone
August 01, 2007
The author is shown here standing on a 10 meter (33 foot) shatter cone, one of the largest found on Earth. It's located on one of the islands forming the central peak of the heavily eroded Slate Island Impact Structure. The islands are located 9.5 kilometers (6 miles) offshore in northern Lake Superior. Canada’s southern-most caribou herd resides here.
Approximately 436 million years ago, a ~1.5 kilometer (~0.9 mile) diameter bolide impacted here at ~25 kilometers (~15 miles) per second, creating a 32 kilometer (20 mile) diameter complex crater. Most of the crater is under the waters of Lake Superior, and only the 9 kilometer (5.7 mile) diameter central peak consisting of a group of islands is exposed. The shock waves generated from the bolide impact formed the many shatter cones found here in the Slate Island’s crystalline rocks.
Shatter cones are shock-deformation features that form from impact pressures of typically 2-10 gigapascals (GPa) up to ~30 GPa. They represent the only distinctive and unique shock-deformation feature from impacts that develop on a megascopic scale (e.g., hand sample to outcrop scale). Shatter cones appear as distinctively curved striated fractures that typically form partial or complete conical structures. They're commonly found beneath impact crater floors, but they may also be observed in isolated rock fragments within brecciated units.